4 clinical applications of basic x ray physics principes

II ‘::: The AAPM/RSNA Physics for Residents : : Tutorial This artide meets for criteria hour Clinical Principles1 the Applications of Basic X-ray Physics 1.0 credIt in category Beth A Scbueler, PhD The of application of basic the AMA Physician’s Recognition Award To obtain credit, see the questionnaire pp quires on consideration any 725-730 given each radiographic of these current, LEARNING After reading (focal and taking reader selection how of x-ray factors density and Be able the I to identify by technique equipment An is principles and factors Be able basic application Be able to compare scatter reduction mechanisms with respect various to image patient technique measures (source-object patient dose contrast, density, mo- patient expo- involves quality and parameter minimizing while of image factors of exposure including often radiation consideration of and exposure quality dose and #{149} INTRODUCTION in a how to identify the types of x-ray generators and to select the appropriate type for a particular radiographic quality, of geometry of tube equipment influence for evaluation unsharpness, of radiographic and For application characteristics also re- voltage, In addition, design) of image and geometric various basic distance) The basis and factors-tube receptor generator optimization between exposure receptor of the radiograph Selection trade-offs x-ray radiography interrelationships understanding the and image use, complex contrast of unsharpness radiograph and learn the visibility of detail four grid is the The to clinical have time-determine source-image unsharpness, sure image causes affected tion exposure influences size, and the quality will: Understand exposure spot principles that proper is essential to the patient distance article the test, the selection tbis physics factors examination, factors and exposure OBJECTIVES x-ray of many understanding production of basic and x-ray interaction to produce physics includes Radiographic an image that knowledge imaging allows the of the involves principles application radiologist of x-ray of these to visualize the basic internal anatomy of the patient so that a diagnosis can be made For each radiographic examination, the operator has control over several parameters that affect the appearance of the image, including the x-ray tube voltage, tube current, exposure time, and distance In addition, the design of the radiographic equipment (x-ray tube and generator) and properties of the particular patient and examination (tissue, contrast media used, and motion) affect radiographic image quality An understanding of how x rays are produced and interact in tissue is essential to determine the appropriate selection of technique factors and equipment design for a particular clinical examination The appearance of a radiograph is described by various image quality elements, which include image density, contrast, blur, and noise These factors describe various characteristics themselves ally Index has RadloGraphics the RSNA ©RSNA, Physics 1998; Physics ceived that provide interrelated a detrimental terms: ‘From are Department November effect on the other and comparing in one image factors Image images The quality factor evaluation must also 55905 From factors gener- include ‘ Radiography 18:731-744 of Diagnostic Tutorial a means for evaluating so that an improvement at the 24; accepted 1996 Radiology, RSNA November scientific 28 Mayo Clinic, assembly Address reprint 200 First Received requests St, SW, March Rochester, MN 3, 1998; revision to the requested the AAPM/ May 27 and re- author 1998 731 Figure Radiographic strate how radiographic was acquired at 70 kVp density and the 10-kVp rule (a-c) Lateral density varies with different milliampere-second and 16 mAs With the kilovoltage unchanged, with half the milliampere-seconds the milliampere-seconds or 32 mAs obtained twice diographic density and 16 mAs With 10-kVp reduction varies with different the miffiampere-seconds to 60 kVp, or mAs, radiograph exposure as tow as possible article examines compromises associated with the choice of various technique and equipment design factors to provide a guide for proper selection of x-ray technique, focal spot size, x-ray generator type, and scatter rejection method Several standard texts provide addiThis information and on image principles quality of basic x-ray (1-7) a SELECTION OF X-RAY EXPOSURE FACTORS X-ray exposure factors include the peak tube voltage, tube current, and time that are selected on the control panel of the x-ray machine to produce the desired radiograph The selection of these factors affects the image density and tient contrast of the radiograph and the pa- exposure Control of Image Density The primary control of image density ing of