Joong Hyun Kim

Comparison of Segmentation-Based Attenuation Correction Methods for PET/MRI: Evaluation of Bone and Liver Standardized Uptake Value with Oncologic PET/CT Data

For attenuation correction (AC) in PET/MRI systems, segmentation-based methods are most often used. However, the standardized uptake value (SUV) of lesions in the bone and liver, which have higher attenuation coefficients than other organs, can be underestimated, potentially leading to misinterpretation of clinical cases. Errors in SUV estimation are also dependent on the segmentation schemes used in the segmentation-based AC. In this study, this potential bias in SUV estimation using 4 different segmentation-based AC methods was evaluated for the PET/CT data of cancer patients with bone and liver lesions. Methods: Forty patients who had spine or liver lesions and underwent 18F-FDG PET/CT participated (18 women and 22 men; 20 spine lesions and 20 liver lesions; mean age (±SD), 60.5 ± 11.4 y; mean body weight, 57.7 ± 10.4 kg). The patient body region was extracted from the CT image and categorized into 5 tissue groups (air, lungs, fat, water, and bone) using Hounsfield unit thresholds, which were determined from the CT histogram. Four segmentation-based AC methods (SLA [soft-tissue/lung/air], WFLA [water/fat/lung/air], SLAB [soft-tissue/lung/air/bone], and WFLAB [water/fat/lung/air/bone]) were compared with CT-based AC. The mean attenuation coefficient for each group was calculated from 40 CT images and assigned to the attenuation maps. PET sinograms were reconstructed using segmentation- and CT-based AC maps, and mean SUV in the lesions was compared. Results: Mean attenuation coefficients for air, lungs, fat, water, and bone were 0.0058, 0.0349, 0.0895, 0.0987, and 0.1178 cm−1, respectively. In the spine lesions, the SUVs were underestimated by 16.4% ± 8.5% (SLA AC) and 14.7% ± 7.5% (WFLA AC) but not to a statistically significant extent for SLAB and WFLAB AC relative to CT AC. In the liver lesions, the SUVs were underestimated by 11.1% ± 2.6%, 8.1% ± 3.0%, 6.8% ± 3.8%, and 4.1% ± 3.8% with SLA, SLAB, WFLA, and WFLAB AC, respectively. Conclusion: Without bone segmentation, the SUVs of spine lesions were considerably underestimated; however, the bias was acceptable with bone segmentation. In liver lesions, the segmentation-based AC methods yielded a negative bias in SUV; however, inclusion of the bone and fat segments reduced the SUV bias. The results of this study will be useful for understanding organ-dependent bias in SUV between PET/CT and PET/MRI.

Recent Advances in Hybrid Molecular Imaging Systems

Nuclear medicine imaging methods that use radionuclides, such as positron emission tomography (PET) and single-photon emission computed tomography (SPECT), offer highly sensitive and quantitative tools for the detection and localization of the biochemical and functional abnormalities associated with various diseases. The introduction of dual-modality PET/CT and SPECT/CT systems to the clinical environment in the late 1990s is regarded as a revolutionary advance in modern diagnostic imaging, bringing precise anatomical localization to conventional PET and SPECT imaging techniques and enhancing the quantitation capabilities of these modalities. The great success of PET/CT has also revived interest in the combination of PET and MR scanners, leading to commercially available clinical PET/MR systems. In this article, we review the recent improvements made in these hybrid molecular imaging systems, which have been dramatic in terms of both hardware and software over the past decade. We focus primarily on the hybrid imaging systems that are currently used in clinical practice and the technologies applied in those systems, with emphasis on the efforts to improve their diagnostic performances for musculoskeletal diseases.

Quantitative positron emission tomography imaging of angiogenesis in rats with forelimb ischemia using 68 Ga-NOTA-c(RGDyK)

