Review of Radiologic Physics Third Edition PDF

P1: PBU LWBK312-FM LWBK312-Huda June 19, 2009 10:22 Review of Radiologic Physics Third Ed...

P1: PBU LWBK312-FM LWBK312-Huda June 19, 2009 10:22 Review of Radiologic Physics Third Edition Walter Huda, Ph.D. Professor of Radiology Medical University of South Carolina (MUSC) Charleston, SC i P1: PBU LWBK312-FM LWBK312-Huda June 19, 2009 10:22 Acquisitions Editor: Brian Brown Managing Editor: Ryan Shaw Marketing Manager: Angela Panetta Production Editor: Beth Martz Creative Director: Doug Smock Compositor: Aptara, Inc. Third Edition Copyright  C 2010, 2003 Lippincott Williams & Wilkins, a Wolters Kluwer business. 351 West Camden Street 530 Walnut Street Baltimore, MD 21201 Philadelphia, PA 19106 Printed in China All rights reserved. This book is protected by copyright. No part of this book may be reproduced or transmitted in any form or by any means, including as photocopies or scanned-in or other electronic copies, or utilized by any information storage and retrieval system without written permission from the copyright owner, except for brief quotations embodied in critical articles and reviews. Materials appearing in this book prepared by individuals as part of their official duties as U.S. government employees are not covered by the above-mentioned copyright. To request permission, please contact Lippincott Williams & Wilkins at 530 Walnut Street, Philadelphia, PA 19106, via email at [email protected], or via website at lww.com (products and services). 9 8 7 6 5 4 3 2 1 Library of Congress Cataloging-in-Publication Data Huda, Walter. Review of radiologic physics / Walter Huda.—3rd ed. p. ; cm. Includes bibliographical references and index. ISBN 978-0-7817-8569-3 1. Radiology, Medical—Outlines, syllabi, etc. 2. Medical physics—Outlines, syllabi, etc. I. Title. [DNLM: 1. Health Physics—Examination Questions. WN 18.2 H883r 2010] R896.5.H83 2010 616.07’54076—dc22 2009008871 DISCLAIMER Care has been taken to confirm the accuracy of the information present and to describe generally accepted practices. However, the authors, editors, and publisher are not responsible for errors or omissions or for any consequences from application of the information in this book and make no warranty, expressed or implied, with respect to the currency, completeness, or accuracy of the contents of the publication. Application of this information in a particular situation remains the professional responsibility of the practitioner; the clinical treatments described and recommended may not be considered absolute and universal recommendations. The authors, editors, and publisher have exerted every effort to ensure that drug selection and dosage set forth in this text are in accordance with the current recommendations and practice at the time of publication. However, in view of ongoing research, changes in government regulations, and the constant flow of information relating to drug therapy and drug reactions, the reader is urged to check the package insert for each drug for any change in indications and dosage and for added warnings and precautions. This is particularly important when the recommended agent is a new or infrequently employed drug. Some drugs and medical devices presented in this publication have Food and Drug Administration (FDA) clearance for limited use in restricted research settings. It is the responsibility of the health care provider to ascertain the FDA status of each drug or device planned for use in their clinical practice. To purchase additional copies of this book, call our customer service department at (800) 638-3030 or fax orders to (301) 223-2320. International customers should call (301) 223-2300. Visit Lippincott Williams & Wilkins on the Internet: http://www.lww.com. Lippincott Williams & Wilkins customer service representatives are available from 8:30 am to 6:00 pm, EST. ii P1: PBU LWBK312-FM LWBK312-Huda June 19, 2009 10:22 To my parents, Stefan and Paraskevia Huda, for their resolute support and encouragement iii P1: PBU LWBK312-FM LWBK312-Huda June 19, 2009 10:22 Ordinary language is totally unsuited for expressing what physics really asserts, since the words of everyday life are not sufficient y abstract. Only mathematics and mathematical logic can say as little as the physicist means to say. —BERTRAND RUSSELL iv P1: PBU LWBK312-FM LWBK312-Huda June 19, 2009 10:22 Contents Preface ix Acknowledgements xi Introduction xiii I. What is Radiologic Physics? xiii II. Why Study Radiologic Physics? xiii III. Review Book Structure xiv IV. Radiology Residents and the ABR Exam xiv V. Radiology Technologists and the ARRT Exam xv 1 X-Ray Production 1 I. Basic Physics 1 II. Electromagnetic Radiation 3 III. X-ray Generators 4 IV. Making X-rays 6 V. X-ray Tubes 9 VI. X-ray Tube Performance 11 Review Test 14 Answers and Explanations 16 2 X-Ray Interactions 17 I. Matter 17 II. X-rays and Matter 19 III. Attenuation of Radiation 22 IV. X-ray Filtration Effects 24 V. Scatter Removal 26 VI. Measuring Radiation 28 Review Test 30 Answers and Explanations 32 3 Projection Radiography I 33 I. Film 33 II. Intensifying Screens 35 III. Digital Basics 37 IV. Digital Detectors 40 V. Digital Radiography 42 VI. Digital Image Data 44 Review Test 48 Answers and Explanations 50 v P1: PBU LWBK312-FM LWBK312-Huda June 19, 2009 10:22 vi Contents 4 Projection Radiography II 51 I. Mammography Imaging Chain 51 II. Clinical Mammography 53 III. MQSA 56 IV. Image Intensifiers 58 V. Television 61 VI. II/TV Imaging 62 Review Test 66 Answers and Explanations 68 5 Computed Tomography 69 I. Hardware 69 II. Images 72 III. Scanner Operation 76 IV. Dosimetry 78 V. Miscellaneous 81 Review Test 84 Answers and Explanations 86 6 Image Quality 87 I. Contrast 87 II. Resolution 89 III. Imaging System Resolution 92 IV. Noise 94 V. Measuring Performance 97 Review Test 100 Answers and Explanations 102 7 Radiobiology/Patient Dosimetry 103 I. Basics 103 II. High-Dose Effects 105 III. Carcinogenesis 107 IV. Hereditary and Teratogenic Effects 109 V. Patient Dosimetry 111 VI. Effective Dose 114 Review Test 118 Answers and Explanations 120 8 Radiation Protection 121 I. Measuring Radiation 121 II. Dose Limits 123 III. Protecting Workers 125 IV. Patient Doses 128 V. Protecting Patients 131 VI. Population Doses 133 Review Test 136 Answers and Explanations 138 9 Nuclear Medicine 139 I. Radionuclides 139 II. Radiopharmaceuticals 143 III. Planar Imaging 146 P1: PBU LWBK312-FM LWBK312-Huda June 19, 2009 10:22 Contents vii IV. Tomography 150 V. Quality Control 153 VI. Image Quality 154 VII. Radiation Doses 155 Review Test 159 Answers and Explanations 161 10 Ultrasound 163 I. Properties 163 II. Interactions 164 III. Transducers 167 IV. Imaging 170 V. Doppler 172 VI. Imaging Performance 175 Review Test 178 Answers and Explanations 180 11 Magnetic Resonance 181 I. Physics 181 II. Relaxation 183 III. Instrumentation 185 IV. Imaging 189 V. Imaging Performance 192 VI. Contrast Agents 194 VII. Advanced Techniques 195 Review Test 197 Answers and Explanations 199 Examination Guide 201 Practice Examination A: Questions 202 Practice Examination A: Answers and Explanations 209 Practice Examination B: Questions 213 Practice Examination B: Answers and Explanations 220 Appendices 223 Glossary 227 Bibliography 239 Index 243 P1: PBU LWBK312-FM LWBK312-Huda June 19, 2009 10:22 viii P1: PBU LWBK312-FM LWBK312-Huda June 19, 2009 10:22 Preface Six years have now passed since the second edition of Review of Radiologic Physics appeared. The focus of this book remains imaging using x-rays (i.e., projection radio- graphy, fluoroscopy, and CT), as well as nuclear medicine, ultrasound, and magnetic resonance (MR). Only essential information is included to help radiology residents and radiologic technologists understand how images are created, as well as the cor- responding risks