the radiograph) is the product current and exposure time, expressed (blackenof tube as milli- ampere-seconds (mAs) Increasing the mifiiampere-seconds wifi proportionally increase the number of x rays that reach the patient and the image receptor Changes in the tube voltage also affect density, but in this the tube voltage will greatly sure to the patient and the rays through changes image the patient case increase increasing the transmission As a result, in tube voltage cause density The relationship and by observing that the overexposed radiographs patient physics phantom demonradiograph (b) radiograph (a) was normal Lateral consideration of the radiation exposure to the patient Unfortunately, image quality improvements frequently result in greater patient exposure Therefore, it is important to consider ways to optimize image quality while keeping tional of a skull values The the underexposed the overexposed radiograph (c) was obtained at radiographs of a skull phantom demonstrate how rakilovoltage values The normal radiograph (e) was acquired at 70 kVp unchanged, the underexposed radiograph (d) was acquired with a (d-f) and the overexposed kVp The 10-kVp rule is demonstrated are similar and the densities of the radiographs expo- of x small large changes in between mdli- (0 was acquired with a 10-kVp densities of the underexposed (c, f) are similar ampere-seconds and tube increase voltage that #{149}maging I & Therapeutic Technology (a, d) results in production of equivalent image density is known as the 10-kVp rule: An increase of 10 kVp is equivalent to doubling the milliampereseconds Figure shows how kilovoltage and milliampere-seconds can be manipulated to change image density As predicted by the 10-kVp rule, a decrease of 10 kVp produces an image with density similar to that achieved by reducing the milliampere-seconds from 16 mAs to mAs, and an increase of 10 kVp produces with density similar to that achieved bling the miuiampere-seconds from an image by dou16 mAs to 32 mAs It should be noted that the 10-kVp rule does not apply for radiographs acquired at 100 kVp or of small body parts such as the extremities Tube Voltage Selection Selection of tube voltage is the primary of controlling contrast in a radiograph contrast is defmed as the difference method Image in radio- graphic density of adjacent anatomic structures The formation of image contrast depends on two independent factors: film contrast and subject contrast Film contrast depends on the characteristics of the film used and how it is processed, which is described by the characteristic curve Subject contrast is defmed as the relative radiation intensities of the x-ray beam exiting the patient The subject contrast is larger if x-ray penetration through an object is much different from the penetration through adjacent background tissue The penetrability, or penetrating power, is determined by the effective energy of the x-ray beam: Higher-energy x-ray beams penetrate matter farther than towenergy beams Because x-ray beam energy directly affected by changing the tube the latter is a major factor in determining graphic contrast 732 to 80 radiographs Volume is voltage, radio- 18 Number a d b - c May-June f 1998 Schueler U RadioGrapbics #{149} 33 - Figure Effect torn was acquired of tube voltage quired at 100 kVp in a large reduction and mAs in patient at 70 kVp is 1 mR 370 mR Figure tem shows anatomic (0.297 x 10’ C/kg), (0.955 x io- C/kg) Characteristic curve change in film contrast areas with different on contrast and 60 mAs In addition exposure whereas with relative represented by the solid and dashed ference in optical densities between Low-contrast to a reduction The entrance the for a screen-film and dose (b) skin exposure (a) High-contrast radiograph of a skull phanradiograph of a skull phantom was ac- in contrast, skin exposure produced the increase in kilovoltage for the low-contrast in the high-contrast radiograph is Shoulder sys- exposure attenuation results radiograph Two are lines A larger the two areas dii- indicates higher contrast is present in the image When the anatomic areas are properly exposed, the optical densities fall within the linear portion of the characteristic curve and the contrast is greatest If the anatomic areas are overexposed, the optical densities fall within the shoulder portion of the curve and contrast is reduced C,) C) C Use of high tube voltage results in a reduc- tion in contrast, compared with that achieved with low kilovoltage techniques This effect is demonstrated by the two radiographs in Figure The image obtained at 100 kVp has substantially reduced contrast, compared with that seen in the 70-kVp image The loss of contrast results in a decrease in the visibility of detail log Relative Exposure in areas such as the frontal sinus However, when