AAPM REPORT NO. 14 PERFORMANCE SPECIFICATIONS AND ACCEPTANCE TESTING FOR X-RAY GENERATORS AND AUTOMATIC EXPOSURE CONTROL DEVICES Published for the American Association of Physics in Medicine by the American Institute of Physics AAPM REPORT No. 14 PERFORMANCE SPECIFICATIONS AND ACCEPTANCE TESTING FOR X-RAY GENERATORS AND AUTOMATIC EXPOSURE CONTROL DEVICES Report of the Diagnostic X-Ray Imaging Committee Task Group on Performance Specifications and Acceptance Testing for X-Ray Generators and Automatic Exposure Control Devices Raymond P. Rossi (Chairman) University of Colorado Health Sciences Center Denver, Colorado 80262 Pei-Jan Paul Lin Northwestern University Medical School Chicago, Illinois 60611 Phillip L. Rauch Henry Ford Hospital Detroit, Michigan 48202 Keith J. Strauss The Childrens Hospital Boston, Massachusetts 02115 January 1985 Published for the American Association of Physicists in Medicine by the American Institute of Physics Further copies of this report may be obtained from Executive Secretary American Association of Physicists in Medicine 335 E. 45 Street New York, NY 10017 Library of Congress Catalog Card Number: 84-72316 International Standard Book Number: 0-88318-461-3 International Standard Serial Number: 0271-7344 Copyright © 1985 by the American Association of Physicists in Medicine All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means (electronic, mechanical, photocopying, recording, or otherwise) without the prior written permission of the publisher. Published by the American Institute of Physics, Inc. 335 East 45 Street, New York, NY 10017 Printed in the United States of America Table of Contents INTRODUCTION 1 PART 1: Principles and characteristics of diagnostic x- 3 ray generators and automatic exposure control devices PART 2: Characteristics and performance specifications 68 for x-ray generators and automatic exposure control devices PART 3: Test methods and performance levels for x-ray 75 generators and automatic exposure control de- vices PERFORMANCE SPECIFICATIONS AND ACCEPTANCE TESTING FOR X-RAY GENERATORS AND AUTOMATIC EXPOSURE CONTROL DEVICES Raymond P. Rossi University of Colorado Health Sciences Center Denver, Colorado 80262 Pei-Jan Paul Lin Northwestern University Medicalchool Chicago, Illinois 60611 Phillip L. Roach Henry Ford Hospital Detroit, Michigan 48202 Keith J. Strauss The Children8 Hospital Boston, Massachusetts 02115 INTRODUCTION This report is the result of a charge from the Diagnostic X-Ray Imaging Committee of the American Association of Physicists in Medicine to develop a document addressing performance specifications and acceptance testing methods for x-ray generators a n d automatic exposure control devices. To execute this charge a task group was formed in November of 1981. This report reflects the collective efforts of the task group to provide a document which would be useful to practicing medical physicists who might be called upon to assist i n the development of bid documents and the evaluation of performance specifications and acceptance testing o f these components of radiological imaging systems. This report is divided into three parts relating to x-ray generators and automatic exposure control devices. The first part contains background information concerning t h e principles of operation of x-ray generators and automatic exposure control devices. The second part identifies relevant characteristics of x-ray generators and automatic exposure control devices for w h i c h performance specifications would be useful vendor to assist in evaluating different types of equipment. The third part outlines details of test methods for evaluating clinically installed equipment for purposes of acceptance testing and levels of performance which might be reasonably anticipated. 1 This report is not presented as definitive but rather as guidance to be used by professional medical physicists as they see fit. It is anticipated t h a t revisions will be needed as technological advances in the areas of x-ray generators and automatic exposure control devices are made. 2 PART 1 PRINCIPLES AND CHARACTERISTICS OF DIAGNOSTIC XRAY GENERATORS AND AUTOMATIC EXPOSURE CONTROL DEVICES T h i s section o f t h e report contains background information concerning t h e b a s i c p r i n c i p l e s a n d characteristics of x-ray generators a n d a u t o m a t i c exposure control devices used in radiological imaging systems. This material is provided to assist the reader i n g a i n i n g a n i n s i g h t i n t o t h e f u n c t i o n o f t h e s e components in an imaging system. 3 X-RAY GENERATORS Diagnostic x-ray generators are designed to provide electrical energy to be converted into X radiation. The quality and quantity of X radiation is pre-determined by the radiographer who sets the appropriate exposure factors at the control desk. There are many varieties of x-ray generators, and these can be classified according to their source of primary power (Table I) and appearance of the secondary voltage waveform (Table II). The voltage applied to the x-ray tube can be observed during the exposure by inserting a calibrated voltage divider into the high tension secondary circuit, and displaying the waveform on a dual-trace storage oscilloscope.1 The single-phase, half-wave rectified waveform [Figure 1 (a)] appears as individual voltage pulses varying from 0 volts to a peak potential, and returning to 0 volts. The voltage ripple is thus 100%. These pulses appear every 1/60th second (16.67 msec), which is one cycle of the mains supply voltage. The single-phase, full-wave rectified waveform [Fiure l(b)] is similar, with 100% ripple, except that 2 pulses appear per 1/60th second. These two voltage waveforms are inefficient for x-ray production, since both the quality and quantity of X radiation is greatly reduced as the voltage decreases below the peak value. In addition, the half-wave rectified waveform produces useful radiation for less than half of the exposure interval. A single-phase power supply can be made to produce x- radiation more efficiently by placing a capacitor in parallel with the x-ray tube. Once charged, the capacitor tends to maintain the voltage during the interval when the power supply drops to 0 volts [Figure 2 (a)]. The voltage ripple is thus reduced, and the quality and quantity of X radiation is greatly increased. Note, however,, that the removal of primary voltage, at termination of the exposure, results in a slow discharge of the capacitor which may contribute unnecessary patient exposure. This unnesseary exposure can be reduced by using a lead shutter to block radiation produced after the exposure is terminated. The lower trace in Figure 2 (a) is the radiation waveform, showing that radiation output is blocked by the lead shutter long before the capacitor is fully discharged. A conventional A.C. line of relatively low power can be utilized to charge a capacitor to rather high potential (e.g., 100 kV). The voltage on the capacitor can then be applied to an x-ray tube to produce x- radiation. For a very short exposure of relatively low mAs, this method produces excellent results. However, for longer exposure both the quality and quantity of X- radiation is severly reduced, as the capacitor is 4 5 6