Nobuya Hashizume

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.

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).

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.

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.

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.