Spiral CT Protocol Optimization & Quality Assurance Ge Wang, Ph.D. Department of Radiology University of Iowa Iowa City, Iowa 52242, USA
Spiral CT Protocol Optimization & Quality Assurance Protocol optimization Techniques & parameters Imaging model Optimal pitch Quality Assurance Monthly test Dose measurement Future
Techniques Tube voltage 137 kv - 100% (20 cm water) 120 kv - 40% 80 kv - 20% Product of tube current & scan time (mas) The higher mas, the weaker noise Reconstruction filters & display settings The higher-pass filter, the sharper image, the stronger noise Window level, window width
P(q,t) Filters P (q,t) P(q,t) f(x,y) f(x,y) g (x,y) Filtering in the projection domain: P, f Filtering in the image domain: P, g Smooth - Standard - Sharp Soft tissue - Standard - High - Ultrahigh
Image Display µ µ HU( µ ) =1000 µ water water 0 255 CT Number -Hounsfield unit Air: -1024 Water: 0 Bone: +175 to +3071 Viewing Parameters Window level (L) Window width (W) Zoom factor -1024 W +3071 L
Parameters Detector collimation Table feed Scanning time Reconstruction interval
Detector Collimation 1 mm: temporal bone 2-3 mm: lung nodule, renal arteries 5 mm: neck, kidney, pancreas 8 mm: chest, liver Source Collimation Detector
Pitch Pitch = Table Feed / Collimation In current practice Generally, pitch=1 Not to exceed pitch=2
Slice Thickness Formula SSP D T σ = 2 D + 12 2 T 2 24 s: Standard deviation of the SSP D: Detector collimation T: Table feed s
Slice Thickness Formula σ = 2 D + 12 2 T 2 24 s: Standard deviation of the SSP D: Detector collimation T: Table feed For reduction of slice thickness Reduction of D is more effective than reduction of T When pitch goes from 1 to 2, slice thickness is degraded by about 40%
Number of Reconstructed Slices per Collimation n = π 1 12 3 + 2 p 24 n: slice number per collimation p: pitch defined as ratio of table feed over collimation To avoid aliasing the MTF, about 2-3 transverse slices should be reconstructed per collimation. Wang and Vannier Medical Physics 24:1635-1639, 1997
Image Noise Due to quantum and other random effects Depending on Collimation Tube voltage mas product Patient Size HVL for water 3.6 cm Voxel size Algorithm
Image Noise 2 η = η c η ID N N h: Standard deviation of the noise I: Tube current D: Detector collimation h: Standard deviation of the noise N: Number of X-ray photons
Maximum Scanning Time & Range t c t = t: Max scanning time I I: Tube current t L = c L Tt L: Max scanning length T: Table increment I
Imaging Model σ t = c I t 2 D + 12 2 2 T = 2 η = c η ID L = 24 c L Tt σ D η I UT t I UD L T Ut D detector collimation T table increment p pitch, T/D s SSP standard deviation h image noise standard deviation I tube current t maximum scanning time L maximum scanning range
Optimization Problems Given slice thickness and image noise, maximize scanning range Given scanning range and image noise, minimize slice thickness Given scanning range and slice thickness, minimize image noise Given scanning range, minimize product of slice thickness and image noise, that is, maximize signal-to-noise ratio
Optimal Pitch p = 2 =1.414
Spiral CT Protocol Optimization & Quality Assurance Protocol optimization Techniques & parameters Imaging model Optimal pitch Quality Assurance Monthly test Dose measurement Future
CT Performance Phantom
Monthly Test For the consistency of the system MTF of a thin wire: 2% Slice sensitivity profile: Full-Width-at-Half-Maximum (FWHM) Image noise Homogeneity Contrast: CTwater / (CTwater - CTair) Position of the light-beam indicator Exactness of the table top position
Spiral CT Protocol Optimization & Quality Assurance Protocol optimization Techniques & parameters Imaging model Optimal pitch Quality Assurance Monthly test Dose measurement Future
Radiation Dose Dose - radiation energy transferred to an anatomic structure during X-ray scanning The unit of dose is Gray (Gy) sometimes Rad (0.01 Gy) Typical values for a CT transaxial scan are in the range of 30 to 50 mgy
Radiation Profile: Single Scan Ideal profile Real profile Radiation spreads outside the designated slice due to scattering
CT Dose Index T CTDI = 1 T 7T 7T D( z) dz CTDI: CT dose index T: slice thickness D(z): local dose z: longitudinal coordinate
Radiation Profile: Multiple Scans Real profile Radiation dose from multiple scans are accumulated in the central slice
Multiple Scan Average Dose I MSAD = / 2 1 I I I / 2 D N, I ( z) dz MSAD: multiple scan average dose I: inter-slice distance N: Number of scans
Dose Measurement Cylindrical phantoms of 16 cm & 32 cm Pencil ionization chamber Dosimeter 16 or 32 cm
MSAD Estimation MSAD is directly proportional to ma directly proportional to scan time increases with kvp as compared to dose at 120 kvp 0.2-0.4 times less at 80 kvp 1.2-1.4 times more at 140 kvp increases slightly with decreasing slice thickness similar at the iso-center and near surface for head significantly less at the iso-center than near surface for body
Spiral CT Protocol Optimization & Quality Assurance Protocol optimization Techniques & parameters Imaging model Optimal pitch Quality Assurance Monthly test Dose measurement Future
Cone-Beam Spiral CT Simultaneous Source rotation Table translation Data acquisition 2D detector array
Cone-Beam Image Reconstruction (From AMIL, SUNY/Buffalo)
Image Analysis Visualization & analysis 3D, 4D Networked, PC-based Image fusion Computer aided diagnosis Image-based surgery
References T. S. Curry III, J. E. Dowdey, R. C. Murry Jr. Christensen s physics of diagnostic Radiology (4th edition), Lea & Febiger (for residents) G. Wang, M. W. Vannier: Computerized tomography. Encyclopedia of Electrical and Electronics Engineering, edited by Webster JG, to be published by John Wiley & Sons (for engineers) http://dolphin.radiology.uiowa.edu/ge (on-line slides & handouts in the Teaching section)