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Image Quality – Bushberg Chapter 10 Diagnostic Radiology Imaging Physics Course 16 December 2004 What Makes for a ‘Quality’ Image? Image Quality – Chapter 10 ¬ Insert audience participation here Brent K Stewart, PhD, DABMP Professor, Radiology and Medical Education Director, Diagnostic Physics a copy of this lecture may be found at: http://courses.washington.edu/radxphys/PhysicsCourse04http://courses.washington.edu/radxphys/PhysicsCourse04-05.html Brent K Stewart, PhD, DABMP Brent K Stewart, PhD, DABMP Take Away: Five Things You should be able to Explain after the Image Quality Lecture ¬ ¬ ¬ ¬ ¬ Image Quality - Motivation The differences between subject, detector and radiographic contrast and what factors affect each How the Modulation Transfer Function (MTF) describes an imaging system’s spatial resolution characteristics How the number of photons used in imaging affects the perceived quantum noise and contrast resolution Why aliasing wraps high frequency image information into lower frequencies and how this affects an image Methods used to describe an imaging system’s performance, including Detective Quantum Efficiency (DQE), ContrastContrast-Detail curves and ROC curves Brent K Stewart, PhD, DABMP Brent K Stewart, PhD, DABMP ¬ ¬ ¬ ¬ ¬ Applies to all types of images ‘Quality’: subjective notion, dependent on image function Bottom line outcome measure of a radiological image is its usefulness in determining an accurate diagnosis Understanding the image characteristics that comprise image quality important so that radiologists can recognize problems and articulate their cause Introduction to the terminology used for various metrics used by physicists and engineers to measure image quality, e.g., contrast, spatial resolution and noise Brent K Stewart, PhD, DABMP Image Quality – Bushberg Chapter 10 Diagnostic Radiology Imaging Physics Course 16 December 2004 Contrast ¬ ¬ ¬ Subject Contrast (Cs) What is contrast? The difference in the image gray scale between closely adjacent regions of the image Medical image contrast the result of many steps during acquisition, processing and display ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ Difference in some aspect of the signal prior to it being recorded Consequence of fundamental differences in the object, e.g., in xx-ray intensity based on attenuation Cs = (A(A-B)/A µ(x+z) For A=N0e-µx and B=N0e-µ(x+z) -µz Cs = 11-e either µ(E) or z to Cs E to µ(E) and thus Cs Why low kVp used in mammography c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 256 ¬ ¬ ¬ ¬ c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., pp 257257-258 Brent K Stewart, PhD, DABMP Brent K Stewart, PhD, DABMP Detector Contrast (Cd) Radiographic Contrast (Cr) A detector’s characteristics play an important role in producing contrast in the final image Cd determined principally by how the detector ‘maps’ detected energy into the output signal Characteristic curve (e.g., H&D curve): input radiation exposure to output value (analog or digital) Output of digital imaging system a digital value for each pixel: gray scale value ¬ ¬ ¬ ¬ ¬ ¬ ¬ c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 260 Brent K Stewart, PhD, DABMP Brent K Stewart, PhD, DABMP For screenscreen-film radiography analog film OD the output Cr = ODA - ODB In radiography the contrast cannot in general be adjusted or enhanced on the analog film How does the final light signal reaching the radiologist’s eyes depend on patient thickness (assuming a constant µ)? X ∝ e-µx OD ∝ g · log10(X) T = 10-OD c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., pp 