1979, British Journal of Radiology, 52, 44- 50
A calibration system for X-ray generators and tube factors By T. Healey, F.R.C.R., M.I.Nuc.E., F.R.A.I., D. G. Dickson, T.Eng.(CEI), F.S.X-ray TM A.F.S.E.R.T., and M. W. B. Greenwood, H.D.C.R., Cert.Hlth.Ed. Barnsley District General Hospital, Barnsley, South Yorkshire {Received September, 1977 and in revised form February, 1978)
ABSTRACT
An apparatus (Machlett's Dynalyzer II system) is described that makes real-time dynamic tests on the output and performance of the X-ray tube and generator so that a single exposure gives information on mA and mAs; exposure time (ms); kVp anode to earth; kVp cathode to earth; kVp cathode to anode; tube filament current; line voltage and radiation output of the tube (mR). The method of use is described together with the results of comparisons made with other test equipment. Some novel design features are of particular interest. The results show that by using this apparatus present design characteristics of X-ray generatorcontrol-tube systems are such that the accuracy of calibration can be improved by at least an order of magnitude.
All progress in diagnostic radiography since 1895 has been toward increasing the dependence of the radiographer on the apparatus producing the primary beam. For instance, exposures are measured in milliseconds rather than minutes and radiographers are expected to carry out many different examination techniques using several types of apparatus, often by more than one manufacturer, usually in a department containing several diagnostic rooms. In order that consistent results can be obtained under these conditions, the radiographer must be reasonably confident that exposure factors that apply with one machine can be used with another, or indeed with all machines. This is an ideal condition that too often is not achieved in practice, yet there is usually little the radiographer can do other than increase or decrease the exposure in accordance with his experience and to report the apparent defect to his superintendent. A physicist experienced in diagnostic radiology could help with these problems, but at present such physicists are relatively uncommon in the United Kingdom. In the absence of such an expert, the superintendent or the hospital physicist should have access to a dosemeter and/or a cathode-ray oscilloscope, but even if the manufacturers' engineers are called in there is comparatively little they can ordinarily find out about the output of the generator and the X-ray output of the tube without going to a great deal of trouble that would take the set out of commission for a relatively long period. Problems can also arise when we consider what is being measured. For instance, an Ardran-Crookes penetrameter cassette measures effective kilovoltage by the radiation produced. A potential divider 44
system measures peak kilovoltage and a cathode-ray oscilloscope shows the waveform. All these measure different aspects of tube performance and some compromise may be needed between the users and the testers as to which standard should be applied in a particular case. A system is now available that makes real-time dynamic tests possible on the output and performance of the X-ray tube and generator, either separately or together. The apparatus is the Dynalyzer II from Machlett X Ray Tubes (Great Britain) Ltd. It is inserted in the secondary side of the high voltage circuit between the high tension generator and the X-ray tube (Fig. 1), and a single exposure can give all the following information: mA and mAs; exposure time (ms); kVp anode to earth; kVp cathode to earth; kVp cathode to anode; tube filament current; line voltage; radiation output of the tube (mR). A paper-tape printer is included in the Dynalyzer system, so that an intermittent fault can be investigated by leaving the apparatus in circuit all day, so that each exposure is logged for off-line analysis. DESCRIPTION OF APPARATUS
The high voltage unit (Figs. 1 and 2) encloses a high-voltage divider network, an anode current sensor and a filament current monitor. The highvoltage divider inserts 1000 megohm resistances between the anode and earth and the cathode and earth, the rated anode-cathode voltage being 150 kVp. The voltage is measured across a 10000:1 stepdown section with ^ 1 % accuracy. The anode current sensor is an optically-coupled device to give a signal that represents the anode waveform. Currents from 1 mA to 2 A can be measured with a claimed accuracy of ± 4 % for currents less than 10 mA and ± 2 % for currents over 10 mA. The output conversion factor is 1 mV per mA of anode current and this output voltage is electronically buffered so that it can be coupled to a cathode ray oscilloscope or other measuring device. The means of producing an isolated input that represents the anode current is an innovation. The anode current is sampled through a 1 ohm resistance. The signal is amplified and then fed to a gallium
JANUARY 1979
A ^calibration system for X-ray generators and tube factors
HIGH
VOLTAGE
UNIT
RADIATION 1
:
D
*
1
O
0
PROBE
* •
0 0
©
O
0
LmJ
DIGITAL
DIGITAL
PRINTER
DISPLAY
FlG. 1. Block diagram to show how the Dynalyzer II system is incorporated in the high tension circuit.
