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A silicon photomultiplier (SiPM) is considered a next generation photo-sensor for low-light applications. In this study, we propose and develop the general concept of a multipurpose submillimeter gamma-ray (MSG) imaging module that is applicable to most gamma-ray imaging devices. The main features of the MSG module are submillimeter intrinsic resolution, geometrically expandable structure, expandable data acquisition system, multi-energy capability and usability in intense magnetic fields. The elements of the gamma-ray detector are pixelated Ce:GAGG ( $31\times 31$ pixels, 0.8 mm pitch), a 1.5-mm acrylic light guide, and one SiPM array ( $8 \times 8$ pixels). The readout electronics are composed of a symmetric charge division resistive network, 16 channel preamplifiers, multiplexing amplifiers, and analog-to-digital converter (ADC) circuits. We use a subminiature field-programmable gate array (FPGA) board as a data acquisition system. To make the system more expandable, we apply a 10 GbE-based tree structure using a FPGA-based data acquisition board and switching hub. To verify multipurpose capabilities, the energy spectra and the flood images are obtained using Co-57, Na-22, and Cs-137 isotopes. We measure temperature characteristics of the detector module. A pinhole image is obtained to confirm high resolution property. The maximal data transfer rate that is measured using dummy gamma-ray event packets is 963.143 Mbps. This detector module can be applied to nuclear medicine imaging devices and radiation monitoring systems.

We have investigated a dual-ended readout detector based on two silicon photo-multipliers (SiPMs) using a 30 mm long cerium-doped lutetium-yttrium oxyorthosilicate ( ${\rm Lu}_{0.5}{{\rm Y}_{1.4}}{\rm Si}_{0.5}:{\rm Ce}$ , LYSO) crystal to study the feasibility of a time-of-flight (TOF) and depth-of-interaction (DOI) positron emission tomography (PET). To improve the timing resolution in the dual-ended readout detector, a novel meantime measurement was employed. The meantime was obtained by averaging two signal arrival times at two SiPMs, each of which was attached to both ends of the LYSO crystal. The meantime method minimizes the arrival time difference along the crystal. Both timing and DOI resolutions using the meantime method were better than those of a single-ended readout detector.

Attenuation correction (AC) and scatter correction (SC) are problematic issues for animal positron emission tomography (PET). In this study, the effects of AC and SC were assessed using PET on a phantom and actual rat brain. Transmission (TX) was performed using 57Co for 15 min. After a 15 min TX scan, emission (EM) PET was performed in list mode for 1 h. To assess the effects of AC and SC, the spillover ratio (SOR) was measured using a rat-sized NEMA NU4 image-quality phantom; statistical parametric mapping (SPM) was performed to assess the effects of AC and SC in the rat brain using 18F-FDG (FDG). In addition, the binding potential (BP) was compared for 18F-FP-CIT (FP-CIT) PET. SPM was used to compare PET images to which AC and SC were applied, and BP was used for FP-CIT PET. The SORs of air and water decreased after AC and SC. SPM for FDG PET after AC showed a significant increase in FDG-measured activity in the cerebellum and occipital cortex. After AC/SC, a significant decrease in FDG-measured activity was observed in the frontal and temporal cortices. For FP-CIT PET of the rat brain, the BP decreased by 26% after AC because the FP-CIT uptake increased more in the cerebellum than in the striatum owing to AC. After AC and SC, the mean BP increased by 61%. AC and AC/SC were found to be necessary components of the artifact correction process for both FDG PET and FP-CIT PET of rat brains.

