Shigeyuki Nakamura

Development of a Dual-Sided Readout DOI-PET Module Using Large-Area Monolithic MPPC-Arrays

We are proposing a novel design for a module with depth of interaction (DOI) capability for gamma rays by measuring the pulse-height ratio of double-sided Multi-Pixel Photon Counters (MPPCs) coupled at both ends of a scintillation crystal block. Thanks to newly developed monolithic MPPC arrays consisting of 4 × 4 channels with a three-side buttable package, the module is very thin and compact, thereby enabling less dead space between each module when arranged into a fully designed gantry. To demonstrate our concept of a DOI measuring technique, we first made a 1-D crystal array consisting of five Ce-doped Gd₃Al₂Ga₃O₁₂ (Ce:GAGG) cubic crystals measuring 3×3×3 mm³ in size, separated by a layer of air approximately 10 μm-thick. When the light signals output from both ends are read with the 3×3 mm² MPPCs, the position of each crystal is clearly distinguished. The same measurements were also made using Ce-doped (Lu,Y)₂(SiO₄)O (Ce:LYSO), achieving a similarly good separation. We then fabricated thin Ce:GAGG 2-D crystal arrays consisting of two types: [A] 4 × 4 matrix of 3×3×3 mm³ pixels, and [B] 10 × 10 matrix of 0.8×0.8×5 mm³ pixels, with each pixel divided by a BaSO₄ reflector 0.2 mm-thick. Then four arrays are laid on top of each other facing the DOI direction through a layer of air 10 μm-thick. We demonstrated that the 3-D position of each Ce:GAGG pixel is clearly distinguished in both the 2-D and DOI directions for type A and B when illuminated by 662 keV gamma rays. Average energy resolutions of 9.8 ± 0.8% and 11.8 ± 1.3% were obtained for types A and B, respectively. These results suggest that our proposed method is simple and offers promise in achieving both excellent spatial and energy resolutions for future medical imaging, particularly in positron emission tomography (PET).

Development of a MPPC-based DOI-PET module with submillimeter 3-D resolution

We are proposing a novel design for a module with depth of interaction (DOI) capability for gamma rays by measuring the pulse-height ratio of double-sided Multi-Pixel Photon Counters (MPPCs) coupled at both ends of a scintillation crystal block. Thanks to newly developed monolithic MPPC arrays consisting of 4 × 4 channels with a three-side buttable package, the module is very thin and compact, thereby enabling less dead space between each module when arranged into a fully designed gantry. To demonstrate our concept of a DOI measuring technique, we first made a 1-D crystal array consisting of five Ce-doped Gd₃Al₂Ga₃O₁₂ (Ce:GAGG) cubic crystals measuring 3 × 3 × 3 mm³ in size, separated by a layer of air. When the light signals output from both ends are read with the MPPCs, the position of each crystal is clearly distinguished with a spatial uncertainty of 0.48 ± 0.03 mm. For 3-D measurements, we then fabricated three different type arrays: [A] 4 × 4 × 4 matrix of 3 × 3 × 3 mm³ pixels, [B] 5 × 5 × 5 matrix of 2 × 2 × 2 mm³ pixels, and [C] 10 × 10 × 10 matrix of 1 × 1 × 1 mm³ pixels, with each pixel divided by a BaSO₄ reflector in the 2-D direction and by a layer of air in the DOI direction. We demonstrated that the 3-D position of each Ce:GAGG pixel was clearly distinguished when illuminated by 662 keV gamma rays uniformly. Average energy resolutions of 9.8 ± 0.8 %,9.8 ± 0.9 %, and 13.2 ± 1.7 % were obtained for types A, B and C, respectively. These results suggest that our proposed method is simple and offers promise in achieving 1 mm 3-D spatial resolution for future medical imaging, partic

Development of silicon hybrid SPAD 1D arrays for lidar and spectrometer applications

