Restoration of lost frequency in OpenPET imaging: comparison between the method of convex projections and the maximum likelihood expectation maximization method

We are developing a new PET scanner based on the "OpenPET" geometry, which consists of two detector rings separated by a gap. One item to which attention must be paid is that OpenPET image reconstruction is classified into an incomplete inverse problem, where low-frequency components are truncated. In our previous simulations and experiments, however, the OpenPET imaging was made feasible by application of iterative image reconstruction methods. Therefore, we expect that iterative methods have a restorative effect to compensate for the lost frequency. There are two types of reconstruction methods for improving image quality when data truncation exists: one is the iterative methods such as the maximum-likelihood expectation maximization (ML-EM) and the other is an analytical image reconstruction method followed by the method of convex projections, which has not been employed for the OpenPET. In this study, therefore, we propose a method for applying the latter approach to the OpenPET image reconstruction and compare it with the ML-EM. We found that the proposed analytical method could reduce the occurrence of image artifacts caused by the lost frequency. A similar tendency for this restoration effect was observed in ML-EM image reconstruction where no additional restoration method was applied. Therefore, we concluded that the method of convex projections and the ML-EM had a similar restoration effect to compensate for the lost frequency

Restoration of the Analytically Reconstructed OpenPET Images by the Method of Convex Projections

We have proposed the OpenPET geometry which has gaps between detector rings and physically opened field-of-view. The image reconstruction of the OpenPET is classified into an incomplete problem because it does not satisfy the Orlov\u27s condition. Even so, the simulation and experimental studies have shown that applying iterative methods such as the maximum likelihood expectation maximization (ML-EM) algorithm successfully reconstruct images in the gap area. However, the imaging process of the iterative methods in the OpenPET imaging is not clear. Therefore, the aim of this study is to analytically analyze the OpenPET imaging and estimate implicit constraints involved in the iterative methods. To apply explicit constraints in the OpenPET imaging, we used the method of convex projections for restoration of the images reconstructed by the analytical way in which low-frequency components are lost. Numerical simulations showed that the similar restoration effects are involved both in the ML-EM and the method of convex projections. Therefore, the iterative methods have advantageous effect of restoring lost frequency components of the OpenPET imaging.Fully ３D　１

Development and progress of the web-based CT exposure dose calculator WAZA-ARIv2

X-ray CT (Computed Tomography) is very popular as a useful diagnostic method, so it is important to assess high exposure dose of CT in terms of justification and optimization. WAZA-ARI version 2 (WAZA-ARIv2, https://waza-ari.nirs.qst.go.jp/) is the web-based open system for X-ray CT dose calculation, which has been developed by National Institute of Radiological Sciences (NIRS), Oita University of Nursing and Health Sciences and the Japan Atomic Energy Agency (JAEA). The organ doses of various CT exposures were calculated using Monte Carlo simulation and voxel phantoms. In WAZA-ARIv2, can provide organ doses taking into consideration of the body type of patient using Japanese voxel phantoms developed by JAEA. And it can also provide exposure doses of children using child voxel phantoms developed by the University of Florida. In this system selectable CT scanners are 31 models. The number of CT models in WAZA-ARIv2 system is still increasing. Furthermore, we added the database function of storing the calculation results in each facility in order to check the exposure level of the CT examination in the facility in the stored distribution data in Japan. In January 2015, WAZA-ARIv2 system started, and currently over one thousand users are registered. In order to acquire more data on the actual situation of medical exposure in Japan, we plan to expand the number of users and to improve functions of WAZA-ARIv2 system. In this presentation, we will report the development and progress of WAZA-ARIv2 system.World Congress on Medical Physics and Biomedical Engineering 2018 (WC2018