Spiral CT. Protocol Optimization & Quality Assurance. Ge Wang, Ph.D. Department of Radiology University of Iowa Iowa City, Iowa 52242, USA

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1 Spiral CT Protocol Optimization & Quality Assurance Ge Wang, Ph.D. Department of Radiology University of Iowa Iowa City, Iowa 52242, USA

2 Spiral CT Protocol Optimization & Quality Assurance Protocol optimization Techniques & parameters Imaging model Optimal pitch Quality Assurance Monthly test Dose measurement Future

3 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

4 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

5 Image Display µ µ HU( µ ) =1000 µ water water CT Number -Hounsfield unit Air: Water: 0 Bone: +175 to Viewing Parameters Window level (L) Window width (W) Zoom factor W L

6 Parameters Detector collimation Table feed Scanning time Reconstruction interval

7 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

8 Pitch Pitch = Table Feed / Collimation In current practice Generally, pitch=1 Not to exceed pitch=2

9 Slice Thickness Formula SSP D T σ = 2 D T 2 24 s: Standard deviation of the SSP D: Detector collimation T: Table feed s

10 Slice Thickness Formula σ = 2 D 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%

11 Number of Reconstructed Slices per Collimation n = π 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: , 1997

12 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

13 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

14 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

15 Imaging Model σ t = c I t 2 D 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

16 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

17 Optimal Pitch p = 2 =1.414

18 Spiral CT Protocol Optimization & Quality Assurance Protocol optimization Techniques & parameters Imaging model Optimal pitch Quality Assurance Monthly test Dose measurement Future

19 CT Performance Phantom

20 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

21 Spiral CT Protocol Optimization & Quality Assurance Protocol optimization Techniques & parameters Imaging model Optimal pitch Quality Assurance Monthly test Dose measurement Future

22 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

23 Radiation Profile: Single Scan Ideal profile Real profile Radiation spreads outside the designated slice due to scattering

24 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

25 Radiation Profile: Multiple Scans Real profile Radiation dose from multiple scans are accumulated in the central slice

26 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

27 Dose Measurement Cylindrical phantoms of 16 cm & 32 cm Pencil ionization chamber Dosimeter 16 or 32 cm

28 MSAD Estimation MSAD is directly proportional to ma directly proportional to scan time increases with kvp as compared to dose at 120 kvp times less at 80 kvp 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

29 Spiral CT Protocol Optimization & Quality Assurance Protocol optimization Techniques & parameters Imaging model Optimal pitch Quality Assurance Monthly test Dose measurement Future

30 Cone-Beam Spiral CT Simultaneous Source rotation Table translation Data acquisition 2D detector array

31 Cone-Beam Image Reconstruction (From AMIL, SUNY/Buffalo)

32 Image Analysis Visualization & analysis 3D, 4D Networked, PC-based Image fusion Computer aided diagnosis Image-based surgery

33 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) (on-line slides & handouts in the Teaching section)