Dynalyzer II system. For fullest utilization of the equipment, a dual-trace cathode-ray oscilloscope and a radiation probe are needed in addition. Any cathode-ray oscilloscope can be used, but the impedance of the output of the ionization chamber needs to be matched to that of the digital display unit. The output of the high-voltage unit is frequency compensated for cable length, high voltage transients and for oscilloscope impedance factors, so that the overall accuracy of the voltage readings on the cathode-ray oscilloscope is normally limited by the accuracy of
arsenide infra-red light-emitting diode. The light signal from this device is proportional to the current sampled. It is optically coupled through a glass lightpipe to a photodiode. The output from this diode is again amplified and then fed to the display unit. The tube-filament current monitor uses an isolating toroidal transformer in the common lead of the filament circuit. It can measure currents up to 20 A. Claimed accuracy is ± 1 % at 50 Hz. The digital display unit (Figs. 1 and 3A) and the paper-tape printer (Figs. 1 and 3B) are part of the 45
VOL.
52, No. 613 T. Healey, D. G. Dickson and M. W. B. Greenwood
the oscilloscope used. The accuracy of the digital display unit is usually at least an order of magnitude higher.
measurements are made and finally triggering can be either external or, if internal, from the kV or mA wave forms. In practice, it was found that an initial 10 ms delay was essential to prevent the apparatus recording the initial spike on any exposure. With regard to the vernier settings, the recommended levels of 10% for single phase and 75% for 3-phase were found to be a good general guide. We did find on some occasions, however, output waveforms that necessitated measurements being taken at the 50% point.
Method of connection The secondary circuit of the high tension supply to the X-ray tube is interrupted by removing the high voltage cables from the tube and connecting them into the Dynalyzer II high-voltage unit (Fig. 1). Jumper cables are then used to connect the high-voltage unit to the tube. A direct connection is then made from the high-voltage unit to the digital display unit, by means of a multi-core cable, and thence to the digital printer. A 240 V supply is taken to the three components of the Dynalyzer II system from a common point. This is to prevent any earth loop becoming established. The oscilloscope takes its electrical supply from the same mains and connections are then made to the appropriate sockets on the digital display unit so as to view the anode and cathode wave forms. Another socket accepts the lead from the radiation probe.
METHOD OF TESTING
During the evaluation of the apparatus, exposures were made utilizing all the variables and noting the effects on the indicated factors. For each set of conditions on the Dynalyzer II system (e.g. automatic
Method of operation The high-voltage waveform is unlikely to be identical from set to set, so that there is a need to select the most useful point on the waveform. This point at which readings are to be taken can be affected in four ways. Firstly, a built-in delay can be switched out when making exposures of less than 3 ms. Secondly, there is a vernier scale to select the triggering point. For single phase units, a point at 10% of the peak is advised, and for 3-phase units a point at 75% of the peak. It is also possible to include an additional delay of up to 20 ms before kVp
FIG. 2. The high voltage unit containing a high voltage divider, an anode current sensor and a filament current monitor.
FIG. 3. (A) The digital display unit and (B) the paper tape printer of the Dynalyzer II system.
46
JANUARY 1979
A calibration system for X-ray generators and tube factors delay "ON", 10% of waveform, kV trigger, 10 ms delay) tests were made as shown in Table I. When each set of tests had been carried out, the whole procedure was repeated with a different set of conditions being selected on the Dynalyzer II. When these results were being obtained, some additional constraints became apparent. Firstly, it was found that the extra cable in the high tension circuit imposed such a load on the X-ray system that
the tube current was significantly lowered, with an accompanying drop in kVp—contrary to what might have been expected. Allowance must be made for this in any calibration of settings on the control panel. It was found that the Dynalyzer II is much more sensitive to variations in the line voltage than is the usual "automatic" compensator on the X-ray apparatus. This meant that a given stud setting
TABLE I
Constant
Time
mA
kVp
Varying
Constant
Varying
Constant y
mA check
y
y
mAs check
y
y y
kV check y
y
Repeatability check
y
y
Varying
Constant
Varying y
y
y
Timer check
mAs
y
y
y y
y
y
110 -i
y
i
Single phase unit Three phase unit
10090 KVP.
90 -\ 80 KV.P. 8070 KVP 70-
60-
60 KVP
100
200
300
400
500
Selected tube M A FIG. 4. Typical results obtained with the Dynalyzer II system showing the variation in actual kVp output against tube mA for various selected values of kVp. 47
VOL.
