Med. & Biol. Eng. & Comput., |977, 15, 168-178
Implementing an automatic control system for dynamic radiography Malcolm
L. H e i m e r
Robert
Shiroff
Department of Electrical Engineering, Pennsylvania State University
Department of Cardiology, PennsylvaniaState UniversityCollegeof Medicine,The Milton S. HersheyMedical Center,PennsylvaniaState University, Hershey, Pa. 17033, USA
Alan M. Jacobs
William
Edward
S. K e n n e y
Department of Nuclear Engineering, Pennsylvania State University
A. Weidner
Department of Radiology, Pennsylvania State University College of Medicine, The Milton S. Hershey Medical Center, Pennsylvania State University, Hershey, Pa. 17033, USA
Abstract--The basic detection system used for dynamic radiography is reviewed. Some practical factors are discussed in the application of dynamic radiography to the mapping of epicardial motion in canines. A fluoroscopic aiming technique is described for hitting specific points on the cardiac silhouette. The development and design of an automatic control system for dynamic radiography is covered. This system accomplishes automatic detector scanning to the proper depth, as well as calibration of the output signal This output displays the surface position in millimetres. Some results from canine studies are discussed.
Keywords--Heart motion, Scattered radiation
1 Introduction DYNAMIC radiography works by detecting a portion of the scattered radiation produced by the interaction between an impinging beam of radiation and a target material. (JACOBS and KENNEY, 1972). The detector output signal is proportional to the density of that target material which occupies a small and spacially well-defined volume, which can be projected inside an opaque structure. This detecting volume (sensitive volume) can be moved from point to point within the target, while the detector output gives an indication of the density distributions at these points. To use the system for motion imaging, the position of the sensitive volume is fixed to enclose a boundary between high- and low-density materials. In this configuration, the detector output is determined by the degree to which the sensitive volume is filled by the highdensity material. A movement of the boundary which changes this degree of filling also causes a proportional change in the detector output. Thus the a.c. component of the detector output can be used to image the motion of such a boundary lying inside an opaque target. The motion imaging capability of t h e dynamic radiography detection system was validated using both in vitro and in vivo targets (HEIMER, 1976). In these tests, independent imaging techniques
showed boundary motion which agreed with the dynamic-radiography outputs. The dynamic-radiography technique was first applied to a biological system with the imaging of canine epicardial motion. In these studies, carried out at the Milton S. Hershey Center, the single-spot dynamic radiography system permitted imaging motion from only one spot at a time on the epicardium (TILLEY e t al, 1976). Detector systems currently being developed will employ arrays of detectors to simultaneously image epicardial motion from many points. The developments described in this article are viewed as an important step in the evolution of dynamic radiography from single-spot detection to area detection. F o r this intermediate stage, sequentially recorded motion signals from a number of heart points were used to approximate the performance of an area-detector system. To do this, it was necessary to develop a format to select target points on the epicardium, a fluoroscopic aiming technique to hit those spots and an electronic control system to optimally position the dynamic radiography detectors. Many of these developments will be used in the later dynamic radiography systems using the area detectors. Also, the results from these canine studies offer a preview of the results that might be expected from these later systems.
Single-spot dynamic radiography detector system First received8th Februaryand in final form 15th July 1976
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The geometry of the present dynamic radiography
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detector assembly is shown in Fig. 1. A narrow beam of X-radiation (8mm diameter) is used in conjunction with two independent detectors. Each scintillation detector consists of a focused collimator, a sodium-iodide crystal and a photomultiplier tube. The field of view of each detector, shown by the broken lines, converges to a span of 10ram along the X-ray beam. The sensitive volume is defined by this intersection of the field of view and the X-ray beam. Each detector is mounted on a linear bearing track and can be driven by a servomotor on a path. parallel to the X-ray beam. Thus the depth positions of the sensitive volumes can be electrically controlled over a 160mm range. F o r these studies, a position slaving servo-circuit forced the sensitive volumes to remain superimposed. This means that the two detectors always imaged the same motion, permitting redundancy and an improvement in the signal-to-noise ratio. I n the earlier canine dynamic radiography studies, epicardial motion was recorded at a single spot on the central left ventricular wall. Amplitude frequency spectral analysis of this motion signal was used to indicate the state of the heart (WEIDNER, et al; 1975). This approach did not utilise the spacial resolution inherent in the system. A motion signal recorded by dynamic radiography is generated by a small (10mm diameter) spot on the epicardium. By recording a number of these spot motions, the spacial variations over the heart's surface can be studied. It is hoped that such a mapping study will eventually make it possible to accurately diagnose ischaemic and infarcted regions.
Practical factors in the use of dynamic radiography The present clinical study series involves mapping epicardial motion over large regions of the heart. To accomplish this, it is necessary to draw the cardiac silhouette, select target spots at which motion is to be recorded and then be able to position the sensitive volume on these spots. The aiming procedure consists of two separate steps: positioning the X-ray beam to hit the epicardium at the desired point; and scanning the detectors to place the sensitive volumes at the correct depth. A fluoroscopic technique was developed for aiming the X-ray beam. The configuration is shown in Fig. 2. consisting of the X-ray source, image intensifier and the television monitor. The X-ray source and image intensifier are positioned exactly 0" 9 m apart, with the X-ray spot centred on the monitor screen. A 10 mm-spaced wire grid is centred in the target plane, halfway between the source and the image intensifier. A permanent overlay template is prepared by tracing the grid projection on the monitor screen. Essentially, this accomplishes a transformation of the true rectilinear co-ordinate system i n t h e target plane to the screen image, which is magnified and slightly distorted. When the canine heart is positioned in the target plane, the cardiac silhouette is drawn on the overlay template. Now, graphically selected target points are translated to grid co-ordinates. Hitting these target points is then reduced to moving the heart 10 mm for each grid increment. In early studies, the detectors were manually scanned while the a.c. component of the output
Fig. I Detector configuration for the single-spot dynamic-radiography system Medical & Biological Engineering & Computing
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signal was monitored on an oscilloscope. The detectors were stopped at the location that appeared to give the maximum amplitude for the epicardial motion signal. This process proved to be slow and unreliable, especially for heart points with small motion amplitudes. It was decided that the d.c.coupled detector signal could be used instead for
this positioning process. As shown in Fig. 3, there is a clear detector response as the sensitive volume crosses the epicardial surface. The present positioning procedure uses this detector response, which is not dependent on any epicardial motion, to bring the detectors to the optimal position. Time duration is an important factor in these
Fig. 2 Fluoroscopic aiming set up with the wire grid placed in the target plane
Fig. 3 Detector-output voltage against sensitive volume depth ZD, during a scan into a canme thorax
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mapping studies. Ideally, all of the spot-motion waveforms should be recorded simultaneously. In this way, motion waveforms from the same heartbeat could be compared for diagnosis. In lieu, of this, it is best to have as short a time as possible between recordings of motions from different points. This reduces the effects of long-term changes in the physiological state of the heart of the subject. However, it still leaves the effect of the beat-tobeat variations in the heart motion. Timing is also an important factor in each individual detector-scanning procedure. During the detector scanning and the subsequent data recording, the respiration of the subject must be halted. This is necessary so that respiratory motion does not shift the heart position. This would cause errors in the aiming procedure and also it would completely mask the much smaller motion due to contraction of the heart. F o r the present studies, using anaesthetised dogs, respiration is halted rather easily by controlling the respirator. However, for projected future use on human subjects, asking a conscious cardiac patient to voluntarily hold his breath for a long period is quite impractical. Another factor to be considered here is that the X-ray beam is irradiating the patient during both scanning and recording of motion waveforms. Thus, it is desirable to keep this total procedure as short as possible to minimise the radiation dosage to the patient.
