Tetsuo Furumiya

Dynamic Time Over Threshold Method

The time over threshold (TOT) method has several advantages over direct pulse height analysis based on analog to digital converters (ADCs). A key advantage is the simplicity of the conversion circuit which leads to a high level of integration and a low power consumption. The TOT technique is well suited to build multi-channel readout systems for pixelated detectors as described in our previous work that also exploits the Pulse Width Modulation (PWM) method. The main limitation of the TOT technique is that the relation between the input charge to be measured and the width of the encoded pulse is strongly non-linear. Dynamic range limitation is also an issue. To address these aspects, we propose a new time over threshold conversion circuit where the threshold of the comparator is dynamically changed instead of being constant. We call this scheme the “dynamic TOT method”. We show that it improves linearity and dynamic range. It also shortens the duration of measured pulses leading to higher counting rates. We present a short analysis that explains how the ideal linear input charge to TOT transfer function can theoretically be obtained. We describe the results obtained with a test circuit built from discrete components and present several of the spectrums obtained with crystal detectors and a radioactive source. The proposed method can be used for applications like Positron Emission Tomography (PET) that require moderate energy resolution.

Novel front-end pulse processing scheme for PET system based on pulse width modulation and pulse train method

For the high-resolution PET system, architecture of multichannel front-end system is very important. We propose a novel front-end pulse processing scheme with pulse width modulation (PWM) for a PET system. This front-end can realize smart, low power dissipation, and multi-channel signal processing for the radiation detector system including PET system.

Novel Front-End Pulse Processing Scheme for PET System Based on Pulse Width Modulation and Pulse Train Method

The architecture of a multi-channel front-end system is important for realizing a high-resolution PET system. We propose a novel front-end pulse processing scheme with pulse width modulation (PWM) and pulse train method for PET systems. Each channel of the proposed system consists of a preamplifier, a shaping amplifier, a comparator, and a digital circuit that generates a pulse train for each event. The preamplifier-shaper-discriminator module first generates a trigger pulse with time-over-threshold (ToT), which contains the energy information. The trigger pulse is then processed through a digital circuit that adds subsequent pulses to form a pulse train. These additional pulses encode channel information, timing information, etc. The digital signal output of each channel can be connected by simple wired-OR logic, and the output is read in one transmission line. This multi-channel, low power consumption front-end scheme can acquire enough pulse height (energy) and position information to realize a PET system with a significantly smaller number of output pins in the front-end ASIC. The pulse width encoding also simplifies the digital processing system. We designed a new ASIC based on this concept. The proposed architecture can be applied to high-resolution PET systems with multi-channel ASICs.

Development of a C-shaped breast PET scanner equipped with four-layer DOI detectors

For diagnosis of very small lesions of breast cancer on very early stage, a dedicated breast positron emission tomography (PET) scanner consisting of four-layer depth of interaction (DOI) detectors is now under development. We are aiming for the spatial resolution of less than 1 mm in this scanner and acquisition time is less than 5 minutes by one breast and 10 minutes in total. The “C” shape of this scanner allows it to be positioned closely around the breast, effectively increasing both resolution and sensitivity. The open end of the detector unit allows the arm to be placed there and the C-shaped design of the scanner accommodates a variety of patient physiques, ensuring inclusion of the entire breast into the effective field of view (FOV).

PET data acquisition (DAQ) system having scalability for the number of detector

In a conventional PET data acquisition (DAQ) system, all detector signals are transferred to and processed by a single coincidence module so that high data processing speed is generally required for it. In other words, scale of a PET system (number of detector modules) was greatly limited by the performance of the coincidence module. In this situation, we developed a new DAQ system having scalability for the number of detector modules. Therefore, the new DAQ system can be applied widely for a small-scale system to a large-scale clinical system. The DAQ system was designed aiming for connecting 256 PET detector modules in maximum and for connecting multiple coincidence modules to form a daisy-chain structure transferring the detector signals to each other. The DAQ system consists of the three modules: one for generating data of position, energy and timing by processing detection pulse signals digitally, one for judging coincidence and generating coincidence event data and one for connecting the coincidence modules and a console Pc. This structure separates paths of single event data and coincidence event data and realizes that the transmission load between coincidence modules can be reduced. We confirmed using Matlab/Simulink that the count loss of coincidence events in the proposed DAQ system was only 5.5 % at the total coincidence count rate of 30 Mcps/system. We also measured the timing jitter of the DAQ system and the result was 97 ps. These results indicate that the DAQ system significantly reduces coincidence count losses and provides the timing performance enough for TOF-PET scanners.

Development of a prototype DOI-TOF-PET scanner

A prototype depth-of-interaction and time-of-flight positron emission tomography (DOI-TOF-PET) scanner was developed to offer enhanced spatial resolution with high sensitivity and high signal-to-noise-ratio (SNR) in the reconstructed images. The detector ring is 775 mm diameter with 48 mm axial field-of-view (FOV) per ring. The system can be expanded up to three rings. The ring comprises 48 detector modules, each of which consists of four layers of 16 × 16 crystal elements and a 64ch PS-PMT (H8500 position sensitive photomultiplier tube, Hamamatsu Photonics K.K.) optically coupled with silicone resin. The size of the crystal elements are 2.9 mm × 2.9 mm and increase in depth through 5, 6, 7, and 8 mm, from the first to fourth layer, to reduce the sensitivity differences between each layer. The crystal material is used Lu2xGd2(1−x)SiO5 (Hitachi Chemical Co., Ltd.) because of its short decay time, high density and high light yield. A data acquisition board was developed to improve the spatial resolution and the timing resolution of the system. The TDC (time-to-digital Converter) chips (TDC-GPX, Acam Messelectronic) mounted on the board operate in high-resolution mode (R-mode, 27 ps/bin). In addition, a new timing correction method to correct the intrinsic timing difference both each detector and each crystal of the detector by using DOI information was developed. As a result, the average timing resolution of this system was 442 ps (FWHM). Reconstructed image quality with-/without-DOI-TOF technique was evaluated in GATE simulation and a preliminary iamge was obtained with the prototype system.

