Detector development for positron emission based real-time tumor tracking

Abstract

Tumor motion limits the accuracy of radiation therapy. Positron emission tracking (PeTrack) is a technique that can track tumors through detecting annihilation gammas from implanted positron emission markers. This thesis focuses on detector development for PeTrack. Due to the intense scattered x rays from a Linac, scintillator (BGO) afterglow and detector gating, i.e. turning off the detector during the intense x-ray pulse need to be addressed. The evaluation of BGO showed very low afterglow. A gating circuit was designed, optimized and tested. Energy resolution of the detector is better than 25% (FWHM) with optimal gating parameters. A data acquisition system for PeTrack was set up and calibrated successfully. The first PeTrack prototype was developed and evaluated. The prototype was able to localize two positron emitting markers with an average precision of 0.16-0.21 mm, and an average accuracy of 0.6 mm on distance between the two markers.

Technical aspects of real time positron emission tracking for gated radiotherapy

“…In addition, the scattered x rays generate intense scintillation light in the PeTrack detectors which saturates the PMTs. 34 Therefore, the PeTrack detectors need to be gated off during each x-ray pulse. A custom phase-lock loop (PLL) system was designed and built (TECFA, Inc., Ottawa, ON).…”

“…A faster PMT gating mechanism has been developed previously. 34 Instead of turning off the entire HV supply to the PMTs and turning it on again, the gating mechanism only reverses the voltage of two dynodes in each PMT, with much smaller voltage disruption. This gating mechanism should be implemented in the future to overcome the current data collection efficiency degradation at higher Linac pulse frequencies.…”

Application of Three-Dimensional Motion Tracking of Low-Activity Fiducial Positron-Emitting Markers in Radiation Therapy and Positron Emission Tomography

“…The peak around channel 230 is the 511 keV annihilation peak; the peak around channel 600 is the 1.274 MeV photopeak of 22 Na. This figure is reproduced from [3] with permission from the author. .…”

“…Performance evaluation of a preliminary version of the prototype detectors was conducted by Soleimani. [3] The energy resolution was measured as (22±1)% at 511 keV from a set of ten crystal elements from a detector block. Figure 2.8 (reproduced from [3] with permission from the author) shows the energy spectrum measured by Energy spectrum of a single crystal element (size 3.9 × 3.9 × 20 mm 3 ) exposed to radiation from a 22 Na source.…”

“…[3] The energy resolution was measured as (22±1)% at 511 keV from a set of ten crystal elements from a detector block. Figure 2.8 (reproduced from [3] with permission from the author) shows the energy spectrum measured by Energy spectrum of a single crystal element (size 3.9 × 3.9 × 20 mm 3 ) exposed to radiation from a 22 Na source. The peak around channel 230 is the 511 keV annihilation peak; the peak around channel 600 is the 1.274 MeV photopeak of 22 Na.…”