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Single Photon Emission Tomography SPECT. Physics CLRS 344. Anger Gamma Camera. X. Y. Positional Circuit. Z. Gate. Output. Collimator. Collimators. Tomography. Shows position and relationship of objects in 3D. Planar Imaging Resolution is depth dependent
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Single Photon Emission Tomography SPECT Physics CLRS 344
Anger Gamma Camera X Y Positional Circuit Z Gate Output Collimator
Tomography • Shows position and relationship of objects in 3D. • Planar Imaging • Resolution is depth dependent • Single Photon Emission Computed Tomography • Resolution is independent on depth • Resolution is inferior to Planar • Reconstruction magnifies noise • Signal-to-noise ratio is less than planar for the same number of acquired counts
Single Photon Emission Computed TomographySPECT Acquire multiple planar views
SPECT Acquisition • Linear Sampling • d 1 note: max is the max obs. frequency (2max) Nyquist Frequency • Angular Sampling • Number of views = D/2d • D: Diameter of view • d: linear sampling distance
SPECT Acquisition Sampling Point Actual signal, 9 Hz False signal, 1 Hz
SPECT Acquisition Angular sampling Minimum acquisition: 1800
Spatial Frequency Line pair (# lines per unit distance)
Modulation Transfer Function Low spatial Frequency Counts Pixel-distance Counts High spatial Frequency Pixel-distance
Modulation Transfer Function Low spatial Frequency Counts MTF= cout()/cin() = 1 Pixel-distance High spatial Frequency Counts MTF= cout()/cin() = 0.33 Pixel-distance
Modulation Transfer Function ideal Collimator-source distance MTF= cout()/cin() 2.5 cm 10 cm Spatial Frequency (cm-1)
Spatial Frequency Spatial Domain Frequency Domain Amplitude Counts Frequency pixel
Spatial Frequency Frequency Domain Amplitude Frequency
Reconstruction Algorithms • Filter Backprojection • Easy to implement • Computational fast • Iterative reconstruction • Number of iterations: hard to determine • Computational intense
Reconstruction Algorithms g4=f1+f2+f3 g5=f4+f5+f6 g6=f7+f8+f9 Radon Equation g2=f2+f5+f8 g1=f3+f6+f9 g3=f1+f4+f7
Reconstruction Algorithms Radon Equation g4 Backprojection Operator g5 Filtered Backprojection g6 g3 g2 g1
Transverse Image 60 60 60 60
Backprojection 120 120 counts counts 120 60 60 120 120 60 60 120 counts 120 120 counts
Backprojection 120 120 counts 20 20 20 20 20 20 20 20 20 20 20 20
Backprojection 120 120 counts counts 40 40 120 40 80 80 40 40 80 80 40 120 40 40 120 40 80 80 40 40 80 80 40 120 40 40 40 40 counts 120 120 counts
Backprojection 40 40 40 80 80 40 40 80 80 40 40 40 40 80 80 40 40 80 80 40 40 40 40 40
Backprojection 80 80 80
RampBackprojection 80 80 80 80 80 80 80 Amp Spatial Freq.
Filter High Freq Noise Resultant freq Low Freq
Filter Amp Cut-off frequency Nyquest Freq Spatial Freq. Smoothing function Butterworth- Low Pass Metz Wiener Parzen Amp Amp Spatial Freq. Spatial Freq.
Filter Backprojection Amp Amp X Spatial Freq. Spatial Freq. Amp Spatial Freq.
Filter Backprojection standard 0.5 mCi 1.0 mCi 6.0 mCi
Iterative Reconstruction x x x Recon Image Initial
Iterative Reconstruction y y y y x Recon Image Initial
Iterative Reconstruction 45 45 y 45 45 x Recon Image Initial
Reconstruction Algorithms Iterative Methods 10 6 5 11 0 0 7 9 8 8 8 8
standard 6mCi/FBP 6mCi/Iterative
Filtered Backprojection Iterative Reconstruction Reconstruction Algorithms Scan Time 7 min 5 min 3 min
Filtered Backprojection Iterative Reconstruction Reconstruction Algorithms
Attenuation Correction • Uniform attenuation • First Order Attenuation Correction • Chang’s Method • Measured (Transmission Image) • Point source Measured Attenuation • Point source Segmented Attenuation
Attenuation Correction • Chang’s Method d1 Contour d2 Assume: uniform Transverse slice Note: Only good for Brain imaging
Attenuation Correction • Transmission Method (point source) • Geometric Mean I0 I2 I1 d2 d1 D
Attenuation Correction • Transmission Method (Measured) For each Line of response (LOR) Obtain; 1, 2, 3, etc
Attenuation Correction • Transmission Method (Measured) Low statistics Poor Image quality Require long transmission scans
Attenuation Correction • Transmission Method (Segmented) Option: Group attenuation factors into 3 groups; Lung, Tissue, and Bone lung tissue bone
Attenuation Correction • Segmented Attenuation Correction Measured Segmented
Attenuation Correction • Segmented Attenuation Correction MEASURED SEGMENTED 3 Minutes 2 Minutes 1 Minute
SPECT Corrections • Partial Volume Concentration: Counts/ROI nCi/cc