Interaction map. X-Ray Computed Tomography Measures Tissue Properties from Macro to Micro. Outline

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1 X-Ray Computed Tomography Measures Tissue Properties from Macro to Micro Michael Andre, Ph.D. Department of Radiology University of California, San Diego San Diego VA Healthcare System Outline Perspective on Computed Tomography (image reconstruction from projections) Scientific basis and timeline Evolution of medical design Principles of image reconstruction Current medical CT scanner design and applications System performance and image display Patient Dose Reduction and Dose Reports Artifacts 1 2 Conventional Radiography: 2D map of 3D object Goals of CT 2D image of 2D object without superposition Uniform object contrast Uniform contrast sensitivity Calibrated image Structure and function 2D map of x-ray attenuation (e -µx ) Superposition Distortion due to non-uniform magnification Non-uniform exposure Widely varying image contrast 3 Cross-sectional anatomy was a new challenge to the medical community Reconstruction from Projections Transmission Emission Transmission Computed Tomography ( Image Reconstruction ) Interaction map I 0 I X-Ray, microwave, ultrasound, optical p r, r, SPECT, PET, MRI f ( x, y) ds 5 Assumed conditions for x-ray CT: Straight-line propagation Monoenergetic x-ray beam 6 1

2 Sum attenuation coefficients Solution: We need more views Incident Intensity I 0 Transmitted Intensity I Interaction Map: I = I 0 e -µx X μ/ρ = Mass attenuation coefficient (cm 2 /gm) Select x for desired resolution and FOV 7 Equivalent transmitted intensity 8 Goals of CT 2D image of 2D object without superposition Uniform object contrast Uniform contrast sensitivity Calibrated image Structure and function 9 Cross-sectional anatomy was a new challenge to the medical community First Commercial CT Scanner EMI Limited, 1973 First Application: Head Trauma Translate I 0 48x64 512x512 I Rotate First Generation Scanner Translate-Rotate design min to acquire singe slice Head scan only 13 min scan, 13 min 2 nd Generation: 30 sec scan, Rigid head holder and water path 11 reconstruction per slice! 1 min reconstruction 12 2

3 Second Generation Scanner Third Generation Scanner Geometry Tube and detector array rotate together Translate-Rotate design Complete detector profile from single x-ray tube pulse More detectors so fewer angles needed sec rotation speed to acquire single slice 30 sec to acquire singe slice All current CT scanners use this design geometry Capable of body imaging with breath hold 13 Number and size of detector elements limits resolution Width of fan beam determines Field of View (FOV) 14 Fourth Generation Scanner Electron Beam CT Tube rotates, detectors stationary Allows oversampling in rotation to increase resolution significantly 1 sec to acquire singe slice 15 No mechanical motion Single slice in msec 16 EBCT Image Reconstruction Methods in Medicine Required two adjacent rooms to house system Remarkable cardiac images, poor for everything else Doomed by the advent of multi-slice helical scanners 17 I. Analytic Methods a. Filtered back-projection (Convolution, Radon, Fourier filtering) 1) Dominant method for decades 2) Very fast reconstruction b. Two- or three-dimensional Fourier reconstruction 1) MRI II. Iterative Numerical Methods a. Slower but more accurate ( X longer than FBP) b. Significant dose reduction c. Simultaneous Iterative Reconstruction Technique (SIRT) d. Algebraic Reconstruction Technique (ART) e. Iterative Least Squares Technique (ILST) 3

4 Backprojection Concept Single scan Object Detector signal profile Detector View = detector profile 19 X-ray tube Object Backprojection of two profiles for rectangular object. For a given point, reconstructed density is the sum of all ray projections that pass through it. 20 Simple Backprojection Concept Backproject multiple detector profiles Superimposing each backprojection leads to messy artifacts! First Medical Backprojection Improved Solution: Filtered Backprojection Single scan Filtered detector signal Detector BP Video 23 X-ray tube Backproject filtered profiles for rectangular object (Shepp-Logan filter) 24 4