of the radiation used to make these images. Basic physics topics relating to the production and interaction of x-rays have been kept to a minimum, while more important topics of radiation biology, radiation protection, and nuclear medicine have been expanded. In this third edition, major changes have been made with respect to the organiza- tion as well as content of the text, tables, figures, and questions. The first two chapters deal with x-ray production and x-ray interactions. Three chapters address how x-rays can be used to generate projection and tomographic images. Image quality (i.e., con- trast, resolution, and noise) is now comprehensively covered in one chapter, which describes both the basic concepts and the specific values of these parameters for all imaging modalities that use x-rays. Radiation biology and radiation protection are both very important topics that now merit their own chapters. Material on nuclear medicine, ultrasound, and MR has been updated, but these chapters continue to fo- cus on basic physics. Accordingly, only minimal information is provided on the more advanced applications of nuclear medicine, ultrasound, and MR that are currently used in clinical practice. One important theme in the revised books is to focus on the material that non- physicists need to understand to permit them to perform routine clinical duties. Selection of material has been guided by whether the material is necessary to re- ally understand three issues: (a) the essentials (but not details) as to how any image is created; (b) the factors that impact on the image quality, and how this feature can be controlled and optimized; (c) the factors that impact on (any) imaging risks and other imaging costs, and how these characteristics can be minimized without adversely affecting diagnostic information. Another important goal has been to sim- plify the material by minimizing superfluous details as well as streamlining all tables and simplifying the figures to convey only the most important features. The author firmly believes that the provision of an approximate conceptual mental picture of image formation is much more valuable than detailed descriptions that may be tech- nically accurate but are of minimal didactic value. Most of the questions in the book have now been revised and, hopefully, im- proved. Each question relates to a specific piece of information that the author believes to be important for residents and technologists to know. In writing these questions, every effort has been made to ensure that they are clear and unambiguous; as such, the author would expect that any physics teacher would grasp why a particular ques- tion was being asked, as well as being able to immediately identify the correct answer. The two practice tests contain 10 questions from each chapter, and are designed to ix P1: PBU LWBK312-FM LWBK312-Huda June 19, 2009 10:22 x Preface permit residents to assess how well they have assimilated the material presented in this review. As with previous editions, readers need to understand that this review book does not explain any topic in full detail. Accordingly, the material covered in this book should be read in conjunction with a more comprehensive textbook on the topic of medical imaging. P1: PBU LWBK312-FM LWBK312-Huda June 19, 2009 10:22 Acknowledgements The author gratefully acknowledges the assistance of S Balter, PhD M Bilgen, PhD SC Bushong, PhD C Daniels, PhD RG Dixon, MD S Elojeimy, MD/PhD GD Frey, PhD C Gadsen, RT EL Gingold, PhD NA Gkantsios, PhD K Green-Donnelly, MBA, RT W He, MEng KR Johnson, PhD EM Leidholdt Jr, PhD E Mah, MS M Mahesh, PhD PS Morgan, PhD KM Ogden, PhD RJ Pizzutiello, PhD TL Pope Jr, MD DW Rickey, PhD DWO Rogers, PhD ML Roskopf, RT R Shaw P Sprawls, PhD NM Szeverenyi, PhD L Theron LK Wagner, PhD AB Wolbarst, PhD CE Willis, PhD MV Yester, PhD xi P1: PBU LWBK312-FM LWBK312-Huda June 19, 2009 10:22 xii P1: PBU LWBK312-FM LWBK312-Huda June 19, 2009 10:22 Introduction I. WHAT IS RADIOLOGIC PHYSICS? Radiology is arguably the most technology-dependent specialty in medicine, and which has seen significant changes over the past decade. Computer integration with constant technical innovations have changed the workplace and influenced the role radiology plays in the diagnosis and treatment of disease. Radiologic physics is not an esoteric subject of abstract equations and memorized definitions, but rather the total process of creating and viewing a diagnostic image. A range of physical principles influence the process of image formation. Radiologists and technologists need to understand the technology and the physical principles that constitute the advantages, govern the limitations, and determine the risks of the equipment they use. Radiologic physics covers the important medical imaging modalities of radio- graphic and fluoroscopic x-ray imaging, computed tomography, magnetic resonance, nuclear medicine, and ultrasound. Radiologic physics provides an understanding of the factors that improve or degrade image quality. Selection of the most appro- priate way of generating a medical image is the responsibility of the radiologic imaging team, consisting of the radiologist, technologist, medical physicist, and equipment manufacturer. Optimizing medical imaging performance requires a solid understanding of how these images are generated, as well as the most important determinants of image quality. All imaging modalities have a cost associated with their use. For modalities that use ionizing radiations, one of the costs is the radiation dose to the patient and staff working with these systems. Accordingly, radiation protection principles are important. Radiologists and technologists should understand the magnitude of the radiation dose to the patient and personnel exposed, and ensure that radiation levels are kept as low as reasonably achievable (ALARA principle) as well as within any relevant regulatory limits. MR and ultrasound do not have any specific risks, and the cost is generally the time required to perform the study. II. WHY STUDY RADIOLOGIC PHYSICS? Radiologists and technologists need to acquire an understanding of the underlying imaging science for each diagnostic modality and be able to pass their respective radiologic physics exams. However, neither will actually practice physics and there is no need to learn how to generate modulation transfer functions in radiographic imaging, write programs to perform filtered back projection algorithms in CT, or design RF pulses in MRI. It is important for well-rounded radiologists and technologists to have a basic understanding of the following: (i) image quality parameters, such as mottle, spatial xiii P1: PBU LWBK312-FM LWBK312-Huda June 19, 2009 10:22 xiv Introduction resolution, and contrast; (ii) how image quality is affected by radiographic techniques; (iii) how to evaluate commercial imaging equipment in terms of its ability to perform the required patient examinations; (iv) the radiation dose and risks associated with radiographic exposure; and (v) how to communicate with medical physicists and service personnel regarding imaging problems. The focus of the text and allied questions is the essential physics underlying the creation of clinical images. Special emphasis has been given to the factors impact- ing on image quality, notably image contrast, spatial resolution, and mottle. Radi- ologists and technologists understand the achievable performance of any imaging equipment and how this equipment should best be used to solve patient imaging problems. III. REVIEW BOOK STRUCTURE This review book is designed to help prepare residents and technologists for the radiologic physics portion of their board and registry exams. It provides a source for comprehensive self-study in the area of diagnostic radiologic physics. The text assumes a