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Objective. For PET imaging systems, a smaller detector ring enables less intrinsic spatial resolution loss due to the photon non-collinearity effect as well as better balance between production cost and sensitivity, and a hemispherical detector arrangement is more appropriate for brain imaging than a conventional cylindrical arrangement. Therefore, we have developed a brain-dedicated PET system with a hemispherical detector arrangement, which has been commercialized in Japan under the product name of VRAINTM. In this study, we evaluated imaging performance of VRAIN. Approach. The VRAIN used 54 detectors to form the main hemispherical unit and an additional half-ring behind the neck. Each detector was composed of a 12 × 12 array of lutetium fine silicate crystals (4.1 × 4.1 × 10 mm3) and a 12 × 12 array of silicon photomultipliers (4 × 4 mm2 active area) with the one-to-one coupling. We evaluated the physical performance of VRAIN according to the NEMA NU 2–2018 standards. Some measurements were modified so as to fit the hemispherical geometry. In addition, we performed 18F-FDG imaging in a healthy volunteer. Main results. In the phantom study, the VRAIN showed high resolution for separating 2.2 mm rods, 229 ps TOF resolution and 19% scatter fraction. With the TOF gain for a 20 cm diameter object (an assumed head diameter), the peak noise-equivalent count rate was 144 kcps at 9.8 kBq ml−1 and the sensitivity was 25 kcps MBq−1. Overall, the VRAIN provided excellent image quality in phantom and human studies. In the human FDG images, small brain nuclei and gray matter structures were clearly visualized with high contrast and low noise. Significance. We demonstrated the excellent imaging performance of VRAIN, which supported the advantages of the hemispherical detector arrangement.

This work presents an iterative method for the estimation of the absolute dose distribution in patients undergoing carbon ion therapy, via analysis of the distribution of positron annihilations resulting from the decay of positron-emitting fragments created in the target volume. The proposed method relies on the decomposition of the total positron-annihilation distributions into profiles of the three principal positron-emitting fragment species - 11C, 10C and 15O. A library of basis functions is constructed by simulating a range of monoenergetic 12C ion irradiations of a homogeneous polymethyl methacrylate phantom and measuring the resulting one-dimensional positron-emitting fragment profiles and dose distributions. To estimate the dose delivered during an arbitrary polyenergetic irradiation, a linear combination of factors from the fragment profile library is iteratively fitted to the decomposed positron annihilation profile acquired during the irradiation, and the resulting weights combined with the corresponding monoenergetic dose profiles to estimate the total dose distribution. A total variation regularisation term is incorporated into the fitting process to suppress high-frequency noise. The method was evaluated with 14 different polyenergetic 12C dose profiles in a polymethyl methacrylate target: one which produces a flat biological dose, 10 with randomised energy weighting factors, and three with distinct dose maxima or minima within the spread-out Bragg peak region. The proposed method is able to calculate the dose profile with mean relative errors of 0.8%, 1.0% and 1.6% from the 11C, 10C, 15O fragment profiles, respectively, and estimate the position of the distal edge of the SOBP to within an average of 0.7 mm, 1.9 mm and 1.2 mm of its true location.

