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Huda and Abrahams Resolution on RadiographsResidents’ Section Physics MinimoduleinRadiologyRes idents ’ Sect ion • Phys ics MinimoduleX-Ray-Based Medical Imaging and ResolutionWalter Huda1 R. Brad Abrahams2Huda W, Abrahams RBKeywords: focal blur, image quality, motion blur, radiographic imaging, receptor blur, resolutionDOI:10.2214/AJR.14.13126Received May 9, 2014; accepted after revision May 30, 2014.1Department of Radiology and Radiological Science, Medical University of South Carolina, 96 Jonathan Lucas St, MSC 23,Charleston, SC 29425-3230. Address correspondence to W. Huda (walterhuda@hotmail.com).2Department of Radiology and Imaging, Georgia Regents University, Augusta GA.WEBThis is a web exclusive article.AJR 2015; 204:W393–W3970361–803X/15/2044–W393© American Roentgen Ray SocietyThiadgee ipnnutgir fpyeo xasabemn ooifnr mmatoaiolsintt irse asdi sii no ltaoo g pieacit tiihemenr-t or to classify a patient as beinghealthy while minimizing radiation exposure [1]. To achieve these goals, it is of obvious im portance to ensure that the radiologicimage quality is sufficient for a given imaging task. Medical image quality is normally character ized in terms of contrast, noise, andspatial resolution. The achieved quality for any im age will depend on both the intrinsic proper ties of the imaging system and themanner in which the images were obtained, including the choices of image acquisition tube voltage (kilovoltage) and output (tubecurrent–expo sure time product).To b e d e t e c t a b l e , a l e s i o n m u s t t r a n s m i t a d i f fe re n t x- r ay b e a m i n t e n s i t y c o m p a re d t o t h e s u r ro u n d i n g n o r m a l t i s s u e s . T h e d i f fe rence in x-ray beam intensity through a lesion relative to the normal background is called subject contrast. The appearance of thissub ject contrast in the resultant image is called image contrast. Noise has both random and structural components, with the latterbeing fixed for a given patient, such as ribs in chest radiographs. The random noise in virtual ly all radiographic imaging is due toquan tum mottle, which can be reduced by using more x-ray photons to create the image, but which will also increase the patientdose [2]. Contrast and noise are directly related to the choice of radiographic techniques, namely x-ray tube voltage (kilovoltage)and output (tube current–exposure time product) [3].The amount of blur, or unsharpness, af fects the appearance of both normal anat omy and pathologic abnormalities. Spatialresolution is the technical term used to re fer to the amount of blur in an image. Spatial resolution performance is an intrinsic property of an imaging system that is generally independent of the selected technique factors (kilovoltage and tube current–exposuretime product). In this article, we describe the es sential characteristics of spatial resolution. Factors that influence spatial resolution,as well as performance of x-ray-based medical imaging modalities, are described [4–7].15/10/2020, 3:47 PMPage 1 of 6

What Is Resolution?Blur and SharpnessMost radiologists have an intuitive sense of what constitutes poor resolution and can easily recognize a blurred image. Resolu tion,however, is sometimes taken to mean the ability to see objects so that when lesions are seen, then the resolution must be good.Such statements are incorrect, as illustrated by considering a single-cell molecule labeled with a radionuclide and imaged with a PETsystem. The activity will be seen, but the blur associated with this point source will be sev eral millimeters, resulting in a blurry imagewith poor resolution.This example can be expanded to illus trate what imaging scientists mean regard ing spatial resolution performance. For two nearbycells with radioactively labeled DNA in their nuclei, a system that can resolve these two entities would show two hot spots as distinctentities (Fig. 1, upper panel). A PET system would show only one very large blurred object, and the activity in cell one would not beresolved from the activity de picted in the nearby cell two (Fig. 1, lower panel). A system with good resolution per mits the detection(and characterization) of two small features, such as microcalcifica tions in mammograms, even when they are physically close toeach other.There are several terms used to describe spatial resolution in imaging. Blur and sharp ness are good descriptors that are universal lyunderstood, and they simply mean that a sharp edge will also appear as sharp (not blurred) in an image obtained with a systemFig. 