Masafumi Furuta

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 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.

Development of a proof of concept system for multi-modal compatible PET: Flexible PET

Recently, various PET-MRI systems with silicon photomultiplier (SiPM) arrays have been developed by many research groups [1-3]. We previously proposed the concept of a multi-modal compatible “flexible” PET scanner [4]. The scanner consists of adjustable two detector heads and scans a patient lying on a bed equipped with another medical device such as CT, MR and radiotherapy device. In this study, we have developed a proof-of-concept (POC) MR-compatible PET system that consists of two detector heads and the data acquisition (DAQ) system. The single detector module consists of 16 × 16 LGSO crystal array, a light guide and a 6 × 6 SiPM array. The size of each crystal element was 2.9 mm × 2.9 mm × 20.0 mm with a special reflector arrangement for four-layer DOI encoding. Two detector heads each consists of six detector modules are positioned in front of a permanent magnet (1.5 T) open MRI scanner. We evaluated the mutual interference between PET and MRI by the sequential PET/MRI experiments, and finally performed small animal imaging studies. These results showed MR-compatibility of our detectors and encouraged the concept of the “flexible” PET.

Development of a Digital Baseline Restorer for high-resolution PET detectors

For a data processing circuit using Anger positioning method on a high-resolution PET detector, it is difficult to measure precisely the position of the interaction, because a signal baseline fluctuates by causes such as temperature dependence of the detector sensitivity and the circuit drift, pulse pileup at high count rates, and scintillator afterglow. Particularly in the case of the number of scintillator elements much greater than the number of photo detector elements, both high signal precision and stability are required because the positioning error is caused by a slight baseline change. Usually AC-coupled circuit is utilized to reduce the influence of the baseline change, but may not work at high count rates. Then we adopted a DC coupling and have developed a DBLR (Digital Baseline Restorer) method to correct the baseline shift in real time. The amount of baseline shift is periodically estimated during the acquisition based on the shrinkage of a 2D position histogram map of the detector. The ratios of the total count of the outermost region and the inner region for each direction of the 2D map are fed back to correct the position calculation on FPGA. We evaluated the proposed method with a four-layer DOI (depth of interaction) detector that consists of 4,096 LGSO crystal elements and a 64-channel PS-PMT (position sensitive photomultiplier tube). The DBLR can effectively correct the baseline shift and reduce the map shrinkage to less than 0.3% even at a high count rate of 243 kcps.