Munetaka Nitta

Development of a small single-ring OpenPET prototype with a novel transformable architecture.

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.

Whole-body dual-ring OpenPET for in-beam particle therapy imaging

The OpenPET is our original idea that realizes the worlds first open-type 3D PET scanner for PET-image guided particle therapy such as in situ dose verification and direct tumor tracking. Even with a full-ring geometry, the OpenPET has an open gap between its 2 detector rings through which the treatment beam passes. Following our initial 2008 proposal, we developed a small prototype in 2010 to show a proof-of-concept. Now, we report the development of a prototype whole-body OpenPET. The key technology which enabled the OpenPET realization is our original, 4-layered depth-of-interaction detector. In order to measure a radiation from the limited activity produced though fragmentation reactions, Zr-doped GSO (GSOZ), which contains less natural radioactivity, was chosen for the scintillators instead of Lu-based scintillators although timing performance was compromised. In order to compensate for the limited light yield, on the other hand, we used 64-channel flat-panel PMTs with a super-bialkali photocathode, which had a 30% higher quantum efficiency. In order to enable stable in-beam PET measurement even under high background radiations, voltage divider circuits were designed to provide 5 times higher linearity. Additionally, to avoid severe radiation damage, we did not use gain control ASICs in the front-end circuits, and position analyzer circuits were placed with a 15-m cable extension. The prototype consists of 2 detector rings, and each detector ring has 2 sub-rings of 40 detectors. Each detector consists of 16 × 16 × 4 array of GSOZ (2.8 × 2.8 × 7.5mm3). The portable gantry has a compact design; each detector ring has a 940 mm outer diameter and 171 mm thickness for the detector inner bore of 640 mm diameter and 113 mm thickness. The system was tested with a carbon beam irradiation at a clinical intensity. Phantom images were obtained by applying a GPGPU-based, list mode iterative reconstruction algorithm with geometrical detector response modeling.

Development of the helmet-chin PET prototype

There is a strong potential demand for high-sensitivity and low-cost brain positron emission tomography (PET) imaging that is applicable to early diagnosis of Alzheimers disease. Therefore, we have proposed a high-sensitivity dedicated brain PET geometry composed of a helmet detector having a hemisphere shape and a chin detector, which we call helmet-chin PET. Because the shape of a human head is a sphere, the hemispherical arrangement of the detectors allows closer positioning of detectors and better sensitivity than the conventional cylindrical arrangement. In addition, adding detectors around the chin position significantly improves the sensitivity at the center where the cerebellum is located. For a proof-of-concept of the helmet-chin PET, we developed the first prototype of the helmet-chin PET using 4-layer depth-of-interaction (DOI) detectors. The helmet detector for the prototype system was realized by multiple rings having different numbers of detectors and a cross-shaped part covering the top. We used in total 54 DOI detectors, each of which consisted of 1,024 Zr-doped GSO crystals with dimensions of 2.8 × 2.8 × 7.5 mm3 and a high-sensitivity, 64-ch flat-panel photomultiplier tube. In performance evaluations, we determined there were uniform spatial resolutions of 3.0 mm by an analytical method and 1.4 mm by an iterative method. Peak sensitivity was measured as 10 % at a region near the top of the head, which was almost equivalent to the central sensitivity of the cylindrical PET composed of 120 DOI detectors. In addition, we performed an initial imaging test with a brain phantom and we reconstructed the images with and without the chin detector. We found the slice near the bottom of the helmet detector had strong noise without the chin detector, while the slice had good imaging performance with it, and the overall image quality was improved. Therefore, we concluded that the helmet-chin PET had high potential for realizing high-sensitivity, low-cost, and accurate brain imaging.