5 Filtered Backprojection is a Fourier Transform Method Points outside the object receive positive and negative contributions from backprojected views to cancel out artifacts 25 P k F k k F ˆ (, ) ( x, y ) ( k x, k y ) k k R km x x p *( x ) km p( x ) p( x ) sin 2 dx 2 2 R x x p(x ) is measured profile, p*(x ) is filtered profile, R = max radius of object, k m represents high frequency cut-off, sin 2 reduces to 0 or 1, simple program results Fourier coeffs. of Back-Projected image are equal to the exact Fourier coeffs F(k x,k y ) divided by the frequency One can compute Back-projected image, take 2-D F.T. and multiply by k (magn of spatial freq) to reconstruct new image Projections p are filtered to obtain p* which are then backprojected Ideally this should be exact reconstruction Very fast since reconstruction can start the moment the first projections are 26 acquired Shepp-Logan filters are used in FBP for X-Ray CT Image Reconstruction Methods in Medicine Lak works best in absence of noise (never so in x-ray). S-L has high freq roll-off. H has extreme roll-off with more noise suppression. 27 I. Analytic Methods a. Filtered back-projection (Convolution, Radon, Fourier filtering) 1) Dominant method for decades 2) Very fast reconstruction b. Two- or three-dimensional Fourier reconstruction 1) MRI II. Iterative Numerical Methods a. Slower but more accurate ( X longer than FBP) b. Significant dose reduction c. Simultaneous Iterative Reconstruction Technique (SIRT) d. Algebraic Reconstruction Technique (ART) e. Iterative Least Squares Technique (ILST) Illustration of Iterative Reconstruction (ART) Now in use for lower dose, quantitative multi-energy reconstructions, artifact suppression Vary: 1) number of samples (rays) in each view 2) number of views (detector profiles) per rotation Object FOV 1 X,Y = rays or samples View = detector profile

6 FOV Object Samples vs. Angles Reconstruction Filters X,Y = rays or samples View = detector profile Common to set pixel size to ½ the desired spatial resolution of the scanner (i.e., FWHM). Following the Nyquist Sampling Theorem, need 2 samples/pixel Example: 0.5 mm pixel for FWHM of 1.0 mm. Number of measurement angles needs to be /2 times the number of samples per x-ray pulse for a fan beam Example Want 25 cm FOV to achieve 1.0 mm FWHM requires 25 cm x 10 mm/cm x 2 samples/mm = 500 samples The number of angles of view = /2 x 500 = 785 angles 25 cm FOV, 512x512 matrix gives (25/512) ~0.5 mm pixel Additional measurements are made to improve image quality, times as many measurements as image pixels Effect of Filter on MTF In CT (and other digital imaging modalities), image postprocessing can do all of the following except: A. Reduce the appearance of noise B. Enhance the appearance of edges C. Reduce artifacts D. Reduce appearance of motion blur E. Extend dynamic range Why use less sharp filter? To reduce impact of noise! The Sinogram and Backprojection In CT (and other digital imaging modalities), image postprocessing can do all of the following except: A. Reduce the appearance of noise B. Enhance the appearance of edges C. Reduce artifacts D. Reduce appearance of motion blur E. Extend dynamic range Dynamic range is determined by the acquisition system. Post-processing cannot extend data that were never recorded

7 CT Number (HU) Sinogram A stack of all measured 1D profiles that are then spatially filtered and backprojected Rotation Computer Reconstruction Output: 2D Map of µ (5120 or more numbers) Video Translation 37 For human image perception: 2D Map of Display Brightness with limited gray scale 38 CT Numbers (Hounsfield Unit) t w HU 1000 µ w = Linear atten coeff of water µ t = Linear atten coeff of tissue w CT Numbers (HU) Tissue Compact bone Cancellous bone Liver Muscle Kidney Water Fat Lung Air CT # (HU) Scale for medical scanners: to 4096 HU Which is true? In a CT image, the CT number: A. Of a material is dependent on its linear attenuation coefficient B. Increases if the window width is increased C. Has units of per cm D. Is reduced if the image matrix is changed from 512x512 to 320x320 E. Increases with mas CT Number (HU) Calibration Highly linear for a modest range In a CT image, the CT number: 1500 A. Of a material is dependent on its linear attenuation coefficient B. Increases if the window width is increased C. Has units of per cm D. Is reduced if the image matrix is changed from 512x512 to 320x320 E. Increases with mas HU have no dimensional units, they are a ratio. Other factors have no effect on HU Relative Attenuation (Density) 42 7

