Noise evaluation of Compton camera imaging for proton therapy

Abstract

Compton Cameras emerged as an alternative for real-time dose monitoring techniques for Particle Therapy (PT), based on the detection of prompt-gammas. As a consequence of the Compton scattering process, the gamma origin point can be restricted onto the surface of a cone (Compton cone). Through image reconstruction techniques, the distribution of the gamma emitters can be estimated, using cone-surfaces backprojections of the Compton cones through the image space, along with more sophisticated statistical methods to improve the image quality. To calculate the Compton cone required for image reconstruction, either two interactions, the last being photoelectric absorption, or three scatter interactions are needed. Because of the high energy of the photons in PT the first option might not be adequate, as the photon is not absorbed in general. However, the second option is less efficient. That is the reason to resort to spectral reconstructions, where the incoming γ energy is considered as a variable in the reconstruction inverse problem. Jointly with prompt gamma, secondary neutrons and scattered photons, not strongly correlated with the dose map, can also reach the imaging detector and produce false events. These events deteriorate the image quality. Also, high intensity beams can produce particle accumulation in the camera, which lead to an increase of random coincidences, meaning events which gather measurements from different incoming particles. The noise scenario is expected to be different if double or triple events are used, and consequently, the reconstructed images can be affected differently by spurious data. The aim of the present work is to study the effect of false events in the reconstructed image, evaluating their impact in the determination of the beam particle ranges. A simulation study that includes misidentified events (neutrons and random coincidences) in the final image of a Compton Telescope for PT monitoring is presented. The complete chain of detection, from the beam particle entering a phantom to the event classification, is simulated using FLUKA. The range determination is later estimated from the reconstructed image obtained from a two and three-event algorithm based on Maximum Likelihood Expectation Maximization. The neutron background and random coincidences due to a therapeutic-like time structure are analyzed for mono-energetic proton beams. The time structure of the beam is included in the simulations, which will affect the rate of particles entering the detector.

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

“…(b), the simulated results show that the LYSO‐based two‐stage CC can measure the initial energy of incident PGs. In addition to our study, several research groups have investigated the use of scintillator‐based scatterers . These groups demonstrated that CCs consisting of a high Z and fast scintillator‐based scatterer have the advantages of high detection efficiency, reasonable angular resolution, and fast timing response, which are essential for proton therapeutic applications.…”

“…The proposed two‐stage CC produced PG images with better quality than the three‐stage CC (Fig. ), possibly because the number of triple‐coincidence events is too low . The poor performance of the studied three‐stage CC may also be due to detector resolution effects .…”

A cost-effective monitoring technique in particle therapy via uncollimated prompt gamma peak integration

“…Collimated systems 2-6,10,11 have already mechanical constraints due to the weight of the collimator, furthermore either a multitude of single detectors, or position sensitive detectors are required. Compton cameras [7][8][9] are even more complex in terms of the number of readout channels and an (iterative) image reconstruction is required.…”

Large surface gamma cameras for medical imaging: characterization of the bismuth germanate blocks

“…Moreover, the absence of internal radioactivity which characterizes other scintillator materials employed in the same field (e.g. LYSO), makes it suitable for low noise detectors, required by a Compton camera to reduce the amount of random coincidences, one of the main sources of background for the application in ion beam therapy monitoring [13].…”