A real time 2-dimensional ultrasonic scanner for clinical use A. SHAW, J. S. PATON, N. L. GREGORY and D. J. WHEATLEY The technical details of a real time 2-dimensional ultrasonic scanning system are described, with particular regard to the improved clinical performance over other 2-dimensional scanners. Our instrument is capable of being used with existing ultrasonic equipment extending certain standard ultrasonic machines for use as real time 2-dimensional scanners conveniently and at little cost. Initial results are given and future technical developments are discussed.

Introduction Since Edler and Hertz first described the ultrasonic examination o f the heart, considerable progress has been made.2, 3, 4 At present most clinical systems for echocardiography use a pulsed ultrasonic beam from a single transducer. From this, A-mode, ECG gated B-scans and time-position displays can be achieved. With some experience in aiming the transducer, the position, dimensions and, to a limited extent, the movement of some cardiac structures can be studied. The normal mitral echogram is well known s and the diseased mitral valve produces patterns which can provide a reasonably quantitative assessment of functional impairment. 6 The aortic valve can be assessed in a similar manner as can other cardiac valves although these are often less readily visualized. 7 The visualization of the ventricular cavity and walls with their dimensions and movement allows assessment of left ventricular function, a, 9 Assessment of congenital heart disease, particularly in the newborn, is another field in which echocardiography promises to play an increasing role. 10 The major advantage of echocardiography is that it is a simple, non-invasive technique which is safe and can, if necessary, be repeatedly applied even to the critically ill.n, 12, 13 It promises to reduce the use of more hazardous and complex cardiological investigations in the future. 2

Although these 2-dimensional images promise to make an important contribution to cardiac diagnosis giving a moving, cross-sectional picture o f the heart with its structures in true anatomical relationship, there are still a number of technical problems. In an attempt to improve on the present 2-dimensional scanning systems a rapid scanner has been developed which scans an arc of 60 ° and has a single probe which oscillates about a point on the chest wall, thus allowhag it to be placed over a rib interspace to reduce obstruction o f the ultrasound by chest wall structures or lung (Fig. 1). This clinically viable instrument was developed from a prototype which has been previously reported. 16

Technical description The instrument can be seen in Fig.2 and it consists o f two parts, the scanning head and the electronic processing unit. The complete system is specially designed so that it can be directly connected to the Nuclear Enterprises Diasonograph (Model 4102), but with appropriate electronic development it can be used with most types of scanners. A diagrammatic view of the scanning head can be seen in Fig.3. The driving motor is a 12 V dc servomotor which rotates at a maximum speed of 130 rev s "1. It is mounted

The main limitation of most current systems is that information is obtained from a single stationary beam and the record represents events in a small section of the heart traversed by the ultrasound beam. The record bears no resemblance to anatomical structures and requires experience in interpretation. Attempts which have been made to extend the information available from ultrasonic examination of the heart include the use of multiple parallel probes to provide a 2-dimensional image of the heart and the use of a single probe oscillating through a 30 ° arc to provide a 2-dimensional image. 14, is A. Shaw, J. S. Paton and N. L. Gregory are in the West of Scotland Health Boards, Department of Clinical Physics and Bio-Engineering, 11 West Graham Street, Glasgow, G4 9LF and D. J. Wheatley is at the Cardiothoracic Unit, Mearnskirk Hospital, Newton Mearns, Glasgow, G77 5RZ. Paper received 18 June 1975.

ULTRASONICS. JANUARY 1976

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35

crank. Most parts were constructed from anodised aluminium alloy or nylon. Those parts which require stronger materials were made from stainless steel and were so shaped to ensure minimum weight, consistent with strength. The motor was specially supplied with an aluminium alloy casing.