the milliampere-seconds is adjusted so that the amount of radiation reaching the image receptor is the same, the 100-kVp technique requires nique more beam a much Milliampere-Seconds Selection Selection of milhiampere-seconds affects image density, as demonstrated in Figure la-ic In addition, milliampere-seconds selection influences contrast in a secondary way For screen- compared 734 Imaging lower with radiation that & Therapeutic exposure needed in the Technology to the 70-kVp patient tech- The higher penetrating, is absorbed kilovoltage x-ray beam is so a smaller fraction of the by the patient Volume 18 Number 4’ J 1)‘ a b Figure Loss of contrast due to improper exposure Underexposed radiograph of a skull phantom acquired at 70 kVp and 30 mAs (a) and the overexposed radiograph acquired at 70 kVp and 20 mAs (c) are lower in contrast cornpared with the normal exposure acquired at 70 kVp and 60 mAs (t) the difference anatomic in optical areas wilt be densities the between largest est contrast Film contrast posure results in densities for the two high- is reduced when exthat lie in the toe or shoulder regions of the curve The effect of Under- and overexposure on contrast in a clinical image is demonstrated in Figure FOCAL SPOT SELECTION The choice of focal spot size primarily influences the amount of geometric unsharpness a radiograph However, focal spot selection C film radiography, both underexposure (mifiiampere-seconds too low) or overexposure (millampere-seconds too high) result in a reduction in film contrast The relationship between film contrast and density can be understood in terms of the characteristic curve (Fig 3) The characteristic curve (or Hurter and Driffield [H&D] curve) describes the relationship between optical density and exposure The curve has three regions that correspond 1998 influences the amount of motion blur in an image, since tube current the selection limits the maximum and tube voltage settings, thereby affecting exposure the time In addition, design of the x-ray tant consideration, tube anode is also because the anode may focal influence the field coverage, vided by the spot and heat x-ray tube size, capacity the an imporangle radiation that are pro- to different exposure levels For low- and high-exposure levels, the slope of the curve is relatively small These portions of the curve are the toe and shoulder regions In between the toe and shoulder, the curve is a straight line with a steep slope Within the straight-line portion, May-June also in Focal Spot The x-ray focal rays Instead, size This Blur spot is not a point it is a rectangular causes a point Schueler source region in an object of x of finite to appear #{149} adioGrapbics R U 735 F Focal Spot I SOD SID 6a Object Plane OlD Image Plane Bf Figures 5, how (5) Focal spot blur Diagram the focal spot blur in the image _ plane (Bj7 increases as the object is moved 6b closer to the focal spot OlD = object-image distance, SID = source-image distance, SOD = source-object distance illustrates (6) Effect of focal spot size and magnification on blur (a, b) Radiograph of the sella turcica, obtained with a small focal spot of nominal size 0.3 mm (measured size, 0.5 mm) (a), exhibits greater detail than the radiograph obtained with a large focal spot of nominal size 1.0 mm (measured size, mm) (b) Both a and b have the same magnification (M = 2) (c) Radiograph obtained with a large focal spot of nominal size mm (measured size, mm) but with the object in contact with the image receptor (M = 1) is relatively sharp cornpared with b, even though a large focal spot was used ing Bf blurred the the on the image The amount of blur by where SID the magnification M (M = source-image distance): Bf0 = = Bf/M =Fx(1 R Imaging & Therapeutic SID/SOD, F x (OlD/SOD), where F = focal spot size, OlD = object-image distance, and SOD = source-object distance To compare the focal spot blur to the size of the object itself, we calculate the blur in the plane of the object (Bf) Bf0 is determined by divid- 736 = in image plane (BJ7 can be calculated from two similar triangles shown in Figure 5: Bf _ Technology - 1/il) The effect of focal spot size and magnification on blur in a clinical image is demonstrated in Figure For the same magnification, the focal spot blur wifi increase as the focal spot size increases (Fig 6a, 6b) In Figure 6b, bone margins are indistinct and some fme structures blend into the background In addition to the Volume 18 Number 0.7 0.6 Composite 0.6 - 0.5 -.