The MPPC, a sort of silicon photomultiplier, has good photon-counting ability and timing accuracy. Recently, a new type of MPPC that has excellent sensitivity in the green region or near-infrared, wide dynamic range, higher photon detection efficiency, and various format configurations (single channel and 1D array) was developed. To utilize its advantageous performance, dedicated readout electronics is required. For LIDAR applications, the red-enhanced MPPC can be used, and the functional for the readout system are estimating the number of photons and recording the precise time-of-arrival. For these requirements, a time-over-threshold circuit that can recognize the incoming energy down to 1 photon and a time-to-digital converter that can record time-of-arrival with 312ps resolution were integrated onto a single die. We have demonstrated that the system has the capability to measure distance with centimeter accuracy. For situations that require higher dynamic range, a high-speed comparator and counter array configuration can be provided. For weak-light-level applications like spectroscopy, a configuration consisting of a SPAD 1D array, active quenching circuit and gate function can be used. We will propose a 1D hybrid SPAD that is the optimal combination for various applications.

Development of an InGaAs SPAD 2D array for flash LIDAR

An InGaAs Single-Photon Avalanche Photodiode (SPAD) array and a hybrid photon-counting image sensor have been developed for time-resolved applications in the near infrared region, especially for Flash LIDAR. The implemented array has a 100 μm pitch 32×32 matrix, and the active area in each pixel has a diameter of 12 μm. A dedicated read-out IC incorporates an active quenching and recharge circuit, and an in-pixel time-to-digital converter with 318 ps resolution for providing stable Geiger-mode operation of the InGaAs SPAD pixels and a simultaneous time-resolving function. The uniformity of the electrical field in the entire two-dimensional area is one of the key characteristics as it affects most performance parameters. The breakdown voltage mapping revealed the SPAD array has an excellent uniformity. Full characterizations of dark count rate, photon detection probability, and timing jitter were performed and will be discussed. Methods of suppressing afterpulses were examined by utilizing an adjustable hold-off function. The results showed that the function worked effectively to reduce the afterpulse probability. Finally we prove that the InGaAs SPAD image sensor has the capability of accurate timing detection for constructing a flash LIDAR in the near future.

Development of InGaAs MPPC for NIR photon counting applications

An InGaAs Multi-Pixel Photon Counter (MPPC) has been developed for detecting the near-infrared and shortwave infrared wavelength. Numerous studies were made to develop silicon-based photomultipliers with sensitivity in the nearinfrared region, so they can be applied in various applications such as distance ranging. However, achieving sensitivity at 1.0 μm and longer wavelengths was hard to implement. Therefore a hybrid photon counting device was designed by improving the InGaAs avalanche photodiode. In our approach, the sensor layer and the circuit layer are prepared separately, and this configuration brings more flexibility to both layers. The 3x3 pixel matrix and the dedicated circuit were fabricated, and characterizations were performed in terms of the sensitivity uniformity and the other essential parameters of a photon counting device such as dark count rate, photon detection probability and so on. An excellent uniformity was observed across the active regions in both a single device and the 3x3 matrix device, which indicated that no any abrupt breakdowns were formed. The DCR was 844 kHz and PDP was 8% under the typical conditions of Ve = 1.1 V and T = 253 K. The 3x3 matrix device also exhibited the capability of resolving photon numbers, and the result means this work can be applied to numerous photon-counting applications in infrared.

Silicon hybrid SPAD with high-NIR-sensitivity for TOF applications

This paper proposes a single-photon avalanche diode (SPAD) sensor array comprised of a hybrid structure which can maximize the fill factor of the active area and be compatible with the other detector layer optimized for various demands. In order to implement the hybrid structure, a 100μm pitch through silicon via (TSV) implementation method has been developed to access the back surface of the sensor layer. The achieved fill factor is up to 60%, thus, photon detection efficiency can be reached 35%. A 32×32 SPAD array and a dedicated application specific IC has been designed. We have proved the concept structure can work successfully through the characterization of the hybridized chip. On the other hand, we realized multi-event detection capability should be considered when we apply the photon counting image sensor to a time-of-flight application in high background intensity, and the new concept of a SiPM-based pixel structure has been considered. In order to prove the concept, fundamental experiments have been performed by using the new SiPMs which have extended sensitivity in the near infrared region, and a current mode front-end ROIC which can mark a time-of-arrival and distinguish a photon quantity. A walk error has been studied and found the plot of the time-of-arrival and the photon quantity can be utilized for the measurement compensation.