SPECT imaging with semiconductor detectors Andy Boston ajboston@liv.ac.uk

Outline of presentation What is SPECT? What detector technology can we consider? The ProSPECTus project & links to fundamental research The future prospects

What is SPECT? Functional imaging modality

What SPECT Radionuclides? t1/2=65.94h t1/2=6.01h 141 keV 910-5% 2.1105y >99% stable

Tomographic Imaging The sinogram is what we aim to measure - Measure of intensity as a function of projection, θ and position, r - Often seen plotted as a 2d grey scale image θ x r y f(x ,y) p(r , θ) Measured result – “Sinogram” (256 projections, 363 positions per projection) Note : We measure from 0 to 180° Underlying source distribution “Shepp-Logan Phantom”

SPECT : Problems/Opportunities Technical Collimator Limits Spatial Resolution & Efficiency Collimator is heavy and bulky Energy of radioisotope limited to low energy NaI:Tl Dominant for >40 Years... MRI  Existing PMTs will not easily operate Would like to be able to image a larger fraction of events. Common radionuclides: 99mTc, 123I, 131I

What are the detector requirements? Ideally would want: Good energy resolution (Good light yield/charge collection) < few% High efficiency (High Z) Position resolution Timing resolution Detector materials: Semiconductors (Si, Ge, CdZnTe) Scintillators (LaBr3, CsI(Tl), NaI(Tl), BaFl, BGO)

Next generation Single Photon Emission Computed Tomography ProSPECTus Next generation Single Photon Emission Computed Tomography Nuclear Physics Group, Dept of Physics, University of Liverpool, Nuclear Physics & Technology Groups, STFC Daresbury Laboratory, MARIARC & Royal Liverpool University NHS Trust

ProSPECTus: What is new? ProSPECTus is a Compton Imager Radical change  No mechanical collimator Utilising semiconductor sensors Segmented technology and PSA and digital electronics (AGATA) Image resolution 7-10mm  2-3mm Efficiency factor ~100 larger Simultaneous SPECT/MRI

What’s new? Conventional SPECT Compton camera Source E0 Gamma rays detected by a gamma camera Inefficient detection method Incompatible with MRI Gamma rays detected by a Compton camera Positions and energies of interactions used to locate the source Factors that limit the performance of a Compton Imager: Energy resolution, Detector position resolution, Doppler Broadening

System Configuration GEANT4 simulations L. Harkness 1cm 2cm 5cm 141keV Si(Li) Ge Total Coincident ~3.49% SPECT ~ 0.025% (typical value) Factor of ~140 Event Type % Single / Single 2.23 Single / Multiple 0.33 Multiple / Single 0.61 Multiple / Multiple 0.04 Not absorbed 0.28

HPGe Germanium Excellent energy resolution Medium Z (32) Lithographic electrode segmentation Requires cooling to LN2 HPGe growth still presents challenges Technology drivers: large scale physics projects (AGATA/GRETA/GERDA/MAJORANA)

AGATA (Advanced GAmma Tracking Array) 4 -array for Nuclear Physics Experiments at European accelerators providing radioactive and high-intensity stable beams Main features of AGATA Efficiency: 43% (M =1) 28% (M =30) today’s arrays ~10% (gain ~4) 5% (gain ~1000) Peak/Total: 58% (M=1) 49% (M=30) today ~55% 40% Angular Resolution: ~1º  FWHM (1 MeV, v/c=50%) ~ 6 keV !!! today ~40 keV Rates: 3 MHz (M=1) 300 kHz (M =30) today 1 MHz 20 kHz Summary List few items Agata very powerful instrument for nuclear spectroscopy Exciting science programme subject of this meeting Aim to propose and start realisation of the full AGATA from 2008 180 large volume 36-fold segmented Ge crystals in 60 triple-clusters Digital electronics and sophisticated Pulse Shape Analysis algorithms allow Operation of Ge detectors in position sensitive mode  -ray tracking

Ingredients of g-Tracking 4 1 Identified interaction points Reconstruction of tracks e.g. by evaluation of permutations of interaction points Highly segmented HPGe detectors (x,y,z,E,t)i g · · · Pulse Shape Analysis to decompose recorded waves · 2 3 What has to be done to perform tracking What are the ingredients Detectors highly segmented, encapsulated, cryostats Electronics Digital PSA to extract Energy Time and Position Algorithm development particularly for position Area where effort is required Thorsten Kroell. See him. Tracking algorithms, reconstruction of the events for full array Obtain full energy Many technical development in all areas. Digital electronics to record and process segment signals reconstructed g-rays

