Fully 3D Implementation of the End-to-end Deep Image Prior-based PET Image Reconstruction Using Block Iterative Algorithm

Deep image prior (DIP) has recently attracted attention owing to its unsupervised positron emission tomography (PET) image reconstruction, which does not require any prior training dataset. In this paper, we present the first attempt to implement an end-to-end DIP-based fully 3D PET image reconstruction method that incorporates a forward-projection model into a loss function. To implement a practical fully 3D PET image reconstruction, which could not be performed due to a graphics processing unit memory limitation, we modify the DIP optimization to block-iteration and sequentially learn an ordered sequence of block sinograms. Furthermore, the relative difference penalty (RDP) term was added to the loss function to enhance the quantitative PET image accuracy. We evaluated our proposed method using Monte Carlo simulation with [$^{18}$F]FDG PET data of a human brain and a preclinical study on monkey brain [$^{18}$F]FDG PET data. The proposed method was compared with the maximum-likelihood expectation maximization (EM), maximum-a-posterior EM with RDP, and hybrid DIP-based PET reconstruction methods. The simulation results showed that the proposed method improved the PET image quality by reducing statistical noise and preserved a contrast of brain structures and inserted tumor compared with other algorithms. In the preclinical experiment, finer structures and better contrast recovery were obtained by the proposed method. This indicated that the proposed method can produce high-quality images without a prior training dataset. Thus, the proposed method is a key enabling technology for the straightforward and practical implementation of end-to-end DIP-based fully 3D PET image reconstruction.Comment: 9 pages, 10 figure

Fabrication of a nanofiber Bragg cavity with high quality factor using a focused helium ion beam

Nanofiber Bragg cavities (NFBCs) are solid-state microcavities fabricated in an optical tapered fiber. NFBCs are promising candidates as a platform for photonic quantum information devices due to their small mode volume, ultra-high coupling efficiencies, and ultra-wide tunability. However, the quality (Q) factor has been limited to be approximately 250, which may be due to limitations in the fabrication process. Here we report high Q NFBCs fabricated using a focused helium ion beam. Whenan NFBC with grooves of 640 periods is fabricated, the Q factor is over 4170, which is more than 16 times larger than that previously fabricated using a focused gallium ion beam

Creation of silicon vacancy color centers with a narrow emission line in nanodiamonds by ion implantation

Nanodiamonds containing silicon-vacancy (SiV) centers with high brightness, high photo-stability, and a narrow zero phonon line (ZPL) have attracted attention for bioimaging, nanoscale thermometry, and quantum technologies. One method to create such nanodiamonds is the milling of diamond films synthesized by chemical vapor deposition (CVD). However, this requires post-processing such as acid treatment and centrifugation after the milling process. Therefore, the number of SiV center-containing nanodiamonds made from an initial CVD diamond is small. An alternative method without these problems is the implantation of Si ions into preselected nanodiamonds. This method, however, has an issue regarding the ZPL linewidths, which are more than twice as broad as those in nanodiamonds synthesized by CVD. In order to reduce the linewidth, we employed annealing treatment at high temperatures (up to 1100°C) and high vacuum after the implantation. For an ion fluence of 1013 ions/cm2, a ZPL with a linewidth of about 7 nm at room temperature was observed for a nanodiamond with a median size of 29.9 ± 16.0 nm. This was close to the linewidth for nanodiamonds created by CVD

Rice immediately adapts the dynamics of photosynthates translocation to roots in response to changes in soil water environment

