Mitsuo Watanabe

A high resolution animal PET scanner using compact PS-PMT detectors

A new high resolution PET scanner dedicated to animal studies has been designed, built and tested. The system utilizes 240 block detectors, each of which consists of a new compact position-sensitive photomultiplier tube (PS-PMT) and an 8/spl times/4 BGO array. A total number of 7,680 crystals (480 per ring) are positioned to form a 508 mm diameter of 16 detector rings with 7.2 mm pitch and 114 mm axial field of view (FOV). The system is designed to perform activation studies using a monkey in a sitting position. The data can be acquired in either 2D or 3D mode, where the slice collimators are retracted in 3D mode. The transaxial resolution is 2.6 mm FWHM at the center of the FOV, and the average axial resolution on the axis of the ring is 3.3 mm FWHM in the direct slice and 3.2 mm FWHM in the cross slice. The scatter fraction, sensitivity and count rate performance were evaluated for a 10 cm diameter cylindrical phantom. The total system sensitivity is 2.3 kcps/kBq/ml in 2D mode and 22.8 kcps/kBq/ml in 3D mode. The noise equivalent count rate with 3D mode is equivalent to that with 2D mode at five times higher radioactivity level. The applicable imaging capabilities of the scanner was demonstrated by animal studies with a monkey.

High resolution block detectors for PET

New high-resolution blocks detectors in which BGO scintillators are coupled to a position-sensitive photomultiplier tube (PMT), were developed for positron emission tomography (PET) applications. The block detectors, which use two different BGO scintillator structures, were constructed and evaluated: one is a 1.7-mm-wide BGO array which provides a coincidence detector resolution of less than 2 mm FWHM (full width at half maximum) and the other is a BGO block having comb-shaped slits in it, which has a potential capability of improving the resolution uniformity of a PET system because of its ability to detect the depth-of-interaction of gamma -rays in the scintillator. The application of these detectors to a PET system for animal studies is discussed. >

A single-ring OpenPET enabling PET imaging during radiotherapy

We develop an OpenPET system which can provide an accessible open space to the patient during PET scanning. Our first-generation OpenPET geometry which we called dual-ring OpenPET consisted of two separated detector rings and it could extend its axial field of view (FOV) therefore enabling imaging the gap region in addition to the in-ring region. However, applications such as dose verification by in-beam PET measurement during particle therapy and real-time tumor tracking by PET require sensitivity focused onto the gap rather than on the wide FOV. In this paper, we propose a second-generation OpenPET geometry, single-ring OpenPET, which can provide an accessible and observable open space with higher sensitivity and a reduced number of detectors than the earlier one. The proposed geometry has a cylinder shape cut at a slant angle, in which the shape of each cut end becomes an ellipse. We provided a theoretical analysis for sensitivity of the proposed geometry, compared with the dual-ring OpenPET and a geometry where the conventional PET was positioned at a slant angle against the patient bed to form an accessible open space, which we called a slant PET. The central sensitivity depends on the solid angle of these geometries. As a result, we found that the single-ring OpenPET has a sensitivity 1.2 times higher than the dual-ring OpenPET and 1.3 times higher than the slant PET when designed for a 600 mm bed width with 300 mm accessible open space and about 200 detector blocks, each with a front area of 2500 mm². In addition, numerical simulation was carried out to show the imaging property of the proposed geometry realized with the ellipsoidal rings and these results indicate that the depth-of-interaction detector can provide uniform resolution even when the detectors are arranged in an ellipsoidal ring.