52, No. 613 T. Healey, D. G. Dickson and M. W. B. Greenwood 600
Single phase unit. Three phase unit.
600
500
400
400 330
300 300 200
200
100
100 —
60
70
80
90
100
Selected K.V.P. FIG. 5. Variation in actual generated mA against the selected kVp for chosen mA values. These are the same X-ray units as were tested to give Fig. 4.
might give a different high-voltage output at different times, due to these variations in the line voltage. This is no fault in the Dynalyzer II system, for it accurately follows these changes, but it does mean that accurate calibration of the control settings can be impossible if the mains voltage is significantly variable. When the kVp and time are kept constant and a series of readings is taken with variation in the mA used, then, in general, as the mA increases the kVp would drop. The apparatus is arranged so as to compensate for this effect but, if the mains voltage is also varying, the compensation may not be exactly proportional to the change in mA. A change of ± 5 % in the actual kVp has frequently been detected in Barnsley, where the mains supply is particularly erratic. Although such a change would probably not be detected on the film, this effect must be remembered whenever the Dynalyzer II is used to check or calibrate stud settings. It was also found that although it is remarkably easy to make deductions as to the shape of the waveform even without a cathode-ray oscilloscope, in 48
practice it is a great advantage to know the shape of the kVp output waveform before undertaking a test exposure. This is in order to determine what delay should be applied before the reading-point and to decide at which point to start the measurement of exposure time. RESULTS
It would be impracticable to list all the results here. Even from one generator and one tube an enormous quantity of data becomes available. Our aim was to assess the Dynalyzer II system rather than the X-ray apparatus, so that we tested many installations and checked wherever possible against other measuring devices. In most of the tests, the equipment suppliers and their engineers gave full co-operation and these units were tested concurrently with their own test equipment. There were a few instances where the X-ray equipment suppliers and we have agreed to differ over slight differences of calibration, but such differences have been minor, especially when the width and intensity of the
JANUARY 1979
A calibration system for X-ray generators and tube factors oscilloscope trace is taken into account. We were fortunate in being able to assess the Dynalyzer II on new equipment being installed in a new department by all four major suppliers, as well as in several older departments containing well-used apparatus. An example of the type of result obtained when making measurements on single- and 3-phase units is shown in Fig. 4. When this type of curve was first obtained, at first sight it seemed that the units under test were poorly aligned, but closer examination of the tube current results suggested that the tube mA was low and that this was giving rise to the unexpectedly high kVp. The tube mA values for these units are shown in Fig. 5, wherein it will be seen that the mA did not vary with selected kVp in the way that the recorded kVp varied with selected tube current. Eventually, the explanation was found to lie in variations in line voltage and the sensitivity of the Dynalyzer II to these variations mentioned above. In summary, our findings were: mA check gives an average reduction of 8% in mA with constant kVp, when both HT cables were connected. The variations between the readings was < 1 %. kV check With both HT cables inserted, the reduction in mA gave a rise inkV P of8-10%. Timer check The variation between readings was < 1 % using kV trigger, average variation 3.2% using mA trigger, average variation 1%. Repeatability check < 1 % variation. When the cathode cable was disconnected from the high-voltage unit and reconnected directly to the X-ray tube, the load on the cathode circuit was removed, and the mA results were accurate to within 1%. Consequently, the indicated kVp rose to the level needed to compensate this effect. The maximum variation between results was found to be 1.15%, both on testing different tubegenerator systems and repeated tests on one apparatus. DISCUSSION
The Dynalyzer II test system is a very sensitive apparatus that gives excellent reproducibility and consistent results. It can be used in two ways, both during the initial calibration and alignment of the X-ray generator-control-tube system and as a running check on the system's performance throughout its working life. The Dynalyzer II is so sensitive that 49