259 and 261 Brent K Stewart, PhD, DABMP Image Quality – Bushberg Chapter 10 Diagnostic Radiology Imaging Physics Course 16 December 2004 Huda 2nd Edition – Chapter – Image Quality Digital Radiographic and Displayed Contrast ¬ ¬ ¬ ¬ ContrastContrast-toto-noise ratio (CNR) = (A(A-B)/σ B)/σ, with σ = image noise Due to the ability to postpostprocess digital images, the CNR is a more relevant description of the contrast potential in the image than simply the contrast itself Alter appearance of image through looklook-upup-table (LUT) transformation LUT slope related to displayed image contrast ¬ The object contrast does not depend on the lesion’s: ¬ A Thickness B Density C Atomic number D Background composition E Temperature ¬ ¬ ¬ ¬ c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 262 Brent K Stewart, PhD, DABMP Brent K Stewart, PhD, DABMP Raphex 2003 Diagnostic Question Huda 2nd Edition – Chapter – Image Quality ¬ D6 D6 Which of the following will increase subject contrast in a screenscreen-film imaging system? ¬ A Decreasing the grid ratio B Decreasing the kVp C Increasing the developer temperature D Increasing the focal spot size E Increasing xx-ray beam filtration ¬ ¬ ¬ ¬ Brent K Stewart, PhD, DABMP Brent K Stewart, PhD, DABMP ¬ Subject contrast depends on: ¬ A Focal spot to film distance B mAs C Tube voltage D Developer temperature E Film gradient ¬ ¬ ¬ ¬ 11 Brent K Stewart, PhD, DABMP 10 12 Image Quality – Bushberg Chapter 10 Diagnostic Radiology Imaging Physics Course 16 December 2004 Huda 2nd Edition – Chapter – Image Quality Raphex 2000 Diagnostic Question ¬ D10 D10 Image receptor contrast (as opposed to subject contrast) depends on: ¬ A H&D characteristic curve of the film B kVp C ScreenScreen-film contact D SourceSource-toto-image receptor distance (SID) ¬ ¬ ¬ ¬ Film contrast, as opposed to subject contrast, is affected primarily by: ¬ A Tube voltage B Iodine contrast C Use of grid D Differences in Z E Optical density ¬ ¬ ¬ ¬ Brent K Stewart, PhD, DABMP 13 Brent K Stewart, PhD, DABMP Spatial Resolution ¬ ¬ ¬ ¬ Spatial Domain: the Point Spread Function 2D image really 3D: H, W (both spatial) and gray scale Spatial resolution is a property that describes the ability of an imaging system to accurately depict objects in the two spatial dimensions of the image (x,y) Classic notion: ability of an imaging system to distinctly depict objects as they become smaller and closer together The spatial domain refers to the two spatial dimensions (x,y) of an image Brent K Stewart, PhD, DABMP Brent K Stewart, PhD, DABMP 14 ¬ One method of measuring the spatial resolution to stimulate system with a pointpoint-spread function (PSF) ¬ Stationary: PSF constant over entire fieldfield-ofof-view (FOV) NonNon-stationary: PSF not const As each small area of the input signal to an image acts as a point stimulus, the output image is just the collection of these point stimuli convolved with the PSF (convolution = ) ¬ ¬ ¬ 15 Isotropic and nonnon-isotropic c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 264 16 Brent K Stewart, PhD, DABMP Image Quality – Bushberg Chapter 10 Diagnostic Radiology Imaging Physics Course 16 December 2004 Spatial Domain: the Point Spread Function Image Processing Based on Convolution ¬ ¬ stationary ¬ non-stationary Input(x,y) PSF(x,y) Output(x,y) Effect: blurring edges and fine detail* ¬ more regarding in the DR lecture c.f Bushberg, et al The Essential Physics of Medical c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 264 Imaging, 2nd ed., p 265 Brent K Stewart, PhD, DABMP 17 Convolution: Ch 11 – Digital Radiography and Ch 13 - CT Defined