Ebook Radiography in the digital age (3/E): Part 1

(BQ) Part 1 book “Radiography in the digital age” has contents: Introduction to radiographic science, basic physics for radiography, unit conversions and help with math, electromagnetic waves, X-ray production, production of subject contrast, visibility qualities of the image, geometrical qualities of the image,… and other contents.

OGRAPHY RADI e h nt i TALAGE GI DI on ti rd Edi Thi CS PHYSI EXPOSURE OGY OL I ON B ATI RADI T R , ED M , l ol r Car nnB Qui RADIOGRAPHY IN THE DIGITAL AGE Dedication To Jason and Stephanie, Melissa and Tim, Chad and Sarah, Tiffani and Nate, Brandon, and Tyson a most remarkable family, and to my cherished wife, Margaret, who made it possible for them all to come into my life THIRD EDITION RADIOGRAPHY IN THE DIGITAL AGE Physics—Exposure— Radiation Biology By Quinn B Carroll, M.ED., R.T CHARLES C THOMAS • PUBLISHER, LTD Springfield • Illinois • U.S.A Published and Distributed Throughout the World by CHARLES C THOMAS • PUBLISHER, LTD 2600 South First Street Springfield, Illinois 62704 This book is protected by copyright No part of it may be reproduced in any manner without written permission from the publisher All rights reserved â 2018 by CHARLES C THOMAS PUBLISHER, LTD ISBN 978-0-398-09214-6 (Hard) ISBN 978-0-398-09215-3 (Ebook) First Edition, 2011 Second Edition, 2014 Third Edition 2018 With THOMAS BOOKS careful attention is given to all details of manufacturing and design It is the Publisher’s desire to present books that are satisfactory as to their physical qualities and artistic possibilities and appropriate for their particular use THOMAS BOOKS will be true to those laws of quality that assure a good name and good will Printed in the United States TO-S-2 Library of Congress Cataloging-in-Publication Data Names: Carroll, Quinn B., author Title: Radiography in the digital age : pysics, exposure, radiation biology / by Quinn B Carroll Description: Third edition | Springfield, Illinois : Charles C Thomas, Publisher, Ltd., 2018 | Includes index Identifiers: LCCN 2017061734 (print) | LCCN 2018000406 (ebook) | ISBN 9780398092153 (ebook) | ISBN 9780398092146 (hard) Subjects: | MESH: Radiography | Physics | Radiology—methods | Radiographic Image Enhancement | Technology, Radiologic | Radiobiology Classification: LCC RC78.7.D53 (ebook) | LCC RC78.7.D53 (print) | NLM WN 200 | DDC 616.07/572—dc23 LC record available at https://lccn.loc.gov/2017061734 REVIEWERS Consulting Physicist Daniel J Sandoval, PhD, DABR Diagnostic Medical Physicist, Dept of Radiology University of New Mexico Health Sciences Center Albuquerque, New Mexico Ajesh Singh Dip, BSc (Med Imaging), MEd, FHEA C William Mulkey, PhD, RT (R), FASRT Lecturer, School of Clinical Science Queensland University of Technology Brisbane, Australia Dean, Dept of Radiologic Sciences (Retired) Midlands Technical College West Columbia, North Carolina Robert L Grossman, MS, RT (R) (CT) Donna Endicott, MEd, RT (R) Instructor, Radiography Middlesex Community College Middletown, Connecticut Director, Radiologic Technology Xavier University Cincinnati, Ohio Patrick Patterson, MS, RT (R) (N), CNMT Miranda Poage, PhD Director, Radiography Program State College of Florida, Manatee-Sarasota Bradenton, Florida Associate Professor, Biology Midland College Midland, Texas Philip Heintz, PhD Dennis Bowman, AS, RT (R) Professor Emeritus, Biomedical Physics University of New Mexico Medical Center Albuquerque, New Mexico Clinical Instructor Community Hospital of Monterey Peninsula Marina, California v PREFACE New to This Edition This 3rd edition was peer-reviewed by four colleagues who brought many valuable corrections and improvements to the text The entire textbook has been converted to metric units, and to Systeme International (SI) units for radiation biology and protection This was done to make it more usable for an international community of educators, and to align with the American Registry of Radiologic Technologists’ adoption of SI units in 2016 Medical imaging informatics was added to PACS in Chapter 36 Applying Radiographic Technique to Digital Imaging, Chapter 33, was substantially strengthened, including revised and updated material on the use of grids and new virtual grid software, all with an eye to reducing patient dose The ability of digital processing not only to generally compensate for scatter radiation, but to correct specific fog patterns in the image is more fully explained Because we deal with several different kinds of “hard” and “soft” matrices, (the DR detector matrix, the light matrix of a CR reader, the “hardware pixel” matrix of a display monitor, and the “soft” matrix of the displayed light image), the relationship between field-of-view (FOV), matrix size, and spatial resolution is now completely covered in all these contexts A new Table 13-1 lists twenty types of digital image noise organized into eight broad categories These important topics relating to noise are comprehensively explored as no other radiography textbook has done to date In radiation biology, the section on radiation units in Chapter 40 has been vastly expanded to include the concepts of air kerma, exposure area product, surface integral exposure, absorbed dose, dose area product, integral dose, dose equivalent, effective dose, and collective effective dose Optically stimulated luminescence (OSL) dosimeters were also added Digital fluoroscopy was significantly strengthened in Chapter 37 Conventional tomography has been eliminated because of its clinical obsolescence Many crisp illustrations have been added, along with helpful tables and refinements to the text designed to make the entire presentation more student-friendly Remarkable clarity and concise descriptions help the student with more complicated topics, especially in the digital domain The practical limitations of digital features such as smoothing and edge enhancement are covered with their direct implications for clinical application Several sections have been deleted, moved or reorganized to provide smoother transitions and development of the topics, with particular focus on the digital imaging chapters Material on rescaling the digital image has been greatly strengthened, and new graphs have been added that make histogram analysis and errors much easier to grasp vii viii Radiography in the Digital Age The math review chapter (Chapter 3) includes a section on basic graphs Along with material on the x-ray beam spectrum, a new section titled Understanding the Digital Histogram has been added which includes foundational support exercises directly related to the later chapters on digital image processing A glossary of technical radiographic and digital imaging terms has been expanded In addition, a deliberate effort has been made to include the content areas identified in the Curriculum Guide published by the American Society of Radiologic Technologists, and to address the Standard Definitions published by the American Registry of Radiologic Technologists Scope and Philosophical Approach The advent of