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P1: PBULWBK312-FMLWBK312-HudaJune 19, 2009IntroductionI. WHAT IS RADIOLOGIC PHYSICS?Radiology is arguably the most technology-dependent specialty in medicine, andwhich has seen significant changes over the past decade. Computer integration withconstant technical innovations have changed the workplace and influenced the roleradiology plays in the diagnosis and treatment of disease. Radiologic physics is not anesoteric subject of abstract equations and memorized definitions, but rather the totalprocess of creating and viewing a diagnostic image. A range of physical principlesinfluence the process of image formation. Radiologists and technologists need tounderstand 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 radiographic and fluoroscopic x-ray imaging, computed tomography, magnetic resonance,nuclear medicine, and ultrasound. Radiologic physics provides an understandingof the factors that improve or degrade image quality. Selection of the most appropriate way of generating a medical image is the responsibility of the radiologicimaging team, consisting of the radiologist, technologist, medical physicist, andequipment manufacturer. Optimizing medical imaging performance requires a solidunderstanding of how these images are generated, as well as the most importantdeterminants of image quality.All imaging modalities have a cost associated with their use. For modalities thatuse ionizing radiations, one of the costs is the radiation dose to the patient andstaff working with these systems. Accordingly, radiation protection principles areimportant. Radiologists and technologists should understand the magnitude of theradiation dose to the patient and personnel exposed, and ensure that radiation levelsare kept as low as reasonably achievable (ALARA principle) as well as within anyrelevant regulatory limits. MR and ultrasound do not have any specific risks, and thecost is generally the time required to perform the study.II. WHY STUDY RADIOLOGIC PHYSICS?Radiologists and technologists need to acquire an understanding of the underlyingimaging science for each diagnostic modality and be able to pass their respectiveradiologic physics exams. However, neither will actually practice physics and thereis no need to learn how to generate modulation transfer functions in radiographicimaging, write programs to perform filtered back projection algorithms in CT, ordesign RF pulses in MRI.It is important for well-rounded radiologists and technologists to have a basicunderstanding of the following: (i) image quality parameters, such as mottle, spatialxiii10:22

P1: PBULWBK312-FMLWBK312-HudaxivJune 19, 2009Introductionresolution, and contrast; (ii) how image quality is affected by radiographic techniques;(iii) how to evaluate commercial imaging equipment in terms of its ability to performthe required patient examinations; (iv) the radiation dose and risks associated withradiographic exposure; and (v) how to communicate with medical physicists andservice personnel regarding imaging problems.The focus of the text and allied questions is the essential physics underlying thecreation of clinical images. Special emphasis has been given to the factors impacting on image quality, notably image contrast, spatial resolution, and mottle. Radiologists and technologists understand the achievable performance of any imagingequipment and how this equipment should best be used to solve patient imagingproblems.III. REVIEW BOOK STRUCTUREThis review book is designed to help prepare residents and technologists for theradiologic physics portion of their board and registry exams. It provides a sourcefor comprehensive self-study in the area of diagnostic radiologic physics. The textassumes a background of instruction in radiologic physics and is not intended toreplace the standard radiologic physics texts. This book is designed, rather, to providea concise yet complete source of review to refresh and reinforce the concepts ofradiologic 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 asummary 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 knowledge and comprehension in each area. The philosophy adopted by the author is thatmaterial comprehension, rather than rote memorization, will guarantee success inthe exam. The review book also contains two practice examinations with questionsthat range over the topics covered in this book. At the end of the book is a glossaryof key terms commonly used in radiologic physics.Radiation quantities are generally provided using SI units. Use of roentgen tospecify 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 medicalimaging literature. In diagnostic radiology, an exposure of 1 R can be taken to be equalto an air kerma of 8.76 mGy. In the text that follows, exposures have been replacedby air kerma, with an air kerma of 10 mGy taken to be approximately equivalent to anexposure of 1 R. In nuclear medicine, non-SI units predominate in clinical practicein the United States, whereas SI units are prevalent outside of the United States andin the scientific literature. In this book use is made of SI units (i.e., MBq), with thenon-SI equivalent (i.e., mCi) provided in parenthesis. Magnetic fields are generallyexpressed in teslas, with recognition given to the fact that MR personnel are muchmore likely to refer to the 5-gauss line than a 0.5-mT line.IV. RADIOLOGY RESIDENTS AND THE ABR EXAMThe physics portion of the American Board of Radiology (ABR) examination is administered in the fall of each year and taken on a computer. Board-eligible residentsmay 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 multiplechoice 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 theallowed time, with no need to perform any calculations beyond trivial additions,multiplications, or divisions.10:22

