Computed Tomography – Chapter 13 Bushberg Diagnostic Imaging Physics Course 10 March – April 2005 Basic Principles - Conventional Radiograph Computed Tomography – Chapter 13 Kalpana Kanal, Ph.D., DABR Lecturer, Diagnostic Physics Dept of Radiology UW Medicine a copy of this lecture may be found at: http://courses.washington.edu/radxphys/PhysicsCourse04http://courses.washington.edu/radxphys/PhysicsCourse04-05.html c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 328 Kanal Basic data acquisition in CT Historical Development of CT X-ray Tube X-ray Beam CT Table Detectors Figure from Dr Mahesh, John Hopkins, MD, AAPM Handout ¬ First Generation CT : Rotate/Translate, Pencil Beam ¬ In 1972, the EMI scanner was the first CT scanner introduced into clinical practice ¬ minutes to generate a pair of images c.f http://www.impactscan.org/slides/impactcourse/1_2_basicprinciples/img5.htm http://www.impactscan.org/slides/impactcourse/1_2_basicprinciples/img5.htm c.f Wolbarst, The Physics of Radiology, Radiology, 2nd Edition, 2005 Kanal Kalpana M Kanal, Ph.D Kanal Computed Tomography – Chapter 13 Bushberg Diagnostic Imaging Physics Course 10 March – April 2005 Historical Development of CT Historical Development of CT ¬ ¬ ¬ Second Generation CT: Rotate/Translate, Narrow Fan Beam ¬ Third Generation CT: Rotate/Rotate, Wide Fan Beam 0.5 to seconds to acquire an image minute to generate a single image c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 328 c.f Wolbarst, The Physics of Radiology, Radiology, 2nd ed., p 409 Kanal Historical Development of CT ¬ Fourth Generation CT: Rotate/Stationary ¬ 0.5 to seconds to acquire an image Historical Development of CT ¬ c.f Wolbarst, The Physics of Radiology, Radiology, 2nd ed., p 410 Kanal Kalpana M Kanal, Ph.D c.f Wolbarst, The Physics of Radiology, Radiology, 2nd ed., p 409 Kanal Kanal Fifth Generation CT: Stationary/Stationary X Electron beam scanner X Primarily for cardiologists, 50 msec scan times X Uses tungsten target and highhigh-energy electron beam c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 337 Computed Tomography – Chapter 13 Bushberg Diagnostic Imaging Physics Course 10 March – April 2005 Formation of a CT image - Tomographic Acquisition Historical Development of CT ¬ Sixth Generation CT: Helical X SlipSlip-ring technology developed (allows gantry to rotate continuously without wires) X Helical CT scanners acquires data while table is moving c.f Seeram, Computed Tomography, Tomography, 2nd Ed., pg 79 ¬ Seventh Generation CT X Multiple Detector Array ¬ A single transmission measurement through the patient made by a single detector at a given moment in time is called a ray c.f www.impactscan.org Kanal Formation of a CT image - Tomographic Acquisition ¬ ¬ Kalpana M Kanal, Ph.D c.f www.sprawls.org, computed tomography lecture 10 Formation of a CT image - Tomographic Acquisition A series of rays that pass through the patient at the same orientation orientation is called a projection or view All modern CT scanners incorporate fan beam geometry Kanal c.f Macovski, Medical Imaging Systems, Systems, pg 114 Kanal ¬ 11 Kanal You could have approx 800 rays taken at 1,000 different projection angles giving 800,000 transmission measurements c.f www.sprawls.org, computed tomography lecture 12 Computed Tomography – Chapter 13 Bushberg Diagnostic Imaging Physics Course 10 March – April 2005 Formation of a CT image – Reconstruction Formation of a CT image - Tomographic Acquisition ¬ ¬ ¬ Before the axial acquisition of the next slice, the table that the the patient is lying on is moved slightly in the cranialcranial-caudal direction or the zz-axis of the scanner This positions a different slice of tissue in the path of the xx-ray beam for the acquisition of the next image High kV of 120 to 140, mA ranging from 10 – 440 and scan times of 0.4 – seconds c.f www.sprawls.org, computed tomography lecture Kanal 13 c.f Seeram, Computed Tomography, Tomography, 2nd Ed., pg.107 Kanal Tomographic Reconstruction Preprocessing & Raw Data ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ Tomographic Reconstruction Each ray is a transmission measurement through the object along a line, where the detector measures an xx-ray intensity, It I0 = unattenuated intensity of xx-ray beam It = I0 e-µt t = thickness of patient along the ray µ = the average linear attenuation coefficient along the ray ln (I0 / It) = µt for each ray This is the preprocessing step performed before image reconstruction Image primarily depends on the patient’s anatomic characteristics characteristics Kanal Kalpana M Kanal, Ph.D 14 ¬ ¬ ¬ ¬ ¬ 15 Kanal After preprocessing the raw data, a CT reconstruction algorithm is used to produce the CT image (attenuation coefficient map) Filtered back projection is most widely used in clinical CT scanners scanners The backprojection method builds up the CT image in the computer by essentially reversing the acquisition steps During backprojection, the µ value for each ray is in essence smeared along the same path in the image of the patient Areas of high attenuation reinforce each other and areas of low attenuation reinforce each other building up the image in the computer 16 Computed Tomography – Chapter 13 Bushberg Diagnostic Imaging