Toshiaki Sakai

Development of a high-resolution four-layer DOI detector using MPPCs for brain PET

A new high-resolution four-layer DOl detector using MPPCs for brain PET scanner has been developed. The new depth of interaction (DOl) detector was designed to compose of four layers of detector units, which were lined up five axially. Each of the detector units consists of a LYSO scintillator array finely segmented of 1.2 mm and an 8 × 8 array of multi-pixel photon counters (MPPCs), which are one of the products of silicon photomultiplier family. The MPPC is so compact and insensitive to gamma-ray that the detector units can be piled up with a small gap between each scintillator array in the depth direction. In order to have the detector in every layer equally sensitive to gamma-ray, the scintillator thickness was designed at 3 mm, 4 mm, 5 mm and 8 mm toward the bottom respectively, and the total thickness was 20 mm. We adopted an internal focused laser processing technique to a monolithic LYSO scintillator and fabricated a 2D segmented array of 32 × 32 with 1.2 mm pitch in 38.4 mm square cross-section. Each detector layer has independently front end circuits including ASICs for MPPCs and signal processing circuits for crystal identification, energy and timing detection. Each data set of four layers are fed into data interface circuits placed behind detector layers and transferred to a data acquisition unit as formatted list-mode data. The performance of the four-layer DOl detector has been evaluated. The coincidence timing resolution of the detector, with a reference BaF2 detector, was obtained 850 ps FWHM. The average energy resolution value was 24.5% at 511 keV. The crystal separation with finely segmented LYSO scintillator was also good enough at each layer.

A novel single-ended readout depth-of-interaction PET detector fabricated using sub-surface laser engraving

We propose a novel scintillation detector design for positron emission tomography (PET), which has depth of interaction (DOI) capability and uses a single-ended readout scheme. The DOI detector contains a pair of crystal bars segmented using sub-surface laser engraving (SSLE). The two crystal bars are optically coupled to each other at their top segments and are coupled to two photo-sensors at their bottom segments. Initially, we evaluated the performance of different designs of single crystal bars coupled to photomultiplier tubes at both ends. We found that segmentation by SSLE results in superior performance compared to the conventional method. As the next step, we constructed a crystal unit composed of a 3  ×  3  ×  20 mm3 crystal bar pair, with each bar containing four layers segmented using the SSLE. We measured the DOI performance by changing the optical conditions for the crystal unit. Based on the experimental results, we then assessed the detector performance in terms of the DOI capability by evaluating the position error, energy resolution, and light collection efficiency for various crystal unit designs with different bar sizes and a different number of layers (four to seven layers). DOI encoding with small position error was achieved for crystal units composed of a 3  ×  3  ×  20 mm3 LYSO bar pair having up to seven layers, and with those composed of a 2  ×  2  ×  20 mm3 LYSO bar pair having up to six layers. The energy resolution of the segment in the seven-layer 3  ×  3  ×  20 mm3 crystal bar pair was 9.3%-15.5% for 662 keV gamma-rays, where the segments closer to the photo-sensors provided better energy resolution. SSLE provides high geometrical accuracy at low production cost due to the simplicity of the crystal assembly. Therefore, the proposed DOI detector is expected to be an attractive choice for practical small-bore PET systems dedicated to imaging of the brain, breast, and small animals.

Fabrication of Optoelectronic Devices in Lithium Fluoride Crystals by Interfering Femtosecond Laser Pulses

Fine-pitched microgratings either on or beneath surface of bulk lithium fluoride (LiF) are holographically fabricated by interfering with the second harmonic (400 nm) of a mode-locked Ti:sapphire oscillator–amplifier laser. The laser-active F2 and F3+ color centers in LiF are excellent candidates for producing visible laser action from the green-to-red spectral range when excited with a single wavelength. Here a green distributed feedback (DFB) laser action with a narrower oscillating linewidth is demonstrated by utilizing simultaneous formation of the F3+ color centers and waveguide with the microgratings encoded by interference of 400 nm femtosecond laser pulses. In addition, the possibility of a dual-beam DFB laser based on these color centers in LiF is discussed.

Multi-pixel photon counter module for MRI compatible application

A new detector for PET using multi-pixel photon counters (MPPCs) was developed for MRI compatible applications. This module has an 8 × 8 MPPC array, each segment has a 3 mm × 3 mm active area, and the pitch of the array is 4.1 mm in both directions. A temperature sensor is attached to the back of the array for temperature compensation. The MPPC array is connected to the front-end circuit with a detachable flexible printed circuit cable (FPC), which provides flexibility for detector arrangement. The front-end circuit consists of preamplifiers, a register network, buffer amplifiers, a built-in high voltage (HV) unit, and an embedded microprocessor unit. The HV unit is a down-regulator and requires an external HV supply. The preamplifier also has a sum output, which can be used for timing pick-off and energy discrimination. With LYSOs, the timing performance was evaluated using flexible printed cables of two different lengths. Set in a copper shield box, energy spectra and flood images were evaluated with a 3T-MRI Very little interference was observed during simultaneous MRI measurements.

Development of a novel DOI detector using laser manufacturing

A novel single-ended-readout depth-of-interaction (DOI) detector is proposed for application in positron emission tomography (PET). The crystal unit of this detector is formed by a transform of the crystal unit of a dual-ended readout detector, which is constructed from a LYSO scintillator pillar segmented using sub-surface laser engraving (SSLE). Specifically, two scintillator pillars are coupled with optical glue at the first (top) segment, and an enhanced specular reflector (ESR) film is inserted between the second segment and the bottom segment. By placing these crystal units in a two-dimensional array and coupling them to an multi-pixel photon counter (MPPC) array, a six-layer block detector can be constructed. We developed a new MPPC array with 64 (8 × 8) channels; each pixel had an active area of 2.0 × 2.0 mm and the pitch was 2.2 mm in both directions. Single-ended-readout DOI detectors were fabricated using these block detectors, which produced clearly resolved position maps with an average energy resolution of 12%. For both the individual crystal units and the block detector, the first layer exhibited the best timing resolution, 715 ps FWHM and 726 ps FWHM, respectively.