digital radiographic imaging has radically changed many paradigms in radiography education In order to bring the material we present completely upto-date, and in the final analysis to fully serve our students, much more is needed than simply adding two or three chapters on digital imaging to our textbooks: First, the entire emphasis of the foundational physics our students learn must be adjusted in order to properly support the specific information on digital imaging that will follow For example, a better basic understanding of waves, frequency, amplitude and interference is needed so that students can later grasp the concepts of spatial frequency processing to enhance image sharpness A more thorough coverage of the basic construction and interpretation of graphs prepares the student for histograms and look-up tables Lasers are also more thoroughly discussed here, since they have not only medical applications, but are such an integral part of computer technology and optical disc storage Second, there has been a paradigm shift in our use of image terminology Perhaps the most disconcerting example is that we can no longer describe the direct effects of kVp upon image contrast; Rather, we can only describe the effects of kVp upon the subject contrast in the remnant beam signal reaching the image detector, a signal whose contrast will then be drastically manipulated by digital processing techniques Considerable confusion continues to surround the subject of scatter radiation and its effects on the imaging chain Great care is needed in choosing appropriate terminology, accurate descriptions and lucid illustrations for this material The elimination of much obsolete and extraneous material is long overdue Our students need to know the electrical physics which directly bear upon the production of x-rays in the x-ray tube - they not need to solve parallel and series circuit problems in their daily practice of radiography, nor they need to be spending time solving problems on velocity MRI is briefly overviewed when radio waves are discussed under basic physics, sonography is also discussed under the general heading of waves, and CT is described along with attenuation coefficients under digital imaging But, none of these subspecialties has a whole chapter devoted to it It is time to bring our teaching of image display systems up to date by presenting the basics of LCD monitors and the basics of quality control for electronic images These have been addressed in this work, as part of ten full chapters dealing specifically with digital and electronic imaging concepts If you agree with this educational philosophy, you will find this textbook of great use Preface Organization The basic layout is as follows: In Part I, The Physics of Radiography, ten chapters are devoted to laying a firm foundation of math and basic physics skills The descriptions of atomic structure and bonding go into a little more depth than previous textbooks have done A focus is maintained on energy physics rather than mechanical physics The nature of electromagnetic waves is more carefully and thoroughly discussed than most textbooks provide Chapters on electricity are limited to only those concepts which bear directly upon the production of x-rays in the x-ray tube Part 2, Production of the Radiographic Image, presents a full discussion of the xray beam and its interactions within the patient, the production and characteristics of subject contrast within the remnant beam, and the proper use of radiographic technique Image qualities are thoroughly covered This is conventional information, but the terminology and descriptions used have been adapted with great care to the digital environment Part 3, Digital Radiography, includes nine chapters covering the physics of digital image capture, extensive information on digital processing techniques, and the practical application issues of both CR and DR PACS and medical imaging informatics are included There is a chapter on mobile radiography, fluoroscopy, and digital fluoroscopy, and an extensive chapter on quality control which includes digital image QC Finally, Part consists of five chapters on Radiation Biology and Protection, including an unflinching look at current issues and practical applications including an unflinching look at current issues and practical applications Feedback For a textbook to retain enduring value and usefulness, professional feedback is always needed Colleagues who have adopted the text are invited to provide continuing input so that improvements might be made in the accuracy of the information as well as the presentation of the material Personal contact information is available in the Instructor and Laboratory Manual on disc or download This is intended to be a textbook written “by technologists for technologists,” with proper focus and scope for the practice of radiography in this digital age It is sincerely hoped that it will make a substantial contribution not only to the practice of radiography and to patient care, but to the satisfaction and fulfillment of radiographers in their career as well Instructional Resources INSTRUCTOR RESOURCES CD FOR RADIOGRAPHY IN THE DIGITAL AGE This disc includes the answer key for all chapter review questions and student workbook questions, and a bank of over 1500 multiple choice questions with permission for instructors’ use It also includes 35 laboratory exercises with 15 demonstrating the applications of CR equipment The manual is available on disc or download from Charles C Thomas, Publisher POWERPOINT SLIDES ON DISC PowerPoint™ slides are available for classroom ix 412 Radiography in the Digital Age up on an electrical capacitor until it reaches the preset threshold amount that corresponds to an ideal amount of radiation exposure The thyratron in the circuit then releases the charge in a surge of electricity that is used to activate an electromagnet The electromagnet pulls open the exposure switch, terminating the exposure When a patient is turned sideways, or when larger patients are radiographed, more radiation is absorbed within their bodies so that there is less radiation per second striking the ion chambers It therefore takes a longer time for the capacitor to reach the preset amount of charge, so that the radiation exposure is lengthened until the desired exposure is attained It should be emphasized that an AEC only controls the exposure time and consequently the total mAs used for an exposure Optimum kVp and optimum mA must still be determined and set by the radiographer when using the AEC, in accordance with all of the principles discussed in Chapters 15 and 16 If the set kVp is insufficient to achieve proper penetration of the body part, sensors will detect a reduction in the exposure rate and the AEC will allow a longer exposure to try to compensate As explained in Chapter 16, no amount of exposure can compensate for inadequate penetration Some areas of the image will still be too light, regardless of the increased exposure time Nor will increasing the density control setting properly