Development of PET Detectors Using Monolithic Scintillation Crystals Processed With Sub-Surface Laser Engraving Technique

New monolithic scintillation detectors for PET have been developed, where the crystals are processed using internal focused laser processing technique, which is called subsurface laser engraving (SSLE) technique. When high intensity light pulses of short duration from a laser are focused into a scintillation crystal, they induce multi-photon absorption at the focal point and result in refractive index changes or micro-cracks inside the crystal. By applying the SSLE technique to a monolithic scintillation block, fine segmentation in the crystal can be formed without inter-pixel gaps. We have fabricated 2D segmented arrays engraved various patterns of micro-cracks inside monolithic LYSO crystal blocks by using a Nd:YAG laser. The processed crystal array segmented to 12 × 12 with 1.67 mm pitch have been evaluated by coupling to a position-sensitive photomultiplier tube (PS-PMT). The 2D position histograms were measured for uniform irradiation of gamma-rays and each crystal segment was clearly separated. The average energy resolution was 9.7%, similar to that of the conventional arrays, so that the laser processed LYSO crystals have kept their primary scintillation properties. We have also evaluated the laser processed crystals by using multi-pixel photon counters (MPPCs) to investigate the possibilities as a future PET detector. These results suggest that it is possible to fabricate high performance PET detectors using the SSLE technique.

Development of 3-D detector system for positron CT

A detector composed of a bundle of BGO (bismuth germanate) pillars and position-sensitive photomultiplier tubes is proposed. This detector has the ability to provide 3-D position information on gamma -ray absorption in the scintillator. It is expected that the use of this detector for positron-emission tomography (PET) will result in high spatial resolution and sensitivity. In particular, uniform and high spatial resolution over the entire field of view can be obtained because of the detectors ability to detect the depth of interaction of the gamma -rays in the scintillator. The fundamental characteristics were investigated by both experiment and simulation, and the results are discussed. >

Development of a high resolution PET

A high-resolution positron emission tomograph (PET) for brain studies has been developed. It consists of five detector rings (240 BGOs/ring). New multisegment photomultiplier tubes (PMTs) were adopted for the system with 5-mm-wide BGOs. The system is designed to examine a patient sitting or lying down on a chair/bed couch. The functions of PMT auto gain control and real-time image display are implemented in the system. The physical performance of the system was evaluated: the spatial resolution is 3.5 mm in the transaxial plane and 5.7 mm in the axial direction, and the total system sensitivity is 109 kcps/ mu Ci/ml for a uniform phantom 20 cm in diameter with a pulse height threshold of 350 keV. >

A new high resolution PET scanner dedicated to brain research

A high-resolution positron emission tomography (PET) scanner dedicated to brain studies has been developed and its physical performance was evaluated. The block detector consists of a new compact position-sensitive photomultiplier tube (PS-PMT, Hamamatsu R7600-C12) and an 8/spl times/4 bismuth germanate (BGO) array. The size of each crystal is 2.8 mm/spl times/6.55 mm/spl times/30 mm. The system has a total of 11 520 crystals arranged in 24 detector rings 508 mm in diameter (480 per ring). The field of view (FOV) is 330 mm in diameter/spl times/163 mm, which is sufficient to measure the entire human brain. The diameter of the scanners opening is equal to the transaxial FOV (330 mm). The system can be operated in three-dimensional (3-D) data acquisition mode, when the slice septa are retracted. The mechanical motions of the gantry and bed are specially designed to measure the patient in various postures; lying, sitting, and even standing postures. The spatial resolution of 2.9 mm in both the transaxial and axial directions is obtained at the center of the FOV. The total system sensitivity is 6.4 kc/s/kBq/ml in two-dimensional (2-D) mode, with a 20-cm-diameter cylindrical phantom. The imaging capabilities of the scanner were studied with the Hoffman brain phantom and with a normal volunteer.

Performance of a PET detector with a 256ch flat panel PS-PMT

A 256ch flat panel position sensitive photomultiplier tube (FP-PMT) is a promising device for a PET detector because of its large opening area, 52mm /spl times/ 52 mm, and small dead space. The useful area of the FP-PMT is 89% to the opening area so that the FP-PMT affords optical coupling with a 16 /spl times/ 16 array or scintillation crystals having 3 mm /spl times/ 3 mm bottom area. Its 14.7 mm thickness will also ensure a compact volume and less weight for the PET apparatus. We evaluated performance by irradiating 511keV gamma ray onto GSO crystals coupled to a prototype of the FP-PMT. The resultant positioning image map assure its capability for crystal identification. In a series of measurements, we used a multilayer polymer mirrors for a reflector of the detector. It was cut or marked for folding in precise sizes using a CO/sub 2/ gas laser. Making folds on the reflector by a laser contributed to easier assembly of the detector composed of many small crystal elements and may potentially be utilized in various shaped detectors.

