BEAM QUALITY MEASUREMENTS IN DIAGNOSTIC ROENTGENOLOGY*

Transcription

1 JULY, 1970 BEAM QUALITY MEASUREMENTS IN DIAGNOSTIC ROENTGENOLOGY* By E 1)ALE TROUT, l)sc, JOHN P KELLEY, BS, and KENNETH E WEAVER, MS CORVALLIS, T THE Annual Meeting of this Society in 1967 we reported on work searching for a method of beam quality description applicable to roen tgenographic procedures #{176} In that discussion we made the proposition that the methoti usetl to describe the charactenistics of the x-ray beam should take illto consideration the fact that only a very small fraction of the incident beam reached the film-screen combination and produced the roentgenognaphic image For that reason, the half-value layer (HVL) was not consitieneti appropriate anti our work to that time indicated that the twentiethvalue layer in aluminum (20th VL) might be the common tienominaton being sought It was possible at that time to report that atlequate i nstnumen tation WIS generali y available to determine the twentieth-value 1aen anti using a phantom pelvis it was possible to report that, on one laboratory installation, it was possible to predict the exposure for this phantom using the twentieth-value layer as the measure of beam quality The present report extentis this stutiy to cover a skull phantom as well as the pelvic phantom It also takes into consideration the results obtained on io more roentgenographic units: 2 in our laboratory, i in a private clinic, 2 in the private office of a group of noentgenologists, and 5 units in 3 hospitals These units ranged in age from i very recently installeti to equipflleflt in use more than 15 years All equipment made use of rotating anotle tubes and all were operated at 100 milliamperes The grids used were those in use at the various installations anti ha(i ratios of 8:i, i 2:1, and 16:1 All equipment hati total filtration in * Presented at the Seventieth Annual Meeting of the American Roentgen Ray Society, Washington, DC,, September 30-October 3, 5969 From the X-ray Science Engineering Iaboratory, Oregon State University, Corvallis, Oregon The research upon which this publication is based was performed pursuant to Contract No PH a with the US Public Health Service, Department of Health, Education, and Velfare 624 OREGON compliance with NCRP Report 33 recommendations No attempt was made to determine the accuracy of kilovoltage settings since the purpose of the study was to determine whether or not a beam quality number woulti reflect the energy characteristics of the radiation in use It was determined that the instruments used to indicate the exposure in m illiampere-seconds were reading correctly On each unit, absorption data were taken in aluminum and plotted to determine the first half-value layer and the twentieth-value layer at an indicated kilovoltage of 6o, 8o, anti ioo kvp and at the maximum kilovoltage used on that unit The geometry anti instrumentation used in taking the absorption tiata were descnibed in our previous paper A summary of the determinations for 6o and 100 kilovolts is shown in Table i Roentgenognams were then made of the pelvic phantom anti the skull phantom All exposures were made using the same cassette and screens and all films were processed in our automatic processor All tube units were equippeti wi th adjustable collimators and they were adjusted for a 14X17 inch field at the 40 inch source-film distance used for all noentgenognams The exposunes used for the pelvis at 6o and ioo kilovolts were those given for the twentieth-value layers shown in Figure 2 in our previous paper1#{176} The exposures useti for the skull were pretlicated on exposures determined on a unit in our laboratory The roentgenograms in our laboratory anti the exposure determinations for the twentieth-value layers had made use of a 16:! grid On those units having grid ratios

2 VOL 109, No 3 Beam Quality Measurements in Diagnostic Roentgenology C 4C 3c 20 Enstal- lation IC TABLE SUMMARY OF BEAM QUALITY DETERMINATIONS Machine mdicated kvp I EXPOSURE, st HVL mm Al 20th mm A I 6o A I A 2 6o A B 3 6o B B 6o B C 5 6o C D 6 6o D 6 ioo D 7 6o D E 8 6o E F 6o F F 10 6o F F II 6o F II VL Al other than i 6 : I, a conversion factor was applied to the exposure to give the number of milliampere-seconds appropriate to that grid ratio The conversion factors used were determined using a unit in our laboratory and are shown in Table ii SKULL TABLE II GRID CONVERSION FACTORS (8o lines/inch grids; 40 inch source-film distance) x - LABORATORY INSTALLATIONS 0 -MEDICAL INSTALLATIONS MILLIAMPERE-SECONDS I I I I I I #{149} Grid Ratio Conversion Factor 16: : :1 o6 A summary of all of the exposures used plotted against the twentieth-value layer is shown in Figure i In our previous paper #{176} a density measurement was made at a specified point on all roentgenognams A similar measurement was made on all roentgenograms of the skull and pelvis made at the various installations Using the exposures determined from our data as shown in Figure i, the maximum deviation in density from the predicted density of i at the specified point on the pelvic film was ±oi In the case of the skull, a density of ii at the specified point had been used in setting up the exposure versus twentieth-value layer graph In the roentgenograms of the skull made using the data from the basic determinations, the maximum density variation from 11 was ±01 Since it is not possible to demonstrate the uniformity of the results by showing all roentgenognams taken, I I #{149} I FIG I Twentieth-value layer in Al vs exposure in milliampere-seconds Medium speed film and screens; 40 inch source-film distance; 16:1 grid 400