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System performance objectives
System design
The clinical factors listed above point at the need for an automatic control system for use in dynamic radiography studies. It was decided to design a closed-loop system which processes the detector signals and generates the drive for the detector scan motor. This system should be able to carry out the complete scan procedure, bringing the detector to the optimum location with no intervention from the operator. The design objective was for this procedure to be carried out within a 10 s span. Another important design objective was automatic calibration of the output signal. This would make it possible to interpret the motion signals directly as millimetres of displacement. It was felt that being able to measure the motion amplitude is important for diagnosis of myocardial contractile states. The basis of the control-system operation is the scan-signal profile shown in Fig. 3. An automatic scan sequence is to begin with the sensitive volume positioned in the lung field and then to scan it into the myocardium, just past the point of peak response. During this scan, peak detectors store both the maximum and minimum signal levels encountered. These stored voltages can then be used to accomplish both optimal detector positioning and calibration of motion signals.
The design of the electronic scan-control system was begun with a detailed examination of the detectors' spacial sensitivity. Fig. 4a shows the measured detector response against the position of a small target in the X-ray beam. This curve, which is characteristic of the detector's focused collimator, defines the sensitive volume along the X-ray beam axis. Clearly, the sensitive volume has no well-defined boundaries on this axis, but it drops Off nearly linearly with distance from the centre of the detector field of view. For designing the control system, the response curve of interest is the spatial response of the detector to a boundary between high- and lowdensity materials. This response is approximated by the broken curve in Fig. 4b, generated by i n tegrating the curve in Fig. 4a. This broken curve is not the true boundary response of the detector, because it neglects the attenuation of the X-ray beam within the target material. To correct this, the dynamic radiography detection process was mathematically modelled and programmed for a Hewlett Packard HP-65 calculator. The detector's spacial response was simulated for a boundary between materials with densities of air and water. This result was empirically duplicated using an actual air-to-water boundary. The two sets of data were normalised and were found to match. giving the solid curve of Fig. 4b. This curve constitutes the transfer function between the boundary position (relative to the detector position) and the resulting detector output. The transfer-function curve of Fig. 4b can be applied to the scanning of the epicardial surface. The shape of this curve is determined by the curve of Fig. 4a and by the X-ray attenuation constant of the high-density material at the boundary. The attenuation constant of water is essentially the same as that of soft tissue, including myocardium. Thus scanning the detector across the epicardial surface generates a spatial-response curve whose shape is identical to the solid curve of Fig. 4b. The height of this curve during an epicardial scan is variable, depending mainly on the amount of X-ray attenuation occurrring in the thoracic walls of the subject. The boundary response of Fig. 4b was used in the control-system design, It demonstrates that when the detector position is centred on the boundary, the signal level is 6 0 ~ of maximum. In several simulated scans over different regions of a sphere, this level varied from 58 to 6 3 ~ of maximum; so 6 0 ~ was chosen. This response curve is also used for calibration, since it comprises the transfer function between the boundary location and the detector response. The automatic calibration system sets the gain of the system so that the height of this curve is a known value. With a known transferfunction curve, and knowing the mean position of
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operation on that curve, the output voltage can be directly translated into millimetres of excursion. Unfortunately, this transfer function is nonlinear, so that, for larger values of motion, correction factors must be included. The automatic control system can be spilt into three sections with different functions. These are: the sequencer; the signal processors; and the scan motor driver. The sequencer counts through the five modes that make up a complete scan sequence. In each of these modes, it generates cohtrol signals which define the operating parameters o f the signal processors and of the scan motor driver. The
INHIBIT >
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sequencer also awaits a specific event which causes the advancing to the next mode of the sequence. The block diagram of a signal processor from one channel is shown in Fig. 5. The detector signal is amplified to drive a meter and a pair of peak detectors. The peak detectors store the maximum and minimum signal levels encountered following the termination of the reset a n d / o r inhibit signalsA pair of signals is generated by differential aml plifiers following these detectors. The AI/ signa. is simply the difference between the outputs of the two peak detectors. This is used to set the digitally controlled attenuator (d.c.a.). The peak-detector
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Fig. 6 Channel-1 scan driver subsystem
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outputs are combined to generate a V,~s...... (as defined earlier, the 60% response corresponding with the optimum detector position) and a differential amplifier subtracts this from the present detector signal. The resulting (Vdetector -- ~/'ref . . . . . . ) is used for the final positioning of the detector. A single-pole/double-throw electronic switch controls the selection of differential-amplifier outputs fed to the d.c.a. The reset signal sets the d.c.a, at maximum gain to begin a scan. As the scan progresses, increasing values of the A V signal cause the attenuation level to increase, keeping the output level of the d.c.a, at exactly 4 V. At the end of tbe scan sequence, the hold signal freezes the attenuation at its present value. Switching the input of the d.c.a. to the detector signal then results in a calibrated signal output. The sequential operation of this section of the system will be discussed in more detail later. The sequencer section of the system is based on a b.c.d, counter and a decoder which counts through modes 0 to 4. These mode signals are fed through logic gates, which generate the control signals for the signal-processor and scan-motor-drive sections. These control signals also gate the sequencer clock, so that only one specific event can cause the sequencer to advance to the next mode. The scan motor drive section is shown in Fig. 6. This is a simple servo loop, with the amplified error signals supplying the motor drive. The bipolar motor voltage is derived from a unipolar output by using a polarity reversing relay. This necessitates the absolute value amplifier and comparator following the error amplifier. A set of analogue electronic switches operated by control signals from the sequencer determine the inputs to the error amplifier. F o r most of the scan sequence, one of two potentiometer output signals is compared with the detector-position signal. In the final positioning mode, both of these potentiometer signals are turned off so that this input is at 0 V. The other input to the error amplifier is now the (V~ete,o, -V,eser.... ) signal. Under these conditions in the normal situation, the system will scan until (V,~ete,o, -- V,ef...... ) = 0. AS described earlier, this results in the detector being centred on the epicardial boundary. The complete operation of the scan sequence will now be described with, reference to the system diagram and the scan signal profile of Fig. 3. In mode-d.c.a.s 0 (system reset), the peak detectors and d.c.a.s are all reset and the detector positions are determined by the output of a potentiometer. A switch causes the sequencer to go to mode-l, where the detector begins scanning out toward a position set by the other potentiometer. The negative peak detector is enabled, and the input to the d.c.a. is the A V signal. The sequencer block in this mode is supplied by a signal labelled 'DENSITY'. This density signal is generated by differentiating the Medical & Biological Engineering & Computing
detector signal. When the rate of increase of the detector signal exceeds a threshold level, this indicates that the sensitive volume has entered the high-density myocardium. The resulting signal then clocks the sequencer to mode-2. In mode-2, the detector scanning continues out as before and the positive peak detectors are now enabled. This early inhibition of the positive peak detector is performed to prevent the storage of any possible peaks from scanning within the thoracic wall. The A V signal continues to increase, causing the attenuation in the d.c.a, to increase also, keeping its output at 4V. The sequencer clock is now waiting for the event that the scan has passed by the peak signal response. This event has occurred when a full cardiac cycle passes with no further increase in the A V signal. The occurrence of this event causes the sequencer to switch to mode-3, the final-positioning mode. In mode-3, all information for calibration and final location has been stored, so the peak detectors and d.c.a, are held in their present states. The signal (Va~t~to, - V r e f . . . . . . ) replaces the AV signal at the input of the d.c.a. As previously stated, the scan control is set to cause the signal (Vaetecto,Vrel.... ~) to go to 0. The scan direction now reverses and proceeds until this occurs, which causes the sequencer to switch to mode-4. In mode-4, the scan drive is shut-off and the calibrated output signal is available for recording. If the scan-control loop were not disabled in mode-4, the detectors would attempt to track the epicardial motion itself.