Physics Course 10 March – April 2005 Formation of a CT image – Filtered back projection ¬ ¬ ¬ Image Reconstruction However, simple backprojection produces an image that is somewhat blurred Raw data must first be filtered using a mathematical filter, or kernel This process is known as the convolution technique c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 352 Kanal 17 Kanal Smooth versus Sharp Filter ¬ Bone algorithm - fine detail but with increased noise ¬ Soft tissue filters - smoothing, which decreases image noise but also decreases spatial resolution ¬ The choice of the best filter to use with the reconstruction algorithm depends on the clinical task c.f Seeram, Computed Tomography, Tomography, 2nd Ed., pg.108 CT Image ¬ ¬ ¬ ¬ Kalpana M Kanal, Ph.D A pixel (picture element) is the basic 2D element of the digital image Each pixel displays brightness information concerning the patient’s anatomy that is located in the corresponding voxel (volume element) The pixel width and height are equal to the voxel width and height The voxel has a third dimension that represents the slice thickness of the CT scan c.f http://www.impactscan.org/slides/impactcourse/1_2_basicprinciples/img21.htm c.f http://www.impactscan.org/slides/xrayct/sld056.htm Kanal 18 19 Kanal 20 Computed Tomography – Chapter 13 Bushberg Diagnostic Imaging Physics Course 10 March – April 2005 What Is Being Measured? CT Image ¬ ¬ ¬ ¬ ¬ ¬ ¬ The rows and columns comprise a matrix Matrix sizes are 512 x 512, 1024 x 1024 etc The technologist selects the field of view (FOV) Pixel size = FOV/matrix size The gray scale range for each pixel is 1212-bits (0(0-4095) Spatial resolution improves with a larger matrix (smaller pixels) or smaller FOV ¬ ¬ ¬ c.f http://www.impactscan.org/slides/impactcourse/1_2_basicprinciples/img21.htm Kanal 21 22 What CT numbers correspond to physically in the patient? ¬ The CT(x,y) number in each pixel, (x,y) of the image is derived from: ⎡ µ ( x, y ) − µ water ⎤ CT ( x, y ) = 1000 ⎢ ⎥ µ water ⎣ ⎦ ¬ ¬ ¬ µ(x, y) is the attenuation coefficient for the voxel, µwater is the attenuation coefficient of water and CT (x,y) is the CT number (or (or Hounsfield unit) that comprises the final clinical CT image Air = -1000, soft tissues range from -300 (lung) to -90 (fat), water = 0, white matter = 30, gray matter = 40, muscle = 50, dense bone and areas filled with high contrast agent range up to +3000 Kanal Kalpana M Kanal, Ph.D c.f http://www.impactscan.org/ Kanal CT Numbers or Hounsfield Units ¬ The CT reconstruction process results in a 2D matrix of floating point numbers in the computer which range from near 0.0 up to value equal to 1.0 These numbers correspond to the average linear attenuation coefficient of the tissue contained in each voxel The CT images are normalized and truncated to integer values that encompass 4096 values, between -1000 and 3095 (typically) CT numbers are rescaled linear attenuation coefficients 23 Kanal CT numbers and hence CT images derive their contrast mainly from the physical properties of tissue that influence Compton scatter ¬ The linear attenuation coefficient tracks linearly with density of tissue and plays the dominant role in forming contrast in medical medical CT CT numbers are quantitative, ¬ pulmonary nodules that are calcified are typically benign, and amount of calcification can be determined from the mean CT number of the nodule ¬ CT is also quantitative in terms of linear dimensions and can be used to accurately access tumor volume or lesion diameter 24 Computed Tomography – Chapter 13 Bushberg Diagnostic Imaging Physics Course 10 March – April 2005 CT Timeline Digital Image Display: Window/Level ¬ ¬ ¬ ¬ ¬ P1 = L – ½ W P2 = L + ½ W CT image voxels utilize a 1212-bit graygray-scale (212=4096 shades) Computer monitors and laser imagers for printing have about bits of display fidelity (28=256) The 1212-bit CT images must be reduced to bits to accommodate most image display hardware The window width (W) determines the contrast of the image, with narrower windows resulting in greater contrast The level (L) is the CT number at the center of the window 64 slice scanners 2005 Figure from Dr Mahesh, John Hopkins, MD, AAPM Handout c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 359 Kanal 25 Kanal 26 Technological Advances That Led To: Helical (Spiral) Acquisition Helical/Spiral CT ¬ Patient is transported continuously through gantry while data are acquired continuously during several 360360-deg rotations ¬ SlipSlip-ring technology ¬ HighHigh-power xx-ray tubes ¬ Interpolation algorithms c.f Seeram, Computed Tomography, Tomography, 2nd Ed., pg 82 c.f Kalender WA, et.al Radiology, 176(1):181176(1):181-3, 1990 Kanal Kalpana M Kanal, Ph.D 27 Kanal 28 Computed Tomography – Chapter 13 Bushberg Diagnostic Imaging Physics Course 10 March – April 2005 Interpolation (Helical) ¬ ¬ ¬ ¬ Helical CT scanning produces a helical trajectory around the patient CT reconstruction algorithms assume that the xx-ray source has negotiated a circular, not a helical path around the patient To compensate for these differences, the helical data set is interpolated into a series of planar image data sets This allows the production of additional overlapping images with no