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What are SPECT basics?

What are SPECT basics?. Anger camera. Hal O. Anger invented the scintillation camera in 1958 Established basic design: NaI(Tl) crystal PMT array Position weighted signals. Hal O. Anger. PULSE HEIGHT ANALYZER. POSITION SIGNALS. ENERGY SIGNAL. X. Y. Z. PMT ARRAY. NaI(Tl) Crystal.

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What are SPECT basics?

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  1. What are SPECT basics?

  2. Anger camera • Hal O. Anger invented the scintillation camera in 1958 • Established basic design: • NaI(Tl) crystal • PMT array • Position weighted signals Hal O. Anger

  3. PULSE HEIGHT ANALYZER POSITION SIGNALS ENERGY SIGNAL X Y Z PMT ARRAY . . . . . . . NaI(Tl) Crystal COLLIMATOR Image Display Overview

  4. Detector NaI(Tl) crystal Photomultiplier tube (PMT) array Analog-to-digital converters (ADCs) Scintillation camera components • Collimator • Low energy • Medium energy • High energy • Axial shields (coincidence imaging) • Pinhole

  5. Computer(s) Acquisition Processing Acquisition & processing Physicians viewing Scintillation camera components • Patient Table • Pallet • Accessories

  6. Nal(TI) Scintillator • Sensitive material for gamma ray detection • Large rectangular (40 x 50 cm), thin (9.5 mm) crystal* • Converts gamma ray energy into visible light (Total absorption of a 140 keV gamma ray yields 5000 photons) • Fragile: Sensitive to trauma and temperature changes

  7. Advantages 85% sensitivity @ 140 keV Moderate energy resolution (9-10% @ 140 keV) Moderate cost Nal(TI) Crystal • Disadvantages • Hygroscopic (requires hermetic seal) • Limiting component in count rate performance (200 nSec scintillation decay time)

  8. Side View PMTs are arranged in a close-packed array to cover the crystal surface PMT Cross Sections Circular Hexangonal Square FOV 3" PMTs 2" PMTs 30 x 40 cm 28 60 40 x 55 cm 55 120 PMT array

  9. Position-based Signal Weights Position Signal (x or y) Normalized Position Signal (x or y) X/Z Y/Z Weighted Sum Normalization Energy Signal (Z) Pulse Height Analyzer Total Sum Analog position electronics

  10. PMT ARRAY 1 PULSE HEIGHT ANALYZER Y POSITION SIGNALS ENERGY SIGNAL Z X X . . . . . . . NaI(Tl) Crystal COLLIMATOR Y Image Display

  11. Collimation • Purpose: To project gamma ray distribution onto the detector • Basic design • Distance performance • Spatial resolution vs. count sensitivity

  12. Collimator design Image forming aperture of the scintillation camera. Limiting component in spatial resolution & count sensitivity. Collimators are fabricated from lead. 25 mm 1.2 mm Gamma rays that hit the septa are absorbed.

  13. Collimator performance • Count sensitivity • ~ 1/5,000 gamma rays are transmitted • Requires short holes with large diameters • Inverse relationship with resolution • Spatial resolution • 6 - 12 mm FWHM @ 10 cm • Requires long holes with small diameters • Distance dependent

  14. Spatial resolution Dependence on source to collimator distance 5 cm 10 cm 15 cm 20 cm 25 cm 30 cm

  15. Energy correction Before energy correction After energy correction • Corrects for the difference in energy responses within and between PMTs • Digitize local spectra (e.g. @ 64 x 64 locations) • Set local photopeak windows • Event must fall within local window

  16. Linearity correction New location x = x’ + Dx’ y = y’ + Dy’ Event location is estimated as x’,y’ Before linearity correction After linearity correction • Image a known rectangular hole pattern • Calculate x & y correction offsets • Interpolate values over entire field

  17. Linearity correction Before correction After correction Correcting the mispositioning of events (spatial linearity) has a profound effect on field uniformity.

  18. Uniformity correction Energy, linearity & uniformity correction Energy & linearity correction • After energy and linearity corrections are performed, residual non-uniformities are corrected using a reference flood image. • The high count reference flood image is used to regionally weight events.

  19. Scintillation camera performance specifications • Field uniformity (2% - 4%) • Intrinsic spatial resolution (3.5-5.5 mm) • System spatial resolution at 10 cm (8-12 mm) • Energy resolution (9-10%) • Multi-energy window spatial registration (< 2 mm)

  20. Spatial resolution Count profile Ideal point FWHM Image of point with real system pixels • Specifies amount of image blur • Quantified by the full-width-at-half maximum (FWHM) of the point or line spread function

  21. Clinical ApplicationsPlanar & SPECT • Cardiac • Whole-body bone • Renal • Gastric • Hepatobiliary • Thyroid • Pulmonary • Brain

  22. Acquisition types • Static • Dynamic • Whole-body • SPECT • Gated SPECT • Dynamic SPECT • Whole-body SPECT • Coincidence imaging

  23. SPECT Reconstruction Algorithms • FBP (Filtered Back Projection) • Iterative reconstruction (MLEM, OSEM) • 2D • 3D

  24. Iterative reconstruction process Estimated Reconstruction Volume Estimated Projections Acquired (measured) Projections Estimate Projections Update Image Estimate Compare Projections If the correct physical model for the collimation is used in estimating the projections, then the feedback of the iterative process drives the convergence with implicit recovery of resolution. Enhancement filtering of projection data is not required.

  25. Bone SPECT comparison FBP Flash 3D 2D - OSEM e.cam 3/8” Hx: 36-year-old female. Indication staging for osteosarcoma

  26. Myocardial perfusion SPECT FBP Flash 3D 2D Iterative

  27. R-R interval R-Wave Gates Gate 1 Gate 2 Gate 3 Gate 4 Gate 5 Gate 6 Gate 7 Gate 8 Gate 1 GATED SPECT • Aufsummierte Datensatz

  28. Clinical software • Organ specific software • (cardiac, renal, gastric, pulmonary, brain, etc.) • Cardiac quantification software • Cedars Sinai QGS, QPS, QBS • 4D-MSPECT (Univ. of MI) • Emory Cardiac Toolbox • 3D display software • Image fusion software • CT-based attenuation correction

  29. Acquisition Processing Physician Review Printing Innovative Workflow Concept e.soft Schedule Administrative Data Data Aquisition Quality Control Automatic Reconstruction Automatic Processing Data Display HARDCOPY Activity SYNGO

  30. Archiving Printing PACS e.station OEM Workstation Fully Automated Data Distribution e.soft Administrative Data Data Aquisition Quality Control Automatic Reconstruction Automatic Processing Data Display HARDCOPY Activity Workflow Complete SYNGO

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