Optimizing light transport in scintillation crystals for time-of-flight PET: an experimental and optical Monte Carlo simulation study

Abstract

Achieving excellent timing resolution in gamma ray detectors is crucial in several applications such as medical imaging with time-of-flight positron emission tomography (TOF-PET). Although many factors impact the overall system timing resolution, the statistical nature of scintillation light, including photon production and transport in the crystal to the photodetector, is typically the limiting factor for modern scintillation detectors. In this study, we investigated the impact of surface treatment, in particular, roughening select areas of otherwise polished crystals, on light transport and timing resolution. A custom Monte Carlo photon tracking tool was used to gain insight into changes in light collection and timing resolution that were observed experimentally: select roughening configurations increased the light collection up to 25% and improved timing resolution by 15% compared to crystals with all polished surfaces. Simulations showed that partial surface roughening caused a greater number of photons to be reflected towards the photodetector and increased the initial rate of photoelectron production. This study provides a simple method to improve timing resolution and light collection in scintillator-based gamma ray detectors, a topic of high importance in the field of TOF-PET. Additionally, we demonstrated utility of our Monte Carlo simulation tool to accurately predict the effect of altering crystal surfaces on light collection and timing resolution.

Keywords: (120.3890) Medical optics instrumentation; (170.2670) Gamma ray imaging; (170.5280) Photon migration; (240.0240) Optics at surfaces; (240.5770) Roughness; (260.1180) Crystal optics.

Fig. 1

Surface roughening sections are indicated by red, polished surfaces by blue. a) Top surface roughened, b) – e) One side roughened 5 – 20 mm, f) Two sides roughened 10 mm, g) All lateral sides roughened 20 mm. The bottom surface of the crystal is coupled to the PMT.

Fig. 2

a) AFM sample of polished surface. b) AFM sample of roughened surface. c) Computed reflectance curves for polished and roughened sections.

Fig. 3

a) Schematic used for testing crystals. Drawing is not to scale. A light absorbing material (not shown) is placed around the crystals outside the reflector to secure the crystals to the PMT window. b) Experimental setup used to acquire detector data.

Fig. 4

Processes involved in generating simulated scintillation pulses.

Fig. 5

a) Experimental 511 keV energy spectrum with Gaussian fit of the photopeak to estimate light collection. b) Changes in photopeak position for roughened crystals relative to all polished surfaces. Error bars represent standard deviation of five measurements for experimental data and three measurements for simulated data.

Fig. 6

a) Histogram of experimental timing data and Gaussian fit used to calculate the timing resolution. b) Changes in timing resolution with roughened surfaces relative to all polished surfaces. Error bars represent standard deviation of five measurements for experimental data and three measurements for simulated data. The green bars represent predicted changes in experimental timing resolution based on Eq. (1).

Fig. 7

Spatial distribution of photons reflected off one lateral surface for a) polished (Media 1), b) top roughened (Media 2), c) one side roughened 5 mm (Media 3), and d) one side roughened 15 mm (Media 4). Photon directions (blue points) were projected on a unit sphere (yellow).

Fig. 8

a) Angles of photons reflected off crystal faces (face in contact with PMT not included). Angles were computed with respect to the horizontal assuming the crystal was vertical. b) Fraction of emitted photons that were detected, escaped through the sides of the crystal, or were absorbed (bulk absorption with absorption length = 800 mm) varied with roughening.

Fig. 9

a) Schematic representation of computing signals from single photon responses. For each photoelectron, a Gaussian pulse is generated (blue), and the sum of all Gaussian pulses forms the signal (red). With a 1 ns rise time, the sum of single photon responses quickly reaches the timing pick-off threshold (Th). b) Photoelectron production rate for 4 surface configurations, showing that the crystals with one side roughened 5 mm and 15 mm produced more photoelectrons around the pick-off time (~1.3 ns). c) Coincidence timing resolution for the 200 events used for photoelectron production rate analysis.