mathematically as passing a NN-dimensional convolution kernel over an NNdimensional numeric array (e.g., 2D image or CT transmission profile) At each location (x, y, z, t, ) in the number array multiply the convolution kernel values by the associated values in the numeric array and sum Place the sum into a new numeric array at the same location c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 312 18 Brent K Stewart, PhD, DABMP 10K 1K 100 10 Physical Mechanisms of Blurring ¬ ¬ ¬ ¬ ¬ ¬ Defocusing (lens) Optical diffusion (intensifying screen) Motion (involuntary, cardiac) Slice thickness (angled features) in tomography Focal spot blurring and magnification (Chapter 6) Other spread functions ¬ ¬ ¬ ¬ ¬ ¬ ¬ Line spread function (LSF) Edge spread function (ESF) LSF = d(ESF)/dx PSF = d(LSF)/dy ¬ c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., pp 266 and 268.19 Brent K Stewart, PhD, DABMP Brent K Stewart, PhD, DABMP Spatial Frequency Domain Temporal frequencies (middle A: ν = 440 Hz) that comprise a timetime-domain audio signal (t) Similarly the objects in an image (audio signal) can be thought of as the superposition of spatial frequencies For objects in an image that are separated by shorter distances (mm, x), these objects correspond to high spatial frequencies (cycles/mm, f) Square wave line pairs per mm (lp/mm) c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 269 20 Brent K Stewart, PhD, DABMP Image Quality – Bushberg Chapter 10 Diagnostic Radiology Imaging Physics Course 16 December 2004 Modulation Transfer Function: MTF(f) ¬ ¬ ¬ ¬ ¬ Modulation Transfer Function: MTF(f) Input constant amplitude sine waves of various frequencies (f) into an imaging system: what is the amplitude of the output wave? MTF(f) As f the MTF(f) Modulation is essentially the output contrast normalized by the input contrast Modulation vs spatial freq plot Complete description of the resolution properties ¬ Imaging chains: ¬ Image Intensifier Example ¬ ¬ ¬ ¬ ¬ ¬ c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., pp 270270-271 Brent K Stewart, PhD, DABMP ¬ ¬ ¬ ¬ Compute the MTF(f) from the LSF(x) using the Fourier Transform (FT) MTF(f) = |FT{LSF(x)}| FT an integral calculus operation that converts a spatial domain (x) signal into a spatial frequency (f) function As LSF width MTF faster Method used in daily practice: line pair phantom and star phantom for quick determination of lp/mm Huda 2nd Edition – Chapter – Image Quality Brent K Stewart, PhD, DABMP ¬ Screen-film resolution can best be improved by changing to: ¬ A Lower tube voltage B Slower film C Higher grid ratio D Thinner screens E Green sensitive film ¬ ¬ ¬ c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., pp 272272-273 23 Weakest link – item in imaging chain with worst MTF often determines the overall system, “dragging” dragging” the system down Brent K Stewart, PhD, DABMP ¬ Brent K Stewart, PhD, DABMP A: Optics MTF(f) B: Image intensifier MTF(f) C: Video camera MTF(f) System (f) = Optics MTF(f) * Image intensifier MTF (f) * Video camera MTF(f) c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 272 22 21 The LSF, MTF and Fourier Transform ¬ MTFtotal(f) = ∏ MTFi(f) Brent K Stewart, PhD, DABMP 24 Image Quality – Bushberg Chapter 10 Diagnostic Radiology Imaging Physics Course 16 December 2004 Huda 2nd Edition – Chapter – Image Quality Raphex 2003 Diagnostic Question ¬ D9 D9 The modulation transfer function (MTF) is a tool for describing the of an imaging system ¬ A Properties of the characteristic (H&D) curve B Sharpness C Noise content D Latitude ¬ ¬ ¬ Brent K Stewart, PhD, DABMP ¬ Spatial resolution cannot be