Akram Mohammadi - Academia.edu (original) (raw)

Papers by Akram Mohammadi

2017 IEEE Nuclear Science Symposium and Medical Imaging Conference (NSS/MIC)

The purpose of this study is to perform a first characterization and proof of principle investiga... more The purpose of this study is to perform a first characterization and proof of principle investigation of a Compton camera setup composed by a scatterer component consisting of a pixelated GAGG crystal read out by a SiPM multi-pixel photon counter (MPPC) and an absorber component consisting of a monolithic LaBrc(Ce) scintillator read out by a 256-fold multianode photomultiplier (PMT). The rationale of the study is to develop a Compton camera system as a future ion beam range verification device during particle therapy, via prompt gamma imaging. The properties to be investigated are the reconstruction efficiency and accuracy achievable with this system for detecting prompt-y rays. The Compton camera system described has been tested with a laboratory radioactive Cesium-137 source, in a certain geometrical configuration. The readout system is based on individual spectroscopy (NIM+VME) electronic modules, digitizing energy and time signals. The data have been analyzed to produce an input for the image reconstruction, performed using the MEGAlib toolkit software. I. In t r o d u c t i o n P ARTICLE beam therapy is nowadays considered an advantageous option in cancer treatment, but the beneficial high dose delivery precision on the other hand demands a high accuracy of the Bragg peak placement: different approaches for the beam range monitoring are worldwide being evaluated.

A combined technique of an imaging plate (IP) and thin absorbers was applied to tritium in nickel... more A combined technique of an imaging plate (IP) and thin absorbers was applied to tritium in nickel and vanadium specimens using copper, aluminum, and gold foil as the absorber. Copper and aluminum foil are used as a K-edge filter with X-ray absorption at 9.0 keV and 1.56 keV, respectively. Gold has L-edges X-ray absorption at around 13 keV. With this technique, photostimulated luminescence (PSL) decay curves are obtained by changing absorber’s thickness. In the nickel specimen, the difference in PSL decay curves between for the copper and gold absorber was clearly observed 20 days after loading, however, all curves became similar single pattern after 388 days. The same curve pattern was obtained in vanadium for all absorbers. The cross section images and depth profiles, which were taken at 468 days and 3.9 years after loading for the nickel and vanadium specimen, respectively, show no significant inclination of tritium concentration for both specimens. These results indicate that uni...

We are developing the OpenPET for in-beam imaging in carbon ion therapy. We have succeeded in sho... more We are developing the OpenPET for in-beam imaging in carbon ion therapy. We have succeeded in showing a proof-of-concept by developing small OpenPET prototypes that we used in the HIMAC. One of the major issues to realize the OpenPET is radiation damage to detectors because a number of fragmented particles are incident on the detectors located downstream from the target. While we are currently using photo-multiplier tubes in the OpenPET, some groups reported feasibility of use of a semiconductor photo-detector in hadron therapy. In this paper, therefore, we tested radiation hardness of the multi-pixel photon counter (MPPC) to show feasibility of its use in the OpenPET for carbon ion therapy. We used a single pixel MPPC (S10931-050) to evaluate radiation hardness. The experiment was performed in the PH2 course of the HIMAC. The energy of the 12C beam was 290MeV/u and beam intensity was 1.6 × 109 particles per second (pps) which was ten times higher than the typical clinical beam inte...

Time-of-fligfrt (TOF) PET improves image quality and quantitative accuracy compared with a conven... more Time-of-fligfrt (TOF) PET improves image quality and quantitative accuracy compared with a conventional PET system. While it is true that TOF works better for larger objects, recent improvement in timing resolution has encouraged application of TOF to brain-dedicated PET systems. Thus, as the second prototype of the helmet-type PET, we have developed a new TOF brain-dedicated PET prototype using detector modules of 12×12 lutetium fine silicate (LFS) crystals (4.1×4.1×10 mm3) connected to a 12×12 (4 mm pitch, 144-ch) MPPC array. In this paper, we investigated another detector module, which has the same outer size but smaller crystals: 16×16 LFS of 3.1×3.1×10 mm3 size coupled to 16×16 (3 mm pitch, 256-ch) MPPC array. The 3-mm 256-ch module showed an energy resolution of 12.0%. For the coincidence response function, the 3-mm module showed a better full width at half maximum (FWHM) of 1.9 mm compared with the 4-mm 144-ch module (2.4 mm). The FWHM was improved by 21%. The coincidence res...

Detectors with depth-of-interaction (DOI) information are utilized in positron emission tomograph... more Detectors with depth-of-interaction (DOI) information are utilized in positron emission tomography (PET) scanners to improve the sensitivity of radiation detection and the uniformity of spatial resolution. We recently developed a series of dual-ended detectors using crystal bars segmented by applying the subsurface laser engraving (SSLE) technique to 7, 13 and 20 DOI segments. It is crucial to achieve the submillimeter level spatial resolution for our detector as the narrow crystal bars are highly fragile after applying the SSLE technique. In this work, we focused on optimizing the surface area of the SSLE-induced layer to the narrow crystal bars in order to resolve the issue of fragility while maintaining sufficient crystal segment separation. The SSLE layers were induced to the middle of the cross section of the five crystal bars with dimensions of 1.5 × 1 . 5 × 20 mm3 with a distance of 0.1, 0.2, 0.3, 0.4 or 0.5 mm from the two opposite lateral edges of each crystal bar. All cry...

