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Prompt X-ray imaging using a low-energy X-ray camera is a promising method for observing the beam shape from outside the subject. However, such imaging has so far been conducted only on static images with relatively long acquisition times without any energy information. Consequently, we performed list-mode prompt X-ray imaging using a newly developed data acquisition system combined with a pinhole YAP(Ce) camera during irradiation of a water phantom with carbon ions. Prompt X-ray imaging was conducted in list mode with a 1-ms time stamp and 128-channel energy bins during irradiation of a water phantom with 241.5 MeV/n carbon ions. After the imaging, list-mode data were sorted to obtain the time-sequential prompt X-ray images and those with different energies. From the images with different energies, we found the energy spectra were different depending on the areas in the images, and the reduction of the background fraction was possible. From the short time-sequential prompt X-ray images, we could even observe the differences in the images depending on the acquisition times, as well as the spill and ripple shapes of the carbon ion beam.

Gold wire is commonly used for quality assurance (QA) of the neutron beam in a boron neutron capture therapy (BNCT) system. It is set in water and irradiated with the neutron beam, and then 412-keV gamma photons from the activated gold wire are measured by a semiconductor detector. Since this procedure takes time and labor, a more efficient method is desired. To reduce the time and labor to measure the radioactivity of an activated gold wire, we carried out imaging of 412-keV gamma photons from the activated gold wire using a developed high-energy gamma camera. After the gold wire was set in the depth direction in a water-filled phantom and irradiated with neutron beams using the BNCT system, gamma photon imaging was conducted with the developed high-energy gamma camera. On the measured image, a depth profile was set to obtain the neutron distribution, and this was compared with the profile sequentially measured with a semiconductor detector. An image of the 412-keV gamma photons was obtained with an imaging time of 1.5 hours. The estimated depth profile of the neutron beam from the gamma camera image closely matched that measured with a semiconductor detector. Imaging of the gamma photons emitted from the activated gold wire was possible, and it offers an efficient method to measure the thermal neutron distribution of the BNCT system. This method has the potential to reduce the time and labor for QA of a BNCT system.

Although the luminescence images of water and an acrylic block during irradiation of proton and carbon ion beams were measured recently, they were not measured for helium ions. Because the mass of helium ions is four times that of protons and one-third that of carbon ions, different results from protons or carbon ions will be obtained by luminescence imaging. We measured the luminescence images of water and an acrylic block during irradiation of helium ion beams. Imaging was conducted using a cooled charge-coupled device (CCD) camera during the irradiation of helium ion pencil beams at three different energies to a water phantom and an acrylic block in addition to mini-beams of helium ions at four different energies. From the measured luminescence images during irradiation of helium ion pencil beams, we could estimate the ranges of the beams for water and the acrylic block. In the luminescence images of the acrylic block during irradiation of mini-beams of helium ions, we observed the luminescence of mini-beams with Bragg peaks in the images. Luminescence imaging of water and an acrylic block during the irradiation of helium ion beams was possible and could be used for range determination of the beams.

Although luminescence imaging of water during irradiation by particle ions is a promising method for dose estimation, it has only been tried for static images in which temporal information is not included. In addition to the positional distribution of the beam, temporal information is also important because the beams from a synchrotron-based therapy system have short pulse shapes called spills. The temporal information is also important for high dose rate, short-time radiotherapy, or so-called FLASH radiotherapy. To measure the particle ion beam distributions with precise temporal information, we conducted short time sequential luminescence imaging of protons. First, we measured short time sequential luminescence images during irradiation of a water phantom by 150-MeV protons using a cooled charge-coupled device (CCD) camera at 0.143-s intervals. With this imaging, the images showed beam distributions, but the shapes of the spill were not precisely evaluated in time intensity curves due to the insufficient sampling rate of the imaging. Then we measured short time sequential optical images with 0.053-s intervals. With this imaging, the images showed that the beam distributions in the spill shape could be measured, but the image and depth profiles evaluated from the images were noisy due to the insufficient light intensity. Consequently, we measured short time sequential luminescence images during irradiation of fluorescein (FS) water by 150-MeV protons. Since FS water produced ∼10 times higher luminescence, we could obtain high-intensity images enabling us to evaluate the time intensity curves based on the shape of the spills during measurement with 0.053-s intervals. The depth profiles of the beam were also obtained from the measured images. With these results, we confirmed that time sequential luminescence imaging was possible and, in such cases, FS water images measured at 0.053-s intervals are most promising to measure the short time sequential luminescence images during irradiation of protons.

Vitamin B2 (V-B2) is a material that converts shorter-wavelength light such as ultraviolet (UV) light to longer-wavelength light. However, it is not clear whether longer-wavelength light is produced by the irradiation of alpha particles. Because UV light such as Cherenkov light is required to produce longer-wavelength light for V-B2 and alpha particles do not produce Cherenkov light in a material, no light is expected to be emitted in V-B2 by the irradiation of alpha particles. However, we irradiated alpha particles to a V-B2 plate and measured the produced light in the plate. During irradiation of alpha particles, a significant amount of longer-wavelength light was observed from the V-B2 plate. The luminescence intensity of the V-B2 plate during irradiation of alpha particles was ∼1/6 of that of the plastic scintillator. The spectrum of luminescence from the V-B2 plate during this irradiation was the same as that emitted by the irradiation of ultraviolet (UV) light, which possibly indicates UV or shorter-wavelength light production in the V-B2 plate by the irradiation of alpha particles. This longer-wavelength light is not attributed to the UV light from the air scintillation by the alpha particles but to the direct irradiation of alpha particles to the V-B2 plate. From these results, we conclude that the luminescence of the V-B2 plate was the scintillation by the irradiation of alpha particles, or other phenomenon that was; UV as well as shorter-wavelength visible light was produced in the V-B2 plate by the irradiation of alpha particles and that the longer-wavelength light was produced from this light. The UV light was produced from the light generated by the dipole interaction of moving secondary electrons with V-B2 or binder molecules and then converted to the longer-wavelength light with a slower decay of the V-B2 in the plate that had high intensity due to the decreased simultaneity of the light emission.

