Kazumasa Inoue

Experimental verification of proton beam monitoring in a human body by use of activity image of positron-emitting nuclei generated by nuclear fragmentation reaction.

Proton therapy is a form of radiotherapy that enables concentration of dose on a tumor by use of a scanned or modulated Bragg peak. Therefore, it is very important to evaluate the proton-irradiated volume accurately. The proton-irradiated volume can be confirmed by detection of pair-annihilation gamma rays from positron-emitting nuclei generated by the nuclear fragmentation reaction of the incident protons on target nuclei using a PET apparatus. The activity of the positron-emitting nuclei generated in a patient was measured with a PET-CT apparatus after proton beam irradiation of the patient. Activity measurement was performed in patients with tumors of the brain, head and neck, liver, lungs, and sacrum. The 3-D PET image obtained on the CT image showed the visual correspondence with the irradiation area of the proton beam. Moreover, it was confirmed that there were differences in the strength of activity from the PET-CT images obtained at each irradiation site. The values of activity obtained from both measurement and calculation based on the reaction cross section were compared, and it was confirmed that the intensity and the distribution of the activity changed with the start time of the PET imaging after proton beam irradiation. The clinical use of this information about the positron-emitting nuclei will be important for promoting proton treatment with higher accuracy in the future.

Absorbed dose rate in air in metropolitan Tokyo before the Fukushima Daiichi Nuclear Power Plant accident.

The monitoring of absorbed dose rate in air has been carried out continually at various locations in metropolitan Tokyo after the accident of the Fukushima Daiichi Nuclear Power Plant. While the data obtained before the accident are needed to more accurately assess the effects of radionuclide contamination from the accident, detailed data for metropolitan Tokyo obtained before the accident have not been reported. A car-borne survey of the absorbed dose rate in air in metropolitan Tokyo was carried out during August to September 2003. The average absorbed dose rate in air in metropolitan Tokyo was 49±6 nGy h(-1). The absorbed dose rate in air in western Tokyo was higher compared with that in central Tokyo. Here, if the absorbed dose rate indoors in Tokyo is equivalent to that outdoors, the annual effective dose would be calculated as 0.32 mSv y(-1).

High-resolution computed tomography of single breast cancer microcalcifications in vivo

Microcalcification is a hallmark of breast cancer and a key diagnostic feature for mammography. We recently described the first robust animal model of breast cancer microcalcification. In this study, we hypothesized that high-resolution computed tomography (CT) could potentially detect the genesis of a single microcalcification in vivo and quantify its growth over time. Using a commercial CT scanner, we systematically optimized acquisition and reconstruction parameters. Two ray-tracing image reconstruction algorithms were tested: a voxel-driven “fast” cone beam algorithm (FCBA) and a detector-driven “exact” cone beam algorithm (ECBA). By optimizing acquisition and reconstruction parameters, we were able to achieve a resolution of 104 μm full width at half-maximum (FWHM). At an optimal detector sampling frequency, the ECBA provided a 28 μm (21%) FWHM improvement in resolution over the FCBA. In vitro, we were able to image a single 300 μm X 100 μm hydroxyapatite crystal. In a syngeneic rat model of breast cancer, we were able to detect the genesis of a single microcalcification in vivo and follow its growth longitudinally over weeks. Taken together, this study provides an in vivo “gold standard” for the development of calcification-specific contrast agents and a model system for studying the mechanism of breast cancer microcalcification.

Improvement of the diagnostic accuracy of lymph node metastases of colorectal cancer in 18F-FDG-PET/CT by optimizing the iteration number for the image reconstruction

