Image Reconstruction With an Electron Tracking Compton Camera

The electron tracking Compton camera (ETCC) system proposed by Tanimori, et al. (2006) facilitates reconstruction of three-dimensional distribution of radioisotopes without a mechanical collimator. This system has been developed for observations in astronomy, but it applicable to the imaging in nuclear medicine as well. The ETCC uses the information obtained by Compton scattered photons and recoiled electrons, which are generated in the process of Compton scattering. Without the information of the recoiled electron, the source position is localized only on a cone whose apex angle corresponds to the Compton scattering angle. However, as the ETCC can measure the energy and direction of the recoiled electron, the source position is fundamentally determined uniquely at a line on the cone surface. The quality of an ETCC image is affected by the uncertainly in the measurement of the energy of a scattered photon and that of a direction and energy of a recoiled electron. The former is affected by the Compton angle (Angular resolution: ARM) and the latter is affected by the dispersion on the cone with a Compton angle (Angular resolution: SPD). The paper studied the effect of the ARM and SPD on the reconstructed image. To evaluate the effect of the angular resolutions ARM and SPD, we assumed three detector geometries using one to three ETCCs. We assumed that the ETCC was composed of a CF4 gas detector with the size of 20 × 20 × 20 cm3 and NaI detector with the size of 20 × 20 cm2. The targeted gamma rays were 300 keV (Ga-67), 511 keV (PET) and 4438 keV (Carbon therapy). The phantom used water in sphere with a diameter of 3 cm. We transported photons with GEANT4 Monte Carlo code, and image reconstruction was performed with a list mode ML-EM algorithm. The results of simulations showed that the blurring of an image was improved with more than two detectors and the quality of images was better when we used high energy gamma rays.