Compton imaging or Compton camera imaging has been studied well, but its advantages in nuclear medicine and molecular imaging have not been demonstrated yet. Therefore, the aim of this work was to compare Compton imaging with positron emission tomography (PET) by using the same imaging platform of whole gamma imaging (WGI). WGI is a concept that combines PET with Compton imaging by inserting a scatterer ring into a PET ring. This concept utilizes diverse types of gamma rays for 3D tomographic imaging. In this paper, we remodeled our previous WGI prototype for small animal imaging, and we developed an image reconstruction method based on a list-mode ordered subset expectation maximization algorithm incorporating detector response function modeling, random correction and normalization (sensitivity correction) for either PET and Compton imaging. To the best of our knowledge, this is the world’s first realization of a full-ring Compton imaging system. We selected 89Zr as an imaging target because a 89Zr nuclide emits a 909 keV single-gamma ray as well as a positron, and we can directly compare Compton imaging of 909 keV photons with PET, a well-established modality. We measured a cylindrical phantom and a small rod phantom filled with 89Zr solutions of 10.3 MBq and 10.2 MBq activity, respectively, for 1 h each. The uniform radioactivity distribution of the cylindrical phantom was reconstructed with normalization in both PET and Compton imaging. Coefficients of variation for region-of-interest values were 4.2% for Compton imaging and 3.3% for PET; the difference might be explained by the difference in the detected count number. The small rod phantom experiment showed that the WGI Compton imaging had spatial resolution better than 3.0 mm at the peripheral region although the center region had lower resolution. PET resolved 2.2 mm rods clearly at any location. We measured a mouse for 1 h, 1 d after injection of 9.8 MBq 89Zr oxalate. The 89Zr assimilated in the mouse bony structures was clearly depicted, and Compton imaging results agreed well with PET images, especially for the region inside the scatterer ring. In conclusion, we demonstrated the performance of WGI using the developed Compton image reconstruction method. We realized Compton imaging with a quality approaching that of PET, which is supporting a future expectation that Compton imaging outperforms PET.

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.

In heavy-ion therapy, the stopping position of primary ions in tumours needs to be monitored for effective treatment and to prevent overdose exposure to normal tissues. Positron-emitting ion beams, such as

Myocardial perfusion imaging (MPI) with PET plays a vital role in the management of coronary artery disease. High sensitivity systems can contribute to maximizing the potential value of PET MPI; therefore, we have proposed two novel detector arrangements, an elliptical geometry and a D-shape geometry, that are more sensitive and more compact than a conventional large-bore cylindrical geometry. Here we investigate two items: the benefits of the proposed geometries for cardiac imaging; and the effects of scatter components on cardiac PET image quality. Using the Geant4 toolkit, we modeled four time-of-flight (TOF) PET systems: an 80-cm-diameter cylinder, a 40-cm-diameter cylinder, a compact ellipse, and a compact D-shape. Spatial resolution and sensitivity were measured using point sources. Noise equivalent count rate (NECR) and image quality were examined using an anthropomorphic digital chest phantom. The proposed geometries showed higher sensitivity and better count rate characteristics with a fewer number of detectors than the conventional large-bore cylindrical geometry. In addition, we found that the increased intensity of the scatter components was a big factor affecting the contrast in defect regions for such a compact geometry. It is important to address the issue of the increased intensity of the scatter components to develop a high-performance compact cardiac TOF PET system.

Simultaneous measurements of multiple physiological parameters are essential for the study of brain disease mechanisms and the development of suitable therapies to treat them. In this study, we developed a measurement system for simultaneous optical imaging and PET for awake mice. The key elements of this system are the OpenPET, optical imaging and fixation apparatus for an awake mouse. The OpenPET is our original open-type PET geometry, which can be used in combination with another device because of the easily accessible open space of the former. A small prototype of the axial shift single-ring OpenPET was used. The objective lens for optical imaging with a mounted charge-coupled device camera was placed inside the open space of the AS-SROP. Our original fixation apparatus to hold an awake mouse was also applied. As a first application of this system, simultaneous measurements of cerebral blood flow (CBF) by laser speckle imaging (LSI) and [(11)C]raclopride-PET were performed under control and 5% CO2 inhalation (hypercapnia) conditions. Our system successfully obtained the CBF and [(11)C]raclopride radioactivity concentration simultaneously. Accumulation of [(11)C]raclopride was observed in the striatum where the density of dopamine D2 receptors is high. LSI measurements could be stably performed for more than 60 minutes. Increased CBF induced by hypercapnia was observed while CBF under the control condition was stable. We concluded that our imaging system should be useful for investigating the mechanisms of brain diseases in awake animal models.