Physics in Medicine & Biology

The purpose of this work is to develop a validated Geant4 simulation model of a whole-body protot... more The purpose of this work is to develop a validated Geant4 simulation model of a whole-body prototype PET scanner constructed from the four-layer depth-of-interaction detectors developed at the National Institute of Radiological Sciences, National Institutes for Quantum and Radiological Science and Technology, Japan. The simulation model emulates the behaviour of the unique depth of interaction sensing capability of the scanner without needing to directly simulate optical photon transport in the scintillator and photodetector modules. The model was validated by evaluating and comparing performance metrics from the NEMA NU 2-2012 protocol on both the simulated and physical scanner, including spatial resolution, sensitivity, scatter fraction, noise equivalent count rates and image quality. The results show that the average sensitivities of the scanner in the field-of-view were 5.9 cps kBq−1 and 6.0 cps kBq−1 for experiment and simulation, respectively. The average spatial resolutions m...

2018 IEEE Nuclear Science Symposium and Medical Imaging Conference Proceedings (NSS/MIC)

In carbon ion therapy, visualization of the range of incident particles in a patient body is impo... more In carbon ion therapy, visualization of the range of incident particles in a patient body is important for treatment verification. In-beam positron emission tomography (PET) imaging in ion therapy is one of the method for verification of treatment due to the high quality of PET images. We have already shown the feasibility of radioactive beams of 15O and 11C using in-beam PET imaging using our OpenPET system. Recently, we have developed a whole gamma imaging (WGI) system which can work as PET, single gamma-ray and triple gamma-ray imaging simultaneously. The WGI system has high potential to detect location of 10C, which emits positron with a simultaneous gamma-ray of 718 keV, and activity of other produced positron emitting nuclei within patient body during ion therapy. In this work, we focus on investigation of performance of WGI system for 10C irradiation. First, the performance of scatterer detector of the WGI system regarding the radiation hardness was studied for 10C irradiation, then the performance of WGI was studied by simulation using the Geant4 code. The scatterer detector consisted of a segmented GAGG crystal and a multi-pixel photon counter (MPPC) and its performance was studied as a PMMA phantom was irradiated with 5 and 400 spills of 10C with energy of 350 MeV/u. No damage to the MPPC (scatterer detector) was observed even though after irradiation of 400 spills (~1.9×105 particle per spill). Sensitivity values of WGI system by simulation of a 10C nuclide at the center of field of view (FOV) for PET mode imaging, single gamma-ray of 718 keV imaging and triple gamma-ray imaging were 7.85%, 0.28% and 0.012% respectively. The performance of WGI system is going to be evaluated for 10C irradiation in the near future.

Physics in Medicine & Biology

Sensitivity and spatial resolution of positron emission tomography (PET) scanners can be improved... more Sensitivity and spatial resolution of positron emission tomography (PET) scanners can be improved by using thicker scintillation crystals with depth-of-interaction (DOI) encoding. Subsurface laser engraving (SSLE) can be used to segment crystals of a scintillation detector in order to fabricate a DOI detector. We previously applied SSLE to crystal bars of 3 × 3 × 20 mm3 and 1.5 × 1.5 × 20 mm3 and developed two dual-ended detectors with DOI segments of 3 mm and 1.5 mm, respectively. To further improve the DOI resolution, our SSLE detector design can be used with smaller pitch crystal bars, making them excellent detector candidates for small animal PET scanners with submillimetre resolution. In the present study, three small crystal bars of 1 × 1 × 20 mm3, 2 × 1 × 20 mm3, and 2 × 1 × 40 mm3 were laser engraved to 12, 20 and 40 segments, respectively, by applying SSLE in their height directions. The segmented crystal bars were characterised in three prototype detector arrangements. First, the 1 × 1 × 20 mm3 crystal bars were characterised in an 8 × 8 crystal array designed for DOI encoding along crystal height in a conventional small animal PET design. Second, a 4 × 8 crystal array of 2 × 1 × 20 mm3 crystal bars was characterised for using the DOI information for crystal interaction positioning along the axial axis of a small animal PET scanner. Finally, the third part of the study was performed on a single 2 × 1 × 40 mm3 crystal bar with 40 segments to investigate the feasibility of DOI estimation in longer crystals for application in a system with extended axial length. We evaluated the capability of segment identification and energy resolution of theses detectors. The 3D position maps of the detectors were obtained using the Anger-type calculation and the crystal identification performance was evaluated for each detector. Clear segment separation was obtained for the crystal arrays with 12 (segment pitch of 1.67 mm) and 20 (segment pitch of 1 mm) segments. Mean energy resolutions of 8.8% ± 0.4% and 9.6% ± 0.8% at 511 keV were obtained for the segments in the central regions of the 8 × 8 array with 12 segments and the 4 × 8 array with 20 segments, respectively. Clear segment identification was found to be difficult for the detector with 40 segments, especially for the segments at the middle of the crystal. Energy and interaction positioning characterisation results suggest that both prototype detectors with 12 and 20 segments are well suited for small animal PET scanners with high spatial resolution.