the user must be aware of the limitations of the Xray equipment that is being tested, because it is very easy to misinterpret the results obtained and to ascribe a permitted variation to an apparatus fault. In most instances, a view of the output waveform is essential. Another finding was that when tests were being made on apparatus that had been aligned in the conventional manner by the supplier's engineers, the load presented by the Dynalyzer II unit was not acceptable and it was found that it was better to operate with only the anode cable connected into the circuit when measuring mA and kVp. This necessitated that both anode and cathode waveforms should be symmetrical with respect to each other. Both cables could be inserted when exposure time was being measured and it was found that the best reading was obtained using an mA trigger because the mA waveform is delayed compared with the kV wave and mA is needed to obtain any X rays. In a similar manner, both cables were connected when readings of the tube filament current were being taken. When the apparatus is used for initial alignment and calibration of a new installation, both hightension cables are connected into the high-voltage unit. When alignment is satisfactory, the cathode cable is removed and final adjustments are made. The accuracy of the final alignment is only limited by the design characteristics of the generator-controltube system. It has hitherto been thought that a tolerance in final alignment of 10% was all that could be obtained and this has been regarded as acceptable. The design characteristics are such that the present investigation has found that this tolerance can be reduced to 1 %, so that previous testing methods were not making full use of the adjustments available. In use, the Dynalyzer II is easy to connect into any secondary circuit using 75 kVp 3-pin federaltype connectors. On some apparatus from Siemens, special adaptors were found necessary because the retaining ring for the high-tension cable has an internal thread. The Dynalyzer II test system can readily be transported by one man using a car. The use is specialized in that any one hospital or even one group or area would find it difficult to justify the relatively high cost by the local use that could be found for it. On the other hand, it would seem to be a highly justifiable purchase at the regional level. This apparatus can eliminate guesswork and it takes very little time to produce a complete analysis of performance.
V O L . 5 2 , No.
613
T. Healey, D. G. Dickson and M. W. B. Greenwood ACKNOWLEDGMENTS It is a pleasure to record the cooperation given by Machlett X Ray Tubes (Great Britain) Ltd. in providing the test apparatus and in always being forthcoming with very useful advice. Dynalyzer is a registered trade-mark of Machlett. The engineers of all the major apparatus suppliers have
assisted in these tests and have enabled us to compare the results obtained on the Dynalyzer II with those of the installation engineers, using their own test equipment. Most of the tests were carried out on new installations in Barnsley District General Hospital (Superintendent Radiographer, A. M. Devine, H.D.C.R.).
Book review Thermo-Radio-Therapie. Animal Experiments Investigating the Combined Treatment with High Frequency Waves and Rontgen Rays of Solid Ehrlich Ascites Tumours in the Mouse. By Dr. med. habil. Franz Dietzel, pp.124 +94 references + 96 Appendix with details of results, 1978. (In German) (Urban & Schwarzenburg, Munich) DM48. ISBN 3-541-08681-5 This book is a published research thesis. It is economically produced in offset-litho with soft covers. It describes experiments carried out over 2-3 years at the University of Giessen on the effect of 461 MHz microwaves and X rays on solid Ehrlich ascites tumours in mice. The experiments were sensibly designed, were well carried out and are well analysed and very clearly described (if I can presume to say so of another language). The literature is adequately listed up to the end of 1976: over 1200 references. The author used rather short bursts of hyperthermia labelled H1, H2, H3, and H4 respectively because the microwave generator was switched on at a constant 150 W for just one, two, three or four minutes. Temperature versus time curves are given, and are nearly triangular. The times above 37°C were only four, six, eight and ten minutes respectively. The peak temperatures reached momentarily were reported to be about 39.5°, 40.5°, 41.5°, and 42.5° respectively. The author concluded "easily" (unschwer) that hyperthermia alone, given with the aim of healing the tumour, was unrealistic. X-ray doses of 10, 20, 30 and 40 G were used alone in the control experiments, with acceptable results. Combined treatments were always with 20 Gy, sometimes with simultaneous heating, at other times with an interval between them up to 96 hours. There was little or no effect of the hyperthermia alone on the tumours. The largest effect was with simultaneous heat and X rays, although intervals shorter than 24 hours were not studied. Dose modifying factors of 1.5 to 2 were found for various parameters of tumour response when heat and radiation were given together, as other authors have found for longer exposures to 42-43 °C for one hour. It is surprising that such large DMFs were found for such short exposures. Perhaps microwaves of 461 MHz do have a stronger effect then "waterbath" heating; or were the temperatures actually higher? These DMFs are not higher than those obtained for normal tissues by other authors, whom Dr. Dietzel correctly quotes. This conclusion does not mean that hyperthermia combined with radiotherapy will provide no therapeutic advantages, but it does mean that the advantages have to be sought for by further research and don't just fall off the tree at the first shake. This book should be on the shelves of researchers into the therapeutic effect of hyperthermia. J. F. FOWLER.