Compton imaging or Compton camera imaging has been studied well, but its advantages in nuclear medicine and molecular imaging have not been demonstrated yet. Therefore, the aim of this work was to compare Compton imaging with positron emission tomography (PET) by using the same imaging platform of whole gamma imaging (WGI). WGI is a concept that combines PET with Compton imaging by inserting a scatterer ring into a PET ring. This concept utilizes diverse types of gamma rays for 3D tomographic imaging. In this paper, we remodeled our previous WGI prototype for small animal imaging, and we developed an image reconstruction method based on a list-mode ordered subset expectation maximization algorithm incorporating detector response function modeling, random correction and normalization (sensitivity correction) for either PET and Compton imaging. To the best of our knowledge, this is the world's first realization of a full-ring Compton imaging system. We selected 89Zr as an imaging target because a 89Zr nuclide emits a 909 keV single-gamma ray as well as a positron, and we can directly compare Compton imaging of 909 keV photons with PET, a well-established modality. We measured a cylindrical phantom and a small rod phantom filled with 89Zr solutions of 10.3 MBq and 10.2 MBq activity, respectively, for 1 h each. The uniform radioactivity distribution of the cylindrical phantom was reconstructed with normalization in both PET and Compton imaging. Coefficients of variation for region-of-interest values were 4.2% for Compton imaging and 3.3% for PET; the difference might be explained by the difference in the detected count number. The small rod phantom experiment showed that the WGI Compton imaging had spatial resolution better than 3.0 mm at the peripheral region although the center region had lower resolution. PET resolved 2.2 mm rods clearly at any location. We measured a mouse for 1 h, 1 d after injection of 9.8 MBq 89Zr oxalate. The 89Zr assimilated in the mouse bony structures was clearly depicted, and Compton imaging results agreed well with PET images, especially for the region inside the scatterer ring. In conclusion, we demonstrated the performance of WGI using the developed Compton image reconstruction method. We realized Compton imaging with a quality approaching that of PET, which is supporting a future expectation that Compton imaging outperforms PET.

Dose and range verification have become important tools to bring carbon ion therapy to a higher level of confidence in clinical applications. Positron emission tomography is among the most commonly used approaches for this purpose and relies on the creation of positron emitting nuclei in nuclear interactions of the primary ions with tissue. Predictions of these positron emitter distributions are usually obtained from time-consuming Monte Carlo simulations or measurements from previous treatment fractions, and their comparison to the current, measured image allows for treatment verification. Still, a direct comparison of planned and delivered dose would be highly desirable, since the dose is the quantity of interest in radiation therapy and its confirmation improves quality assurance in carbon ion therapy. In this work, we present a deconvolution approach to predict dose distributions from PET images in carbon ion therapy. Under the assumption that the one-dimensional PET distribution is described by a convolution of the depth dose distribution and a filter kernel, an evolutionary algorithm is introduced to perform the reverse step and predict the depth dose distribution from a measured PET distribution. Filter kernels are obtained from either a library or are created for any given situation on-the-fly, using predictions of the [Formula: see text]-decay and depth dose distributions, and the very same evolutionary algorithm. The applicability of this approach is demonstrated for monoenergetic and polyenergetic carbon ion irradiation of homogeneous and heterogeneous solid phantoms as well as a patient computed tomography image, using Monte Carlo simulated distributions and measured in-beam PET data. Carbon ion ranges are predicted within less than 0.5 mm and 1 mm deviation for simulated and measured distributions, respectively.

Positron annihilation spectroscopy (PAS) spectra of biological tissue in 11C irradiation are reported and spatial resolution coefficient of positron emission tomography (PET) obtained from the PAS spectrum is discussed for 11C irradiation. A PAS spectrum of the biological tissue with water is the same as that of the water pool phantom in 11C irradiation. However, a PAS spectrum of the biological tissue with less water differs from that of the water pool phantom. The PET spatial resolution coefficient depends on the kind of biological tissue. However, the PET spatial resolution coefficient, 0.00243±0.00014, can be used as a common value of maximum limit.

