Because of the ability to present molecular and functional information in organisms, nuclear medical imaging (NMI) is attracting more and more attention. Among NMI modalities, X-ray fluorescence computed tomography (XFCT) has the advantage that the tracers used in XFCT are not spontaneously decayed. The synthesis, storage of contrast agents is more convenient, the price of XFCT is much lower as well. However, XFCT usually has mechanical collimation to tell the incident photon direction, which results in the reduction of the detection efficiency. The Compton camera is an imaging modality, which does not need mechanical collimators in its structure, which makes Compton cameras have high detection efficiency. Therefore, it is a great idea to use Compton camera-based imaging systems to realize X-ray fluorescence (XF) imaging. In this work, the first XFCC imaging system in the laboratory environment is established, which consists of a 150keV X-ray tube and a single-layer Compton camera system based on the Timepix3 photoncounting detector (PCD). The element Gd (43keV) is used as the XF element. The first imaging reconstruction results of the XFCC system are represented.

Objective. The Compton cameras have been researched for medical applications and radioactive material detection. It is challenging for the Compton camera to realize high-resolution reconstruction when the incident photon energy is below 200keV. However, multiple kinds of nuclear medical radionuclides are in this energy range, such as 201Tl, 67Ga, 99mTc, and 123I. In this work, we propose an improved probabilistic model with correction of detector energy resolution, detector spatial resolution, and Doppler broadening effect. The proposed model is used for numerical calculation of the system matrix in the list-mode maximum likelihood expectation maximization (LM-MLEM) algorithm. Approach. The model can improve the imaging resolution of LM-MLEM reconstruction by taking Doppler broadening effect into account. It performs well, especially in the following situations: low-energy photon incidence below 200keV or (and) small distance between scattering and absorbing positions. Main results. Firstly, three main factors that affect the angular resolution of the Compton camera are theoretically analyzed and quantitatively calculated. The results of the analysis indicate the necessity of including the Doppler broadening effect in the model. Secondly, the details and derivation of the proposed probabilistic model are described. Thirdly, both Monte Carlo (MC) simulations and experiments are carried out to verify the performance of the proposed algorithm. The simulations focus on the low-energy reconstruction in which 201Tl (70keV) and 99mTc (141keV) are simulated. And the experiments are based on a single-layer Compton camera composed of a Timepix3 detector. Significance. The results of the simulations and the Timepix3-based experiments are presented to verify the effectiveness of the proposed algorithm. The model improves the Compton imaging resolution when the photon energy is below 200keV.

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