additional dose to the patient (interleaved reconstruction) Helical CT - Pitch Pitch = Table increment per rotation (mm) Beam collimation (mm) ¬ Pitch is a parameter than comes to play when helical scan protocols protocols are used Typical Pitch Ratio - 0.5, 1.0, 1.5, 2.0 ¬ Pitch 1 implies extended imaging and reduced patient dose ¬ c.f Seeram, Computed Tomography, Tomography, 2nd Ed., pg 218 Kanal 29 Kanal 30 Pitch Slice Thickness: Single Detector Array Scanners Slice Sensitivity Profile ¬ ¬ c.f Bushberg, et al The Essential Physics of Medical Imaging, 1st ed., p 261 ¬ The slice thickness in single slice CT is determined by the physical collimation of the incident xx-ray beam with two lead jaws The SSP describes how thick a section is imaged and to what extent details within the section contribute to the signal When a small object is placed in the center of the CT slice X X Kanal Kalpana M Kanal, Ph.D 31 Kanal it produces greater contrast from background (greater difference in CT number) than when the same object is positioned near the edge of the slice volume c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 343 32 Computed Tomography – Chapter 13 Bushberg Diagnostic Imaging Physics Course 10 March – April 2005 Detectors and Detector Arrays Slice Sensitivity Profile for Conventional and Helical CT Most modern CT systems use either Xenon detectors (old technology) or solidsolid-state scintillator detectors ¬ c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 339 & 340 Kanal 33 Kanal 34 Take Home Points ¬ ¬ ¬ ¬ ¬ ¬ ¬ Take Home Points CT image voxels utilize a 1212-bit graygray-scale (4096 shades) The window width (W) determines the contrast of the image, with narrower windows resulting in greater contrast ¬ The level (L) is the CT number at the center of the window ¬ generations of CT scanner Filtered back projection is most widely used in clinical CT scanners scanners A pixel (picture element) is the basic 2D element of the digital image The pixel width and height are equal to the voxel width and height height The voxel has a third dimension that represents the slice thickness thickness of the CT scan Matrix sizes are 512 x 512, 1024 x 1024 etc Pixel size = FOV/matrix size ¬ Table increment per rotation (mm) ¬ ⎡ µ ( x, y ) − µ water ⎤ CT ( x, y ) = 1000 ⎢ ⎥ µ water ⎣ ⎦ Kanal Kalpana M Kanal, Ph.D 35 Kanal Pitch = Beam collimation (mm) ¬ Pitch 1 implies extended imaging and reduced patient dose 36 Computed Tomography – Chapter 13 Bushberg Diagnostic Imaging Physics Course 10 March – April 2005 Raphex 2001 Diagnostic Questions ¬ ¬ ¬ ¬ ¬ ¬ ¬ Raphex 2000 Diagnostic Questions D35 The CT reconstruction kernel or algorithm chosen by the operator affects: A Only pixel noise B Only spatial resolution C Only patient radiation dose D Pixel noise and spatial resolution E Pixel noise and spatial resolution and radiation dose D36 A small lung nodule (8 mm in diameter) is noticed in a lung CT It has a CT number of 60 If the slice thickness is reduced from 10 mm to mm, the CT number of the nodule would probably: ¬ A Increase B Decrease C Remain the same ¬ ¬ ¬ D The reconstruction algorithm affects both noise and resolution, but has no affect on dose A smooth algorithm decreases noise and and resolution A sharp algorithm increases noise and resolution Kanal ¬ 37 Kanal 38 Raphex 2003 Diagnostic Questions ¬ ¬ ¬ ¬ ¬ ¬ ¬ Raphex 2003 Diagnostic Questions D45 If a CT bone window is set at a width of 1000 with the center at 500, the range of CT numbers that will be displayed as black is _ A Greater than 500 B Less than 500 C Less than -500 D Less than E Less than 1000 ¬ D The center is at 500, and the width is 1000, i.e., -1000 CT numbers below are outside the window, and are displayed as black ¬ Kanal Kalpana M Kanal, Ph.D A If properly centered in the slice, the CT number will go up because of less volume averaging with air in adjacent lung ¬ ¬ ¬ ¬ ¬ 39 Kanal D43 The CT value of white matter is 40 HU, and that of gray matter is 45 HU The approximate subject contrast between white and gray gray matter is _ A 0.12 B 0.5 C 1.2 D 5.0 E 12.0 B For low CT numbers (-200 to 200) the percent contrast can be approximated by: % contrast = (CT number difference)/10 = (4540)/10 = 0.5 40 10 Computed Tomography – Chapter 13 Bushberg Diagnostic Imaging Physics Course 10 March – April 2005 Signal to Noise Ratio (SNR) Huda Question ¬ The main advantage of helical CT over conventional (axial) CT is improved: A Spatial resolution B Low contrast detection C Data acquisition rate D Patient dose E Image reconstruction time ¬ ¬ ¬ ¬ ¬ ¬ ¬ C Fast patient data acquisition is the major benefit of helical CT ¬ Kanal 77 Kanal 78 Radiation Dose in CT Radiation Dose in CT ¬ Signal, N = mean photons used to produce the image/unit area Noise injects a random or stochastic component into an image – many sources Quantum noise is the statistical fluctuation in the photons detected, detected, and is given by = √N Can adjust the noise ( ) in an image by adjusting the mean number of photons used to produce the image Relative noise = coefficient of variation = /N = 1/√ 1/√N (decreases with increase in N) SNR = signal/noise = N/ = N/√ N/√N = √N (increased with increased N) Quantum noise and structure noise both affect