In-beam positron emission tomography (PET) can enable visualization of an irradiated field using positron emitters (β+ decay). In particle therapies, many kinds of secondary particles are produced by nuclear interactions, which affect PET imaging. Our purpose in this work was to evaluate effects of secondary particles on in-beam PET imaging using the Monte Carlo simulation code, Geant4, by reproducing an experiment with a small OpenPET prototype in which a PMMA phantom was irradiated by a (11)C beam. The number of incident particles to the detectors and their spectra, background coincidence for the PET scan, and reconstructed images were evaluated for three periods, spill-time (beam irradiation), pause-time (accelerating the particles) and beam-off time (duration after the final spill). For spill-time, we tested a background reduction technique in which coincidence events correlated with the accelerator radiofrequency were discarded (RF gated) that has been proposed in the literature. Also, background generation processes were identified. For spill-time, most background coincidences were caused by prompt gamma rays, and only 1.4% of the total coincidences generated β+ signals. Differently, for pause-time and beam-off time, more than 75% of the total coincidence events were signals. Using these coincidence events, we failed to reconstruct images during the spill-time, but we obtained successful reconstructions for the pause-time and beam-off time, which was consistent with the experimental results. From the simulation, we found that the absence of materials in the beam line and using the RF gated technique improved the signal-to-noise ratio for the spill-time. From an additional simulation with range shifter-less irradiation and the RF gated technique, we showed the feasibility of image reconstruction during the spill-time.

We are developing a three-dimensional (3D) position-sensitive detector with isotropic spatial resolution, the X'tal cube. Originally, our design consisted of a crystal block for which all six surfaces were covered with arrays of multi-pixel photon counters (MPPCs). In this paper, we examined the feasibility of reducing the number of surfaces on which a MPPC array must be connected with the aim of reducing the complexity of the system. We evaluated two kinds of laser-processed X'tal cubes of 3 mm and 2 mm pitch segments while varying the numbers of the 4 × 4 MPPC arrays down to two surfaces. The sub-surface laser engraving technique was used to fabricate 3D grids into a monolithic crystal block. The 3D flood histograms were obtained by the Anger-type calculation. Two figures of merit, peak-to-valley ratios and distance-to-width ratios, were used to evaluate crystal identification performance. Clear separation was obtained even in the 2-surface configuration for the 3 mm X'tal cube, and the average peak-to-valley ratios and the distance-to-width ratios were 6.7 and 2.6, respectively. Meanwhile, in the 2 mm X'tal cube, the 6-surface configuration could separate all crystals and even the 2-surface case could also, but the flood histograms were relatively shrunk in the 2-surface case, especially on planes parallel to the sensitive surfaces. However, the minimum peak-to-valley ratio did not fall below 3.9. We concluded that reducing the numbers of MPPC readout surfaces was feasible for both the 3 mm and the 2 mm X'tal cubes.

The accumulation of induced radioactivity within in-beam PET scanner scintillators is of concern for its long-term clinical usage in particle therapy. To estimate the effects on OpenPET which we are developing for in-beam PET based on GSOZ (Zi doped Gd2SiO5), we measured the induced radioactivity of GSO activated by secondary fragments in a water phantom irradiation by a (12)C beam with an energy of 290 MeV u(-1). Radioisotopes of Na, Ce, Eu, Gd, Nd, Pm and Tb including positron emitters were observed in the gamma ray spectra of the activated GSO with a high purity Ge detector and their absolute radioactivities were calculated. We used the Monte Carlo simulation platform, Geant4 in which the observed radioactivity was assigned to the scintillators of a precisely reproduced OpenPET and the single and coincidence rates immediately after one treatment and after one-year usage were estimated for the most severe conditions. Comparing the highest coincidence rate originating from the activated scintillators (background) and the expected coincidence rate from an imaging object (signal), we determined the expected signal-to-noise ratio to be more than 7 within 3 min and more than 10 within 1 min from the scan start time. We concluded the effects of scintillator activation and their accumulation on the OpenPET imaging were small and clinical long-term usage of the OpenPET was feasible.

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.