Imaging of positrons induced by nuclear reactions with a proton beam is a possible method for observing the beam shape from outside the subject. However, such imaging of induced positrons has so far been conducted for solid materials. The induced positron distribution in liquid water has not been measured or reported. To clarify the distribution of induced positrons in liquid water, we conducted Cherenkov-light imaging after irradiation by protons to a water phantom. After irradiation by a 117-MeV proton beam to a phantom containing liquid water, Cherenkov-light imaging of the induced positrons was conducted using a cooled charge-coupled device (CCD) camera following the decay of the positrons. We also imaged the luminescence of water during irradiation by the proton beam to compare the distributions. We could measure the distribution of Cherenkov-light from the induced positrons in liquid water. Positron distributions kept their beam shapes in water but were different from that of the luminescence image; positron distribution was wider in the deep area of the beams in the lateral as well as depth direction. The distributions' shapes were only slightly changed with time. We conclude that Cherenkov-light imaging from the induced positrons after irradiation by a proton beam in water was possible, and we found that the induced positrons kept their beam shape in water with different shape from that of dose. These findings may provide new insights for imaging in particle therapy.

Astatine-211 (At-211) is a promising alpha particle emitter for targeted radionuclide therapy. Since its daughter isotope (polonium-211(Po-211)) emits characteristic X-rays of about 80 keV, the distribution of At-211 in the body can be imaged by detecting the X-rays with a scintillation camera. However, the isotopes also emit high-energy gamma photons that are collimated with difficulty for a parallel-hole collimator of a clinical scintillation camera system, and thus the selection of a collimator is important. In this study, we compared the performances of low-energy high-resolution (LEHR), low-energy all-purpose (LEAP), medium-energy (ME), and high-energy (HE) parallel-hole collimators for At-211 using Monte Carlo simulation. We simulated a clinical scintillation camera system with the collimators using the Geant4 toolkit. The energy spectra, sensitivities, and spatial resolutions for the point source of At-211 were evaluated. Moreover, we simulated imaging of six sphere sources of At-211 in a 1-cm-thick cylindrical phantom filled with At-211 solution to evaluate image contrast. All of the results in this study are simulation data. The spatial resolution with LEHR was 7.6 mm full width at half maximum (FWHM) and the highest between collimators, while the sensitivity with LEAP was 85 cps/MBq and the highest. The image contrast acquired with the ME collimator was superior to those with the other collimators. We concluded that the LEHR, LEAP, and ME collimators had their advantages, so an optimum collimator should be selected depending on the purpose of imaging of At-211, although there was no advantage in using the HE collimator for the imaging of At-211.

Reactive hydroxyl radicals (·OH) play important roles in the biological effects of radiation exposure or radiation therapy, and the distribution of ·OH in water during irradiation is of interest to researchers. However, real-time ·OH distribution measurement during irradiation has so far not been achieved due to the difficulty of detecting ·OH. To make these distribution measurements possible, we attempted the imaging of light emitted from luminol water during irradiation with low-energy X-rays. Imaging of the light emitted from luminol water was conducted using a cooled charge-coupled device (CCD) camera during X-ray irradiation to luminol water at lower energy than the Cherenkov-light threshold. The light emission of luminol water was 25 times higher than that of water, and clear images of light distributions were measured for the luminol water. By carrying out the imaging of luminol water with the addition of a radical scavenger to the luminol water, we could confirm that the emitted light was from ·OH produced in water. With this addition of the radical scavenger, the light intensity decreased as the weight of the scavenger increased. With these results, we confirmed that the detected light distribution in luminol water could be attributed to the ·OH produced by the X-ray irradiation.

In the development of new scintillators for X-ray imaging, a high-resolution and highly efficient system is required to evaluate the performance of the scintillator plates. For this purpose, we developed a high-resolution X-ray microscope system. The developed compact X-ray microscope system is based on a magnifying unit and a cooled charge-coupled device (CCD) camera, combined with a small industrial X-ray irradiation system. Using this system, we carried out imaging of three scintillator plates and evaluated their spatial resolution. Each scintillator plates was set in front of the lens of the objective, X-rays were irradiated to the scintillator plates, and transmission images of masks were acquired. The measured spatial resolution of the scintillator plates varied from 16 μm to 30 μm, depending on the type of scintillator plate. The focus size of the X-ray tube had an almost negligible effect on the spatial resolution of the images for the evaluated scintillator plates.

