V. Maxim

A Tracking Compton-Scattering Imaging System for Hadron Therapy Monitoring

Hadron therapy for, e.g., cancer treatment requires an accurate dose deposition (total amount and location). As a consequence, monitoring is crucial for the success of the treatment. Currently employed PET imaging systems are not able to provide information about the deposed dose fast enough to allow stopping the therapy in case of a discordance with the treatment plan. We are currently investigating an imaging system based on a combined Compton scattering and pair creation camera capable of imaging gamma rays up to 50 MeV. The camera would be able to measure the complete spectrum of emitted gamma rays during the therapy session. We have performed Monte Carlo simulations for three different proton beam energies in a typical hadron therapy scenario. They show that the location of the gamma-ray distribution decay and the falloff region of the deposed dose are related. The reconstructed images prove that the proposed system could provide the required imaging and dose location capabilities.

Design study of the absorber detector of a compton camera for on-line control in ion beam therapy

The goal of this study is to tune the design of the absorber detector of a Compton camera for prompt γ-ray imaging during ion beam therapy. The response of the Compton camera to a photon point source with a realistic energy spectrum (corresponding to the prompt γ-ray spectrum emitted during the carbon irradiation of a water phantom) is studied by means of Geant4 simulations. Our Compton camera consists of a stack of 2 mm thick silicon strip detectors as a scatter detector and of a scintillator plate as an absorber detector. Four scintillators are considered: LYSO, NaI, LaBr3 and BGO. LYSO and BGO appear as the most suitable materials, due to their high photo-electric cross-sections, which leads to a high percentage of fully absorbed photons. Depth-of-interaction measurements are shown to have limited influence on the spatial resolution of the camera. In our case, the thickness which gives the best compromise between a high percentage of photons that are fully absorbed and a low parallax error is about 4 cm for the LYSO detector and 4.5 cm for the BGO detector. The influence of the width of the absorber detector on the spatial resolution is not very pronounced as long as it is lower than 30 cm.

Probabilistic models and numerical calculation of system matrix and sensitivity in list-mode MLEM 3D reconstruction of Compton camera images.

This paper addresses the problem of evaluating the system matrix and the sensitivity for iterative reconstruction in Compton camera imaging. Proposed models and numerical calculation strategies are compared through the influence they have on the three-dimensional reconstructed images. The study attempts to address four questions. First, it proposes an analytic model for the system matrix. Second, it suggests a method for its numerical validation with Monte Carlo simulated data. Third, it compares analytical models of the sensitivity factors with Monte Carlo simulated values. Finally, it shows how the system matrix and the sensitivity calculation strategies influence the quality of the reconstructed images.

Low Statistics Reconstruction of the Compton Camera Point Spread Function in 3D Prompt-

The Compton camera is a relevant imaging device for the detection of prompt photons produced by nuclear fragmentation in hadrontherapy. It may allow an improvement in detection efficiency compared to a standard gamma-camera but requires more sophisticated image reconstruction techniques. In this work, we simulate low statistics acquisitions from a point source having a broad energy spectrum compatible with hadrontherapy. We then reconstruct the image of the source with a recently developed filtered backprojection algorithm, a line-cone approach and an iterative List Mode Maximum Likelihood Expectation Maximization algorithm. Simulated data come from a Compton camera prototype designed for hadrontherapy online monitoring. Results indicate that the achievable resolution in directions parallel to the detector, that may include the beam direction, is compatible with the quality control requirements. With the prototype under study, the reconstructed image is elongated in the direction orthogonal to the detector. However this direction is of less interest in hadrontherapy where the first requirement is to determine the penetration depth of the beam in the patient. Additionally, the resolution may be recovered using a second camera.

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The Compton camera is an imaging device relevant for the detection of prompt photons produced during nuclear fragmentation in hadrontherapy. It allows a considerable improvement in detection compared to a standard gamma-camera but also requires more sophisticated image reconstruction techniques. In this work we apply a recently developed filtered backprojection algorithm to data simulated from a realistic Compton camera prototype designed for hadrontherapy on-line monitoring. We reconstruct images of a point source having a broad energy spectrum compatible with the up-mentioned application. Preliminary results indicate that the achievable resolution in directions parallel to the camera, thus including the beam direction, are compatible with the quality control requirements. With the prototype under study, the reconstructed image is elongated in the direction orthogonal to the camera. However this direction is of less interest and the resolution may be easily recovered using a second camera.

Imaging of Ion Beam Therapy

Prompt secondary radiations such as gamma rays and protons can be used for ion-range monitoring during ion therapy either on an energy-slice basis or on a pencil-beam basis. We present a review of the ongoing activities in terms of detector developments, imaging, experimental and theoretical physics issues concerning the correlation between the physical dose and hadronic processes.

Image reconstruction for Compton camera applied to 3D prompt γ imaging during ion beam therapy

Iterative image reconstruction of data measured by a Compton scattering camera has to overcome various difficulties, e.g. a large amount of data, noise arising from both low counts recorded and imaging response, etc. Several approaches were proposed trying to attain an efficient Compton scattered data reconstruction. A forward step is introduction of list-mode data acquisition, which represents an efficient way of measurements allowing to preserve full precision of events as well as to gain computation efficiency. Iterative approaches include e.g. ML based algorithms, Bayesian methods, etc. Image estimation by ML criterion brings noise amplification when an iterative algorithm is applied, so a regularization step is needed. The proposed solution is a regularization technique using wavelet-based thresholding of the OSEM correction factors, called WOSEM. Methods of wavelet thresholding are efficient for edge-preserving image de-noising. The threshold used depends on the wavelet decomposition level. The method is illustrated by computer experiments.

Real-time online monitoring of the ion range by means of prompt secondary radiations

Hadron therapy for e.g. cancer treatment requires an accurate dose deposition (total amount and location). As a consequence, monitoring is crucial for the success of the treatment. Currently employed PET imaging systems are not able to provide information about the deposed dose fast enough to allow stopping the therapy in case of a discordance with the treatment plan. We are currently investigating an imaging system based on a combined Compton scattering and pair creation camera capable of imaging gamma rays up to 50 MeV. The camera would be able to measure the complete spectrum of emitted gamma rays during the therapy session. We have performed Monte-Carlo simulations for three different proton beam energies in a typical hadron therapy scenario. They show that the location of the gamma-ray distribution decay and the falloff region of the deposed dose are related. The reconstructed images show that the proposed system could provide the required imaging and dose location capabilities.