Multiple Modality $\gamma$ -Ray Imager Using LaCl $_{3}$ (Ce) Scintillators With Active Collimation

Multiple modality γ-ray imaging using mechanical and electronic collimation with different detection methods was proposed and tested by simulation. Although conventional mechanical collimators, which are made from high atomic number and density materials, passively filter the incident radiation, the mechanical collimator reported in this study consisted of LaCl₃(Ce) scintillators, which actively collimate the radiation and are used as the 1st detector of an electronic collimator to reconstruct a Compton image. A detector box made from six planar LaCl₃(Ce) scintillators was positioned behind the active collimator. The LaCl₃(Ce) scintillator arrays used as the active collimator and box detector consisted of voxels, each 2 mm × 2 mm × 5 mm³ in size. A combination of active collimators and box detectors can produce multiple modalities for both mechanical and electronic collimation. The information from each modality was combined effectively using the maximum likelihood expectation maximization (MLEM) method. Therefore, a reconstructed image from multiple modalities has inherently higher efficiency than each conventional modality. The reconstructed images were evaluated using the resolution-variance curve as a quantitative method. Although mechanical collimation is effective at low γ-ray energy and electronic collimation shows high performance at high γ-ray energy, multiple modalities are superior to the conventional single modalities for intermediate γ-ray energies (364 keV). The angular resolution and absolute efficiency were calculated for each modality and source energy.