8 Window Level / Window Width On a CT scanner, if the Hounsfield unit for water is zero and for air is -1024, the HU for cortical bone, fat, lung and muscle are typically (in order): 256 grey levels is typical Linear transformation Non-linear look-up tables are rarely if ever used in CT A. 1600, 30, -600, -100 B. -600, 30, 1600, -100 C. 1600, -100, -600, 30 D. -600, -100, 30, On a CT scanner, if the Hounsfield unit for water is zero and for air is -1024, the HU for cortical bone, fat, lung and muscle are typically (in order): Innovations have Launched a CT Revolution in Medicine A. 1600, 30, -600, -100 B. -600, 30, 1600, -100 C. 1600, -100, -600, 30 D. -600, -100, 30, 1600 Bone is highest HU of any tissue. Fat has slightly lower density and µ than water while muscle is slightly higher. Lung has high air content, low density and µ. 45 Continuous rotation slip ring gantries Multi-detector multi-slice scanning Dose reduction techniques Iterative reconstuctions Multi-energy scanners 46 Helical CT Scanner Continuous rotation Video Contiguous slices permit multi-planar reformatting May not have isotropic voxels Tube and detector array rotate together continuously (slip ring) Complete detector profile from single x-ray tube pulse sec rotation speed All current CT scanners use this design geometry Number and size of detector elements determines resolution Width of fan beam determines Field of View (FOV)

9 Volumetric Rendering Assign colors to tissues, segment by HU Variable transparency Some tissues are difficult to segment uniquely Pretty pictures but 3D not widely adopted in clinical practice until very recently First applications used Pixar animation computer Pitch = d/w = 1 as depicted w d 49 table movement 50 Multi-slice Multi-Detector CT Scanner MDCT Pitch 4- to 320-detector arrays 51 Pitch = Table Speed / (Beam Width = Detector size x # Detectors) Increasing pitch reduces dose, increases blurring and partial volume What is the Pitch? Table Speed = mm/rotation 64 x mm = 40 mm Detector Width = mm Number of Detectors = 64 Pitch = / 40 = Dynamic Volume CT Toshiba Detector Evolution Scintillator on Photodiode array (99% absorption efficiency) 16 cm coverage per rotation Aquilion ONE 0.5mm x X 0.5mm detector elements 350 msec rotation time cm of coverage 650 lb patient couch 53 9

10 Cardiac: Whole heart cardiac perfusion in one beat Virtual CT Colonography Video of polyp 2D 3D LAD Stenosis Courtesy of, Dr Katada & Dr Anno, Fujita Polyp 56 Computed Tomography is Quantum Noise Limited The smallest image contrast difference (ΔCT#) that can be detected is ~ 0.25%. But we almost never operate the scanner at this setting due to excessive patient dose. All medical x-ray imaging should be quantum noise limited using the minimum dose required to address the medical question mas 80 mas CT phantom low contrast module imaged under two different noise conditions on the same scanner with: 1 1 Quantum Noise mas Dose Therefore, if the mas is reduced by 1/3, then noise should increase by 3 =1.73 (73% increase). 58 Contrast Factors (Noise) X-Ray flux (dose, mas) Slice thickness X-ray scatter rejection at the detector Computational noise Low-frequency response of convolution filter Reconstruction interpolation Machine round-off error X-ray Exposure (mas or Dose) and Noise 10 ma 40 ma 160 ma 640 ma Noise Standard deviation (HU) σ% = x Example σ% = ±5 on ±1000 HU scale σ% = 5 x 100/1000 = ±0.5% 59 Quantum Noise Mean: HU SD: 18.9 HU 10 ma 64 = 8 1 mas 1 Dose 640 ma Mean: HU SD: 3.1 HU 60 10

