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Brain PET, which has led research in molecular imaging and diagnosis of brain cancer, epilepsy and neurodegenerative disorders, is being spotlighted again to promote earlier diagnosis of dementia with the advent of amyloid and tau tracers. To meet this demand, in this paper, we developed a brain-dedicated PET imaging device with a hemispherical detector arrangement, which provides comparable sensitivity with fewer detectors than conventional cylindrical geometries. The introduction of the time-of-flight (TOF) measurement capability was a key point for the development to get a gain in the image signal-to-noise ratio. Currently, whole-body PET scanners with around 200-400 ps coincidence resolving time (CRT) are commercially available. In order to obtain the same TOF gain which can be obtained with 400 ps CRT for a 30 cm diameter object, 267 ps CRT will be required for a 20 cm diameter object such as the human head. In this work, therefore, we aimed at developing a TOF brain-dedicated PET prototype with the hemisphere detector arrangement and the CRT faster than 267 ps. The detector was composed of a 12 × 12 lutetium fine silicate (LFS) array coupled with a 12 × 12 multi-pixel photon counter (MPPC) array. Each LFS crystal with a size of 4.14 × 4.14 × 10 mm3 was individually coupled to a separate MPPC. Singles list-mode data from each detector were stored, and coincidences were identified using a coincidence-detection software algorithm. The CRT of 245 ps was finally achieved as the system average after a fine timing correction. For image reconstruction, we implemented the list-mode TOF-OSEM. For a small rod phantom, rods of 3 mm diameter were clearly separated. Also, images of the 3D Hoffman brain phantom, which demonstrated clear contrast between gray and white matter, supported the effect of TOF information.

Background: The concrete walls inside the vaults of cyclotron facilities are activated by neutrons emitted by the targets during radioisotope production. Reducing the amount of radioactive waste created in such facilities is very important in case they are decommissioned. Thus, we proposed a strategy of reducing the neutron activation of the concrete walls in cyclotrons during operation. Materials and Methods: A polyethylene plate and B-doped Al sheet (30 wt% of B and 2.5 mm in thickness) were placed in front of the wall in the cyclotron room of a radioisotope production facility for pharmaceutical use. The target was Xe gas, and a Cu block was utilized for proton dumping. The irradiation time, proton energy, and beam current were 8 hours, 30 MeV, and 125 μA, respectively. To determine a suitable thickness for the polyethylene plate set in front of the B-doped Al sheet, the neutron-reducing effects achieved by inserting such sheets at several depths within polyethylene plate stacks were evaluated. The neutron fluence was monitored using an activation detector and 20-g on de Au foil samples with and without 0.5-mm-thick Cd foil. Each Au foil sample was pasted onto the center of a polyethylene plate and B-doped Al sheet, and the absolute activity of one Au foil sample was measured as a standard using a Ge detector. The resulting relative activities were obtained by calculating the ratio of the photostimulated luminescence of each foil sample to that of the standard Au foil. Results and Discussion: When the combination of a 4-cm-thick polyethylene plate and Bdoped Al sheet was employed, the thermal neutron rate was reduced by 78%. Conclusion: The combination of a 4-cm-thick polyethylene plate and B-doped Al sheet effectively reduced the neutron activation of the investigated concrete wall.

Time-of-fligfrt (TOF) PET improves image quality and quantitative accuracy compared with a conventional PET system. While it is true that TOF works better for larger objects, recent improvement in timing resolution has encouraged application of TOF to brain-dedicated PET systems. Thus, as the second prototype of the helmet-type PET, we have developed a new TOF brain-dedicated PET prototype using detector modules of 12×12 lutetium fine silicate (LFS) crystals (4.1×4.1×10 mm3) connected to a 12×12 (4 mm pitch, 144-ch) MPPC array. In this paper, we investigated another detector module, which has the same outer size but smaller crystals: 16×16 LFS of 3.1×3.1×10 mm3 size coupled to 16×16 (3 mm pitch, 256-ch) MPPC array. The 3-mm 256-ch module showed an energy resolution of 12.0%. For the coincidence response function, the 3-mm module showed a better full width at half maximum (FWHM) of 1.9 mm compared with the 4-mm 144-ch module (2.4 mm). The FWHM was improved by 21%. The coincidence resolving time obtained by the 3-mm 256-ch module was 241 ps. The energy resolution and coincidence resolving time were almost the same between the two modules. The 3-mm 16x16 MPPC-based TOF-PET module can be expected to improve spatial resolution without compromising the energy resolution and coincidence resolving time compared with the 4-mm 12×12 module.

We have developed the brain-dedicated compact PET scanner which has a hemispherical helmet detector unit and an add-on detector unit located at the neck position (helmet-neck PET). To realize clinical applications of the helmet-neck PET scanner, we need to evaluate the performance regarding their absolute quantitation, image contrast, noise and uniformity using appropriate phantoms. However, existing standard phantoms are not applicable for the helmet-neck PET scanner because of its hemisphere geometry. Therefore, in this study, we developed two new phantoms, a small sphere contrast phantom and a 3dimensional hemispherical Hoffman brain phantom, which model a brain tumor and static regional cerebral blood flow, respectively. These phantoms have an adaptive structure for use with the PET scanner with hemispherical detector arrangement. In addition, we tested the developed contrast phantom using the helmet-neck PET prototype. The PET image obtained by the helmet-neck PET prototype showed high image contrast and acceptable noise level. These phantoms are useful for evaluating the imaging performance of compact helmet-shape scanners.