the conspicuity of a target Three aspects of radiation dose in CT that are unique in comparison comparison to xx-ray projection imaging: X Single CT image obtained in a highly collimated manner X Even distribution of dose due to rotational acquisition compared to a chest radiography X CT acquisition requires a high SNR to achieve high contrast resolution and therefore the dose to the slice volume is higher because the techniques used are higher X PA Chest xx-ray – 120 kVp, mAs X Chest CT – 120 kVp, 200 mAs Kanal Kalpana M Kanal, Ph.D 79 Kanal McNittMcNitt-Gray Multislice CT Workshop 80 20 Computed Tomography – Chapter 13 Bushberg Diagnostic Imaging Physics Course 10 March – April 2005 Radiation Dose in CT Radiation Dose ¬ Compton scattering is the principal interaction mechanism in CT, so the radiation dose attributable to scattered radiation is considerable ¬ The acquisition of a CT slice delivers a considerable dose from scatter to adjacent tissues, outside the primary beam c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 363 McNittMcNitt-Gray Multislice CT Workshop Kanal 81 Kanal 82 Radiation Dose Multiple Scan Average Dose (MSAD) Kanal Kalpana M Kanal, Ph.D ¬ The MSAD is the dose to tissue that includes the dose attributable to scattered radiation emanating from all adjacent slices ¬ The MSAD is defined as the average dose, at a particular depth from the surface, resulting from a large series of CT slices McNittMcNitt-Gray Multislice CT Workshop Radiation Dose Dose Measurement - CTDI 83 Kanal ¬ As estimate of the MSAD can be accomplished with a single scan by measuring the CT dose index (CTDI) ¬ The CTDI can be measured using a pencilpencil-type ionization chamber in phantoms that simulate heads (16 cm diameter acrylic) and bodies (32 cm diameter acrylic) 84 21 Computed Tomography – Chapter 13 Bushberg Diagnostic Imaging Physics Course 10 March – April 2005 Radiation Dose Dose Measurement - CTDI ¬ ¬ ¬ Quality Assurance Radiation Dose CTDIFDA is defined by the FDA as the radiation dose to any point in the patient including the scattered radiation contribution from CT slices in both directions, for a total of 14 slices Doses at the patient surface may be higher than the dose at the center of the patient ¬ In head scans, the surfacesurface-toto-center ratio is approximately 1:1 ¬ In body scans, the surfacesurface-toto-center ratio is approximately 2:1 CTDI measurements are made at the surface, CTDIperipheral and at the center, CTDIcenter of the phantom and combined to give CTDIw (2/3 CTDIperipheral + 1/3 CTDIcenter) Kanal 85 Kanal 86 Radiation Dose Dose Measurement - CTDI ¬ ¬ ¬ Radiation Dose kVp CTDI values for body scans are lower than those for head scans because of the greater attenuation of xx-rays in body CTDI does not quantify the patient risk because it takes no account account of the number of sections scanned, or the radiosensitivity of irradiated organs ¬ CTDI increases with tube voltage ¬ kVp not only controls the image contrast but also controls the amount of penetration that the xx-ray beam will have as it traverses the patient Parameter 80 kV 120 kV 140 kV Image Contrast Best Intermediate Poor Noise Most Average Least Penetration Least Average Most Patient Dose per mAs Lowest Intermediate Highest decreasing kVp will reduce dose with other factors constant Kanal Kalpana M Kanal, Ph.D 87 Kanal 88 22 Computed Tomography – Chapter 13 Bushberg Diagnostic Imaging Physics Course 10 March – April 2005 Dose Considerations in Helical Scanning Dose Considerations in Helical Scanning Dose (helical) = Dose (axial) × Dose (helical) = Dose (axial) × ¬ ¬ ¬ collimator pitch Helical scanning with a collimator pitch of 1.0 is physically similar similar to performing a conventional (nonhelical (nonhelical)) axial scan with contiguous slices Collimator pitch of 1.5, dose is 67% (33% less) that of the conventional CT dose (assuming mAs remains the same) Collimator pitch of 0.75, dose is 133% (i.e., 33% greater) that of the conventional CT dose (assuming mAs remains the same) Kanal 89 Kanal 90 Factors that influence dose – Tube Current, mA and Time (sec), mAs Factors that influence dose - Pitch CTDIvol = CTDIw / pitch ¬ collimator pitch pitch = 0.75 133% of dose at pitch = pitch = 1.5 67% of dose at pitch = pitch = 2.0 50% of dose at pitch = mAs Pitch – increasing pitch holding all other factors constant reduces dose Kanal Kalpana M Kanal, Ph.D ¬ 91 Kanal CTDIw - Head CTDIw - Body 5.7 mGy 100 13 mGy 200 26 mGy 12 mGy 300 40 mGy 18 mGy 400 53 mGy 23 mGy mA and time – Dose increases LINEARLY with mAs 92 23 Computed Tomography – Chapter 13 Bushberg Diagnostic Imaging Physics Course 10 March – April 2005 Factors that influence dose - Collimation mm CTDIw - Head CTDIw - Body 35.8 mGy 15.9 mGy 35.4 mGy 15.7 mGy 34.8 mGy 15.4 mGy 34.4 mGy 15.3 mGy 10 34.2 mGy 15.2 mGy mm ¬ Factors that influence dose – Patient Size MultiMulti-Slice Detector (Other factors constant at 140 kVp, 200 mA and second), GE 16-slice 1.25 ¬ SingleSingle-Slice Detector (Other factors constant at 140 kVp, 200 mA and second), GE single-slice ¬ ¬ ¬ ¬ ¬ CTDIw - Head CTDIw - Body 76.3 mGy 41.8 mGy 73.5 mGy 40.3 mGy 10 58.4 mGy 32.0 mGy 15 54.7 mGy 30.0 mGy 20 50.0 mGy 27.4 mGy Kanal 93 ¬ ¬ American College