3 626 F Dale Trout, John P Kelle\ and Kenneth E \Veaver jui,v, G 2 I elvis roentgenograms of beam quality extremes Left to right: Installation D, machine 6, 6o kvp; InStallation, machine i, 6o kvp; Installation D, machine 6, 100 kvp; Installation A, machine, ioo kvp those iiiade OIl the 2 installations llavillg the maxinlurn and I flinimul n twentietiivalue layers are showii in Figures 2 and 3 Ihe beam quality for these 2 installations is shown in Fable III Note the great differences Ill both first half-value layer and twen tietil-valu e layer These differences woulti be expected to reflect diflenences in kvp and wave form as well as total filtnation SUMNI\R\ Our studs I)aSed Oil roentgenogranls niatie 011 skull atiti pelvis phantoms incorporating hunian skeletal niatenial imbedded in tissue-equivalent material (31\i) indicates that tile twentieth-value layer is a meaningful number from which roentgenographic exposures can be predicted \Ve know that tile indicated kilovoltage was not correct on some of the units used in this study These diflerences were reflected in the twentieth-value layer and so were compensated for in tile predicted exposure values It will i)c noted from lable I that there we e great variations in the first half-value Instal- lation 1,ABIE 111 EXTR ENS ES EAM Ql \ lily Machine, Jndicated kvp 1st HVL mm Al 20th mm A i 6o A i ioo D 6 6o D 6 TOO VL Al la ers, nesultitig partly from variations in kilovoltage but langel\ tlue to variations in the total filtration in tile beam These vanations, while having a pronounced effect on incident exposure rate, have a decreasing effect OIi exposure rate at increasing depths in tissue For tllis reason it is our concliision that the half-value laen is oflittle value in predicting the technique in roentgenographic procedures We would recommend that tile half-value laen continue to be useti only to determine compliance with nlinirnum filtration recommendations The twen tieth-value layer accou n ts for those variables which determine effective beam energy incident to the grid, and with some modification by the grid, to the film-screen combination It is the measure of this, not that of the incident beam, that is important in determining the exposure (mas) requined to produce the roentgenognam In the coming year this study will be expanded to cover three-phase equipment, other phantoms, anti to make use of the information SO tleniveti in an installation where patiellt roentgenograms are matie l,i)ale Trout, 1)Sc Oregon State ttniversitv X-Ray Science Engineering Laboratory Corvallis, Oregon We wish to acknowledge the cooperation of the following: Albany General Hospital, Albany, Oregon; Corvallis Clinic, Corvallis, Oregon; Good Samaritan Hospital, Convalus, Oregon; Lebanon Community Hospital, Lebanon, Oregon; and Drs Ovregaard,

4 Voi 109, No Betm Quality Measurements in Diagnostic Roentgenologv 627 FIG 3 Skull roentgenograms of beam quality extremes Left to right: Installation D, machine 6, 6o kvp; Installation A, machine i, 6o kvp; Installation D, machine 6, 100 kvp; Installation A, machine 1, 100 kvp Merrill and Brauti, Corvallis, Oregon; and the people in these institutions whose cooperation meant so much in making this investigation possible REFERENCES 1 ADRAN, G M, and CROOKS, H E Checking diagnostic x-ray beam quality Brit 7 Radiol, 1968, 4!, BENOIST, L Radiochrometer for roentgen rays Compi rend hebdom Seances d Academie des Sciences, 1902, 134, CHRISTEN, TH Messung und l)osierung der R#{246}ntgenstrahlen Archiv und Atlas der Normalen und Pathologischen Anatomie Lucas, Grafe und Sillem, Hamburg, PEEK, F W, JR Dielectric Phenomena in High Voltage Engineering Third edition McGraw- Hill Book Company, New York, 1929 #{231} RODERICK, J F Plastic roentgen-ray phantoms AM J ROENTGENOL, RAD THERAPY & Nu- CLEAR MED, 1959,81, SHONKA, 1 R, ROSE, J E, and l Iii?, ( Conducting plastics equivalent to tissue, air, and polystyrene Progr?,T,u/car 11ed, i 95$, 1, IROUT, E I), and KE1iEv, J P The Reduction of Patient l)ose by I)iagnostic Radiologic Instrumentation Edited by Nloseley and Rust Charles C Thomas, Publisher, Springfield, Ill, TROUT, E I), KELLEV, J P, and LuCS, A C Influence of cable length on (IOSC rate and half-value layer in diagnostic x-ray procedures Radiology, 1960, 74, ROUT, E D, KELLEY, J P, anti LUCAS, A C 1)etermination of half-value layer AM j ROENTGENOL, Rw THERAPY & NUClEAR MED, 1960, 84, TRou r, E 1), KELIEY, J P, and GRoss, R E Beam quality measurements in diagnostic roentgenology AM j ROENTGENOI, Rn Therapy & NUCIEAR MED, 1968, 103,

5 This article has been cited by: 1 B Guille, J P Ducourtieux, M Bernard 1982 The determination of the X-ray beam quality from a continuous ( kv) high voltage generator using a pn junction Physics in Medicine and Biology 27:2, [Crossref]