background of instruction in radiologic physics and is not intended to replace the standard radiologic physics texts. This book is designed, rather, to provide a concise yet complete source of review to refresh and reinforce the concepts of radiologic physics expected of residents and technologists. The text is divided into 11 chapters, each with approximately six subsections, covering everything from basic physics to image quality. Each chapter begins with a summary of the key information in point form pertaining to the area under review. This is followed by 30 questions designed to provide a self-test of the reader’s knowl- edge and comprehension in each area. The philosophy adopted by the author is that material comprehension, rather than rote memorization, will guarantee success in the exam. The review book also contains two practice examinations with questions that range over the topics covered in this book. At the end of the book is a glossary of key terms commonly used in radiologic physics. Radiation quantities are generally provided using SI units. Use of roentgen to specify radiation exposure is problematic, and use of the correct conversion factor (i.e., 1 R = 2.58 × 10−4 C/kg) would be inappropriate given current practice in medical imaging literature. In diagnostic radiology, an exposure of 1 R can be taken to be equal to an air kerma of 8.76 mGy. In the text that follows, exposures have been replaced by air kerma, with an air kerma of 10 mGy taken to be approximately equivalent to an exposure of 1 R. In nuclear medicine, non-SI units predominate in clinical practice in the United States, whereas SI units are prevalent outside of the United States and in the scientific literature. In this book use is made of SI units (i.e., MBq), with the non-SI equivalent (i.e., mCi) provided in parenthesis. Magnetic fields are generally expressed in teslas, with recognition given to the fact that MR personnel are much more likely to refer to the 5-gauss line than a 0.5-mT line. IV. RADIOLOGY RESIDENTS AND THE ABR EXAM The physics portion of the American Board of Radiology (ABR) examination is ad- ministered in the fall of each year and taken on a computer. Board-eligible residents may register 1 year in advance to take the examination in September of their second, third or fourth year of training. The 4-hour examination contains about 130 multiple- choice questions, similar to the format used in this book. Calculators are not allowed, but one is available on the PC. Most residents comfortably finish the exam in the allowed time, with no need to perform any calculations beyond trivial additions, multiplications, or divisions. P1: PBU LWBK312-FM LWBK312-Huda June 19, 2009 10:22 Introduction xv About 60% of the questions cover x-ray-based imaging, image quality, ultra- sound, and MR. Approximately 20% of the questions relate to nuclear medicine, and the remaining 20% to issues relating to radiation biology and protection. Results in the form of quartiles are provided to candidates who have taken the physics examination and pertain to each of these three syllabus categories. Further information regarding the American Board of Radiology and the written physics examination can be obtained at the ABR web site (theabr.org). V. RADIOLOGY TECHNOLOGISTS AND THE ARRT EXAM The American Registry of Radiologic Technologists (ARRT) administers credential- ing examinations and provides continuing registration for radiologic technologists within the United States. A multiple-choice test format, consisting of approximately 200 questions, is used to assess the following categories: (i) radiation protection, (ii) equipment operation and maintenance, (iii) image production and evaluation, (iv) radiographic procedures, and (v) patient care. Examinees are given 3.5 hours to complete the exam, and testing centers provide an erasable board and pen (no scratch paper is allowed). A scientific calculator is available on the computer, or one will be provided if requested. Each version of the ARRT exam is score scaled, based on the overall difficulty accounting for slight variations in exam versions. A scaled test score of 75 is required to pass the ARRT exam. Further information on the American Registry of Radiologic Technologists can be obtained at the ARRT web site (www.arrt.org). P1: PBU LWBK312-FM LWBK312-Huda June 19, 2009 10:22 xvi P1: OSO LWBK312-01 LWBK312-Huda March 10, 2009 10:17 Chapter 1 X-RAY PRODUCTION I. BASIC PHYSICS A. Forces –The mass of a body is a measure of its resistance to acceleration. –Mass is measured in kilograms (kg). –Velocity is the speed of a body moving in a given direction. –Velocity is measured in meters per second (m/s). –Acceleration is the rate of change of velocity. –Acceleration is measured in meters per second squared (m/s2 ). –A force causes a body to deviate from a state of rest or constant velocity (push or pull). –Force = mass × acceleration, measured in newtons (N). –The four physical forces in the universe are gravitational, electrostatic, strong, and weak. –Relative strengths of these four forces are listed in Table 1.1. –Gravity pulls objects to the Earth, and is important in cosmology. –At the atomic level, effects of gravity are extremely small and are ignored. –The electrostatic force causes protons and electrons to attract each other. –Electrostatic forces hold atoms together. –Strong forces hold the nucleus together. –Weak forces are involved in beta decay. B. Energy –Energy is the ability to do work. –Energy is measured in joules (J). –Energy takes on various forms including electrical, nuclear, chemical, and thermal. –One common form of energy is kinetic energy (KE) caused by motion. –A bullet with mass m and velocity v has a kinetic energy of 1/2 mv2. –Another form of energy is potential energy (PE), which is the energy of position. –A raised ball has potential energy. –Energy cannot be created or destroyed. –When a ball is released at a height, potential energy is converted into kinetic energy as the ball’s velocity increases. –Einstein showed that mass and energy are interchangeable. –E = mc2 where E is energy, m is mass, and c is the velocity of light. –Rest mass energy is the energy equivalence of a particle. –In diagnostic radiology, the electron volt (eV) is a convenient unit of energy. –1 eV = 1.6 × 10−19 J –One electron volt (1 eV) is the kinetic energy gained by an electron when it is accelerated across an electric potential of 1 volt (V) as depicted in Figure 1.1. –An electron gains 1,000 eV (1 keV) when accelerated across an electric potential of 1,000 V. –An electron gains 1 MeV (1,000 keV) when accelerated across an electric potential of 1,000,000 V. –1 MeV = 103 keV = 106 eV C. Electricity –Electrons are negatively charged and protons are positively charged. –Electric charge of an electron (or proton) is 1.6 × 10–19 coulomb (C). 