11 61 Assumes we wish to maintain same noise value per pixel 62 Public Concern with Patient Doses in CT Patient Doses in CT October 15, 2009: Radiation Overdoses Point Up Dangers of CT Scans For reasons not yet fully understood, the X-ray technologist... activated the CT scan 151 times on the same area. December 8, 2009: More Radiation Overdoses Reported The number of hospitals where suspected stroke patients were over-radiated while undergoing CT scans has risen to three in California, with an unconfirmed case at a fourth hospital in Alabama, Alabama Brain Perfusion Patient Threshold for epilation 3 Sv Typical CT Perfusion today 12 msv EDE Natural background 3 msv/yr 64 Radiation Sievert is the unit of effective dose, which accounts for the tissue absorbed dose (Gy), type of radiation, relative sensitivity of the organs exposed (conversion factor k) Natural background radiation from all sources: 3 msv/year The effective dose of a single abdomen and pelvis CT scan is greater than three times that of a year of background radiation The most radiosensitive organs are red bone marrow, lungs, breast, stomach, colon Medical radiation is associated with increased risk of malignancy and is much higher in infants and children EPA Radon Map of US (red is 2X yellow) Radiation in Nature Source Radon Average Natural Background Radiation Cosmic Rays Gamma Rays (Soil Radionuclides) Internal Radionuclides ( 40 K, 14 C) Total Transcontinental Flight 0.03 msv Level 2 msv/year 0.3 msv/year 0.3 msv/year 0.4 msv/year ~3 msv/year Example of Non-Medical Radiation Centers for Disease Control and Prevention, Healthy Housing Reference Manual, Chapter Walter Huda, Review of Radiological Physics, Chapter 8, Radiation Protection 66 11

12 Patient Doses from CT Average Annual Effective Dose to the US Population Early 1980s 2006 Average Annual Effective Dose from All Sources = 6 msv Medical = 50% A major reason for this increase is greater utilization of CT imaging. Background Medical Background Medical CT High priority effort in Radiology to reduce all patient doses has been successful especially in pediatrics Medical Sources of Radiation DEXA scan The effective dose of a single abdomen and pelvis CT scan is greater than three times that of a year of background radiation (what is that number again?) Adapted from ICRP Publication 102: Managing Patient Dose in Multi-Detector Computed Tomography (MDCT), 102 Annals of the ICRP Volume 37/1, Chapter 4 (2007) Patient Dose Reports in CT Dose Indices in CT: CT Dose Index (volumetric) Dose-Length Product CTDI VOL and DLP indicate measured doses to 100 mm ion chamber in a cylindrical plastic phantom CT head phantom, 16 cm CT body phantom, 32 cm Not a measured dose to the patient

13 Images CT Dose Report Definitions CTDIvol mgy DLP mgy cm Dose Eff % Phantom cm Head Body Body Body 32 Projected series DLP: Accumulated exam DLP: mgy cm mgy cm Scanner Display CTDIvol (mgy) Weighted avg measurement in phantom w/pencil chamber DLP (mgy cm) CTDIvol x scan z-axis length Dose Efficiency (%) Measure of z-axis beam usage Projected Series DLP Based on tabulated measures Accumulated Exam DLP Sum of series DLPs 73 CT Exam Methods to Reduce CT Dose Reference CTDIvol Notification CTDIvol Typical Dose Equiv Head 75 mgy 80 mgy 2 msv Adult Abdomen 25 mgy 50 mgy 8 msv Pediatric Abdomen (5 yo) 20 mgy 25 mgy 2 msv Brain Perfusion 500 mgy 600 mgy 12 msv Background Radiation Dose 3.1 msv/yr Reduce ma Dose is proportional to ma Use adaptive ma (less for thinner pats or body part) Results in more Reduce kvp for thinner patients/pediatrics noise Dose increases exponentially with kvp Increase pitch, gantry speed or detector aperture Use iterative reconstruction Reduce number of series -- Do you really need pre-contrast? Angle gantry to avoid direct exposure of eyes, breast, gonads 74 Automatic Tube Current Modulation 3D Automatic Tube Current Modulation Decreasing Dose Fixed ma technique uses constant dose: too low or too high. Auto-mA modulation may be vulnerable to user error. Four adjustments per rotation: Sectors 1-4 Noise index and slice thickness determine ma and thus dose. Reducing slice thickness without adjusting the noise index can cause enormous patient doses. Positioning is critical Iterative Reconstruction to Reduce Dose and Noise Image pairs were generated from the same raw CT data Adaptive Statistical Iterative Reconstruction (ASIR) 87 mas 2.6 msv Original FBP Interative 20 mas.68 msv 77 X-Ray System Optics FBP ignores the geometry of the focal spot and detector Assumed to be a point beam Interaction of beam and detector cell assumed to take place at geometric center (pencil-beam) Thibault JB, Sauer KD et al. A three-dimensional statistical approach to improved image quality for multislice CT. Med. Phys (11)