IEEE Transactions on Radiation and Plasma Medical Sciences

We developed a human-scale single-ring OpenPET (SROP) system, which had an open space allowing us... more We developed a human-scale single-ring OpenPET (SROP) system, which had an open space allowing us access to the subject during measurement. The SROP system consisted of 160 4-layer depth-of-interaction detectors. The open space with the axial width of 430 mm was achieved with the ring axial width of 214 mm and the ring inner diameter of 660 mm. The detectors were axially shifted to each other so that the detector ring was aligned along a plane horizontally tilted by 45° against the axial direction. The system was developed as a mobile scanner to be used not only in clinical PET rooms but also in charged-particle therapy treatment rooms as well as animal experiment rooms. Almost uniform spatial resolution better than 3 mm throughout the entire FOV was realized with an iterative image reconstruction method. Peak absolute sensitivity was 3.1%, and there was a region with sensitivity better than 0.8% for a length of more than 700 mm. An in-beam imaging experiment conducted at the HIMAC showed that the system was operable even at the highest beam intensity available for heavy-ion therapy. In addition, we conducted entirebody monkey dynamic imaging utilizing the long region inside the gantry by positioning a monkey along the direction having the longest FOV tilted by 45° against the axial direction. We concluded the developed system realizes versatile PET applications because of its wide-open space and mobility as well as high spatial resolution with sufficiently good sensitivity.

Physics in Medicine & Biology

This work presents an iterative method for the estimation of the absolute dose distribution in pa... more This work presents an iterative method for the estimation of the absolute dose distribution in patients undergoing carbon ion therapy, via analysis of the distribution of positron annihilations resulting from the decay of positron-emitting fragments created in the target volume. The proposed method relies on the decomposition of the total positron-annihilation distributions into profiles of the three principal positron-emitting fragment species - 11C, 10C and 15O. A library of basis functions is constructed by simulating a range of monoenergetic 12C ion irradiations of a homogeneous polymethyl methacrylate phantom and measuring the resulting one-dimensional positron-emitting fragment profiles and dose distributions. To estimate the dose delivered during an arbitrary polyenergetic irradiation, a linear combination of factors from the fragment profile library is iteratively fitted to the decomposed positron annihilation profile acquired during the irradiation, and the resulting weight...

Medical Physics

PURPOSE This work has two related objectives. The first is to estimate the relative biological ef... more PURPOSE This work has two related objectives. The first is to estimate the relative biological effectiveness of two radioactive heavy ion beams based on experimental measurements, and compare these to the relative biological effectiveness of corresponding stable isotopes to determine whether or not they are therapeutically equivalent. The second aim is to quantitatively compare the quality of images acquired post-irradiation using an in-beam whole-body positron emission tomography scanner for range verification quality assurance. METHODS The energy deposited by monoenergetic beams of 11 C at 350 MeV/u, 15 O at 250 MeV/u, 12 C at 350 MeV/u and 16 O at 430 MeV/u were measured using a cruciform transmission ionisation chamber in a water phantom at the Heavy Ion Medical Accelerator in Chiba (HIMAC), Japan. Dose-mean lineal energy was measured at various depths along the path of each beam in a water phantom using a silicon-on-insulator mushroom microdosimeter. Using the modi_ed microdosimetric kinetic model, the relative biological effectiveness at 10% survival fraction of the radioactive ion beams was evaluated and compared to that of the corresponding stable ions along the path of the beam. Finally, the post-irradiation distributions of positron annihilations resulting from the decay of positron-emitting nuclei were measured for each beam in a gelatin phantom using the in-beam whole-body positron emission tomography scanner at HIMAC. The depth of maximum positron-annihilation density was compared with the depth of maximum dose deposition and the signal-to-background ratios were calculated and compared for images acquired over 5 minutes and 20 minutes post irradiation of the phantom. RESULTS In the entrance region, the RBE10 was 1.2 ± 0.1 for both 11 C and 12 C beams, while for 15 O and 16 O it was 1.4 ± 0.1 and 1.3 ± 0.1, respectively. At the Bragg peak, the RBE10 was 2.7 ± 0.4 for 11 C and 2.9 ± 0.4 for 12 C, while for 15 O and 16 O it was 2.7 ± 0.4 and 2.8 ± 0.4, respectively. In the tail region, RBE10 could only be evaluated for carbon; the RBE10 was 1.6 ± 0.2 and 1.5 ± 0.1 for 11 C and 12 C, respectively. Positron emission tomography images obtained from gelatin targets irradiated by radioactive ion beams exhibit markedly improved signal-to-background ratios compared to those obtained from targets irradiated by non-radioactive ion beams, with 5-fold and 11-fold increases in the ratios calculated for the 15 O and 11 C images compared with the values obtained for 16 O and 12 C, respectively. The difference between the depth of maximum dose and the depth of maximum positron annihilation density is 2.4 ± 0.8 mm for 11 C, compared to -5.6 ± 0.8 mm for 12 C and 0.9 ± 0.8 mm for 15 O versus -6.6 ± 0.8 mm for 16 O. CONCLUSIONS The RBE10 values for 11 C and 15 O were found to be within the 95% confidence interval of the RBEs estimated for their corresponding stable isotopes across each of the regions in which it was evaluated. Furthermore, for a given dose, 11 C and 15 O beams produce much better quality images for range verification compared with 12 C and 16 O, in particular with regards to estimating the location of the Bragg peak.