1 P1: OSO LWBK312-01 LWBK312-Huda March 10, 2009 10:17 2 X-ray Production TABLE 1.1 Relative Strength of Physical Forces Type of Force Relative Strength Description Gravitational 1 Binds earth to the sun Weak ∼1024 Involved in beta decay Electrostatic ∼1035 Binds electrons and protons in atoms Strong ∼1038 Binds protons and neutrons in the nucleus –Applying a voltage in an electrical circuit causes electrons to move. –The positive region of an electrical circuit is called the anode. –The negative region is called the cathode. –Electrons are repelled from the cathode and attracted to the anode. –Any voltage source in a complete circuit results in a flow of electrons in the circuit. –Electric current, measured in amperes (A), is the flow of electrons through a circuit. –An ampere is the amount of charge that flows divided by time. –1 ampere = 1 coulomb per second –Power supplies in any domestic home have a minimum of two wires and are single phase. –Single-phase power supplies have one wire that has an oscillating voltage, with the other carrying no voltage. –If there is a third wire, this is an “earth connection’’ for safety. –In the United States, the electric power supply from utility companies is normally 110 volts (V). –U.S. electricity is an alternating current (AC) that oscillates at a frequency of 60 cycles per second (60 Hz). –In Britain, AC voltage is 220 V and oscillates at a frequency of 50 cycles per second (50 Hz). –Three-phase power supplies have three lines of voltage, each 120 degrees out of phase with the others. –Three-phase power supplies provide much more power than single phase. D. Power –Power is the rate of performing work. –Power is the energy used divided by time, measured in watts (W). –1 watt = 1 joule per second –Table 1.2 lists the power and energies of a range of sources. –1 horsepower (HP) corresponds to 750 W. –In electric circuits, the power (P) dissipated is the product of electric current (I) and voltage (V). –Power (watt) = current (ampere) × voltage (volt) –If the voltage is 100,000 V (100 kV) and the current is 1 A (1,000 mA), the power dissipated is 100,000 W (100 kW). –A typical household in North America uses a few kW of electrical power. –X-ray generators use up to 100 kW of electrical power, or the power required for ∼30 U.S. households. FIGURE 1.1 At the negatively charged plate, the electron has potential energy of 1 eV, which is converted into a kinetic energy of 1 eV as the electron is accelerated from the cathode to the anode. P1: OSO LWBK312-01 LWBK312-Huda March 10, 2009 10:17 Electromagnetic Radiation 3 TABLE 1.2 Common Power Sources Source Power (W) Energy Used per Second (J) Flashlight 2 2 Domestic light bulb 50 50 Microwave 500 500 Average U.S. home 3,000 3,000 X-ray generator 100,000 100,000 –The total energy generated is the product of power and time. –Energy (joule) = power (watt) × time (second) –X-ray generators are only switched on for short periods of time. –A typical exposure time for a chest x-ray examination is 10 ms. –Energy utilization in making x-rays is therefore low because of the very short exposure times that are used. II. ELECTROMAGNETIC RADIATION A. Waves –A wave is an entity that varies in space and time. –A common example of a wave is the variation of the water level in the ocean. –Waves are characterized by a wavelength, frequency, and velocity. –Wavelength (λ) is the distance between successive crests of waves. –Wavelengths are measured in meters (m). –Frequency (f) is the number of wave oscillations per unit of time. –Frequencies are measured in cycles per second, where one cycle per second is equal to one hertz (Hz). –The wave period is the time required for one wavelength to pass. –Wave period is 1/f. –The wave velocity (v) is the product of the wavelength and frequency, and measured in meters per second (m/s). –Velocity (m/s) = frequency (Hz) × wavelength (m) –Electromagnetic radiation is a wave that is associated with oscillating electric and mag- netic fields. –Visible light is a form of electromagnetic radiation. –The sun emits (loses) the energy that it generates in nuclear processes by radiating visible light. B. X-rays –X-rays are a form of electromagnetic radiation. –Electromagnetic radiation represents a transverse wave, in which the electric and mag- netic fields oscillate perpendicular to the direction of the wave motion. –Electromagnetic radiation travels in a straight line at the speed of light (c). –The value of c is 3 × 108 m/s in a vacuum. –The product of the wavelength (λ) and frequency (f) of electromagnetic radiation is equal to the speed of light (c = fλ). –Low-frequency electromagnetic radiation has a long wavelength. –High-frequency electromagnetic radiation has a short wavelength. –Figure 1.2 shows the electromagnetic spectrum that ranges from radio waves to gamma rays. C. Photons –Electromagnetic radiation is quantized, meaning that it exists in discrete quantities called photons. –Photons may behave as waves or particles but have no mass. –Photon energy (E) is directly proportional to frequency. –Photon energy is inversely proportional to wavelength. –Photon energy is E = h f = h (c/λ), where h is Plank’s constant. –A 10-keV photon has a wavelength of 0.1 nm, comparable to the size of a small atom. –A 100-keV photon has a wavelength of 0.01 nm. P1: OSO LWBK312-01 LWBK312-Huda March 10, 2009 10:17 4 X-ray Production FIGURE 1.2 Electromagnetic spectrum ranging from radio waves to gamma rays showing that the photon energy is directly proportional to frequency. –Radio waves have low frequencies (low photon energies) and gamma waves have high frequencies (high photon energies) as depicted in Figure 1.2. –High energy photons are called x-rays if produced by electron interactions but gamma rays if produced in a nuclear process. –There are no physical differences between x-rays and gamma rays of the same energy. D. Inverse square law –The intensity of an x-ray beam is proportional to the number of photons crossing a given area (e.g., square millimeter). –X-ray beam intensity decreases with distance from the x-ray tube because of the diver- gence of the x-ray beam. –The decrease in intensity is proportional to the square of the distance from the source. This nonlinear falloff in intensity with distance is called the inverse square law. –Doubling the distance from the x-ray source decreases the x-ray beam intensity by a factor of 4. –Halving the distance increases the x-ray beam intensity by a factor of 4. –Table 1.3 shows how increasing and decreasing the distance from a source of radiation changes the radiation intensity. –In general, if the distance from the x-ray source is changed from x1 to x2 , then the x-ray beam intensity changes by (x1 /x2 )2. III. X-RAY GENERATORS A. Generator role –X-ray generators provide electrical power to the x-ray tube. –A small fraction of this power (∼1%) is converted into x-rays. –Virtually all current x-ray generators in Radiology departments use three-phase power supplies. TABLE 1.3 Relative Radiation Intensity as a Function of Distance from the Radiation Source (Inverse Square Law) Distance from Radiation Source (m) Intensity (Relative to Intensity at 1 m) 0.1 100 0.25 16 0.5 4 1 1 2 1/4 4 1/16 10 1/100 The intensity at 1 m has been arbitrarily set to 1.0. P1: OSO LWBK312-01 LWBK312-Huda March 10, 2009 10:17 X-ray Generators 5 –A generator uses a transformer to increase the voltage that is applied across the x-ray tube. –The generator also rectifies the waveform from AC to direct current (DC). –Generators permit x-ray operators to control three key parameters of x-ray operation. –The tube voltage (kV) that is applied across the x-ray tube. –The current (mA) that flows through the x-ray tube. –The total exposure time (seconds) for which the tube current flows. –The power dissipated equals the product of tube voltage (V) in volt and current (I) in amps, or VI, and is measured in watts (kW). –Typical transformer ratings in x-ray departments are 100 kV and 1,000 mA, which cor- respond to a power of 100 kW. B. Generator types –Generators consist of an input power supply, transformer, and rectification circuit. –Single-phase generators use a single-phase power supply. –Single-phase generators use a bridge rectifier circuit that directs the alternating flow of high-voltage electrons so that flow is always from cathode to anode. –Single-phase generators have been replaced by three-phase generators for use in diag- nostic radiology. –Single-phase generators are common for dental radiography where teeth are rela- tively thin and longer exposure times are tolerable (no moving parts). –Three-phase generators use a three-phase power supply. –High-frequency inverter generators transform an AC input into low-voltage DC, then into high-frequency AC, and finally into high-frequency