of Radiology guidelines ¬ Exam CTDIvol X Adult Head CT < 60 mGy X Peds Abdomen CT < 25 mGy X Adult Abdomen CT < 35 mGy Kanal 94 Reducing Patient Radiation Dose – Impact on Diagnostic Image Quality Reducing Patient Radiation Dose ¬ Patient size – dose more for smaller patients For same technical factors, CTDIw – Head > CTDIw – Body because of the greater attenuation of x-rays in body Reduce technique factors when scanning smaller adults and pediatric patients FDA notice dated 1111-2-01: www.fda.gov/cdrh/safety.html For Pediatric and small patients X Reduce tube mA (current) X Increase pitch X Develop mA settings based on patient weight or diameter and body region X Reduce number of multiple scans without contrast X Eliminate inappropriate referrals for CT ¬ Decrease mA or current ¬ Increase noise ¬ Increase pitch ¬ Increase volume averaging ¬ Increase axial increment ¬ Introduce gaps NCI published a guideline and circulated to all ACR members X www.cancer.gov/cancerinfo/causes/radiationwww.cancer.gov/cancerinfo/causes/radiation-risksrisks-pediatricpediatric-CT Kanal Kalpana M Kanal, Ph.D 95 Kanal 96 24 Computed Tomography – Chapter 13 Bushberg Diagnostic Imaging Physics Course 10 March – April 2005 Effective Dose Comparison with Chest PA Exam Effective doses in CT and Radiographic Examinations CT examination Effective dose [mSv] Radiographic examination Procedures Eff Dose [mSv] Equivalent no of chest xx-rays Approx period of background radiation Chest PA 0.02 days Pelvis 0.7 35 months Abdomen 1.0 50 months CT Chest 400 3.6 years CT Abdomen or Pelvis 1010-20 500 4.5 years Effective dose [mSv] Head Skull 0.07 Chest Chest PA 0.02 Abdomen 1010-20 Abdomen 1.0 Pelvis 1010-20 Pelvis 0.7 Ba swallow 1.5 Ba enema Typical Background Radiation - mSv per year Kanal 97 Kanal 98 Current Modulation in CT Organ Doses in CT ¬ ¬ ¬ ¬ Head CT X Thyroid - 1.9 mGy X Eye lens - 40 mGy Chest CT X Breast - 21 mGy Abdomen CT X Uterus – mGy X Gonads - mGy Pelvis CT X Uterus – 26 mGy X Gonads - 23 mGy Kanal Kalpana M Kanal, Ph.D ¬ ¬ Patient skin doses are typically between 20 (body) and 40 mGy (head) or to rad Induction of erythema is typically Gy 99 Kanal ¬ Modern CT scanners are capable of modulating the mA (current) during the scan ¬ The rationale behind this technique is that it takes fewer photons photons (lower mA) to penetrate thinner tissue, and more xx-ray photons (higher mA) are needed to penetrate thicker projections through the body ¬ Dose can be reduced ¬ Because mA is reduced per gantry rotation, xx-ray tube loading is reduced and helical scans can be performed for longer periods and and with greater physical coverage ¬ Auto mA on GE Scanners 100 25 Computed Tomography – Chapter 13 Bushberg Diagnostic Imaging Physics Course 10 March – April 2005 Davis Question Take Home Points ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ Radiation Dose in CT is given by CTDI ¬ MSAD, CTDIw, CTDIvol, DLP CTDI values for body scans are lower than those for head scans Use lower techniques for pediatric and small patients to reduce dose Dose (helical) = dose (axial) / pitch CTDI increases with tube voltage and mAs, keeping all other factors factors constant CTDI increases with decreasing slice thickness for MSCT, is same for SSCT except at smaller slice thicknesses Increasing pitch holding all other factors constant reduces dose Remember effective dose for head, chest and pelvis CT Good idea to keep in mind the organ doses Kanal If the mAs is reduced to half, with no other changes: A the patient dose will go down by oneone-half B the visible noise in the image will increase by about 40% C the contrast resolution will decrease D all of the above E none of the above ¬ 101 Kanal 102 Davis Question ¬ Huda Question Which of the following changes will result in the same primary dose dose per slice (before and after the change)? A Halving slice thickness, doubling mAs B Halving slice thickness, halving mAs C Doubling slice thickness, no other change D Halving kV, doubling time, no change in mAs ¬ C: Any time you change the mAs in CT you will change the dose By increasing the collimator separation and doubling the slice thickness, twice the photons will be used but will be absorbed in in twice the volume (and thus mass) of tissue Dose is energy/mass More tissue will be irradiated, but the primary dose will be the same Kanal Kalpana M Kanal, Ph.D D: All of the above X Dose is linear with mAs (at the same kV), and thus halving mAs will halve dose X The visible (or relative) noise is related to the number of photons, N, by 1/√ 1/√N Thus, by halving N the noise will go from 1/√ 1/√N to 1/√ 1/√(N/2), an increase of √2 = 1.4 = 40% increase X When the visible noise goes up, the image gets noisier, and the contrast resolution decreases CT fluoroscopy minimizes radiation doses by using lower: A Filtration B Voltage C Current D Collimator thickness E Field of view C Low current values are normally used in CT fluoroscopy (20(20-50 mA) to reduce doses to patients and operators 103 Kanal 104 26 Computed Tomography – Chapter 13 Bushberg Diagnostic Imaging Physics Course 10 March – April 2005 Huda Question ¬ Huda Question Representative patient doses in CT are expected to include all the the following except: