Time of Flight in Positron Emission Tomography using Fast
Sampling
Dan Herbst
Henry Frisch
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Summary
• Overview of PET
• Fast sampling capabilities
• Experimental setup
• Data
• Analysis
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PET
• Metabolically-active positron tracer
• Antiparallel 511 kEv photon emission
• Detector ring
http://www.scq.ubc.ca/looking-inside-the-human-body-using-positrons/
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Fast Sampling
• Tektronix– 40 Gs/sec– $142K retail– Continuous fast
sampling
• BLAB1– ~5.12 Gs/sec– ~$10/channel in bulk– Triggered burst of fast
sampling
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Experimental Setup
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Hardware Work
• Uploaded drivers onto BLAB’s FPGA
• Plateaued tubes
• Setup coincidence detection
• Setup delay lines to BLAB
• Collected data
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Data
• Oscilloscope & BLAB pulses (different event)
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Filtering on Energy• Many photons will
Compton scatter off of scintillation crystal, only depositing partial energy
• Keep only events where both pulses are fully absorbed
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Pulse Smoothing
• Experimented with different algorithms• Ended up using: f(t) such that is minimized. • Parameter ‘c’ determines smoothness
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A Typical Time Extraction Algorithm
• Fit the leading-edge points to a function (i.e. linear fit), and take where that function crosses the baseline
Qingguo Xie, UChicago Departmentof Radiology
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My Objections
• Why weight all points on the leading edge equally?
• Why fit to a line or other arbitrary function?
• Make these things parameters and feed to an optimization algorithm– Quality measure: standard deviation of timing
difference over a large set of representative pulse pairs
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Why Pulse Shape Optimizations May Have Failed in the Past
• Many degrees of freedom– Valleys become narrow, must scale
parameters – Time extraction must be fast to give optimizer
many attempts– Bias in stepping unless careful
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My Timing Extractor
• Normalize pulses• Fit the template to the
pulse under the transformations:– Time shift– Time scale (about a
given point)– y-scale (optional)
• …using least squares (horizontal!)
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Advantage
• Since least squares fitting is in horizontal direction, time-shift, time-scale, and scale-about point (global) are calculated analytically
Disadvantage• Pulse is only “sampled” at a limited
number of points– Working on a new version to fix this problem
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Results (scope data)
• ~300 p.s. FWHM without y-scaling
• ~270 p.s. with y-scaling (need to confirm)
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Results (BLAB data)
• 957 p.s. FWHM assuming 5.12 Gs/sec
• Obviously there was a malfunction somewhere
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Where to Proceed
• Short term:– Shorten travel distances in photo-tube base– Finish full-sampling version of pulse-shape
optimizer– Understand BLAB results
• Long term:– Simulate and optimize phototube design– Improve fast sampling board
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Questions?
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Appendix
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scan time = 5 min 3 min 2 min 1min
35-cm diameter phantom 10, 13, 17, 22-mm hot spheres (6:1 contrast); 28, 37-mm cold spheres background activity concentration of 0.14 Ci/ml
TOF achieves better contrast, with shorter scan
#iter = 10
#iter = 5
nonTOF
TOF
Slide by Joel Karp, University of Pennsylvania Dept. of Radiology & PhysicsMarch 27, 2008
How does Time of Flight improve tumor detection?