- 1.0 mm Focal 0.5 Spot 0.4 E E 0.4 0.3 I- 0.3 0.2 0.2 Detail Screen 0.1 1.2 1.4 1.6 1.8 2.2 2.4 1.2 1.4 Magnification 2.2 2.4 Magnification Figure Blur in the object plane as a function of magnification cal spot and a high speed screen has a minimum composite blur 0-mm focal spot and a detail screen has a minimum composite spot blur also depends on the 6c) When there is no magnification (M = 1), the focal spot blur is zero if magnification is increased by either moving the object away from the receptor or moving the focal spot closer to the object, the focal spot blur will increase Blur due to the image receptor will also contribute to the total image blur in a radiograph Receptor blur is primarily caused by the spreading of light photons formed by x rays interacting with the intensifying screen Because the spreading of emitted light increases as the distance between the x-ray interaction and film increases, the amount of blur depends on the magnification thickness size, the (Fig of the screen phosphor layer A thick, high-speed screen has an inherent blur (Br) of approximately 0.7 mm, whereas the blur from a thin, detail screen is 0.2-0.3 mm As with focal spot blur, it is more clinically retevant to calculate the amount of blur in the ohject plane because it can be compared with the size of the object itself The receptor blur in the object plane (Br0) is determined by dividing the inherent blur in the image plane by the magnification: Br = Br/M 1998 (a) A radiographic system with a 0-mm at a magnification of 1.5 (b) A system with blur when no magnification is used foa the sum of the two components squared The contribution to B from the two sources depends on the magnification of the object As magnification increases, focal spot blur increases while receptor blur decreases The relationship between total image fication can be demonstrated cal spot-receptor combination graph (8) Figure 7a shows blur and magni- for a particular foin the form of a the composite im- age blur for a radiographic system when a 1.0mm focal spot and a high-speed screen are used The curve representing focal spot blur shows how geometric unsharpness increases with magnification The curve representing receptor blur shows improvement in detail with magnification The composite of the two mdicates that the total image blur decreases then increases with nification is used, magnification receptor When the magnification size becomes the major When blur little mag- dominates is large, the determining focal factor spot in the total image blur For this system, a magnification of will produce the sharpest radiograph Figure 7b demonstrates the blur-magnification relationship when a detail screen and 0-mm focal spot are used The detail screen results in tess receptor blur compared with that produced by the high-speed screen For this equation shows that receptor blur in the object plane wifi decrease as an object is magnified The total image blur in a radiograph (H) is a composite of the focal spot blur and the receptor blur It is calculated as the square root of This May-June 1.8 b a focal 1.6 system, produced ing the receptor tance the sharpest if magnification object as close and increasing Schueler radiograph wifi be is minimized by placas possible to the image the source-image ills- #{149} adioGrapbks R #{149} 37 Motion Blur Another component tal image blur that minimized by using possible However, factors to produce may result contributes is patient in an to the motion shortest the Motion exposure the selection the shortest increase to- blur is Focal of technical exposure time in focal spot Spot blur Rotating Anode We have seen that geometric unsharpness is decreased by using a small focal spot A small focal spot concentrates heat on a smaller area of the anode and results in a tower heat capacity A tow heat capacity limits technique settings to low power or tow tube voltage and tube current With technique selection limited to low values, exposure time must to increased to produce adequate image density A large focal spot can be used with higher tube voltage and tube current settings for a shorter exposure and minimized motion blur In clinical practice, the compromise tween geometric unsharpness and Track time Anode Angle I I I I time Cathode bemotion blur can be handled by tailoring the focal spot lection to the requirements of the particular amination When image detail is important, se- small focal focal spot spot blur When large focal spot should tube voltage the exposure be motion with values time blur higher should used to reduce is a problem, tube be used current exthe I I I’ the Focal Size I and Figure to minimize ing Anode Angle The surface of an x-ray tube anode is angled with respect to the central axis of the x-ray beam (Fig 8) Tubes are produced with anode angles that range from 7#{176} 20#{176} to This angulation permits larger heat loading while minimizing the effective focal spot size, which is the size of the x-ray source as viewed from the image The angled surface increases the width of the anode focal spot track, which is defined by the surface area impacted by electrons from