AC waveforms that are rec- tified to yield a nearly constant voltage waveform. –High-frequency generators are smaller and more efficient than three-phase genera- tors. –Constant potential generators provide a nearly constant voltage across the x-ray tube. –Constant potential generators are expensive, require more space, and are used in inter- ventional radiology. C. Transformers –A transformer changes the size of the input voltage and is capable of producing high and low voltages. –Step-up transformers increase the voltage. –A step-down transformer decreases the voltage. –If two wire coils are wrapped around a common iron core, current in the primary coil produces a current in the secondary coil by electromagnetic induction. –The voltages in the two circuits (Vp and Vs ) are proportional to the number of turns in the two coils (Np and Ns ) –Np /Ns = Vp /Vs , where p refers to the primary and s to the secondary coils –For an ideal transformer, the power in the primary and secondary circuits will be equal. –Vp Ip = Vs Is –The step-up transformers used in x-ray generators have a secondary coil with many more turns (500:1) to produce a high voltage, which is applied across the tube. –Generators also have a step-down transformer with fewer turns in the secondary coil. –The step-down transformer produces a low voltage (10 V), which is applied across the x-ray tube filament circuit. –An autotransformer permits adjustment of the output voltage using movable contacts to change the number of windings in the circuit. D. Rectification –The electric current from an AC power supply flows alternately in both directions, re- sulting in a voltage waveform shaped like a sine wave. –Rectification changes the AC voltage into a DC voltage across the x-ray tube. –Rectification is achieved using diodes, which permit current to flow in only one direction. –Rectification for single-phase power supply normally uses four diodes and is called full-wave rectification. –In full-wave rectification, there are two pulses per cycle of 1/60 second. –AC electricity oscillation is 60 cycles per second. –Each pulse ranges from zero volts to a peak (maximum) voltage. –The maximum voltage is known as the kVp (p stands for peak). –Rectification circuits in three-phase power supplies use a large number of diodes. P1: OSO LWBK312-01 LWBK312-Huda March 10, 2009 10:17 6 X-ray Production FIGURE 1.3 A three-phase generator (left) transforms and rectifie the input voltage to produce a high output voltage. –Three-phase rectification circuits are arranged in combinations of delta and wye cir- cuits. –Many three-phase power supplies generate waveforms that have either 6 or 12 pulses per cycle of 1/60 second. E. Voltage waveform –Voltage waveform is a plot of voltage over time. –A constant high voltage is desired across the x-ray tube for x-ray production. –In practice, there is some variation in the voltage called ripple. –The peak voltage or kilovolt peak (kVp ) is the maximum voltage that crosses the x-ray tube during a complete waveform cycle. –The voltage waveform ripple is the maximum voltage minus the minimum voltage per cycle expressed as a percentage of the maximum voltage. –Single-phase systems have 100% ripple. –Three-phase 6-pulse systems have ∼13% ripple. –Three-phase 12-pulse systems have ∼4% ripple. –High-frequency generators have ripple comparable to 12-pulse systems. –Figure 1.3 shows how the waveform is created for a three-phase generator, and the corresponding ripple. –The average (or effective) voltage will be slightly lower than the peak voltage. –Most ripples in diagnostic radiology are relatively small (70 keV, which is sufficient to eject K-shell electrons that have a binding energy of 70 keV. –Following the ejection of a K-shell electron, the excess energy may also be emitted as an Auger electron. –K-shell characteristic x-ray energies are always slightly lower than the K-shell binding energy. –L-shell characteristic x-rays always accompany K-shell x-rays, but these have very low energies and are absorbed by the x-ray tube glass envelope. D. X-ray spectra –X-ray beams in diagnostic radiology generally have a wide range of photon energies. –A graph of x-ray tube output showing the number of photons at each x-ray energy is called an x-ray spectrum. –For most radiologic imaging (e.g., radiography, fluoroscopy, and CT), the effective pho- ton energy is between one third and one half of the maximum photon energy. –Effective energy is also called the average energy. –Each target material emits characteristic x-rays of specific discrete energies as shown in Figure 1.4B. –Tungsten (Z = 74; K-shell binding energy 70 keV) has characteristic x-ray energies of 58 to 67 keV. –Molybdenum (Z = 42; K-shell binding energy 20 keV) has characteristic x-ray energies of 17 to 19 keV (Fig. 1.4B). –In Figure 1.4B, if the K-shell vacancy is filled by an electron in the L shell, the characteristic x-ray has an energy of 17.4 keV. –When the K-shell vacancy is filled by an electron in the M shell, the characteristic x-ray energy is 19.5 keV. –K-shell characteristic x-rays contribute less than 10% of the whole spectrum at 100 kV. –For voltages 2,200◦ C), resulting in the thermionic emission of electrons. P1: OSO LWBK312-01 LWBK312-Huda March 10, 2009 10:17 10 X-ray Production FIGURE 1.5 Major components of an x-ray tube. The inset shows a magnifie view of the target and illustrates the line focus principle, whereby the focal spot size (F ) is smaller than the electron beam (L) because of the anode angle (θ ◦ ). C. Tube current –Electrons emitted from a heated filament form a negative cloud around the filament called a space charge, which prevents further emission of electrons. –The tube current is the flow of electrons from the filament to the target embedded in the anode. –Electrons flow from the negative filament toward the positive anode. –At low voltages, the potential is insufficient to cause all the electrons to be pulled away from the filament, and a residual space charge remains (space charge limited). –At the saturation voltage, all electrons are immediately pulled away from the filament, and the x-ray tube current is maximized. –Above 40 kV, the filament current is proportional to and determines the tube current (emission limited). P1: OSO LWBK312-01 LWBK312-Huda March 10, 2009 10:17 X-ray Tube Performance 11 TABLE 1.5 Nominal Focal Spot Sizes in Mammography and Radiography Focal Spot Size (mm) Clinical Application 0.1 Magnific tion mammography 0.3 Mammography; magnific tion radiography 0.6 Fluoroscopy; extremity radiography (95% of all radiographs) –Tube currents are normally increased by increasing the filament heating (i.e., increasing the filament current). –Tube currents range between 1 mA and 1,200 mA. –Tube currents of a few mA are used in fluoroscopy and a few hundred mA in radiogra- phy and CT. D. Focal spots –The area of the target struck by the electrons is determined by the filament size and focusing cup. –The focal spot is the size of the source of x-rays as viewed by the patient (Fig. 1.5). –The line focus principle is used to permit larger heat loading while minimizing the size of the focal spot (see inset in Fig. 1.5). –Note that in Figure 1.5 inset, the length of the target that is irradiated (L) is much larger than the focal spot size (F). –The anode angle (θ ◦ in Fig. 1.5 inset) is an important factor in determining the focal spot size. –The anode angle is the angle between the target surface and the central beam. –Typical anode angles range from 7 degrees to 20 degrees. –Radiation field coverage increases with increasing target angle. –Focal spots need to be small to produce sharp images. –Focal spots need to be large to tolerate a high heat loading. –The choice of the focal spot size is achieved by balancing the conflicting need for sharp images, and