Hideaki Tashima

Development of a small prototype for a proof-of-concept of OpenPET imaging

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.

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.

Real-Time Imaging System for the OpenPET

The OpenPET and its real-time imaging capability have great potential for real-time tumor tracking in medical procedures such as biopsy and radiation therapy. For the real-time imaging system, we intend to use the one-pass list-mode dynamic row-action maximum likelihood algorithm (DRAMA) and implement it using general-purpose computing on graphics processing units (GPGPU) techniques. However, it is difficult to make consistent reconstructions in real-time because the amount of list-mode data acquired in PET scans may be large depending on the level of radioactivity, and the reconstruction speed depends on the amount of the list-mode data. In this study, we developed a system to control the data used in the reconstruction step while retaining quantitative performance. In the proposed system, the data transfer control system limits the event counts to be used in the reconstruction step according to the reconstruction speed, and the reconstructed images are properly intensified by using the ratio of the used counts to the total counts. We implemented the system on a small OpenPET prototype system and evaluated the performance in terms of the real-time tracking ability by displaying reconstructed images in which the intensity was compensated. The intensity of the displayed images correlated properly with the original count rate and a frame rate of 2 frames per second was achieved with average delay time of 2.1 s.

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.

A proposed helmet-PET with a jaw detector enabling high-sensitivity brain imaging

The advanced depth-of-interaction positron emission tomography (PET) detectors have been improved to provide three-dimensionally uniform detector resolution. These detectors allow a geometry that places the detectors very close to imaging subjects, because they can maintain high spatial resolution even in the peripheral region in the field-of-view (FOV) without loss of sensitivity. In this study, we are proposing a helmet-PET geometry consisting of a hemispheric-shaped detector and a jaw detector. The key points of the geometry are the closely positioned detectors and the detector covering the jaw. For each point in the FOV, we calculated the geometrical sensitivity as the relative coverage of the solid angle where the coincidence measurement was possible. We found that the hemispheric-shaped geometry had high sensitivity for the cerebrum region. In addition, the jaw detector significantly improved the sensitivity at the center region of the hemisphere, where the cerebellum is positioned during PET measurement. High sensitivity and quantitative accuracy for the cerebellum region are important especially for functional brain imaging because this region may be used as a reference value in kinetic analysis. Geometrical analysis showed that the proposed geometry has good potential for highly sensitive and accurate measurement of brain functions. In addition, numerical simulations showed the proposed geometry increases image quality especially for the central region.

A prototype of a novel transformable single-ring OpenPET

We are developing the worlds first, open-type 3D PET scanner “OpenPET” for PET-image guided particle therapy such as in situ dose verification and direct tumor tracking. Following our first idea of a dual-ring Open PET (DROP), we proposed our second-generation geometry, single-ring OpenPET (SROP), which is more efficient than DROP in terms of manufacturing cost and sensitivity. In this paper, we have developed a SROP prototype based on a novel detector arrangement, in which block detectors originally forming a conventional PET scanner were axially shifted little by little. Sixteen detector units each of which consists of two depth-of-interaction detectors are arranged to form a perfect circle, 25cm in diameter. Detector units have an axial shifting mechanism so that they can be transformed into the SROP; adding this mechanism to the units allows us to use the scanner as a conventional (i.e., non-open) PET when in-beam PET measurements are not required. After confirming its basic imaging performance using a phantom filled with 18F solution, we carried out in-beam imaging tests in the Heavy Ion Medical Accelerator in Chiba (HIMAC). In addition to the usual carbon (12C) beam, we applied RI beams of 11C and 10C. Stopping positions of primary particles were directly imaged with the RI beam irradiation, while the stopping position distribution of secondary particles was imaged with the 12C beam irradiation. Phantom study results with pencil beam irradiation of about 2.5Gy showed that beam stopping positions can be measured with the precision better than 2mm with the 11C beam irradiation followed by 20 min PET measurement. With the 10C beam, PET measurement time could be reduced to 1/10 while still maintaining the precision. For both 11C and 10C, there is room for further reduction of PET measurement time.