Development of a whole-body single-ring OpenPET for in-beam particle therapy imaging

One of the challenging applications of PET is implementing it for in-beam PET, which is an in situ monitoring method for charged particle therapy. For this purpose, we have previously proposed two geometries for our original open-type PET scanners named OpenPET. Following our initial proposal of the dual-ring OpenPET (DROP) in 2008, we developed a whole-body prototype of DROP (WBDROP) in 2014 to show a proof-of-concept. On the other hand, in 2011, we also proposed the single-ring OpenPET (SROP), which is more efficient than DROP in terms of manufacturing cost, width of open gap. In this paper, we developed the whole-body prototype of SROP (WBSROP). The WBSROP prototype was designed with 4 axial shifted detector rings of 40 depth-of-interaction (DOI) detectors. The detector rings were each 66 cm in diameter. The rings were slanted by 45 deg from the axial direction to obtain large open gap of 43-cm width. The DOI detectors consist of 1024 Zr-doped GSO crystals which are arranged in 4 layers of 16 × 16 arrays, coupled to a 64-ch flat panel position sensitive PMT with a super-bialkali photocathode, which had a 30% higher quantum efficiency. Each crystal element is 2.8 × 2.8 × 7.5 mm3. In order to enable stable in-beam PET measurement even under high background radiations, voltage divider circuits were designed so as to have 5 times higher linearity. Front-end circuit equipped only resister chain and amplifier and the data acquisition system was separated from the gantry by 7-m coaxial cables for protecting electronics circuits from radiation damage by secondary particles. For sensitivity comparison between WBSROP and WBDROP prototypes, both scanners were simulated using Monte-Carlo simulation. Predicted sensitivity of the WBSROP prototype was 3.1%. The WBSROP prototype promises high sensitivity although wide open gap.

Activation measurement for material selection of OpenPET components in particle therapy

In-beam positron emission tomography (PET) is expected to be suitable as an in situ dose verification technique in particle therapies. We have developed the worlds first open-type PET, OpenPET, to monitor radiation field in heavy ion radiation therapy. In using in-beam PET, the number of annihilation radiation particles to be measured is generally low. On the other hand, OpenPET devices can be activated by secondary particles such as neutrons generated through the nuclear reactions between primary particles and irradiated objects. This causes an increase of single counts and that could possibly decrease the signal-to-noise ratio. Thus we should carefully select materials for devices with less activation. We are going to use aluminum or stainless steel for the body of the equipment in our next development of a human size OpenPET. To that goal, in this work, we measured activation of an aluminum plate and a stainless plate to estimate the influence on PET scanning. The sample plates were placed at downstream of a water phantom and was irradiated by a 12C beam with 290 MeV/u. to activate with effect, we shoot higher intensity beam than clinical usage. After the irradiation, we measured radioactivity of the both samples using a germanium detector. We evaluated the radioactivity especially gamma rays with 511 keV and isotopes created by activation. Both aluminum sample and stainless sample had not less than 400 Bq and 1300 Bq of radioactivity immediately after the irradiation respectively. Comparing activity of the both sample, we found that aluminum is thus one of the promising candidates for the better material of OpenPET components. We also evaluated the activation of Gd2SiO5 scintillator in the same way.

A new four-layered DOI detector with quadrisected top layer crystals

We developed a modified, more practical four-layered depth-of-interaction (DOI) detector based on the light sharing method. Reflectors, which are inserted in every two lines of crystal segments and shifted differently depending on each layer, project 3-D crystal positions onto a 2-D position histogram without any overlapping after applying an Anger-type calculation. The best crystal separation we have ever made based on this method was the 4-layered 32 × 32 array of LYSO crystals sized at 1.45 × 1.45 × 5 mm3. However, assembling crystals of a tiny size tends to cost a lot, and fine tuning of the light guide and the front-end circuit is required to have fine crystal identification from photo sensor signals of coarser pixel pitch. In this paper, therefore, we proposed a more practical 4-layered DOI detector. The key idea is that the crystals in the top layer, which have the highest detection efficiency, mostly contribute to PET spatial resolution. We applied two new ideas: (1) use of 1/4 size crystals only for the 1st (top) layer and (2) inserting a thin light guide between the 1st and the 2nd layers of crystal array. In the developed prototype detector, the 1st layer used 32 × 32 LYSO crystals of quarter size (1.4 × 1.4 × 5.0 mm3) compared with the other layers (16 × 16 arrays of crystals of 2.8 × 2.8 × 5.0 mm3). For better crystal identification of small crystals in the 1st layer, we optimized the optical condition between crystals such as use of an optical glue or air. Also, a thin light guide was inserted between the 1st and the 2nd layers for improvement of crystal identification of the 1st layer. With the appropriate insertion of the light guide, all crystals of the 1st layer were identified as well as the crystals in the other layers.

