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It is commonly believed that when electron beams with energy of more than MeV scale are irradiated to water, a Cerenkov-ring is formed and observed in the forward direction of the beam. We tried to observe the Cerenkov-ring in water during irradiation of high-energy electrons or X-ray beams. We irradiated 1-cm-diameter electron beams to water with energy of 6 to 12 MeV from a linear accelerator (LINAC), and then we imaged the produced light with a translucent plate in water at the forward position. We also irradiated 0.5 x 0.5-cm 6-MV and 10-MV X-rays to water, and imaging was conducted with the same set-up. For all electron energies, the formed images did not have any ring shape but only a round shape with highest intensity at the center of the circles. Monte Carlo simulation also showed that no ring formed and that the distributions were similar to those of the experimental results. For X-ray irradiation, high intensity spots were observed at the center of the image, and no ring shape was observed. Our results indicate that electrons or X-ray beams do not produce any Cerenkov-ring in the forward direction with the beam energies we used for the experiments. This is because the scattered electrons produced Cerenkov-light not only in the forward direction but also in many directions, which made it impossible to observe the ring shape, in addition to the effect of the water’s luminescence having lower energy than the Cerenkov-light threshold.

Imaging of the secondary electron bremsstrahlung (SEB) X-rays emitted during particle-ion irradiation is a promising method for beam range estimation. However, the SEB X-ray images are not directly correlated to the dose images. In addition, limited spatial resolution of the X-ray camera and low-count situation may impede correctly estimating the beam range and width in SEB X-ray images. To overcome these limitations of the SEB X-ray images measured by the X-ray camera, a deep learning (DL) approach was proposed in this work to predict the dose images for estimating the range and width of the carbon-ion beam on the measured SEB X-ray images. To prepare enough data for the DL training efficiently, 10,000 simulated SEB X-ray and dose image pairs were generated by our in-house developed model function for different carbon-ion beam energies and doses. The proposed DL neural network consists of two U-nets for SEB X-ray to dose image conversion and super-resolution. After the network being trained with these simulated X-ray and dose image pairs, the dose images were predicted from simulated and measured SEB X-ray testing images for performance evaluation. For the 500 simulated testing images, the average mean squared error (MSE) was 2.5 × 10^-5 and average structural similarity index (SSIM) was 0.997 while the error of both beam range and width was within 1 mm FWHM. For the three measured SEB X-ray images, the MSE was no worse than 5.5 × 10^-3 and SSIM was no worse than 0.980 while the error of the beam range and width was 2 mm and 5 mm FWHM, respectively. We have demonstrated the advantages of predicting dose images from not only simulated data but also measured data using our deep learning approach.

We investigated an estimation method of Bragg-peak shifts via machine learning using proton-beam images obtained by measurement of secondary electron bremsstrahlung (SEB) by Monte Carlo simulation. Proton beams having energy of 139 MeV were incident on a water phantom with randomly placed air spheres inside, and 6400 pairs of “proton-beam images” and “a Bragg-peak shift” were prepared and then multiple linear regression analysis was carried out. A good agreement was found between the actual Bragg-peak shifts and predicted values in both the training and test sets. The coefficients of determination of the obtained prediction model were 0.899 for the training set and 0.894 for the test set. Consequently, we found that a prediction model with small variance and high prediction performance could be obtained using the SEB data.

We evaluated an estimation ability of shifts of therapeutic carbon-ion beams owing to cavities in a polyethylene target by measuring prompt X-rays emitted from beam trajectories. Carbon-12 beams having the energy of 241.5 MeV/u were irradiated on a polyethylene target. The target had a square-prism-shaped cavity in it. The thickness of the cavity was changed from 3.0 to 0.0 cm with 0.3-cm steps. For each setup of the cavity, 7.5 × 10^10 carbon ions were irradiated. A pinhole-type X-ray camera was placed beside the target and utilized to acquire the beam images. The beam trajectory and a gap on the trajectory clearly appeared in the acquired images. The actual beam shifts well coincided with the estimated beam shifts from the acquired images. The maximum fluctuation of the estimated shifts was approximately 0.2 cm. It was confirmed that the internal cavity can be imaged and the range can be accurately evaluated.