except: A Head skin dose of 40 mGy (4 rad) B Head central axis dose of 40 mGy (4 rad) C Body skin dose of 20 mGy (2 rad) D Body central axis dose of 40 mGy (4 rad) E Embryo dose (abdomen CT) of 15 mGy (1.5 rad) ¬ A B C D E D A body central axis dose would be about 10 mGy (1 rad), which is about half the skin dose Kanal The dose to the fetus during an abdominal CT scan would not increase with increasing: Patient size Tube voltage Tube current Scan time Number of sections A Increasing the patient size will always reduce the embryo dose because there is more attenuation of the xx-ray beam 105 Kanal 106 Image Quality Huda Question ¬ A B C D E The scattered radiation dose meter from a patient undergoing a head CT scan is: Less than 0.04 mGy (below mrad) mrad) About 0.04 mGy (4 mrad) mrad) About 0.4 mGy (40 mrad) mrad) About mGy (400 mrad) mrad) More than mGy (over 400 mrad) mrad) B Skin dose to patient will be about 40 mGy (4 rad), and the scatter will be about 0.1% of this level at a distance of meter meter Kanal Kalpana M Kanal, Ph.D 107 Kanal ¬ Spatial resolution or highhigh-contrast spatial resolution is the ability to discriminate between adjacent objects and is a function of pixel size Typical spatial resolution range from 0.5 – 1.5 lp/mm ¬ LowLow-contrast or tissue resolution refers to the ability of an imaging imaging procedure to reliably depict very subtle differences in contrast, contrast, is the difference in the HU values between tissues 108 27 Computed Tomography – Chapter 13 Bushberg Diagnostic Imaging Physics Course 10 March – April 2005 Image Quality More noise, less dose Image Quality ¬ CT demonstrates contrast resolution of about 0.5% whereas screenscreenfilm radiography is approximately 5% ¬ There is a compromise between spatial resolution (decreasing pixel pixel size reduces SNR unless tube current is increased which would increase dose) and contrast resolution ¬ In CT, there is a wellwell-established relationship between radiation dose, pixel dimensions, SNR and slice thickness, D∝ ¬ CT image noise is determined primarily by the number of photons used to make an image (quantum mottle) (square root of N) ¬ Quantum mottle decreases as the number of photons increases X To improve both contrast and spatial resolution, the radiation dose must be increased so that more photons are available and noise is reduced SNR ∆3T Kanal 109 Kanal 110 Image Quality ¬ CT noise is generally reduced by increasing the tube voltage, current or scan time, if all other parameters are constant ¬ CT noise is also reduced by increasing voxel size (i.e., by decreasing matrix size, increasing FOV, or increasing section thickness) ¬ The typical noise in a modern CT system is approximately HU (i.e., 0.3% difference in attenuation coefficient) ¬ At a fixed technique, small patients transmit more radiation and thus noise will be reduced, this permits reduction of technique with small patients ¬ Noise is also affected by reconstruction filters used Kanal Kalpana M Kanal, Ph.D less noise, more dose Factors Affecting Spatial Resolution 111 Kanal ¬ Focal spot size ¬ If the focal spot size increases, details in the object are distributed over several detectors, thus decreasing spatial resolution ¬ Detector aperture width ¬ Higher spatial resolution can be obtained for smaller aperture sizes ¬ Objects can be resolved when the aperture size is smaller than the spacing between objects 112 28 Computed Tomography – Chapter 13 Bushberg Diagnostic Imaging Physics Course 10 March – April 2005 Factors Affecting Spatial Resolution Factors Affecting Spatial Resolution 10 mm ¬ Number of projections: More projections, more data available for image reconstruction and spatial resolution improves Kanal ¬ 113 ¬ ¬ Kalpana M Kanal, Ph.D 114 Factors Affecting Spatial Resolution At same kVp and mAs, number of detected photons increases linearly with slice thickness, SNR goes up Larger slice thickness at same technique yields better contrast resolution (higher SNR) but spatial resolution in the slice thickness dimension is reduced Smaller slice thickness improves spatial resolution in slice thickness dimension and reduces partial volume averaging ¬ Noise will increase unless mAs is also increased to compensate for loss of xx-ray photons from collimation Kanal Slice thickness: Smaller slice thickness improves spatial resolution resolution since partial volume effect is less Kanal Important tradeoffs with slice thickness ¬ mm ¬ 115 Kanal Helical pitch ¬ Greater pitches reduce spatial resolution (width of slice sensitivity profile increases) 116 29 Computed Tomography – Chapter 13 Bushberg Diagnostic Imaging Physics Course 10 March – April 2005 Factors Affecting Spatial Resolution Soft tissue filter X Factors Affecting Spatial Resolution ¬ Pixel matrix X The number of pixels used to reconstruct the CT image has a direct influence (for a fixed FOV) on spatial resolution Increasing matrix size for fixed FOV (decreases pixel size) will improve spatial resolution ¬ Field of view (FOV) ¬ FOV influences the pixel dimension of each pixel A 1010-cm FOV in a 512x512 matrix = 0.2 mm pixel, 3535-cm FOV (512x512 matrix) = 0.7 mm ¬ Patient motion X Motion will cause blurring and hence spatial resolution will degrade Bone