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SPECT/CT Basics, Technology Updates, Quality Assurance, and Applications S. Cheenu Kappadath, PhD Department of Imaging Physics University of Texas M D Anderson Cancer Center, Houston, Texas [email protected] Educational Objectives 1. 2. 3. Understand the underlying principles of SPECT/CT image acquisition, processing and reconstruction Understand current and future clinical applications of SPECT/CT imaging Familiarization with commercially-available SPECT/CT systems S. Cheenu Kappadath, PhD AAPM 2009 - Anaheim, CA Outline Review of SPECT principles Iterative SPECT reconstruction Hybrid SPECT/CT imaging SPECT/CT quality assurance Commercial SPECT/CT systems SPECT/CT clinical applications S. Cheenu Kappadath, PhD AAPM 2009 - Anaheim, CA SPECT Basics Single Photon Emission Computed Tomography Radio-pharmaceutical administration – injected, ingested, or inhaled Bio-distribution of pharmaceutical – uptake time Decay of radionuclide from within the patient – the source of information Gamma camera detects gamma rays and images (tomography) the radio-pharmaceutical distribution within the patient – SPECT Used for visualization of functional information based on the specific radio-pharmaceutical uptake mechanism S. Cheenu Kappadath, PhD AAPM 2009 - Anaheim, CA SPECT Hardware Anatomy of a Gamma camera 1. 2. 3. 4. 5. Collimator Scintillation Detector Photomultiplier Tubes Position Circuitry Data Analysis Computer © U of British Columbia S. Cheenu Kappadath, PhD AAPM 2009 - Anaheim, CA SPECT Back-Projection Model g(s,) = f(x,y) along an in-plane line integral © Bruyant, P. P., J Nucl Med 2002; 43:1343-1358 S. Cheenu Kappadath, PhD AAPM 2009 - Anaheim, CA Crystal Thickness Thinner crystals spatial resolution interactions occur at a better defined depth multiple interactions less likely less light spread interaction likelihood for higher energy ’s Thicker crystals sensitivity interaction likelihood (esp. for higher E ’s) likelihood of multiple interactions greater light spread spatial resolution S. Cheenu Kappadath, PhD AAPM 2009 - Anaheim, CA Spatial Resolution Intrinsic Spatial and Energy Resolution # of scintillation photons, N Gamma-ray energy, E Spatial Resolution = 100 /N 1/N 1/E Energy Resolution = 100 FWHM/E 1/E B Le H Collimator Resolution System Resolution S. Cheenu Kappadath, PhD D ( Le H B ) Rg Le Rs2 Ri2 Rg2 Le D AAPM 2009 - Anaheim, CA SPECT Acquisitions SPECT acquires 2-D projections of a 3-D volume © SPECT in the year 2000: Basic principles, JNMT 24:233, 2000 © Yale School of Medicine S. Cheenu Kappadath, PhD AAPM 2009 - Anaheim, CA Radon transform angular symmetry violated in SPECT P() Anterior View S. Cheenu Kappadath, PhD ≠ P(+) horizontally flipped Posterior AAPM 2009 - Anaheim, CA Radon transform angular symmetry violated in SPECT Why ? Due to Differential Attenuation L b a i c I0 I(i+) b I(i) I(i) = I0 e-a (L)dL c I(i+) = I0 e-a (L)dL Other mediating factors: S. Cheenu Kappadath, PhD distance-dependent resolution depth-dependent scatter AAPM 2009 - Anaheim, CA SPECT Acquisitions SPECT projections acquired over 360° Exception: Cardiac SPECT acquired over 180° 0° 0° 180° S. Cheenu Kappadath, PhD AAPM 2009 - Anaheim, CA SPECT images have isotropic voxel size 2-D filter of projections 3-D post-reconstruction filter No volume smoothing transverse sagittal coronal Butterworth: 0.6 Nyquist, 10th order S. Cheenu Kappadath, PhD AAPM 2009 - Anaheim, CA SPECT Filtered BackProjection FBP based on ideal Radon inversion formula SPECT imaging systems are neither angularly symmetric nor shift-invariant assumes a linear, shift-invariant system and angular symmetry of projections SPECT projection data affected by attenuation, scatter, and spatial resolution that are all depth-or distance-dependent Thus, FBP reconstruction cannot adequately model the physics of SPECT S. Cheenu Kappadath, PhD AAPM 2009 - Anaheim, CA Conventional SPECT Corrections Attenuation: Chang post-processing algorithm L(x,y,i) I(x,y) = SPECT image w/o AC I(x,y,i) = IAC(x,y).e-L(x,y,i) IAC (x,y) = I(x,y) / {(1/M).i e-L(x,y,i)}; I(x,y,i) i i = 1, M IAC(x,y) Scatter: Energy window subtraction Lower Scatter Window PhotoPeak Window Upper Scatter Window STD in acrylic 20000 STD in air Counts P(x,y) = projections w/ scatter PLE(x,y) = projection at lower energy PHE(x,y) = projection at higher energy PSC (x,y) = P(x,y) – kL.PLE(x,y) – kH.PHE(x,y) Energy Spectrum of Sm-153 30000 STD in acrylic with TEW Scatter Correction 10000 0 25 50 75 100 125 150 Photon Energy [keV] S. Cheenu Kappadath, PhD AAPM 2009 - Anaheim, CA Outline Review of SPECT principles Iterative SPECT reconstruction Hybrid SPECT/CT imaging SPECT/CT quality assurance Commercial SPECT/CT systems SPECT/CT clinical applications S. Cheenu Kappadath, PhD AAPM 2009 - Anaheim, CA SPECT Iterative Reconstruction Maximum Likelihood-Expectation Maximization (ML-EM) Accounts for the statistical nature of SPECT imaging Incorporates the system response p(b,d) – the probability that a photon emitted from an object voxel b is detected by projection pixel d voxel b detector d p(b,d) captures… 1. Depth-dependent resolution 2. Position-dependent scatter 3. Depth-dependent attenuation Use a measured attenuation map along with models of scatter and camera resolution to perform a far more accurate reconstruction S. Cheenu Kappadath, PhD AAPM 2009 - Anaheim, CA SPECT Iterative Recon: Attenuation Modeling b along a line integral … g(s,) = f(x,y) * pattn(x,y,s,) pattn(x,y,s,) = probability due to attenuation pattn(x,y,s,) = exp(-ab(x’,y’)x’,y’)) S. Cheenu Kappadath, PhD a AAPM 2009 - Anaheim, CA SPECT Iterative Recon: System Resolution Modeling Distance-dependent collimator beam ________ Rs = Ri2 + Rc2 r Pencil Beam (FBP) Intrinsic Detector Resolution Ri - iterative) Fan Beam (2D Cone Beam (3D iterative) S. Cheenu Kappadath, PhD AAPM 2009 - Anaheim, CA SPECT Iterative Recon: Resolution Modeling 2D: g(s,) = f(x,y) * pres(x,y,s,) 3D: g(s,) = f(x,y,z) * pres(x,y,z,s,) pres = probability due to resolution “fan of acceptance” (2D fan beam model) “cone of acceptance” (3D cone beam model) S. Cheenu Kappadath, PhD AAPM 2009 - Anaheim, CA SPECT Imaging: Scatter Scatter compensation occurs before attenuation the photopeak window contains scatter attenuation accounts for the removal of photopeak photons Scatter contribution estimated as a weighted sum of one or more adjacent energy window images, Ci(x,y,) S(x,y,) = i ki × Ci(x,y,) Subtract scatter prior to reconstruction Pcorr(x,y,) P(x,y,) - S(x,y,) Incorporate scatter into forward projection P(x,y,) Pcorr(x,y,) + S(x,y,) S. Cheenu Kappadath, PhD SC techniques: DEW TEW ESSE AAPM 2009 - Anaheim, CA SPECT Iterative Reconstruction True projection intensity = sum of true voxel intensities weighted by detection probabilities Forward Projection True voxel intensity = sum of true detector intensities weighted by detection probabilities Back Projection S. Cheenu Kappadath, PhD B y (d ) (b) p (b, d ) b 1 D (b) y (d ) p (b, d ) d 1 AAPM 2009 - Anaheim, CA Iterative Reconstruction Flow Diagram D (b ) [k ] [ k 1] (b ) d 1 y ( d ) p (b , d ) [k ] b '1 (b ') p (b ', d ) B D p (b , d ) d 1 In clinical practice, the stopping criteria is number of iterations (a time constraint) instead of a convergence criteria. S. Cheenu Kappadath, PhD AAPM 2009 - Anaheim, CA Ordered Subset EM (OSEM) Each OSEM iteration is a ML-EM iteration using an ordered subset of n (out of N) projections (eg: 4/36 views - 9 subsets, start with 0°,90°,180°,270° views) The next OSEM iteration starts with the result of the previous OSEM iterations but uses a different ordered subset of n projections (next set uses 10°,100°,190°,280° views) rate of convergence by using an ordered subset of all N projections for each iteration m OSEM iterations with n subsets