Circuit design In the designing and building of the circuitry for this system, low cost was a most important factor. The circuitry includes an analogue-signalprocessing section and digital sections for control and sequencing. The analogue circuitry was built using the L M 324 and 741 operational amplifiers and using 0"25 watt., _+5 ~ carbon-composition resistors. This type of design was considered adequate, because of the relatively low signal-tonoise ratio of the signals being processed. Most of the digital circuitry was built using SN7400 series integrated circuits. This logic family was chosen primarily because of low cost, easy availability of the parts and previous design experience with the circuits. The logic signals in this system are based on relatively slow mechanical events, so the SN7400 series is much faster than needed. Some circuitry is implemented using the c.o.s.-m.o.s. 4000 series of intregrated circuits, which provide more efficient package density than does the SN7400 series. It also includes an electronic switching function which cannot be attained from either of the other two logic familities. This cos-mos 4000 series was used primarily in the peak detectors, analogue switches and the d.c.a.s. March 1977
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Much of the circuitry is just the straightforward application of standard designs. However, considerable design work went into the peak detectors and the d.c.a.s. A schematic diagram of a positive peak detector is shown in Fig. 7. Basically, it consists of a c.o.s./m.o.s, analogue switch, a unitygain amplifier and a comparator. Referring to Fig. 7, the circuit operates as follows. Whenever Vt, exceeds Vo,,, the comparator turns on the analogue switch to increase the voltage on the capacitor and therefore increase Vo,t. The circuitly involving resistors R3 and R4 causes the comparator input signal to be approximately 10 mV greater than Vo,t. This compensates for the input offset voltages in the amplifier and the comparator,
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which could cause the circuit to lock-in the ON state. This 10 mV constitutes an error in the operation of the peak detector but this is relatively small compared with the 1 to 5 V signal levels. Resistor R2 provides some filtering, so that the peak detector does not respond to noise spikes in the input signal. Resistor R1 allows external signals to control the analogue switch, over-riding the comparator signal. The RESET signal causes the analogue switch to be on at all times, so that the Vo., = Vt.. The H O L D signal keeps the analogue switch off, so that V,,.t stays at its present value, regardless of the level of Vi,,. In the hold mode, the decay rate of the output is very low, being less than 1 mV per minute. This is because of the
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Fig. 8 Circuit diagram of a digitally controlled attenuator used for automatic calibration
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extremely low bias current of the Teledyne 1029 operational amplifier < 1 pA and the low O F F state leakage current of the CD 4016 switch (10 pA). A schematic diagram of the d.c.a, is shown in Fig. 8. The gain of the amplifier is controlled by the state of the c.o.s./m.o.s, binary counter, CD 4020. The values of resistors R3 through R8 are binary weighted, and they can be switched in parallel with feedback resistor R2 by the analogue switches. Since the output of the binary counter controls the switches, it can set the amplifier gain over a range of 64 to 1. The function of the d.c.a, is to keep the output level at 4 V as the input A V monotonically increases during the scan. The operation of this circuit to accomplish that is as follows: The scan sequence began with the counter reset so that all analogue switches are open and the amplifier gain is at its maximum. As Vi. increases and causes Vo.t to exceed 4 V, the comparator gates clock pulses into the counter, increasing the count and consequently reducing the gain until the output falls to 4 V. As the scan continues, the input signal increases until the transition from lung field to myocardium is complete. At this point, calibration is complete, with the voltage swing from lung field to myocardium resulting in exactly 4 V at the output
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of the d.c.a. Then the HOLD signal disables the clock oscillator to ensure no further changes in attenuation level when the motion signal (Vd,re~tor -Vref. . . . . . ) is passed through this same amplifier. There were time factors to be considered in the design of the d.c.a. The clock oscillator operates at a frequency of 200 Hz. This means that the d.c.a, can sequence through its entire range of 64 levels in approximately 300 ms. Since the highest heart rates encountered are normally around 3 beats/s, this means that the system is capable of changing over its entire range over one cardiac cycle. For worst-case design, this is a desirable condition. Capacitor C1 provides filtering in the amplifier of the d.c.a, and prevents the appearance of voltage spikes on the output from the switching action. If these spikes were allowed to pass through and appear on the input of the comparator, they could cause extra clock pulses which could result in erroneous operation of the d.c.a. However, if C1 is made too large, it can also result in errors in operation. For each switching of resistors, Vo,t does not change instantaneously, but changes to its new level exponentially at a time constant determined by CI in parallel with the combination of resistors. For reliable operation, the time constant of this amplifier circuit should be less than the period of the clock oscillator. The circuit was designed with a time constant varying from 4 ms at maximum gain to 60 ps at mimimum gain. Since the clock period is 5 ms, this is felt to be a safe design. The comparator output is a logic signal labelled DCA GATE, which is used in the sequencer circuitry. This signal is used in sensing that the scan has passed into the myocardium past the point of peak signal level. During this scanning, an increasing signal level will always result in an increased attenuation in the d.c.a, and thus in an appearance of the DCA GATE signal. The condition of passing through an entire cardiac cycle with no appearance of the DCA-GATE signal is used as an indication of having passed peak response.
System performance The operation of this system together with the dynamic-radiography detector assembly has been evaluated both with a simulated target and in clinical canine studies. This simulated target consists of a plastic cylinder filled with water. This is mounted on an electric solenoid which moves it between two positions, producing a square motion waveform. Both the range of the motion and the rate of switching can be adjusted by the operator. This presents a controlled air-to-water boundary, which can be scanned to test the positioning and calibration performance of the system. Medical & Biological Engineering & Computing
The accuracy of the motion calibration was tested by measuring the output signals from a known displacement of the simulated target and comparing these voltages to the theoretical values from the curve of Fig. 4b. For this testing, many scans were made to allow for variations in the calibration due to noise and due to variations in the final detector location. Also, the detector signals themselves were varied, over a ratio of more than 2:1, by changing the voltage to the photomultiplier tubes in the detectors. This simulates variations in shielding due to the make-up of the subject. The largest data base was taken with 4 mm of target motion. In this case, 40 samples were taken; resulting in a mean output of 1 "76 V_+ 0.09 V standard deviation. The theoretical signal level for this condition from the curve is 1" 69 V. The results of tests of channel-1 at different motion amplitudes are shown in Table 1. Table 1. Channel-1 calibration tests
Theoretical Motion
Samples
2mm 4 6 8
10 40 10 10
Output 1"13_+O'09volts 1 -76+_0'09 2"46+0'18 2'64+0"25
output 0.09 1 -69 2"39 2"78
Another series of test scans was made using the simulated target to determine the positioning accuracy of the system. The scan-motor-drive electronics includes a digital position readout derived from a potentiometer connected to the scan-drive assembly. This allows the position of the channel-1 detector to be read out to within 0" 1 mm. For this test, the final detector positions was read out over many scans, both with and without target motion, As previously stated, the final positioning relies on the difference between the detector signal and a reference signal to generate an error voltage which drives the scan motor. As the detector is scanning back in the final phase of the scan sequence, the scan is terminated t h e first time, this error goes to 0. Since the detector signal includes an a.c. component owing to the motion of the target, this allows for variations in the final location of the scan. To give the system operation more flexibility, a manual rescan switch permits the operator to reactivate this final positioning loop, even though the automatic sequence is terminated. The loop includes a lowpass filter to reduce the tendency of the detector to attempt to track the cyclic boundary motion itself. Unfortunately, this filter introduces a lag which causes the positioning loop to be unstable. As a result, with the rescan button depressed, the detector will hunt over a range of about + 3 mm around the true boundary position. March 1977
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In these positioning tests, scans were made using the simulated target both with no motion and with 4 mm of motion. For each run, the detector position at the end of the automatic sequence was recorded. Then the manual rescan mode was used to vary the position for five more readings. The variations in final detector position at the end of the automatic scan sequence was 3 mm, with no target motion, and it was 2' 1 mm with 4 mm of target motion. When the manual rescan mode was utilised, the final positions had a standard deviation of 0" 8 mm with no target motion and a standard deviation of 1.5 mm with 4 mm of target motion. The position of the detector during data recording is important for two reasons. First, this information allows 3-dimensional mapping of the epicardium. Secondly, the signal level is changed by the boundary positions within the detector field of view. This can be seen from Fig. 4b. For example, a 4 m m boundary motion with the boundary centred produces 1" 69 V output. But if it is moved out 2 mm, the output is reduced to 1"22 V. Any such errors in the final position of the detector during recording can be picked up, since they will result in a shift of the d.c. level of the output signal. Thus corrections can be made for the true boundary location and for the calibration of the signal. For the next-generation system, it is felt that the calibrated output should be passed through a nonlinear amplifier to correct the curve of Fig. 4b to a linear voltage-against-position relationship. This would eliminate both the dependence of the calibration on the detector position and the compression of signal output for larger motions.
