Time of Flight in Positron Emission Tomography using Fast Sampling Dan Herbst Henry Frisch

2 Summary Overview of PET Fast sampling capabilities Experimental setup Data Analysis

3 PET Metabolically-active positron tracer Antiparallel 511 kEv photon emission Detector ring /

4 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

5 Experimental Setup

6 Hardware Work Uploaded drivers onto BLAB’s FPGA Plateaued tubes Setup coincidence detection Setup delay lines to BLAB Collected data

7 Data Oscilloscope & BLAB pulses (different event)

8 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

9 Pulse Smoothing Experimented with different algorithms Ended up using: f(t) such that is minimized. Parameter ‘c’ determines smoothness

10 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 Department of Radiology

11 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

12 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

13 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!)

14 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

15 Results (scope data) ~300 p.s. FWHM without y-scaling ~270 p.s. with y- scaling (need to confirm)

16 Results (BLAB data) 957 p.s. FWHM assuming 5.12 Gs/sec Obviously there was a malfunction somewhere

17 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

18 Questions?

19 Appendix

20 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 & Physics March 27, 2008 How does Time of Flight improve tumor detection?