We are developing a novel, general purpose isotropic-3D PET detector X'tal cube which has high spatial resolution in all three dimensions. The research challenge for this detector is implementing effective detection of scintillation photons by covering six faces of a segmented crystal block with silicon photomultipliers (SiPMs). In this paper, we developed the second prototype of the X'tal cube for a proof-of-concept. We aimed at realizing an ultimate detector with 1.0 mm(3) cubic crystals, in contrast to our previous development using 3.0 mm(3) cubic crystals. The crystal block was composed of a 16 × 16 × 16 array of lutetium gadolinium oxyorthosilicate (LGSO) crystals 0.993 × 0.993 × 0.993 mm(3) in size. The crystals were optically glued together without inserting any reflector inside and 96 multi-pixel photon counters (MPPCs, S10931-50P, i.e. six faces each with a 4 × 4 array of MPPCs), each having a sensitive area of 3.0 × 3.0 mm(2), were optically coupled to the surfaces of the crystal block. Almost all 4096 crystals were identified through Anger-type calculation due to the finely adjusted reflector sheets inserted between the crystal block and light guides. The reflector sheets, which formed a belt of 0.5 mm width, were placed to cover half of the crystals of the second rows from the edges in order to improve identification performance of the crystals near the edges. Energy resolution of 12.7% was obtained at 511 keV with almost uniform light output for all crystal segments thanks to the effective detection of the scintillation photons.

The OpenPET geometry is our new idea to visualize a physically opened space between two detector rings. In this paper, we developed the first small prototype to show a proof-of-concept of OpenPET imaging. Two detector rings of 110 mm diameter and 42 mm axial length were placed with a gap of 42 mm. The basic imaging performance was confirmed through phantom studies; the open imaging was realized at the cost of slight loss of axial resolution and 24% loss of sensitivity. For a proof-of-concept of PET image-guided radiation therapy, we carried out the in-beam tests with (11)C radioactive beam irradiation in the heavy ion medical accelerator in Chiba to visualize in situ distribution of primary particles stopped in a phantom. We showed that PET images corresponding to dose distribution were obtained. For an initial proof-of-concept of real-time multimodal imaging, we measured a tumor-inoculated mouse with (18)F-FDG, and an optical image of the mouse body surface was taken during the PET measurement by inserting a digital camera in the ring gap. We confirmed that the tumor in the gap was clearly visualized. The result also showed the extension effect of an axial field-of-view (FOV); a large axial FOV of 126 mm was obtained with the detectors that originally covered only an 84 mm axial FOV. In conclusion, our initial imaging studies showed promising performance of the OpenPET.

The long patient port of a PET scanner tends to put stress on patients, especially patients with claustrophobia. It also prevents doctors and technicians from taking care of patients during scanning. In this paper, we proposed an 'open PET' geometry, which consists of two axially separated detector rings. A long and continuous field-of-view (FOV) including a 360 degrees opened gap between two detector rings can be imaged enabling a fully 3D image reconstruction of all the possible lines-of-response. The open PET will become practical if iterative image reconstruction methods are applied even though image reconstruction of the open PET is analytically an incomplete problem. First we implemented a 'masked' 3D ordered subset expectation maximization (OS-EM) in which the system matrix was obtained from a long 'gapless' scanner by applying a mask to detectors corresponding to the open space. Next, in order to evaluate imaging performance of the proposed open PET geometry, we simulated a dual HR+ scanner (ring diameter of D = 827 mm, axial length of W = 154 mm x 2) separated by a variable gap. The gap W was the maximum limit to have axially continuous FOV of 3W though the maximum diameter of FOV at the central slice was limited to D/2. Artifacts, observed on both sides of the open space when the gap exceeded W, were effectively reduced by inserting detectors partially into unnecessary open spaces. We also tested the open PET geometry using experimental data obtained by the jPET-D4. The jPET-D4 is a prototype brain scanner, which has 5 rings of 24 detector blocks. We simulated the open jPET-D4 with a gap of 66 mm by eliminating 1 block-ring from experimental data. Although some artifacts were seen at both ends of the opened gap, very similar images were obtained with and without the gap. The proposed open PET geometry is expected to lead to realization of in-beam PET, which is a method for an in situ monitoring of charged particle therapy, by letting the beams pass through the gap. The proposed open PET geometry will also allow simultaneous PET/CT measurements of the same PET FOV as the CT FOV, in contrast to the conventional PET/CT where each FOV is separated by several tens of centimeters.