In some recently developed brain-dedicated PET systems, a seated position is selected for the patient. The PET scanners with the seated measurement style have a merit of allowing system downsizing. However, the effect of the seated position for head motion suppression compared with the conventional supine position is not clear. Although motion correction methods have been developed, they are not in practical use at a routine level. Even when using the motion correction method, it is desirable to make the head motion as small as possible. In this study, we developed a contactless head motion tracking system, and we conducted a volunteer study of the head motion tracking to assess the optimum measurement position for the brain PET. We developed the tracking system using the Microsoft Kinect sensor as a range sensor. As a result of the accuracy evaluation, translation accuracy of about 1 mm and rotation accuracy of less than 1 deg were achieved. In the volunteer study, we measured the head motion of volunteers with supine, normal sitting, and reclining. Additionally, we measured these positions with and without head fixation. As a result, the normal sitting position without head fixation had the largest head motion, and the reclining position had smaller motion than the supine position. The head fixation was effective for motion suppression in all cases. The lowest head motion was achieved with the reclining and supine positions (average displacement in 60 s was about 0.5 mm). In order to assess the effect of the head motion, we added the motion information obtained with the volunteer study to brain phantom data measured with our helmet-chin PET prototype. In the case of the reclining with head fixation, we could obtain reconstruction images of equal quality to the case without motion even though we did not apply any motion correction. We concluded that the reclining position had a high head motion suppression effect equal to or better than that of the supine position. （ M-15-058）, IEEE NSS-MIC2017

Time-of-flight (TOF) is now a standard technology for positron emission tomography (PET), but its effective use for small diameter PET systems has not been studied well. In this paper, we simulated a brain-dedicated TOF-PET system with a hemispherical detector arrangement. We modeled a Hamamatsu TOF-PET module (C13500-4075LC-12) with 280 ps coincidence resolving time (CRT), in which a 12 x 12 array of multi pixel photon counters (MPPCs) is connected to a lutetium fine silicate (LFS) crystal array of 4.1 x 4.1 mm2 cross section each, based on one-to-one coupling. On the other hand, spatial resolution degradation due to the parallax error should be carefully addressed for the small diameter PET systems. The ideal PET detector would have both depth-of-interaction (DOI) and TOF capabilities, but typical DOI detectors that are based on light sharing tend to degrade TOF performance. Therefore, in this work, we investigated non-DOI detectors with an appropriate crystal length, which was a compromise between suppressed parallax error and decreased sensitivity. Using GEANT4, we compared two TOF detectors, a 20 mm long non-DOI and a 10 mm long non-DOI, with a non-TOF, 4-layer DOI detector with a total length of 20 mm (i.e., 5 x 4 mm). We simulated a contrast phantom and evaluated the relationship between the contrast recovery coefficient (CRC) and the noise level (the coefficient of variation, COV) for reconstructed images. The 10 mm long non-DOI, which reduces the parallax error at the cost of sensitivity loss, showed better imaging quality than the 20 mm long non-DOI. For example, the CRC value of a 10 mm hot sphere at COV = 20% was 72% for the 10 mm long non-DOI, which was 1.2 times higher than that of the 20 mm long non-DOI. The converged CRC values for the 10 mm long non-DOI were almost equivalent to those of the non-TOF 4-layer DOI, and the 10 mm long non-DOI converged faster than the non-TOF 4-layer DOI did. Based on the simulation results, we evaluated a one-pair prototype system of the TOF-PET detectors with 10 mm crystal length, which yielded the CRT of 250 ± 8 ps. In summary, we demonstrated support for feasibility of the brain-dedicated TOF-PET system with the hemispherical detector arrangement.

A strong demand is expected for high-sensitivity, high-resolution and low-cost brain positron emission tomography (PET) imaging for early diagnosis of dementia, as well as for general neuroscience studies. Therefore, we have proposed novel geometries of a hemisphere detector arrangement for high-sensitivity brain imaging, in which an add-on detector at the chin position or neck position helps in sensitivity uniformity improvement. In this study, we developed the first prototype system for proof-of-concept using four-layer depth-of-interaction detectors, each of which consisted of 16 × 16 × 4 Zr-doped GSO crystals with dimensions of 2.8 × 2.8 × 7.5 mm3 and a high-sensitivity 64-channel flat-panel photomultiplier tube. We used 47 detectors to form a hemisphere detector with a hemisphere shape of 25 cm inner diameter and 50 cm outer diameter, and we used seven detectors for each of the add-on detectors. The total detector number of 54 was about one-fourth that of a typical whole-body PET scanner. The hemisphere detector for the prototype system was realized by multiple rings having different numbers of detectors and a cross-shaped top detector unit covering the top. Performance evaluation showed uniform spatial resolutions of 3-4 mm by the filtered back-projection method. Imaging tests of a hot-rod phantom done with an iterative method were able to resolve 2.2 mm rods. Peak sensitivity was measured as more than 10% at a region near the top of the head, which was achieved with the help of the top detector unit. In addition, using the prototype system, we performed the first FDG clinical test with a healthy volunteer. The results showed that the proposed geometries had high potential for realizing high-sensitivity, high-resolution, and low-cost brain PET imaging. As for the add-on detector position, it was shown that the neck position resulted in higher sensitivity and wider field of view (FOV) than the chin position because the add-on detector at the neck position can be placed continuously to the hemisphere detector and close to the FOV.