the filament as the anode rotates As a result, the amount of anode angulation influences the heat capacity of the x-ray tube In addition, the anode angle determines the size of the area Diagram assembly anode tween covered by the x-ray beam, since the edge the anode wilt limit the angle of the emitted Effective Spot the anode depicts the side view of a rotatof an x-ray tube The angle be- surface and the central field coverage is limited An x-ray duced with an anode with a large a larger area, low spot track but because anode rate on examination high such For heat capacity and as cine angiography, small anode angles are phy requires large field angles are of the the the procovers dissipation width practice, depends beam angle of heat small In clinical angle graphic the of the ode of axis is de- fined as the anode angle The effective focal spot size is the length and width of the x-ray beam projected down the central axis is focal choice particular of radio- applications requiring small field coverage, x-ray tubes with used General coverage, radiograso large x rays For a given effective focal spot size, the choice of the anode angulation is a compromise between heat capacity and field coverage As shown in Figure 9, anodes with small angles provide the highest heat capacity, but radiation U GENERATOR The x-ray generator ity and technique generator promise patient SELECTION design exposure and focal selection of various affects Just image as with #{149}maging I & Therapeutic Technology qual- x-ray spot selection, the goat of is to choose the best comfactors depending on particular radiographic examination The basic types of generators are single phase, three phase, high frequency, and constant 738 an- needed Volume 18 the four po- Number Large Anode Angle Small Anode Angle I Focal Track Spot Width Focal Spot Track Width / / / / I / I I I / I I Field Coverage Field a Coverage b Figure Comparison size, the choice of the age (a) Diagram vides a large of small and large anode angles For anode angulation is a trade-off between depicts focal spot the side view track width of an anode for a high heat with a given effective heat capacity a small capacity, angle but the focal spot and field cover- The small resultant angle radiation profield coverage is limited (b) Diagram depicts the side view of an anode with a large angle The large angle provides larger radiation field coverage, but the rate of heat dissipation is low because of the small width of the focal spot track I fsJ#{149}SstJ#{149}SsJ#{149}CsJ\ I - - - Single full wave phase rectified (two pulse) Three phase Three pulse) (six phase I Patient Exposure and Exposure Time Considerations of patient dose and exposure time can be evaluated by examining the generator voltage waveform Figure 10 shows representative voltage waveforms for each generator type The voltage ripple is defmed as the percentage (twelve pulse) and difference minimum single-phase between voltages the in the generator maximum waveform exhibits The a 100% voltage ripple, since the voltage varies from zero to the peak value The three-phase, six-pulse generaHigh frequency tor has a lower Three-phase, voltage ripple 12-pulse of generators 3%-25% have a ripple Figure 10 Diagram illustrates representative voltage waveforms for single-phase, three-phase (sixpulse and 12-pulse), high-frequency, and constant of 3%- 10%, which is similar to the voltage ripple in high-frequency generators (4%-15%) Constant potential generators have no ripple A generator with a large voltage ripple requires a higher patient exposure to produce a radiograph at a certain kilovoltage selection These types of generators produce many low- potential energy 0, - Constant potential > Time x-ray generators tential Selection criteria of patient exposure and exposure reproducibility, low unit cost May-June 1998 include exposure compact minimization time, good size, and x rays that not contribute age because they are absorbed Therefore, the highest patient needed when a single-phase Schueler to the by the exposure generator, U im- patient is which RadioGraphics U 739 has 100% stant voltage potential substantial reduction Generator also is used longer in greater Use provides in patient with large types require sults ripple, generator Primary ripple times, blur most dose voltage exposure motion of a con- the This X-rays which re- is because the low-voltage portion of the exposure pulses does not deliver a significant exposure to the image receptor, thus the exposure must be lengthened to produce proper image density The total exposure time required when a single-phase generator used is longest of the four generator types Applications in which rapidly moving structures are imaged, such as cine angiography, need a generator capable of producing very short exposure times A constant