Physics in Medicine & Biology

Positron emission tomography (PET) has been used for dose verification in charged particle therap... more Positron emission tomography (PET) has been used for dose verification in charged particle therapy. The causes of washout of positron emitters by physiological functions should be clarified for accurate dose verification. In this study, we visualized the distribution of irradiated radioactive beams,11C and15O beams, in the rabbit whole-body using our original depth-of-interaction (DOI)-PET prototype to add basic data for biological washout effect correction. We also collected expired gas of the rabbit during beam irradiation and the energy spectrum was measured with a germanium detector. Irradiated radioactive beams into the brain were distributed to the whole body due to the biological washout process, and the implanted11C and15O ions were concentrated in the regions which had high blood volume. The11C-labbled11CO2was detected in expired gas under the11C beam irradiation, while no significant signal was detected under the15O beam irradiation as a form of C15O2. Results suggested that the implanted11C ions form molecules that diffuse out to the whole body by undergoing perfusion, then, they are incorporated into the blood-gas exchange in the respiratory system. This study provides basic data for modelling of the biological washout effect.

Physics in Medicine & Biology

Page 4, figure 2 (top row) A white rectangular line in figure 2 erroneously surrounds the blue te... more Page 4, figure 2 (top row) A white rectangular line in figure 2 erroneously surrounds the blue text 'C-11, O-15'. Page 4, figure 3(a) A white rectangular line in figure 3(a) erroneously surrounds the text 'optical axis (distance = 44 cm)'.

Physics in Medicine & Biology

In advanced ion therapy, the visualization of the range of incident ions in a patient's body ... more In advanced ion therapy, the visualization of the range of incident ions in a patient's body is important for exploiting the advantages of this type of therapy. It is ideal to use radioactive ion beams for in-beam positron emission tomography (PET) imaging in particle therapy due to the high quality of PET images caused by the high signal-to-noise ratio. We have shown the feasibility of this idea through an in-beam PET study for 11C and 15O ion beams using the dedicated OpenPET system. In this work, we investigate the potential difference between the Bragg peak position and the position of the maximum detected positron-emitting fragments by a PET system for the radioactive beams of 11C and 15O. For this purpose, we measured the depth dose in a water phantom and performed PET scans of an irradiated PMMA phantom for the available beams of 11C and 15O at the Heavy Ion Medical Accelerator in Chiba (HIMAC). Then, we simulated the depth dose profiles in the water phantom and the yield of the positron-emitting fragments in a PMMA phantom for both available beams using the Monte Carlo code PHITS. The positions of the Bragg peak and maximum positron-emitting fragments from the measurements were well reproduced by simulation. The effect of beam energy broadening on the positional differences between two peaks was studied by simulating an irradiated PMMA phantom. The differences in position between the Bragg peak and the maximum positron-emitting fragments increased when the beam energy spread was broadened, although the differences were zero for the ideal mono-energetic beams. Greater differences were observed for 11C ion beams compared to 15O ion beams, although both beams had the same range in water, and the higher energy corresponded to a larger difference. For the known energy spread of the beams, the predicted differences between two peaks from the simulation were consistent with the measured data within submillimetre agreement.

Physics in Medicine and Biology

Heavy ion therapy is a promising cancer therapy technique due to the sharper Bragg peak and small... more Heavy ion therapy is a promising cancer therapy technique due to the sharper Bragg peak and smaller lateral scattering characteristics of heavy ion beams as compared to a proton therapy. Recently, the potential for radioactive ion beam therapy has been investigated in combination with the OpenPET system to improve the accuracy of in vivo beam range verification. However, the characteristics of the radioactive ion beams have not been investigated thoroughly. Optical imaging has been proposed as a novel high-resolution beam range estimation method for heavy ion beams. In this study, high-resolution luminescence imaging and Cerenkov light imaging were performed for the range estimation of radioactive ion beams such as 11C and 15O in the Heavy Ion Medical Accelerator in Chiba (HIMAC) secondary beam line. A polymethyl methacrylate (PMMA) phantom (10.0 × 10.0 × 9.9 cm3) was irradiated by 11C and 15O ion beams. In order to obtain the in-beam luminescence and off-line beam Cerenkov light images, an optical system was used that consisted of a lens and a cooled CCD camera. The Bragg peaks and stopping positions of the 11C and 15O ion beams could be visualized by using the luminescence and Cerenkov light imaging, respectively. The Bragg peaks showed a good correlation with the peak of the luminescence profile with a positional discrepancy of 1 mm and 0.4 mm for the 11C and 15O ion beams, respectively. In conclusion, optical imaging using luminescence and Cerenkov light could be used for the precise range estimation of radioactive ion beams.

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment

Abstract The quality and uniformity of positron emission tomography (PET) images can be improved ... more Abstract The quality and uniformity of positron emission tomography (PET) images can be improved significantly using a PET scanner, which has both time-of-flight (TOF) and depth-of-interaction (DOI) capabilities. We have already developed a DOI dual-end detector using segmented crystal bars by applying a subsurface laser engraving (SSLE) technique, and the DOI was determined by the ratio of the detected light between two readouts using the Anger calculation. In this study, we investigated the influence of the number of DOI segments on the performance of DOI identification and the coincidence timing performance of the detector. The detector consisted of a single lutetium fine silicate (LFS) crystal bar (3 × 3 × 20 mm3) with various numbers of DOI segments that were made by applying the SSLE technique and Hamamatsu silicon photomultiplier (SiPM) modules. The maximum number of DOI segments was six, and the SiPM module included one hundred forty-four 4 mm × 4 mm SiPM readouts. The coincidence resolving time (CRT) of each DOI segment was obtained from the side irradiation of the dual-end detector. All DOI segments of the detector with 2, 3, 4, 5 or 6 DOI segments were clearly identified, and average energy resolutions of 9.8 ± 0.5% and 12.5 ± 1.4% were obtained at the 511 keV photo peak for the detectors with 2 DOI and 6 DOI segments, respectively. The minimum and maximum estimated CRT of 180 ± 6 ps and 236 ± 6 ps were obtained for the detectors with 2 DOI and 6 DOI segments, respectively. Insignificant differences were observed between the CRT values of different segments of one detector. Greater CRT values were obtained for detectors with larger DOIs. The results of this study prove that there is a high potential for segmented crystal bars using the SSLE technique as a good candidate for PET scanners with TOF and DOI capabilities, which can significantly improve the quality of PET images.