correct this problem because it has no effect on x-ray beam penetration Optimum mA was defined in Chapter 15 as the maximum mA available for a given focal spot size, which does not overload the x-ray tube heat capacity Defining the optimum mA is a bit more complicated for automatic exposure control, because there is the additional consideration of minimum response time for the AEC circuit to properly operate MINIMUM RESPONSE TIME All electronic devices require a minimum amount of time and signal (input) in order to operate The automatic exposure control is no exception It takes time, albeit thousandths of a second, for the circuit to detect and react to the radiation received Minimum response times vary greatly from one radiographic unit to another and between manufacturers When a new unit is installed, it is a good idea for the quality control technologist to post or otherwise ensure that staff radiographers are made aware of its minimum response time (MRT), especially if the unit will be frequently used for pediatric radiography Typical MRT’s range from 0.002 seconds for stateof-the-art equipment to 0.02 seconds for older units High power generators are often employed to reduce exposure times for pediatric radiography, sometimes in combination with increased digital processing speeds It is possible for the actual exposure time to be reduced to such an extent that it is too short for the AEC circuit to respond to When the machine does not shut off until it reaches the MRT, overexposure to the patient results In such a circumstance, the best alternative is to decrease the mA station until sufficient exposure times are produced For example, let us assume an MRT of 0.005 seconds for a new high-power generator To keep exposure times short, the radiographers are in the habit of using the 300 mA station At this mA, the minimum total mAs that the machine can produce is: 300 mA × 0.005 seconds = 1.5 mAs Suppose that an ideal technique for a PA chest on a child is 65 kVp and 0.8 mAs If the radiographer uses the AEC and the 300 mA station on this child, an overexposure of almost twice too much radiation will be delivered to the child Note that in this situation, adjusting the density knob to a “minus” setting will not help The machine is not capable of making a shorter exposure The proper solution is to reduce the mA to 100, since 200 mA would still produce a minimum mAs of 1.0 Optimum mA takes on a new meaning, then, when applied to AEC exposures: It is defined as an mA high enough at a given focal spot size to minimize motion, but not so high that resulting exposure times are shorter than the MRT BACK-UP mAs OR TIME Although it is a rare occurrence, it is possible for the AEC circuit to fail A “back-up time” or “back-up Using Automatic Exposure Controls (AEC) mAs” must be set to prevent excessive duration of the exposure in this event There are two important reasons for taking this precaution: One is to prevent excessive heat overload of the x-ray tube which may damage the anode, but more importantly, excessive and unnecessary radiation exposure to the patient must be prevented On older equipment, the regular electronic timer should be set as a “back-up timer.” Most newer x-ray units set a total back-up mAs rather than a back-up time Some authors recommend a back-up time of 50% more, or 11⁄2 times the expected exposure time for the anatomy Generally, the back-up time should never be set to more than times the expected exposure time or mAs for a particular projection For example, if a typical manual technique for an AP abdomen were 15 mAs, what would be an appropriate back-up time when using the AEC at 300 mA? The manual exposure time at this mA would be: 15 mAs / 300 mA = 0.05 second Multiply this exposure time by for a resulting back-up time of 0.1 second Using the same rule for total mAs, an appropriate back-up mAs for this projection would be (2 × 15 =) 30 mAs The fourcentimeter rule (Chapter 19) can be used for different thicknesses of patients in estimating adjusted back-up times An appreciation for the importance of back-up time or mAs is gained by examining the extremely short exposures required for frontal chest projections: Let us assume that a particular chest projection requires an exposure time of 1/40 (0.025) second The AEC fails, and a very quick radiographer realizes the exposure is continuing beyond normal and releases the manual exposure switch after just seconds The patient will have received the equivalent of 80 chest x-rays! Clearly, radiographers should be certain that a back-up time or mAs is always preset An old practice was to always set the back-up time at or seconds for all AEC exposures This is unacceptable, since in the scenario just given even second would result in 40 times too much exposure A common error while using the AEC is to forget to activate the correct bucky mechanism, such as when performing a chest radiograph at the vertical chest board but leaving the table bucky on The bucky selection button also activates the AEC detectors for 413 that bucky In this case, the exposure would continue indefinitely at the chest board, while the detectors at the table “wait” for an adequate exposure level to be reached Once again, an appropriate back-up time or mAs is the only way to prevent excessive exposure to the patient Always check all stations at the console before making an AEC exposure, including the bucky selection, and the density control to make sure it has not been left on a plus or minus setting from the previous patient Preset Automatic Back-up mAs or Time Most modern units have all of the back-up mAs values preset by the manufacturer upon installation, but many modern x-ray machines are preset to excessive back-up times or mAs values A department survey recently conducted by the author discovered two dramatic examples: Example #1: A DR unit of Brand A displays the back-up mAs at the “manual” mAs knob when the AEC is engaged Setting AEC for a PA chest projection, the back-up mAs displayed is 80 mAs The average mAs for the PA chest listed on the technique chart for this unit (also provided by the manufacturer upon installation) is mAs The back-up mAs is 20 times the average mAs for a PA chest If the wrong bucky were activated, the patient would receive the equivalent of 20 chest x-rays before the exposure was terminated Example #2: In the same department, a CR unit of Brand B displays the back-up mAs on a touch-screen read-out under the heading max whenever the AEC is engaged Maximum or back-up mAs values listed include: 500 mAs for all barium procedures 1000 mAs for the abdomen and IVP ᭤ 1000 mAs for the PA chest, 1250 for the lateral ᭤ 800 mAs for the C-spine, shoulder girdle, and L-spine ᭤ 2000 mAs for the oblique sternum, 1600 for AC joints ᭤ 800 mAs for the PA skull, but 2000 mAs for the lateral ᭤ ᭤ 414 Radiography in the Digital Age If we take 50 mAs (intentionally overstated) as an average for an abdomen projection, with other body parts following the ratios of proportional anatomy from that value, we can characterize the above