Hideo Murayama

Design of a depth of interaction detector with a PS-PMT for PET

A detector unit using a depth encoding scheme was designed and evaluated. The detector unit consists of four crystal blocks in a 2/spl times/2 array coupled to a position-sensitive photomultiplier tube (PS-PMT) having metal channel dynodes and 4/spl times/4 multianodes. Each crystal block consists of three stages of 2/spl times/2 GSO arrays of which element size is 3.8 mm/spl times/3.8 mm/spl times/10 mm. Each scintillation event is mapped on a two dimensional positioning histogram according to the relative ratio of the output signals of the PS-PMT. With appropriate light control in the crystal block, good positioning performance for Cs-137 photons was obtained.

Count rate analysis of PET scanner designs based on a GSO depth of interaction detector with a large area PS-PMT

We have estimated count rate properties of three-dimensional (3-D) positron emission tomography (PET) scanners based on a Gd/sub 2/SiO/sub 5/:Ce (GSO) detector with depth of interaction (DOI) capability using a large-area position-sensitive photomultiplier tube (PS-PMT). The proposed detector unit consists of 64 crystal blocks with four stages of 2/spl times/2 GSO arrays coupled to a 52-mm square PS-PMT which has small dead space. With appropriate light control in the crystal block, DOI information can be obtained using simple Anger-type positioning logic. Thus, dead-time factors can be calculated using a count rate model with standard acquisition architecture. Compton and photoelectric interactions in the scintillator and uniform cylindrical phantoms were tracked by Monte Carlo simulation programs. Since the DOI detector can provide high resolution throughout the entire field of view, 3-D PET scanners with a large solid angle covered by the detectors with relatively small ring diameters were simulated. The preliminary results suggest that, compared to current PET scanners, high noise equivalent count rate can be obtained by the proposed scanner designs despite the relatively large size of the detector module. The count rate performance can be improved by the reduction of single events that cause block dead-time losses at the cost of a slight decrease in sensitivity.

A study of external end-shields for PET

The effects of external end-shields in screening radioactivity out-of the field-of-view were studied by phantom experiments with a help of Monte-Carlo simulation for ECAT EXACT HRS. Random coincidence rates were measured for a standard cylindrical phantom at out-of the field-of-view with and without an external end-shield. The results were well reproduced with Monte-Carlo simulation. Various single count rates were calculated as a function of the detector ring number by Monte-Carlo simulation.

A 2D/3D hybrid PET scanner with rotating partial slice-septa and its quantitative procedures

This paper presents a PET scanner capable of acquiring projection data in three-dimensional (3D) and two-dimensional (2D) modes simultaneously. The scanner has rotating partial slice-septa, and coincidence events are stored as 2D data or as 3D data depending on whether the lines of response are collimated by the septa or not. 68Ge/Ga rod sources can be set on the rotating septa, and a transmission scan for attenuation correction is performed in the 2D mode. The scanner allows simultaneous 3D-emission/2D-transmission scanning or post-injection transmission scanning with little cross-talk. A blank scan for detector normalization is also performed with the rotating rod sources in the 2D mode, from which we can derive the normalizing factors in both modes. The 3D/2D difference method is available for scatter correction, even in a dynamic study where the source distribution is changing. A summation method is proposed as a new image reconstruction algorithm, in which the high- and low-frequency components of images are reconstructed from the 3D and 2D data respectively. In this method, most of the scatter contribution in the 3D data is removed by high-pass filtering, not by subtracting estimated scatter distribution, and hence the method is expected to be robust for scatter from outside the axial field of view. Computer simulations revealed that the rotating partial septa offer a single-scatter to true ratio similar to that of the conventional full septa if the depth of the partial septa is properly lengthened, with a small increase in multiple scattering.

Monte Carlo simulation for depth encoding multicrystal detector for PET

At the EEEE MIC 1999, we presented a Monte Carlo simulator made for designing a depth encoding multicrystal detector and reported some simulation results. While the basic performance of the simulator was validated from the comparison between the simulator outputs and experimental results using a prototype detector, many factors to be implemented were still remaining for simulating real phenomena inside the detector. This time, we improved the simulator by including Compton scattering, more accurate migration of optical photons at every interface of materials, and so on. Using this simulator, discrimination characteristics were investigated with respect to thickness of scintillator block, refractive index of inter-stage material, reflectance of optical reflector and surface roughness of scintillator. From these investigations, useful information for detector design was obtained.

A phantom and simulation study on body-shields to cope with radioactivity outside the field of view in 3D PET

Basic physics characteristics of body-shields to screen out radioactivity outside the field of view were analyzed by phantom experiments and Monte-Carlo simulation. The shielding effect depended primarily on the geometrical condition as in the case of end-shields. However, body-shields became more effective by increasing the shield thickness more than 10 mm as not in the case of end-shields. This characteristic difference was related with the fractions of un-scattered and shield-scattered components. A body-shield could be as effective as an end-shield if a good geometrical condition was kept with sufficient shield thickness.