K. Heidel

Test of Compton camera components for prompt gamma imaging at the ELBE bremsstrahlung beam

In the context of ion beam therapy, particle range verification is a major challenge for the quality assurance of the treatment. One approach is the measurement of the prompt gamma rays resulting from the tissue irradiation. A Compton camera based on several position sensitive gamma ray detectors, together with an imaging algorithm, is expected to reconstruct the prompt gamma ray emission density map, which is correlated with the dose distribution. At OncoRay and Helmholtz-Zentrum Dresden-Rossendorf (HZDR), a Compton camera setup is being developed consisting of two scatter planes: two CdZnTe (CZT) cross strip detectors, and an absorber consisting of one Lu2SiO5 (LSO) block detector. The data acquisition is based on VME electronics and handled by software developed on the ROOT framework. The setup has been tested at the linear electron accelerator ELBE at HZDR, which is used in this experiment to produce bunched bremsstrahlung photons with up to 12.5 MeV energy and a repetition rate of 13 MHz. Their spectrum has similarities with the shape expected from prompt gamma rays in the clinical environment, and the flux is also bunched with the accelerator frequency. The charge sharing effect of the CZT detector is studied qualitatively for different energy ranges. The LSO detector pixel discrimination resolution is analyzed and it shows a trend to improve for high energy depositions. The time correlation between the pulsed prompt photons and the measured detector signals, to be used for background suppression, exhibits a time resolution of 3 ns FWHM for the CZT detector and of 2 ns for the LSO detector. A time walk correction and pixel-wise calibration is applied for the LSO detector, whose resolution improves up to 630 ps. In conclusion, the detector setup is suitable for time-resolved background suppression in pulsed clinical particle accelerators. Ongoing tasks are the quantitative comparison with simulations and the test of imaging algorithms. Experiments at proton accelerators have also been performed and are currently under analysis.

Comparison of LSO and BGO block detectors for prompt gamma imaging in ion beam therapy

A major weakness of ion beam therapy is the lack of tools for verifying the particle range in clinical routine. The application of the Compton camera concept for the imaging of prompt gamma rays, a by-product of the irradiation correlated to the dose distribution, is a promising approach for range assessment and even three-dimensional in vivo dosimetry. Multiple position sensitive gamma ray detectors arranged in scatter and absorber planes, together with an imaging algorithm, are required to reconstruct the prompt gamma emission density map. Conventional block detectors deployed in Positron Emission Tomography (PET), which are based on Lu2SiO5:Ce (LSO) and Bi4Ge3O12 (BGO) scintillators, are suitable candidates for the absorber of a Compton camera due to their high density and absorption efficiency with respect to the prompt gamma energy range (several MeV). We compare experimentally LSO and BGO block detectors in clinical-like radiation fields in terms of energy, spatial and time resolution. The high energy range compensates for the low light yield of the BGO material and boosts significantly its performance compared to the PET scenario. Notwithstanding the overall superiority of LSO, BGO catches up in the field of prompt gamma imaging and can be considered as a competitive alternative to LSO for the absorber plane due to its lower price and the lack of intrinsic radioactivity.

The CBM Time-of-Flight wall — a conceptual design

Charged hadron identification in the Compressed Baryonic Matter experiment (CBM) is realized via the Time-of-Flight method [1]. For this purpose the CBM-ToF collaboration designed a Time-of-Flight wall composed of Multi-gap Resistive Plate Chambers (MRPCs). Due to the high interaction rate in CBM of 10 MHz the key challenge is the development of high rate MRPCs above 25 kHz/cm2 which become possible after the development of low resistive glass with extremely good quality. In this article we present the actual conceptual design of the ToF-wall which is subdivided in three parts namely the outer wall, the inner wall and the forward zone that are discussed in detail.

Compton imaging in a high energetic photon field

Through the well defined range of charged particles in matter, cancer irradiation by means of ions can be very tumor conformal. However, external range verification is needed to fully exploit the advantages of ion beam therapy. Nuclear interactions between the projectiles and targets result in excited nuclei which emit photons in the MeV energy range during deexcitation. With a Compton camera, it should be possible to image the origin of these photons which is correlated to the beam position. A prototype Compton camera comprising CdZnTe layers and scintillation detectors has been developed and tested with radioactive point sources. In this work, the performance of the camera is tested at a tandetron beam line in a clean radiation field of 4.44 MeV photons. It was shown that Compton imaging at this energy is feasible.