Prompt secondary electron bremsstrahlung X-ray (prompt X-ray) imaging using a low-energy X-ray camera is a promising method for observing a beam shape from outside the subject. However, the images measured within short times suffer from statistical noise. Consequently, we performed prompt X-ray imaging with higher sensitivity using a larger-diameter pinhole collimator and compared the results with those of a conventional collimator. Prompt X-ray imaging was conducted during irradiation with pencil beams of 241.5-MeV/n carbon ions to a water phantom. A newly developed X-ray camera with a 4-mm diameter as well as conventional 1.5-mm-diameter pinhole collimators was used for the imaging in list mode, and we compared the prompt X-ray images, energy spectra, and time count rate curves between 1.5-mm-diameter and 4-mm-diameter pinhole collimators. The prompt X-ray images taken with the 4-mm-diameter pinhole collimators had ∼7 times higher sensitivity with 70 % lower offset fractions originating from the prompt gamma photons. Furthermore, the ranges were more precisely estimated with the 4-mm collimator than with the 1.5-mm collimator. The energy spectra showed less contamination by tungsten-characteristic X-rays for the 4-mm pinhole collimator. Even for images measured with 0.1-s intervals, the beam shapes and time count rate curves could be obtained with less statistical noise using the 4-mm-diameter pinhole collimators. The use of the 4-mm-diameter pinhole collimator attached to the X-ray camera had advantages for prompt X-ray imaging with high sensitivity and low background, enabling us to image the beams even with short-time measurements.

The imaging of the luminescence of water and Cherenkov-light with fluorescein during irradiation is a useful method to increase intensity; however, the magnitudes of the increase in intensity for Cherenkov-light and luminescence of water have been neither evaluated nor compared. Therefore, we measured the Cherenkov-light and luminescence of water with and without fluorescein during irradiation, and then we compared the magnitudes of the intensity increase with fluorescein. For 6 and 10 MV X-rays as well as 6 and 12 MeV electrons, the intensity of the Cherenkov-light images increased only 2 to 3 times with fluorescein water compared with that of water. Meanwhile, for 100 kV X-rays and 150 MeV protons, the intensity of the luminescence images increased more than 10 times with fluorescein water compared with that of water. The difference could probably be attributed to the difference in the light-emission mechanisms and coherency between Cherenkov-light and the luminescence of water.

Silver-doped zinc sulfide (ZnS(Ag)) is an opaque powder scintillator that is mainly used for detection or imaging of charged particles such as alpha particles. Since ZnS(Ag) is not transparent, the thickness of ZnS(Ag) was limited to ∼10 μm. If a thicker ZnS(Ag) scintillator could be developed, it would be useful for studies such as high-energy particle ion detection as well as beta particle or gamma photon detection. We developed a ZnS(Ag) fiber-structured scintillator using a capillary plate in which ZnS(Ag) powder was encapsulated in the capillaries. The thickness of the capillary plate was 400 μm, and the light produced in ZnS(Ag) escaped from the capillaries, spread through the transparent lead glass area, and reached the opposite side of the plate; consequently, the opaque character and absorption of light could be avoided. The amount of light emitted from the capillary plate based fiber-structured ZnS(Ag) was almost the same as that of a commercially available ZnS (Ag) film, but the detection efficiency was about 1/5 (∼ 20%). The amount of light emitted from beta particles and gamma photons per MeV was less than 1% of that from alpha particles. The spatial resolution of the developed capillary plate based fiber-structured ZnS(Ag) scintillator for 5.5 MeV alpha particles was ∼200 μm FWHM. Imaging of the slits and light spots from alpha particles could be achieved with the developed scintillator combined with an electron-multiplied charge-coupled device (EM-CCD) camera. The developed capillary plate based fiber-structured ZnS(Ag) will be useful for detecting high-energy particle ions.

Although optical imaging of electron beams and X-rays from medical linear accelerators (LINAC) is a possible method for dose distribution measurements, it has been limited to two-dimensional (2D) projection images. For the precise measurement of an optical image of electron beams and X-rays, three-dimensional (3D) imaging is desired. To measure 3D dose distributions, we conducted imaging of electron beams and X-rays using a plastic scintillator plate set in a water phantom. When this plate was immersed in the water phantom, irradiation with electron beams or X-rays was carried out from along the plate's sides. Optical images of the scintillator plate were acquired using a charge-coupled device (CCD) camera from the side during irradiation with electron beams and X-rays. Measurements were conducted at 6 MeV, 9 MeV and 12 MeV for electron beams and at 6 MV and 10 MV for X-rays. The imaging system was set on the bed of the LINAC and moved at 10-mm steps perpendicular to the beam direction to acquire a set of sliced optical images of the beams. A set of these sliced images were stacked and interpolated to form 3D optical images. For the 3D images, after the correction of the Cherenkov-light component in the images, the relative depth and lateral doses were evaluated. From the relative depth doses of electron beams, the half-value depths could be evaluated within an error of 1.3 mm. Lateral widths could be evaluated within an error of less than 2 mm parallel to the plastic scintillator and less than 6.5 mm perpendicular to it. From the relative depth doses of X-rays, the average difference between the measured value and that by a planning system was within an error of 2 %. Lateral widths could be evaluated within an error of less than 0.68 mm parallel to the plastic scintillator and less than 2.6 mm perpendicular to it. We confirmed that 3D imaging of electron beams and X-rays using plastic scintillator plate is feasible and is a promising method for measuring dose images at any position.

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.

