US3515873A - Method and apparatus for analyzing and calibrating radiation beams of x-ray generators - Google Patents

Description

June 2, 1970 HI ms ET AL 3,515,873

METHOD AND APPARATUS FOR ANALYZING AND CALIBRATING RADIATION BEAMS OF X-RAY GENERATORS Filed Jan. 11, 1968' a Sheets-Sheet 1 I 1-1 so 29 "l1 'li IETZA 35728 A INVENTORS EDWARD M. mssms HAROLD D. ROSENBAUM WALDO D. DEVORE M1,? M A ORNEY 'Jun 2, 1970 ADIATION E. M. HIGGINS T L 7 METHOD AND APPARATUS FOR ANALYZING AND CALIBRATING R BEAMS OF X-RAY GENERATORS 2 Sheets-Sheet 2 Filed Jan. 11, 1968 n Y O E m M m .w W ME m 5 s WW MA 3 EVW 5 1 A v m ID i lm MDOZ Dnu AQL Z0.w .m24r o O W A A m m .ZI .IIIII.II

United States Patent 3,515,873 METHOD AND APPARATUS FOR ANALYZING AND CALIBRATING RADIATION BEAMS 0F X-RAY GENERATORS Edward M. Higgins, Harold D. Rosenbaum, and Waldo D. De Vore, Lexington, Ky., assignors to The University of Kentucky Research Foundation, Lexington, Ky., a corporation of Kentucky Filed Jan. 11, 1968, Ser. No. 697,233 Int. Cl. G01t 1/24 US. Cl. 25083.3 4 Claims ABSTRACT OF THE DISCLOSURE A method for calibrating an X-ray generator by passing its radiation beam through two semi-conductor detector elements, one of which is partially shielded to the beam and the other of which is unshielded, the outputs of the detector elements being depicted simultaneously as separate traces on an oscilloscope. The generator is adjusted until the traces substantially coincide with standard traces taken on a suitably calibrated generator whose beam has previously passed through the same detector elements. An apparatus for use in carrying out the method is disclosed.

BACKGROUND OF THE INVENTION Field of the invention Radiology departments of hospitals frequently have several X-ray machines which are used interchangeably. It is desirable that all such machines, regardless of their make, be used in a uniformly calibrated condition so that radiographs of identical density and contrast may be secured. Kilovoltage variations among X-ray machines should be especially minimized not only because of resulting variations in quality of the beams, but also because of variations in the radiation output. As is known, the output is approximately a function of the square of the voltage. Although present day X-ray film has been improved to a point where a small change in exposure does not significantly change the radiograph density, the lati tude of exposure change still is not great enough to permit use of improperly calibrated machines with the result that in some instances repeated examinations of the patient have been necessitated due to lack of proper calibration.

Description of the prior art Manufacturers of conventional X-ray generators gen erally supply calibration data with the machine, and technicians employ voltage and current indicating meters to adjust the machine into conformity with such data during the initial calibration of the machine at the hospital. However, this does not signify that a plurality of generators calibrated according to factory specifications will have, or will retain, the same output of beam energy. For exam ple, a weak or gassy valve in the machine, relay contact bounce, faulty rectifiers, dirty contacts and other electrical faults in the generator will influence the quality and output of the beam, as will variations of power in the transmission line and transformers serving the machine. It is a purpose of the present invention to reduce substantially, if not to overcome completely, handicaps of the above mentioned nature in connection with the calibration and use of X-ray generators.

SUMMARY OF THE INVENTION The invention comprises a portable apparatus which A accomplished. The apparatus includes a combination of a radiation detector preamplifying assembly compactly arranged in a box-like housing and with a cable carrying conductors extending from the housing to a power supply and to a dual sweep oscilloscope. The preamplifying assembly includes a pair of semiconductor detector elements, one of which is relatively partially shielded to the radiation "beam, the other of which is relatively non-shielded. The respective signals from the detector elements are amplified and transmitted simultaneously to the oscilloscope while the elements are subjected to the beam of radiation.

