| dc.description.abstract |
<p>Coincidence Timing Calibration (CTC) is an essential part of ensuring proper PET
scanner function. The purpose of CTC is to account for timing differences in detector
modules. The importance and precision in which this calibration needs to work is even
more stringent for Time-of-Flight (TOF) PET. In this work, we looked to investigate
the CTC process by which the TOF capable GE PET/CT Discovery-690 (D690) operates.
Currently, it uses a 68Ge rotating pin source (RPS) to perform the calibration. The
purpose of this work was to investigate the use of a centrally located source to perform
the calibration. The timing resolution of the D690 was determined and used as a metric
to evaluate both methods. </p><p> Two cylindrical 18F filled phantoms of 7.5 and 10
cm diameter were used to perform the CTC. The RPS and system table motion had to
be disabled in order to use the centrally located sources in the CTC. All CTCs started
with the default calibration file in place. Iterations of the CTC were performed until
convergence of the calibration was observed on the review screen. Even after convergence,
more iterations were performed for further analysis. At the end of the CTC with the
centrally located sources, a follow-up iteration with the RPS was performed to see
what adjustments would be made. Next, the timing resolution of the system was measured
using a 68Ge line source. An apparatus with known locations to support the source
allowed for the evaluation of the timing resolution off the central axis. The importance
of this was that it allowed for non-centrally located lines of response to be evaluated.
Furthermore, the timing resolution was measured with specific calibration files enabled
that corresponded to particular iterations. In addition, a novel way of measuring
the timing resolution (propagated method) for a particular calibration result without
an actual measurement with that calibration enabled was developed and implemented.
This greatly reduced the number of resolution measurements needed, which was particularly
helpful for evaluating the improvement for each iteration. </p><p> The timing resolution
of the system improved as more iterations were done. The difference between the propagated
and measured timing resolution was under 2% most of the time. The cases in which the
discrepancy was larger than 2% corresponded to one of the first iterations performed.
After 15 iterations were performed for both centrally located scanners, the timing
resolution of the system was measured through propagation to be 610 ps. The 15 iterations
amounts to 15 minutes of acquisition time. After one iteration for the RPS, the timing
resolution was measured to be 585 ps (587 ps in the propagated measurement). The single
iteration of the RPS corresponded to 8 minutes of acquisition time. When following
up the final iterations of the centrally located sources with the RPS, there was a
change observed that improved the timing resolution to that measured after only one
iteration of the RPS.</p><p> Conclusively, trends in the data showed that the centrally
located sources did bring opposing detectors into good timing alignment with one another.
These trends also indicated that the current CTC algorithm is not optimized for centrally
located sources for the diameters tested. Finally, the method of propagating the change
in calibration files illustrated a new CTC process method.</p>
|
|