being able to tolerate high heat loadings. –Large focal spots are favored when a short exposure time is important, and small focal spots are needed to obtain the best spatial resolution. –Focal spot sizes, as quoted by manufacturers of x-ray tubes, range from about 0.1 mm to ∼1.2 mm. –Focal spot sizes can be measured using pinhole cameras, star or bar test patterns, or slit cameras. –Measured focal spot sizes may be up to 50% larger than the nominal values listed in Table 1.5. E. Anodes –Electrons striking the target produce heat and x-rays. –The target is embedded in an anode material, which temporarily stores the heat energy deposited into the target. –A stationary anode usually consists of a tungsten target embedded in a copper block. –Although copper is a good heat conductor, heat dissipation is limited. –Stationary anodes are used in portable x-ray units. –A rotating anode greatly increases the effective target area used during an exposure and therefore raises the heat capacity. –To maintain the vacuum required inside the x-ray tube, rotating anodes employ an electric induction motor. –The rotor (inside the envelope) turns in response to the changing electric current in the stator electric windings (outside the envelope). VI. X-RAY TUBE PERFORMANCE A. X-ray techniques –In manual mode, the operator selects the kV, x-ray tube current, and exposure time on the control panel. –In automatic exposure mode, the operator chooses a kV while the generator circuit controls the tube current and exposure time. P1: OSO LWBK312-01 LWBK312-Huda March 10, 2009 10:17 12 X-ray Production TABLE 1.6 Representative Techniques (kV/mA), Power Loadings, and Energy Deposition in X-ray Imaging Type of Exposure Power Energy Examination Techniques Time (kW) (kJ) Chest x-ray 140 kV/500 mA 5 ms 70 0.35 Abdominal x-ray 80 kV/1,000 mA 50 ms 80 4 Fluoroscopy 80 kV/3 mA Continuous 0.24 0.24 per second CT 120 kV/750 mA 0.5 s rotation 90 45 per x-ray tube rotation –Tube current in radiography ranges between 100 and 1,000 mA. –Radiographic exposure times range between tens and hundreds of milliseconds. –Fluoroscopy tube currents range between 1 and 5 mA. –For small body parts, such as the extremities, x-ray tube voltages are 55 to 65 kV. –Most radiographic and fluoroscopy imaging is performed at x-ray tube voltages between 70 and 90 kV. –Higher voltages may be used to penetrate excessively larger patients. –Chest radiography is often performed at higher x-ray tube voltages of about 120 kV. –High voltages (>100 kV) are also used in some fluoroscopy performed with barium contrast agents to provide sufficient penetration. –Table 1.6 shows typical radiographic techniques used for a range of x-ray imaging modal- ities. B. Energy deposition –Only about 1% of the electric energy supplied to the x-ray tube is converted to x-rays. –Approximately 99% of the electrical energy supplied to an x-ray tube is converted to heat. –Heat energy deposited during an x-ray exposure is known as tube loading. –X-ray tube loading depends on the peak kV, voltage waveform, tube current, and ex- posure time. –The total energy deposited in the anode also depends on the number of exposures. –For a constant x-ray tube voltage (V) and current (I), the energy deposited during an x-ray exposure is V × I × t joules, where t is the exposure time measured in seconds. –Table 1.6 shows typical energy deposition rates for common radiologic examinations. –This energy is temporarily stored in the anode, which has a heat capacity of several hundred thousand joules. –Anodes in CT x-ray tubes have a capacity of several million joules. –When the tube voltage is not constant, calculation of energy deposition is complicated. –For systems with single-phase power supplies and full-wave rectification, the quantity (kVp ) × (mA) × (time) is given in terms of heat units. –One heat unit is ∼0.7 J. –Single-phase generators are no longer used in Radiology departments and heat units are an anachronism. –Energy deposition in the focal spot, anode, and x-ray tube housing must be considered to ensure none of these components overheat. C. Tube rating –The rating of an x-ray tube is based on maximum allowable kilowatts (kW) at an expo- sure time of 0.1 second. –For example, a tube with a rating of 100 kW (100,000 W) tolerates a maximum exposure of 100 kV and 1,000 mA for an exposure lasting 0.1 second. –Typical x-ray tube ratings are between 5 and 100 kW and depend on focal spot size. –In radiography, power loading is ∼100 kW for a large focal spot size. –Power loading is ∼25 kW for the small focal spot. –Increasing the exposure time or using a larger focal spot size may be required to achieve the required x-ray tube output without overheating. –In fluoroscopy, power loadings are very low, and typically between 100 and 500 W. –Table 1.6 shows power loadings for x-ray imaging modalities encountered in diagnostic radiology. D. X-ray tube heat dissipation –X-ray tubes are designed to efficiently dissipate heat. –Modern anodes are circular and rotate at high speeds (3,000 to 10,000 rpm) to spread heat loading over a large area. P1: OSO LWBK312-01 LWBK312-Huda March 10, 2009 10:17 X-ray Tube Performance 13 FIGURE 1.6 The solid curve shows how heat energy stored in the anode is reduced when starting at the maximum anode capacity (i.e., 200 kJ); the dashed line shows the increase of heat energy in the anode when 300 W (300 J/s) is continuously being added during fluo oscopy. –Heat is transferred from the focal spot by radiation to the tube housing and by conduc- tion into the anode. –Radiation is the primary way that anodes transfer heat to the housing. –Anodes get white hot during the x-ray exposure and lose their acquired energy by emitting light photons. –X-ray tubes are usually immersed in oil, which aids heat dissipation by convection. –Air fans are sometimes used to increase the rate of heat loss. –Taking a large number of radiographs, or performing long CT scans can saturate the anode heat capacity. –When the anode heat capacity is reached, anodes must cool down before additional exposures are allowed. –Figure 1.6 shows the cooling/heating curve. –It takes several minutes for a hot x-ray tube anode to cool. –In fluoroscopy, power deposition in the anode is only a few hundred watts, which is dissipated without reaching the maximum anode capacity (Fig. 1.6). –The low power loading in fluoroscopy permits the use of the small (0.6 mm) focal spot size. E. Radiation from x-ray tubes –X-rays produced in an x-ray tube are emitted isotropically. –Isotropic means that the intensity is equal in all directions. –The primary x-ray beam goes through the x-ray tube window, which is directed toward the patient. –The radiation that is incident on the patient is also known as the useful x-ray beam. –Primary beams produce radiographic and fluoroscopic images. –X-ray tubes are surrounded by lead to absorb unwanted radiation. –Leakage radiation is radiation that is transmitted through the x-ray tube housing. –Leakage radiation should not exceed 1 mGy per hour at a distance of 1 m from the x-ray tube. –Leakage radiation is measured with the x-ray tube operated at the maximum techniques (kV and mA) and the collimators fully closed. –Scattered radiation has been deviated in direction after leaving the tube. –Secondary radiation is the sum of the leakage and scattered radiation. –Secondary radiation contributes no useful information, but will result in unnecessary exposure to any personnel in the x-ray room (radiologists, technologists, etc.). –Operators working within an x-ray room need to wear protective apparel to minimize their exposure to secondary radiation (see Chapter 8, Section III). P1: OSO LWBK312-01 LWBK312-Huda March 10, 2009 10:17 14 X-ray Production REVIEW TEST 1.1 Which of the following is not consid- d. 100 ered a force? e. 1,000 