The method of calibration includes providing a graph, or a photograph of a pair of traces on the oscilloscope, representative of a properly calibrated generator, and then observing the ratio of pulse heights shown on the oscilloscope while a generator under calibration is being operated, and finally adjusting such generator until the percent of transmission through the respective detector elements coincides with a standard peak voltage indicated by the graph or until the traces on the oscilloscope coincide with the traces shown on the photograph. The graph or the photograph are prepared using the same apparatus which is later used in calibrating other generators to the predetermined standard. Observation of the traces on the oscilloscope, or of photographs made of the same, provide information from which malfunctioning of the generator may :be deduced.

Among the objects of the invention are the provision of a reliable, safe and portable apparatus which can be used in X-ray generator calibrating and the provision of a method for analyzing and for calibrating the radiation beams of a number of such generators.

BRIEF DESCRIPTION OF THE DRAWINGS The objects of the invention will become more apparent as the description proceeds and when considered in conjunction with the accompanying drawings in which:

FIG. 1 is a diagrammatic view indicating a typical usage of the invention.

FIG. 2 is a top plan view of the housing for the preamplifying assembly, with the top cover removed, and with the detector elements and the amplifier mounts shown schematically.

FIG. 2A is an inside view of the cover for the housing of FIG. 2.

FIG. 2B is a side elevation view of the bottom plate for the housing of FIG. 2.

FIG. 3 is a wiring diagram for the assembly contained in the housing.

FIG. 4 is a view of the oscilloscope graticule showing traces similar to those of a properly calibrated and operating generator.

FIG. 4A is a view of the graticule showing traces deviating from a properly operating generator and illustrating a situation wherein a faulty high voltage rectifier is present, and

FIG. 5 is a graph for reference to determine the peak voltage of the calibrated machine following an inspection of the traces such as indicated in FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring first to FIG. 1, a conventional X-ray generator indicated generally at 10 is mounted for movement above a table 11 on which a patient is to be placed. A portable apparatus, preferably in the form of a vehicular support, includes a conventionaldual sweep oscilloscope 12, a conventional decade type glow tube timing means 13, acamera 14 preferably of the Polaroid type pivotally mounted adjacent the graticule of the oscilloscope, a

power supply for a preamplifying assembly 16, and conventional conductors 17 for electricity supplied to the timer and to the oscilloscope. Acable 18 having a length suflicient to displace the operator at a safe distance from the radiation beam extends from the assembly 16 to the power supply.

The oscilloscope which forms a significant feature of the combination of apparatus may be of any conventional type, as for example the Type 545B dual sweep oscilloscope available from Tektronix Inc., Beaverton, Oregon,

and having a conventional graticule on which the output signals derived from the preamplifying assembly, later to be described, will be simultaneously depicted.

As seen in FIGS. 2 to 2B, the housing for the preamplifying assembly comprises a generally rightparallelepiped form of box having aside plate 20 with anintegral side wall 21 through whichcable 18 extends, and with theplate 20 providing support for block structures indicated at 22, 23 and serving as the mounting of the operational amplifiers, later to be described. Mounted side by side at a suitable spacing, for example about 1.5 cm., and supported fromplate 20 in any suitable manner are afirst detector element 24 and asecond detector element 25, the latter being shielded by asuitable sheet 26 of beam filtering material such as aluminum. Another member providinwalls 27, 28 for the box is adapted to engage withplate 20 andwall 21 when the box is assembled. Abottom plate 29 having a pair of threadedrods 30, 31 projecting upwardly therefrom is adapted to abut against the lower ends of the box walls, and the rods are adapted to extend through holes in thetop plate 32 and to be engaged by nuts or other fastening means disposed on the upper surface of the top plate. Afiixed to the inside surface of the top plate is a sheet 33 of shielding material such as lead and projecting inwardly from the top plate is a U-shapedwall 34 of similar shielding material which is adapted to prevent backscatter into the detector elements. The sheet 33 has an opening therein corresponding to the confines ofwall 34 thus to provide awindow 35 through which the beam passes and which window is covered by the outer layer of top plate material. Preferably the walls, bottom and top plate are constructed of thin gauge aluminum sheet.

