Yuichi Morimoto

Development of a Prototype 3D PET Scanner Using Semiconductor Detectors and Depth of Interaction Information

A prototype brain positron emission tomography (PET) scanner using semiconductor detectors and depth of inleraclion (DOI) information has been developed to achieve high spatial resolulion and reduced scalier fraction. At the first step of the development, we created a two-dimensional prololype PET scanner composed of a single-slice full-ring detector unit to confirm the feasibility of the basic technologies that are necessary lo realize a semiconductor PET scanner. Through phantom and small-animal studies, the feasibility of the semiconductor PET was confirmed and the results showed that the semiconductor PET could produce quantitative imaging with high spatial resolution. Based on these achievements, a prototype brain PET scanner was developed to demonstrate the high spatial resolution and quantitative imaging capability required in human imaging.

Evaluation of delineation of image details in semiconductor PET utilizing the normalized mutual information technique.

ObjectivePET using semiconductor detectors provides high-quality images of the human brain because of its high spatial resolution. To quantitatively evaluate the delineation of image details in clinical PET images, we used normalized mutual information (NMI) to quantify the similarity with images obtained through MRI. NMI is used to evaluate image quality by determining similarity with a reference image. The aim of this study was to evaluate quantitatively the delineation of image details provided by semiconductor PET. Materials and methodsTo quantitatively evaluate anatomical delineation in clinical PET images, MRI scans of patients were used as T1-weighted images. [18F]-fluorodeoxyglucose (18F-FDG) PET brain images were obtained from six patients using (a) a Hitachi semiconductor PET scanner and (b) a ECAT HR+ scintillator PET scanner. The NMI calculated from the semiconductor PET and MRI was denoted by NMIsemic, whereas the NMI calculated from conventional scintillator PET and MRI was denoted by NMIconve. The higher the value of NMI, the greater the similarity to MRI. ResultsNMIsemic ranged from 1.22 to 1.29, whereas NMIconve ranged from 1.13 to 1.18 (P<0.05). Furthermore, all the NMI values of the semiconductor PET were higher than those of the conventional scintillator PET. ConclusionUtilizing NMI, we quantitatively evaluated the delineation of image details in clinical PET images. The results reveal that semiconductor PET has superior anatomical delineation and physical performance compared with conventional scintillator PET. This improved delineation of image details makes semiconductor PET promising for clinical applications.

Metabolic Activity of Red Nucleus and Its Correlation with Cerebral Cortex and Cerebellum: A Study Using a High-Resolution Semiconductor PET System

The red nucleus (RN) is a pair of small gray matter structures located in the midbrain and involved in muscle movement and cognitive functions. This retrospective study aimed to investigate the metabolism of human RN and its correlation to other brain regions. Methods: We developed a high-resolution semiconductor PET system to image small brain structures. Twenty patients without neurologic disorders underwent whole-brain scanning after injection of 400 MBq of 18F-FDG. The individual brain 18F-FDG PET images were spatially normalized to generate a surface projection map using a 3-dimensional stereotactic surface projection technique. The correlation between the RN and each voxel on the cerebral and cerebellar cortices was estimated with Pearson product-moment correlation analysis. Results: Both right and left RNs were visualized with higher uptake than that in the background midbrain. The maximum standardized uptake values of RN were 7.64 ± 1.92; these were higher than the values for the dentate nucleus but lower than those for the caudate nucleus, putamen, and thalamus. The voxel-by-voxel analysis demonstrated that the right RN was correlated more with ipsilateral association cortices than contralateral cortices, whereas the left RN was equally correlated with ipsilateral and contralateral cortices. The left RN showed a stronger correlation with the motor cortices and cerebellum than the right RN did. Conclusion: Although nonspecific background activity around RNs might have influenced the correlation patterns, these metabolic relationships suggested that RN cooperates with association cortices and limbic areas to conduct higher brain functions.

Improvement of a Simple Evaluation Method for Duct Streaming by Taking Account of the Deep Penetration Component

A simplified formula for radiation streaming through bent ducts was previously derived by optics analogy, considering only components passing through the duct and without enormous preparatory calculations. Now, by taking account of deep penetrating component, the streaming formula is improved in order to estimate not only γ-ray flux but also neutron flux with accuracy, factor 2–3, applicable to preliminary shielding designs. Dose rates at the exit estimated by the improved formula are compared with experimental values. As for γ-ray evaluated values become conservative by improvement, while evaluated values were underestimated before improvement. Also for neutron, the discrepancy between the calculated values by the present formula and observed ones becomes within factor 2 after improvement from factor 3 before improvement. From these results, it is concluded that the improved formula is applicable to preliminary shielding designs.

Dogleg Diet Streaming Experiment with 14 MeV Neutron Source

There are several vacant channels for diagnosis, RF heating, etc. through the shielding structure in fusion reactors. Some of them consist of dogleg ducts, and the streaming effect has not been experimentally investigated yet. The present work was conducted to study the behavior of neutrons streaming through the ducts and estimate the uncertainties of calculations for the design of fusion reactors such as ITER on the basis of the experiments with the FNS facility at JAERI. The experimental assembly was an iron slab 180 cm in thickness. A doubly bent duct 30 cm X 30 cm in the cross section was shaped in the assembly The lengths of the 3 legs were 115 cm, 60 cm and 65 cm respectively The experiment was analyzed with the Monte Carlo transport calculation codes MCNP-4B, -4C, the nuclear data libraries FENDL-2 and JENDL-3.3. In the analysis, the calculated neutron spectra agreed with measured data within the statistical errors. The reaction rates for 93Nb(n,2n)92mNb and 115In(n,n’)115mIn showed relatively favorable agreement within 30 - 70 % between calculated and measured results, while those for 197Au(n,γ)198Au did within a factor of 2 pointing out the difficulties of the calculation in the low energy range. As a result, the present calculation method turned out to be sufficient enough for the design of fusion reactors.

A New PET Scanner with Semiconductor Detector Reveals Intratumoral Inhomogeneous Cell Activity with High Spatial and Energy Resolution

Objective Intensity-modulated radiation therapy. an advanced mode of high-precision radiotherapy developed to deliver precise radiation doses to specific areas within a tumor, has become popular in the clinical situation. With this modality, it has become particularly important in the clinical situation to estimate accurate cell activity with positron emission tomography (PET) scanner. We developed a new PET scanner with a semiconductor detector. Phantom images and [18F] fluorode-oxyglucose ([18F] FDG)-PET images of patients were measured to evaluate this new scanner’s capacity to identify intratumoral inhomogeneous cell activity.