Clinical use of the system This system has been used in several clinical studies with canine subjects (TILLEY et al., 1976). One study has involved open-chested dogs with coronary artery ligations. After a thoracotomy, ligatures are positioned around one or more coronary artery branches. Epicardial motion is recorded at eight to ten points on the heart, followed by tightening the ligatures to produce occlusion of these arteries. Then epicardial motion is again recorded 20 and 90 min post ligation. The motion waveforms resulting from one such study are shown in Fig. 9. The points at which motion was recorded and the location of the ischemic myocardium are shown on the cardiac silhouette. For each point, the motion waveforms are shown for preligation, 20 and 90 min post ligation. The horizontal lines on the waveforms represent 1 mm of displacement and the vertical lines coincide with the R-wave of the e.c.g, waveform. The application of dynamic radiography to openchested dogs has presented some physical problems. During these acute studies, the chest must be left open to allow access to the ligatures without moving the dog or its heart. However, the pericardial sac must be left intact enough to prevent too much shift in the heart position. There have also been some problems with high-density accumulations in the lower part of the thoracic cavity. These seem to be attributable to either postoperative bleeding or to fluid accumulation in the exposed lung. This system has operated best in studies with intact dogs. Fig. 10 shows the epicardial motion map of the same dog as for Fig. 9, taken two weeks
Fig. 9 Graphical display of an open-chest canine study
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prior to the open-chest procedure. The difference in motion waveforms between these two Figures may be attributed to mechanical differences due to the open-chest condition in Fig. 9 and to the differing heart rates between the two studies. A clinical study is presently being undertaken in which coronary-artery occlusion is produced in closed-chest dogs using a catheter. Dynamic radiography is then used in conjunction with other diagnostic techniques to evaluate the size and extent of any myocardial infarction resulting from these occlusions. Another clinical study was performed using eight dogs which had undergone coronary-artery ligation four weeks earlier in a student surgery course. It was found that these dogs had adhesions and fibrosis around the ligation sites which interfered with the automatic scan sequence. By reverting to manual control, it was possible to get uncalibrated dynamic-radiography signals for most cases. However, the low-density gradient at the epicardial boundary resultes in very poor signalto-noise ratios. This study points up the requirement for a low density adjacent to the epicardium to achieve good performance. The system performance during these clinical canine studies has been good. The automatic scan sequence, averaged about 12s in duration. In control studies, the motion amplitude has been reproducible to within about + 107o. This is about the order of variability indicated in the simulation tests. Perhaps the most serious problem encountered has been the appearance in some cases o f high densities in the lung field of the subject. Some
manual controls have been added to the system to allow the operator to aid the automatic sequence. In these cases, however, the poor density gradient results in a low signal-to-noise ratio and the usefulness of such signals is questionable.
Summary A dynamic-radiography system, consisting of a detector assembly and control electronics, can be used to map epicardial motion. 2-dimensional aiming at points on the heart is performed by specially developed fluoroscopic techniques. The detectors are scanned along the third dimension by the control electronics. This 12s sequence correctly positions the detectors and automatically calibrates the output signals. By combining the data, a 3-dimensional map of the epicardium can be generated with calibrated motion waveforms shown for each target point. Tests have indicated that the motion-amplitude calibration is accurate to within + 1 0 ~ . The system has worked successfully in both open- and closed-chest canine studies. Problems have occurred when high-density masses are present around the epicardial surface. Manual controls permit the recording of degraded motion signals in these cases. Clinical studies are underway to determine the changes in epicardial motion with coronary-artery occlusion. This should lead to being able to evaluate the extent of infarcts using this noninvasive technique.
Fig. 10 Graphical display of closed-chest canine wave-forms
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References JACOBS, A. M. and KENNEY, E. S. (1972) Dynamic radiography. A new imaging technique using penetrating radiation. Proc. Soc. Photo-Opt. Instr. Engrs. 29, 17-22 HEIMER, M. L. (1976) Developing a control system for mapping epicardial motion with dynamic radiography. Ph.D. thesis, Department of Electrical Engineering at the Pennsylvania State University.
WEIDNER,W. A., JACOaS,A. M., KENNEY,E. S., MILLER, K. L. and TILLEY,D. G. (1975) Dynamic radiography-An evaluation of cardiac motion by the analysis of scattered radiation during fluoroscopy. Investigative Radiology 10, 132-139 TILLEY, D. G., JACOBS,A. M., KENNEY, E. S., WEIDNER, W. A. and MILLER,K. M. Dynamic radiography-A technique employing scattered radiation to monitor surface motion. Med & BioL Eng. 14, 141-150
Implementation eines automatischen Regelsystems f/Jr dynamische Radiographie Zusammenfassung--Das f~ir dynamische Radiographic verwendete grundlegende Aufdeckungs-system wird besprochen. Einige praktische Faktoren bei dcr Anwcndung von dynamischer Radiographic auf die Abbildung epikardialer Bewegung bei Hunden werden diskutiert. Es wird cin fluoroskopisches Zielverfahren zum Treffen spezifischer Punkte auf der Herz-Silhoutette beschricben. Die Entwicklung und Konstruktion eines automatischen Regelsystems flit D R G wird eingeschlossen. Dieses System erzielt automatische Detektorabtastung auf die richtige Tiefe, wie auch Kalibrierung des Ausgangssignals. Dieser Ausgang zeigt die Oberfl/~chenlage in mm an. Einige Resultate aus Untersuchungen an Hunden werden besprochen.
Mise en oeuvre d'un syst&me de rdglage automatique pour la radiographie dynamique Sommaire--Le syst~mc de d6tection de base utilis6 en radiographic dynamique cst pass6 en revue. On 6tudie certains facteurs d'ordre pratique intervenant dans l'application de la radiographic dynamique au trac6 du mouvement 6picardiaque chez les chiens. On expose une technique de vis6e fluoroscopique pour atteindre des points particuliers de la silhouette cardiaque. La mise au point et la conception d'un syst~me de r6glage automatique en radiographie dynamique sont trait6es. Ce syst~me permet une d6tection par balayage automatiquc h la profondeur voulue ainsi que l'6talonnage du signal de sortie. Le signal de sortie permet de montrer la position en millim&res de la surface. Certains r6sultats obtenus par des 6tudcs sur des chiens sont 6valu6s.