a. Electrostatic 1.9 Which of the following is not a type of b. Weak x-ray generator? c. Strong a. Single phase d. Gravity b. Double phase e. Electricity c. Six pulse 1.2 Which of the following is not a unit of d. Twelve pulse energy? e. High frequency a. Erg b. Joule 1.10 The purpose of x-ray transformers is c. Watt most likely to change the: d. Calorie a. magnetic field e. eV b. electrical voltage 1.3 Which of the following would most c. power level likely be attracted to an anode? d. waveform frequency a. Proton e. current intensity b. Neutron 1.11 When a secondary coil has 500 more c. Electron turns than a primary coil, the ratio of d. Positron the secondary voltage to the primary e. Alpha particle voltage is most likely: 1.4 Which quantity is the best measure of a. 500 power? b. 5000.5 a. Joule c. 1/500 b. Tesla d. 1/5000.5 c. Watt e. Depends on AC frequency d. Coulomb 1.12 Which of the following generators is e. Newton likely to have the largest waveform 1.5 Which of the following is/are likely to ripple? have the longest wavelength? a. Constant potential a. Gamma rays b. High frequency b. Microwaves c. Single phase c. Radio waves d. Six pulse d. Ultraviolet e. Twelve pulse e. Visible light 1.13 Electrons passing through matter lose 1.6 For electromagnetic radiation, which energy primarily by producing: increases with increasing photon en- a. bremsstrahlung ergy? b. characteristic x-rays a. Wavelength c. atomic ionizations b. Frequency d. Compton electrons c. Velocity e. photoelectrons d. Charge e. Mass 1.14 Tungsten is most likely used as an 1.7 If the distance from a radiation source x-ray target because it has a high: is halved, the radiation intensity in- a. physical density creases by a factor of: b. electron density a. 2−2 c. electrical resistance b. 2−1 d. melting point c. 20 e. ionization potential d. 2+1 1.15 The maximum photon energy in x-ray e. 2+2 beams is determined by the x-ray tube: 1.8 X-ray generators have a power level a. current (kW) of approximately: b. exposure time a. 0.1 c. target material b. 1 d. anode–cathode voltage c. 10 e. total filtration P1: OSO LWBK312-01 LWBK312-Huda March 10, 2009 10:17 Review Test 15 1.16 The most likely characteristic x-ray en- c. 45 ergy (keV) from x-ray tubes used in d. 60 chest radiography is: e. 75 a. 19 1.24 X-ray tube output would likely in- b. 33 crease the most when increasing the c. 65 x-ray tube: d. 75 a. voltage e. 140 b. anode angle 1.17 At 65 kV and with a tungsten target, c. target Z the percentage (%) of K-shell x-rays in d. current the x-ray beam is most likely: e. exposure time a. 0 b. 1 1.25 A chest x-ray examination on a dedi- c. 10 cated chest unit would be least likely d. 50 to use: e. 99 a. 60-kV voltage b. 800-mA tube current 1.18 The average photon energy of an x-ray c. 10-ms exposure time beam is least likely to be affected by d. 1-mm focus changes in the: e. 5-mm Al filtration a. tube current b. tube voltage 1.26 For specification of anode heat capac- c. voltage waveform ities, one heat unit corresponds to en- d. target composition ergy (J) of: e. beam filtration a. 0.9 b. 0.8 1.19 The number of electrons accelerated c. 0.7 across an x-ray tube is most strongly d. 0.5 influenced by: e. 0.3 a. anode speed b. focus size 1.27 At the same peak voltage, which genera- c. filament current tor likely deposits most energy into an d. tube filtration anode? e. tube voltage a. Constant potential 1.20 The most likely x-ray tube filament b. High frequency current (mA) is: c. Three phase (12 pulse) a. 0.4 d. Three phase (6 pulse) b. 4 e. Single phase c. 40 1.28 Heat stored in x-ray tube anodes is d. 400 most likely dissipated by: e. 4,000 a. convection 1.21 Changing x-ray tube current (mA) b. conduction most likely changes the x-ray: c. radiation a. field of view d. air cooling b. maximum energy e. oil cooling c. average energy 1.29 In a standard x-ray tube, the maximum d. anode angle power loading (kW) on the 0.6 mm fo- e. beam intensity cal spot is most likely: 1.22 The large focus dimension is most a. 1 likely larger (%) than that of the small b. 2 focus by: c. 5 a. 10 d. 10 b. 25 e. 25 c. 50 1.30 Radiation transmitted through the d. 75 x-ray tube housing is referred to e. 100 as: 1.23 The anode angle (degrees) in an x-ray a. useful tube used for chest radiography is b. secondary most likely: c. stray a. 15 d. leakage b. 30 e. scattered P1: OSO LWBK312-01 LWBK312-Huda March 10, 2009 10:17 16 X-ray Production ANSWERS AND EXPLANATIONS 1.1e. Electricity is the flow of charge, and 1.17a. There will be no characteristic is measured in amps (C/s). x-rays, as the electron kinetic energy 1.2c. The watt is a unit of power, (65 keV) is insufficient to eject W measured in J/s. K-shell electrons that have a binding energy of 70 keV. 1.3c. Electron, since it has a negative charge that is attracted to the 1.18a. The tube current does not affect the positive anode. average (or maximum) photon energy in x-ray beams. 1.4c. Watt is a unit of power, where 1 W = 1 J/s. 1.19c. The filament current affects the temperature of the filament and 1.5c. Radio waves have the lowest thereby how many electrons the frequencies and longest filament “bubbles off’’. wavelengths. 1.20e. X-ray tube filaments are about 4 A, 1.6b. Frequency, which is directly or 4,000 mA. proportional to the photon energy 1.21e. Tube current controls the x-ray 1.7e. 22 (i.e., 4). Halving the distance beam intensity, or the total number quadruples the radiation intensity of x-ray photons produced. (inverse square law). 1.22e. The large focal spot is typically 1.2 1.8d. 100 kW is typical of the power of mm, and the small focal spot is 0.6 x-ray generators in radiography and mm (i.e., 100% larger). CT. 1.23a. 15 degrees is a typical anode angle. 1.9b. There are no double-phase generators. 1.24a. The x-ray tube output is (approximately) proportional to the 1.10b. Transformers change (increase or square of the x-ray tube voltage. decrease) voltages. 1.25a. Chest x-rays are performed at high 1.11a. 500. The increase in voltage is voltage (120 kV). directly proportional to the increase in the number of turns. 1.26c. The heat unit is 0.7 joule, and is an anachronism in modern 1.12c. The ripple on a single-phase radiology. generator is 100%. 1.27a. Constant potential, since it has 1.13c. Electrons lose most of their kinetic negligible ripple and the voltage energy by knocking out (or exciting) across the x-ray tube is always the outer shell electrons. maximum possible value. 1.14d. Tungsten can tolerate very high 1.28c. Anodes get to be white hot and lose temperatures, which makes it an energy by radiation (light) to the attractive target material in x-ray tube housing. tubes. 1.29e. The small focal spot can tolerate 1.15d. The voltage across the x-ray tube power levels of 25 kW (higher determines the kinetic energy power would require the large focal imparted to the electrons that are spot). accelerated from the cathode (filament) to the anode (target), and 1.30d. Leakage radiation escapes through thereby the maximum x-ray photon a fully closed collimator (the energy. regulatory limit in the United States is 1.0 result in subject contrast being amplified. –Radiographic films have gradients of ∼2, but in mammography, characteristic curves have gradients >3. C. Contrast and latitude –Film latitude is the range of air kerma values that results in a satisfactory image con- trast. –Latitude is known as dynamic range in engineering. –The latitude (dynamic range) of film is ∼40:1. –Film latitude and film gradient (contrast) are inversely related. –The higher the film gradient, the narrower the range of air kerma values that result in a good image contrast. 