Gallium-68-labeled 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA)—cyclic Arg-Gly-Asp-D-Tyr-Lys (c(RGDyK)) was developed for αvβ3 targeting, and is a promising agent for imaging of cancer and disorders related to angiogenesis. In this study, we performed kinetic analysis of 68Ga-NOTA-c(RGDyK) in rats with surgically induced forelimb ischemia, and immunohistochemical analysis was also performed to assess αvβ3 immuno-staining level. Animal models were created by excision of the left brachial vessels, and a sham operation was performed on the right brachial region under 2xa0% isoflurane anesthesia. Using an animal positron emission tomography/computed tomography (PET/CT) scanner, a list mode PET scan (120xa0min) was started with the injection of 68Ga-NOTA-c(RGDyK) via the tail vein at 3, 5 and 7xa0days after ischemic surgery. Volumes of interest were drawn on the left ventricle, sham operation, control, and ischemic regions. Compartmental and two graphical analyses (Logan and RE plots) were performed for kinetic parameter estimation. The immunohistochemical analysis was also performed after the last PET scan, and cell components were scored on a six point scale for quantification of immuno-staining level (0-negative to 5-very high). A 3-compartment model with reversible binding best described the tissue time-activity curves. The distribution volume of the ischemic region was significantly higher than that of the sham operation (Pxa0<xa010−6) and control region (Pxa0<xa010−9). Both the Logan and RE plots showed high correlation with compartmental analysis (R2xa0=xa00.96 and 0.95 for Logan and RE, respectively). The temporal changes in distribution volume and binding potential were not significant. The immuno-staining level of the ischemic region was significantly higher than that of sham operation (Pxa0<xa010−4) and control region (Pxa0<xa010−8). Kinetic modeling studies with dynamic 68Ga-NOTA-c(RGDyK) PET scan are feasible based on an image-derived input function in a rat ischemia model. The kinetic modeling analysis performed in this study will be useful for the quantitative evaluation of 68Ga-NOTA-c(RGDyK) binding to αvβ3 in angiogenic tissues.

Tracking Performance of the Scintillating Fiber Detector in the K2K Experiment

The K2K long-baseline neutrino oscillation experiment uses a Scintillating Fiber Detector (SciFi) to reconstruct charged particles produced in neutrino interactions in the near detector. We describe the track reconstruction algorithm and the performance of the SciFi after 3 years of operation.

Whole-Body Voxel-Based Personalized Dosimetry: The Multiple Voxel S-Value Approach for Heterogeneous Media with Nonuniform Activity Distributions

Personalized dosimetry with high accuracy is becoming more important because of the growing interest in personalized medicine and targeted radionuclide therapy. Voxel-based dosimetry using dose point kernel or voxel S-value (VSV) convolution is available. However, these approaches do not consider the heterogeneity of the medium. Here, we propose a new method for whole-body voxel-based personalized dosimetry in heterogeneous media with nonuniform activity distributions—a method we refer to as the multiple VSV approach. Instead of using only a single VSV, as found in water, the method uses multiple numbers (N) of VSVs to cover media of various density ranges, as found in the whole body. Methods: The VSVs were precalculated using GATE Monte Carlo simulation and were convoluted with the time-integrated activity to generate density-specific dose maps. CT-based segmentation was performed to generate a binary mask image for each density region. The final dose map was acquired by the summation of N segmented density-specific dose maps. We tested several sets of VSVs with different densities: N = 1 (single water VSV), 4, 6, 8, 10, and 20. To validate the proposed method, phantom and patient studies were conducted and compared with the direct Monte Carlo approach, which was considered the ground truth. Finally, dosimetry on 10 patients was performed using the multiple VSV approach and compared with the single VSV and organ-based approaches. Errors at the voxel and organ levels were reported for 8 organs. Results: In the phantom and patient studies, the multiple VSV approach showed significant decreases in voxel-level errors, especially for the lung and bone regions. As the number of VSVs increased, voxel-level errors decreased, although some overestimations were observed at the lung boundaries. For the multiple VSVs (N = 8), we achieved a voxel-level error of 2.06%. In the dosimetry study, our proposed method showed greatly improved results compared with single VSV and organ-based dosimetry. Errors at the organ level were −6.71%, 2.17%, and 227.46% for single VSV, multiple VSV, and organ-based dosimetry, respectively. Conclusion: The multiple VSV approach for heterogeneous media with nonuniform activity distributions offers fast personalized dosimetry at the whole-body level, yielding results comparable to those of the direct Monte Carlo approach.

A simplified geometric calibration method for rotating triple head pinhole SPECT system using point source

Micro-pinhole SPECT system with conventional multiple-head gamma cameras has the advantage of high magnification factor for imaging of small animals such as rodents. However, several geometric factors should be calibrated to obtain the SPECT image with good image quality. We developed a simplified geometric calibration method for rotating triple- head pinhole SPECT system and assessed the effects of the calibration using several phantom and rodent imaging studies. Trionix Triad XLT9 triple-head SPECT scanner with 1.0 mm pinhole apertures were used for the experiments. Approximately centered one point source was scanned to track the angle- dependent positioning errors. The centroid of point source was determined by the center of mass calculation. Axially departed two point sources were scanned to calibrate radius of rotation from pinhole to center of rotation. To verify the improvements by the geometric calibrations, we compared the spatial resolution of the reconstructed image of Tc-99m point source with and without the calibration. SPECT image of micro performance phantom with hot rod inserts was acquired and several animal imaging studies were performed. Exact sphere shape of the point source was obtained by applying the calibration and axial resolution was improved. Lesion detectability and image quality was also much improved by the calibration in the phantom and animal studies. Serious degradation of micro-pinhole SPECT images due to the geometric errors could be corrected using a simplified calibration method using only one or two point sources.