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Medical & Biological Engineering & Computing
March 1977
Implementing an automatic control system for dynamic radiography Malcolm
L. H e i m e r
Robert
Shiroff
Department of Electrical Engineering, Pennsylvania State University
Department of Cardiology, PennsylvaniaState UniversityCollegeof Medicine,The Milton S. HersheyMedical Center,PennsylvaniaState University, Hershey, Pa. 17033, USA
Alan M. Jacobs
William
Edward
S. K e n n e y
Department of Nuclear Engineering, Pennsylvania State University
A. Weidner
Department of Radiology, Pennsylvania State University College of Medicine, The Milton S. Hershey Medical Center, Pennsylvania State University, Hershey, Pa. 17033, USA
Abstract--The basic detection system used for dynamic radiography is reviewed. Some practical factors are discussed in the application of dynamic radiography to the mapping of epicardial motion in canines. A fluoroscopic aiming technique is described for hitting specific points on the cardiac silhouette. The development and design of an automatic control system for dynamic radiography is covered. This system accomplishes automatic detector scanning to the proper depth, as well as calibration of the output signal This output displays the surface position in millimetres. Some results from canine studies are discussed.
Keywords--Heart motion, Scattered radiation
1 Introduction DYNAMIC radiography works by detecting a portion of the scattered radiation produced by the interaction between an impinging beam of radiation and a target material. (JACOBS and KENNEY, 1972). The detector output signal is proportional to the density of that target material which occupies a small and spacially well-defined volume, which can be projected inside an opaque structure. This detecting volume (sensitive volume) can be moved from point to point within the target, while the detector output gives an indication of the density distributions at these points. To use the system for motion imaging, the position of the sensitive volume is fixed to enclose a boundary between high- and low-density materials. In this configuration, the detector output is determined by the degree to which the sensitive volume is filled by the highdensity material. A movement of the boundary which changes this degree of filling also causes a proportional change in the detector output. Thus the a.c. component of the detector output can be used to image the motion of such a boundary lying inside an opaque target. The motion imaging capability of t h e dynamic radiography detection system was validated using both in vitro and in vivo targets (HEIMER, 1976). In these tests, independent imaging techniques
showed boundary motion which agreed with the dynamic-radiography outputs. The dynamic-radiography technique was first applied to a biological system with the imaging of canine epicardial motion. In these studies, carried out at the Milton S. Hershey Center, the single-spot dynamic radiography system permitted imaging motion from only one spot at a time on the epicardium (TILLEY e t al, 1976). Detector systems currently being developed will employ arrays of detectors to simultaneously image epicardial motion from many points. The developments described in this article are viewed as an important step in the evolution of dynamic radiography from single-spot detection to area detection. F o r this intermediate stage, sequentially recorded motion signals from a number of heart points were used to approximate the performance of an area-detector system. To do this, it was necessary to develop a format to select target points on the epicardium, a fluoroscopic aiming technique to hit those spots and an electronic control system to optimally position the dynamic radiography detectors. Many of these developments will be used in the later dynamic radiography systems using the area detectors. Also, the results from these canine studies offer a preview of the results that might be expected from these later systems.
Single-spot dynamic radiography detector system First received8th Februaryand in final form 15th July 1976
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The geometry of the present dynamic radiography
Medical & Biological Engineering & Computing
March 1977
detector assembly is shown in Fig. 1. A narrow beam of X-radiation (8mm diameter) is used in conjunction with two independent detectors. Each scintillation detector consists of a focused collimator, a sodium-iodide crystal and a photomultiplier tube. The field of view of each detector, shown by the broken lines, converges to a span of 10ram along the X-ray beam. The sensitive volume is defined by this intersection of the field of view and the X-ray beam. Each detector is mounted on a linear bearing track and can be driven by a servomotor on a path. parallel to the X-ray beam. Thus the depth positions of the sensitive volumes can be electrically controlled over a 160mm range. F o r these studies, a position slaving servo-circuit forced the sensitive volumes to remain superimposed. This means that the two detectors always imaged the same motion, permitting redundancy and an improvement in the signal-to-noise ratio. I n the earlier canine dynamic radiography studies, epicardial motion was recorded at a single spot on the central left ventricular wall. Amplitude frequency spectral analysis of this motion signal was used to indicate the state of the heart (WEIDNER, et al; 1975). This approach did not utilise the spacial resolution inherent in the system. A motion signal recorded by dynamic radiography is generated by a small (10mm diameter) spot on the epicardium. By recording a number of these spot motions, the spacial variations over the heart's surface can be studied. It is hoped that such a mapping study will eventually make it possible to accurately diagnose ischaemic and infarcted regions.
Practical factors in the use of dynamic radiography The present clinical study series involves mapping epicardial motion over large regions of the heart. To accomplish this, it is necessary to draw the cardiac silhouette, select target spots at which motion is to be recorded and then be able to position the sensitive volume on these spots. The aiming procedure consists of two separate steps: positioning the X-ray beam to hit the epicardium at the desired point; and scanning the detectors to place the sensitive volumes at the correct depth. A fluoroscopic technique was developed for aiming the X-ray beam. The configuration is shown in Fig. 2. consisting of the X-ray source, image intensifier and the television monitor. The X-ray source and image intensifier are positioned exactly 0" 9 m apart, with the X-ray spot centred on the monitor screen. A 10 mm-spaced wire grid is centred in the target plane, halfway between the source and the image intensifier. A permanent overlay template is prepared by tracing the grid projection on the monitor screen. Essentially, this accomplishes a transformation of the true rectilinear co-ordinate system i n t h e target plane to the screen image, which is magnified and slightly distorted. When the canine heart is positioned in the target plane, the cardiac silhouette is drawn on the overlay template. Now, graphically selected target points are translated to grid co-ordinates. Hitting these target points is then reduced to moving the heart 10 mm for each grid increment. In early studies, the detectors were manually scanned while the a.c. component of the output
Fig. I Detector configuration for the single-spot dynamic-radiography system Medical & Biological Engineering & Computing
March 1977
169
signal was monitored on an oscilloscope. The detectors were stopped at the location that appeared to give the maximum amplitude for the epicardial motion signal. This process proved to be slow and unreliable, especially for heart points with small motion amplitudes. It was decided that the d.c.coupled detector signal could be used instead for
this positioning process. As shown in Fig. 3, there is a clear detector response as the sensitive volume crosses the epicardial surface. The present positioning procedure uses this detector response, which is not dependent on any epicardial motion, to bring the detectors to the optimal position. Time duration is an important factor in these
Fig. 2 Fluoroscopic aiming set up with the wire grid placed in the target plane
Fig. 3 Detector-output voltage against sensitive volume depth ZD, during a scan into a canme thorax
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mapping studies. Ideally, all of the spot-motion waveforms should be recorded simultaneously. In this way, motion waveforms from the same heartbeat could be compared for diagnosis. In lieu, of this, it is best to have as short a time as possible between recordings of motions from different points. This reduces the effects of long-term changes in the physiological state of the heart of the subject. However, it still leaves the effect of the beat-tobeat variations in the heart motion. Timing is also an important factor in each individual detector-scanning procedure. During the detector scanning and the subsequent data recording, the respiration of the subject must be halted. This is necessary so that respiratory motion does not shift the heart position. This would cause errors in the aiming procedure and also it would completely mask the much smaller motion due to contraction of the heart. F o r the present studies, using anaesthetised dogs, respiration is halted rather easily by controlling the respirator. However, for projected future use on human subjects, asking a conscious cardiac patient to voluntarily hold his breath for a long period is quite impractical. Another factor to be considered here is that the X-ray beam is irradiating the patient during both scanning and recording of motion waveforms. Thus, it is desirable to keep this total procedure as short as possible to minimise the radiation dosage to the patient.