Fig.2

The scanning head and electronic processing unit

in line with a gearbox which reduces the maximum output speed to about 13 rev s q . The output shaft from the gearbox carries the encoding disc and driving crank. Radial slots cut into the periphery of the encoding disc pass light from light emitting diodes to two photo-diodes, the outputs from which are used to control the electronic display system. The driving crank carries a single pin on which is mounted a small ball-bearing. This locates in a curved cam which is in turn firmly attached to the probe carrier. There is sufficient clearance between this cam and the ball-bearing so that the bearing will rotate when the mechanism is in operation. As the driving crank rotates, the cam is driven in a sinusoidal oscillatory motion. The cam and probe carrier are made from molybdenum - disulphide impregnated nylon. This material combines lightness, stability and strength with low friction properties. The probe carrier is fitted with two sets of wheels, which run on a curved track machined in parallel stainless steel guide plates attached to the main frame. As with other parts of the mechanism these wheels are a precision fit in the tracks. The probe is positioned and the tracks are so shaped that when the cam oscillates back and forward, the probe moves in an arc about its face and through its centreline. The geometry of the systetn is such that the probe is swept through an angle of 60 ° by means of this compact, hand-held instrument. Because of the relatively high angle of oscillation it would have been difficult to transfer the acoustic signals to and from the patient by direct skin contact with the probe. To overcome this problem a constant volume oil-filled cell was interposed between the face of the probe and the front cover of the instrument. This was achieved by attaching one end of a rubber sleeve to the probe and the other end to a specially shaped collar on the face of the instrument. After filling this cavity with de-aerated oil a separate membrane is secured over the front of this collar, trapping the oil and protecting the probe. Reliable electrical connection to the crystal in the probe is made by means of a fine coaxial cable which is threaded through a small-diameter nylon tube formed into a spiral, with one end fixed to the moving probe carrier and the other end mounted on the side of one of the guide plates. Movement is evenly distributed throughout the spiral, eliminating points at which excessive wear or fatigue could otherwise occur. Care was taken to ensure that out of balance forces within the mechanism are minimal. This was achieved by ensuring that the oscillating parts were as light as possible. In addition, balance weights were incorporated in the driving

36

The principle function of the electronic unit which couples the rapid scanner to the display unit is to control the sector scan display, making the display follow the scanning action of the probe. Subsidiary functions include control of the motor, a facility for setting the probe angle as required for static A-scans or time-position scans, and the completion of other functions which were not included on the electronic console of the Diasonograph (type 4102). A block diagram of the interface electronics for use with this particular electronic console is shown in Fig.4. The display vector is produced by applying signals Vx and Vy to the deflection amplifiers where: Vx = a Vvt cos 0 Vy = a Vvt sin 0 a is a constant, depending on deflection sensitivity. Vv is the 'velocity voltage' which sets the scale calibration. 0 is the angle to be reproduced. In normal operation of the Diasonograph, signals Vv cos 0 and Vv sin 0 are derived from potentiometers in the manually operated mechanical assembly and these control the rate of run up, or down, of integrators in the electronic module. The integrators are gated in synchronism with the transmission pulse, giving outputs: Vx = a f t ° V v cos 0 dt and Vy = aftto Vv sin 0 dt In our 2-dimensional scanner we deliberately used a noncontact optical system for determining the position of the probe. This type of system uses less power than that which would be required to drive a potentiometer and is not so prone to wear at the high operating speeds employed. The encoding disc interrupts the light paths of two optically

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Photo diode and led Encoder disc Roller track Ultrasonic probe 0il filled window