50
A calibration system for X-ray generators and tube factors By T. Healey, F.R.C.R., M.I.Nuc.E., F.R.A.I., D. G. Dickson, T.Eng.(CEI), F.S.X-ray TM A.F.S.E.R.T., and M. W. B. Greenwood, H.D.C.R., Cert.Hlth.Ed. Barnsley District General Hospital, Barnsley, South Yorkshire {Received September, 1977 and in revised form February, 1978)
ABSTRACT
An apparatus (Machlett's Dynalyzer II system) is described that makes real-time dynamic tests on the output and performance of the X-ray tube and generator so that a single exposure gives information on mA and mAs; exposure time (ms); kVp anode to earth; kVp cathode to earth; kVp cathode to anode; tube filament current; line voltage and radiation output of the tube (mR). The method of use is described together with the results of comparisons made with other test equipment. Some novel design features are of particular interest. The results show that by using this apparatus present design characteristics of X-ray generatorcontrol-tube systems are such that the accuracy of calibration can be improved by at least an order of magnitude.
All progress in diagnostic radiography since 1895 has been toward increasing the dependence of the radiographer on the apparatus producing the primary beam. For instance, exposures are measured in milliseconds rather than minutes and radiographers are expected to carry out many different examination techniques using several types of apparatus, often by more than one manufacturer, usually in a department containing several diagnostic rooms. In order that consistent results can be obtained under these conditions, the radiographer must be reasonably confident that exposure factors that apply with one machine can be used with another, or indeed with all machines. This is an ideal condition that too often is not achieved in practice, yet there is usually little the radiographer can do other than increase or decrease the exposure in accordance with his experience and to report the apparent defect to his superintendent. A physicist experienced in diagnostic radiology could help with these problems, but at present such physicists are relatively uncommon in the United Kingdom. In the absence of such an expert, the superintendent or the hospital physicist should have access to a dosemeter and/or a cathode-ray oscilloscope, but even if the manufacturers' engineers are called in there is comparatively little they can ordinarily find out about the output of the generator and the X-ray output of the tube without going to a great deal of trouble that would take the set out of commission for a relatively long period. Problems can also arise when we consider what is being measured. For instance, an Ardran-Crookes penetrameter cassette measures effective kilovoltage by the radiation produced. A potential divider 44
system measures peak kilovoltage and a cathode-ray oscilloscope shows the waveform. All these measure different aspects of tube performance and some compromise may be needed between the users and the testers as to which standard should be applied in a particular case. A system is now available that makes real-time dynamic tests possible on the output and performance of the X-ray tube and generator, either separately or together. The apparatus is the Dynalyzer II from Machlett X Ray Tubes (Great Britain) Ltd. It is inserted in the secondary side of the high voltage circuit between the high tension generator and the X-ray tube (Fig. 1), and a single exposure can give all the following information: mA and mAs; exposure time (ms); kVp anode to earth; kVp cathode to earth; kVp cathode to anode; tube filament current; line voltage; radiation output of the tube (mR). A paper-tape printer is included in the Dynalyzer system, so that an intermittent fault can be investigated by leaving the apparatus in circuit all day, so that each exposure is logged for off-line analysis. DESCRIPTION OF APPARATUS
The high voltage unit (Figs. 1 and 2) encloses a high-voltage divider network, an anode current sensor and a filament current monitor. The highvoltage divider inserts 1000 megohm resistances between the anode and earth and the cathode and earth, the rated anode-cathode voltage being 150 kVp. The voltage is measured across a 10000:1 stepdown section with ^ 1 % accuracy. The anode current sensor is an optically-coupled device to give a signal that represents the anode waveform. Currents from 1 mA to 2 A can be measured with a claimed accuracy of ± 4 % for currents less than 10 mA and ± 2 % for currents over 10 mA. The output conversion factor is 1 mV per mA of anode current and this output voltage is electronically buffered so that it can be coupled to a cathode ray oscilloscope or other measuring device. The means of producing an isolated input that represents the anode current is an innovation. The anode current is sampled through a 1 ohm resistance. The signal is amplified and then fed to a gallium
JANUARY 1979
A ^calibration system for X-ray generators and tube factors
HIGH
VOLTAGE
UNIT
RADIATION 1
:
D
*
1
O
0
PROBE
* •
0 0
©
O
0
LmJ
DIGITAL
DIGITAL
PRINTER
DISPLAY
FlG. 1. Block diagram to show how the Dynalyzer II system is incorporated in the high tension circuit.
