Norihito Yanagita

Basic Performance Test of a Prototype PET Scanner Using CdTe Semiconductor Detectors

A prototype positron emission tomography (PET) scanner using CdTe semiconductor detectors was developed, and its initial evaluation was conducted. The scanner was configured to form a single detector ring with six separated detector units, each having 96 detectors arranged in three detector layers. The field of view (FOV) size was 82 mm in diameter. Basic physical performance indicators of the scanner were measured through phantom studies and confirmed by rat imaging. The system-averaged energy resolution and timing resolution were 5.4% and 6.0 ns (each in FWHM) respectively. Spatial resolution measured at FOV center was 2.6 mm FWHM. Scatter fraction was measured and calculated in a National Electrical Manufacturers Association (NEMA)-fashioned manner using a 3-mm diameter hot capillary in a water-filled 80-mm diameter acrylic cylinder. The calculated result was 3.6%. Effect of depth of interaction (DOI) measurement was demonstrated by comparing hot-rod phantom images reconstructed with and without DOI information. Finally, images of a rat myocardium and an implanted tumor were visually assessed, and the imaging performance was confirmed.

Impulse breakdown mechanism based on discharge propagation process under non-uniform electric field in air

In this paper, we aim to clarify the discharge mechanism leading to breakdown under non-uniform electric field in air, especially on the propagation process of discharge current channel. The discharge measurement was performed in a needle (φ1 mm)-plane electrode system with gap length g=15-100 mm in 0.1 MPa dry air under applying a positive impulse voltage. We observed discharge current waveform and fast-framing images of light emission with image intensifier controlled by a nanosecond pulse. We found out two types of breakdown mechanisms from streamer initiation to breakdown (BD); secondary streamer progress and leader progress. In addition, we investigated the transition region between both types of BD mechanism, focusing on the case of gap length (g=30 mm). Finally, we found that the secondary streamer propagation would dominate the formation process of the discharge channel and the mechanism leading to BD. Also, we found the criterion of impulse breakdown mechanism in air.

Physical performance of a prototype 3D PET scanner using CdTe detectors

We developed a prototype 3D PET scanner that uses CdTe semiconductor detectors. The scanner is dedicated to human brain imaging. A semiconductor detector should have advantages when used as a PET detector (e.g. good quantitative imaging and high-spatial resolution). The detectors incorporate three important technical features: stacked crystals, no contact with the board, and direct soldering onto the board. Our scanner comprises 18 detector units with a 350-mm-diameter patient port that has a transaxial field of view of 310 mm and an axial field of view of 250 mm. We demonstrated the scanner had a timing resolution of 6.8 ns (FWHM), an energy resolution of 4.1% (FWHM), and a spatial resolution such that hot rods down to 2.5 mm in diameter were visually separated. These results showed that our scanner should produce quantitative imaging with a high-spatial resolution.

Positive streamer propagation and breakdown characteristics in non-uniform air gap

Discharge phenomena in a needle-plane electrode in air were studied with nano-second time resolution by means of synchronous measurement of streak image, light intensity, discharge current and fast-framing image. When positive impulse voltage was applied, a primary streamer initiated from the needle tip and propagated to the grounded plane electrode. A secondary streamer initiated after the primary streamer arrived at the plane electrode. There are close relations between the applied voltage, the primary and the secondary streamer current and the initiation time of the secondary streamer. The breakdown can start when the secondary streamer have arrived at the plane electrode. Moreover, the time to breakdown becomes shorter with the increase in the secondary streamer current.

Streamer development mechanism under non-uniform electric field in air

We discussed the development mechanism of the streamer channel leading to breakdown in a short air gap, by using an ultra-high speed measuring system. We clarified that the initiation of the secondary streamer leading to breakdown is attributed to an electron injection from the grounded plane electrode to the residual channel made by the primary streamer. In addition, by selective measurement of the discharge current waveform of single streamer channel leading to breakdown, the breakdown was verified to be dominated by the development of the secondary streamer and the subsequent Joule heating process focusing on the single streamer channel.