filter Reconstruction Kernel ¬ Shape of the reconstruction kernel affects spatial resolution Bone filters have the best spatial resolution, and soft tissue filters filters have lower spatial resolution Kanal 117 Kanal 118 Factors Affecting Contrast Resolution Factors Affecting Contrast Resolution ¬ mAs (tube current x scan time) X mAs increases, number of photons increase, SNR increases and contrast resolution improves X Doubling mAs increases SNR by 41% improving contrast resolution ¬ Pixel size (FOV) X If patient size and all other scan parameters are fixed, as FOV increases, pixel dimensions increase, and the number of xx-rays passing through each voxel increases leading to improvement in contrast resolution Kanal Kalpana M Kanal, Ph.D 119 Kanal ¬ Slice thickness X Thicker slices use more photons and have better SNR Double the slice thickness, increases SNR by square root of or 41% ¬ Reconstruction filter X Bone filters produce lower contrast resolution (higher spatial resolution) and soft tissue filters improve contrast resolution but decrease spatial resolution ¬ Patient size X For same xx-ray technique, larger patients attenuate more xxrays resulting in detection of fewer xx-rays, reducing SNR and therefore contrast resolution 120 30 Computed Tomography – Chapter 13 Bushberg Diagnostic Imaging Physics Course 10 March – April 2005 Quality Assurance ¬ ¬ ¬ Quality Assurance In order to assure consistent image quality over the system’s lifetime, establish and maintain a regular Quality Assurance (QA) (QA) program Manufacturers provide a QA phantom to assess system performance and establish an ongoing QA program This phantom can be used to provide maximum performance information with minimum effort Kanal ¬ 121 Kanal 122 Artifacts Artifacts ¬ ¬ Beam Hardening ¬ LowerLower-energy xx-rays are attenuated to a greater extent than higherhigher-energy xx-rays as xx-rays pass through a patient ¬ Shape of the spectrum becomes more skewed towards higher energies Kanal Kalpana M Kanal, Ph.D Image Quality parameters that are tested, X High contrast spatial resolution X Low contrast detectability X Laser Alignment Accuracy X Noise and Uniformity X Slice thickness X CT number accuracy and linearity X Radiation Dose c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 370 123 Kanal Beam Hardening ¬ Attenuation of bone is greater than that of soft tissue, bone causes more beam hardening than an equivalent thickeness of soft tissue ¬ Petrous bones in the head, where spider webweb-like artifact connects the two bones on the image ¬ Simple beambeam-hardening correction algorithms correct the problem c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 370 124 31 Computed Tomography – Chapter 13 Bushberg Diagnostic Imaging Physics Course 10 March – April 2005 Artifacts Artifacts ¬ ¬ Motion Artifact ¬ Due to patient motion during acquisition ¬ Artifacts appear as blurring (small motions), image ghosting or double images (large motions) Partial Volume Averaging ¬ For voxels containing the same tissue type, the linear attenuation coefficient is representative of that tissue ¬ For voxels containing different tissue types, the linear attenuation coefficient is a weighted average of the two different values c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 372 Kanal 125 Kanal 126 Artifacts ¬ ¬ ¬ ¬ Raphex 2000 Diagnostic Question Partial volume imaging is pronounced for softly rounded structures that are almost parallel to the CT slice Ex., where the cranium shares a substantial number of voxels with brain tissue, causing details of the brain parenchyma to be lost because the large linear attenuation coefficient of bone dominates This is easily recognizable situation Partial volume averaging can lead to misdiagnosis when the presence of adjacent anatomic structures is not suspected Obvious approach to reducing partial volume averaging is use thinner CT slices Kanal Kalpana M Kanal, Ph.D ¬ D34 The contrast resolution of CT is approximately % ¬ A 0.3 to 0.6 B 1.0 to 2.0 C 2.0 to 3.0 D 4.0 to 5.0 E 5.0 to 10.0 ¬ ¬ ¬ ¬ 127 Kanal 128 32 Computed Tomography – Chapter 13 Bushberg Diagnostic Imaging Physics Course 10 March – April 2005 Raphex 2000 Diagnostic Question ¬ Raphex 2001 Diagnostic Question D37 After injection of a contrast medium, the CT number for a region of the brain changes from 10 to 20 This represents an increase in the linear attenuation coefficient of % relative relative to water ¬ D33 Some CT units offer 0.5 mm slice thickness The primary advantage of a thin slice is: ¬ A Reduced volume averaging B Smaller pixels are possible C Quantum mottle is less D Faster scan times E Lower patient radiation dose ¬ ¬ ¬ ¬ ¬ ¬ A B C 10 D 20 E 100 ¬ ¬ ¬ Kanal 129 Kanal 130 Raphex 2002 Diagnostic Question Raphex 2001 Diagnostic Question ¬ D36 A CT image is abnormally noisy; this could be from: ¬ A Using a higher than normal kVp B Using a thicker than normal slice width C Using a smoothing reconstruction algorithm D Using a larger than normal pitch E Scanning a head using a body scan mAs setting ¬ ¬ ¬ ¬ ¬ D40 For CT images, all of the following factors improve the visibility visibility of large, lowlow-contrast lesions except: ¬ A Smaller pixel sizes B Thicker CT slices C Higher mAs D