Dynalyzer II system. For fullest utilization of the equipment, a dual-trace cathode-ray oscilloscope and a radiation probe are needed in addition. Any cathode-ray oscilloscope can be used, but the impedance of the output of the ionization chamber needs to be matched to that of the digital display unit. The output of the high-voltage unit is frequency compensated for cable length, high voltage transients and for oscilloscope impedance factors, so that the overall accuracy of the voltage readings on the cathode-ray oscilloscope is normally limited by the accuracy of
arsenide infra-red light-emitting diode. The light signal from this device is proportional to the current sampled. It is optically coupled through a glass lightpipe to a photodiode. The output from this diode is again amplified and then fed to the display unit. The tube-filament current monitor uses an isolating toroidal transformer in the common lead of the filament circuit. It can measure currents up to 20 A. Claimed accuracy is ± 1 % at 50 Hz. The digital display unit (Figs. 1 and 3A) and the paper-tape printer (Figs. 1 and 3B) are part of the 45
VOL.
52, No. 613 T. Healey, D. G. Dickson and M. W. B. Greenwood
the oscilloscope used. The accuracy of the digital display unit is usually at least an order of magnitude higher.
measurements are made and finally triggering can be either external or, if internal, from the kV or mA wave forms. In practice, it was found that an initial 10 ms delay was essential to prevent the apparatus recording the initial spike on any exposure. With regard to the vernier settings, the recommended levels of 10% for single phase and 75% for 3-phase were found to be a good general guide. We did find on some occasions, however, output waveforms that necessitated measurements being taken at the 50% point.
Method of connection The secondary circuit of the high tension supply to the X-ray tube is interrupted by removing the high voltage cables from the tube and connecting them into the Dynalyzer II high-voltage unit (Fig. 1). Jumper cables are then used to connect the high-voltage unit to the tube. A direct connection is then made from the high-voltage unit to the digital display unit, by means of a multi-core cable, and thence to the digital printer. A 240 V supply is taken to the three components of the Dynalyzer II system from a common point. This is to prevent any earth loop becoming established. The oscilloscope takes its electrical supply from the same mains and connections are then made to the appropriate sockets on the digital display unit so as to view the anode and cathode wave forms. Another socket accepts the lead from the radiation probe.
METHOD OF TESTING
During the evaluation of the apparatus, exposures were made utilizing all the variables and noting the effects on the indicated factors. For each set of conditions on the Dynalyzer II system (e.g. automatic
Method of operation The high-voltage waveform is unlikely to be identical from set to set, so that there is a need to select the most useful point on the waveform. This point at which readings are to be taken can be affected in four ways. Firstly, a built-in delay can be switched out when making exposures of less than 3 ms. Secondly, there is a vernier scale to select the triggering point. For single phase units, a point at 10% of the peak is advised, and for 3-phase units a point at 75% of the peak. It is also possible to include an additional delay of up to 20 ms before kVp
FIG. 2. The high voltage unit containing a high voltage divider, an anode current sensor and a filament current monitor.
FIG. 3. (A) The digital display unit and (B) the paper tape printer of the Dynalyzer II system.
46
JANUARY 1979
A calibration system for X-ray generators and tube factors delay "ON", 10% of waveform, kV trigger, 10 ms delay) tests were made as shown in Table I. When each set of tests had been carried out, the whole procedure was repeated with a different set of conditions being selected on the Dynalyzer II. When these results were being obtained, some additional constraints became apparent. Firstly, it was found that the extra cable in the high tension circuit imposed such a load on the X-ray system that
the tube current was significantly lowered, with an accompanying drop in kVp—contrary to what might have been expected. Allowance must be made for this in any calibration of settings on the control panel. It was found that the Dynalyzer II is much more sensitive to variations in the line voltage than is the usual "automatic" compensator on the X-ray apparatus. This meant that a given stud setting
TABLE I
Constant
Time
mA
kVp
Varying
Constant
Varying
Constant y
mA check
y
y
mAs check
y
y y
kV check y
y
Repeatability check
y
y
Varying
Constant
Varying y
y
y
Timer check
mAs
y
y
y y
y
y
110 -i
y
i
Single phase unit Three phase unit
10090 KVP.
90 -\ 80 KV.P. 8070 KVP 70-
60-
60 KVP
100
200
300
400
500
Selected tube M A FIG. 4. Typical results obtained with the Dynalyzer II system showing the variation in actual kVp output against tube mA for various selected values of kVp. 47
VOL.