In dose verification techniques of particle therapies based on in-beam positron emission tomography (PET), the causes of washout of positron emitters by physiological effects should be clarified to correct washout for accurate verification. As well, the quantitative washout rate has a potential usefulness as a diagnostic index which should be explored. Therefore, we measured washout rates of rat brain after vasodilator acetazolamide loading to investigate the possible effects of blood flow on washout. Six rat brains were irradiated by a radioisotope (11)C beam and time activity curves on the whole brains were obtained with a small single-ring OpenPET prototype. Then, washout rates were calculated with the Mizuno model, where two washout rates (k 2m and k 2s ) were assumed, and a two-compartment model including efflux from tissue to blood (k 2) and influx (k 3) and efflux (k 4) between the two tissue compartments. Before the irradiations, we used laser-Doppler flowmetry to confirm that acetazolamide increased cerebral blood flow (CBF) of a rat. We compared means of k 2m , k 2s and k 2, k 3 and k 4 without acetazolamide loading (Rest) and with acetazolamide loading (ACZ). For all k values, ACZ values were lower than Rest values. In other words, though CBF increased, washout rates were decreased. This may be attributed to the implanted (11)C reacting to form (11)CO2. Because acetazolamide increased the concentration of CO2 in brain, suppressed diffusion of (11)CO2 and decomposition of (11)CO2 into ions were prevented.

The single-ring OpenPET (SROP), for which the detector arrangement has a cylinder shape cut by two parallel planes at a slant angle to form an open space, is our original proposal for in-beam PET. In this study, we developed a small prototype of an axial-shift type SROP (AS-SROP) with a novel transformable architecture for a proof-of-concept. In the AS-SROP, detectors originally forming a cylindrical PET are axially shifted little by little. We designed the small AS-SROP prototype for 4-layer depth-of-interaction detectors arranged in a ring diameter of 250 mm. The prototype had two modes: open and closed. The open mode formed the SROP with the open space of 139 mm and the closed mode formed a conventional cylindrical PET. The detectors were simultaneously moved by a rotation handle allowing them to be transformed between the two modes. We evaluated the basic performance of the developed prototype and carried out in-beam imaging tests in the HIMAC using (11)C radioactive beam irradiation. As a result, we found the open mode enabled in-beam PET imaging at a slight cost of imaging performance; the spatial resolution and sensitivity were 2.6 mm and 5.1% for the open mode and 2.1 mm and 7.3% for the closed mode. We concluded that the AS-SROP can minimize the decrease of resolution and sensitivity, for example, by transforming into the closed mode immediately after the irradiation while maintaining the open space only for the in-beam PET measurement.

A high-performance brain PET scanner, jPET-D4, which provides four-layer depth-of-interaction (DOI) information, is being developed to achieve not only high spatial resolution, but also high scanner sensitivity. One technical issue to be dealt with is the data dimensions which increase in proportion to the square of the number of DOI layers. It is, therefore, difficult to apply algebraic or statistical image reconstruction methods directly to DOI-PET, though they improve image quality through accurate system modelling. The process that requires the most computational time and storage space is the calculation of the huge number of system matrix elements. The DOI compression (DOIC) method, which we have previously proposed, reduces data dimensions by a factor of 1/5. In this paper, we propose a transaxial imaging system model optimized for jPET-D4 with the DOIC method. The proposed model assumes that detector response functions (DRFs) are uniform along line-of-responses (LORs). Then each element of the system matrix is calculated as the summed intersection lengths between a pixel and sub-LORs weighted by a value from the DRF look-up-table. 2D numerical simulation results showed that the proposed model cut the calculation time by a factor of several hundred while keeping image quality, compared with the accurate system model. A 3D image reconstruction with the on-the-fly calculation of the system matrix is within the practical limitations by incorporating the proposed model and the DOIC method with one-pass accelerated iterative methods.