potential generator is capable of the shortest exposure pulses of approximately J 111111111 Image Receptor msec An additional generator property affecting exposure time is the generator power rating Use of a generator with a higher power rating allows for selection of higher tube voltage and tube current exposure factors so that exposure times can be shorter for a desired milliampereseconds Exposure Reproducibility Good exposure reproducibility produce images with uniform reduce the number of retakes is also phy critical because for digital High-frequency and erators provide superior to that phase generators three-phase can potential reproducibility with singleis because output the ability to compensate for variations from the desired that discussion, provide the conlowest high cost of the are less system High-fre- much more compact expensive In addition, generators can be in designed from either singleor three-phase line age or from a battery or charged capacitor for mobile radiographic units to voltbank and directly for radiation run is on are in- time sudden line voltage changes High-frequency generators use closed-loop regulation to sense the tube voltage and tube current continuously and to correct and quency generators size and relatively in- that or three- the input line voltage Voltage regulators cluded in these circuits, but the response limits size gen- single- depends preceding generators high-frequency constant This potential of scattered patient exposure, shortest exposure time, and good reproducibility However, disadvantages of constant potential generators are the large be- cause the amount receptor Size and Cost As evident from the angiogravoltage images exposure available power helps reduce reaches the is required to contrast and to Reproducibility in tube tween mask and contrast complete subtraction 11 Cross-sectioned diagram shows how a grid placed between the patient and image receptor Figure stant subtraction differences 11111 0.5 settings U SCATI’ER A large fraction undergo REJECTION of the x rays entering Compton interactions, which a patient produce scattered x rays The scattered photons are emitted in all directions, but they tend to be scattered in a more forward direction, as the energy of the primary beam is increased When the primary x-ray beam enters hone surrounded by soft tissue, the radiographic density change between the soft tissue and bone should be very large However, the high contrast is reduced by scattered x rays, which strike the image receptor 740 U Imaging & Therapeutic Technology Volume 18 Number a b Comparison of grid and “nongrid” techniques Both radiographs of a skull phantom were with 90 kVp, 105-cm source-image distance, and 80-cm source-object distance Radiograph obtained with a grid (grid ratio of 12:1 [grid thickness:interspace width]) (a) demonstrates a noticeable improvement in contrast compared with the nongrid radiograph (b) In addition, a substantial increase in patient dose was required for the grid radiograph (150 mR [0.387 x 10 C/kg]) compared with the nongrid radiograph (33 mR [0.085 x 10’ C/kg]) Figure 12 obtained can the shadow of the bone Several methods be used to reduce the amount of scattered x The increase in contrast is achieved, ever, at the expense of increased patient howdose rays that The lead strips of the grid absorb some radiation that would have reached the receptor; thus, an increase in exposure quired to achieve the same film density entrance skin exposure for the radiograph taken with the grid is 150 mR (0.387 x kg), whereas the skin exposure produced obtaining the radiograph without a grid of the image is reThe stantially C/kg) within reach the receptor oils are the use of grids tion to limit the volume reduces the production Two of these meth- or an air gap Collimaof irradiated tissue also of scattered x rays Grids The most common method of reducing the level of scattered radiation reaching the image receptor is use of grids (9) A grid is constructed of alternating strips oftead and nonabsorbing interspace material and is placed between the patient and image receptor The strips are arranged a line rected to transmit from at an the only x-ray angle are those source x rays (Fig preferentially directed 1) X rays absorbed in 1998 at 33 mR (0.085 x 10 Cl in is sub- The ratio of the exposure required with grid use and without grid use is called the Bucky factor The Bucky factor is higher for higher tio grids and higher energy exposures ra- diby the grid Because most scattered x rays are emitted at an angle to the primary beam direction, a large fraction of the scattered radiation is absorbed Figure 12 demonstrates the contrast improvement that can be obtained by using a grid May-June tower 10 Air Gap Technique Another method of reducing tered radiation that reaches