Development of a TOF-PET detector capable of four-layer DOI encoding with a single-layer crystal array

In positron emission tomography (PET), depth of interaction (DOI) detectors are preferable for providing both high spatial resolution and high sensitivity. In previous work, we developed a four-layer DOI detector and invented and proved it works successfully for a time-of-flight (TOF) PET scanner. The detector consists of four layers of a 16 × 16 Lu1.8Gd0.2SiO5 (LGSO) crystal array. Each crystal element is 3.0 mm square and 5, 6, 7, and 8 mm in depth, and the four-layer DOI encoding is allowed by changing the reflector arrangement for each layer. However, the gaps between layers degrade the performance of the detector such as the energy and time resolution. In addition, the increase in the number of crystal elements increases the assembly time of the crystal blocks. In this work, we have developed a new TOF-PET detector capable of four-layer DOI encoding with a single-layer 16 × 16 crystal array. Each crystal element of LGSO is 3.0 mm square and 26 mm in depth, but the reflector arrangement is same as in the conventional DOI-TOF detector. The crystal block is coupled to a 64-channel flat panel position sensitive photomultiplier tube which has 8 × 8 multi anodes at intervals of 6.08 mm. The proposed detector was arranged opposite a BaF2 detector and was uniformly irradiated using 511 keV gamma rays from a 22Na point source. All crystal elements and DOI layers were expressed on a two-dimensional position histogram without overlapping. In comparison with the conventional DOI-TOF detector, the energy resolution was improved from 13.0% to 11.2% and the time resolution was improved from 476 ps to 455 ps. These results demonstrated the proposed DOI-TOF detector is suitable for practical use by reducing cost while improving energy and time resolutions

A new dynamic time over threshold method

A Time Over Threshold (TOT) system has advantage over pulse height measurements on its high integrity and low power dissipation because of its binary readout and circuit simplicity. However the relation between TOT and input charge is strongly nonlinear and dynamic range is limited. We propose a new dynamic TOT system which converts the pulse height to pulse width with a dynamically changing threshold. This kind of TOT system can enable wider dynamic range and improves linearity since the threshold follows the input signal and even shorten the width of TOT pulse. We show the concept of dynamic TOT system and results with discrete circuits. It can improve the dynamic range and theoretically it is possible to desired relation between TOT and input charge by using dedicated threshold function. We also designed and fabricated 48channel dynamic TOT ASIC with 0.25um TSMC CMOS technology.

A new clamp amplifier suitable for PET scanner’s front-end electronics based on integrated circuit

We have proposed a new architecture of the clamp amplifier that is one of the important components of analog front-end ASIC (application specific integrated circuits) for high-resolution PET (positron emission tomography) scanner’s data acquisition system. It consists of a comparing and switching block, an offset correction block, and an operational amplifier. The proposed technique enables us to fabricate high speed and small size clamp amplifier in the standard semiconductor process. Using this amplifier, the high-performance front-end ASIC can be achieved. We have evaluated the basic characteristics of the proposed clamp amplifier by the commercial components based test circuit, and have confirmed that it has excellent performance as a clamp amplifier.

Development of a dual-head mobile DOI-TOF PET system having multi-modality compatibility

We previously proposed the concept of “FlexiblePET”, a dual-head mobile DOI-TOF PET system which scans the patient lying on a bed equipped by another imaging/therapy device. Following the development of a small prototype with dual-head SiPM-based detectors showing a proof-of-concept for MR compatibility, we are now developing a human prototype with DOI-TOF detectors and a scalable data acquisition system. Each detector module consists of a 2-D crystal array (2.9 mm × 2.9 mm × 20 mm LGSO in a 16 × 16 array), a light guide and a 4 × 4 4-ch SiPM array. The detector has a four-layer DOI capability by a special reflector arrangement and is expected to have <; 500 ps coincidence timing resolution. The scanner has two arced detector heads (central angle: 135 degree, diameter: 778 mm), and each consists of 18 detector modules in transaxial direction and 3 rings in axial direction. This geometric configuration provides 715 mm transaxial FOV and 150 mm axial FOV. The detector head arrangement is changeable into four types: Top-Bottom, Left-Right, Side-C and Top-C, depending on imaging purpose. In addition, high-sensitivity imaging is possible by moving detector heads close to the patient. To compensate image quality degradation caused by the limited angle coincidence measurement, which is inherent in stable dualhead scanning, a new regularized TOF list-mode reconstruction algorithm that combines weighted maximum likelihood estimation and projection-space regularization was also developed. In this study, we will report initial results of physical performance evaluations of the prototype FlexiblePET system according to the NEMA NU 2-2007 standards. The experimental results support that the developed dual-head DOI-TOF PET protoptype system has the MR-compatibility and the acceptable image quality from the incomplete TOF projection measurement.