52, No. 613 T. Healey, D. G. Dickson and M. W. B. Greenwood 600
Single phase unit. Three phase unit.
600
500
400
400 330
300 300 200
200
100
100 —
60
70
80
90
100
Selected K.V.P. FIG. 5. Variation in actual generated mA against the selected kVp for chosen mA values. These are the same X-ray units as were tested to give Fig. 4.
might give a different high-voltage output at different times, due to these variations in the line voltage. This is no fault in the Dynalyzer II system, for it accurately follows these changes, but it does mean that accurate calibration of the control settings can be impossible if the mains voltage is significantly variable. When the kVp and time are kept constant and a series of readings is taken with variation in the mA used, then, in general, as the mA increases the kVp would drop. The apparatus is arranged so as to compensate for this effect but, if the mains voltage is also varying, the compensation may not be exactly proportional to the change in mA. A change of ± 5 % in the actual kVp has frequently been detected in Barnsley, where the mains supply is particularly erratic. Although such a change would probably not be detected on the film, this effect must be remembered whenever the Dynalyzer II is used to check or calibrate stud settings. It was also found that although it is remarkably easy to make deductions as to the shape of the waveform even without a cathode-ray oscilloscope, in 48
practice it is a great advantage to know the shape of the kVp output waveform before undertaking a test exposure. This is in order to determine what delay should be applied before the reading-point and to decide at which point to start the measurement of exposure time. RESULTS
It would be impracticable to list all the results here. Even from one generator and one tube an enormous quantity of data becomes available. Our aim was to assess the Dynalyzer II system rather than the X-ray apparatus, so that we tested many installations and checked wherever possible against other measuring devices. In most of the tests, the equipment suppliers and their engineers gave full co-operation and these units were tested concurrently with their own test equipment. There were a few instances where the X-ray equipment suppliers and we have agreed to differ over slight differences of calibration, but such differences have been minor, especially when the width and intensity of the
JANUARY 1979
A calibration system for X-ray generators and tube factors oscilloscope trace is taken into account. We were fortunate in being able to assess the Dynalyzer II on new equipment being installed in a new department by all four major suppliers, as well as in several older departments containing well-used apparatus. An example of the type of result obtained when making measurements on single- and 3-phase units is shown in Fig. 4. When this type of curve was first obtained, at first sight it seemed that the units under test were poorly aligned, but closer examination of the tube current results suggested that the tube mA was low and that this was giving rise to the unexpectedly high kVp. The tube mA values for these units are shown in Fig. 5, wherein it will be seen that the mA did not vary with selected kVp in the way that the recorded kVp varied with selected tube current. Eventually, the explanation was found to lie in variations in line voltage and the sensitivity of the Dynalyzer II to these variations mentioned above. In summary, our findings were: mA check gives an average reduction of 8% in mA with constant kVp, when both HT cables were connected. The variations between the readings was < 1 %. kV check With both HT cables inserted, the reduction in mA gave a rise inkV P of8-10%. Timer check The variation between readings was < 1 % using kV trigger, average variation 3.2% using mA trigger, average variation 1%. Repeatability check < 1 % variation. When the cathode cable was disconnected from the high-voltage unit and reconnected directly to the X-ray tube, the load on the cathode circuit was removed, and the mA results were accurate to within 1%. Consequently, the indicated kVp rose to the level needed to compensate this effect. The maximum variation between results was found to be 1.15%, both on testing different tubegenerator systems and repeated tests on one apparatus. DISCUSSION
The Dynalyzer II test system is a very sensitive apparatus that gives excellent reproducibility and consistent results. It can be used in two ways, both during the initial calibration and alignment of the X-ray generator-control-tube system and as a running check on the system's performance throughout its working life. The Dynalyzer II is so sensitive that 49
