Yoshihiko Kumazawa

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.

Reconstruction of 4-Layer DOI detector equipped C-shaped PEM via list-mode iterative algorithm

This C-shaped PEM unit currently under development is designed for the very early stage detection of very small lesions, such as breast cancers. This PEM units scanner is equipped with 4-layer Depth of Interaction (DOI) detectors, each of which contains small scintillator crystals. The scanner is positioned closely around the breast, providing both high 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). In this work, list-mode iterative algorithms can be utilized for image reconstruction using incomplete acquisition datasets, due to this scanners module gap and large number of LORs. To evaluate the effect of using DOI detectors on C-shaped PEM images, and the resulting effective FOV of this PEM, Monte- Carlo simulations of the acquisitions from this PEM scanner were used for image reconstruction via list-mode DRAMA algorithms. Results indicate that while non-DOI detector contrast near the detector gap deteriorates substantially, use of a DOI detector system preserves a high level of contrast.

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.

Sophisticated 32 × 32 × 4-layer DOI detector for high resolution PEM scanner

We are now developing a sophisticated 32times32times4-layer depth of interaction (DOI) detector for a positron emission mammography (PEM) scanner which has both high resolution and high sensitivity. The candidate of scintillator is Lu1.8Gd0.2SiO5 (LGSO) or Lu1.9Y0.1SiO5 (LYSO) and each crystal size is 1.45 mm times 1.45 mm times 4.5 mm. The white plastic reflectors with thickness of 0.125 mm are inserted into a crystal block with different patterns for each DOI layer. The crystal block is coupled to a 256-channel flat panel position sensitive photomultiplier tube which has 16 times 16 at intervals of 3.04 mm. In this work, we evaluated the performance of a DOI detector with LGSO crystals by obtaining a two-dimensional (2D) position histogram of all crystal elements with 511 keV gamma-ray irradiation. Results show that 4,096 crystal elements (32 x 32 x 4) can be clearly identified on the 2D position histogram. As the average over all crystal elements, the energy resolution is 14.7% and the timing resolution is 849 ps in FWHM.

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).

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.

Random correction using singles count rates for DOI Positron Emission Mammography

With the aim of realizing the early diagnosis of breast cancer, we are promoting the research and development of a high-resolution Positron Emission Mammography (PEM). In order to obtain both high resolution and high sensitivity, this scanner uses for-layer DOI (depth of interaction) detectors consisting of small crystals being positioned close to the breast Since the total number of crystals in the scanner is extremely large and it is predicted that the total number of LORs is greater than the total number of events, a list-mode iterative image reconstruction method is required. Furthermore, a random correction method based on singles counting is beneficial for reducing statistical noise in randoms estimation at each LOR. However, an extremely large number of counters are required to count single events for each crystal. We also developed a random correction method calculating the count for each crystal. This method makes it possible to reduce the number of counters. We evaluated the efficacy of this technique incorporated in a list-mode iterative reconstruction using simulation data. Results demonstrated that random correction using conventional delayed coincidence method does not work well for the list-mode iterative reconstruction having an extremely large number of LORs, whereas random correction using singles count rates is effective for improving the image quality.

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.

Advantage of the four-layer DOI information in the time resolution for a TOF-PET detector

Time-of-flight position emission tomography (TOF-PET) is a promising technique for greatly improving image quality. It is well known that depth of interaction (DOI) information can reduce parallax error and uniform the spatial resolution in whole FOV. In this work, we developed three types of DOI detector (non-DOI, 2-layer and 4-layer) for TOF-PET and confirmed whether DOI information can also improve time resolution. The crystal blocks consist of a 16 × 16 Lu1.8Gd0.2SiO5 (LGSO) array. To become the same sensitivity for all crystal blocks, they had the total length in depth direction of the crystal blocks are the same. The size of each crystal element was 3 mm square. The crystal blocks were optically coupled to a 64-channel position sensitive photomultiplier tube, which has 8 × 8 multi anodes at intervals of 6.08 mm. Irradiating 511 keV gamma rays uniformly, All crystal elements of each block are clearly separated on a two-dimensional position histogram. Energy resolutions of non-DOI, 2-layer and 4-layer DOI detectors are 11.1%, 12.0% and 13.0% and time resolutions are 450 ps, 455 ps and 476 ps, respectively.