Test of a compton imaging prototype at the ELBE bremsstrahlung beam

In the context of particle therapy, particle range verification is a major challenge for the quality assurance of the treatment. One approach is the measurement of the prompt gamma rays resulting from the tissue irradiation. A Compton camera based on several position sensitive gamma ray detectors, together with an imaging algorithm, is expected to reconstruct the prompt gamma ray emission density map, which is correlated with the dose distribution. At Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and OncoRay, a Compton camera setup has been developed consisting of two scatter planes (CdZnTe cross strip detectors) and an absorber (Lu2SiO5 block detector). The data acquisition is based on VME electronics and handled by software developed on the ROOT framework. The setup was tested at the linear electron accelerator ELBE at HZDR, which was used to produce bunched bremsstrahlung photons with up to 12.5MeV. Their spectrum has similarities with the one expected from prompt gamma rays in the clinical case, and the flux is also bunched with the accelerator frequency. The spatial resolution for the CZT and LSO detector is analyzed and it showed a trend to improve for low and high energy depositions respectively. The time correlation between the pulsed prompt photons and the measured signals to be used for background discrimination exhibits a time resolution of 3 ns (2 ns) FWHM for the CZT (LSO) detector. A time walk correction and pixel calibration is applied for the LSO detector, whose resolution improved up to 630 ps. In conclusion, the detectors are suitable for time-resolved background suppression in pulsed clinical particle accelerators. Ongoing tasks are the test of the imaging algorithms and the quantitative comparison with simulations. Experiments at proton accelerators have been also performed and are now under analysis.

A compton imaging prototype for range verification in particle therapy

During the 2012 AAPM Annual Meeting 33 percent of the delegates considered the range uncertainty in proton therapy as the main obstacle of becoming a mainstream treatment modality. Utilizing prompt gamma emission, a side product of particle tissue interaction opens the possibility of in-beam dose verification, due to the direct correlation between prompt gamma emission and particle dose deposition. Compton imaging has proven to be a technique to measure three dimensional gamma emission profiles ([1], [2]) and opens the possibility of adaptive dose monitoring and treatment correction.

Particle range retrieval in heterogeneous phantoms with the prompt gamma ray timing method at a clinical proton accelerator

The characteristic dose profile of accelerated ions has opened up new horizons in the context of cancer treatment. However, particle range uncertainties strongly constrain the potentialities of ion beam therapy. In spite of worldwide efforts, a detector system for range and dose delivery assessment in real-time is not yet available for clinical routine.

Timing of pulsed prompt gamma rays for background discrimination

In the context of particle therapy, particle range verification is a major challenge for the quality assurance of the treatment. One approach is the measurement of the prompt gamma rays resulting from the tissue irradiation. A Compton camera based on several planes of position sensitive gamma ray detectors, together with an imaging algorithm, is expected to reconstruct the prompt gamma ray emission density profile, which is correlated with the dose distribution. At Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and OncoRay, a camera prototype has been developed consisting of two scatter planes (CdZnTe cross strip detectors) and an absorber plane (Lu2SiO5 block detector). The data acquisition is based on VME electronics and handled by software developed on the ROOT platform. The prototype was tested at the linear electron accelerator ELBE at HZDR, which was set up to produce bunched bremsstrahlung photons. Their spectrum has similarities with the one expected from prompt gamma rays in the clinical case, and these are also bunched with the accelerator frequency. The time correlation between the pulsed prompt photons and the measured signals was used for background discrimination, achieving a time resolution of 3 ns (2 ns) FWHM for the CZT (LSO) detector. A timewalk correction was applied for the LSO detector and improved its resolution to 1 ns. In conclusion, the detectors are suitable for time-resolved background discrimination in pulsed clinical particle accelerators. Ongoing tasks are the test of the imaging algorithms and the quantitative comparison with simulations. Further experiments will be performed at proton accelerators.