amounts as generally 20 times the average or more What is worse, some appear to have not been calibrated even in the right direction: Note that the skull settings (listed last) double for the lateral projection over the PA, when the lateral skull is thinner anatomy Perhaps the installer of this equipment was focused on heat-load to the x-ray tube rather than on patient exposure—what is clear is that these settings are not consistent with the anatomy These trends point up a serious need for radiographers to get involved in the interest of their patients First, on an individual level, it is important for each radiographer to appreciate the magnitude of overexposure to the patient that failure of the AEC system or engaging the wrong bucky can cause Many of these preset back-up times or mAs values can be overridden and reduced to a more appropriate level at the touch of a knob or button Back-up values of to times the expected exposure should cover nearly all contingencies that might arise Second, upon installation of new equipment, quality control technologists and managers should get personally involved with the manufacturer in determining appropriate preset values for back-up times or mAs We should take ownership of the issue as a profession, and insist upon having input into this process THE AEC INTENSITY (DENSITY) CONTROL A knob or series of buttons may be found on the console labeled density which applies to the AEC circuit This control increases or decreases the preset sensitivity of the thyratron by specific percentages, so that the exposure time will automatically be extended or shortened by those amounts With digital imaging, a more appropriate term for this device would be the intensity control, since it actually determines the intensity of exposure at the receptor plate rather than the end result density of the postprocessed image So far, however, the label “density” control has continued to be used by manufacturers There are various formats for this control; some have only three settings, for small, average, and large patients In this case the small setting will usually cut the exposure time to one-half, and the large setting will double it Others are labeled as 1⁄2, 3⁄4, N, 11⁄2 and 2, in which the N represents normal or average and the other numbers are fractions or factors of that amount (For example, the 11⁄2 setting is 50% more than average, or 11⁄2 times average.) Many have seven settings labeled as –3, –2, –1, N, +1, +2 and +3 or even a range from –5 to +5 In some cases there is no labeling but only symbols showing a bar or light that becomes wider to indicate an increase and narrower to indicate a decrease In this format, unless otherwise specifically labeled, each of the seven stations usually represents a 25 percent change (For example, the +2 setting would not be a doubling of the exposure time but rather sets of 25% increases for a total increase of 50% from N.) Remember from Chapter 19 the minimum change rule which states that overall technique must be increased by at least 35 percent (or more than 1⁄3) in order to make a significant difference This means that a density setting of +1, if it translates to a 25 percent increase, is not likely to result in any substantial improvement in exposure to the image receptor—this was dramatically demonstrated in Historical Sidebar 19-1 (page 287) in Chapter 19 using AEC exposures We conclude that a minimum intensity control setting of +2 is recommended when more exposure is needed for any reason (The –1 setting usually does result in a significant decrease because –25% results in 3⁄4 exposure, the exact inverse of 4⁄3 or a 33% increase.) As long as the AEC circuit is functioning properly, the intensity (density) control should only need to be used infrequently, for special projections as noted in this chapter, and for special circumstances When constant adjustments are made for routine positions, it is indicative of either a calibration problem with the equipment or poor positioning by the radiographer For example, Figure 27-1 illustrates how poor centering over the lumbar spine can result in a Using Automatic Exposure Controls (AEC) significant portion of the detector cell having soft tissue rather than bone tissue over it Since more radiation penetrates through the soft tissue, this can result in the AEC shutting off the exposure somewhat early, and underexposure of the tissue of interest which is the bony spine Figure 27-2 is an example of how setting the intensity control can be an integral part of automatic exposure control for some procedures In this unilateral “frog” position for the hip, you can see that the detector, centered over the neck portion of the femur, is doing a perfect job of maintaining image density there, but that in doing so the acetabulum and head of the femur (black arrow) are much too light These are essential anatomy of interest for a hip series, but are in a corner of the view where the detector cannot properly measure and adjust for the exposure One solution is to maintain proper centering over the neck of the femur as shown, but to use a +2 or higher setting on the intensity control to darken this area At this point, we must emphasize that digital imaging systems can generally compensate for overexposure at the receptor plate, but cannot compensate for information which is simply missing due to underexposure at the receptor plate If an AEC exposure is too light in some portions of the remnant beam image, insufficient data or skewed data is provided to the computer regarding the overall image; rather than just coming out too light in this one area, as an old film radiograph would, different histogram analysis and post-processing errors can occur that result in a bizarre image that is too light or too dark, has too low or too high contrast, or which manifests extensive mottle To prevent processing errors, sufficient exposure must be provided to the receptor plate across all areas of the image field It is therefore critical to identify those projections, such as this “frog” lateral hip, where a plus setting at the density control is indicated for routine use When equipment is out of calibration, the intensity control setting can provide a temporary coping tool while waiting for service The intensity control itself can be out of calibration, and there is a simple way for a radiographer to check it: Most AEC units include a mAs indicator on the console which reads out the actual total mAs used after the exposure is 415 Figure 27-1 Off-centering of the oblique spine by only cm places about one-fourth of the detector cell area outside the tissue of interest (bone) Underexposure of the spine will result from the AEC shutting off too soon (From Quinn B Carroll, Practical Radiographic Imaging, 8th ed Springfield, IL: Charles C Thomas, ,Publisher, Ltd., 2007 Reprinted by permission.) Figure 27-2 Using the AEC for a “frog” lateral hip projection, the anatomy over the detector cell is properly exposed on this film radiograph, but anatomy of interest includes the head of the femur and acetabulum (arrow), which are underexposed, because they are in the corner of the field and no detector cell is activated there Digital systems cannot correct for this lack of information in the image (From Quinn B Carroll, Practical Radiographic Imaging, 8th ed Springfield, IL: Charles C Thomas, Publisher, Ltd., 2007 Reprinted by permission.) 