Physics in Medicine & Biology

Depth of interaction (DOI) information is indispensable to improving the sensitivity and spatial ... more Depth of interaction (DOI) information is indispensable to improving the sensitivity and spatial resolution of positron emission tomography (PET) systems, especially for small field-of-view PET such as small animal PET and human brain PET. We have already developed a series of X'tal cube detectors for isotropic spatial resolution and we obtained the best isotropic resolution of 0.77 mm for detectors with six-sided readout. However, it is still challenging to apply the detector for PET systems due to the high cost of six-sided readout electronics and carrying out segmentation of a monolithic cubic scintillator in three dimensions using the subsurface laser engraving (SSLE) technique. In this work, we propose a more practical X'tal cube with a two-sided readout detector, which is made of crystal bars segmented in the height direction only by using the SSLE technique. We developed two types of prototype detectors with a 3 mm cubic segment and a 1.5 mm cubic segment by using 3 × 3 × 20 mm3 and 1.5 × 1.5 × 20 mm3 crystal bars segmented into 7 and 13 DOI segments, respectively, using the SSLE technique. First, the performance of the detector, composed of one crystal bar with different DOI segments and two thorough silicon via (TSV) multi-pixel photon counters (MPPCs) as readout at both ends of the crystal bar, were evaluated in order to demonstrate the capability of the segmented crystal bars as a DOI detector. Then, performance evaluation was carried out for a 4 × 4 crystal array of 3 × 3 × 20 mm3 with 7 DOI segments and an 8 × 8 crystal array of 1.5 × 1.5 × 20 mm3 with 13 DOI segments. Each readout included a 4 × 4 channel of the 3 × 3 mm2 active area of the TSV MPPCs. The three-dimensional position maps of the detectors were obtained by the Anger-type calculation. All the segments in the 4 × 4 array were identified very clearly when there was air between the crystal bars, as each crystal bar was coupled to one channel of the MPPCs; however, it was necessary to optimize optical conditions between crystal bars for the 8 × 8 array because of light sharing between crystal bars coupled to one channel of the MPPCs. The optimization was performed for the 8 × 8 array by inserting reflectors fully or partially between the crystal bars and the best crystal identification performance was obtained with the partial reflectors between the crystal bars. The mean energy resolutions at the 511 keV photo peak for the 4 × 4 array with air between the crystal bars and for the 8 × 8 array with partial reflectors between the crystal bars were 10.1% ± 0.3% and 10.8% ± 0.8%, respectively. Timing resolutions of 783 ± 36 ps and 1.14 ± 0.22 ns were obtained for the detectors composed of the 4 × 4 array and the 8 × 8 array with partial reflectors, respectively. These values correspond to single photon timing resolutions. Practical X'tal cubes with 3 mm and 1.5 mm DOI resolutions and two-sided readout were developed.

Physics in Medicine and Biology

Dose monitoring and range verification are important tools in carbon ion therapy. For their imple... more Dose monitoring and range verification are important tools in carbon ion therapy. For their implementation, positron emission tomography (PET) can be used to image the beta+\beta^{+}beta+-activation of tissue during treatment. Predictions of these beta+\beta^{+}beta+-activity distributions are usually obtained from Monte Carlo simulations, which demands high computational time and thus limits the applicability of this technique in clinical scenario. Nevertheless, it is desirable to explore faster approaches able to give such a prediction, since only its comparison with the measured distributions allows a definite assessment of potential range deviations from the planned treatment. For the first time, we present an approach to perform deconvolution from PET data in carbon ion therapy and reconstruct the dose. A filtering method is used to predict positron emitter profiles from dose profiles in short time. In order to reverse the convolution and estimate a dose distribution from a positron emitter distribution, we apply an evolutionary algorithm. Filters are obtained from either a library or are created in advance for a specific problem, assuming that a prediction of the positron emitter distribution is available. To perform the latter method and find the best filter for a specific problem, we use another evolutionary algorithm, hence optimizing the filter on-the-fly for the given treatment scheme. The application of our method is shown for dose and positron emitter distributions in homogeneous phantoms using simulated and newly measured online PET data. Carbon ion ranges can be predicted within 2 mm and the shape of the dose distribution is reconstructed with an overall promising agreement.

