3/27/2012 WHY SPECT / CT? SPECT / CT Basic Principles. Advantages of SPECT. Advantages of CT. Dr John C. Dickson, Principal Physicist UCLH

Transcription

1 3/27/212 Advantages of SPECT SPECT / CT Basic Principles Dr John C. Dickson, Principal Physicist UCLH Institute of Nuclear Medicine, University College London Hospitals and University College London john.dickson@uclh.nhs.uk Advantages of CT WHY SPECT / CT? 1

2 3/27/212 Localisation Corrections: Attenuation Complementary Information SPECT / CT BY REGISTRATION 2

3 3/27/212 SPECT / CT by Registration SPECT / CT by Registration Acquire SPECT Acquire CT Overlay Data SIMPLE.. Match Patient Positioning Arms up or down? Sometimes not important Cushions, arm, and leg support Keep it as simple and reproducible as possible Breathing Protocol SPECT acquired over 3 minutes CT can be acquired less than 2 seconds Use end inspiration / end expiration / free breathing? Table top 3

4 3/27/212 Image Registration Image registration methodology Manually shift data Difficult and prone to errors Computer algorithm to shift data Manually define three or more anatomical landmarks on both data sets Fully automatic method using measures of similarity between data All data or a subsection of data Translate Rotate Translate Rotate Warp SPECT / CT by Registration Works best for simple structures e.g brain More difficult for Limbs Extremely difficult for moving or deformable structures e.g. lung, breast Proven successful for cardiac perfusion and CT coronary angiography Technical, co-ordination and workflow challenges Patient positioning critical 4

5 3/27/212 Transition to SPECT / CT The SPECT / CT System CT SPECT SPECT / CT Systems SPECT BASICS 5

6 3/27/212 SPECT SPECT Adapted from Sharp, Gemmell, Murray Filtered Back Projection (FBP) BP original NO CHANGE estimate patient original projections update (x ratio) Back Projection Filtered Back Projection ML-EM reconstruction Courtesy B. Hutton (UCL) estimated projections FP current estimate 6

7 3/27/212 Iterative Reconstruction Single Photon Emission Tomography ML-EM Computationally very expensive (Many projections) OS ML-EM (OSEM) Groups projection data into ordered sequence of subsets E.g. Subset size of 4: 1, 17,33,49;... Is an order of magnitude quicker than ML-EM Defined by number of iterations and subsets e.g. 2 iterations of 2 subsets (subset size of 6 for 12 projections) is equivalent to 4 iterations of ML-EM Comparing CT with SPECT CT BASICS 7

8 3/27/212 CT Scanner Comparing CT with SPECT CT Radiation can be turned on/off as required Radiation in required direction only Large number of photons Projections: Sum of attenuation Accurate Quantification SPECT Radiation present until decayed / excreted Isotopic radiation (most not detected) Fewer photons (~1 5 ) Projections: Sum of attenuated emissions Difficult to quantitate CT Reconstruction FBP Iterative (now) idose (Philips) IRIS (Siemens) ASIR (GE) Scanning Mode Axial, Helical, Cine Rotation time Slice Thickness Table Speed Helical Pitch Tube Current (Modulation) Dose Information Imaging Extent Shaping Filter kv Field of View (Zoom) Field Position Recon Filter 8

9 3/27/212 CT Parameters Scanning Mode Helical: Continuous sweep down subject Axial: Step and shoot. Cine: Used for dynamic studies e.g. using contrast Table speed / Pitch (Helical only) Determines tightness of spiral Large pitch means more interpolation to determine slices Shaping Filter (aka Bow-tie filter) Used to control dose to extremities Centre of field of view has more attenuation than edges, so we shape the beam to give more flux through centre CT parameters kvp: Potential difference across CT tube Higher kvp produce more penetrating X-rays Higher kvp increases dose to patient Tube Current (ma) Intensity/Flux of photons High ma, more photons, better image High ma, more dose Modulating ma can: Increase ma for high attenuation slices e.g. Pelvis Increase ma for lateral views compared to AP views BOTH Also ECG Modulation for cardiac studies CT Parameters CT Parameters Field of View (DFOV) Gives zoom facility to allow e.g. 25cm field of view for brain studies Can move field of view Reconstruction filters Standard, Soft (Tissue), Bone Predefined sharp or smooth filters Don t name using type and cut-off like SPECT CTDIvol Gives estimate of dose per cm of patient scanned Defined on phantoms e.g. Body DLP (Dose Length Product) Product of CTDIvol and length of body scanned Dose effectiveness (%) Amount of dose which is useful to produce image 9

10 3/27/212 CT Parameters used in SPECT / CT SPECT / CT systems available Low dose CT (< 5 mas per slice) Attenuation Correction Limited anatomical mapping > 5mm slice thickness Mid dose CT (5-1 mas per slice) Improved Anatomical Mapping Simple diagnostic imaging > 2.5 mm slice thickness High dose CT (> 1 mas per slice) Diagnostic Imaging Contrast Thin slices Gated and dynamic acquisitions Inherent Registration SPECT / CT: LOCALISATION CT SPECT 1

11 3/27/212 Inherent Registration Known mechanical displacement Software tweaks Registered SPECT/CT Inherent Registration? System Related misregistration Poor alignment calibration x Bed Sag Issues - Misregistration Comparison of breath hold and tidal breathing 12 second Patient related misregistration Patient movement SPECT scan duration 2-4 minutes CT scan duration 2-1 minutes (GE Hawkeye) 1-4 seconds (Philips, Siemens) Breath hold CT, 12kVp, 5mA, 1ms pulse width,.8mgy 6 second Tidal breathing CT, 12kVp, 2.5mA, continuous CT, 1.2mGy Images courtesy of Radiological March 212: Associates BIR Londonof Sacramento 11

12 3/27/212 Software solution for Misregistration SPECT / CT: CORRECTIONS NB: Misregistration affects alignment and attenuation correction Iterative Reconstruction System matrix Estimate Activity Distribution Prior Knowledge Attenuation Resolution loss [Scatter] Forward Project Data to give Estimated Projections Compare Estimated and Measured Projections 1 pixel i 1 Difference? 1 No Yes Stop voxel j 12

13 3/27/212 Attenuation Correction System matrix: with attenuation.2.5 μ.9 Future? Scatter Correction Advantages of Scatter Correction Hutton BF et al: Phys Med Biol 56 (211) R85-R112 13

14 3/27/212 Thank you! 14