14 Adaptive Statistical Iterative Reconstruction (ASIR) 1 ASIR 10% ASIR System Optics Most time consuming portion (> x FBP) Improved spatial resolution System Statistics Affects noise of resulting image ASIR 100% High ASIR level decreases noise, and moderately degrades spatial resolution (increases blur) 1. ent.obj 2. Thibault JB, Sauer KD et al. A three-dimensional statistical approach to improved image quality for multislice CT. Med. Phys (11) Illustration of Iterative Reconstruction (ART) Now in use for lower dose, quantitative multi-energy reconstructions, artifact suppression Recall speed and accuracy depend on quality of the starting estimate Latest Advance: Dual-Source, Dual-Energy Very fast scans (75 msec temporal resolution) Very low dose possible (<1 msv) Accurate measures of attenuation coefficient Physics-based Iterative reconstruction to reduce artifacts X-ray beam filter 80 kv kv Dual-Energy CT 80 kvp 120 kvp Why Dual Source, Dual-Energy? 80 kv 120 kv overlap 120 kv 83 Remarkable Applications! Chemical tissue analysis Quantitative lung perfusion Bone removal, bone mineral Vascular imaging Aortic dissection w/ stent Fast trauma diagnosis 84 14

15 Dual Energy, Multi-Spectral Reconstruction 85 CT Performance Summary Spatial Resolution ~0.25 mm maximum (isotropic) in smallest FOV Micro CT capable of mm resolution Contrast Detectability 0.25% (ΔHU) Quantum noise limited (dose), iterative methods help May require iv contrast media (Iodine, Xenon) for many soft tissues 0.25 sec minimum rotation speed EKG gating allows shorter time intervals for image formation 75 msec temporal resolution with Dual E-Dual Source Patient Dose: <1 to 500+ msv Potential for high doses (1 year background radiation ~ 3 msv) 86 CT Image Artifacts Motion artifact due to peristalsis of a very high contrast interface (air-tissue) Originate in Patient Motion Missing rays due to radio-opaque metal Partial volume Edge ringing Originate in Scanner Detector failure or out of calibration ( detector ring ) Beam hardening Other stationary high contrast edges do not show this effect X-Ray Beam is not Monoenergetic Lower energy x-rays preferentially absorbed/scattered Beam hardening Energy spectrum New clinical tool: Iterative reconstruction and/or dual- or multi-energy CT for tissue-specific imaging and quantitative tissue characterization 89 Beam hardening is greater with higher attenuating materials Small motions cause blur, large motions may produce ghosts 90 15

16 Iterative reconstruction techniques reduce metal artifacts Partial Volume Artifact In this example it produces phantom lesion in liver More pronounced with thicker slices May vary with location in the FOV (center vs. edge) Streak artifact from biopsy needle outside the FOV during CTguided abdominal biopsy Shading from missing data in shadow of biopsy needle Iterative reconstruction, Needle tip visible Artifacts due to Equipment Failure Video Ring artifact due to defective detector that interrupted x-ray detection Ring-like artifact due to x-ray arc that interrupted x-ray output