1that has good resolution performance. Oth er terms that may be used include resolution, high-contrast resolution, unsharpness, anddetail visibility.Measuring ResolutionImaging scientists consider the spatial reso lution performance to be an intrinsic charac teristic property of an imaging system. Ingen eral, resolution cannot be evaluated by looking at line pair phantoms because the visibility (detectability) in phantoms dependson the amount of noise in the image. It is only when noise is somehow made negligible that one is left with the intrinsic blurassociated with a given imaging system. Two common ways of assessing the intrinsic resolution properties of an imaging system arethe line spread function and the modulation transfer function.A line spread function is measured using an image obtained of a narrow line, which is simply a blurred version of the initial line (Fig.2). The amount of blur can be measured as the full width half maximum, which for a line is the physical distance in millimetersbetween the two points where the line intensity has been reduce to half of the central (maximum) value. Different measures of thespread in the image are also possible, such as the full width tenth maximum, where the value of full width tenth maximum isobviously greater than the full width half maximum (Fig. 2). The advan tage of these measures is that they are intuitive and provide aquantitative sense of the amount of blur in a given imaging system.A modulation transfer function curve can be estimated by looking at the images ob tained at differing spatial frequencies, as depicted in Figure 3. For low spatial frequen cies, corresponding to very large bars and gaps, the region behind the lead will be purewhite and the region in the gap between the bars will be pure black. The difference be tween the lead and gap regions is called modulation and is essentially close to 100% (black vs white) at the lowest spatial frequencies. This modulation is always reduced asspatial frequencies increase because of the blurring effects of an imaging system. At some highFig. 2spatial frequency, the image of the bars and gaps blends into a uniform gray. At this point, there is zero modulation in the image, andad jacent bars cannot be differentiated (Fig. 3).Line Pair Phantoms and Pixel SizeA line pair phantom contains strips of lead, with gaps of the same size as the lead. When the lead (and gap) are 0.5 mm, we can fitone line pair into 1 mm and call this a spatial fre quency of one line pair per millimeter. High spatial frequencies correspond to smallob jects and vice versa. When a line pair phan tom is imaged, the large objects are readily resolved from each other because there isa large gap between adjacent lines (Fig. 3). As the spatial frequency increases, however, the lines start to blur into each other andthe lim iting spatial resolution occurs when distinct lines are no longer visible (Fig. 3). When the images are obtained at highexposures so that noise is negligible, this spatial frequency is known as the limiting spatial frequency and is expressed in line pairsper millimeter.15/10/2020, 3:47 PMPage 2 of 6

The use of large pixels would clearly re duce the limiting spatial resolution perfor mance. In general, however, the use of smallFig. 3er pixels is not expected to improve spatial resolution performance. The reason for this is that there are intrinsic sources of blur inthe imaging chain, such as the size of the focal spot, that cannot be overcome by making pix els smaller. Vendors of imaging systemswill make the pixels as small as they need to be, and pixel size is not further reduced when in trinsic factors become the determinantfactors of achievable image sharpness.What Affects Resolution?Focal SpotThe size of the focal spot affects the amount of focal spot blur in all radiologic imaging, as depicted in Figure 4 [8–10]. Inradiography, fluoroscopy, and CT, the x-ray tube normally has two focal spot sizes, with nominal sizes of 0.6 and 1.2 mm. When thelarge focal spot is used, the total power load ing that may be used is 100 kW, whereas the small focal spot can only tolerate 25 kW.The use of small focal spots will thus generally result in longer exposures, which, in turn, in crease the likelihood of motion blur (seedis cussion later in this article).In mammography, the x-ray tube normal ly also has two focal spot sizes, with nomi nal sizes of 0.1 and 0.3 mm. The maximum tubecurrent with the small focal spot is only 25 mA but can be four times higher with the large focal spot. As with radiography, the use ofa small focal spot is generally associated with much longer exposures and the associ ated problems of motion blur.In mammography, geometric magnification is used to investigate suspicious regions, which is achieved by moving the compressedbreast closer to the x-ray tube (Fig. 4). In intervention al neuroradiology, the visibility of very small blood vessels can be improvedthrough the use of geometric magnification. In both of these cases, the use of geometric magnification will always require the use ofsmaller focal spots to minimize focal spot blur.MotionThe amount of motion blur is directly re lated to the speed of any motion, which mayFig. 