[Fundamental Studies on Improvement of Count Rate Performance of Scintillation Detectors].

The purpose of this article is to introduce fundamental studies on improvement of count rate performance of scintillation detectors which Dr. Eiichi Tanaka dedicated himself to carrying out. He proposed a new technique based on the combination of pulse shortening and selective integration in which the integration period is not fixed but shortened by the arrival of the following pulse.

Development of 1.45-mm resolution four-layer DOI-PET detector for simultaneous measurement in 3T MRI.

Recently, various types of PET-MRI systems have been developed by a number of research groups. However, almost all of the PET detectors used in these PET-MRI systems have no depth-of-interaction (DOI) capability. The DOI detector can reduce the parallax error and lead to improvement of the performance.

Efficient one-pair experimental system for spatial resolution demonstration of prototype PET detectors.

In the development of depth-of-interaction (DOI)-positron emission tomography (PET) detectors, one of the important steps toward their practical use is an evaluation of their imaging performance, such as the spatial resolution as measured by use of a point source and a one-pair experimental system which simulates actual PET geometries. The DOI-PET detectors have a broad field of view providing good imaging performance compared with conventional detectors. Therefore, evaluation including the region from the center to the periphery close to the detector ring is required in an effort to show their advanced performance regarding uniform spatial resolution.

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.

Monte Carlo simulation of small OpenPET prototype with (11)C beam irradiation: effects of secondary particles on in-beam imaging.

In-beam positron emission tomography (PET) can enable visualization of an irradiated field using positron emitters (β+ decay). In particle therapies, many kinds of secondary particles are produced by nuclear interactions, which affect PET imaging. Our purpose in this work was to evaluate effects of secondary particles on in-beam PET imaging using the Monte Carlo simulation code, Geant4, by reproducing an experiment with a small OpenPET prototype in which a PMMA phantom was irradiated by a (11)C beam.

Reduction method for intrinsic random coincidence events from (176)Lu in low activity PET imaging.

For clinical studies, the effects of the intrinsic radioactivity of lutetium-based scintillators such as LSO used in PET imaging can be ignored within a narrow energy window. However, the intrinsic radioactivity becomes problematic when used in low-count-rate situations such as gene expression imaging or in-beam PET imaging. Time-of-flight (TOF) measurement capability promises not only to improve PET image quality, but also to reduce intrinsic random coincidences.

Performance evaluation of a depth-of-interaction detector by use of position-sensitive PMT with a super-bialkali photocathode.

Our purpose in this work was to evaluate the performance of a 4-layer depth-of-interaction (DOI) detector composed of GSO crystals by use of a position-sensitive photomultiplier tube (PMT) with a super-bialkali photocathode (SBA) by comparing it with a standard bialkali photocathode (BA) regarding the ability to identify the scintillating crystals, energy resolution, and timing resolution. The 4-layer DOI detector was composed of a 16 × 16 array of 2.9 × 2.

Simulation study optimizing the number of photodetection faces for the X'tal cube PET detector with separated crystal segments.

We are developing a novel PET detector with 3D isotropic resolution called a crystal (X'tal) cube. The X'tal cube detector consists of a crystal block all 6 surfaces of which are covered with silicon photomultipliers (SiPMs). We have developed a prototype detector with 3D isotropic 1 mm resolution.

GPU-based optical propagation simulator of a laser-processed crystal block for the X'tal cube PET detector.

The X'tal cube is a next-generation DOI detector for PET that we are developing to offer higher resolution and higher sensitivity than is available with present detectors. It is constructed from a cubic monolithic scintillation crystal and silicon photomultipliers which are coupled on various positions of the six surfaces of the cube. A laser-processing technique is applied to produce 3D optical boundaries composed of micro-cracks inside the monolithic scintillator crystal.

Potential for reducing the numbers of SiPM readout surfaces of laser-processed X'tal cube PET detectors.

