Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

Radiation therapy by means of protons or heavier ions can improve the performance of radiotherapy for cancer treatment by delivering dose more locally to tumor tissue. Due to the inherent precision of this irradiation modality a dose deposition monitoring is desirable. It has been shown that positron emission tomography (PET) can be used for that by exploiting the decay of positron emitters which arise from nuclear fragmentations of projectile and target nuclei. This monitoring was implemented and successfully applied to the treatment by carbon ions for more than 440 patients at the pilot facility at GSI Helmholtz Centre for Heavy Ion Research, Darmstadt, Germany. However, it comes with inherent drawbacks, e.g. spatial blurring of the reconstructed dose distribution due to unpredictable metabolically driven transport of the positron emitters within the patient.

Alternatively, the imaging of prompt gamma-rays -- photons emitted at time and location of the beam-tissue interaction -- is proposed to estimate dose deposition distributions more precisely and probably in real-time. Determined by their origin, these gamma-ray emission distributions have certain properties, i.e. smoothly extended in the order of a decimeter and a continuous energy spectrum up to 10 MeV.

A Compton camera is a single photon imaging device measuring position and energy deposition according to incoherent scattering (Compton scattering) of a photon, and its trajectory afterwards. Since the scattering angle is related to the photon energy before and after scattering a conical surface can be spanned in patient space which covers all possible source locations of the gamma-ray. This surface of response is the Compton camera equivalent to the well known line of response in PET.

Spanning this surface (which means performing a backprojection and constructing the system matrix) is the major part of the image reconstruction process of Compton camera imaging (CCI) in terms of design and implementation efforts, and computation time. As shown previously, its precision also strongly impacts the quality of the image reconstruction results. Furthermore, in CCI a realistic system matrix (SM) has to take into account the physical and technological characteristics of the camera like limited energy and spatial resolution as well as Compton camera specific features as: (i) the event and image space are of high dimensionality, i.e. due to the energy of the gamma-rays and multi attribute measurements, (ii) the backprojections are surfaces instead of lines (Anger SPECT, PET) and therefore the SM occupancy is high, and (iii) incompletely absorbed photons may lead to incorrect conclusions on the initial gamma energies and scattering angles and subsequently to misarranged backprojections.

These challenges of CCI including their impacts will be discussed. Approaches to overcome them will be proposed and evaluated. This is done by means of measurements using radioactive sources as well as via in-beam tests at the proton beam facility AGOR in at KVI Groningen, Netherlands. Complementary results by means of simulations will be shown.