416 Radiography in the Digital Age completed If a plexiglass “phantom” of the knee, skull or torso is available, or some other absorber to simulate a body part, the post-exposure readouts can be written down for the different intensity control settings, starting with the N (normal), average or zero setting Ensure that no other variables are changed between exposures, and write down the mAs readout immediately after each exposure Now, for each plus or minus setting, make the following calculation: N – A × 100 N where N is the mAs readout for the N or average setting, and A is the mAs readout for each plus or minus setting This calculation simply yields the percentage by which the technique was changed up or down from “N.” Observe the following results for one x-ray machine tested by the author: +1 = +17% –1 = –14% +2 = +42% –2 = –51% +3 = +75% –3 = –67% It seems clear that this machine was designed for each setting to be an increment of 25 percent; the –2 and +3 settings fit almost exactly The +2 and –3 settings each fall about percent short of a 25 percent increment Most interesting are the +1 and –1 settings: The +1 setting is only increasing technique by 17 percent We have stated that even a 25 percent increase is not likely to make a substantial difference in exposure intensity, so a 17 percent increase would certainly fit this description The –1 setting is 44 percent short of the intended incremental decrease Radiographers using this machine would well to go right to the +2 or –2 settings when needed On another machine tested by the author, it was discovered that both the –2 and -3 settings actually increased the mAs readouts by nearly double! This should serve as a wake-up call to radiographers and quality control technologists, that intensity controls can be and often are seriously out of calibration The procedure to check them is easy to perform and well worth the short time it takes Stations that are far out of calibration should be marked to avoid until a service technician corrects the problem LIMITATIONS OF AEC The AEC circuit is automatically engaged when the machine is set for spot-filming during fluoroscopic procedures For overhead radiographs, AEC can be used at the radiographer’s discretion While repeat rates have been reduced by the use of AEC for many procedures, it was never intended to be used on all procedures AEC does have its limitations Radiographers who use AEC as a “crutch,” as an escape from the mental work needed to set manual techniques where appropriate, may cause repeats rather than prevent them The major constraints on the use of AEC are as follows: AEC should never be used on anatomy that its too small or narrow to completely cover at least one detector cell This includes most distal extremities, and extremities in general on small children Detectors measure the average amount of radiation striking the area they cover Portions of the detector cell not covered will receive too much radiation and the AEC will shut off too soon, resulting in underexposure In digital x-ray imaging, this can be a major cause of mottle in the final image Care must be taken when radiographing anatomy that is peripheral, that is, close to the edge of the body, such as the clavicle (Fig 27-3), the mandible, and lateral projections of the sternum or scapula In each of these situations the CR is normally centered close to the edge surface of the body part, so that portions of the detector cell may extend beyond the part into the raw x-ray beam As it averages the measured exposure, the detector cell will terminate the exposure early, resulting in underexposure and mottling of the digital image Radiographers have been known to adapt a position to the point where the CR is centered or cm away from the “textbook” centering point, just to be sure the detector cell is covered so that the AEC can be used and “manual technique” avoided Proper positioning should never be compromised merely to allow use of the AEC Even when AEC is used for proper applications, positioning and centering must be perfected For an ideal exposure, the tissue of interest, not just any anatomy, must cover most of the detector cells used For some procedures, the demands of nearly Using Automatic Exposure Controls (AEC) 417 Figure 27-3 Film radiograph using AEC for an AP clavicle projection, in which this peripheral anatomy resulted in the detector cell extending above the anatomy and receiving “raw” x-ray exposure This resulted in the AEC shutting off too soon, underexposing the image (From Quinn B Carroll, Practical Radiographic Imaging, 8th ed Springfield, IL: Charles C Thomas, Publisher, Ltd., 2007 Reprinted by permission.) perfect positioning may not be within reasonable limits For example, as shown in Figure 27-1, the lumbar spine is about the same width as a detector cell Therefore, in order to cover the cell with bone tissue rather than soft tissue, the centering must be nearly perfect This is not difficult to achieve for the AP projection, but is much more difficult for the oblique positions, especially on large patients The x-ray field must be well-collimated to the anatomy of interest When the field is too large, excessive amounts of scatter radiation from the table and body will cause the AEC to shut off prematurely Digital imaging is particularly vulnerable to this problem A frequent result is a “washed-out” appearance to the digital radiograph in which it appears light overall and begins to manifest mottle Side-to-side collimation for cervical spine projections, “swimmers” views, and “groin lateral” (cross-table lateral) hips must be tighter than the plate size to prevent these effects when using the AEC The AEC should never be used when there is any type of radiopaque surgical apparatus, orthopedic corrective devices, extensive orthodontic dental work, or other large metal artifacts which cannot be readily removed from the area of interest Such artifacts leave large areas over the detector cell where almost no exposure is being received (see Fig 27-4 in Historical Sidebar 27-1) The detector cell averages the exposure rate across its entire area The effect of large radiopaque objects is to lower this measured rate, such that the AEC will stay on much longer in order to reach its preset cut-off value Historical Sidebar 27-1 gives two demonstrations of how this resulted in gross overexposure of the surrounding anatomy of interest for film-based radiography Digital imaging systems are able to compensate for general overexposure and restore an image to diagnostic quality, but this scenario with AEC presents a very special case in which only a portion of the image is grossly overexposed, while another section of the image is nearly devoid of all data