ObjectiveImprovement in the diagnostic accuracy of lymph node (LN) metastases of colorectal cancer by optimizing the reconstruction parameters was studied, including a phantom study and clinical studies.MethodsIn the experimental study, the contrast ratio was evaluated using a standard image quality phantom, changing the iteration number of ordered subsets expectation maximization algorithm from 2 to 6. In the clinical study, 89 patients with preoperative colorectal cancer who received 18F-2-deoxy-2-fluoro-d-glucose positron emission tomography/computed tomography (PET/CT) were studied. Their LN metastases were visually assessed when systematically changing the iteration number, and the optimal iteration number was determined. After the appropriate cut-off value of maximum standardized uptake value was determined, the improvement of the diagnostic accuracy of LN metastases was evaluated for the proximal nodes and the distal ones using the reconstructed images with the optimal iteration number. This was compared with the conventional method that had an iteration number of 2.ResultsIn the phantom study, it was confirmed that the contrast ratio improved when the iteration number increased. In clinical evaluation, the optimal iteration number was determined to be 5 by visual assessment. When the cut-off value of 1.5 was used, which happened to be the best number, the sensitivity/specificity/accuracy improved from 52%/91%/72% to 77%/89%/83% for the proximal nodes, and from 63%/90%/84% to 91%85%/87% for the distal nodes, respectively.ConclusionsWhen the iteration number of the reconstruction algorithm was optimized, the sensitivity of LN metastasis improved by more than 20%, and the accuracy exceeded 80%. Optimization of the image reconstruction parameters in the diagnosis of LN metastases using PET/CT is clinically important.

Natural variation of ambient dose rate in the air of Izu-Oshima Island after the Fukushima Daiichi Nuclear Power Plant accident

The ambient dose rate in air and radioactivity concentration in soil samples collected on Izu-Oshima Island were observed in 2012, 2013 and 2014, i.e. 1, 2 and 3 years after the severe accident at the Fukushima Daiichi Nuclear Power Plant. A car-borne survey for the ambient dose rate in air was carried out for the entire island. Soil samples were collected for the radioactivity concentration measurements from 22 points. The ambient dose rates in air were 36 nGy h(-1) in 2012, 34 nGy h(-1) in 2013 and 29 nGy h(-1) in 2014. The corresponding radioactivity concentrations in those years for (134)Cs were 53, 39 and 29 Bq kg(-1) and for (137)Cs, 87, 73 and 75 Bq kg(-1). All the values have decreased every year.

In Vivo Visualization of Heterogeneous Intratumoral Distributionof Hypoxia-Inducible Factor-1α Activity by the Fusion of High-Resolution SPECT and Morphological Imaging Tests

Purpose. We aimed to clearly visualize heterogeneous distribution of hypoxia-inducible factor 1α (HIF) activity in tumor tissues in vivo. Methods. We synthesized of 125I-IPOS, a 125I labeled chimeric protein probe, that would visualize HIF activity. The biodistribution of 125I-IPOS in FM3A tumor-bearing mice was evaluated. Then, the intratumoral localization of this probe was observed by autoradiography, and it was compared with histopathological findings. The distribution of 125I-IPOS in tumors was imaged by a small animal SPECT/CT scanner. The obtained in vivo SPECT-CT fusion images were compared with ex vivo images of excised tumors. Fusion imaging with MRI was also examined. Results. 125I-IPOS well accumulated in FM3A tumors. The intratumoral distribution of 125I-IPOS by autoradiography was quite heterogeneous, and it partially overlapped with that of pimonidazole. High-resolution SPECT-CT fusion images successfully demonstrated the heterogeneity of 125I-IPOS distribution inside tumors. SPECT-MRI fusion images could give more detailed information about the intratumoral distribution of 125I-IPOS. Conclusion. High-resolution SPECT images successfully demonstrated heterogeneous intratumoral distribution of 125I-IPOS. SPECT-CT fusion images, more favorably SPECT-MRI fusion images, would be useful to understand the features of heterogeneous intratumoral expression of HIF activity in vivo.

Diagnosis supporting algorithm for lymph node metastases from colorectal carcinoma on 18F-FDG PET/CT.