A multipixel photon counter (MPPC) features excellent photon-counting capability as a radiation detector. In particular, a two-plane Compton camera consisting of Ce:GAGG scintillators coupled with MPPC arrays has significant application potential owing to its compact size and low weight. For example, the camera can be easily mounted on a commercial drone to identify radiation hot spots from the sky. In Fukushima, we demonstrated that a Cs-137 distribution within a 100 m diameter can be mapped correctly within a couple of tens of minutes. The advanced use of the Compton camera is also anticipated in the field of proton therapy. We evaluated an image of 511 keV annihilation gamma-rays emitted from a PMMA phantom irradiated by 200 MeV protons to mimic an in-beam monitor for proton therapy. Finally, we developed an ultracompact Compton camera (weight = 580 g), for 3-D multicolor molecular imaging. In order to demonstrate the performance capabilities of the device, I-131 (365 keV) , Sr-85 (SrCl2, 514 keV), and Zn-65 (ZnCl2, 1116 keV) were injected into a living mouse and the data were taken from 12 angles with a total acquisition time of 2 h. We confirmed that all tracers had accumulated on the target organs of the thyroid, bone, and liver, and that the obtained 3-D image was quantitatively correct with an accuracy of ±20%.

We have developed a simple method for the long-term monitoring of the seasonal variation of the radiocaesium radioactivity in trees using commercially available integrating dosimeters. Experiments and Monte Carlo simulations were performed to assess whether the proposed dosimeter-based method could measure the seasonal variation. From the experimental results, the performance variation among individual dosimeters was found to be 1.6%, which is sufficiently small compared with the expected seasonal variation. Moreover, the simulation results indicated that the measured value of the dosimeter increased owing to the photons passing through a stem, and a correction factor was necessary to compensate the influence of photon absorption and scattering in stem. We suggested that the simple and easy dosimeter-based method could be useful to measure the seasonal variations of radioactivity in trees.

We studied feasibility of detection of a gap which is located across a beam track by measuring low-energy (63-68 keV) photons generated by beam irradiation. An experiment was performed with the Heavy Ion Medical Accelerator in Chiba (HIMAC). A C-12 beam having 290 MeV/u was injected on a target consisting of two acrylic blocks. These two blocks were placed with a 10-mm gap along the beam axis. A detection system consisting of a semiconductor detector, a lead collimator having a slit, and borated polyethylene blocks was placed on a movable stage to detect low-energy photons emitted perpendicularly to the beam axis. The position of the detection system was moved at 2-mm intervals along the beam axis. It was found that the yield of 63--68~keV photons was clearly correlated with the position of the detection system. The position at which the yield curve had the lowest value agreed with the gap position. We also confirmed that the experimental result was well reproduced by a Monte Carlo simulation that includes generation of secondary electron bremsstrahlung.

2012年にPhys. Med. Biol.誌にアクセプトされた原著論文 ( "Beam range estimation by measuring bremsstrahlung”, Phys. Med. Biol. 57 (2012) 2843 ) について、計算コード PHITS を用いた水中での炭素イオンの飛程に関して再計算したところ、飛程の値に間違いが見つかった。今後、制動輻射を用いたビームモニタリングに関する研究を行う上で無視することができない差異であるため、関連する記述内容も含め、訂正原稿の投稿を行い 4 月に正式に出版された。

Low-energy X-ray imaging of secondary electron bremsstrahlung X-rays emitted during carbon-ion irradiation is a promising method for range estimation and could be used for imaging with almost clinical dose levels of carbon ion. However, the number of counts in images with clinical dose levels is relatively small, making it difficult to obtain precise range estimations. Since improving the sensitivity of the X-ray camera may solve this issue, we developed two new types of X-ray cameras. One uses a 1-mm thick, 40 mm × 40-mm cerium-doped yttrium aluminum perovskite (YAP(Ce)) scintillator plate combined with a 2-inch square flat panel photomultiplier tube (FP-PMT) contained in a 2-cm thick tungsten shield with a pinhole collimator positioned 50 mm from the scintillator; the other uses a 0.5-mm thick, 20 mm × 20-mm YAP(Ce) scintillator plate combined with a 1-inch square position sensitive photomultiplier tube (PS-PMT) contained in the same tungsten shield with a pinhole collimator, but with the scintillator positioned closer (30 mm) to the pinhole collimator to obtain a similar field-of-view (FOV). For both cameras, we used a wider angle (~55 degrees) pinhole collimator to measure the phantom closer to improve sensitivity. Although the 40 mm × 40-mm YAP(Ce) camera had high system spatial resolution, the background count fractions were high and produced a high count area at the center of the images due to the pulse pileup of the signals. With the 20 mm × 20-mm YAP(Ce) camera, we obtained X-ray images with low background counts without a high count area at the image center. By smoothing the measured images, we were able to estimate the ranges even for clinical dose levels. We therefore confirmed that one of our newly developed YAP(Ce) cameras had high sensitivity and is promising for the imaging of secondary electron bremsstrahlung X-rays during irradiation of carbon ions in clinical conditions.