assessed using: ¬ A Line pair phantom B LSF image C Full-width half maximum D MTF curve E Pixel standard deviation ¬ ¬ ¬ ¬ 25 Brent K Stewart, PhD, DABMP Davis Notes - Image Quality ¬ Referring to Figure (right), which demonstrates three different line spread functions (LSF), which LSF will yield the best spatial resolution? ¬ A LSF A B LSF B C LSF C ¬ ¬ Davis Notes - Image Quality ¬ 10 Referring to Figure which shows LSFs, and Figure which shows the corresponding modulation transfer functions (MTFs), which MTF corresponds to LSF C? ¬ A MTF number B MTF number C MTF number ¬ ¬ Brent K Stewart, PhD, DABMP Brent K Stewart, PhD, DABMP 26 27 Brent K Stewart, PhD, DABMP 28 Image Quality – Bushberg Chapter 10 Diagnostic Radiology Imaging Physics Course 16 December 2004 Davis Notes - Image Quality ¬ ¬ ¬ ¬ ¬ ¬ Noise 11 Referring to Figure illustrating MTFs, the axes should be labeled for the y-axis and for the x-axis: ¬ Noise injects a random or stochastic component into an image – many sources Definitions, first: ¬ Mean: ¬ Standard Deviation – measure of variability, either naturally occurring or random fluctuation: N ¬ A Relative amplitude, distance (mm) B Spatial frequency (lp/mm), distance (mm) C Lateral dimension (mm), Fresnel ratio D Relative amplitude, spatial frequency (lp/mm) E Relative amplitude, relative amplitude σ = Brent K Stewart, PhD, DABMP Gaussian (normal) distribution: G( x ) = ke ¬ ¬ ¬ 1⎛ x − x ⎞ − ⎜ ⎝ σ ⎟⎠ ∑(x − x ) i =1 i N −1 c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 273 30 Brent K Stewart, PhD, DABMP Poisson Probability Distribution Function ¬ Poisson distribution: Poisson Distribution m x −m P( x ) = e x! x and describe the shape ¬ Many commonly encountered measurements of people and things make for this kind of distribution (Gaussian) hence the term “normal” e.g., the height of 1000 third grade school children approximates a Gaussian ¬ ¬ ¬ ¬ c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 275 31 0.4 0.35 m = mean, shape governed by one variable P(x) difficult to calculate for large values of x due to x! X-ray and γ-ray counting statistics obey P(x) Used to describe ¬ Brent K Stewart, PhD, DABMP Brent K Stewart, PhD, DABMP N ∑ xi N i =1 29 Gaussian Probability Distribution Function ¬ x= 0.3 m=1 m=2 0.25 m=4 m=6 0.2 m=8 0.15 m=10 m=20 0.1 0.05 0 10 20 30 40 Radioactive decay Quantum mottle Brent K Stewart, PhD, DABMP 32 Image Quality – Bushberg Chapter 10 Diagnostic Radiology Imaging Physics Course 16 December 2004 Probability Distribution Functions ¬ Probability of observing an observation in a range: integrate area (for G): ¬ ¬ ¬ ¬ ¬ ¬ Quantum Noise ¬ ¬ ¬ = 68.25% 1.96 = 95% 2.58 = 99% ¬ ¬ Error bars and confidence intervals P(x) very similar to G(x) when ≈ √x use G(x) as approx Can adjust the noise ( ) in an image by adjusting the mean number of photons used to produce the image c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., pp 276 - 277 33 ¬ ¬ ¬ ¬ ¬ N = mean photons/unit area = √N, from P(x) (variance) = N P(x) Relative noise = coefficient of variation = /N = 1/√ 1/√N ( with N) SNR = signal/noise = N/ = N/√ N/√N = √N ( with N) TradeDose 4x Trade-off between SNR and radiation dose: SNR 2x c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 278 34 Brent K Stewart, PhD, DABMP Brent K Stewart, PhD, DABMP Contrast Resolution Noise Frequency Ability to detect a lowlowcontrast object Related to how much noise there is in the image SNR As SNR the CR Rose criterion: SNR > to reliably identify an object Quantum noise and structure noise both affect the conspicuity of a target ¬ ¬ ¬ ¬ ¬ c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 