Low-energy X-ray imaging of prompt secondary electron bremsstrahlung X-rays (prompt X-rays) emitted during particle-ion irradiation is a promising method for range estimation. However, measurements have so far been conducted mainly for uniform phantoms of water or an acrylic block. Prompt X-ray imaging for phantoms with air cavities has not yet been extensively measured or evaluated with realistic conditions. Consequently, we conducted imaging of prompt X-rays using a pinhole YAP(Ce) camera during irradiation by protons as well as carbon ions to non-uniform acrylic phantoms with small cavities and then evaluated the images and estimated the ranges from the measured prompt X-ray images. The non-uniform acrylic phantom used for imaging had a cylindrical cavity with a 20-mm or 10-mm diameter in the phantom. During irradiation by protons or carbon ions, imaging of one of the phantoms was conducted using the pinhole YAP(Ce) camera with an air cavity as well as filling the cavity with an acrylic rod. For the phantom with a 20-mm-diameter cavity, the prompt X-ray images measured for both protons and carbon ions showed the shape of the cavity in the images, and the ranges could be estimated from the images. For the phantom with a 10-mm-diameter hole, although the shape of the hole could not be clearly observed, the ranges could also be estimated from the images. Furthermore, Monte Carlo simulated prompt X-ray images with different spatial resolution of the X-ray camera showed similar images to the measured images. We confirmed that prompt X-ray imaging of phantoms with air cavities using the pinhole YAP(Ce) cameras was possible and that prompt X-ray imaging is a promising approach for estimating the ranges for both protons and carbon ions, even for phantoms with air cavities.

Although optical imaging of decayed positrons and muons can provide promising methods, it has been attempted only for muons without a collimator, and the beam characteristics with collimators, such as peak position or beam spread in depth and lateral directions, have not yet been evaluated. Therefore, we conducted optical imaging of decayed positrons and muons with different collimators. For the imaging of decayed positrons, Cherenkov-light imaging in fluorescein (FS) water was used, while imaging of a plastic scintillator block was used for the imaging of muons. We conducted these imaging trials during irradiation with 84.5-MeV/c positive muons to an FS water phantom or a plastic scintillator block using a cooled charge-coupled device (CCD) camera with each collimator of a different diameter attached to the beam port. We could measure the Cherenkov-light images of FS water of decayed positrons and optical images of muons using the plastic scintillator block for all collimators. The depth profiles of the Cherenkov-light images were slightly wider for the muons with the collimators of larger diameters, although the estimated peak depths were nearly the same for all collimators. The lateral profiles of the Cherenkov light were wider for the muons when using collimators of larger diameters. Asymmetry in the directions of positron emissions from the muons was observed for all collimators. The depth profiles of the optical image of muons using a plastic scintillator block had nearly the same shape. The estimated lateral widths of the optical images of the plastic scintillator block were the same sizes as the collimator diameters within a 1.1-mm difference at a 10-mm depth of the scintillator block, and the widths were wider at the Bragg peak. With these measured optical images, we conclude that Cherenkov-light imaging of decayed positrons in water and optical imaging of muons using a plastic scintillator block with collimators are useful methods for determining not only peak position but also beam width as well as the asymmetry of the directions of positron emissions from the muons.

Although luminescence imaging during irradiation of protons is a useful method for range and dose estimations, it has only been attempted for uniform phantoms. Since luminescence imaging inherently has high resolution, we will be able to measure the images with non-uniform phantoms. Therefore, we measured the luminescence images for a non-uniform structured acrylic block during irradiation of protons. Imaging was conducted using a cooled charge-coupled device (CCD) camera during the irradiation of 129-MeV protons to an acrylic block with a 10-mm diameter hole in which an acrylic rod, air, or a titanium rod was inserted. Under these three different conditions, we could obtain different luminescence images for protons. The luminescence images with the acrylic rod showed uniform Bragg peaks in the lateral direction. In the luminescence images with air, a part of the Bragg peak under the hole was elongated. In the luminescence images with the titanium rod, the part of the Bragg peak under the hole was shortened. The ranges estimated from the depth profiles of the measured images matched the simulated results within a 1-mm difference. We confirmed that the luminescence imaging of non-uniform phantoms was possible and that ranges could be estimated from the images. We conclude that luminescence imaging is promising not only for uniform phantoms but also non-uniform phantoms.

Cerium doped Gd3(GaAl)5O12 (GGAG) scintillator has a character in which the decay time for alpha particles differs from that of gamma photons. This is a good point for an imaging detector used for alpha autoradiography of a mouse administered Astatine-211 (At-211) in radionuclide therapy research. Thus, we developed an alpha camera system using a GGAG imaging detector combined with a charge-coupled device (CCD) camera. Our GGAG imaging detector is made of a 0.5 mm-thick GGAG plate optically coupled to a position sensitive photomultiplier tube (PSPMT) and set in a stand with the CCD camera positioned on the upper side of the imaging detector to take optical photos. We first measured the performance of our alpha camera system using an Am-241 alpha source and natural alpha emitting radionuclides with radon daughters. We then conducted imaging of the non-sliced organs of a mouse. After the imaging detector surface was covered with 2.5 µm-thick Mylar film, the organs of the At-211-administered mouse was set on the imaging detector after split or opened and the alpha emitter distributions were measured. We used pulse shape analysis to separate the alpha particles from the beta particles or gamma photons. We successfully measured the alpha particle distributions of the wet non-sliced organs of the At-211-administered mouse. Using the pulse shape spectrum, we could separate the alpha particle images and beta particle or gamma photon images. We conclude that the developed alpha camera system is promising for the imaging of alpha particles and separating of the images using pulse shape discrimination.