4be voluntary or involuntary. Examples of in voluntary motion include cardiac motion and peristalsis. The amount of motion blur is alsodirectly proportional to the exposure time of any radiographic examination. Motion blur can be estimated by multiplication of thespeed of any motion by the corresponding exposure time. When increasing x-ray tube output, the x-ray tube currents should beincreased when this is technically feasible, so that exposure times are kept as short as possible.Chest radiographic imaging has very short exposure times, of the order of milliseconds, which is easy to achieve as the chest is notdif ficult to penetrate and only low x-ray tube out puts (≈ 1 mAs) are used. In abdominal imag ing, exposure times must be increasedto tens of milliseconds to ensure that enough radia tion is incident on the image receptor, which is achieved by use of much higherintensities (≈ 20 mAs). The longest exposures are generally encountered in mammography, where a typi cal exposure time isapproximately 500 ms or more and up to a factor of 3 higher when per forming magnification mammography [11].There are a number of practical steps that may be taken to minimize patient motion. Im mobilization devices include head supports inCT and compression paddles in mam mography [12, 13]. Patients can be asked to hold their breath and stay still. In cardiac CT,β-blockers can be prescribed to reduce the heart rate [14]. Sedation can be used when im aging newborns and young infants [15].Receptor SizeScintillators used in radiology include ce sium iodide used in flat-panel detectors and image intensifiers and gadolinium oxysulfide inmammographic screens. When x-rays are absorbed by any scintillator, approximate ly 10% of the absorbed energy is converted intolight, which spreads out when traveling to the light detector, as depicted in Figure 5 [16]. This spreading of light between the x ray15/10/2020, 3:47 PMPage 3 of 6

interaction and the corresponding light detector (film or a digital equivalent) may result in significant image blur. Although acolumnlike structure can be used to reduce diffusion of light, detector blur is nonethe less present in all scintillators. The amount ofblur is directly related to the scintillator thickness as depicted in Figure 5.In photoconductors, charge created after a photoelectric or Compton interaction is collect ed directly by the application of anelectric field [17]. Because low-energy electrons do not trav el very far, there will be very little blur (Fig. 6). Consequently,photoconductors are expect ed to have excellent resolution performance and are attractive for use in digital mammog raphy, wheregood resolution performance isFig. 5Fig. 6essential. By contrast, in computed radiogra phy, light beams are used to release the energy stored in an exposed photostimulablephosphor. As shown in Figure 7, there will be scattering of the incident light, which will result in increased image blur because of therelease of light from adjacent pixels. For this reason, there is a limit to the thickness of any photostimulable phos phor. When thindetectors are used, this in creases patient doses because of the reduction in x-ray absorption in thinner detectors.In fluoroscopy, the number of television lines determines the nominal width of each line and thereby affects the achievable spa tialresolution performance. When the num ber of television lines increases from 500 to 1000, the limiting resolution performance willalso improve by a factor of 2. In CT, it is the physical size of each detector (length and width) that is directly related to the achievable spatial resolution performance [18]. When corrected for image magnification, the nominal detector dimension in both direc tionson a CT scanner may be taken to be a nominal 0.5 mm or so.Resolution Performance in Radiographic ImagingRadiography and MammographyDigital radiographs generally have a ma trix size of 2000 × 2500. For a 35 × 43-cm cassette, this digital matrix corresponds to a pixelsize of 175 μm and a limiting resolution of three line pairs per millimeter. For a 20 × 24-cm cassette, the pixel size would be 100 μm,and the limiting resolution would be five line pairs per millimeter. Extremity radio graphs, where good resolution is important fordetecting hairline fractures, should thereFig. 