281 35 Brent K Stewart, PhD, DABMP Brent K Stewart, PhD, DABMP Although noise appears random, the noise has a frequency distribution Example: ocean waves Take a flatflat-field xx-ray image (still has noise variations) Fourier Transform (FT) the flat image Noise Power Spectrum: NPS(f) NPS(f) is the noise variance ( 2) of the image expressed as a function of spatial freq (f) c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 282 36 Brent K Stewart, PhD, DABMP Image Quality – Bushberg Chapter 10 Diagnostic Radiology Imaging Physics Course 16 December 2004 Huda 2nd Edition – Chapter – Image Quality Detective Quantum Efficiency (DQE) ¬ ¬ ¬ DQE: metric describing overall system SNR performance and dose efficiency SNRout DQE = SNRin2 SNR2in = N ( ¬ 15 Which of the following is not true for Poisson distributions? ¬ A They are used to describe radioactive decay B They are used to describe quantum mottle C The variance is equal to the mean D They are always symmetrical E They are approximate to a Gaussian for means greater than 10 ¬ ¬ SNR = √N) ¬ [MTF (f )] ¬ SNR2out = ¬ NPS(f ) k [MTF (f )] DQE(f) = N ⋅ NPS(f ) ¬ DQE(f=0) = QDE c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 282 37 Brent K Stewart, PhD, DABMP Brent K Stewart, PhD, DABMP Raphex 2000 Diagnostic Question Huda 2nd Edition – Chapter – Image Quality ¬ D9 The degree of mottle in a screenscreen-film xx-ray image is: ¬ A Usually determined by the number of primary xx-ray photons absorbed in the film B Independent of the mAs C An inherent property of the AgBr structure in the film emulsion D Increased by increasing the film speed E Measured in line pairs per millimeter (lp/mm) ¬ ¬ ¬ ¬ Brent K Stewart, PhD, DABMP Brent K Stewart, PhD, DABMP ¬ 17 The speed of screen/film can be increased without increasing noise by: ¬ A Using a faster film B Using phosphor with a higher conversion efficiency C Increasing the processor developer temperature D Increasing the phosphor absorption efficiency E Decreasing screen thickness ¬ ¬ ¬ ¬ 39 Brent K Stewart, PhD, DABMP 38 40 10 Image Quality – Bushberg Chapter 10 Diagnostic Radiology Imaging Physics Course 16 December 2004 Huda 2nd Edition – Chapter – Image Quality Davis Notes - Image Quality ¬ A student technologist has a cadaver on the table and is practicing manual technique factors for a digital photospot system that is not phototimed Circle each of the following techniques that will result in higher quantum mottle in the image: ¬ 19 Image contrast-to-noise ratio could not be increased by using: ¬ A Lower tube voltages B Higher-ratio grids C Larger x-ray beam areas D Screens with lower conversion efficiency E Slower films ¬ ¬ ¬ ¬ ¬ ¬ A Higher kVp, higher mAs B Lower mAs, same kVp C Higher mAs, same kVp D Higher kVp, same mAs E Lower kVp, lower mAs Brent K Stewart, PhD, DABMP ¬ ¬ ¬ 41 Brent K Stewart, PhD, DABMP Raphex 2000 Diagnostic Question ¬ Sampling and Aliasing in Digital Images D8 Low contrast detectability refers to the ability of a system to distinguish: ¬ Array of detector elements ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ A A calcified lung nodule B A nonnon-calcified lung nodule C Between overlying and underlying tissues D The size of a small fracture E Vessels during the arterial phase of a normal angiogram ¬ ¬ ¬ ¬ Brent K Stewart, PhD, DABMP Brent K Stewart, PhD, DABMP 42 43 Sampling (pixel) pitch Detector aperture width The spacing between samples determines the highest frequency that can be imaged Nyquist frequency: FN = 1/2∆ 1/2∆ If a frequency component in an sampled < image > FN 2x/cycle: aliasing Wraps back into the image as a lower frequency