ObjectiveWe studied the improvement of the detect-ability of lymph node (LN) metastases from colorectal cancer in 18F-fluorodeoxyglucose positron emission tomography (FDG PET)/computed tomography (CT) by analyzing the acquired counts with a statistical method.MethodsThirty-nine metastatic LNs from 32 cases with colorectal cancer were included in this study. “Uptake region” was defined as the site where counts were higher than the average plus 3 standard deviations (SDs) on each transaxial image of FDG PET. After the initial uptake regions were selected, these high accumulation areas were automatically excluded from consideration thereafter. This method was repeated and new uptake regions were identified. This method was repeated up to five times. After that, the stacked-up uptake regions were compared with computed tomography (CT) images, and the high accumulation areas that were superimposed on the normal structures, such as intestine, vessels, and ureters, were excluded from the consideration. The remaining uptake regions were diagnosed as metastatic LNs, and the detectability of LN metastases was calculated. We then compared these statistical results with the results obtained on the basis of visual assessments by radiologists.ResultsOur proposed method showed the best results when the procedures were repeated three times in the light of detectability. After being repeated three times, this method detected 15/23 (65.2%) metastatic LNs in the first LN group, 16/16 (100%) in the second-third LN groups and 31/39 (79.4%) in the total LNs, whereas the radiologists diagnosed 8/23 (34.8%) of metastatic first LNs, 12/16 (75.0%) in the second-third LNs and 20/39 (51.3%) in the total LNs. A statistically significant difference was observed between the result of iteration number 3 and that by radiologists as for the second—third LNs and the total LNs.ConclusionsThis study suggests that our proposed statistical method could improve the detectability of LN metastases from colorectal cancer. Our method will help radiologists to detect small metastatic lesions such as LN metastases.

Environmental radiation at Izu-Oshima after the Fukushima Daiichi nuclear power plant accident

Environmental radiation at Izu-Oshima Island was observed 6 months after the accident at the Fukushima Daiichi Nuclear Power Plant (F1-NPP). A car-borne survey of the dose rate in air was conducted over the entire island and the results were compared with measurements performed in 2005 (i.e. before the accident). The activity concentrations of (134)Cs and (137)Cs were also measured using a germanium detector. The dose rate in air was found to be 2.9 ± 1.2 times higher than that in 2005 and (134)Cs was detected on Izu-Oshima Island. These results are attributed to the accident at the F1-NPP.

A simulation study for estimating scatter fraction in whole-body 18F-FDG PET/CT

AbstractWhereas Monte Carlo (MC) simulation is widely utilized in estimation of the scatter component, a simulation model which can calculate the scatter fraction (SF) of each patient is needed for making an accurate image quality assessment for clinical PET images based on the noise equivalent count. In this study, an MC simulation model was constructed which can calculate the SF for various phantoms. We utilized the Geant4 toolkit based on MC simulation to make a model of a PET scanner with a scatter phantom, and SFs calculated with this model were compared with the SF (SFconstant: 44%) measured with use of an actual PET scanner. Additionally, the SF values for an anthropomorphic phantom were calculated from its voxel phantom. Furthermore, we evaluated the impact on the SF due to the difference in the source distribution inside the phantom. The SF calculated from the scatter phantom in the MC simulation was 44%, the same as the SFconstant value. The average SF for the anthropomorphic phantom was 41%, but there was a maximum of 14 percentage points difference between each scan range, and the maximum difference in the SF was 8 percentage points for the difference in the source distribution. We constructed an MC simulation model which can calculate SFs for various phantoms. The SF was confirmed to be affected significantly by the source distribution. We judged that the actually measured SFconstant obtained from the PET scanner with the scatter phantom was not suitable for the assessment of the quality of all patient images.

Estimation of local statistical noise in PET images induced by attenuation inside the body

ObjectivesIn positron emission tomography/computed tomography (PET/CT) examinations, the standardized uptake value (SUV) is a commonly used index to evaluate the activity of cancer. The precision of SUV is directly affected by the local statistical noise in PET images because SUV is calculated based on the counts on PET image data. The purpose of this study was to estimate the local statistical noise in the PET image caused by attenuation of annihilation photons inside the body.MethodsTo estimate the local statistical noise, we defined the number proportional to the standard deviation of a voxel caused by attenuation using CT image data. The conversion table between the number obtained from the CT image data and the actually measured standard deviation of a voxel was made by performing experiments with different forms or sizes of phantoms and scattering bodies. We compared the standard deviation of a voxel estimated by the method we proposed with actually measured ones.ResultsThe standard deviation of a voxel in the PET image varied according to the size of the phantom. The effect of attenuation on the standard deviation of a voxel was more significant in the 3D mode than in the 2D mode. The correlation coefficients between the estimated standard deviation and actually measured standard deviation were 0.98 and 0.99 in the 2D mode and 3D mode, respectively.ConclusionsOur algorithm favorably estimated the local statistical noise in PET image data using CT image data, regardless of object size, although the consideration was limited for phantoms with homogeneous interiors.