Rice is susceptible to abiotic stresses such as drought stress. To enhance drought resistance, elucidating the mechanisms by which rice plants adapt to intermittent drought stress that may occur in the field is an important requirement. Roots are directly exposed to changes in the soil water condition, and their responses to these environmental changes are driven by photosynthates. To visualize the distribution of photosynthates in the root system of rice plants under drought stress and recovery from drought stress, we combined X-ray computed tomography (CT) with open type positron emission tomography (OpenPET) and positron-emitting tracer imaging system (PETIS) with 11C tracer. The short half-life of 11C (20.39 min) allowed us to perform multiple experiments using the same plant, and thus photosynthate translocation was visualized as the same plant was subjected to drought stress and then re-irrigation for recovery. The results revealed that when soil is drier, 11C-photosynthates mainly translocated to the seminal roots, likely to promote elongation of the root with the aim of accessing water stored in the lower soil layers. The photosynthates translocation to seminal roots immediately stopped after rewatering then increased significantly in crown roots. We suggest that when rice plant experiencing drought is re-irrigated from the bottom of pot, the destination of 11C-photosynthates translocation immediately switches from seminal root to crown roots. We reveal that rice roots are responsive to changes in soil water conditions and that rice plants differentially adapts the dynamics of photosynthates translocation to crown roots and seminal roots depending on soil conditions

First Imaging Tests of an OpenPET Prototype for Small Animals

Introduction: The OpenPET geometry is our new idea to visualize a physically opened space between two detector rings (Yamaya, PMB 757, 2008). The OpenPET is expected to enable 1) PET image-guided radiation therapy by letting the beams pass though the gap, 2) real-time multimodal imaging by inserting another imaging device in the gap, and 3) extension of an axial field-of-view with the limited number of detectors. In this paper, we developed the first prototype for small animals to show a proof-of-concept of OpenPET imaging. Methods: The prototype was designed as a compact system so as to be easily carried between PET areas and therapy areas such as the Heavy Ion Medical Accelerator in Chiba (HIMAC) (figure (a)). Two detector rings of 110 mm in diameter composed of 8 block-detectors were placed with a gap of 42 mm. Actual gap was limited to 27 mm by the gantry structure (but the gap will become over 20 cm, which is enough for radiation therapy, if the gantry is extended to the human size in scale). Each block-detector, which had 4-layer depth-of-interaction capability based on our method (Tsuda, IEEE TNS 2537, 2004), was composed of 2.9 x 2.9 x 5 mm3 LGSO crystals and a Hamamatsu H8500 PMT. Colon-26 cancer cells (2.0 x 10^5) were inoculated subcutaneously into a female BALB/c nude mouse (16.5 g weight) and allowed to grow for 7 days. 18F-FDG (1.6 MBq) was injected intravenously via tail vein. After 70min, the animal was placed so that the tumor located in the gap, and measured for 10 min. For an initial proof-of-concept of real-time multimodal imaging, an optical image of the surface was taken during PET imaging by inserting a digital camera in the gap. Results: As shown in figure (b), the tumor in the gap was clearly visualized. In addition, the large axial FOV of 126 mm was obtained with the detectors originally covering only an 84 mm axial FOV. Conclusion: Our initial imaging studies showed promising performance of the OpenPET prototype. In a molecular imaging field, we expect novel applications of the OpenPET which make the best use of the gap.2010 World Molecular Imaging Congres

Impact of TOF information in OpenPET imaging

We are developing an open-type PET geometry that we named OpenPET. One possible geometry is the dual-ring OpenPET, which consists of two detector rings separated by a gap for entrance of the radiation beam. The gap can also be used for different modalities such as X-ray CT to realize a simultaneous multi-modality system. In addition, the dual-ring OpenPET has an effect to enlarge the axial field-of-view. In our previous simulations and experiments the OpenPET imaging geometry was shown to be feasible by applying iterative reconstruction methods. However, the gap violates Orlov\u27s completeness condition for accurate tomographic reconstruction. Therefore, the gap could lead to artifacts caused by lost low-frequency components in the projection data depending on the shapes of imaging subjects. In this study, we investigate the effect of time-of-flight (TOF) information, which is expected to compensate for the lost low-frequency components in OpenPET imaging. We simulated the dual-ring OpenPET geometry with a numerical phantom and analyzed frequency components of the reconstructed images. The result showed that the low-frequency components were not fully estimated in the gap region of the dual-ring OpenPET while those components were well recovered with the TOF information.The 2013 IEEE Nuclear Science Symposium and Medical Imaging Conference, and Room-Temperature Semiconductor X-Ray and Gamma-Ray Detectors worksho

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