We are developing a three-dimensional (3D) position-sensitive detector with isotropic spatial resolution, the X'tal cube. Originally, our design consisted of a crystal block for which all six surfaces were covered with arrays of multi-pixel photon counters (MPPCs). In this paper, we examined the feasibility of reducing the number of surfaces on which a MPPC array must be connected with the aim of reducing the complexity of the system.

Intrinsic spatial resolution evaluation of the X'tal cube PET detector based on a 3D crystal block segmented by laser processing.

The X'tal cube is a depth-of-interaction (DOI)-PET detector which is aimed at obtaining isotropic resolution by effective readout of scintillation photons from the six sides of a crystal block. The X'tal cube is composed of the 3D crystal block with isotropic resolution and arrays of multi-pixel photon counters (MPPCs). In this study, to fabricate the 3D crystal block efficiently and precisely, we applied a sub-surface laser engraving (SSLE) technique to a monolithic crystal block instead of gluing segmented small crystals.

System design of a small OpenPET prototype with 4-layer DOI detectors.

We have proposed an OpenPET geometry which consists of two axially separated detector rings. The open gap is suitable for in-beam PET. We have developed the small prototype of the OpenPET especially for a proof of concept of in-beam imaging.

A SiPM-based isotropic-3D PET detector X'tal cube with a three-dimensional array of 1 mm(3) crystals.

We are developing a novel, general purpose isotropic-3D PET detector X'tal cube which has high spatial resolution in all three dimensions. The research challenge for this detector is implementing effective detection of scintillation photons by covering six faces of a segmented crystal block with silicon photomultipliers (SiPMs). In this paper, we developed the second prototype of the X'tal cube for a proof-of-concept.

Semi-quantitative and simulation analyses of effects of γ rays on determination of calibration factors of PET scanners with point-like (22)Na sources.

The uncertainty of radioactivity concentrations measured with positron emission tomography (PET) scanners ultimately depends on the uncertainty of the calibration factors. A new practical calibration scheme using point-like (22)Na radioactive sources has been developed. The purpose of this study is to theoretically investigate the effects of the associated 1.

Basic performance of a large area PET detector with a monolithic scintillator.

Conventionally, block detectors, which consist of a two-dimensionally segmented scintillator array with inserted reflectors, are often used for PET. On the other hand, PET detectors with a monolithic block have been investigated because they are expected to offer higher resolution than do segmented crystal arrays. However, previous reports focused on detectors dedicated as small-animal PET, and the thickness was not good enough to stop 511-keV radiation.

Development of a small prototype for a proof-of-concept of OpenPET imaging.

The OpenPET geometry is our new idea to visualize a physically opened space between two detector rings. In this paper, we developed the first small prototype to show a proof-of-concept of OpenPET imaging. Two detector rings of 110 mm diameter and 42 mm axial length were placed with a gap of 42 mm.

Creation and application of three-dimensional computer-graphic animations for introduction to radiological physics and technology.

Physics-related subjects are important in the educational fields of radiological physics and technology. However, conventional teaching tools, for example texts, equations, and two-dimensional figures, are not very effective in attracting the interest of students. Therefore, we have created several multimedia educational materials covering radiological physics and technology.

A practical method of determining cross-calibration factors of PET scanners by moving a point-like (22)Na radioactive source.

Purpose: Thus far, cylindrical phantoms with (18)F or (68)Ge/(68)Ga have been used in the standard techniques for determining the cross-calibration factors (CCFs) of PET scanners. This paper describes a new practical method that uses a point-like (22)Na radioactive source for determining CCFs.

Methods: A point-like (22)Na radioactive source (about 700 kBq) was equipped with a spherical aluminium absorber capsule to ensure the symmetry of the emitted photons.

Point-like radioactive source with multiple absorber capsules for evaluating PET scanners.

Objective: Radioactive sources for evaluating sensitivity and uncertainty in the radioactivity measurements performed using PET scanners must be equipped with absorber materials that ensure the annihilation of positrons. Attenuation and scattering owing to the absorber materials produce uncertainty in the performance evaluation. The aim of this study is to propose a point-like radioactive source with multiple absorber capsules, for which evaluation can be independent of scatter and attenuation owing to the source absorbers.