Assessment of shielding materials for the add-on PET at different magnetic field strengths of mri

We are developing a novel PET/MRI system. In this system, PET detectors are closely located to the MR RF-coil. To reduce the electromagnetic interaction between the PET detectors and the RF-coil, the PET detectors are covered with conductive shield boxes. However, when the magnetic field around the shield box is changed by MRI field gradient pulses, an eddy current is generated in the shield box. The eddy current produces the secondary magnetic field (ΔB0), resulting in degraded MR image quality. The interference effect may depend on the static magnetic field strength of the MRI. In our previous work, we evaluated the signal-to-noise ratio and the eddy current for several shield materials with a 3 T clinical MRI system. The results showed that carbon fiber roving had high shielding performance and suppression capacity for the eddy current this paper, we evaluated performance of the carbon fiber roving in different static magnetic field strengths of the MRI. The results showed that carbon fiber roving had high shield performance regardless of the static magnetic field strength. From the experiment using the 3 T MRI, we clarified that the ΔB0 of carbon fiber roving was equivalent to ΔB0 of FRP. The slew rate of the gradient magnetic field of 0.3 T MRI is lower than that of the 3 T MRI, therefore, the eddy current and ΔB0 of the 0.3 T MRI are lower than those of the 3 T MRI. Thus, we judged that carbon fiber roving was valid as the shield material of shield boxes regardless of magnetic field strength of the MRI.

Induced radioactivity of a GSO scintillator by secondary fragments in carbon ion therapy and its effects on in-beam OpenPET imaging

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.

Positron annihilation spectroscopy of biological tissue in 11C irradiation

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.

Influence of misalignment of a scintillator array and a multi-anode PMT for 4-layer DOI PET detector

We are developing a depth of interaction (DOI) detector having a crystal block composed of four layers of a scintillation crystal array. By removing reflectors between crystal elements for every two lines and shifting reflector patterns depending on layers, responses of all crystal elements in the four layers can be mapped to a 2-dimentional (20) position histogram after applying conventional Anger-type calculation. Because this method projects four times more number of crystal responses onto the position histogram than conventional non-DOI detectors do. Slight difference of light distribution on a multi-anode photomultiplier tube (MA-PMT) discriminates each crystal responses. In particular, location of the crystal responses in the position histogram is sensitive to spatial relationship between the crystal block and the MA-PMT at the edge side of the responses. In this paper, to set up the crystal block appropriate position, we developed an optical camera-based system for positioning of the crystal block. Using this system, we studied the influence of positioning accuracy of the crystal block on the crystal responses in the 20 position histogram. We used a MA-PMT with a 49 × 49 mm2 effective area in 52 × 52 mm2 opening window and anode interval of 6 mm. We prepared two types of crystal blocks, a non-DOI crystal block and the four-layer DOI crystal block, and obtained 20 position histograms with shifting each crystal block from a corner of the MA-PMT to the center. The crystal blocks were composed of 2.8 x 2.8 x 7.5 mm3 GSO crystal elements. We found that all responses of the non-DOI detector can be discriminated, even it was on the corner of the MA-PMT, however, responses of the DOI detector are not able to be discriminated on the edge sides of the 2D position histogram e