2017 IEEE Nuclear Science Symposium and Medical Imaging Conference (NSS/MIC)

The purpose of this study is to perform a first characterization and proof of principle investiga... more The purpose of this study is to perform a first characterization and proof of principle investigation of a Compton camera setup composed by a scatterer component consisting of a pixelated GAGG crystal read out by a SiPM multi-pixel photon counter (MPPC) and an absorber component consisting of a monolithic LaBrc(Ce) scintillator read out by a 256-fold multianode photomultiplier (PMT). The rationale of the study is to develop a Compton camera system as a future ion beam range verification device during particle therapy, via prompt gamma imaging. The properties to be investigated are the reconstruction efficiency and accuracy achievable with this system for detecting prompt-y rays. The Compton camera system described has been tested with a laboratory radioactive Cesium-137 source, in a certain geometrical configuration. The readout system is based on individual spectroscopy (NIM+VME) electronic modules, digitizing energy and time signals. The data have been analyzed to produce an input for the image reconstruction, performed using the MEGAlib toolkit software. I. In t r o d u c t i o n P ARTICLE beam therapy is nowadays considered an advantageous option in cancer treatment, but the beneficial high dose delivery precision on the other hand demands a high accuracy of the Bragg peak placement: different approaches for the beam range monitoring are worldwide being evaluated.

A combined technique of an imaging plate (IP) and thin absorbers was applied to tritium in nickel... more A combined technique of an imaging plate (IP) and thin absorbers was applied to tritium in nickel and vanadium specimens using copper, aluminum, and gold foil as the absorber. Copper and aluminum foil are used as a K-edge filter with X-ray absorption at 9.0 keV and 1.56 keV, respectively. Gold has L-edges X-ray absorption at around 13 keV. With this technique, photostimulated luminescence (PSL) decay curves are obtained by changing absorber’s thickness. In the nickel specimen, the difference in PSL decay curves between for the copper and gold absorber was clearly observed 20 days after loading, however, all curves became similar single pattern after 388 days. The same curve pattern was obtained in vanadium for all absorbers. The cross section images and depth profiles, which were taken at 468 days and 3.9 years after loading for the nickel and vanadium specimen, respectively, show no significant inclination of tritium concentration for both specimens. These results indicate that uni...

We are developing the OpenPET for in-beam imaging in carbon ion therapy. We have succeeded in sho... more We are developing the OpenPET for in-beam imaging in carbon ion therapy. We have succeeded in showing a proof-of-concept by developing small OpenPET prototypes that we used in the HIMAC. One of the major issues to realize the OpenPET is radiation damage to detectors because a number of fragmented particles are incident on the detectors located downstream from the target. While we are currently using photo-multiplier tubes in the OpenPET, some groups reported feasibility of use of a semiconductor photo-detector in hadron therapy. In this paper, therefore, we tested radiation hardness of the multi-pixel photon counter (MPPC) to show feasibility of its use in the OpenPET for carbon ion therapy. We used a single pixel MPPC (S10931-050) to evaluate radiation hardness. The experiment was performed in the PH2 course of the HIMAC. The energy of the 12C beam was 290MeV/u and beam intensity was 1.6 × 109 particles per second (pps) which was ten times higher than the typical clinical beam inte...

Time-of-fligfrt (TOF) PET improves image quality and quantitative accuracy compared with a conven... more Time-of-fligfrt (TOF) PET improves image quality and quantitative accuracy compared with a conventional PET system. While it is true that TOF works better for larger objects, recent improvement in timing resolution has encouraged application of TOF to brain-dedicated PET systems. Thus, as the second prototype of the helmet-type PET, we have developed a new TOF brain-dedicated PET prototype using detector modules of 12×12 lutetium fine silicate (LFS) crystals (4.1×4.1×10 mm3) connected to a 12×12 (4 mm pitch, 144-ch) MPPC array. In this paper, we investigated another detector module, which has the same outer size but smaller crystals: 16×16 LFS of 3.1×3.1×10 mm3 size coupled to 16×16 (3 mm pitch, 256-ch) MPPC array. The 3-mm 256-ch module showed an energy resolution of 12.0%. For the coincidence response function, the 3-mm module showed a better full width at half maximum (FWHM) of 1.9 mm compared with the 4-mm 144-ch module (2.4 mm). The FWHM was improved by 21%. The coincidence res...

Detectors with depth-of-interaction (DOI) information are utilized in positron emission tomograph... more Detectors with depth-of-interaction (DOI) information are utilized in positron emission tomography (PET) scanners to improve the sensitivity of radiation detection and the uniformity of spatial resolution. We recently developed a series of dual-ended detectors using crystal bars segmented by applying the subsurface laser engraving (SSLE) technique to 7, 13 and 20 DOI segments. It is crucial to achieve the submillimeter level spatial resolution for our detector as the narrow crystal bars are highly fragile after applying the SSLE technique. In this work, we focused on optimizing the surface area of the SSLE-induced layer to the narrow crystal bars in order to resolve the issue of fragility while maintaining sufficient crystal segment separation. The SSLE layers were induced to the middle of the cross section of the five crystal bars with dimensions of 1.5 × 1 . 5 × 20 mm3 with a distance of 0.1, 0.2, 0.3, 0.4 or 0.5 mm from the two opposite lateral edges of each crystal bar. All cry...

Physics in Medicine & Biology

The purpose of this work is to develop a validated Geant4 simulation model of a whole-body protot... more The purpose of this work is to develop a validated Geant4 simulation model of a whole-body prototype PET scanner constructed from the four-layer depth-of-interaction detectors developed at the National Institute of Radiological Sciences, National Institutes for Quantum and Radiological Science and Technology, Japan. The simulation model emulates the behaviour of the unique depth of interaction sensing capability of the scanner without needing to directly simulate optical photon transport in the scintillator and photodetector modules. The model was validated by evaluating and comparing performance metrics from the NEMA NU 2-2012 protocol on both the simulated and physical scanner, including spatial resolution, sensitivity, scatter fraction, noise equivalent count rates and image quality. The results show that the average sensitivities of the scanner in the field-of-view were 5.9 cps kBq−1 and 6.0 cps kBq−1 for experiment and simulation, respectively. The average spatial resolutions m...