is to place a gap between Schueler the level the image the patient U of scatreceptor and RadioGraphics the #{149} 741 receptor (10) Because are emitted at an angle most scattered x rays to the direction of the primary beam, a large fraction will not strike the receptor if it is separated from the patient by a sufficient distance (Fig 13) However, Primary X-rays pri- mary x rays source wifi directed in a line from the x-ray not be affected The typical air gap distance is 5-45 cm, which wifi also introduce some magnification and limit the field of view of the subject The change in contrast that results from use of an air gap is shown in Figure 14 Patient Clinical Applications Both grid and air gap techniques are effective means of controlling scattered radiation and improving contrast in a radiograph To select S which method is best for a specific application, we must examine the trade-offs involved Grids not require use of magnification, so focal spot blur is reduced When an air gap is used, a small focal spot is generally needed to minimize the geometric unsharpness The advantages of the air gap technique include use of lower milliampere-second values, compared with that needed for the grid technique, which results in less tube tient exposure may gap, but the amount the source-to-patient loading In addition, the be reduced by using of reduction depends distance used Air Gap $ Figure 13 Cross-sectioned an air gap placed between diagram shows how the patient and image receptor helps reduce the amount of scattered radiation that reaches the receptor pa- an air on For most radiographic examinations, grid use is common However, there are several applications in which the air gap technique offers some advantages over grid use One such application is cerebral angiography (1 1) When the air gap technique is used in cerebral angiography, the geometric magnification is generally adjusted to 5- Some radiologists prefer the magnified images, which are free of grid lines Compared with a grid technique, the increase in patient exposure resulting from positioning the patient closer to the x-ray source approximately the same as the exposure increase required when a grid is used Another air gap application is chest radiography (12) this case, because a large source-image tance (6 or 10 feet) is typically used, the an air gap can substantially reduce patient and the resulting magnification is slight is SUBJECT CONTRAST The amount of subject contrast produced is affected by both physical characteristics of the object ray and penetrating beam ness, Object physical number (Z) different sorb In use of dose density, Two voltage erator, Imaging & Therapeutic Technology atomic have either or Z will ab- of radiation X-ray tube and voltage beam the waveform produced by the x-ray genplus filtration (ie, added and inherent material in the path of the x-ray beam) As demonstrated in Figure 2, kilovoltage has a substantial effect on image contrast In addition, factors such as filtration and voltage waveform shape also alter the distribution of x-ray energies in the beam The way in which to produce mined #{149} x- thick- that densities, include gether 742 effective areas amounts characteristics of the include and tissue thicknesses, different attenuating dis- characteristics characteristics by Compton these subject and factors contrast photoelectric Volume come to- is deter- interac- 18 Number a b Figure 14 Comparison of air gap and “non-air gap” tom were acquired at 90 kVp without a grid Radiograph strates a noticeable improvement in contrast compared ample, the source-image distance was adjusted so that proximately the Effective Densities Materials same to better compare techniques Both numbers, and ties are ton very Effective Atomic Number Physical Density (g/cm3) beam 1.00 1.00 0.91 3.50 4.93 0.0013 7.5 5.9 56.0 53.0 7.6 not for mize ray are the tissue Compton in tissue interactions manly on dence on atomic tissue occurrence strong ray probability occurring density, number with energy The teractions increases as energy on of x-ray depen- being interactions atomic has number probability and a of photoelectric as Z increases increases sity Radiography tissue requires to radiographic technique ids, May-June and fat 1998 have relatively so that The special Muscle, attention has Because tissue effective x- of softin which the the a and we need barium dennormally material maximize Use of 60-75 it produces many iodine and structures contrast to is in- contrast number of the iodinated the object contrast used atomic size into of the subject iodine absorption because introduced commonly a high the with above most contain small, able low imaged imaged de- example attenuation usually and ential Soft-Tissue Imaging of soft be the Iodine in- anode can materials x- maxi- a tow-energy