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?