2018 IEEE Nuclear Science Symposium and Medical Imaging Conference Proceedings (NSS/MIC)

In carbon ion therapy, visualization of the range of incident particles in a patient body is impo... more In carbon ion therapy, visualization of the range of incident particles in a patient body is important for treatment verification. In-beam positron emission tomography (PET) imaging in ion therapy is one of the method for verification of treatment due to the high quality of PET images. We have already shown the feasibility of radioactive beams of 15O and 11C using in-beam PET imaging using our OpenPET system. Recently, we have developed a whole gamma imaging (WGI) system which can work as PET, single gamma-ray and triple gamma-ray imaging simultaneously. The WGI system has high potential to detect location of 10C, which emits positron with a simultaneous gamma-ray of 718 keV, and activity of other produced positron emitting nuclei within patient body during ion therapy. In this work, we focus on investigation of performance of WGI system for 10C irradiation. First, the performance of scatterer detector of the WGI system regarding the radiation hardness was studied for 10C irradiation, then the performance of WGI was studied by simulation using the Geant4 code. The scatterer detector consisted of a segmented GAGG crystal and a multi-pixel photon counter (MPPC) and its performance was studied as a PMMA phantom was irradiated with 5 and 400 spills of 10C with energy of 350 MeV/u. No damage to the MPPC (scatterer detector) was observed even though after irradiation of 400 spills (~1.9×105 particle per spill). Sensitivity values of WGI system by simulation of a 10C nuclide at the center of field of view (FOV) for PET mode imaging, single gamma-ray of 718 keV imaging and triple gamma-ray imaging were 7.85%, 0.28% and 0.012% respectively. The performance of WGI system is going to be evaluated for 10C irradiation in the near future.

Physics in Medicine & Biology

Sensitivity and spatial resolution of positron emission tomography (PET) scanners can be improved... more Sensitivity and spatial resolution of positron emission tomography (PET) scanners can be improved by using thicker scintillation crystals with depth-of-interaction (DOI) encoding. Subsurface laser engraving (SSLE) can be used to segment crystals of a scintillation detector in order to fabricate a DOI detector. We previously applied SSLE to crystal bars of 3 × 3 × 20 mm3 and 1.5 × 1.5 × 20 mm3 and developed two dual-ended detectors with DOI segments of 3 mm and 1.5 mm, respectively. To further improve the DOI resolution, our SSLE detector design can be used with smaller pitch crystal bars, making them excellent detector candidates for small animal PET scanners with submillimetre resolution. In the present study, three small crystal bars of 1 × 1 × 20 mm3, 2 × 1 × 20 mm3, and 2 × 1 × 40 mm3 were laser engraved to 12, 20 and 40 segments, respectively, by applying SSLE in their height directions. The segmented crystal bars were characterised in three prototype detector arrangements. First, the 1 × 1 × 20 mm3 crystal bars were characterised in an 8 × 8 crystal array designed for DOI encoding along crystal height in a conventional small animal PET design. Second, a 4 × 8 crystal array of 2 × 1 × 20 mm3 crystal bars was characterised for using the DOI information for crystal interaction positioning along the axial axis of a small animal PET scanner. Finally, the third part of the study was performed on a single 2 × 1 × 40 mm3 crystal bar with 40 segments to investigate the feasibility of DOI estimation in longer crystals for application in a system with extended axial length. We evaluated the capability of segment identification and energy resolution of theses detectors. The 3D position maps of the detectors were obtained using the Anger-type calculation and the crystal identification performance was evaluated for each detector. Clear segment separation was obtained for the crystal arrays with 12 (segment pitch of 1.67 mm) and 20 (segment pitch of 1 mm) segments. Mean energy resolutions of 8.8% ± 0.4% and 9.6% ± 0.8% at 511 keV were obtained for the segments in the central regions of the 8 × 8 array with 12 segments and the 4 × 8 array with 20 segments, respectively. Clear segment identification was found to be difficult for the detector with 40 segments, especially for the segments at the middle of the crystal. Energy and interaction positioning characterisation results suggest that both prototype detectors with 12 and 20 segments are well suited for small animal PET scanners with high spatial resolution.

IEEE Transactions on Radiation and Plasma Medical Sciences

We developed a human-scale single-ring OpenPET (SROP) system, which had an open space allowing us... more We developed a human-scale single-ring OpenPET (SROP) system, which had an open space allowing us access to the subject during measurement. The SROP system consisted of 160 4-layer depth-of-interaction detectors. The open space with the axial width of 430 mm was achieved with the ring axial width of 214 mm and the ring inner diameter of 660 mm. The detectors were axially shifted to each other so that the detector ring was aligned along a plane horizontally tilted by 45° against the axial direction. The system was developed as a mobile scanner to be used not only in clinical PET rooms but also in charged-particle therapy treatment rooms as well as animal experiment rooms. Almost uniform spatial resolution better than 3 mm throughout the entire FOV was realized with an iterative image reconstruction method. Peak absolute sensitivity was 3.1%, and there was a region with sensitivity better than 0.8% for a length of more than 700 mm. An in-beam imaging experiment conducted at the HIMAC showed that the system was operable even at the highest beam intensity available for heavy-ion therapy. In addition, we conducted entirebody monkey dynamic imaging utilizing the long region inside the gantry by positioning a monkey along the direction having the longest FOV tilted by 45° against the axial direction. We concluded the developed system realizes versatile PET applications because of its wide-open space and mobility as well as high spatial resolution with sufficiently good sensitivity.