Medical & Biological Engineering & Computing
March 1977
System performance objectives
System design
The clinical factors listed above point at the need for an automatic control system for use in dynamic radiography studies. It was decided to design a closed-loop system which processes the detector signals and generates the drive for the detector scan motor. This system should be able to carry out the complete scan procedure, bringing the detector to the optimum location with no intervention from the operator. The design objective was for this procedure to be carried out within a 10 s span. Another important design objective was automatic calibration of the output signal. This would make it possible to interpret the motion signals directly as millimetres of displacement. It was felt that being able to measure the motion amplitude is important for diagnosis of myocardial contractile states. The basis of the control-system operation is the scan-signal profile shown in Fig. 3. An automatic scan sequence is to begin with the sensitive volume positioned in the lung field and then to scan it into the myocardium, just past the point of peak response. During this scan, peak detectors store both the maximum and minimum signal levels encountered. These stored voltages can then be used to accomplish both optimal detector positioning and calibration of motion signals.
The design of the electronic scan-control system was begun with a detailed examination of the detectors' spacial sensitivity. Fig. 4a shows the measured detector response against the position of a small target in the X-ray beam. This curve, which is characteristic of the detector's focused collimator, defines the sensitive volume along the X-ray beam axis. Clearly, the sensitive volume has no well-defined boundaries on this axis, but it drops Off nearly linearly with distance from the centre of the detector field of view. For designing the control system, the response curve of interest is the spatial response of the detector to a boundary between high- and lowdensity materials. This response is approximated by the broken curve in Fig. 4b, generated by i n tegrating the curve in Fig. 4a. This broken curve is not the true boundary response of the detector, because it neglects the attenuation of the X-ray beam within the target material. To correct this, the dynamic radiography detection process was mathematically modelled and programmed for a Hewlett Packard HP-65 calculator. The detector's spacial response was simulated for a boundary between materials with densities of air and water. This result was empirically duplicated using an actual air-to-water boundary. The two sets of data were normalised and were found to match. giving the solid curve of Fig. 4b. This curve constitutes the transfer function between the boundary position (relative to the detector position) and the resulting detector output. The transfer-function curve of Fig. 4b can be applied to the scanning of the epicardial surface. The shape of this curve is determined by the curve of Fig. 4a and by the X-ray attenuation constant of the high-density material at the boundary. The attenuation constant of water is essentially the same as that of soft tissue, including myocardium. Thus scanning the detector across the epicardial surface generates a spatial-response curve whose shape is identical to the solid curve of Fig. 4b. The height of this curve during an epicardial scan is variable, depending mainly on the amount of X-ray attenuation occurrring in the thoracic walls of the subject. The boundary response of Fig. 4b was used in the control-system design, It demonstrates that when the detector position is centred on the boundary, the signal level is 6 0 ~ of maximum. In several simulated scans over different regions of a sphere, this level varied from 58 to 6 3 ~ of maximum; so 6 0 ~ was chosen. This response curve is also used for calibration, since it comprises the transfer function between the boundary location and the detector response. The automatic calibration system sets the gain of the system so that the height of this curve is a known value. With a known transferfunction curve, and knowing the mean position of
DETECTOR OUTPUT
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March 1977
171
operation on that curve, the output voltage can be directly translated into millimetres of excursion. Unfortunately, this transfer function is nonlinear, so that, for larger values of motion, correction factors must be included. The automatic control system can be spilt into three sections with different functions. These are: the sequencer; the signal processors; and the scan motor driver. The sequencer counts through the five modes that make up a complete scan sequence. In each of these modes, it generates cohtrol signals which define the operating parameters o f the signal processors and of the scan motor driver. The
INHIBIT >
1
sequencer also awaits a specific event which causes the advancing to the next mode of the sequence. The block diagram of a signal processor from one channel is shown in Fig. 5. The detector signal is amplified to drive a meter and a pair of peak detectors. The peak detectors store the maximum and minimum signal levels encountered following the termination of the reset a n d / o r inhibit signalsA pair of signals is generated by differential aml plifiers following these detectors. The AI/ signa. is simply the difference between the outputs of the two peak detectors. This is used to set the digitally controlled attenuator (d.c.a.). The peak-detector
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Fig. 5 Signal processor subsystem
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Fig. 6 Channel-1 scan driver subsystem
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Medical & Biological Engineering & Computing
March 1977
outputs are combined to generate a V,~s...... (as defined earlier, the 60% response corresponding with the optimum detector position) and a differential amplifier subtracts this from the present detector signal. The resulting (Vdetector -- ~/'ref . . . . . . ) is used for the final positioning of the detector. A single-pole/double-throw electronic switch controls the selection of differential-amplifier outputs fed to the d.c.a. The reset signal sets the d.c.a, at maximum gain to begin a scan. As the scan progresses, increasing values of the A V signal cause the attenuation level to increase, keeping the output level of the d.c.a, at exactly 4 V. At the end of tbe scan sequence, the hold signal freezes the attenuation at its present value. Switching the input of the d.c.a. to the detector signal then results in a calibrated signal output. The sequential operation of this section of the system will be discussed in more detail later. The sequencer section of the system is based on a b.c.d, counter and a decoder which counts through modes 0 to 4. These mode signals are fed through logic gates, which generate the control signals for the signal-processor and scan-motor-drive sections. These control signals also gate the sequencer clock, so that only one specific event can cause the sequencer to advance to the next mode. The scan motor drive section is shown in Fig. 6. This is a simple servo loop, with the amplified error signals supplying the motor drive. The bipolar motor voltage is derived from a unipolar output by using a polarity reversing relay. This necessitates the absolute value amplifier and comparator following the error amplifier. A set of analogue electronic switches operated by control signals from the sequencer determine the inputs to the error amplifier. F o r most of the scan sequence, one of two potentiometer output signals is compared with the detector-position signal. In the final positioning mode, both of these potentiometer signals are turned off so that this input is at 0 V. The other input to the error amplifier is now the (V~ete,o, -V,eser.... ) signal. Under these conditions in the normal situation, the system will scan until (V,~ete,o, -- V,ef...... ) = 0. AS described earlier, this results in the detector being centred on the epicardial boundary. The complete operation of the scan sequence will now be described with, reference to the system diagram and the scan signal profile of Fig. 3. In mode-d.c.a.s 0 (system reset), the peak detectors and d.c.a.s are all reset and the detector positions are determined by the output of a potentiometer. A switch causes the sequencer to go to mode-l, where the detector begins scanning out toward a position set by the other potentiometer. The negative peak detector is enabled, and the input to the d.c.a. is the A V signal. The sequencer block in this mode is supplied by a signal labelled 'DENSITY'. This density signal is generated by differentiating the Medical & Biological Engineering & Computing
detector signal. When the rate of increase of the detector signal exceeds a threshold level, this indicates that the sensitive volume has entered the high-density myocardium. The resulting signal then clocks the sequencer to mode-2. In mode-2, the detector scanning continues out as before and the positive peak detectors are now enabled. This early inhibition of the positive peak detector is performed to prevent the storage of any possible peaks from scanning within the thoracic wall. The A V signal continues to increase, causing the attenuation in the d.c.a, to increase also, keeping its output at 4V. The sequencer clock is now waiting for the event that the scan has passed by the peak signal response. This event has occurred when a full cardiac cycle passes with no further increase in the A V signal. The occurrence of this event causes the sequencer to switch to mode-3, the final-positioning mode. In mode-3, all information for calibration and final location has been stored, so the peak detectors and d.c.a, are held in their present states. The signal (Va~t~to, - V r e f . . . . . . ) replaces the AV signal at the input of the d.c.a. As previously stated, the scan control is set to cause the signal (Vaetecto,Vrel.... ~) to go to 0. The scan direction now reverses and proceeds until this occurs, which causes the sequencer to switch to mode-4. In mode-4, the scan drive is shut-off and the calibrated output signal is available for recording. If the scan-control loop were not disabled in mode-4, the detectors would attempt to track the epicardial motion itself.
