Hiromichi Tonami

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).

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 high resolution whole-body DOI PET system

High spatial resolution Positron Emission Tomography (PET) imaging provides functional data about a patients body and provides us with a range of new possibilities, including medicine development, new diagnostic methods, and others. The use of Depth of Interaction (DOI) detectors is one effective technique for the improvement of whole-body PET imaging, as is evident in the fact that most small animal PET images are taken using a DOI system. In this study, we developed a prototype whole-body DOI- equipped PET system and evaluated its performance. This scanners DOI detectors consist of dual-layer (front and rear) 9 x 10 array of GSO/GSO crystals, a light guide and two rectangular double-anode photo multiplier tubes (PMTs). The DOI layers are discriminated according to decay time differences, which are controlled by Ce concentrations. The DOI detectors are arranged in a circular detector ring, with a diameter of 664 mm. In order to avoid parallax errors, coincidental events, including rear layer detection, were addressed to the nearest neighbor LOR front-layer coincidence pair. To evaluate the spatial resolution of this PET system at various radial positions, 22Na point sources at 1, 10, 20, and 25 cm offset from the center were reconstructed, both with and without DOI information. Results showed that spatial resolution degradation at 25 cm improved from 195 % to 154 % as a result of using DOI data. During a visual evaluation using a Derenzo phantom, image reconstruction using DOI data clarified the image and corrected hot rod shapes. The results presented here show that DOI detectors were useful in creating an effective whole-body PET 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

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.

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 SiPM based MR-compatible DOI-TOF-PET detector

We developed a SiPM based MR-compatible DOI-TOF-PET detector. The detector consists of a scintillator block, a light guide, a 64-channels SiPM array, and front-end electronics (FEE) boards. The scintillator block consists of 2.9 mm × 2.9 mm × 20 mm LGSO crystals in a 16 × 16 array containing light reflectors arranged for four-layer DOI encoding by Anger logic calculation. The FEE including a full-custom mixed-signal ASIC was optimized for SiPM readout and TOF capability. 64-SiPM channels are segmented into four-blocks (16-ch/block) and a trigger signal is generated in each block for reducing degradation of timing resolution caused by dark counts. The ASIC consists of 16-readout channels, 16-DAC channels, an Anger type position encoder, a fast summing amplifier for timing signals, and a slow summing amplifier for energy signals. The readout channel is connected to AC-coupling to the anode of the SiPM and is composed of a fast voltage amplifier (>200 MHz), a slow voltage amplifier (shaper), and an offset controller. A two-dimensional position histogram, which was obtained under uniform irradiation from a 22Na point source, showed that all crystal positions, including DOI direction, were clearly identified and an energy resolution of 13.4 % was achieved. A timing histogram obtained from a quarter block (16-ch) of the developed detector in coincidence with a PMT based detector showed that a timing resolution of 495 ps is expected in the actual system. Based on the developed detectors, we are now developing an MR-compatible DOI-TOF-PET system.

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.

Optimization of enhanced energy window on a whole-body DOI PET system

The use of depth of interaction (DOI) detectors is one effective technique to improve the spatial resolution of whole-body PET imaging. We have developed a whole-body PET system with DOI detectors that achieves less than 3 mm (FWHM) uniform spatial resolution, independent of the radial position in the transaxial field of view. On the other hand, improvements in sensitivity and noise equivalent count rate (NECR) were expected by implementing the DOI-dependent extended energy window (DEEW) method, employing a different energy window for each layer. Furthermore, the energy-based selective coincidence (ESC) method reduces multiple coincidences. In this study, we investigated sensitivity and NECR improvements using the DEEW method and the optimized enhanced energy window setting. The DOI detector consists of dual-layer GSO crystals, and the DOI detectors are arranged into a circular detector ring with a diameter of 664 mm. We evaluated the sensitivity and NECR performance based on the NEMA NU2-2001 standard. ESC and DEEW (1st layer: 412-624 keV, 2nd layer: 200-300 keV + 400-624 keV) resulted in better system performance than the conventional method.