18F -FP-CIT PET image was useful for diagnosis of Parkinson's disease (PD). Although spatial normalization step is necessary in SPM analysis, it was difficult to apply the spatial normalization on 18F-FP-CIT PET due to the lack of reference for spatial normalization. Therefore, additional data acquisition such as MRI or FDG PET scans is needed. In this study, we developed the voxel based statistical analysis method for 18F-FP-CIT PET using early phase of 18F-FP-CIT. 18F-FP-CIT PET data was acquired from 7 normal and 1 PD model rat brain. Emission PET data was collected for 1 h at the start of 1 mCi of 18F-FP-CIT. We used the early phase uptake image (from 0 to 1200 s) including perfusion phase as referential information for spatial normalization. Additionally, 18F-FDG PET was acquired as referential data to validate the substitution of 18F-FP-CIT. To validate early phase 18F-FP-CIT PET data for spatial normalization, correlation coefficient between early phase 18F-FP-CIT and 18F-FDG were assessed. Using our proposed method, SPM for 18F-FP-CIT PET was successfully implemented. Early phase 18F-FP-CIT PET corresponding early phase and 18F-FDG PET were highly correlated. When we applied our developed method for the comparison between normal and PD, we can identify the difference of 18F -FP-CIT between the normal and PD in the striatum (P 0). The SPM method for 18F -FP-CIT PET data could provide high localization accuracy in PD analysis. This method may be useful for assessment of PD.

124I has a complex decay scheme with high gamma energy and low positron abundance. In this study, comparative performance measurements of 124I were performed in terms of spatial resolution, sensitivity, and image quality. All measurements were performed using both 2D and 3D PET in both brain mode and whole body mode. The transverse and axial spatial resolutions at 1 cm were 5.56 and 6.07 mm for 124I, and were 4.58 and 4.77 mm and for 18F, respectively. Sensitivities were 0.5 kcps/MBq (2D) and 3.4 kcps/MBq (3D) for 124I, and 1.8 kcps/MBq (2D) and 9.8 kcps/MBq (3D) for 18F. The %contrast of 3D was higher than that of 2D in 124I. For 124I PET imaging, 3D acquisition with brain mode was highest achievable imaging acquisition mode with finer spatial resolution and higher contrast. This result will be useful for 124I PET imaging.

Positron emission tomography (PET) images suffer from low spatial resolution. To improve the spatial resolution, we previously proposed a sinogram-based super-resolution (SR) algorithm for a whole-body PET scanner, by assuming space invariant blur. However, since the spatial resolution of a sinogram varies along the radial direction due to parallax error, this algorithm is not appropriate for providing a high-resolution sinogram with reduction of parallax error. In this paper, we propose a novel and efficient sinogram-based SR algorithm that is suitable even for a small animal PET scanner by using space variant blur matrices. In the algorithm, we estimate the space variant blur matrices through a Monte Carlo simulation and use them for the SR process to obtain a high-resolution sinogram. Using a Derenzo phantom and a line source, we demonstrate in a real PET scanner, microPET R4, that the proposed SR algorithm noticeably improves the spatial resolution while alleviating its space variance. By applying the proposed SR algorithm, the full width at half-maximum (FWHM) value reaches 1.2 mm at the system center and 1.63 mm with a considerable parallax error reduction at a radial position of 4 cm.

Small animal PET scanning with 18F-FDG has been increasingly used in the studies using murine tumor models. The aim of this study was to obtain multimodal tumor images with the use of small animal PET and clinical CT by using hardware guidance and a registration method. PET imaging studies were performed with a dedicated small animal PET scanner. Static PET images of mice with liver and lung tumors were scanned for 20 minute by the use of hardware guidance with 1% activity of the injected dose. The CT images obtained with the use of a clinical CT scanner can be used to improve the anatomical localization of uptake in PET images. 124I -FIAU-PET was obtained from mice containing an orthotopic hepatocellular carcinoma (HCC). Contrast enhanced CT images were obtained at three hours after injection of contrast agent. The PET and CT images were fused using hardware fiducial markers that were manually identified in both data sets. The PET and CT images were precisely registered by the use of a small animal contour point in both data sets to perform a point-based rigid registration. The registered images in the HCC model and a lung metastatic tumor model showed good agreement of tumor region on the both PET and CT images. The use of PET and contrast enhanced CT allowed a precise and improved detection of tumors in liver and lung models. Multimodal imaging with small animal PET and clinical CT images was useful in the detection of metastatic tumors and this multimodal imaging registration method can improve the quantitative accuracy and interpretation of tracer.