7fore be obtained using smaller cassettes. The limiting resolution of a typical film-screen system used for chest radiography wasabout six line pairs per millimeter, or a factor of 2 better than a typical digital chest radiograph ic imaging system. In digitalmammography, the pixel size typically ranges between 50 and 100 μm, so that the corresponding limiting spatial reso lution rangesbetween 10 and five line pairs per millimeter. A common pixel size of 70 μm corresponds to a limiting resolution of close to seven linepairs per millimeter. The total number of pixels in a digital mammogram is about 12 million, assuming a 3000 × 4000 matrix size.Most mammography worksta tions are capable of displaying up to 5 million pixels, so that seeing all the available detail in a digitalmammogram will require the use of image zoom capabilities [19]. Film-screen mammography typically achieved a limiting spatialresolution of 15 line pairs per millimeter, which is a factor of 2 better than digital mammography. This example illustrates that thebenefits of digi tal mammography relate to image processing rather than any resolution issue per se. Mam mography film-screensystems are superior to CTthe capabilities of the human visual system, which is about five line pairs per millimeter at a 25-cm viewing distance [19]. Thisexplains why mammographers in the days of film al ways had a magnifying glass in their hands for viewing mammograms onviewboxes.Fluoroscopy and Digital PhotospotFluoroscopy can be performed with image intensifiers or flat-panel detectors and does not generally require high-resolution performance. With a standard 500-line television system, the nominal limiting fluoroscopy spatial resolution performance is about one linepair per millimeter. Flat-panel detec tors with a pixel size of 175 μm can achieve a limiting resolution of three line pairs per millimeter.When these are used for per forming fluoroscopy, binning four pixels into one large pixel is common for larger FOVs, which results ina limiting resolution of 1.5 line pairs per millimeter.Digital photospot images using an image intensifier–based imaging chain are general ly obtained using a high-quality (1000 line)television system. For a 25-cm FOV, the re sultant pixel size is 250 μm, and the corre sponding limiting resolution will be about twoline pairs per millimeter. Commercial 2000-line television systems are available, but the benefits of providing improved spa tial15/10/2020, 3:47 PMPage 4 of 6

resolution performance were not deemed to be worthwhile. For flat-panel detectors, spatial resolution for digital photospot im agesis identical to that for radiographs ob tained with these detectors—namely, three line pairs per millimeter.In a head CT scan, the FOV is 250 mm, the matrix size is 512 × 512 and the pixel size is 0.5 mm. The best achievable resolu tion forthis FOV and matrix size is thus one line pair per millimeter. Increasing the FOV to 500 mm for a large patient doubles the pix el size(1 mm) and halves the limiting spa tial resolution performance (i.e., 0.5 line pair per millimeter). Spatial resolution in CT is thus anorder of magnitude worse than film screen combination and four times worseTABLE : 1Imaging SystemCT Fluoroscopy Digital subtraction angiography or photospot Radiography MammographyLimiting Resolution, Line Pairs per Millimeter≈ 0.7 ≈ 1≈ 2≈ 3≈ 7CommentsImproved by zoom reconstruction of the central region of acquired image Improved by the use of electron magnification modes, aswell as high-quality televisionsystems (1000 line)Improved by the use of electronic magnification modes of image intensifiers (e.g.,magnification 1 or 2)Improved performance can be achieved by the use of smaller cassette sizes (5 line pairsper millimeter for 20 × 25 cm)Use of smaller (50 μm) pixels would improve resolution, whereas large pixels (100 μm)would degrade resolutionthan digital chest radiography. CT has ex cellent imaging characteristics and anatomic localization compared with any type of projection imaging but also has inferior spatial resolution performance.In CT, the choice of the mathematic recon struction filter affects the amount of blur in the resultant image [20]. Image reconstructionfil ters offer