Imaging of secondary electron bremsstrahlung x-ray emitted during proton irradiation is a possible method for measurement of the proton beam distribution in phantom. However, it is not clear that the method is used for range estimation of protons. For this purpose, we developed a low-energy x-ray camera and conducted imaging of the bremsstrahlung x-ray produced during irradiation of proton beams. We used a 20 mm × 20 mm × 1 mm finely grooved GAGG scintillator that was optically coupled to a one-inch square high quantum efficiency (HQE)-type position-sensitive photomultiplier tube to form an imaging detector. The imaging detector was encased in a 2 cm-thick tungsten container, and a pinhole collimator was attached to its camera head. After performance of the camera was evaluated, secondary electron bremsstrahlung x-ray imaging was conducted during irradiation of the proton beams for three different proton energies, and the results were compared with Monte Carlo simulation as well as calculated value. The system spatial resolution and sensitivity of the developed x-ray camera with 1.5 mm-diameter pinhole collimator were estimated to be 32 mm FWHM and 5.2 × 10−7 for ~35 keV x-ray photons at 100 cm from the collimator surface, respectively. We could image the proton beam tracks by measuring the secondary electron bremsstrahlung x-ray during irradiation of the proton beams, and the ranges for different proton energies could be estimated from the images. The measured ranges from the images were well matched with the Monte Carlo simulation, and slightly smaller than the calculated values. We confirmed that the imaging of the secondary electron bremsstrahlung x-ray emitted during proton irradiation with the developed x-ray camera has the potential to be a new tool for proton range estimations.

Luminescence of water during irradiation with particles having energies below the Cerenkov-light threshold was recently found for various types of radiations. However, the relation between the intensities of Cerenkov light and of the luminescence of water at the beam energy below the Cherenkov threshold is not well known. To clarify this point, we measured the produced light irradiating a water sample with electron beams having maximum energies above and below the Cerenkov-light threshold. The intensities of light during irradiation of electron beams with different energies increased rapidly at higher energy than ~260 keV while very small intensity was observed at the beam energy below the Cerenkov-light threshold. We conclude that the intensities of light produced in water during irradiation of electron beams with energies below the Cherenkov threshold from the luminescence of water are much lower than those of Cerenkov light.

Although light emission from an acrylic block during irradiations of carbon-ions lower energy than the Cerenkov-light threshold was found recently, the origins and fractions of the light in the images were not yet obvious. Since light spectra may provide information to estimate the origins and fractions of the light emission, we conducted the optical imaging of acrylic block during irradiations of carbon-ions using charge coupled device (CCD) camera with optical filters. We measured the light images of acrylic block with optical filters of different wavelengths using an ultraviolet (UV) sensitive CCD camera during carbon-ion irradiations with the energy slightly higher energy to produce secondary electrons to emit Cerenkov-light (241-MeV/u). From the images, we derived the depth profiles with different wavelengths and calculated the spectra of the emitted light from acrylic block. The depth profiles showed higher intensity due to Cerenkov-light at the shallow area distributed in the images of longer wavelengths. By the calculations between depth profiles of different wavelengths, we could derive the depth profiles for possible origins of the produced light: luminescence, Cerenkov light and scintillation of acrylic block. Using the derived fractions of these components, we could estimate the depth profiles of these three components with different wavelengths.

Range, momentum and deviation of momentum determinations of muons are important for quality assessment (QA) of beams. Imaging of annihilation radiations emitted from positrons decayed from positive muons and that of bremsstrahlung x-rays emitted from positrons and secondary electrons from positive muons are possible methods of imaging muons. However, the energies and intensities as well as position distributions of these radiations have not been obvious. Thus we calculated the energy spectrum and the distributions of annihilation radiations as well as bremsstrahlung x-rays produced in water during irradiation of positive muons using Monte Carlo simulation. The calculations were conducted for 84.5 MeV /c positive muons, which is the same beam condition used in an experimental facility at the Japan Proton Accelerator Research Complex (J-PARC). We were able to calculate the energy spectrum as well as the position distributions of annihilation radiations and bremsstrahlung x-rays. The energy spectrum showed a broad distribution of bremsstrahlung x-rays, mainly from decayed positrons with an energy range up to 50 MeV with higher intensity in low-energy bremsstrahlung x-rays. The spectrum also showed a sharp peak at 511-keV from annihilation radiations. The position distribution of annihilation radiations was wider than those of the bremsstrahlung x-rays. The position distribution of the bremsstrahlung x-rays were nearly identical to the Cerenkov-light position distribution emitted by the decayed positrons in water. We conclude that imaging of bremsstrahlung x-rays from decayed positrons by using an x-ray camera is a promising method for the QA of positive muons and that higher spatial resolution images of positron distributions will be measured than those measured by annihilation radiations.

Radioactive ion (RI) beams combined with in-beam positron emission tomography enable accurate in situ beam range verification in heavy ion therapy. However, the energy spread of the radioactive beams generated as secondary beams is wider than that of conventional stable heavy ion beams which causes Bragg peak region and distal falloff region broadening. Therefore, the energy spread of the RI beams should be measured carefully for their quality control. Here, we proposed an optical imaging technique for the energy spread estimation of radioactive oxygen ion beams. A polymethyl methacrylate phantom (10.0 × 10.0 × 9.9 cm3) was irradiated with an 15O beam (mean energy = 247.7 MeV u−1, standard deviation = 6.8 MeV u−1) in the Heavy Ion Medical Accelerator in Chiba. Three different momentum acceptances of 1%, 2% and 4% were used to get energy spreads of 1.9 MeV u−1, 3.4 MeV u−1 and 5.5 MeV u−1, respectively. The in-beam luminescence light and offline beam Cerenkov light images were acquired with an optical system consisting of a lens and a cooled charge-coupled device camera. To estimate the energy spread of the 15O ion beams, we proposed three optical parameters: (1) distal-50% falloff length of the prompt luminescence signals; (2) full-width at half maximum of the Cerenkov light signals in the beam direction; and (3) positional difference between the peaks of the Cerenkov light and the luminescence signals. These parameters estimated the energy spread with the respective mean squared errors of 2.52 × 10−3 MeV u−1, 5.91 × 10−3 MeV u−1, and 0.182 MeV u−1. The distal-50% falloff length of the luminescence signals provided the lowest mean squared error among the optical parameters. From the findings, we concluded optical imaging using luminescence and Cerenkov light signals offers an accurate energy spread estimation of 15O ion beams. In the future, the proposed optical parameters will be used for energy spread estimation of other RI beams as well as stable ion beams.