68Ge has conventionally been used for the normalization of 18F PET. However, because 124I PET has different characteristics such as a low positron branching ratio (23%), the presence of higher gamma energy (602 keV and 723 keV), and a larger positron range, the use of 68Ge should be validated for 124I PET. To the best of our knowledge, there has been no previous report on the effect of normalization between conventional 68Ge and 124I. In this study, we examined image quality to assess the effect of normalization. For this purpose, we measured the non-uniformities (NUs) and recovery coefficients (RCs) using a NEMA NU4 image quality phantom (NU4 phantom) and compared the values for the conventionally used 68Ge and 124I. A Siemens Inveon PET scanner was used throughout. Emission data were obtained using the NU4 phantom filled with 124I solution (3.7 MBq) for 20 min within 250-750 keV. The normalization data were acquired for 40 h using a cylindrical phantom (diameter: 6 cm, length: 13.5 cm) filled with 124I in a water solution (9.25 MBq). To match the energy window of an emission scan, the energy window setting for the normalization was 250-750 keV. The obtained list mode data were sorted into 3D sinograms. For comparison, normalization data were acquired using a cylindrical 68Ge normalization phantom (diameter: 6 cm, length: 15 cm). The data with no normalization correction were also acquired. The list mode emission PET data were reconstructed using FBP with a ramp filter and normalization, and attenuation and scatter correction were applied. The image quality of the reconstructed images was assessed in terms of the radial, axial, and volume NUs and RCs as a standard using the NEMA NU4 protocol. There were no significant differences in the NUs and RCs between 68Ge and 124I. These results indicate that conventional 68Ge can be used for 124I PET.

Motion of the head during brain positron emission tomography (PET) acquisitions has been identified as a source of artifact in the reconstructed image. A number of techniques have been proposed to correct for this motion artifact, but they are unable to correct for a motion during an acquisition. The aim of this study was to develop a sinogram-based motion correction (SBMC) technique to correct directly the head motion during a PET scan using a motion tracking system and list-mode data acquisition. This method uses a rebinning procedure whereby the lines of response (LOR) are geometrically transformed according to the current values of six-dimensional motion data. A Michelogram was recomposed using the rebinned LOR, and the motion-corrected sinogram was generated. In the motion corrected image, the blurring artifact due to the motion was reduced by the SBMC technique. This technique was applied to actual PET data acquired in the list-mode, and demonstrated the potential for real-time motion correction of head movements during a PET acquisition.

Transmission-dependent convolution subtraction (TDCS) is a promising technique in quantitative SPECT for subtracting the scatter components from emission images. Usually, a 20% photopeak energy window is used in SPECT acquisitions. To date, no investigation has assessed the effects of energy windows in the subtraction of scatter components from emission images with the TDCS technique. To evaluate the energy dependence of the TDCS technique, we analyzed photopeak energy windows of 5%, 10%, 15%, 20%, 25%, 30%, and 35% using /sup 99m/Tc radionuclide. The scatter fractions for a point source placed in a water phantom were estimated using the triple-energy-window (TEW) technique. These estimates were used to establish the parameters of the TDCS equation as a function of transmission. The estimated parameters were applied to the TDCS technique to subtract the scatter components from data of corresponding energy windows using nonuniform and uniform phantoms. All data were reconstructed with the OSEM algorithm. The energy window dependence of the TDCS technique was verified by comparing the coefficients of variance (COV) of the data from the uniform phantom of each energy window. Ten Poisson deviates were generated from each projection's data from the uniform phantom. The nonuniform phantom results showed that the estimated parameters effectively estimated the scatter component from the projection image for each energy window. The COV for the 5%, 10%, 15%, 20%, 25%, 30%, and 35% windows were 2.10%, 1.76%, 1.44 %, 1.39%, 1.34%, 1.37%, and 1.61%, respectively. In the TDCS technique, any photopeak energy window from 15% to 30% may work almost as well as judged by the COV measured for a uniform phantom in this study. The best window by that measure was the 25% window.