reduced image noise but at the ex pense of more image blur. Common names for such high-resolution reconstructionfilters in clude “bone,” “detail,” “high resolution,” and “lung.” These filters are used to get excellent detail for structures such as bone(vs air or tis sue) or lung (vs air), where the high intrinsic contrast negates the importance of higher lev els of noise. Reconstructionfilters with names such as “standard” or “soft tissue” reduce im age noise at the price of inferior spatial reso lution performance andare used where the in trinsic lesion contrast is low.CT spatial resolution can be improved by up to a factor of about 2 by reducing the re construction FOV (zoom). This requires that theoriginal projection data are used to re construct a smaller anatomic region, which thereby differs from simple magnification, whichuses reconstructed image data. Ob taining the best possible resolution may also require the use of a reduced focal spot (0.6 vs 1.2mm), which will also require a reduc tion of the focal spot power loading (25 vs 100 kW). However, when a small focal spot is usedwith reduced power loading, the scan time may need to be increased, which also increases the likelihood of motion blur.ConclusionResolution is the ability of an imaging system to faithfully reproduce a sharp edge that is present in the object. The key factors thatinfluence the sharpness of an image re late to the size of the source of x-rays (fo cal spot), the physical characteristics of the x-raydetector system (area and thickness), and the presence of any motion blur because of the finite duration of all radiographic exposures. Image processing, especially in CT through the filtered back projection recon struction, can also affect spatial resolution15/10/2020, 3:47 PMPage 5 of 6

performance. The limiting resolution that is routinely achieved in clinical practice ranges from about 0.7 line pair per millimeter for CTto about seven line pairs per millimeter for digital mammography (Table 1). The val ues listed in Table 1 may be compared to thehuman visual system, which can resolve up to five line pairs per millimeter at a normal viewing distance of 25 cm.References1. Huda W. Understanding (and explaining) imag- ing performance metrics. AJR 2014; 203:[web] W1–W22. Huda W. Kerma area product in diagnostic radiol-gy. AJR 2014; 203:[web]W565–W5693. Huda W, Abrahams RA. Radiographic tech- niques, contrast, and noise in x-ray imaging. AJR2015. 204:W126–W1314. Bushberg JT, Seibert JA, Leidholdt EM, BooneJ. The essential physics of medical imaging, 3rdK. Philadelphia, PA: Lippincott, Williams & Wilkins, 20125. Huda W. Review of radiologic physics, 3rd ed. Philadelphia, PA: Lippincott Williams & Wilkins, 20106. Nickoloff EL. Radiology review: radiological physics. Philadelphia, PA: Elsevier Saunders, 20057. Wolbarst AB. Physics of radiology, 2nd ed. Madi- son, WI: Medical Physics Publishing, 2005B. Clinical applications of ba-sic x-ray physics principles. RadioGraphics 1998; 18:731–744; quiz, 7299. Villafana T. Generators, x-ray tubes, and expo- sure geometry in mammography. RadioGraphics1990. 10:539–55410. Zink FE. X-ray tubes. RadioGraphics 1997; 17:1259–126811. Hogge JP, Palmer CH, Muller CC, et al. Quality assurance in mammography: artifact analysis. RadioGraphics 1999; 19:503–52212. Ayyala RS, Chorlton M, Behrman RH, KornguthP. Slanetz PJ. Digital mammographic artifacts on full-field systems: what are they and how do I fix them? RadioGraphics2008; 28:1999–200813. Barrett JF, Keat N. Artifacts in CT: recognition and avoidance. RadioGraphics 2004; 24:1679–169114. Pannu HK, Alvarez W Jr, Fishman EK. Beta- blockers for cardiac CT: a primer for the radiolo- gist. AJR 2006; 186(suppl2):S341–S34515. Macias CG, Chumpitazi CE. Sedation and anes- thesia for CT: emerging issues for providing high- quality care. Pediatr Radiol2011; 41(suppl 2):517–52216. Haus AG. The AAPM/RSNA physics tutorial for residents: measures of screen-film performance. RadioGraphics 1996; 16:1165–118117. Rowlands JA, Zhao W, Blevis IM, Waechter DF, Huang Z. Flat-panel digital radiology with amor- phous selenium and active-matrix readout. Radio- Graphics 1997; 17:753–76018. Sprawls P. AAPM tutorial: CT image detail and noise. RadioGraphics 1992; 12:1041–104619. Samei E. AAPM/RSNA physics tutorial for resi- dents: technological and psychophysical consider- ations for digitalmammographic displays. Radio- Graphics 2005; 25:491–50120. Goldman LW. Principles of CT: radiation dose and image quality. J Nucl Med Technol 2007; 35:213–225; quiz, 226–2188. Schueler15/10/2020, 3:47 PMPage 6 of 6