GPU-Based PET Image Reconstruction Using an Accurate Geometrical System Model

In positron emission tomography (PET), 3D iterative image reconstruction methods have a huge computational burden. In this paper, we developed a list-mode image reconstruction method using graphics processing units (GPUs). Efficiency of acceleration for GPU implementation largely depends on the method chosen, where a reduced number of conditional statements and a reduced memory size are required. On the other hand, accurate system models are required to improve the quality of reconstructed images. Various accurate system models for conventional CPU implementation have been proposed, but these models basically require many conditional statements and huge memory size. Therefore, we developed a new system model which matches GPU implementation better. In this model, the detector response functions, which vary depending on each line of response (LOR), are pre-computed in CPUs and modeled by sixth-order polynomial functions in order to reduce the memory size occupied in GPUs. Each element of a system matrix is obtained on-the-fly in GPUs by calculating the distance between an LOR and a voxel. Therefore the developed system model enables efficient GPU implementation of the accurate system modeling with a reduced number of conditional statements and a reduced memory size. We applied the developed method to a small OpenPET prototype, in which 4-layered depth-of-interaction (DOI) detectors were used. For image reconstruction, we used the dynamic row-action maximum likelihood algorithm (DRAMA). Compared with a conventional model for GPU implementation, in which DRFs are given as a Gaussian function of fixed width, we saw no remarkable difference for DOI data, but for non-DOI data the proposed model outperformed the conventional at the peripheral region of the field-of-view. The proposed model had almost the same calculation time as the conventional model did. For further acceleration, we tried parallel GPU implementation, and we obtained 3.8-fold acceleration by using 4 GPUs.

Proposed helmet PET geometries with add-on detectors for high sensitivity brain imaging.

For dedicated brain PET, we can significantly improve sensitivity for the cerebrum region by arranging detectors in a compact hemisphere. The geometrical sensitivity for the top region of the hemisphere is increased compared with conventional cylindrical PET consisting of the same number of detectors. However, the geometrical sensitivity at the center region of the hemisphere is still low because the bottom edge of the field-of-view is open, the same as for the cylindrical PET. In this paper, we proposed a helmet PET with add-on detectors for high sensitivity brain PET imaging for both center and top regions. The key point is the add-on detectors covering some portion of the spherical surface in addition to the hemisphere. As the location of the add-on detectors, we proposed three choices: a chin detector, ear detectors, and a neck detector. For example, the geometrical sensitivity for the region-of-interest at the center was increased by 200% by adding the chin detector which increased the size by 12% of the size of the hemisphere detector. The other add-on detectors gave almost the same increased sensitivity effect as the chin detector did. Compared with standard whole-body-cylindrical PET, the proposed geometries can achieve 2.6 times higher sensitivity for brain region even with less than 1/4 detectors. In addition, we conducted imaging simulations for geometries with a diameter of 250 mm and with high resolution depth-of-interaction detectors. The simulation results showed that the proposed geometries increased image quality, and all of the add-on detectors were equivalently effective. In conclusion, the proposed geometries have high potential for widespread applications in high-sensitivity, high-resolution, and low-cost brain PET imaging.

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.

Dosimetry by means of in-beam PET with RI beam irradiation

In situ visualization of deposited dose distribution is necessary to exploit the advantages of Carbon-ion therapy. Therefore we are developing the worlds first, open-type PET “OpenPET” to verify the field irradiated. In addition, a method of utilizing activity measurement in the target irradiated with the beam with positron-emitting radioisotopes (RI) such as 11C and 10C has been proposed. This method has advantage in the amount of activity as well as direct visualization of primary particles themselves, compared with the irradiation with stable nuclei 12C beam. With the RI beam, however, the profile of the dose was still different from that of the activity. Therefore interpretation from activity distribution to dose distribution is necessary to confirm the irradiation field precisely. In this paper, we developed a method of estimating the dose distribution from PET measurements. To utilize the activity measurement of the target, we used the code which calculated the activation distribution and dose distribution taking initial beam energy as a free parameter. Then the maximum likelihood parameter of the initial energy was determined by comparing the measured and the calculated distributions. By using the parameter determined, the dose distribution in the target was calculated as the estimated distribution for the actual one. The uniform PMMA target was irradiated with 11C beam for 10 s. The target was measured from the time beam irradiation started and to 60 s after finishing the irradiation by using a small single-ring OpenPET prototype developed for a proof-of-concept of the in-beam monitoring for charged particle therapy. As a result, the dose distribution, which was originally different from primary particles distribution, was successfully estimated. The peak depth of the estimated dose distribution was in good agreement with the measured dose peak.