Moiré Moiré pattern, spoke wheels c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 284 44 Brent K Stewart, PhD, DABMP 11 Image Quality – Bushberg Chapter 10 Diagnostic Radiology Imaging Physics Course 16 December 2004 Aliasing due to Reciprocating Grid Failure Sampling and Aliasing in Digital Images Example: sampling pitch of 100 µm FN = cycles/mm When input f > FN then the spatial frequency domain signal at f is aliased down to: fa = 2F 2FN – f Not noticeable with patient Antiscatter grids Aperture blurring - signal averaging across the detector aperture sin(aπ f) MTF(f)=FT{rect(a)}=sinc(af)= aπ f ¬ ¬ ¬ ¬ ¬ ¬ Brent K Stewart, PhD, DABMP ContrastContrast-Detail (C(C-D) Curves ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ Brent K Stewart, PhD, DABMP Receiver Operating Characteristic (ROC) Curves ¬ Spatial resolution: MTF(f) Contrast resolution: SNR Combined quantitative: DQE(f) Qualitative: CC-D curve C-D phantom: holes in plastic of depth and diameter What depth hole at which diameter can just be visualized Connect the dots C-D line Better spatial resolution: highhighcontrast, small detail Better contrast resolution: lowlowcontrast ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 287 47 Brent K Stewart, PhD, DABMP Brent K Stewart, PhD, DABMP c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., pp 285285-286 46 45 Diagnostic task: separate abnormal from normal Usually significant overlap in histograms Decision criterion or threshold Based on threshold: either normal (L) or abnormal (R) N cases: x decision matrix TPF= TP/(TP+FN)= Sensitivity FPF = FP/(FP+TN) Specificity = (1(1-FPF) = TNF ROC curve: sensitivity vs 1-specificity usu @ five threshold levels c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., pp 288288-289 48 Brent K Stewart, PhD, DABMP 12 Image Quality – Bushberg Chapter 10 Diagnostic Radiology Imaging Physics Course 16 December 2004 ROC Questionnaire: Point Confidence Scale Receiver Operating Characteristic Curves ¬ ¬ ¬ ¬ ¬ Brent K Stewart, PhD, DABMP The ROC curve is essentially a way of analyzing the SNR associated with a specific diagnostic task Az: area under the curve – concise description of the diagnostic performance of the systems (including observers) being tested Measure of detectability Az = 0.5 guessing Az = 1.0 perfect c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 291 50 49 Brent K Stewart, PhD, DABMP Huda 2nd Edition – Chapter – Image Quality Huda 2nd Edition – Chapter – Image Quality ¬ 21 Sensitivity is given by: ¬ 22 Specificity is given by: ¬ A False positive fraction (FPF) B True positive fraction (TPF) C False negative fraction (FNF) D True negative fraction (TNF) E Area under ROC curve (Az) ¬ A The true-negative fraction (TNF) B The true-positive fraction (TPF) C (1 – TPF) D (1 + TNF) E Area under the ROC curve (Az) ¬ ¬ ¬ ¬ Brent K Stewart, PhD, DABMP Brent K Stewart, PhD, DABMP ¬ ¬ ¬ ¬ 51 Brent K Stewart, PhD, DABMP 52 13 Image Quality – Bushberg Chapter 10 Diagnostic Radiology Imaging Physics Course 16 December 2004 Huda 2nd Edition – Chapter – Image Quality Davis Notes - Image Quality ¬ 23 A ROC curve is used to measure diagnostic imaging: ¬ A Performance B Accuracy C Specificity D Sensitivity E Cost benefit ratio ¬ ¬ ¬ ¬ ¬ 26 In Figure 5, showing an ROC curve, the X-axis should be labeled (circle all that are correct): ¬ A True Positive Fraction B False Positive Fraction C Sensitivity D Specificity E – Specificity ¬ ¬ ¬ ¬ Brent K Stewart, PhD, DABMP 53 Brent K Stewart, PhD, DABMP Davis Notes - Image Quality ¬ ¬ ¬ ¬ ¬ ¬ Davis Notes - Image Quality 27 In Figure showing the ROC curves, the Y-axis should