Fig.3

Section through the scanning head

ULTRASONICS. JANUARY 1976

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coupled modules (SDA20/2 Texas), so generating two sets of pulse chains which control the electronic logic circuits shown in block A in Fig.4. The probe angle 0 is obtained by assuming 0 (t) = On where On is one of a series of predetermined discrete angles. The display thus becomes a series of discrete lines. The angular intervals at which the slots are cut in the encoding disc are such as to give equally spaced lines on the display. Paired values of sin On and cos On are set up on two potentiometer chains which are supplied from the potential Vv (block B Fig.4). As the probe angle passes through zero, a reference pulse is generated from one optical module setting the counting logic to zero. The phase modulated pulse chain from the second module, clocks the logic system through each revolution of the shaft rotation, selecting appropriate sine, cosine pairs by the action of analogue switches (AM3705CD, National) which are represented in block C. Sixteen values are used and, as these are selected repeatedly through four quadrants, 64 display lines are produced during a complete rotation of the shaft. The lines are spaced at 1.9 ° intervals on the display. The system is independent of motor speed and consequently the operator can select an appropriate speed o f scan at any time. In particular it allows the motor to be driven very slowly to obtain a single line scan for the more conventional A-mode or time-position scans. The motor control unit, block D, provides speed control by voltage regulation and by monopolar pulsing at 6 V 50 Hz, facilitating very slow movement of the scanning line in either the forward or reverse direction.

ULTRASONICS

. JANUARY

1976

In the standard commercial ultrasonic scanner the probe is manually rotated about a point 60 mm above its front face. The electronic circuitry compensating for this by starting the integration of the sine and cosine signals at a fixed time interval, 78/as, before the transmission pulse. In our 2-dimensional scanner the probe rotates about a fulcrum point which corresponds to the centre of the front face of the transducer and therefore the integrator must start at the same time as the transmission pulse. This is accomplished within our electronic processing unit and without modification to the Diasonograph, by gating the sine and cosine outputs (block E). These signals are clamped at zero until the transmission pulse is generated, when they assume their true value for the period of the scan. As the probe moves in one direction 32 lines are generated and a similar 32 lines are generated for the probe returning to its original position. Provision is made (block F) to offset the sine voltage by one quarter of a normal increment in each direction of scan. This produces an evenly-spaced 64 line scan. A greatly enhanced display in terms of visual quality is therefore produced at the expense of some precision. A maximum tangential error of 0.8 mm and a radial error of 0.5 mm is introduced at a distance of 100 mm from the origin. These displacements are visible on an electronically generated image but are considered unlikely to be important in ultrasonic imaging. A facility is included to switch out this interlacing if desired. Our system allows 2-dimension images to be obtained on an unmodified commercial machine. The only preparation needed is to remove the coaxial cable to the transducer, the single plug which links the console to the mechanical frame, and to replace these by the corresponding plugs on our electronic processing unit. The frequency selection switches in the mechanical frame are replaced by links in the processing unit. Additional features which we intend to include later are facilities for increasing the number of lines of display, and direct replay of the scans on the existing display screen by means of a video-tape recorder. Our device, unlike existing commercial 2-dimensional scanning systems, is relatively inexpensive to produce. The total cost of the electronic and mechanical parts of the system is approximately £350. Results

Fig.5 shows a polaroid photograph of the scan obtained over an interval of about 1/8 s. The interpretation of the various cardiac structures and the "anatomical position of the scanner is given. Notice the good definition of the mitral valve and outline of part of the left ventricle and the indication of the aortic valve. The latter identification can be made with confidence after viewing the characteristic movement on the real time display. Fig.6 shows views during systole and diastole. In systole the papillary muscle is clearly visible and also the closed mitral valve, the aortic valve being open. During diastole the mitral valve can be seen in the open position, with the papillary muscles relaxed and the aortic valve closed. These results were obtained with our scanner coupled to a general purpose version of the Diasonograph 4102 which has no special provision for cardiac work. In particular the pulse repetition frequency, prf, was fixed at 600 pps, which is too low to allow full speed operation of the scanner. The scanner was operated at about 8 rev s "l and missing lines in

37

greater clinical interest at the present time, eg for measurement of left ventricular volume and for observing the relative positions of the mitral and aortic valves. We can, of course, interpose a bath of oil or water between our transducer and the surface of the skin, to improve the area of view in the proximal regions. Unlike other systems we can obtain an almost complete view of the left ventricle from a single position, the fourth left interspace. This is of particular significance in obtaining an estimation of left ventricular volume and hence the index of cardiac output. Unlike systems employing multi-element probes, the surface area over which we need to maintain good acoustic coupling is small, being about 2 cm in diameter. This permits the scanning position and direction to be chosen within the limits of the intercostal spaces, to include features lying behind the sternum. Manually rotating the scanning head enables us to obtain various sections through the heart, quickly and conveniently.

