Kyoko Saito

Point-like radioactive source with multiple absorber capsules for evaluating PET scanners

ObjectiveRadioactive sources for evaluating sensitivity and uncertainty in the radioactivity measurements performed using PET scanners must be equipped with absorber materials that ensure the annihilation of positrons. Attenuation and scattering owing to the absorber materials produce uncertainty in the performance evaluation. The aim of this study is to propose a point-like radioactive source with multiple absorber capsules, for which evaluation can be independent of scatter and attenuation owing to the source absorbers.MethodsThe point-like source consists of a small spherical radioactive part and a set of successively sized cylindrical aluminum absorber capsules. Data were collected for different total absorber thicknesses. By an extrapolation technique, the effects of the source absorbers were eliminated. Sensitivity and uncertainty in the radioactivity measurements of PET scanners were evaluated with this technique.ResultsSensitivity and uncertainty of radioactivity measurement to the point-like radioactive source were evaluated successfully with this method.ConclusionThe proposed point-like radioactive source is useful for evaluating performance characteristics of PET scanners in a way that is independent of the effects of the source absorbers.

Evaluation and calibration of PET scanners with a specially designed point-like radioactive source

Background: In order to evaluate the uncertainty in the radioactivity measurements performed using PET scanners and to determine the calibration factors of these scanners, it is essential that the radioactive sources are equipped with absorber materials that ensure the annihilation of positrons. Therefore, the evaluation and calibration depend on scatter and attenuation corrections in conventional techniques based on phantoms. Although an extrapolation method that involves the use of multiple absorber sleeves for a line radioactive source is effective in eliminating the dependence, the long line shape limits its application range. Objective: The objective of this study is to propose a point-like radioactive source with multiple absorber capsules, which can be used to evaluate the uncertainty in the radioactivity measurements performed using a PET scanner and to determine a calibration factor in a way that is independent of scatter and attenuation corrections. Methods: The proposed radioactive source consists of a small spherical radioactive part and a set of successively sized aluminum absorber capsules. Extrapolation over the total absorber thickness made it possible to estimate the results that were approximately free from scatter and absorption caused by the absorber materials. As an example application, the proposed method was used to a research PET scanner. In addition, the basic characteristic of the point-like radioactive source was investigated using a Monte Carlo simulation code. Results and Conclusion: Performance characteristics and a calibration factor of the PET scanner were successfully evaluated and determined. Therefore, the proposed method is useful for this purpose. Further studies are expected to improve the design of the radioactive source and enhance the practicability and traceability of our proposed method.

New calibration and evaluation method for PET scanners using point-like radioactive sources

Objective: In order to improve the convenience and reliability of the calibration procedure with respect to the quantitative aspects of PET scanners, we have been developing new methods that rely on the use of point-like radioactive sources. The aim of this study is to report the results of the application of the proposed method for obtaining the cross-calibration factors (CCFs) of a clinical PET scanner for the first time. Methods: A 22Na point-like source was moved in the axial direction to cover the axial field-of-view (FOV) of a commercial PET scanner, SET-2400W (Shimadzu). From reconstructed images, saturated regions-of-interest (ROI) values were used to calculate the CCFs that were defined slice by slice. Results: It was possible to obtain CCFs that were practically equivalent to those obtained by the standard method with cylindrical water phantoms. The variations in the CCFs obtained over about a half-year were well reproduced by the proposed method. Conclusion: The proposed calibration method, which is based on the use of a point-like radioactive source, is useful for determining CCFs and their variations for a clinical PET scanner.

Effect of Scatter, Attenuation and Resolution Correction on a Pediatric Myocardial Perfusion SPECT Image

Scatter correction, attenuation correction, and resolution correction are commonly used to improve the quantify ability of a SPECT image. However, almost none of these are discussed specifically for the pediatric patient. This study aims to suggest practical image processing techniques to improve pediatric SPECT reconstructions. We chose to use phantoms based on the size of a 3-year-old according to the body surface area (BSA). This age group has much postoperative follow-up. For correction methods, we chose triple energy window (TEW) scatter correction, segmentation with scatter and photo peak window data for attenuation correction (SSPAC) technique, and collimator broad correction (CBC) resolution correction. The phantom studies achieved 10counts/pixel/projection/rotation for the target area. Continuous mode acquisition was employed. Data from multiple sequential rotations were added together to provide data sets with from 10 to 100 counts/pixel/projection. Also, the effect of the corrections on a patient image was evaluated qualitatively. The noise level of their constructed phantom images was compared using the normalized mean square error (NMSE) metric. The error was computed for lesser count images relative to the image for the highest count level. For the 10, 20, 30, 40, 50, 60, 70, 80, and 90 counts/pixel/projection data: the pediatric myocardial phantom with a defect had a NMSE of 0.7587, 0.5997, 0.4627, 0.3519, 0.2546, 0.1700, 0.0976, 0.0412, and 0.0016, respectively. This series for the metric shows a tendency for the image to rapidly get less noisy as there are more counts per pixel. Increasing the acquisition time to get sufficient counts may be really difficult in pediatric myocardial nuclear medicine, however. We have seen that image quality deteriorates for a rapid, low-count acquisition, as a compromise between image quality and practicality; we recommend at least 70 counts/pixel/projection to obtain the desirable low-noise image.

A New Tool for Real-Time Monitoring of Photon-Beam Irradiation Using Positron Production in Radiotherapy

A method for monitoring the real-time irradiation field and irradiation dose of energetic photons has been developed for use in cancer radiotherapy that uses photoproduced positrons. A new detector system consisting of a pair of collimators and a fast crystal scintillator was designed and fabricated. It was successfully operated to confirm the irradiation field by real-time measurements of counting rates, which were found to be proportional to the intensity of the photon beam. These results indicate that this method can be used to monitor the irradiation field as well as the irradiation dose.