17 For which CT exam would the following settings be appropriate: 80 kvp, 0.5 sec, 3 mm slice, pitch = 1.3? A. Large adult abdomen B. Small pediatric abdomen C. Adult head D. Pediatric head E. High resolution chest For which CT exam would the following settings be appropriate: 80 kvp, 0.5 sec, 3 mm slice, pitch = 1.3? A. Large adult abdomen B. Small pediatric abdomen C. Adult head D. Pediatric head E. High resolution chest Sievert is a unit of: A. Exposure B. Effective dose C. Absorbed dose D. Dose rate E. Energy Low kvp is used with small pediatric patients to reduce dose. Pediatric risk of cancer may increase 25% with CT. Low kvp is inappropriate for larger adult patients due to excessive quantum noise. Typical adult abdomen = 120 kvp, 300 mas. Chest requires ~1 mm Sievert is a unit of: A. Exposure B. Effective dose (or effective dose equivalent) C. Absorbed dose D. Dose rate E. Energy Which of the following situations does not reduce the dose to the patient? A. Reducing ma, with no changes to kvp and pitch B. Increasing the ma by 10% and increasing the pitch by 20% C. Reducing kvp, use Auto-mA to maintain noise, no change to pitch D. Increasing the kvp, with no changes to ma and pitch E. Increasing the pitch in helical scanning, with no changes to kvp and ma F. Increasing the gantry rotation speed with no changes to kvp and ma

18 Which of the following situations does not reduce the dose to the patient? A. Reducing ma, with no changes to kvp and pitch B. Increasing the ma by 10% and increasing the pitch by 20% C. Reducing kvp, use Auto-mA to maintain noise, no change to pitch D. Increasing the kvp, with no changes to ma and pitch E. Increasing the pitch in helical scanning, with no changes to kvp and ma F. Increasing the gantry rotation speed with no changes to kvp and ma Approximate CTDIvol values for adult head and abdomen scans are: A. 5 rad head, 1 rad abdomen B. 5 mgy head, 1 mgy abdomen C. 50 mgy head, 10 mgy abdomen D. 500 mgy head, 100 mgy abdomen Occupational Limit Approximate CTDIvol values for adult head and abdomen scans are: A. 5 rad head, 1 rad abdomen (CTDIvol is defined only for mgy) B. 5 mgy head, 1 mgy abdomen C. 50 mgy head, 10 mgy abdomen D. 500 mgy head, 100 mgy abdomen 105 For a Radiation Worker, such as a Radiologist, Nuclear Medicine Physician or technologist, the maximum permissible effective dose is msv/year. A. 5 B. 10 C.50 D.100 Occupational Limit For a Radiation Worker, such as a Radiologist, Nuclear Medicine Physician or technologist, the maximum permissible effective dose is msv/year. A. 5 B. 10 C.50 D.100 Reducing the slice thickness in CT with no change in technique (i.e., same kvp and mas) will: A. Improve spatial resolution B. Increase the signal to noise ratio C. Increase the dose from primary radiation to tissue in the slice D. Improve contrast resolution E. Change CT number calibration

19 Reducing the slice thickness in CT with no change in technique (i.e., same kvp and mas) will: A. Improve spatial resolution B. Increase the signal to noise ratio C. Increase the dose from primary radiation to tissue in the slice D. Improve contrast resolution E. Change CT number calibration Reducing ma by one-half in CT with no other changes will: A. Improve contrast resolution B. Increase spatial resolution C. Decrease patient dose by one-half D. Increase tube heat loading E. Decrease image noise by about 40% Reducing ma by one-half in CT with no other changes will: A. Improve contrast resolution B. Increase spatial resolution C. Decrease patient dose by one-half D. Increase tube heat loading E. Decrease image noise by about 40% Fini Any?? Clinical applications toshiba.com Metal artifact Acute Stroke This case shows a complete acute stroke evaluation acquired on 320-slice scanner in 60 seconds. Using a dynamic volume CT scan sequence this technique provides a fast and comprehensive neurological assessment when time to diagnosis is critical. Note the abnormal cerebral blood flow (CBF), cerebral blood volume (CBV) and mean transit time (MTT) within the left temporal region. There is an additional perfusion deficit along the vertex of the brain which unfortunately could have been missed using a conventional MDCT perfusion protocol. Digital radiograph: Scout Video