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Fig.5 A n e x a m p l e o f a sector scan using a standard c o m m e r c i a l machine. A o -- a o r t a ; A . V . -- a o r t i c valve; R . V . -- r i g h t v e n t r i c l e ; L . A . -- left a t r i u m ; A ; M . V . L . -- a n t e r i o r m i t r a l valve leaflet; P . M . V . L . -- p o s t e r i o r m i t r a l valve l e a f l e t ; P . L . V . W . -- p o s t e r i o r l e f t v e n t r i c u l a r w a l l ; L . V . -- l e f t v e n t r i c l e

the pictures are due to inexact coincidence between the rotational period and the shutter speed. The camera was not triggered from the ECG. Considerably enhanced performance is to be expected when working with a Diasonograph 4102A, equipped with a 'cardiac module' and giving a prf of 1 800 pps. A typical time-position scan, taken with the probe stationary, can be seen in Fig.7.

Discussion Our scanner is specially designed to minimize the limitations of other 2-dimensional scanners for cardiac work. For example, unlike the scanner of Griffiths et al. our specification is based on a sector scan of 60°. Is We consider that smaller angles of scan impose considerable limitations on the clinical usefulness of the technique. Fig.8 shows the relationship between the angle of view of the various systems present available. The multiscan system gives improved field of view close to the surface of the skin, although this will depend on the masking effect of adjacent ribs. 14 For deeper cardiac structures our single element scanner offers a substantially inrproved field of view. The imaging of these deeper cardiac structures we consider to be of

38

With other single probe scanners there has been the difficulty that the moving probe makes direct contact with the patient. In our experience this is often unsatisfactory since the coupling medium, usually gel, tends to be pushed away by the movement of the transducer. Also in the case of a sector scanner with a relatively large angle of sweep and probe diameter, there is almost always loss of acoustic contact at each end of the cycle. In our design the probe is encapsulated behind a fixed plastic window so that there is no fast moving component in direct contact with the patient. This static window is simply smeared with a suitable gel to obtain the necessary acoustic coupling. To determine the effect of the plastic window on the overall performance of our instrument the ultrasonic probe (standard unfocussed 2.5 MHz probe manufactured by Nuclear Enterprises Limited) was checked before and after mounting in our instrument. Our test consisted of adjusting the transmitter power supplied to the probe to give a standardized echo amplitude from a fixed surface at a depth of 11 cm in water. In a series of tests the sensitivities of four additional and similar commercial probes were checked, These gave a range of +-1½ dB. Our probe without the window assembly deviated +½ dB from the mean value. With the window assembly in position the sensitivity was - 3 dB from this mean. This indicates that the window impaired the sensitivity by a small amount. In actual use the ease with which a time-position scan can be recorded (Fig.7) from the mitral valve indicated that this degradation in performance is not serious. Additionally, this window prevents the drive mechanism or the electric motor being overloaded because of changes in the contact force and avoids the additional complication of having to include tachometer-feedback, is Our design ensures that the scanner is reliable. This is due to the use of a well-established method of drive, the precision engineering in the transmission mechanism, the specification of a motor gearbox of adequate strength and attention to such details as ensuring that the mechanism is properly sealed against ingress of dirt. The importance of avoiding direct contact of the rapidly oscillating probe with the patient is significant in ensuring the additional benefit of improved patient comfort. Our instrument is more bulky than some other systems but not significantly heavier. Unlike a multi-probe scanner it contains moving parts which will be subjected to wear. We