Smaller patient size E Smooth reconstruction algorithm ¬ ¬ ¬ ¬ Kanal Kalpana M Kanal, Ph.D 131 Kanal 132 33 Computed Tomography – Chapter 13 Bushberg Diagnostic Imaging Physics Course 10 March – April 2005 Raphex 2002 Diagnostic Question Raphex 2003 Diagnostic Question ¬ D43 The major advantage of a CT scanner over a screenscreen-film radiography system is ¬ D42 For an increase from 200mA to 400mA in a CT exam the result would be to _ ¬ A Lower radiation dose B Higher spatial resolution C Less geometrical unsharpness D Better lowlow-contrast discrimination E Less motion blur ¬ A Increase high contrast (spatial) resolution B Increase low contrast resolution (decrease noise) C Change neither the high or low contrast resolution ¬ ¬ ¬ ¬ Kanal ¬ ¬ 133 Kanal 134 REFERENCES ¬ Bushberg et al, al, The Essential Physics of Medical Imaging, Imaging, 2nd Edition ¬ Mahesh, Mahesh, John Hopkins, MD, AAPM Handout ¬ Seeram, Seeram, Computed Tomography, Tomography, 2nd Edition ¬ www.impactscan.org ¬ Multislice CT workshop handout, MTMI, 2004 ¬ GE Ultra CT manual, Quality Assurance Section Kanal Kalpana M Kanal, Ph.D 135 34 [...]... 2005 Effective Dose Comparison with Chest PA Exam Effective doses in CT and Radiographic Examinations CT examination Effective dose [mSv] Radiographic examination Procedures Eff Dose [mSv] Equivalent no of chest xx-rays Approx period of background radiation Chest PA 0.02 1 3 days Pelvis 0.7 35 4 months Abdomen 1.0 50 6 months CT Chest 8 400 3.6 years CT Abdomen or Pelvis 1010-20 500 4.5 years Effective... 7 April 2005 Davis Question Take Home Points ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ Radiation Dose in CT is given by CTDI ¬ MSAD, CTDIw, CTDIvol, DLP CTDI values for body scans are lower than those for head scans Use lower techniques for pediatric and small patients to reduce dose Dose (helical) = dose (axial) / pitch CTDI increases with tube voltage and mAs, keeping all other factors factors constant CTDI increases... spacing between objects 112 28 Computed Tomography – Chapter 13 Bushberg Diagnostic Imaging Physics Course 10 March – 7 April 2005 Factors Affecting Spatial Resolution Factors Affecting Spatial Resolution 10 mm ¬ Number of projections: More projections, more data available for image reconstruction and spatial resolution improves Kanal ¬ 113 ¬ ¬ Kalpana M Kanal, Ph.D 114 Factors Affecting Spatial Resolution... 0.07 Chest 8 Chest PA 0.02 Abdomen 1010-20 Abdomen 1.0 Pelvis 1010-20 Pelvis 0.7 Ba swallow 1.5 Ba enema 7 Typical Background Radiation - 3 mSv per year Kanal 97 Kanal 98 Current Modulation in CT Organ Doses in CT ¬ ¬ ¬ ¬ Head CT X Thyroid - 1.9 mGy X Eye lens - 40 mGy Chest CT X Breast - 21 mGy Abdomen CT X Uterus – 8 mGy X Gonads - 8 mGy Pelvis CT X Uterus – 26 mGy X Gonads - 23 mGy Kanal Kalpana M... guidelines ¬ Exam CTDIvol X Adult Head CT < 60 mGy X Peds Abdomen CT < 25 mGy X Adult Abdomen CT < 35 mGy Kanal 94 Reducing Patient Radiation Dose – Impact on Diagnostic Image Quality Reducing Patient Radiation Dose ¬ Patient size – dose more for smaller patients For same technical factors, CTDIw – Head > CTDIw – Body because of the greater attenuation of x-rays in body Reduce technique factors when scanning... approximately 2:1 CTDI measurements are made at the surface, CTDIperipheral and at the center, CTDIcenter of the phantom and combined to give CTDIw (2/3 CTDIperipheral + 1/3 CTDIcenter) Kanal 85 Kanal 86 Radiation Dose Dose Measurement - CTDI ¬ ¬ ¬ Radiation Dose kVp CTDI values for body scans are lower than those for head scans because of the greater attenuation of xx-rays in body CTDI does not quantify... April 2005 Single Slice vs Multislice CT SDCT versus MDCT c.f Seeram Computed Tomography, 2nd ed., p 258 *Rydberg et al., Radiographics 2000, 20:1787 Kanal 41 Kanal Slice Thickness: Multiple Detector Array Scanners 42 Slice Thickness: Multiple Detector Array Scanners ¬ The slice thickness of multiple detector array CT scanners is determined by the width of the detectors in the slice thickness dimension... http://www.impactscan.org/slides/impactcourse/1_2_spiralandmultislice/img31.htm Kanal 49 Kanal 50 Flexible Image Reconstruction Pitch in MultiMulti-slice CT Kanal Kalpana M Kanal, Ph.D c.f http://www.impactscan.org/slides 51 Kanal ¬ Two pitch definitions seen in MSCT ¬ Pitchx = table travel per rotation X-ray beam width ¬ Pitchd = table travel per rotation detector width c.f http://www.impactscan.org/slides... also affected by reconstruction filters used Kanal Kalpana M Kanal, Ph.D less noise, more dose Factors Affecting Spatial Resolution 111 Kanal ¬ Focal spot size ¬ If the focal spot size increases, details in the object are distributed over several detectors, thus decreasing spatial resolution ¬ Detector aperture width ¬ Higher spatial resolution can be obtained for smaller aperture sizes ¬ Objects can... difficult to reproduce ¬ If too aggressive, actual protruding structures can be lost from view because of partial volume effects ¬ (a) SurfaceSurface-rendered view from a multiplemultiple-row detector CT series demonstrates an aneurysm (arrow) arising at the origin of the posterior inferior cerebellar artery (arrowhead) from the vertebral artery (circle) (b) On a view from the interior of the artery, the