each mn ML-EM iterations using all N each time S. Cheenu Kappadath, PhD AAPM 2009 - Anaheim, CA OSEM Iterative SPECT Reconstruction: Attenuation and Scatter Correction Un-Corrected Corrected Note the “hot-rim” artifact S. Cheenu Kappadath, PhD AAPM 2009 - Anaheim, CA OSEM Iterative SPECT Reconstruction: Collimator Resolution Modeling 99mTc Bone Scan (osteosarcoma), LEHR Collimator Standard Filtered Backprojection 2-D OSEM w/ fan beam modeling (m=12,n=10) 2-D pre-filter: Butterworth, fc = 0.6 Nyquist, order = 10 3-D OSEM w/ cone beam modeling (m=25,n=10) S. Cheenu Kappadath, PhD 3-D Gaussian Post-Filter (7.8 mm FWHM) AAPM 2009 - Anaheim, CA Outline Review of SPECT principles Iterative SPECT reconstruction Hybrid SPECT/CT imaging SPECT/CT quality assurance Commercial SPECT/CT systems SPECT/CT clinical applications S. Cheenu Kappadath, PhD AAPM 2009 - Anaheim, CA SPECT/CT Hybrid Imaging: Why? Non-uniform attenuation maps required Previous methods used constant maps that work for brain but are problematic for thorax and pelvis radioactive source-based transmission CT – time penalty Functional-anatomical overlay (image fusion) Improve localization of uptake regions Increase confidence in interpretation S. Cheenu Kappadath, PhD AAPM 2009 - Anaheim, CA CT-based AC for SPECT/CT CT CTAC μ‐map CT noise reduced Smooth, re‐bin CT to match SPECT Register CT w/ SPECT Apply bi‐linear transform on pixel‐by‐pixel basis Reconstructed SPECT Transition Matrix aijk S. Cheenu Kappadath, PhD Other factors: ‐SPECT projections ‐Scatter estimates ‐Collimator response AAPM 2009 - Anaheim, CA CT-based values Material attenuation versus Energy Air Muscle Bone Photoelectric effect Compton scatter dominant dominant 0.3 (cm2/g) 0.2 0.1 CT 0 0 100 200 Energy S. Cheenu Kappadath, PhD 300 400 m = k ¥ CT-HU (simple but not accurate) Compton Scatter probability proportional to e- density Photoelectric effect probability proportional to (Z/E)3 Attenuation mismatch between PE and CS with energy for high Z 500 (keV) AAPM 2009 - Anaheim, CA CT-based values - HU-to-cm-1 conversion - not linearly related - piece-wise linear - bi- or tri-modal - Effective energy differences - CT (~ 70 – 80 keV) - SPECT (nuclide dependent) eg: 140 keV for Tc-99m CT Number-to-Tc-99m v alue Function 0.3 value (cm-1) 0.25 0.2 0.15 0.1 0.05 1000 200 0 -1000 0 CT Number (HU) S. Cheenu Kappadath, PhD AAPM 2009 - Anaheim, CA SPECT/CT Hybrid Imaging: Iterative Reconstruction FBP w/ Butterworth 0.4/5 99mTc EC-DG (NSCLC) 3-D OSEM w/ resolution modeling 3-D OSEM w/ resolution and attenuation modeling S. Cheenu Kappadath, PhD AAPM 2009 - Anaheim, CA Outline Review of SPECT principles Iterative SPECT reconstruction Hybrid SPECT/CT imaging SPECT/CT quality assurance Commercial SPECT/CT systems SPECT/CT clinical applications S. Cheenu Kappadath, PhD AAPM 2009 - Anaheim, CA SPECT/CT QA/QC Planar (AAPM Reports 6 and 9; NEMA NU 1-1994) SPECT (AAPM Report 22 and 52) Inherently includes all planar gamma camera QA Energy/Spatial resolution, uniformity, deadtime, sensitivity, rotational uniformity, opposed-head registration, etc. Uniformity and Contrast Resolution SPECT/CT (AAPM TG 177: Jim Halama) NM-CT registration CT-HU to linear attenuation () transformation S. Cheenu Kappadath, PhD AAPM 2009 - Anaheim, CA NM-CT Registration Use Co-57 button sources w/ SPECT phantom S. Cheenu Kappadath, PhD AAPM 2009 - Anaheim, CA CT-HU to -map transformation Use an electron density phantom CIRS Inc. CT image: -790 to 235 HU S. Cheenu Kappadath, PhD AAPM 2009 - Anaheim, CA Outline Review of SPECT principles Iterative SPECT reconstruction Hybrid SPECT/CT imaging SPECT/CT quality assurance Commercial SPECT/CT systems SPECT/CT clinical applications S. Cheenu Kappadath, PhD AAPM 2009 - Anaheim, CA Commercial SPECT/CT systems Siemens SymbiaT (1-, 2-, 6, 16-slice CT) GE Hawkeye (1- or 4-slice CT) S. Cheenu Kappadath, PhD Philips BrightView (Flat-panel