This huge discrepancy can lead to a number of different histogram analysis and postprocessing errors that result in an image that is too light or too dark, or has too low or too high contrast In addition there is the important issue of excessive patient exposure when the AEC overextends the exposure time while trying to compensate for a large radiopaque artifact For example, when attempting automatic exposure for “frog” or “groin” lateral hip projections with a hip prosthesis present, exposure times have been known to run all the way to the back-up time or mAs Overexposure to the patient is likely to be from to 10 times the necessary radiation, and, as discussed under Back Up mAs or Time, it can be as much as 20 times the average The unacceptability of such levels of radiation goes without saying To prevent any risk of this happening to the patient, “manual” technique should be set for all such situations AEC should never be used when large radiopaque artifacts are present Radiographers must be conscientious enough to screen patient’s charts and x-ray requisitions, and to communicate verbally 418 Radiography in the Digital Age Figure 27-4 HISTORICAL SIDEBAR 27-1: Figures 27-4 and 27-5 are both examples of large radiopaque artifacts within the patient that caused an AEC exposure too stay on much too long, overexposing the surrounding anatomy of interest Since these are both film radiographs, the predictable result was much too dark an image For modern digital imaging, the effects upon the resulting final image are not predictable, because they are the result of various possible mathematical misinterpretations by the computer processing algorithms The best way to avoid these errors is to employ manual technique In Figure 27-4, a radiographer unwisely attempted an AEC exposure for a “frog” lateral hip projection on a patient with a large hip prosthesis In Figure 27-5, a radiographer attempted to use AEC on an AP projection for the odontoid process through the open mouth The area of the detector cell is outlined, which included an entire metal tooth and extensive orthodontic bridges These still appear blank white even after the AEC grossly extended the time in an attempt to average the overall exposure For a film-based radiograph, the result was nearly pitch-black density of the cervical spine and odontoid, rendering the image diagnostically useless Large radiopaque artifacts such as this hip pin leave large areas over the AEC detector cell in which almost no exposure is being received Because the detector cell averages the measured exposure across its entire area, the exposure time will be extended and overexposure will result at the receptor plate and to the patient (From Quinn B Carroll, Practical Radiographic Imaging, 8th ed Springfield, IL: Charles C Thomas, Publisher, Ltd., 2007 Reprinted by permission.) Figure 27-5 A film radiograph of the odontoid process in which xray absorption by dental hardware resulted in the exposure time being extended by the AEC, resulting in extreme overexposure for the anatomy of interest (the upper cervical spine) (From Quinn B Carroll, Practical Radiographic Imaging, 8th ed Springfield, IL: Charles C Thomas, Publisher, Ltd., 2007 Reprinted by permission.) Using Automatic Exposure Controls (AEC) with the patient so that the presence of any hardware can be anticipated prior to radiographing the patient 419 Figure 27-6 DETECTOR CELL CONFIGURATION The location of the detector cells is normally demarcated by a triad of rectangles drawn on the radiographic tabletop or on a wall-mounted “chest board” (Fig 27-6) Sometimes the detectors are demarcated by dark lines within the field light, projected from a plastic insert in the collimator For this type, the indicated size of the cells will be accurate only at a specified distance (usually 180 cm for chest units) The actual detectors are made of very thin aluminum and located behind the tabletop, but in front of the grid, imaging plate and bucky tray At extremely low kVp, these chambers and the very thin wires leading from them can show up on the finished image, but at most radiographic techniques with anatomy in the image they are not normally visible Positioning should be such that the detector cells are covered as much as possible with the tissue of interest, and the thickest portions of the anatomy are over an energized cell This will assure adequate exposure to the image receptor plate prior to the termination of exposure When using AEC, selection of the best configuration of the three detector cells to energize is part and parcel of the positioning effort When a radiographer has developed a keen sense of the relative The typical location of the triad of AEC detector cells for a vertical bucky or x-ray table (small rectangles) densities of the tissues and is able to visualize the location of the internal anatomy well, he or she can be more creative in the selection of the detector cells used, and to great advantage A classic example of this principle is provided by the “frog” lateral hip radiograph, Figure 27-2 (page 415) In order to produce sufficient exposure over the acetabulum and head of the femur, the side detector cell which lies under the thicker, medial portion of the hip area can be energized, either alone, or in combination with the center detector, as shown in Figure 27-7 (Even with this configuration, an intensity control setting of +2 is also recommended.) Figure 27-7 Two options for a more proper configuration of the energized AEC cells for a “frog” lateral hip (See Figure 26-2), placing the cells over the thickest portion of the anatomy to ensure adequate exposure at the receptor plate (From Quinn B Carroll, Practical Radiographic Imaging, 8th ed Springfield, IL: Charles C Thomas, Publisher, Ltd., 2007 Reprinted by permission.) 420 Radiography in the Digital Age Chest radiography with digital systems presents a unique situation in selecting the best configuration of AEC detector cells For the PA projection, many radiographers are in the habit of using only the two side cells, a practice held over from the days of filmbased radiography Remembering that two-thirds of all chest x-rays are done primarily for evaluation of the heart, it is essential that the heart and mediastinum be penetrated and demonstrated in light shades of gray rather than as a blank white silhouette image For digital images, such underexposed portions can also cause mottle to appear Remember that digital systems cannot compensate where no information penetrates through to the receptor plate in the first place It is simply better to assure adequate exposure through the heart and mediastinum, and let the digital processor then lighten the lung areas As illustrated in Figure 27-8, the detector cell A to the patient’s right, lies primarily over lung tissue, whereas the left cell, B typically has a considerable portion of the heart (dotted area) overlying it The center cell, C, is over the densest anatomy, overlying the spine as well as heart and some abdominal tissue (shaded area) The lined area over detector