A new compact position-sensitive PMT for scintillation detectors

A new compact position-sensitive photomultiplier tube (PS-PMT), Hamamatsu R7600-C12, has been developed for scintillation detectors. The PS-PMT has 11 stages of metal channel dynodes [2] and 6(X)+6(Y) crossed plate anodes in a 25.7 mm square/spl times/20 mm high metal can package, where the photo-sensitive area is 22 mm square. The performance of the PS-PMT was evaluated in terms of applicability to radiation imaging systems. In comparison with the former type of PS-PMT (Hamamatsu R5900-C8), the new PS-PMT provides smaller light spread and better position response. Also, by removing the flange at the bottom of the PS-PMT, the ratio of the effective area to the outward area is increased. The spatial resolution capability was demonstrated by imaging a stratified LSO array having an element of 1.8 mm/spl times/1.8 mm/spl times/10 mm. Each crystal element is clearly identified on the image map with 662 keV uniform gamma-ray irradiation. This paper describes the characteristics of the new PS-PMT and the experimental results for a gamma-ray imaging detector.

Alterations in α4β2 nicotinic receptors in cognitive decline in Alzheimer’s aetiopathology

Nicotinic acetylcholine receptor subtype α4β2 is considered important in the regulation of attention and memory, and cholinergic degeneration is known as one pathophysiology of Alzheimers disease. Brain amyloid-β protein deposition is also a key pathological marker of Alzheimers disease. Recent amyloid-β imaging has shown many cognitively normal subjects with amyloid-β deposits, indicating a missing link between amyloid-β deposition and cognitive decline. To date, the relationship between the α4β2 nicotinic acetylcholine receptor and amyloid-β burden has not been elucidated in vivo. In this study we investigated the relation between α4β2 nicotinic acetylcholine receptor availability in the brain, cognitive functions and amyloid-β burden in 20 non-smoking patients with Alzheimers disease at an early stage and 25 age-matched non-smoking healthy elderly adults by measuring levels of α4β2 nicotinic acetylcholine receptor binding estimated from a simplified ratio method (BPRI) and Logan plot-based amyloid-β accumulation (BPND) using positron emission tomography with α4β2 nicotinic acetylcholine receptor tracer (18)F-2FA-85380 and (11)C-Pittsburgh compound B. The levels of tracer binding were compared with clinical measures for various brain functions (general cognition, episodic and spatial memory, execution, judgement, emotion) using regions of interest and statistical parametric mapping analyses. Between-group statistical parametric mapping analysis showed a significant reduction in (18)F-2FA-85380 BPRI in the cholinergic projection region in patients with Alzheimers disease with a variety of (11)C-Pittsburgh compound B accumulation. Spearman rank correlation analyses showed positive correlations of (18)F-2FA-85380 BPRI values in the medial frontal cortex and nucleus basalis magnocellularis region with scores of the Frontal Assessment Battery (a test battery for executive functions and judgement) in the Alzheimers disease group (P < 0.05 corrected for multiple comparison), and also positive correlations of the prefrontal and superior parietal (18)F-2FA-85380 BPRI values with the Frontal Assessment Battery score in the normal group (P < 0.05 corrected for multiple comparison). These positive correlations indicated an in vivo α4β2 nicotinic acetylcholine receptor role in those specific functions that may be different from memory. Both region of interest-based and voxelwise regression analyses showed a negative correlation between frontal (11)C-Pittsburgh compound B BPND and (18)F-2FA-85380 BPRI values in the medial frontal cortex and nucleus basalis magnocellularis region in patients with Alzheimers disease (P < 0.05 corrected for multiple comparison). These findings suggest that an impairment of the cholinergic α4β2 nicotinic acetylcholine receptor system with the greater amount of amyloid deposition in the system plays an important role in the pathophysiology of Alzheimers disease.