CT) AAPM 2009 - Anaheim, CA GE – Millennium VG Hawkeye NM 3/8” and 1” NaI(Tl) crystals 16 simultaneous energy windows Slip-ring gantry Body-contouring based on infrared-based transmitters CT Co-planar, dental tube, 4-slice 20 mm beam no additional real estate needed Resolution: 3.5 or 1.75 mm (transaxial); 5 or 10 mm (axial) Time-averaged: 23 s per rotation (slow-scan) kVp: 120 – 140; mA: 1 – 2.5 S. Cheenu Kappadath, PhD AAPM 2009 - Anaheim, CA Phillips – BrightView XCT NM 3/8” and ¾” NaI(Tl) crystals Energy-independent flood calibration (up to 300 keV) 15 simultaneous energy windows Body-contouring based on tissue impedance CT Co-planar, flat-panel detector, 14 cm axial FOV no additional real estate needed High-resolution: 0.33 mm isotropic voxels Time-averaged: 12 s or 24 s per rotation (slow-scan) kVp: 120; mA: 5 – 80 S. Cheenu Kappadath, PhD AAPM 2009 - Anaheim, CA Siemens - SymbiaT NM 3/8” and 5/8” NaI(Tl) crystals Energy-independent flood calibration (up to 300 keV) 6 simultaneous energy windows Body-contouring based on infrared-based transmitters CT Diagnostic CT scanner kVp: 80/110/130; mA: 20 – 345 (T16) & 30 – 240 (T6) Scan time: 0.5, 0.6, 1, 1,5 s per rotation 1-, 2-, 6-, and 16-slice CT scanners S. Cheenu Kappadath, PhD AAPM 2009 - Anaheim, CA Outline Review of SPECT principles Iterative SPECT reconstruction Hybrid SPECT/CT imaging SPECT/CT quality assurance Commercial SPECT/CT systems SPECT/CT clinical applications S. Cheenu Kappadath, PhD AAPM 2009 - Anaheim, CA Clinical SPECT/CT Imaging Stress/Rest Myocardial Perfusion Imaging Stress: 99mTc-sestaMIBI or 99mTc-Tetrafosmin Rest: 99mTc-labeled agents or 201Tl-chloride 99mTc-MDP: bone diseases, bone metasteses 99mTc-sestaMIBI: parathyroid adenomas 99mTc-sulphur colloid: liver/spleen, lymphoscintigraphy 111In-Pentetreotide: neuroendocrine cancers 111In-ProstaScint: prostate cancer 123I/131I-MIBG: pheochromocytoma, neuroblastoma 131I-NaI: thyroid cancer S. Cheenu Kappadath, PhD AAPM 2009 - Anaheim, CA Clinical SPECT/CT Imaging 99mTc-CEA: colorectal cancer 99mTc-RBCs: hemangioma 99mTc-HMPAO, -ECD: brain perfusion 111In-WBC: infection 67Ga-citrate: inflammation, lymphoma 201Tl-chloride: tumor perfusion S. Cheenu Kappadath, PhD AAPM 2009 - Anaheim, CA Clinical Benefits of SPECT/CT Visualization, diagnosis and interpretation of primary and metastatic diseases higher sensitivity and contrast than Planar imaging CT scan increases confidence in interpretation of SPECT examination Surgical planning and IMRT treatment planning 90Y-microspheres radio-embolotherapy (selective internal RT or micro-brachytherapy) Internal radio-pharmaceutical therapy planning S. Cheenu Kappadath, PhD AAPM 2009 - Anaheim, CA SPECT/CT: Limitations Patient motion Contrast CT between SPECT and CT scans respiratory and cardiac motion during SPECT acquisitions contrast introduces electron density-material mismatch map algorithms do not yet account for contrast CT Absolute quantification (Bq/ml) not yet fully developed radionuclide-dependent acquisition/reconstruction technique-dependent calibration techniques not yet standardized S. Cheenu Kappadath, PhD AAPM 2009 - Anaheim, CA SPECT/CT: Future Applications Whole body SPECT/CT (analogous to PET/CT) Quantification of absolute activity (like PET) Compensation for CT contrast in map Compensation for respiratory, cardiac motion SPECT/CT-based 3-D dosimetry/treatment planning S. Cheenu Kappadath, PhD AAPM 2009 - Anaheim, CA Future: Whole-body Bone SPECT/CT Tc-99m MDP Bone Imaging S. Cheenu Kappadath, PhD AAPM 2009 - Anaheim, CA 99m-Tc MDP SPECT/CT: Fused Coronal views S. Cheenu Kappadath, PhD AAPM 2009 - Anaheim, CA Future: Multi-nuclide SPECT/CT Maximum Intensity Projection (MIP) of a dual-isotope (Tc-99m and I-123) SPECT/CT mouse study. Published by the Molecular Imaging Center for Excellence newsletter, SNM publication Volume 2, 2008 S. Cheenu Kappadath, PhD AAPM 2009 - Anaheim, CA