Next generation Single Photon Emission Computed Tomography ProSPECTus Next generation Single Photon Emission Computed Tomography Nuclear Physics Group, Dept of Physics, University of Liverpool, Nuclear Physics & Technology Groups, STFC Daresbury Laboratory, MARIARC & Royal Liverpool University NHS Trust

The SmartPET DSGSD detectors Detector Specification Depletion at -1300V, Operation at -1800V 12 x12 Segmentation, 5mm strip pitch 1mm thick Aluminium entrance window Warm FET configuration, 300mV/MeV pre-amps Average energy resolution ~ 1.5keV FWHM @ 122keV

Am-241 AC x-y surface intensity distribution AC01 AC12 DC12 DC1 The results are presented for 60 keV with 2 minutes of data per position.

Pulse Shape Analysis PSA techniques developed through characterisation measurements Calibration of variation in detector pulse shape response with position Real Charge Image Charge Parameterisation of these pulse shapes provides increased position sensitivity

SmartPET detector depth response “superpulse” pulse shapes for 137Cs (662 keV) events versus depth DC signals AC signals DC signals AC signals

Image Reconstruction Sensors have excellent energy & position information. Uniformity of sensor response Optimise existing: Analytical Iterative Stochastic Requirement for GPU acceleration

Compton Imaging Use of the SmartPET detectors in Compton Camera configuration Typical measurements: 10μCi 152Eu 6 cm from SPET 1 Source rotated Zero degrees in 15º steps up to 60º Detector separation 3 – 11cm in 2cm steps Gates set on energies 2 sources 152Eu and 22Na at different x and y

Compton Imaging Compton Cones of Response projected into image space

Compton Imaging Compton Cones of Response projected into image space

Compton Imaging Compton Cones of Response projected into image space

Compton Imaging Compton Cones of Response projected into image space

Compton Imaging Compton Cones of Response projected into image space

Compton Camera measurements (Ge/Ge) E = 1408 keV, 30 keV gate 6 cm source to crystal 30 mm crystal to crystal No PSA (5x5x20) Iterative reconstruction FWHM ~ 8mm

Multi-nuclide imaging Compton Imaging Multi-nuclide imaging ~7º Angular Resolution FWHM, central position 152Eu E = 1408 keV 22Na E = 1274 keV 152Eu 2cm source separation No PSA (5x5x20) Cone back projection

MRI compatibility & Status Test existing gamma-ray detector in an MRI scanner Does the detector cause distortions in the MRI image? No Does the MRI system degrade the detector performance? In certain positions (which can be minimised) Encouraging results! ProSPECTus final construction stage System in ~6 months

What are the next steps? Immediate priorities For the future: We (almost) have an integrated Compton Gamma camera optimised for <500keV Demonstrate sensitivity with phantoms Commence trials including clinical evaluation For the future: Consider electron tracking Si scatterer Possible use of large CZT analyser (requires large wafer material with 1cm thickness)

ProSPECTus : The Implication Patient benefits: Earlier and more effective diagnosis of tumours (higher probability of effective treatment). Higher sensitivity offering the scope for shorter imaging time (more patients through one machine per day) or lower doses of radio pharmaceuticals. Cardiac and brain imaging Image larger patients SPECT/MRI: Functional/Anatomical Image co-registration

Credit STFC Daresbury Laboratory, Daresbury, WA4 4AD, UK Department of Physics, University of Liverpool, L69 7ZE, UK MARIARC, University of Liverpool, RLUH NHS Trust, UK Industries Funding agencies STFC, EPSRC, MRC Many people have made significant contributions Lots of UK PhD’s and Post Docs Laura Harkness University of Liverpool 2010 Shell and Institute of Physics Very Early career Woman Physicist of the Year

MRI imaging Preliminary analysis of MRI images acquired with the detector at the entrance of the bore (b) show that the detector does not degrade the MRI performance. FLASH images and TSE MRI images were acquired. Image (a) is when there is no detector present. FLASH - Fast Low Angle Shot TSE - Turbo Spin Echo

Assessing Image Quality Geant4 energy and position Experimental Factors Generate realistic images (i) 2 point sources (ii) 5 point sources (iii) 1 line source Slice of a projection

5 point sources: 1cm between each, alternating 141keV and 159keV Point Source Images 5 point sources: 1cm between each, alternating 141keV and 159keV