Circuit design In the designing and building of the circuitry for this system, low cost was a most important factor. The circuitry includes an analogue-signalprocessing section and digital sections for control and sequencing. The analogue circuitry was built using the L M 324 and 741 operational amplifiers and using 0"25 watt., _+5 ~ carbon-composition resistors. This type of design was considered adequate, because of the relatively low signal-tonoise ratio of the signals being processed. Most of the digital circuitry was built using SN7400 series integrated circuits. This logic family was chosen primarily because of low cost, easy availability of the parts and previous design experience with the circuits. The logic signals in this system are based on relatively slow mechanical events, so the SN7400 series is much faster than needed. Some circuitry is implemented using the c.o.s.-m.o.s. 4000 series of intregrated circuits, which provide more efficient package density than does the SN7400 series. It also includes an electronic switching function which cannot be attained from either of the other two logic familities. This cos-mos 4000 series was used primarily in the peak detectors, analogue switches and the d.c.a.s. March 1977
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Much of the circuitry is just the straightforward application of standard designs. However, considerable design work went into the peak detectors and the d.c.a.s. A schematic diagram of a positive peak detector is shown in Fig. 7. Basically, it consists of a c.o.s./m.o.s, analogue switch, a unitygain amplifier and a comparator. Referring to Fig. 7, the circuit operates as follows. Whenever Vt, exceeds Vo,,, the comparator turns on the analogue switch to increase the voltage on the capacitor and therefore increase Vo,t. The circuitly involving resistors R3 and R4 causes the comparator input signal to be approximately 10 mV greater than Vo,t. This compensates for the input offset voltages in the amplifier and the comparator,
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which could cause the circuit to lock-in the ON state. This 10 mV constitutes an error in the operation of the peak detector but this is relatively small compared with the 1 to 5 V signal levels. Resistor R2 provides some filtering, so that the peak detector does not respond to noise spikes in the input signal. Resistor R1 allows external signals to control the analogue switch, over-riding the comparator signal. The RESET signal causes the analogue switch to be on at all times, so that the Vo., = Vt.. The H O L D signal keeps the analogue switch off, so that V,,.t stays at its present value, regardless of the level of Vi,,. In the hold mode, the decay rate of the output is very low, being less than 1 mV per minute. This is because of the
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Fig. 8 Circuit diagram of a digitally controlled attenuator used for automatic calibration
174
extremely low bias current of the Teledyne 1029 operational amplifier < 1 pA and the low O F F state leakage current of the CD 4016 switch (10 pA). A schematic diagram of the d.c.a, is shown in Fig. 8. The gain of the amplifier is controlled by the state of the c.o.s./m.o.s, binary counter, CD 4020. The values of resistors R3 through R8 are binary weighted, and they can be switched in parallel with feedback resistor R2 by the analogue switches. Since the output of the binary counter controls the switches, it can set the amplifier gain over a range of 64 to 1. The function of the d.c.a, is to keep the output level at 4 V as the input A V monotonically increases during the scan. The operation of this circuit to accomplish that is as follows: The scan sequence began with the counter reset so that all analogue switches are open and the amplifier gain is at its maximum. As Vi. increases and causes Vo.t to exceed 4 V, the comparator gates clock pulses into the counter, increasing the count and consequently reducing the gain until the output falls to 4 V. As the scan continues, the input signal increases until the transition from lung field to myocardium is complete. At this point, calibration is complete, with the voltage swing from lung field to myocardium resulting in exactly 4 V at the output
M edical & Biological Engineering & Computing
March 1977
of the d.c.a. Then the HOLD signal disables the clock oscillator to ensure no further changes in attenuation level when the motion signal (Vd,re~tor -Vref. . . . . . ) is passed through this same amplifier. There were time factors to be considered in the design of the d.c.a. The clock oscillator operates at a frequency of 200 Hz. This means that the d.c.a, can sequence through its entire range of 64 levels in approximately 300 ms. Since the highest heart rates encountered are normally around 3 beats/s, this means that the system is capable of changing over its entire range over one cardiac cycle. For worst-case design, this is a desirable condition. Capacitor C1 provides filtering in the amplifier of the d.c.a, and prevents the appearance of voltage spikes on the output from the switching action. If these spikes were allowed to pass through and appear on the input of the comparator, they could cause extra clock pulses which could result in erroneous operation of the d.c.a. However, if C1 is made too large, it can also result in errors in operation. For each switching of resistors, Vo,t does not change instantaneously, but changes to its new level exponentially at a time constant determined by CI in parallel with the combination of resistors. For reliable operation, the time constant of this amplifier circuit should be less than the period of the clock oscillator. The circuit was designed with a time constant varying from 4 ms at maximum gain to 60 ps at mimimum gain. Since the clock period is 5 ms, this is felt to be a safe design. The comparator output is a logic signal labelled DCA GATE, which is used in the sequencer circuitry. This signal is used in sensing that the scan has passed into the myocardium past the point of peak signal level. During this scanning, an increasing signal level will always result in an increased attenuation in the d.c.a, and thus in an appearance of the DCA GATE signal. The condition of passing through an entire cardiac cycle with no appearance of the DCA-GATE signal is used as an indication of having passed peak response.
System performance The operation of this system together with the dynamic-radiography detector assembly has been evaluated both with a simulated target and in clinical canine studies. This simulated target consists of a plastic cylinder filled with water. This is mounted on an electric solenoid which moves it between two positions, producing a square motion waveform. Both the range of the motion and the rate of switching can be adjusted by the operator. This presents a controlled air-to-water boundary, which can be scanned to test the positioning and calibration performance of the system. Medical & Biological Engineering & Computing
The accuracy of the motion calibration was tested by measuring the output signals from a known displacement of the simulated target and comparing these voltages to the theoretical values from the curve of Fig. 4b. For this testing, many scans were made to allow for variations in the calibration due to noise and due to variations in the final detector location. Also, the detector signals themselves were varied, over a ratio of more than 2:1, by changing the voltage to the photomultiplier tubes in the detectors. This simulates variations in shielding due to the make-up of the subject. The largest data base was taken with 4 mm of target motion. In this case, 40 samples were taken; resulting in a mean output of 1 "76 V_+ 0.09 V standard deviation. The theoretical signal level for this condition from the curve is 1" 69 V. The results of tests of channel-1 at different motion amplitudes are shown in Table 1. Table 1. Channel-1 calibration tests
Theoretical Motion
Samples
2mm 4 6 8
10 40 10 10
Output 1"13_+O'09volts 1 -76+_0'09 2"46+0'18 2'64+0"25
output 0.09 1 -69 2"39 2"78
Another series of test scans was made using the simulated target to determine the positioning accuracy of the system. The scan-motor-drive electronics includes a digital position readout derived from a potentiometer connected to the scan-drive assembly. This allows the position of the channel-1 detector to be read out to within 0" 1 mm. For this test, the final detector positions was read out over many scans, both with and without target motion, As previously stated, the final positioning relies on the difference between the detector signal and a reference signal to generate an error voltage which drives the scan motor. As the detector is scanning back in the final phase of the scan sequence, the scan is terminated t h e first time, this error goes to 0. Since the detector signal includes an a.c. component owing to the motion of the target, this allows for variations in the final location of the scan. To give the system operation more flexibility, a manual rescan switch permits the operator to reactivate this final positioning loop, even though the automatic sequence is terminated. The loop includes a lowpass filter to reduce the tendency of the detector to attempt to track the cyclic boundary motion itself. Unfortunately, this filter introduces a lag which causes the positioning loop to be unstable. As a result, with the rescan button depressed, the detector will hunt over a range of about + 3 mm around the true boundary position. March 1977
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In these positioning tests, scans were made using the simulated target both with no motion and with 4 mm of motion. For each run, the detector position at the end of the automatic sequence was recorded. Then the manual rescan mode was used to vary the position for five more readings. The variations in final detector position at the end of the automatic scan sequence was 3 mm, with no target motion, and it was 2' 1 mm with 4 mm of target motion. When the manual rescan mode was utilised, the final positions had a standard deviation of 0" 8 mm with no target motion and a standard deviation of 1.5 mm with 4 mm of target motion. The position of the detector during data recording is important for two reasons. First, this information allows 3-dimensional mapping of the epicardium. Secondly, the signal level is changed by the boundary positions within the detector field of view. This can be seen from Fig. 4b. For example, a 4 m m boundary motion with the boundary centred produces 1" 69 V output. But if it is moved out 2 mm, the output is reduced to 1"22 V. Any such errors in the final position of the detector during recording can be picked up, since they will result in a shift of the d.c. level of the output signal. Thus corrections can be made for the true boundary location and for the calibration of the signal. For the next-generation system, it is felt that the calibrated output should be passed through a nonlinear amplifier to correct the curve of Fig. 4b to a linear voltage-against-position relationship. This would eliminate both the dependence of the calibration on the detector position and the compression of signal output for larger motions.