Radioactive ion (RI) beams combined with in-beam positron emission tomography enable accurate in situ beam range verification in heavy ion therapy. However, the energy spread of the radioactive beams generated as secondary beams is wider than that of conventional stable heavy ion beams which causes Bragg peak region and distal falloff region broadening. Therefore, the energy spread of the RI beams should be measured carefully for their quality control. Here, we proposed an optical imaging technique for the energy spread estimation of radioactive oxygen ion beams. A polymethyl methacrylate phantom (10.0 × 10.0 × 9.9 cm3) was irradiated with an 15O beam (mean energy = 247.7 MeV u−1, standard deviation = 6.8 MeV u−1) in the Heavy Ion Medical Accelerator in Chiba. Three different momentum acceptances of 1%, 2% and 4% were used to get energy spreads of 1.9 MeV u−1, 3.4 MeV u−1 and 5.5 MeV u−1, respectively. The in-beam luminescence light and offline beam Cerenkov light images were acquired with an optical system consisting of a lens and a cooled charge-coupled device camera. To estimate the energy spread of the 15O ion beams, we proposed three optical parameters: (1) distal-50% falloff length of the prompt luminescence signals; (2) full-width at half maximum of the Cerenkov light signals in the beam direction; and (3) positional difference between the peaks of the Cerenkov light and the luminescence signals. These parameters estimated the energy spread with the respective mean squared errors of 2.52 × 10−3 MeV u−1, 5.91 × 10−3 MeV u−1, and 0.182 MeV u−1. The distal-50% falloff length of the luminescence signals provided the lowest mean squared error among the optical parameters. From the findings, we concluded optical imaging using luminescence and Cerenkov light signals offers an accurate energy spread estimation of 15O ion beams. In the future, the proposed optical parameters will be used for energy spread estimation of other RI beams as well as stable ion beams.

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−1 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 × 1010 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.

Radioluminescence by protons and carbon ions of energy lower than the Cherenkov threshold (∼260 keV) in water has been observed. However, the origin of the luminescence has not been investigated well. In the present work, we imaged radioluminescence in water using synchrotron radiation that was of sufficiently lower energy (11 keV) than the Cherenkov threshold and we measured its spectrum using a high-sensitivity cooled CCD camera and optical longpass filters having 5 different thresholds. In addition, to determine effects of impurities in water, the water target was changed from ultrapure water to tap water. Monte Carlo simulation (Geant4) was also performed to compare its results with the experimentally obtained radioluminescence distribution. In the simulation, photons were generated in proportion to the energy deposition in water. As a result, the beam trajectory was clearly imaged by the radioluminescence in water. The spectrum was proportional to λ −3.4±0.4 under an assumption of no peaks. In the spectrum and distribution, no differences were observed between ultrapure water and tap water. TOC (total organic carbon) contents of ultrapure water and tap water as an impurity were measured and these were 0.26 mg l−1 and 2.3 mg l−1, respectively. The radioluminescence seemed to be attributable to water molecules not impurities. The radioluminescence distribution of the simulation was consistent with the experimental distribution and this suggested that radioluminescence was proportional to dose, which is expected to allow use for dose measurement.

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 (YA1O3: 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.

Tritium (H-3) is a pure beta-emitting radionuclide and beta particles have extremely low energy (maximum energy: 18.6 keV). Thus the in-vivo imaging of H-3 is thought to be impossible. However, beta particles emit bremsstrahlung X-rays in subjects that may be imaged from outside of the subjects. We tried to image the bremsstrahlung X-rays from H-3 water using a newly developed radiation imaging system. The developed imaging system used a pixelated Ce-doped (Gd, La)2Si2O7 (LaGPS) scintillator plate optically coupled to a flat-panel position-sensitive photomultiplier tube (FP-PMT). Using the imaging system, we conducted bremsstrahlung X-ray imaging from H-3 water in a plastic bag with 37-MBq radioactivity. We obtained tungsten slit mask images with a spatial resolution of ~3 mm full width at half maximum (FWHM). The energy spectrum of the bremsstrahlung X-rays from the H-3 water showed a broad distribution with an average energy of ~10 keV. The measured sensitivities of the LaGPS imaging system for bremsstrahlung X-rays from H-3 water in a plastic bag were 1.8 × 10−7. We conclude that the imaging of bremsstrahlung X-rays from H-3 water was really possible and it has a potential to be a new method for the in-vivo H-3 imaging of small animals, plants, or materials.

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.

Cerium doped gadolinium pyrosilicate (GPS (Ce), Gd2Si2O7), which is a scintillator with a high light output, is a likely candidate to develop high-resolution radiation imaging detectors. Thus we fabricated a GPS (Ce) imaging detector using a single-crystal GPS (Ce) plate in combination with a position-sensitive photomultiplier tube (PSPMT). Through a performance evaluation, we found that the spatial resolution was ~ 0.5 mm FWHM for Am-241 α particles (5.5 MeV) with an energy resolution of 15% FWHM. The decay time difference of GPS (Ce) between 5.5 MeV α particles and 662 keV Cs-137 γ photons was 7 ns, which was insufficient to separate each image using pulse-shape discrimination. However, the α/γ ratio was large (0.35), so it was possible to separate 5.5 MeV α particles and 662 keV Cs-137 γ photons by the energy spectra. We conclude that the imaging detector using single-crystal GPS (Ce) is promising for radiation, especially for α particles.