In-beam positron emission tomography (PET) is expected to enable visualization of a dose verification using positron emitters (β+ decay). For accurate dose verification, correction of the washout of the positron emitters should be made. In addition, the quantitative washout rate has a potential usefulness as a diagnostic index, but modeling for this has not been studied yet. In this paper, therefore, we applied compartment analyses to in-beam PET data acquired by our small OpenPET prototype, which has a physically opened field-of-view (FOV) between two detector rings. A rat brain was located at the FOV and was irradiated by a (11)C beam. Time activity curves of the irradiated field were measured immediately after the irradiations, and the washout rate was obtained based on two models: the two-washout model (medium decay, k2m; slow decay, k2s) developed in a study of rabbit irradiation; and the two-compartment model used in nuclear medicine, where efflux from tissue to blood (k2), influx (k3) and efflux (k4) from the first to second compartments in tissue were evaluated. The observed k2m and k2s were 0.34 and 0.005 min(-1), respectively, which was consistent with the rabbit study. Also k2m was close to the washout rate in cerebral blood flow (CBF) measurements by dynamic PET with (15)O-water, while, k2, k3, and k4 were 0.16, 0.15 and 0.007 min(-1). Our present work suggested the dynamics of (11)C might be relevant to CBF or permeability of a molecule containing (11)C atoms might be regulated by a transporter because the k2 was relatively low compared with a simple diffusion tracer.

Annihilation photon acollinearity is a fundamental but little investigated problem in positron emission tomography (PET). In this paper, the cause of the angular deviation from 180.00 degrees is described as well as how to evaluate it under conditions of a spatially distributed radiation source and a limited acquisition time for the human body. A relationship between the shape of the photopeak spectrum and the angular distribution is formulated using conservation laws of momentum and energy over the pair annihilation. Then the formula is used to evaluate the acollinearity for a pool phantom and the human body with FDG injected. The angular distribution for the pool phantom agrees well with that for pure water which had been directly measured by Colombino et al in 1965 (Nuovo Cimento 38 707-23), and also with that for the human body determined in this study. Pure water can be considered as a good approximation of the human body regarding the angular deviation. The blurring coefficient to be multiplied by the ring diameter in calculations of the PET spatial resolution is experimentally determined for the first time as 0.00243 +/- 0.00014; this is 10% larger than the value widely used by investigators.

The jPET-D4 is the first PET scanner to introduce a unique four-layer depth-of-interaction (DOI) detector scheme in order to achieve high sensitivity and uniform high spatial resolution. This paper compares measurement and Monte Carlo simulation results of the static physics performance of this prototype research PET scanner. Measurement results include single and coincidence energy spectra, point and line source sensitivities, axial sensitivity profile (slice profile) and scatter fraction. We use GATE (Geant4 application for tomographic emission) as a Monte Carlo radiation transport model. Experimental results are reproduced well by the simulation model with reasonable assumptions on characteristic responses of the DOI detectors. In a previous study, the jPET-D4 was shown to provide a uniform spatial resolution as good as 3 mm (FHWM). In the present study, we demonstrate that a high sensitivity, 11.3 +/- 0.5%, is provided at the FOV centre. However, about three-fourths of this sensitivity is related to multiple-crystal events, for which some misidentification of the crystal cannot be avoided. Therefore, it is crucial to develop a more efficient way to identify the crystal of interaction and to reduce misidentification in order to make use of these high performance values simultaneously. We expect that effective sensitivity can be improved by replacing the GSO crystals with more absorptive crystals such as BGO and LSO. The results we describe here are essential to take full advantage of the next generation PET systems that have DOI recognition capability.

This paper proposes a new count-loss model that is applicable to radiation detection systems in the field of nuclear medicine. The proposed model, represented by n = (1 - exp(-tau(n)0))/tau(n)0: input rate, n: output rate,tau: system clock duration), gives higher count rates than the well-known standard nonparalyzable (non-extensible) model represented by n = n0/(1 + tau(n)0) does, being equally simple. This model assumes that a part of the data processing is performed on a fixed system clock, requiring a data buffering function (latch) to retain digital event information temporarily, and that input timing is random. The model's simple interpretation and expression also offers an advantage over various other advanced count-loss models proposed so far. This model was found to be a better fit to a data-acquisition system for a positron emission tomography scanner.