P1: PBULWBK312-FMLWBK312-HudaJune 19, 2009IntroductionxvAbout 60% of the questions cover x-ray-based imaging, image quality, ultrasound, and MR. Approximately 20% of the questions relate to nuclear medicine, and theremaining 20% to issues relating to radiation biology and protection. Results in the formof quartiles are provided to candidates who have taken the physics examination andpertain to each of these three syllabus categories. Further information regarding theAmerican Board of Radiology and the written physics examination can be obtainedat the ABR web site (theabr.org).V. RADIOLOGY TECHNOLOGISTS AND THE ARRT EXAMThe American Registry of Radiologic Technologists (ARRT) administers credentialing examinations and provides continuing registration for radiologic technologistswithin the United States. A multiple-choice test format, consisting of approximately200 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 hoursto complete the exam, and testing centers provide an erasable board and pen (noscratch paper is allowed). A scientific calculator is available on the computer, orone 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. Ascaled test score of 75 is required to pass the ARRT exam. Further information on theAmerican Registry of Radiologic Technologists can be obtained at the ARRT web site(www.arrt.org).10:22

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P1: OSOLWBK312-01LWBK312-HudaMarch 10, 20091ChapterX-RAY PRODUCTIONI. BASIC PHYSICSA. 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 orpull).–Force mass acceleration, measured in newtons (N).–The four physical forces in the universe are gravitational, electrostatic, strong, andweak.–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 energyas 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 acceleratedacross 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 1,000V.–An electron gains 1 MeV (1,000 keV) when accelerated across an electric potential of1,000,000 V.–1 MeV 103 keV 106 eVC. Electricity–Electrons are negatively charged and protons are positively charged.–Electric charge of an electron (or proton) is 1.6 10–19 coulomb (C).110:17

P1: OSOLWBK312-01LWBK312-Huda2March 10, 2009X-ray ProductionTABLE 1.1 Relative Strength of Physical ForcesType of ForceGravitationalWeakElectrostaticStrongRelative Strength1 1024 1035 1038DescriptionBinds earth to the sunInvolved in beta decayBinds electrons and protons in atomsBinds 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 singlephase.–Single-phase power supplies have one wire that has an oscillating voltage, with theother 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 110volts (V).–U.S. electricity is an alternating current (AC) that oscillates at a frequency of 60 cyclesper 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 phasewith 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) andvoltage (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 dissipatedis 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 30U.S. households.FIGURE 1.1 At the negatively charged plate, the electron has potential energy of 1 eV, which isconverted into a kinetic energy of 1 eV as the electron is accelerated from the cathode to the anode.10:17

P1: OSOLWBK312-01LWBK312-HudaMarch 10, 2009Electromagnetic Radiation3TABLE 1.2 Common Power SourcesSourceFlashlightDomestic light bulbMicrowaveAverage U.S. homeX-ray generatorPower (W)Energy Used per Second (J)2505003,000100,0002505003,000100,000–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 exposuretimes that are used.II. ELECTROMAGNETIC RADIATIONA. 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 equalto 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 inmeters per second (m/s).–Velocity (m/s) frequency (Hz) wavelength (m)–Electromagnetic radiation is a wave that is associated with oscillating electric and magnetic fields.–Visible light is a form of electromagnetic radiation.–The sun emits (loses) the energy that it generates in nuclear processes by radiating visiblelight.B. X-rays–X-rays are a form of electromagnetic radiation.–Electromagnetic radiation represents a transverse wave, in which the electric and magnetic 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 equalto 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 gammarays.C. Photons–Electromagnetic radiation is quantized, meaning that it exists in discrete quantitiescalled 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.10:17

P1: OSOLWBK312-01LWBK312-Huda4March 10, 2009X-ray ProductionFIGURE 1.2 Electromagnetic spectrum ranging from radio waves to gamma rays showing that thephoton energy is directly proportional to frequency.–Radio waves have low frequencies (low photon energies) and gamma waves have highfrequencies (high photon energies) as depicted in Figure 1.2.–High energy photons are called x-rays if produced by electron interactions but gammarays 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 givenarea (e.g., square millimeter).–X-ray beam intensity decreases with distance from the x-ray tube because of the divergence 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 afactor 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 radiationchanges the radiation intensity.–In general, if the distance from the x-ray source is changed from x1 to x2 , then the x-raybeam intensity changes by (x1 /x2 )2 .III. X-RAY GENERATORSA. 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 powersupplies.TABLE 1.3 Relative Radiation Intensity as a Function of Distance from the RadiationSource (Inverse Square Law)Distance from Radiation Source (m)0.10.250.512410The intensity at 1 m has been arbitrarily set to 1.0.Intensity (Relative to Intensity at 1 m)10016411/41/161/10010:17

P1: OSOLWBK312-01LWBK312-HudaMarch 10, 2009X-ray Generators5–A generator uses a transformer to increase the voltage that is applied across the x-raytube.–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) inamps, or VI, and is measured in watts (kW).–Typical transformer ratings in x-ray departments are 100 kV and 1,000 mA, which correspond 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

IV. Radiology Residents and the ABR Exam xiv V. Radiology Technologists and the ARRT Exam xv 1 X-RayProduction 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 Explanati