Ce doped (YGd)3(GaAl)5O12:Ce (YGAG) is a ceramic scintillator with high light output, and its decay times are different between alpha particles and gamma photons or beta particles. This characteristic may be applied to the direct measurement of alpha particles in water where the gamma photons or beta particles become background counts. Thus we proposed a radiation detector using a YGAG plate dipped in water that contained alpha radionuclides to detect alpha particles in water. The scintillation photons from the YGAG plate were detected by a position sensitive photomultiplier tube (PSPMT) set in water that contained alpha radionuclides. A YGAG plate (20 × 20 × 0.5 mm) was set on the bottom of a glass cup filled with water containing alpha radionuclides that emit alpha particles. The alpha particles near the YGAG plate were detected by the scintillator, and the scintillation light was detected by the PSPMT . For radon-containing water sampled from a hot spring, pulse shape spectra showed two peaks: one for the alpha particles and another for the beta particles and the gamma photons. By pulse shape discrimination for the peak of the alpha particles, their count rate decreased with a decay of Rn-222 (3.8 days). For tap water, the pulse shape distribution showed only one peak and the count rate was almost constant, indicating these counts were mainly from the background beta particles or gamma photons. We could show a detection principle for alpha radionuclides in water and this detection principle for alpha radionuclides in water might be a new method for estimating the alpha radionuclide concentration in water.

The measurements of the three-dimensional (3D) dose distribution of proton beams in water are critical for proton therapy for quality assessment (QA). Although ionization chambers are commonly used for this purpose, such measurements take a long time to calculate precise 3D dose distribution. To solve this problem, we measured 3D dose distributions using a glass plate. We placed a 1-mm thick float glass plate on the upper inside of a black box with a water phantom set above the float glass plate outside the black box and irradiated the proton beam to the water phantom from the upper side. The attenuated proton beam by water in the phantom was detected by the float glass plate and a scintillation image was formed in the plate. The image was reflected by a surface mirror set below the float glass plate and detected by a cooled charge-coupled device (CCD) camera from the side. We changed the water depths in the phantom and measured the scintillation images at each depth. Then we calculated the 3D scintillation images from the measured images by stacking them in the depth direction. Measurements were made for 71.2- and 100-MeV proton pencil beams and a spread-out Bragg peak (SOBP) using the imaging system. From the images, we successfully formed 3D scintillation images without quenching. The depth profiles measured from the scintillation images showed almost identical distribution with those measured by the ionization chamber within a difference less than 5%. The lateral profiles were also almost identical within width differences less than 2 mm. We conclude that our proposed method is promising for the 3D dose distribution measurements of proton beams.

Recently we found that the luminescence imaging of water during carbon-ion irradiation was possible using a cooled charge-coupled device (CCD) camera and the method could be used for range estimation of the beam. In the luminescence image, we found luminescence from the fragment particles produced by the nuclear spallation reaction of carbon ions. The luminescence may be used for the estimation of the distribution of the fragment particles by the nuclear spallation. For this purpose, we irradiated carbon ions of 241.5 MeV u-1 to a water phantom and measured the luminescence image of water using a CCD camera. Then, we carefully observed the luminescence distribution after the Bragg peak to find the luminescence from the nuclear spallation reaction. In the luminescence image, we could clearly observe the luminescence from the fragment particles produced by the nuclear spallation reaction during irradiation of carbon ions. The beam widths of the luminescence image of the nuclear spallation were compared with those measured by the ionization chamber. The relative difference of the beam width at FWHM between luminescence image and ionization chamber was 23%. With these results, we conclude that the luminescence image of water during carbon-ion irradiation has a potential to be a new and efficient method for the width estimation of the fragment particles by the nuclear spallation reaction.

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.

The in-vivo imaging of C-14 is thought to be impossible because it is a pure beta-emitting radionuclide and the energy of beta particles is low. However, beta particles emit bremsstrahlung x-rays in subjects that may be imaged from the outside subjects. Thus, we imaged bremsstrahlung x-rays from a C-14 solution using a high resolution Ce-doped (Gd, La)2Si2O7 (LaGPS) radiation imaging system. With a LaGPS radiation imaging system, we conducted bremsstrahlung x-ray imaging from a C-14 solution in a plastic container with 37 MBq radioactivity with and without a parallel hole collimator. Without it, images were obtained with a spatial resolution of ~1 mm FWHM in less than 1 min of acquisition time. The energy spectrum of the bremsstrahlung x-rays from a C-14 solution showed a broad distribution with an average energy of ~30 keV. With a parallel hole collimator, images of the C-14 solutions were also possible although the detected counts decreased to 2.4% of those without a collimator. The measured sensitivities of the LaGPS imaging system without and with a collimator for bremsstrahlung x-rays from C-14 solutions in a plastic container were 3.5 × 10−5 and 8.5 × 10−7, respectively. We conclude that the imaging of bremsstrahlung x-rays from C-14 might be a new method for the in-vivo distribution imaging of small animals or plants.

