Junichi Ohi

Performance Evaluation of a New Dedicated Breast PET Scanner Using NEMA NU4-2008 Standards

The aim of this work was to evaluate the performance characteristics of a newly developed dedicated breast PET scanner, according to National Electrical Manufacturers Association (NEMA) NU 4-2008 standards. Methods: The dedicated breast PET scanner consists of 4 layers of a 32 × 32 lutetium oxyorthosilicate–based crystal array, a light guide, and a 64-channel position-sensitive photomultiplier tube. The size of a crystal element is 1.44 × 1.44 × 4.5 mm. The detector ring has a large solid angle with a 185-mm aperture and an axial coverage of 155.5 mm. The energy windows at depth of interaction for the first and second layers are 400–800 keV, and those at the third and fourth layers are 100–800 keV. A fixed timing window of 4.5 ns was used for all acquisitions. Spatial resolution, sensitivity, counting rate capabilities, and image quality were evaluated in accordance with NEMA NU 4-2008 standards. Human imaging was performed in addition to the evaluation. Results: Radial, tangential, and axial spatial resolution measured as minimal full width at half maximum approached 1.6, 1.7, and 2.0 mm, respectively, for filtered backprojection reconstruction and 0.8, 0.8, and 0.8 mm, respectively, for dynamic row-action maximum-likelihood algorithm reconstruction. The peak absolute sensitivity of the system was 11.2%. Scatter fraction at the same acquisition settings was 30.1% for the rat-sized phantom. Peak noise-equivalent counting rate and peak true rate for the ratlike phantom was 374 kcps at 25 MBq and 603 kcps at 31 MBq, respectively. In the image-quality phantom study, recovery coefficients and uniformity were 0.04–0.82 and 1.9%, respectively, for standard reconstruction mode and 0.09–0.97 and 4.5%, respectively, for enhanced-resolution mode. Human imaging provided high-contrast images with restricted background noise for standard reconstruction mode and high-resolution images for enhanced-resolution mode. Conclusion: The dedicated breast PET scanner has excellent spatial resolution and high sensitivity. The performance of the dedicated breast PET scanner is considered to be reasonable enough to support its use in breast cancer imaging.

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).

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 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

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.

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.