Passing now to FIG. 3 the preamplifying apparatus includes thedetectors 24 and 25 comprising semi-conductor detector elements of the lithium-ion-drift type, such as type 5X5-LID-1000-100A available from Solid State Radiations, Inc., Los Angeles, Calif, and which are fed in parallel at a suitable positive DC. voltage frompower supply 15. As radiant energy is absorbed in the depletion layer of these detectors their resistance is decreased and they can be used to develop an electrical signal proportional to the absorbed radiant energy. The two detectors are substantially identical as regards their ratings and their construction, and both have a high quiescent impedence, for example about 10 megaohrnsv In the load circuit of thedetectors voltage dividers 40A are interposed and the signals developed at the load resistors which, for example, have a rating of 1 megaohm, are coupled directly intooperational amplifiers 41, 41A connected as voltage followers. Depending upon the distance D (FIG. 1) between the X-ray generator and the housing 16 a signal ranging from about 0.1 volt for low PKV- low MA radiation to about 2.5 volts at high PKVhigh MA radiation of thegenerator 10 can be developed and transmitted to the oscilloscope throughconductors 42 and 42A. The operational amplifiers are of a suitable type such as the SQ-l 0A units available from Nexus Reserach Laboratory, lnc., Canton, Mass.

Provision is made for individual zeroing of each operational amplifier bysuitable potentiometers 43, 43A, and variable resistors 44, 44A are employed to trim these amplifiers.Feedback loops 45, 45A provide linear, consistent and distortion-free amplification of signals.Conductors 46, 46A leading to the power supply carry suit able current, for example, at about +15 volt DC, to the amplifiers as doconductors 47, 47A carrying about 15 volt DC. andconductor 48 carrying 0 to -15 volt D.C. Suitable control means on the power supply serve to regulate the settings of the respective potentiometers and variable resistors. The entire preamplifying assembly can be housed in a box as described having dimensions of about three to two by one inches.

METHOD OF ANALYZING AND CALIBRATING A preferred usage of the invention involves the prior preparation of a reference graph such as shown in FIG. 5 representative of the radiation absorption characteristics of the detector element combination and wherein the abscissa represents the peak generating voltages in kilovolts of radiation beams used in diagnostic radiology, and the ordinate represents the transmission of the beam through added absorber 26 as a ratio of the output signal of the shielded orfirst detector 25 so that of the nonshielded orsecond detector 24. Having at hand a properly calibrated and functioninggenerator 10, the user will place the box with itswindow 35 upright on table 11 at the reference distance D from the generator and then actuate the generator at a series of known peak kilovoltage settings. For each such setting the relative heights of the traces 50, 51 (FIG. 4) shown on thegraticule 52 of the oscilloscope are determined and the ratio as a percent of transmission is plotted for the corresponding peak kilovoltage thereby to form a curve 53 for future reference. Moreover, a photograph of the traces shown on the oscilloscope is taken bycamera 14 to provide a permanent record of a properly calibrated and functioning generator against which comparisons with other generators may be made.

The photograph serves further purposes in that it indicates waveform of the signals which upon analysis can be used to detect actual or incipient malfunctioning of the generator. As will be understood, the number of individual pulses indicative of the time of an exposure is displayed along the X-axis of the oscilloscope graticule. The time of the exposure is determined by counting the pulses on the oscilloscope or by the decade readout tubes of theunit 13 which show the number of impulses in any exposure. Normally, in a full wave, rectified, single phase unit type of generator there will be pulses per second.

Should the observed number of pulses deviate from the proper value, the timer of the generator will then be adjusted as part of its calibration.

Furthermore, if a generator being calibrated by means of the identical apparatus. which was employed to formulate the standard graph or to provide the standard photograph, should not develop the proper PKV, the trans former of the same would then be adjusted.