Ce doped (YGd)3(GaAl)5O12:Ce (YGAG) is a ceramic scintillator originally developed for X-ray computed tomography (CT). It has high light output and is also a promising candidate for high resolution event-by-event radiation imaging detectors. Thus we developed a radiation imaging detector using a YGAG plate combined with a position sensitive photomultiplier tube (PSPMT) and evaluated the performance. The spatial resolution for Am-241 alpha particles (5.5 MeV) was ~0.4 mm FWHM and the energy resolution was 12 % FWHM. The spatial resolution for Sr-Y-90 beta particles, Am-241 gammas (60 keV) and Cs-137 X-ray (~35 keV) was 0.8 mm FWHM, 1.6 mm FWHM and 2.0 mm FWHM, respectively. The decay time difference of YGAG between 5.5 MeV alpha particles and 662 keV Cs-137 gammas was 37 ns and the pulse shape spectra for 5.5 MeV alpha particles and 662 keV Cs-137 gammas were separated without overlap with the figure of merit (FOM) of 1.14. It was possible to separate the alpha particle and gamma photon images using pulse shape discrimination. Using pulse shape discrimination, it was also possible to separate the alpha particles and gamma photons in the electrostatically collected radon daughters on the imaging detector. We conclude that the developed YGAG imaging detector is useful for high resolution radiation detector with pulse shape discrimination capability to discriminate the different types of radiations.

The silicon photomultiplier (Si-PM) is a promising photodetector for a high-resolution PET scanner due to its small size, high gain and lower sensitivity to magnetic fields. There are several commercially available Si-PM arrays with different pixel sizes and fill factors, and these parameters can affect the performance of a PET block detector read out by these devices. We compared the performance of block detectors using 4 × 4 Si-PM arrays with 25 µm (Hamamatsu S11064-025P) and 50 µm (S11064-050P) pixels combined with the same 15 × 15 matrix LGSO block made of 0.7 × 0.7 × 6 mm(3) scintillator pixels. Evaluated characteristics include photopeak linearity, energy resolution and positioning performance. Although the photopeak linearity and energy resolution are slightly better for the Si-PM with 25 µm pixels, the position performance measured by the separation of the position histogram is significantly better for the Si-PM with 50 µm pixels. We conclude that using the Si-PM with 50 µm pixels will provide a better solution for the development of ultrahigh-resolution PET systems.

The silicon-photomultiplier (Si-PM) is a promising photodetector for the development of new PET systems due to its small size, high gain and relatively low sensitivity to the static magnetic field. One drawback of the Si-PM is that it has significant temperature-dependent gain that poses a problem for the stability of the Si-PM-based PET system. To reduce this problem, we developed and tested a temperature-dependent gain control system for the Si-PM-based PET system. The system consists of a thermometer, analog-to-digital converter, personal computer, digital-to-analog converter and variable gain amplifiers in the weight summing board of the PET system. Temperature characteristics of the Si-PM array are measured and the calculated correction factor is sent to the variable gain amplifier. Without this correction, the temperature-dependent peak channel shifts of the block detector were -55% from 20 °C to 35 °C. With the correction, the peak channel variations were corrected within ±8%. The coincidence count rate of the Si-PM-based PET system was measured using a Na-22 point source while monitoring the room temperature. Without the correction, the count rate inversely changed with the room temperature by 10% for 1.5° C temperature changes. With the correction, the count rate variation was reduced to within 3.7%. These results indicate that the developed temperature-dependent gain control system can contribute to improving the stability of Si-PM-based PET systems.

A Geiger-mode avalanche photodiode (Si-PM) is a promising photodetector for PET, especially for use in a magnetic resonance imaging (MRI) system, because it has high gain and is less sensitive to a static magnetic field. We developed a Si-PM-based depth-of-interaction (DOI) PET system for small animals. Hamamatsu 4 × 4 Si-PM arrays (S11065-025P) were used for its detector blocks. Two types of LGSO scintillator of 0.75 mol% Ce (decay time: ∼45 ns; 1.1 mm × 1.2 mm × 5 mm) and 0.025 mol% Ce (decay time: ∼31 ns; 1.1 mm × 1.2 mm × 6 mm) were optically coupled in the DOI direction to form a DOI detector, arranged in a 11 × 9 matrix, and optically coupled to the Si-PM array. Pulse shape analysis was used for the DOI detection of these two types of LGSOs. Sixteen detector blocks were arranged in a 68 mm diameter ring to form the PET system. Spatial resolution was 1.6 mm FWHM and sensitivity was 0.6% at the center of the field of view. High-resolution mouse and rat images were successfully obtained using the PET system. We confirmed that the developed Si-PM-based PET system is promising for molecular imaging research.

Although luminescence of water during irradiations of proton and carbon-ion lower energy than the Cerenkov-light threshold were found recently, the sources of the luminescence were not yet obvious. To estimate the sources of the luminescence, we measured the light spectrum of the luminescence of water during carbon-ion irradiations and estimated the sources of the luminescence. Using an ultraviolet (UV) light sensitive charge coupled device (CCD) camera, we measured the luminescence images of water during carbon-ion beam irradiations by changing optical filters, derived the light spectra of the luminescence of water and compared with the calculated results. The intensity of the measured light spectrum of the luminescence of water at the Bragg peak region was decreased as the wavelength of light proportional to ~ λ to the -2.0th power where λ is the wavelength of the light, indicating the source of the luminescence of water can be electromagnetic pulse produced by the dipole displacement inside the water molecules. In the shallow part of the water prior to the Bragg peak, where the Cerenkov-light is included, the spectrum showed steeper curve that is proportional to ~ λ to the -2.6th power, which was similar to the calculated spectrum of Cerenkov-light including the refractive index changes of water with the wavelength of light. From these results, the luminescence of water is thought to be mainly come from electromagnetic pulse produced by the dipole displacement inside the water molecules.