87 P1: OSO LWBK312-06 LWBK312-Huda April 7, 2009 12:30 88 Image Quality –A wide-latitude film has a low film gradient, which results in a low contrast. –Wide-latitude films are used for chest radiographs because of large differences in air kerma between lungs and mediastinum. –High-contrast films are used in mammography. –Breast compression reduces variation of air kerma in mammography, and permits the use of high-contrast film. D. Contrast and digital imaging –Image contrast in digital imaging is the difference in the monitor brightness of a lesion in comparison to the monitor brightness of the adjacent tissues. –Displayed image contrast is the result of subject contrast, together with the effect of the recording device and digital image processing. –Monitor display characteristics and window control settings also affect display con- trast. –Displayed contrast in digital imaging can be controlled by the operator by adjusting the display level and display window width. –Increasing the window width will generally reduce display contrast and vice versa. –It is possible to record images with a wide dynamic range receptor and then display them with a narrow window to enhance contrast in the displayed image. –In chest CT, a narrow window offers excellent soft tissue contrast, but the lungs become invisible (all black). –Use of a wider window in chest CT permits visualization of most tissues, but the contrast between soft tissues is markedly reduced. –Digital imaging modalities permit good contrast over all the image data by viewing multiple images. E. Contrast and photon energy –For a given lesion, subject contrast is primarily affected by the photon energy. –The average photon x-ray energy is increased by increasing the x-ray tube voltage (kV) or by adding filters. –Reducing the ripple on the x-ray tube voltage (e.g., using a constant potential) also increases the average photon energy. –Low photon energies result in high subject contrast and vice versa. –Figure 6.1 shows how subject contrast depends on the kV. –At low kV, intensity differences between adjacent tissues are relatively high, but these differences are markedly reduced at higher photon energies. –As photon energy increases, contrast decreases because of increased x-ray photon penetration. –For correctly exposed (and displayed) imaging systems, changes in subject contrast will generally result in corresponding changes in image contrast. FIGURE 6.1 X-ray penetration increases with increasing photon energy, which reduces x-ray intensity differences (i.e., contrast) between bone, muscles, and lung. P1: OSO LWBK312-06 LWBK312-Huda April 7, 2009 12:30 Resolution 89 TABLE 6.1 CT Image Contrast of a Lesion in a Water Background as a Function of X-ray Photon Energy (Normalized to 100% at 50 keV) X-ray Energy Soft Tissue Lesion Iodinated Vessel (keV) (Z = 7.6) (Z = 53) 50 100 100 60 93 68 70 88 48 80 84 37 –Reduced subject contrast generally results in a reduction of image contrast, and vice versa. –Photon energy also affects contrast in CT imaging (Table 6.1). –Increasing the photon energy from 50 to 80 keV reduces soft tissue contrast (i.e., soft-tissue HU) by 16% and reduces iodine contrast by 63%. –Reducing CT tube voltage improves visibility of iodinated contrast agents. –For large patients, reducing kV may not be practical because of insufficient patient penetration. F. Contrast agents –Contrast agents including air, barium, and iodine are used to improve subject contrast. –Barium is administered as a contrast agent for visualization of the GI tract on radio- graphic examinations. –Barium attenuation is high because of its high density and high atomic number (Z = 56) that places the K-edge at 37 keV. –The barium K-edge energy matches the mean photon energies used in fluoroscopy. –Iodine (Z = 53) is also an excellent contrast agent for similar reasons to those for barium (i.e., K-edge = 33 keV). –Iodinated contrast agents can be injected intravenously or arterially. –Dilution and the osmolar limitations of intravascular fluids limit the achievable iodine concentration. –Air is a negative contrast agent and increases subject contrast because it is less attenuating than tissue. –Carbon dioxide is also sometimes used as a contrast agent in angiography. II. RESOLUTION A. What is resolution? –Resolution is the ability of an imaging system to display two adjacent objects as discrete entities. –Resolution is also known as spatial resolution, high-contrast resolution, sharpness, or blur. –Two small adjacent objects such as microcalcifications will appear sharp and distinct in an image obtained with a system that has good resolution. –Adjacent microcalcifications might appear as one blurred entity in images obtained with a system that has poor resolution. –Resolution may be quantified using a parallel line bar phantom. –Bar phantoms possess very high intrinsic contrast. –One line pair per millimeter (1 lp/mm) is a bar phantom that has 0.5 mm lead (Pb) bars separated by 0.5 mm of radiolucent material. –A 2 lp/mm bar phantom has 0.25 mm Pb bars separated by 0.25 mm of radiolucent material, and so on. –Large objects correspond to low values of line pairs/mm –Smaller structures correspond to higher values of line pairs/mm. –The limiting spatial resolution is the maximum number of line pairs per millimeter that can be recorded by the imaging system. –Table 6.2 shows the limiting resolution of x-ray based imaging modalities. –The human eye resolves ∼5 lp/mm at a viewing distance of ∼25 cm. –Humans can resolve up to ∼30 lp/mm on close inspection. –Focal spot size, detector blur, and patient motion affect resolution in radiography. P1: OSO LWBK312-06 LWBK312-Huda April 7, 2009 12:30 90 Image Quality TABLE 6.2 Approximate Values of Limiting Resolution in Radiologic Imaging Limiting Spatial Resolution Imaging Modality (lp/mm) Screen–fil mammography 15 Screen–fil (200 speed) 5 Digital chest imaging 3 Digital photospot/DSA 2 Fluoroscopy (525-line TV) 1 CT 0.7 B. Focal spot blur –The finite size of a focal spot results in blurred images. –The blurred margin at the edge of objects produced by a finite focal spot is called a penumbra. –The penumbra is the result of x-rays arriving from slightly different locations in the focal spot. –The resultant loss of sharpness is called focal spot blur or geometric unsharpness. –Focal spot blur increases with increasing focal spot size as shown in Figure 6.2. –A point focal spot, or one that is negligibly small, produces no focal spot blur. –There is no focal spot blur in contact radiography (i.e., no magnification) as shown in Figure 6.2. –Focal spot blur is minimal in extremity radiography (i.e., negligible magnification). –In magnification radiography, it is always very important to use small focal spot sizes. –Reducing the focal spot size in magnification imaging increases the sharpness of edges by minimizing the penumbra. –Magnification in mammography improves visibility of microcalcifications but needs a 0.1-mm focal spot to minimize geometric unsharpness. –Magnification is sometimes used in angiograms to improve the visibility of very small blood vessels and makes use of a 0.3-mm focal spot. C. Detector blur –The physical size of any radiation detector will limit the ability to resolve small objects. –Screen thickness introduces a limit on the achievable spatial resolution performance in radiography. –Light produced by absorbed x-rays in a screen produces a blurred image because the light diffuses before being absorbed by a film. FIGURE 6.2 Focal spot blur in radiography showing that in contact radiography (left), the edge is very sharp with negligible blur but becomes less sharp (blurrier) with magnific tion (middle), and the blur further increases with a larger focal spot size (right). P1: OSO LWBK312-06 LWBK312-Huda April 7, 2009 12:30 Resolution 91 –A screen that is 0.4-mm thick will introduce a blur in the resultant image that is compa- rable to the screen thickness. –Capturing an image with x-ray film alone without any sc