be labeled (circle all that are correct): ¬ ¬ A True Positive Fraction B False Positive Fraction C Sensitivity D Specificity E – Specificity Brent K Stewart, PhD, DABMP Brent K Stewart, PhD, DABMP 54 ¬ 55 C 28 Curve letter _ represents pure guessing 29 Curve letter _ A represents the best diagnostic approach D 30 Curve letter _ represents an Az value of about 0.3 Brent K Stewart, PhD, DABMP 56 14 Image Quality – Bushberg Chapter 10 Diagnostic Radiology Imaging Physics Course 16 December 2004 Huda 2nd Edition – Chapter – Image Quality End of Lecture, Additional Questions Follow ¬ 16 Quantum mottle is determined primarily by which of the following factors? ¬ A X-ray beam filtration B X-ray photons absorbed in screen C X-ray photon energy D Screen conversion efficiency E Screen thickness ¬ ¬ ¬ ¬ Brent K Stewart, PhD, DABMP 57 Brent K Stewart, PhD, DABMP Huda 2nd Edition – Chapter – Image Quality Raphex 2003 Diagnostic Question ¬ D5 Which of the following will increase the image contrast that is due to the screenscreen-film image receptor? ¬ A Decreasing the grid ratio B Decreasing the kVp C Increasing the developer temperature D Increasing the focal spot size E Increasing xx-ray beam filtration ¬ ¬ ¬ ¬ Brent K Stewart, PhD, DABMP Brent K Stewart, PhD, DABMP 58 ¬ Gastrointestinal tract contrast could be improved by all of the following except: ¬ A Infusion of barium B Reduced tube voltage C Increased tube current D Increased grid ratio E Reduced field size ¬ ¬ ¬ ¬ 59 Brent K Stewart, PhD, DABMP 60 15 Image Quality – Bushberg Chapter 10 Diagnostic Radiology Imaging Physics Course 16 December 2004 Davis Notes - Image Quality Davis Notes - Image Quality ¬ 12 In a 9” (23 cm) image intensifier with a 0.9” (2.3 cm) output phosphor, a resolution cell measured at the input plane is 0.5 mm At the output phosphor, the resolution cell dimension is now ¬ A 0.50 mm B No change C mm D 50 mm E 50 àm ơ ơ Referring to Figure showing three line spread functions, the best choices for the axes labels are for the y-axis and _ for the x-axis: ¬ A Frequency, amplitude B Blur distance (mm), frequency C Relative amplitude, frequency D Relative amplitude, distance (mm) E Distance (mm), relative amplitude ¬ ¬ ¬ ¬ Brent K Stewart, PhD, DABMP 61 Brent K Stewart, PhD, DABMP Huda 2nd Edition – Chapter – Image Quality ¬ 18 The major contributor to noise in a fluoroscopic image is variations in the: ¬ A Input phosphor thickness B Accelerating tube voltage C Output phosphor thickness D Display screen brightness E Quantum mottle ¬ ¬ ¬ ¬ Davis Notes - Image Quality ¬ 13 For a non-phototimed system, a technologist gets a properly exposed film but it lacks good bone contrast They decide to increase the mAs by a factor of 2, allowing them to reduce the kVp If they was originally at around 100 kVp, what kVp should the technologist choose to get a properly exposed film of the same patient? ¬ A 115 kVp B 85 kVp C 70 kVp D 50 kVp E Need technique charts to determine ¬ ¬ ¬ ¬ Brent K Stewart, PhD, DABMP Brent K Stewart, PhD, DABMP 62 63 Brent K Stewart, PhD, DABMP 64 16 Image Quality – Bushberg Chapter 10 Diagnostic Radiology Imaging Physics Course 16 December 2004 Huda 2nd Edition – Chapter – Image Quality Davis Notes - Image Quality ¬ Assume that a properly calibrated phototimed (automatic exposure control) system is used to produce a clinical radiograph Increased quantum mottle will occur with: ¬ 11 Poor screen/film contact will primarily result in a significant loss of: ¬ A Contrast B Magnification C Image detail D X-ray absorption efficiency E Conversion efficiency ¬ ¬ ¬ ¬ ¬ A A thinner screen B A screen-film using a