ProSPECTus A look into future SPECT/MRI imaging

Transcription

1 ProSPECTus A look into future SPECT/MRI imaging Laura Harkness ljh@ns.ph.liv.ac.uk Intl Workshop on Image Processing Techniques and Applications CMIT 23/06/11

2 Outline Conventional Medical Imaging SPECT vs Compton imaging System Design ProSPECTus Image reconstruction D.S. Judson

3 Conventional medical Imaging Anatomical Imaging + Functional Imaging X-rays CT Computed Tomography MRI Magnetic Resonance Imaging SPECT Single Photon Emission Computed Tomography PET Positron Emission Tomography

4 Nuclear medicine - SPECT Single Photon Emission Computed Tomography (SPECT) Diagnosis/monitoring of cancer and neurological conditions Biological information complements MRI structural information Mechanical collimator 1 x 10-4 Scintillator detector with photomultiplier tubes Patient injected with radiopharmaceutical Radiopharmaceutical accumulates in organ of interest Gamma-rays emitted from organ and detected outside body by gamma camera

5 Conventional SPECT Limitations Compromise between sensitivity and image resolution Maximum gamma ray energy limit Collimator bulky and heavy, incompatible with magnetic fields Existing detector readout technology incompatible with magnetic fields Small animal SPECT collimator Dual-isotope imaging difficult due to poor energy resolution of conventional scintillator detectors

6 Compton Imaging Compton imaging is used to detect sources of gamma radiation Locate radiation: imaging methods Identify what the source is: spectroscopy Wide range of fields of view in a lorry, in a room, in a body Homeland security, nuclear decommissioning, medicine

7 How does it work? Gamma rays interact in both detectors (scatterer and absorber) The path for each gamma ray is reconstructed as a cone Source located at max cone overlap Source E 0 θ E 1 θ E 2 cos 1 m e c E E 0

8 How does it work? Gamma rays interact in both detectors (scatterer and absorber) The path for each gamma ray is reconstructed as a cone Source located at max cone overlap Source E 0 θ E 1 θ E 2 cos 1 m e c E E 0

9 How does it work? Gamma rays interact in both detectors (scatterer and absorber) The path for each gamma ray is reconstructed as a cone Source located at max cone overlap Source E 0 θ E 1 θ E 2 cos 1 m e c E E 0

10 How does it work? Gamma rays interact in both detectors (scatterer and absorber) The path for each gamma ray is reconstructed as a cone Source located at max cone overlap Source E 0 θ E 1 θ E 2 cos 1 m e c E E 0

11 SPECT vs Compton imaging Conventional SPECT Compton imaging Source E 0 θ E 1 θ E 2 Gamma-rays detected by a gamma camera Inefficient detection method Use 1 gamma ray in every 3000 Incompatible with MRI Gamma-rays detected by a Compton camera Use 1 gamma ray in every 30 Semiconductor detectors compatible with MRI

12 System Design L J Harkness et. al, AIP Conf Proc (2009) 1194, Criteria System for use with current medical radionuclides Excellent energy resolution for dual isotope imaging High sensitivity (reduced dose or increased patient throughput) Excellent image quality for clinical diagnosis MRI compatibility for dual-modality imaging Final Design Optimised for imaging 141keV gamma rays from 99m Tc Si(Li) scatter detector and a HPGe absorber detector Custom built cryostat

13 ProSPECTus MRI images Photo Courtesy of Semikon Photo Courtesy of ORTEC Planar Si(Li) detector (60 x 60 x 9) mm crystal 15 strips on each detector face, 4mm pitch Planar HPGe detector (60 x 60 x 20) mm crystal 12 strips on each detector face, 5mm pitch Custombuilt cryostat MRI compatible

14 Image Reconstruction D.S. Judson 1. In Compton imaging, the source is located at the position of maximum cone intersection Take 2D projections of the cone 2.

15 Image Reconstruction D.S. Judson 2. Take 2D projections of the cone Simple trigonometry to calculate projections on image plane 3. 2.

16 Image Reconstruction D.S. Judson Example: cone perpendicular to detector face Intersection of cone with imaging plane produces perfect circle The size of which can be deduced with r=ztanɵ Position in x and y is the same as the interaction positions within the detectors

17 Image Reconstruction D.S. Judson Image reconstruction via a C++ algorithm Almost, real-time, ~few seconds for several thousands of events Open-Source: can be used on any machine with a compatible compiler Cs point sources separated by 20 mm in y postioned at z=175 mm 42k events Time taken ~ 7 s (530, 530) is centre of detectors

18 Image Quality as function of cones D.S. Judson Cs point source located at (330,330) and at z=175mm 20 cones 50 cones 100 cones

19 Image Quality as function of cones D.S. Judson Cs point source located at (330,330) and at z=175mm 500 cones 1000 cones 5000 cones

20 Image Quality as function of cones D.S. Judson For each z depth, profiles in x and y can be produced Profiles fitted assuming a Lorentizan peak on a quadratic background Full Width at Half Maximum (FWHM) of the peak calculated Allows image quality to be quantified for a reconstructed image of a point source

21 Conclusions and Future Work ProSPECTus aims to replace SPECT in nuclear medicine ProSPECTus will facilitate high sensitivity imaging of multiple radioisotopes The system is designed to be compatible with MRI Image reconstruction algorithms have been developed and tested on experimental data. Currently optimising using simulations. Characterisation of detectors imminent First Compton imaging measurements Autumn 2011 Compton imaging with MRI system Winter 2011

22 The ProSPECTus Collaboration Department of Physics, The University of Liverpool, UK AJ Boston, HC Boston, JR Cresswell, DS Judson, PJ Nolan, JA Sampson, DP Scraggs STFC Daresbury Laboratory, UK I Burrows, N Clague, M Cordwell, J Groves, J Headspith, A Hill, IH Lazarus, V Pucknell, J Simpson MARIARC, The University of Liverpool, UK W Bimson, G Kemp