A new input function monitoring system has been developed using the GSO detector assembly for both PET and SPELT quantitative studies. Due to the rapid rise time of the pulse (about 10nsec), the coincidence time window can be reduced < 1nsec, reducing contribution of randoms associated with the high background activity surrounding the detector. Large light output improved the energy resolution of approximately 11% for 511keV photons, and 20% at 140 keV, as compared with the BGO detector, enabling the use of this system also in SPELT studies. The paired assembly of crystals provided the absolute sensitivity of approximately 7% for PET and 75% for PET tracers. Multiple arrangement of the paired detectors provided possibility of correcting for the transit time of radioactivity through the catheter tube. This study demonstrated that the present system can be of use in both clinical and small animal studies using SPECT and PET tracers.

We investigated the effects of attenuation correction and scatter correction on rCBF values with /sup 99m/Tc-ECD SPECT imaging. Scans were performed on seven subjects, in the presence of /sup 99m/Tc-ECD. Quantitative K/sub 1/ images were computed using the reconstructed images and the input function obtained with the frequent arterial blood sampling method. The images were reconstructed by the ordered subset expectation maximization (OSEM) reconstruction in which uniform and segmented /spl mu/ maps were used for attenuation correction with and without scatter correction. The transmission-dependent convolution subtraction technique was utilized for scatter correction. Segmented and uniform /spl mu/ maps were generated from magnetic resonance (MR) images. We also produced uniform /spl mu/ maps using ECD images obtained at various threshold levels and /spl mu/ values (0.11, 0.15, and 0.172 cm/sup -1/). Scatter correction improved the image contrast dramatically. There were no significant differences between K/sub 1/ images with attenuation and scatter corrections assuming a uniform /spl mu/ map (not 0.15 but 0.172 cm/sup -1/) and those corrected with segmented /spl mu/ maps for most regions. However, in the former images, values were overestimated for deep structures (e.g., overestimation of 9.5% in the striatum and 7.3% in the central semi oval). This small but significant error was also observed in phantom studies and Monte Carlo simulations. We show that the overestimation using uniform /spl mu/ maps is due to the weight of the path length in the bone. Absolute K/sub 1/ values were sensitive to the threshold level when the edge of the brain was determined from the ECD images, but the variation of the estimated K/sub 1/ was /spl plusmn/9.0% when the optimal threshold level was selected. This study suggests that the use of uniform attenuation /spl mu/ maps provides reasonable accuracy, despite a small but significant error in deep structure regions, and that uniform /spl mu/ maps may be provided from the emission data alone in this patient population.

Scatter correction is important for absolute quantification in SPECT. Among the approaches taken to achieve scatter correction, some methods using transmission information for scatter estimation require the scatter models based on the physical characteristics of the scattered photons. The influence of the SPECT collimator on the characteristics of the scattered photon, however, has not been well studied yet. Here, the authors investigate the relationship between scatter and collimator design for the use of the transmission-dependent convolution subtraction (TDCS) method. Monte Carlo simulations with various collimator acceptance angles and experiments with four collimators of two SPECT cameras were performed to obtain the scatter fraction (SF) and the line-spread function (LSF) of Tc-99m. The experiments with I-123 also used six collimators of three SPECT systems to obtain the SF and the LSF. In the simulations, the SF of Tc-99m did not change with the collimator acceptance angles. The LSFs measured for Tc-99m showed an excellent agreement between collimators. In the experiments for I-123, the SF was practically the same for the six collimators, although a small difference could be observed due to the septal penetration. Also, the shape of the LSF was very similar between the collimators, and the counts in the background region of reconstructed images did not differ when using different sets of TDCS parameters. These results reveal that scatter is independent of SPECT collimator design for Tc-99m even I-123, and that the TDCS could be applicable to scatter correction of I-123 using a unique collimator-independent set of parameters.