reflective layer in the screens C A thinner patient D Faster film, same screen Brent K Stewart, PhD, DABMP ¬ ¬ ¬ 65 Brent K Stewart, PhD, DABMP Huda 2nd Edition – Chapter – Image Quality Huda 2nd Edition – Chapter – Image Quality ¬ 25 In screen/film radiography, raising the kilovolt peak will increase all of the following except: ¬ 13 Which of the following factors would have the least effect on image sharpness? ¬ A Half-value layer B Scatter C Patient transmission (%) D Subject contrast E Grid penetration ¬ A Film type B Focal spot size C Motion D Screen thickness E Screen/film contact ¬ ¬ ¬ ¬ Brent K Stewart, PhD, DABMP Brent K Stewart, PhD, DABMP 66 ¬ ¬ ¬ ¬ 67 Brent K Stewart, PhD, DABMP 68 17 Image Quality – Bushberg Chapter 10 Diagnostic Radiology Imaging Physics Course 16 December 2004 Huda 2nd Edition – Chapter – Image Quality ¬ Davis Notes - Image Quality ¬ 10 Compared with a regular screen, a detail screen of the same phosphor will have a lower: ¬ ¬ ¬ ¬ ¬ ¬ A Spatial resolution B Speed C Noise level D Conversion efficiency E Linear attenuation coefficient Brent K Stewart, PhD, DABMP ¬ ¬ ¬ 69 Brent K Stewart, PhD, DABMP Davis Notes - Image Quality ¬ ¬ ¬ ¬ ¬ ¬ ¬ D8 Geometric magnification can improve the detection of high contrast objects The fundamental limitation on useful magnification is: ¬ A Blurring due to focal spot size B Blurring due to removal of the grid C H&D curve of the image receptor D MTF of the image receptor E Size of the image receptor ¬ A Higher kVp, higher mAs B Lower mAs, same kVp C Higher mAs, same kVp D Higher kVp, same mAs E Lower kVp, lower mAs Brent K Stewart, PhD, DABMP 70 Raphex 2001 Diagnostic Question A student technologist has a cadaver on the table and is practicing manual technique factors for a digital photospot system that is not phototimed Which techniques would result in a higher absorbed dose to the cadaver? : Brent K Stewart, PhD, DABMP 15 MTF number demonstrates the best spatial resolution 16 MTF number is probably for an image intensifier (9” (9” II in 9” 9” mode) 17 MTF number is probably for a general screenscreenfilm system has 18 MTF number _ a maximum resolving power of about 100 mm has 19 MTF number _ a cutcut-off resolution of about lp/mm ¬ ¬ ¬ 71 Brent K Stewart, PhD, DABMP 72 18 Image Quality – Bushberg Chapter 10 Diagnostic Radiology Imaging Physics Course 16 December 2004 Huda 2nd Edition – Chapter – Image Quality Raphex 2002 Diagnostic Question ¬ D12 A newly installed bucky radiographic system produces abdominal images that are of acceptable density over the spine and progressively lighter toward both lateral edges of the film The most likely reason for this finding is improper: ¬ 14 The MTF is not: ¬ A A description of any imaging system resolution performance B The ratio of image to subject contrast at each spatial frequency C Equal to the unity when the spatial resolution is perfect D Usually lower at high spatial frequencies E Fifty percent at half the limiting spatial resolution ¬ ¬ ¬ ¬ ¬ ¬ A Collimator tracking B Focal distance for grid C Grid ratio D kVp calibration of the system E Programming of the AEC system Brent K Stewart, PhD, DABMP ¬ ¬ ¬ 73 Brent K Stewart, PhD, DABMP 74 Raphex 2002 Diagnostic Question ¬ D13 The impression of noise in an x-ray image is: ¬ A Increased by increasing the film speed in a screenfilm cassette B Decreased by increasing the film speed in a screenfilm cassette C Increased by decreasing the focal-spot size D Decreased by decreasing the focal-spot size E Mainly determined by imperfections in the-image receptor ¬ ¬ ¬ ¬ Brent K Stewart, PhD, DABMP Brent K Stewart, PhD, DABMP 75 19