the user must be aware of the limitations of the Xray equipment that is being tested, because it is very easy to misinterpret the results obtained and to ascribe a permitted variation to an apparatus fault. In most instances, a view of the output waveform is essential. Another finding was that when tests were being made on apparatus that had been aligned in the conventional manner by the supplier's engineers, the load presented by the Dynalyzer II unit was not acceptable and it was found that it was better to operate with only the anode cable connected into the circuit when measuring mA and kVp. This necessitated that both anode and cathode waveforms should be symmetrical with respect to each other. Both cables could be inserted when exposure time was being measured and it was found that the best reading was obtained using an mA trigger because the mA waveform is delayed compared with the kV wave and mA is needed to obtain any X rays. In a similar manner, both cables were connected when readings of the tube filament current were being taken. When the apparatus is used for initial alignment and calibration of a new installation, both hightension cables are connected into the high-voltage unit. When alignment is satisfactory, the cathode cable is removed and final adjustments are made. The accuracy of the final alignment is only limited by the design characteristics of the generator-controltube system. It has hitherto been thought that a tolerance in final alignment of 10% was all that could be obtained and this has been regarded as acceptable. The design characteristics are such that the present investigation has found that this tolerance can be reduced to 1 %, so that previous testing methods were not making full use of the adjustments available. In use, the Dynalyzer II is easy to connect into any secondary circuit using 75 kVp 3-pin federaltype connectors. On some apparatus from Siemens, special adaptors were found necessary because the retaining ring for the high-tension cable has an internal thread. The Dynalyzer II test system can readily be transported by one man using a car. The use is specialized in that any one hospital or even one group or area would find it difficult to justify the relatively high cost by the local use that could be found for it. On the other hand, it would seem to be a highly justifiable purchase at the regional level. This apparatus can eliminate guesswork and it takes very little time to produce a complete analysis of performance.
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T. Healey, D. G. Dickson and M. W. B. Greenwood ACKNOWLEDGMENTS It is a pleasure to record the cooperation given by Machlett X Ray Tubes (Great Britain) Ltd. in providing the test apparatus and in always being forthcoming with very useful advice. Dynalyzer is a registered trade-mark of Machlett. The engineers of all the major apparatus suppliers have
assisted in these tests and have enabled us to compare the results obtained on the Dynalyzer II with those of the installation engineers, using their own test equipment. Most of the tests were carried out on new installations in Barnsley District General Hospital (Superintendent Radiographer, A. M. Devine, H.D.C.R.).
Book review Thermo-Radio-Therapie. Animal Experiments Investigating the Combined Treatment with High Frequency Waves and Rontgen Rays of Solid Ehrlich Ascites Tumours in the Mouse. By Dr. med. habil. Franz Dietzel, pp.124 +94 references + 96 Appendix with details of results, 1978. (In German) (Urban & Schwarzenburg, Munich) DM48. ISBN 3-541-08681-5 This book is a published research thesis. It is economically produced in offset-litho with soft covers. It describes experiments carried out over 2-3 years at the University of Giessen on the effect of 461 MHz microwaves and X rays on solid Ehrlich ascites tumours in mice. The experiments were sensibly designed, were well carried out and are well analysed and very clearly described (if I can presume to say so of another language). The literature is adequately listed up to the end of 1976: over 1200 references. The author used rather short bursts of hyperthermia labelled H1, H2, H3, and H4 respectively because the microwave generator was switched on at a constant 150 W for just one, two, three or four minutes. Temperature versus time curves are given, and are nearly triangular. The times above 37°C were only four, six, eight and ten minutes respectively. The peak temperatures reached momentarily were reported to be about 39.5°, 40.5°, 41.5°, and 42.5° respectively. The author concluded "easily" (unschwer) that hyperthermia alone, given with the aim of healing the tumour, was unrealistic. X-ray doses of 10, 20, 30 and 40 G were used alone in the control experiments, with acceptable results. Combined treatments were always with 20 Gy, sometimes with simultaneous heating, at other times with an interval between them up to 96 hours. There was little or no effect of the hyperthermia alone on the tumours. The largest effect was with simultaneous heat and X rays, although intervals shorter than 24 hours were not studied. Dose modifying factors of 1.5 to 2 were found for various parameters of tumour response when heat and radiation were given together, as other authors have found for longer exposures to 42-43 °C for one hour. It is surprising that such large DMFs were found for such short exposures. Perhaps microwaves of 461 MHz do have a stronger effect then "waterbath" heating; or were the temperatures actually higher? These DMFs are not higher than those obtained for normal tissues by other authors, whom Dr. Dietzel correctly quotes. This conclusion does not mean that hyperthermia combined with radiotherapy will provide no therapeutic advantages, but it does mean that the advantages have to be sought for by further research and don't just fall off the tree at the first shake. This book should be on the shelves of researchers into the therapeutic effect of hyperthermia. J. F. FOWLER.
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