ULTRASONICS. JANUARY 1976

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believe that our close attention to the detailed design of the scanning mechanism compensates for this. The main disadvantage of any single transducer scanner of this type is that it is very difficult to eliminate entirely mechanical vibration. Careful design keeps this at an acceptable level. The advantages of single element scanners over fixed multielements are, we believe, significant; the most important advantages are the ability to achieve a larger number of lines of real information and a greater angle of view. Our scanner was designed for use with a standard ultrasonic machine which is widely used in Britain and elsewhere. It enables good quality 2-dimensional scans to be achieved at little cost, although initial results indicate that far superior image quality is achieved with the faster ultrasonic pulse rate of the cardiac version of the commercial machine. Conclusions

2-dimensional ultrasonic scanning can be used with considerable benefit in echocardiography. Present machines for this work are limited in performance and early indications are that our single transducer rapid scanner offers very good image performance. This device was designed to ensure sufficient angle of view and reliability. The importance of a precision mechanism and a motor-gearbox of adequate power is vital as is the need to protect the oscillating transducer from direct contact with the patient. The various methods of display such as polaroid photography, video-tape recordings and fibre optic displays require to be investigated in greater depth. We suggest that the high performance obtained from our system will allow

ULTRASONICS • J A N U A R Y 1976

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39

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the full potentials o f the various display systems to be more fully exploited and that 2-dimensional echocardiography will b e c o m e just as useful as time-position scanning of the heart. A provisional patent, application n u m b e r 2 0 2 0 / 7 5 has been filed and we are assessing the full clinical uses o f the device in various hospital units.

References 1

40

Edict, I., Hertz, C. H. The use of ultrasonic reflectoscope for the continuous recording of movements of heart walls,

13 14

15 16

Kingliga Fysiogiafiska Sallskapets i Lund Forhanglingar 5 (1954) 24 Editorial, Ultrasounding the Heart, British Medical Journal 1 (1974) 83 Feigenbaum, H. Echocardiography, Lea and Febiger, Philadelphia (1972) Gramiak, R., Shah, P. H. Cardiac ultrasonography A review of current applications, Radiol Clin N A met 9 (19 ? 1) 469 Layton, C., Gent, G., Pridie, R., McDonald, A., Brigden, W. British Heart Journal 35 (1973) 1066 Gustafson, A. Correlation between Ultrasoundcardiography, Haemodynamics and Surgical Findings in Mitral Stenosis, American Journal o f Cardiology 19 (1967) 32 Feizi, O., Symons, C., Yacoub, M. Echocardiography of the aortic valve. I Studies of normal aortic valve, aortic stenosis, aortic regurgitation, and mixed aortic valve disease, British Heart Journal 36 (1974) 341-35 l Murray, J. A., Johnston, W., Reid, J. M. Echocardiographic determinations of Left Ventricular Dimensions, Volumes and Performance, American Journal o f Cardiology 30 (1972) 252 Fortuin, N. J., Hood, W. F., Jr., Craige, E. Evaluation of Left Ventricular Function by Echocardiography, Circulation 46 (1972) 26 Godman, M. J., Tham, P., Kidd, B. S. L. Echocardiography in the evaluation of the cyanotic newborn infant, British Heart Journal 36 (1974) 154 Woodward, B., Pond, J. B., Warwick, R. How safe is diagnostic sonar? British Journal o f Radiology 43 (1970) 719 Hill, C. R. The possibility of hazard in medical and industrial applications of ultrasound, British Journal o f Radiology 41 (1968) 561 Lyon, M. F., Simpson, G.M. An investigation into the possible genetic hazards of ultrasound, British Journal o f Radiology 47 (1974) 712

Roelandt, J., Kloster, F. E., Ten Cate, F. J., Van Dorp, W. G., Honkoop, J., Born, N., Hugenholtz, P. G. Multidimensional Echocardiography An appraisal of its clinical usefulness, British Heart Journal 36 (1974) 29 Griffith, J. M., Henry, W. L., Epstein, S. E. Real Time Two Dimensional Echocardiography, Circulation 48 (Suppl IV 1973) 124 McDicken, W. M., Bruff, K. and Paton, J.S. An ultrasonic instrument for rapid B-scanning of the heart, Ultrasonics I I (1974) 269

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