cell A represents the additional breast tissue for female patients which overlies the lower portions of both cells A and C With all this in mind, we can formulate the following list of detector cell configurations with the relative exposures that result Each step results in only slightly more radiation exposure reaching the receptor plate than the previous step Least exposure Increased exposure Increased exposure Increased exposure Increased exposure Greatest exposure = = = = = = Right cell only Right and left cells All three cells Left cell only Left and center cells Center cell only With digital systems, it is recommended that the exposure level achieved by using all three cells be considered a minimum, and that the use of the two side cells only be abandoned In fact, many radiographers have come to prefer using the center cell only, which you will find produces consistent quality images while maintaining the exposure index number within perfectly acceptable limits CHECKLIST OF AEC PRECAUTIONS Digital imaging systems such a great job of covering up errors that in most cases it is not apparent from the image itself that the AEC has been used incorrectly Where film-based images once turned out obviously too light or too dark, the only thing radiographers really have to go on now is the exposure Figure 27-8 For digital radiography of the chest, activation of only the two side cells will result in underpenetration through the mediastinum, because approximately 3⁄4 of the cell area is over lung tissue To ensure adequate exposure through the mediastinum, adding the center detector cell, at a minimum, is recommended (Many prefer the center cell only.) (From Quinn B Carroll, Practical Radiographic Imaging, 8th ed Springfield, IL: Charles C Thomas Publisher, Ltd., 2007 Reprinted by permission.) Using Automatic Exposure Controls (AEC) indicator (index) number Yet, since this is an indication of the exposure to the patient, it has become a critical ethical and professional issue for radiographers to develop a habit of monitoring the exposure index One staff radiographer made the comment that, “I don’t care what the exposure index is, as long as that image looks right.” Such a remark is tantamount to saying, “I don’t care how much radiation the patient receives.” Our very mission as a profession is to obtain the highest quality images with the least radiation exposure to the patient The day that we lose sight of this will be the day when people with no training can be hired to operate x-ray equipment Each individual student and radiographer must be willing to make a personal commitment to minimize patient exposure, then act on that commitment in daily practice High exposure index numbers indicate overexposure to the patient for a particular projection However, it is just as important to monitor for low index numbers, because these can indicate insufficient penetration of the x-ray beam or underexposure that can lead to a mottled image, to name just two examples that might require repeating the projection Since every repeated exposure essentially doubles the amount of radiation to the patient for that view, underexposures are just as important to monitor as overexposures in the overall scheme of minimizing radiation to the patient Tables 27-1 and 27-2 form a complete checklist of errors that would explain high or low exposure index numbers Whenever the exposure indicator is unusually high or low, the radiographer should go over this checklist mentally, in a process of elimination, to isolate the probable cause of the incorrect exposure AEC TECHNIQUE CHARTS Use of the AEC does not obviate the need for the radiographer to set technique factors The only thing that an AEC controls automatically is the exposure time, and therefore the resulting total mAs An appropriate kVp must still be set “manually” to ensure adequate penetration An optimum mA station must 421 Table 27-1 Causes of Overexposure Using AEC • Wrong bucky activated • Needed exposure time less than minimum response time (small anatomy, high mA) • Density control left on plus setting from previous patient • Electronic malfunction of the AEC (Back-up buzzer sounds) • Incorrect detector cell configuration, such that activated cell(s) lie under tissue denser or thicker than the tissue of interest • Presence of radiopaque artifacts or appliances within the anatomy (hip or knee prosthesis) • Presence of external radiopaque artifacts such as lead sheets or sandbags over the sensor be selected by considering the minimum response time for the machine, the associated focal spot size, the probability of motion, and heat load to the x-ray tube The small focal spot should be selected for extremity procedures An intelligent choice for the configuration of the detector cells must be made An appropriate back-up time should be assured For all these reasons, it makes good sense to construct and use AEC technique charts for all AEC units An example is provided in Table 27-3, showing Table 27-2 Causes of Underexposure Using AEC • Backup time shorter than needed exposure time (esp on large patient) • Density control left on minus setting from previous patient • Inadequate collimation (excessive scatter radiation reaching sensors • Incorrect detector cell configuration, such that activated cell(s) lie under tissue less dense or thinner than the tissue of interest • Detector cells not fully covered by the tissue of interest: • Anatomy too peripheral • Anatomical part too small • Specific tissue area too small • Specific tissue area not centered over selected detector cells 422 Radiography in the Digital Age Table 27-3 AEC Technique Chart View kVp mA Backup Time(s) Detector Selection Density Setting Notes PA/AP 106 200 0.1 N 180 cm LAT 116 200 0.2 N 180 cm 6-YR CHEST PA 90 200 0.05 –2 180 cm RIBS DIAPH AP/OBL 60 400 0.2 –1 180 cm RIBS DIAPH AP 76 400 0.2 N 180 cm ABDOMEN AP 80 400 0.2 IVP AP/OBL 80 400 0.2 G.B / Coned AP/OBL 80 400 0.2 PELVIS AP 80 400 0.2 HIP Unilateral AP/LAT 80 400 0.2 AP 80 400 0.2 PB: 80 400 0.4 LAT 80 400 1.0 L5 / S1 90 400 1.0 Ⅲ Ⅲ ▫ ▫ ▫ Ⅲ ▫ ▫ Ⅲ ▫ ▫ Ⅲ ▫ ▫ Ⅲ Ⅲ Ⅲ Ⅲ Ⅲ Ⅲ ▫ ▫ ▫ Ⅲ ▫ ▫ Ⅲ Ⅲ ▫ ▫ ▫ ▫ Ⅲ ▫ ▫ Ⅲ ▫ ▫ Ⅲ ▫ ▫ Ⅲ ▫ ▫ Ⅲ ▫ ▫ Ⅲ ▫ ▫ Ⅲ ▫ ▫ Ⅲ ▫ ▫ Ⅲ ▫ ▫ Ⅲ ▫ ▫ Ⅲ ▫ ▫ Ⅲ ▫ ▫ Ⅲ ▫ ▫ Ⅲ ▫ ▫ Ⅲ ▫ ▫ Ⅲ ▫ ▫ Ⅲ ▫ ▫ Ⅲ ▫ ▫ Ⅲ ▫ ▫ Ⅲ ▫ ▫ Ⅲ Procedure CHEST LUMBAR SPINE THORACIC SPINE AP 74 400 0.2 LAT 60 10 TWINING C/T LAT 76 400 0.4 AP 76 200 0.2 Odontd 76 200 0.2 Lat/Obl 76 200 0.5 PA/Cald 80 400 0.2 Waters 80 400 0.2 Townes 80 400 0.3 LAT 76 400 0.1 Mastoid/Coned LAT 76 400 0.1 SHOULDER AP 76 200 0.2 FEMUR AP/LAT 76 200 0.2 KNEE / LEG All 70 200 0.1 U.G.I.-A.C All 90 400 0.2 AP/OBL 90 400 0.2 Sigm/Lat 90 400 0.2 CERVICAL SPINE SKULL (SINUS) (FACIAL) B.E.