Media creased The tube to change little energy To kVp are used with and filter to produce of 25-30 agents body very An inter- types is mammography Contrast pri- interac- x rays Contrast depends or x-ray of photoelectric dependence creases The tissue used factors when x-ray Compton effect, radiography to occur as photoelectric distinguishing technique special likely However, be densiComp- a high-energy as effective must and As a result, more photoelectric beam numbers with keY) are actions 7.4 of x rays this exis ap- (Table) is imaged (>40 tions atomic similar low-energy tions (b) For radiographs their interactions a patient Water Muscle Fat Barium Iodine Air of a skull phanair gap (a) demon- contrast Atomic Numbers and Physical of Human Tissues and Contrast Substance radiographs obtained with a 25-cm with a contact radiograph magnification in the two kVp K-absorption are the x rays edge differ- is preferjust of 33 keV fluatomic Schueler U RadioGraphics I 743 x-ray K-absorption absorption ergy is equal (The edge is an abrupt increase that occurs when the x-ray to or energy binding The atomic slightly greater of the K-shell number than in en- case, normally fluid-filled cavities are 233-266 Bushberg JT, Seibert JA, Leidholdt EM, Boone JM The essential physics of medical imaging Baltimore, Md: Williams Bushong SC Radiologic gists: physics, biology, a absorption difference from the application clinical large of x-ray radiography many interrelated technique, scatter factors focal rejection generally for these quality without to of including type, exposure generator size, method involves selecting of image posure principles consideration factors, spot should be based on the tics of a given radiographic basis physics requires and Selection of these compromises that patient cx- 10 P Physical principles of medical imag- Va: American College of Radiology, Bucky 1 Arnmann E X-ray generator and AEC design In: Seibert JA, Barnes GT, Gould RG, eds Specification, acceptance testing, and quality control of diagnostic x-ray imaging equipment 12 G A grating diaphragm to cut off sec- ondary rays from the object Arch Roentgen Ray 1913; 18:6-9 Gould RG, Hale J Control of scattered radiation REFERENCES U Sprawls x-ray imPhysics, 1984 is optimization excessive considerations ing 2nd ed Gaithersburg, Md: Aspen, 1993 Sprawls P, Lamel DA Principles of imaging, Sequence 142 (case no 8042) Diagnostic Radiological Health Sciences Learning Laboratory Reston, particular characterisexamination The factors imaging designs In: film mamand mcdiMadison, Wis: physics responsibilities Medical Physics, 1991; 47-66 Hasegawa BH The physics of medical aging 2nd ed Madison, Wis: Medical 1991 cat CONCLUSIONS The St Louis, Mo: Mosby, 1988 Curry TS, Dowdey JE, Murry RC Christensen’s physics of diagnostic radiology 4th ed Philadetphia, Pa: Lea & Febiger, 1990 Gauntt DM Mammography x-ray generators: mography: in density results & Wilkins, 1994 science for technoloand protection 4th ed conventional and high frequency Barnes GT, Frey GD, eds Screen filled with air Even though the effective atomic number of air is similar to that of soft tissue, differential 20 WoodPhysics, 1994; of electrons.) density of barium and No the are similar to those of iodine, but the size of the structures normally imaged with barium contrast agents are generally large Therefore, high kilovoltage technique is used to penetrate the contrast agent and better visualize the lumen Air can also be used as a contrast agent In this Medical Physics Monograph bury, NY: American Institute by air gap techniques: application to chest radiography AJR 1974; 122:109-114 Barnes GT, Feretti JM, Lamet DA Principles of imaging, sequence 27 (case no 8027) Diagnostic Radiological Health Sciences Learning Laboratory Reston, Va: American College of Radiology, 1984 Barnes GT, Fraser RG Principles of imaging, sequence 128 (case no 8028) Diagnostic Ridiological Health Sciences Learning Laboratory Reston, Va: American College of Radiology, 1984 This Award 744 U hnaging article meets To obtain & Therapeutic the credit, criteriafor see 1.0 the credIt questionnaire Technology hour in category on pp oftbe AMA Physician’s Recognition 725-730 Volume 18 Number ... size, x- ray generator type, and scatter rejection method Several standard texts provide addiThis information and on image principles quality of basic x- ray (1-7) a SELECTION OF X- RAY EXPOSURE... transmit from at an the only x- ray angle are those source x rays (Fig preferentially directed 1) X rays absorbed in 1998 at 33 mR (0.085 x 10 Cl in is sub- The ratio of the exposure required with grid... be used current exthe I I I’ the Focal Size I and Figure to minimize ing Anode Angle The surface of an x- ray tube anode is angled with respect to the central axis of the x- ray beam (Fig 8) Tubes