Clinical use of the system This system has been used in several clinical studies with canine subjects (TILLEY et al., 1976). One study has involved open-chested dogs with coronary artery ligations. After a thoracotomy, ligatures are positioned around one or more coronary artery branches. Epicardial motion is recorded at eight to ten points on the heart, followed by tightening the ligatures to produce occlusion of these arteries. Then epicardial motion is again recorded 20 and 90 min post ligation. The motion waveforms resulting from one such study are shown in Fig. 9. The points at which motion was recorded and the location of the ischemic myocardium are shown on the cardiac silhouette. For each point, the motion waveforms are shown for preligation, 20 and 90 min post ligation. The horizontal lines on the waveforms represent 1 mm of displacement and the vertical lines coincide with the R-wave of the e.c.g, waveform. The application of dynamic radiography to openchested dogs has presented some physical problems. During these acute studies, the chest must be left open to allow access to the ligatures without moving the dog or its heart. However, the pericardial sac must be left intact enough to prevent too much shift in the heart position. There have also been some problems with high-density accumulations in the lower part of the thoracic cavity. These seem to be attributable to either postoperative bleeding or to fluid accumulation in the exposed lung. This system has operated best in studies with intact dogs. Fig. 10 shows the epicardial motion map of the same dog as for Fig. 9, taken two weeks
Fig. 9 Graphical display of an open-chest canine study
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prior to the open-chest procedure. The difference in motion waveforms between these two Figures may be attributed to mechanical differences due to the open-chest condition in Fig. 9 and to the differing heart rates between the two studies. A clinical study is presently being undertaken in which coronary-artery occlusion is produced in closed-chest dogs using a catheter. Dynamic radiography is then used in conjunction with other diagnostic techniques to evaluate the size and extent of any myocardial infarction resulting from these occlusions. Another clinical study was performed using eight dogs which had undergone coronary-artery ligation four weeks earlier in a student surgery course. It was found that these dogs had adhesions and fibrosis around the ligation sites which interfered with the automatic scan sequence. By reverting to manual control, it was possible to get uncalibrated dynamic-radiography signals for most cases. However, the low-density gradient at the epicardial boundary resultes in very poor signalto-noise ratios. This study points up the requirement for a low density adjacent to the epicardium to achieve good performance. The system performance during these clinical canine studies has been good. The automatic scan sequence, averaged about 12s in duration. In control studies, the motion amplitude has been reproducible to within about + 107o. This is about the order of variability indicated in the simulation tests. Perhaps the most serious problem encountered has been the appearance in some cases o f high densities in the lung field of the subject. Some
manual controls have been added to the system to allow the operator to aid the automatic sequence. In these cases, however, the poor density gradient results in a low signal-to-noise ratio and the usefulness of such signals is questionable.
Summary A dynamic-radiography system, consisting of a detector assembly and control electronics, can be used to map epicardial motion. 2-dimensional aiming at points on the heart is performed by specially developed fluoroscopic techniques. The detectors are scanned along the third dimension by the control electronics. This 12s sequence correctly positions the detectors and automatically calibrates the output signals. By combining the data, a 3-dimensional map of the epicardium can be generated with calibrated motion waveforms shown for each target point. Tests have indicated that the motion-amplitude calibration is accurate to within + 1 0 ~ . The system has worked successfully in both open- and closed-chest canine studies. Problems have occurred when high-density masses are present around the epicardial surface. Manual controls permit the recording of degraded motion signals in these cases. Clinical studies are underway to determine the changes in epicardial motion with coronary-artery occlusion. This should lead to being able to evaluate the extent of infarcts using this noninvasive technique.
Fig. 10 Graphical display of closed-chest canine wave-forms
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References JACOBS, A. M. and KENNEY, E. S. (1972) Dynamic radiography. A new imaging technique using penetrating radiation. Proc. Soc. Photo-Opt. Instr. Engrs. 29, 17-22 HEIMER, M. L. (1976) Developing a control system for mapping epicardial motion with dynamic radiography. Ph.D. thesis, Department of Electrical Engineering at the Pennsylvania State University.
WEIDNER,W. A., JACOaS,A. M., KENNEY,E. S., MILLER, K. L. and TILLEY,D. G. (1975) Dynamic radiography-An evaluation of cardiac motion by the analysis of scattered radiation during fluoroscopy. Investigative Radiology 10, 132-139 TILLEY, D. G., JACOBS,A. M., KENNEY, E. S., WEIDNER, W. A. and MILLER,K. M. Dynamic radiography-A technique employing scattered radiation to monitor surface motion. Med & BioL Eng. 14, 141-150
Implementation eines automatischen Regelsystems f/Jr dynamische Radiographie Zusammenfassung--Das f~ir dynamische Radiographic verwendete grundlegende Aufdeckungs-system wird besprochen. Einige praktische Faktoren bei dcr Anwcndung von dynamischer Radiographic auf die Abbildung epikardialer Bewegung bei Hunden werden diskutiert. Es wird cin fluoroskopisches Zielverfahren zum Treffen spezifischer Punkte auf der Herz-Silhoutette beschricben. Die Entwicklung und Konstruktion eines automatischen Regelsystems flit D R G wird eingeschlossen. Dieses System erzielt automatische Detektorabtastung auf die richtige Tiefe, wie auch Kalibrierung des Ausgangssignals. Dieser Ausgang zeigt die Oberfl/~chenlage in mm an. Einige Resultate aus Untersuchungen an Hunden werden besprochen.
Mise en oeuvre d'un syst&me de rdglage automatique pour la radiographie dynamique Sommaire--Le syst~mc de d6tection de base utilis6 en radiographic dynamique cst pass6 en revue. On 6tudie certains facteurs d'ordre pratique intervenant dans l'application de la radiographic dynamique au trac6 du mouvement 6picardiaque chez les chiens. On expose une technique de vis6e fluoroscopique pour atteindre des points particuliers de la silhouette cardiaque. La mise au point et la conception d'un syst~me de r6glage automatique en radiographie dynamique sont trait6es. Ce syst~me permet une d6tection par balayage automatiquc h la profondeur voulue ainsi que l'6talonnage du signal de sortie. Le signal de sortie permet de montrer la position en millim&res de la surface. Certains r6sultats obtenus par des 6tudcs sur des chiens sont 6valu6s.
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