-A.C N N N N +2 *medial cell N N N N N N breathing N N 100 cm N 100 cm N 100 cm N N N N N N N N N air N air N air From Quinn B Carroll, Practical Radiographic Imaging, 8th Ed Springfield, IL: Charles C Thomas Publisher, Ltd., 2007 Reprinted by permission Using Automatic Exposure Controls (AEC) columns for all this information and additional notes By providing technique charts like this, AEC practices within a department can be more standardized, and the learning curve for students and new radiographers shortened Modern radiographers must learn both AEC and “manual” technique skills thoroughly, and maintain clinical proficiency in both Because of the advent of AEC, there is more, not less, to learn about radiographic technique For radiography of the distal extremities, cross-table projections, adaptations for trauma situations, and the presence of unavoidable artifacts in the projection, “manual” techniques must be used There will always be situations in which “manual” technique is more appropriate than using the AEC These skills must not be lost PROGRAMMED EXPOSURE CONTROLS Many modern x-ray machines have been designed to simplify technique manipulation for the radiographer by programming preselected technical factors into computer memory for each type of procedure At the control console, each procedure is listed with a drop-down menu for each specific projection The radiographer simply selects the projection, and a kVp and mAs are displayed Additional settings for large, average, and small patients may be available, which modify the kVp and mAs up or down by preset percentages The important thing for the student to understand is that these listed techniques are not the “final word” in setting technique They may each be overridden, and should be when appropriate, by manually turning the kVp or mAs up or down to refine a final technique setting These techniques should be thought of only as suggested starting points or basic guides Preprogrammed techniques cannot take into account all of the many variations of body habitus and conditions that occur from patient to patient They lack flexibility The patient size indicators are often limited to large, average, and small which certainly not cover all possibilities for patient size While programmed techniques may expedite or simplify the setting of technique, 423 they not and never can replace the need for the independent judgment of the radiographer in refining those techniques to obtain the best possible exposure results SUMMARY AECs were developed to improve the consistency of radiographic exposures, but they are not suited for every procedure Some procedures, particularly those with complex anatomy, large artifacts present, anatomy that is peripheral or too small to fully cover the detector cells are better radiographed using “manual” technique Proper positioning should never be compromised merely to allow the use of AEC When using AEC, the engaged detector cells should be fully covered with the tissue of interest Especially for digital image, when using AEC, the x-ray field must be well-collimated Optimum kVp and mA must still be set by the radiographer when using AEC Adequate penetration must be assured and ample remnant beam signal must reach all portions of the receptor plate to provide sufficient data to correctly process a digital image High-power generators, high-speed digital processing, and high mA stations can combine to bring needed exposure times down to less than the minimum response time of the AEC In this case, overexposures will occur The back-up time or mAs should be set at 2–4 times the expected exposure time Many automatic settings are excessive, and should be overridden and reduced by the radiographer Managers and quality control technologists should be involved in determining appropriate automatic settings upon installation of equipment It is appropriate to adjust the intensity (density) control for special projections and situations When increasing exposure with the intensity (density) control, change to at least +2 to make a significant difference 10 The intensity (density) control can frequently be out of calibration, and can be easily checked by a radiographer 424 Radiography in the Digital Age 11 The correct configuration of AEC detector cells is essential for proper exposure The thickest and densest portion of the anatomy should be positioned over the correctly engaged cells For the PA chest, the middle cell should be engaged to ensure adequate exposure through the mediastinum 12 Radiographers should be adept at both “manual” technique and AEC technique 13 AEC technique charts are just as important as “manual” technique charts and should be provided for every x-ray unit capable of AEC 14 Preprogrammed exposure controls can and should be over-ridden for unusual circumstances REVIEW QUESTIONS What is the disadvantage of preprogrammed exposure controls? It takes longer for the AEC to shut off the exposure on a thicker body part because the is reduced A radiographer uses a 180 cm SID for an AP cervical spine at the vertical “chest board.” If the AEC is used, why will an underexposure not result? How can extremely high-speed imaging systems lead to overexposure when the AEC is engaged? As a rule, the back-up time should be set at the expected exposure List four items that should be included on an AEC technique chart The needed total mAs for a particular procedure is 0.8 mAs The minimum response time for the x-ray machine is 0.006 seconds Using the AEC at the 200 mA station, will the exposure time be too long, too short, or correct? When increased exposure is needed, why is the +1 setting at the intensity (density) control never recommended? (Continued) Using Automatic Exposure Controls (AEC) REVIEW QUESTIONS (Continued) What is the formula for finding the percentage change from N for each station of the intensity (density) control? 10 What is the change in resulting exposure when the wrong bucky is activated during an AEC exposure? 11 Many x-ray units are preset to back-up times, and should be overridden 12 During an AEC exposure, those detector cells activated should be covered by the (thinnest, average, thickest) portion of the anatomy 13 During an AEC exposure, the detector cell should be covered not just by the anatomical area of interest, but by the of interest 14 A low-contrast and light digital image can result from insufficient during an AEC exposure 15 For digital imaging systems, extreme overexposure of parts of the image combined with extreme underexposure of other portions of the image will cause (predictable or unpredictable) results in the final image 16 What are two reasons why back-up times are still very important even if the digital image were unaffected by them? 17 A low exposure indicator (index number) is important to note because it may be an indication of insufficient of the x-ray beam 18 If the image turns out OK, why is a high exposure indicator (index number) still important? 425 426 Radiography in the Digital Age ... 8 01 8 01 8 01 802 803 805 806 806 807 809 811 811 812 813 814 816 817 817 817 818 819 8 21 Appendix 1: Answers to Chapter Exercises Appendix 2: ARRT Standard Definitions... 610 610 611 612 612 613 614 614 614 615 616 616 617 618 618 619 620 620 623 35 Display Systems and Electronic Images 627 Liquid Crystal... 304 306 307 307 308 308 309 309 309 310 310 311 311 312 312 313 315 316 317 21 The Anode Bevel and Focal Spot 3 21 Line-Focus Principle