20 Micro CT of Mouse 25 micron resolution Micro CT Scanner Cone beam 2D CCD Very long acquisitions 5 min or more Mouse must be anesthetized or euthanized Very high doses Fantastic for early drug discovery 115 Geometric magnification of mouse on detector Feldkamp cone beam reconstruction algorithm Dual-energy methods available Mouse squeezed into small cylinder 116 Results are Worth the Long Wait Vascular Imaging Neonatal mouse Mouse femoral head Lungs Implanted tumor 3D Rendered Airways Quantitative with Extra Effort Lung Tumor Model Living and breathing (but motion artifact) Euthanized (but totally cooperative) 119 Methods to Reduce CT Dose Reduce ma Dose is proportional to mas Use Auto-mA (less dose for thinner patient or body part) Accept more quantum noise in the image Reduce kvp when appropriate Dose increases exponentially with kvp for same mas Best when used with iodinated contrast media to increase object contrast exploiting the 33 kev k-edge Use Iterative Reconstruction with lower ma Increase pitch Reduce number of series Do you really need pre-contrast? Angle gantry to avoid direct exposure of eyes, breasts or gonads

21 For the same reconstructed slice widths, a 64-slice CT compared to a 16-slice CT generally: For the same reconstructed slice widths, a 64-slice CT compared to a 16-slice CT generally: A. Has better system resolution B. Will have improved cardiac imaging C. Has a lower radiation dose D. Can use a lower concentration of iodine contrast E. Has a larger diameter field of view (FOV) A. Has better system resolution B. Will have improved cardiac imaging C. Has a lower radiation dose D. Can use a lower concentration of iodine contrast E. Has a larger diameter field of view (FOV) 64-slice covers more Z-direction in a single rotation so less time needed. All other factors are equivalent In CT, the ROI tool placed over a uniform area of an image will return values of mean and standard deviation of the CT numbers within the ROI. The standard deviation is most closely associated with the: In CT, the ROI tool placed over a uniform area of an image will return values of mean and standard deviation of the CT numbers within the ROI. The standard deviation is most closely associated with the: A. X-ray attenuation B. Spatial resolution C. Image noise D. Image contrast E. Atomic number of the tissue in the ROI A. X-ray attenuation B. Spatial resolution C. Image noise D. Image contrast E. Atomic number of the tissue in the ROI Concerning image artifacts in x-ray CT: Concerning image artifacts in x-ray CT: A. Patient movement results in streak artifacts B. Beam hardening leads to higher CT numbers in center of the image C. Ring artifacts are associated with the presence of metallic objects in the reconstruction circle D. Detectors made of high-density material may cause streak artifacts A. Patient movement results in streak artifacts B. Beam hardening leads to higher CT numbers in center of the image C. Ring artifacts are associated with the presence of metallic objects in the reconstruction circle D. Detectors made of high-density material may cause streak artifacts 125 Beam hardening: reduced CT # s. Ring artifacts: detector or x-ray problems. Metal objects in patient

22 Radiation Risk Cancer mortality of C-A-P CT of 45 yo man is ~0.08% Yearly screening x 30 yrs ~2% chance of cancer Cancer incidence in humans ~25-30% Example: Nationwide VA performs ~1.5 M CTs/year Could induce 800 malignancies/year (fewer are actually expressed due to elevated age of patients) Skin erythema results with dose ~2.0 Gy Temporary epilation with dose ~3.0 Gy Background 0.3 Gy/year CT Calcium Measurement Dual Energy CT provides higher accuracy and sensitivity for spine/hip bone densitometry and aortic/coronary calcium assessment CT Number (HU) Dual Energy CT Density Standard CT 127 Calibration Phantom 128 CT or DEXA for Body Composition? Dual-Energy X-Ray Absorptiometry: Bone Mineral Density Total body lean/fat mass, BMD 0.7% precision for BMD Lumbar scan in 10 sec Low cost (<$100) Very low dose: msv (1/5 Chest XR, 1/20 low dose CT) Widely available Huge clinical base 2D projection so superposition Body Composition: VAT/SAT, BMD CT is more specific so it may be required for some research DEXA is preferred for population studies Slice Sensitivity Profile New clinical tool: Iterative reconstruction to reduce metal artifact

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