著者人数: 12名, Accepted: 2018-09-17, 資料番号: SA1180180000

別紙参照, SNMMI2017 Annual Meeting

The Compton camera can simultaneously acquire images of multiple isotopes injected in a body; therefore, it has the potential to introduce a new subfield in the field of biomedical imaging applications. The objective of this study is to assess the ability of a prototype semiconductor-based silicon/cadmium telluride (Si/CdTe) Compton camera to simultaneously image the distributions of technetium (99mTc)-dimercaptosuccinic acid (DMSA) (141 keV emission) and 18F-fluorodeoxyglucose (FDG) (511 keV emission) injected into a human volunteer. 99mTc-DMSA and 18F-FDG were injected intravenously into a 25-year-old male volunteer. The distributions of 99mTc-DMSA and 18F-FDG were simultaneously made visible by setting a specified energy window for each radioisotope. The images of these radiopharmaceuticals acquired using the prototype Compton camera were superimposed onto computed tomography images for reference. The reconstructed image showed that 99mTc-DMSA had accumulated in both kidneys, which is consistent with the well-known diagnostic distribution determined by clinical imaging via single-photon emission computed tomography. In the 18F-FDG image, there is broad distribution around the liver and kidneys, which was expected based on routine clinical positron emission tomography imaging. The current study demonstrated for the first time that the Si/CdTe Compton camera was capable of simultaneously imaging the distributions of two radiopharmaceuticals, 99mTc-DMSA and 18F-FDG, in a human body. These results suggest that the Si/CdTe Compton camera has the potential to become a novel modality for nuclear medical diagnoses enabling multi-probe simultaneous tracking.

Low-energy X-ray imaging of the secondary electron bremsstrahlung X-ray emitted during carbon-ion irradiation is a promising method for range estimation. However, it remains unclear whether the method can be used for imaging with the clinical dose levels of carbon-ion and whether the bremsstrahlung X-ray can be detected from the deeper part of the body. To clarify these points, we developed a new high resolution low-energy X-ray camera and conducted imaging of the secondary electron bremsstrahlung X-ray during the irradiation of carbon-ions of different energies and intensities. Imaging was also tried with an X-ray camera using a human-head-sized, 17-cm diameter cylindrical phantom. To develop a high resolution imaging detector for a low-energy X-ray, we used a 20 × 20 × 0.5 mm thick cerium-doped yttrium aluminum perovskite, YA1O3 (YAP(Ce)) scintillator plate, which was optically coupled to a 25mm square high quantum efficiency (HQE) type position sensitive photomultiplier tube (PSPMT). The imaging detector was encased in a 2 cm thick tungsten container and a pinhole collimator was attached to its camera head. After evaluating the camera’s performance, secondary electron bremsstrahlung X-ray imaging was tried during irradiation of the carbon-ion and compared the results with a Monte Carlo simulation. We imaged the beam tracks by the secondary electron bremsstrahlung X-ray in real-time during irradiation of the carbon-ion and imaging and range estimation were possible even with near clinical dose level of 7.5 × 108 particles of carbon-ion. Clear images of a secondary electron bremsstrahlung X-ray were also obtained for a 17-cm diameter cylindrical phantom. The measured images were good agreement with the Monte Carlo simulation. We confirmed that our developed YAP(Ce) camera is promising for imaging secondary electron bremsstrahlung X-rays during irradiation of carbon-ions even near clinical conditions.