In addition, in the event that the milliamperage of the radiation beam, which can be determined by known calculations from the amplitude of either oftraces 50, 51 shown on the oscilloscope, should depart from the proper value as indicated by comparison with a trace from the standard calibrated generator, then the generator likewise will be adjusted in known manner.

As an additional advantage of the invention, the oscilloscope traces from a generator being tested serve to indicate to a skilled technician the type of abnormality which may be present in that generator. For example, as shown in FIG. 4A, the 'dilference in heights of successive pulses and the displacement of the traces downwardly on the graticule is indicative of a faulty high voltage rectifier in the generator. The fact that four pulses, however, are shown on the trace would indicate that the timer of the generator is operating satisfactorily.

Other abnormalities in waveform are found from experience to be characteristic of certain malfunctioning of various components of the generator and when observed on the oscilloscope provide the necessary clue as to required maintenance or adjustment of the generator which is to be calibrated.

The apparatus embodying the invention is particularly well suited for carrying out the described method steps.Camera 14 is pivotally mounted adjacent the graticule of the oscilloscope and when a photographic record of the traces is desired, it is moved into and clamped in operating position for rapidly taking the record. Use of a camera such as the Polaroid type permits the operator to obtain a permanent record in a short time. The entire apparatus, moreover, is self-contained except for the readily detachable power conductors 17, and may be easily moved from one generator to another in a hospital, and into and out of storage.

Having thus described the method of practicing the invention and the combination of coordinated apparatus by means of which it may be practiced, it will be understood that the invention may also be embodied in other forms without departing from the scope of the appended claims.

What is claimed is:

1. The method of calibrating an X-ray generator to secure a standard peak voltage of its radiation beam comprising, passing said beam simultaneously through a relatively partially shielded preamplifying detector element and through a substantially similar but relatively nonshielded detector element, depicting as exposure-rate time functions the output signals from said elements in the form of respective first and second traces simultaneously on an oscilloscope graticule, determining the ratio of the pulse height of said first to said second trace, and adjusting the generator until said ratio corresponds to a predetermined standard value.

2. The method of calibrating an X-ray generator to secure a standard peak voltage of its radiation beam comprising, providing as a standard a comparative photograph of the traces of a dual sweep oscilloscope showing small and large signal outputs as exposure-rate time functions derived from passing a beam from a suitably calibrated X-ray generator simultaneously and respectively through a relatively partially shielded preamplifying detector element and through a substantially similar but relatively non-shielded preamplifying detector element,

6 and thereafter adjusting the generator to be calibrated until the respective signal outputs derived from its beam passing through the same detector elements used in preparation of such photograph substantially match the respective outputs shown on the photograph.

3. For use in the calibrating of an X-ray generator, a portable apparatus comprising a radiation detector preamplifying assembly, a dual sweep oscilloscope, and a power supply for said assembly; said preamplifying assembly including a housing adapted to be located in the beam of said generator and containing a second detector element relatively partially shielded to said beam and a first detector element substantially similar to said first element but relatively non-shielded to said beam, and means for transmitting the respective small and large signal outputs as exposure-rate time functions from said detector elements, when subjected to said beam of radiation, to said oscilloscope to provide simultaneous traces on the graticule thereof, whereby the value of the peak voltage of said generator may be readily determined as a function of the ratio of the small to the large pulse heights shown by the oscilloscope traces.

4. Apparatus as defined inclaim 3 wherein said housing encloses a pair of substantially similar operational amplifiers receiving signals from the respective detector elements and feeding said signals in amplified form to said oscilloscope through a cable extending through one side wall of said housing.

References Cited UNITED STATES PATENTS 2,258,593 10/1941 Black 250- 3,433,954 3/1969 Bowman et al 250-833 3,435,220 3/1969 Hanken 250=83.3

OTHER REFERENCES Hewlett-Packard Co. Catalogue No. 24, 196 3.

ARCHIE R. BORCHELT, Primary Examiner M. I. FROME, Assistant Examiner U.S. Cl. X.R. 250-65, 83, 103

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