The effects of Doppler broadening and detector resolution on the performance of three‐stage Compton cameras

Abstract

Purpose:The authors investigated how the characteristics of the detectors used in a three-stage Compton camera (CC) affect the CC's ability to accurately measure the emission distribution and energy spectrum of prompt gammas (PG) emitted by nuclear de-excitations during proton therapy. The detector characteristics they studied included the material (high-purity germanium [HPGe] and cadmium zinc telluride [CZT]), Doppler broadening (DB), and resolution (lateral, depth, and energy). Methods:The authors simulated three-stage HPGe and CZT CCs of various configurations, detecting gammas from point sources with energies ranging from 0.511 to 7.12 MeV. They also simulated a proton pencil beam irradiating a tissue target to study how the detector characteristics affect the PG data measured by CCs in a clinical proton therapy setting. They used three figures of merit: the distance of closest approach (DCA) and the point of closest approach (PCA) between the measured and actual position of the PG emission origin, and the calculated energy resolution. Results: For CCs with HPGe detectors, DB caused the DCA to be greater than 3 mm for 14% of the 6.13 MeV gammas and 20% of the 0.511 MeV gammas. For CCs with CZT detectors, DB caused the DCA to be greater than 3 mm for 18% of the 6.13 MeV gammas and 25% of the 0.511 MeV gammas. The full width at half maximum (FWHM) of the PCA in theẑ direction for HPGe and CZT detectors ranged from 1.3 to 0.4 mm for gammas with incident energy ranging from 0.511 to 7.12 MeV. For CCs composed of HPGe detectors, the resolution of incident gamma energy calculated by the CC ranged from 6% to 1% for gammas with true incident energies from 0.511 to 7.12 MeV. For CCs composed of CZT detectors, the resolution of gamma energy calculated by the CC ranged from 10% to 1% for gammas with true incident energies from 0.511 to 7.12 MeV. For HPGe and CZT CCs in which all detector effect were included, the DCA was less than 3 mm for 75% and 68% of the detected gammas, respectively, and restricting gammas to those having energy greater than 2.0 MeV increased these percentages to 83% and 77% for HPGe and CZT, respectively. Distributions of the true gamma origins and the PCA after detector characteristics had been included showed good agreement on beam range and some loss of resolution for the lateral profile of the PG emission. Characteristic energy lines were evident in the calculated gamma energy spectrum. Conclusions: The authors found the following: (1) DB is the dominant source of spatial and energy resolution loss in the CCs at all energy levels; (2) the largest difference in the spatial resolution of HPGe and CZT CCs is that the spatial resolution distributions of CZT have broader tails. The differences in the FWHM of these distributions are small; (3) the energy resolution of both HPGe and CZT three-stage CCs is adequate for PG spectroscopy; and (4) restricting the gammas to those having energy greater than 2.0 MeV can improve the achievable image resolution.

Use of a LYSO‐based Compton camera for prompt gamma range verification in proton therapy

“…Several studies focused on optimizing the geometry and materials of three‐stage CCs . Moreover, Mackin et al evaluated the effects of Doppler broadening and detector resolution on the performance of three‐stage CCs. Polf et al showed the feasibility of using a prototype CC to image PG rays emitted during proton beam irradiation.…”

Feasibility Studies of a New Event Selection Method to Improve Spatial Resolution of Compton Imaging for Medical Applications

“…The DCA is defined as the shortest distance between the source position and the cone-of-origin surface [15, 24]. In previous applications [15, 24], DCA has been used as a metric to understand how the physical characteristics of a CC affect characteristics of a detected γ cone-of-origin, and how these effects impact image reconstruction.…”

“…In previous applications [15, 24], DCA has been used as a metric to understand how the physical characteristics of a CC affect characteristics of a detected γ cone-of-origin, and how these effects impact image reconstruction. In this work DCA is used to process the measured data in two different ways.…”

“…Historically, signal-to-noise and spatial resolution of images produced using this method have been limited by the finite size of the detector pixels as well as the effects of the finite energy resolution of the CC detector components and Doppler broadening [15]. Additionally, determining E 0 requires the γ to deposit its full energy (double scatter event) or to interact at least three times (triple scatter event), limiting the detector’s efficiency.…”

Benchmarking Geant4 hadronic models for prompt‐γ monitoring in carbon ion therapy

“…Experimental approaches considered so far are based on tracking secondary protons, [2][3][4][5] detecting prompt-c emission, 6,7 or performing online PET gamma reconstruction. [8][9][10] Several modalities for prompt-c imaging have been investigated: collimated cameras, [11][12][13][14][15] compton cameras [16][17][18][19][20][21][22][23] and prompt-c timing. 24 One important parameter is the production yield of secondary particles used in these methods during treatment, as this will affect the accuracy of the reconstructed dose distribution.…”