Physics in Medicine & Biology

This work presents an iterative method for the estimation of the absolute dose distribution in pa... more This work presents an iterative method for the estimation of the absolute dose distribution in patients undergoing carbon ion therapy, via analysis of the distribution of positron annihilations resulting from the decay of positron-emitting fragments created in the target volume. The proposed method relies on the decomposition of the total positron-annihilation distributions into profiles of the three principal positron-emitting fragment species - 11C, 10C and 15O. A library of basis functions is constructed by simulating a range of monoenergetic 12C ion irradiations of a homogeneous polymethyl methacrylate phantom and measuring the resulting one-dimensional positron-emitting fragment profiles and dose distributions. To estimate the dose delivered during an arbitrary polyenergetic irradiation, a linear combination of factors from the fragment profile library is iteratively fitted to the decomposed positron annihilation profile acquired during the irradiation, and the resulting weight...

Medical Physics

PURPOSE This work has two related objectives. The first is to estimate the relative biological ef... more PURPOSE This work has two related objectives. The first is to estimate the relative biological effectiveness of two radioactive heavy ion beams based on experimental measurements, and compare these to the relative biological effectiveness of corresponding stable isotopes to determine whether or not they are therapeutically equivalent. The second aim is to quantitatively compare the quality of images acquired post-irradiation using an in-beam whole-body positron emission tomography scanner for range verification quality assurance. METHODS The energy deposited by monoenergetic beams of 11 C at 350 MeV/u, 15 O at 250 MeV/u, 12 C at 350 MeV/u and 16 O at 430 MeV/u were measured using a cruciform transmission ionisation chamber in a water phantom at the Heavy Ion Medical Accelerator in Chiba (HIMAC), Japan. Dose-mean lineal energy was measured at various depths along the path of each beam in a water phantom using a silicon-on-insulator mushroom microdosimeter. Using the modi_ed microdosimetric kinetic model, the relative biological effectiveness at 10% survival fraction of the radioactive ion beams was evaluated and compared to that of the corresponding stable ions along the path of the beam. Finally, the post-irradiation distributions of positron annihilations resulting from the decay of positron-emitting nuclei were measured for each beam in a gelatin phantom using the in-beam whole-body positron emission tomography scanner at HIMAC. The depth of maximum positron-annihilation density was compared with the depth of maximum dose deposition and the signal-to-background ratios were calculated and compared for images acquired over 5 minutes and 20 minutes post irradiation of the phantom. RESULTS In the entrance region, the RBE10 was 1.2 ± 0.1 for both 11 C and 12 C beams, while for 15 O and 16 O it was 1.4 ± 0.1 and 1.3 ± 0.1, respectively. At the Bragg peak, the RBE10 was 2.7 ± 0.4 for 11 C and 2.9 ± 0.4 for 12 C, while for 15 O and 16 O it was 2.7 ± 0.4 and 2.8 ± 0.4, respectively. In the tail region, RBE10 could only be evaluated for carbon; the RBE10 was 1.6 ± 0.2 and 1.5 ± 0.1 for 11 C and 12 C, respectively. Positron emission tomography images obtained from gelatin targets irradiated by radioactive ion beams exhibit markedly improved signal-to-background ratios compared to those obtained from targets irradiated by non-radioactive ion beams, with 5-fold and 11-fold increases in the ratios calculated for the 15 O and 11 C images compared with the values obtained for 16 O and 12 C, respectively. The difference between the depth of maximum dose and the depth of maximum positron annihilation density is 2.4 ± 0.8 mm for 11 C, compared to -5.6 ± 0.8 mm for 12 C and 0.9 ± 0.8 mm for 15 O versus -6.6 ± 0.8 mm for 16 O. CONCLUSIONS The RBE10 values for 11 C and 15 O were found to be within the 95% confidence interval of the RBEs estimated for their corresponding stable isotopes across each of the regions in which it was evaluated. Furthermore, for a given dose, 11 C and 15 O beams produce much better quality images for range verification compared with 12 C and 16 O, in particular with regards to estimating the location of the Bragg peak.

Physics in Medicine & Biology

Positron emission tomography (PET) has been used for dose verification in charged particle therap... more Positron emission tomography (PET) has been used for dose verification in charged particle therapy. The causes of washout of positron emitters by physiological functions should be clarified for accurate dose verification. In this study, we visualized the distribution of irradiated radioactive beams,11C and15O beams, in the rabbit whole-body using our original depth-of-interaction (DOI)-PET prototype to add basic data for biological washout effect correction. We also collected expired gas of the rabbit during beam irradiation and the energy spectrum was measured with a germanium detector. Irradiated radioactive beams into the brain were distributed to the whole body due to the biological washout process, and the implanted11C and15O ions were concentrated in the regions which had high blood volume. The11C-labbled11CO2was detected in expired gas under the11C beam irradiation, while no significant signal was detected under the15O beam irradiation as a form of C15O2. Results suggested that the implanted11C ions form molecules that diffuse out to the whole body by undergoing perfusion, then, they are incorporated into the blood-gas exchange in the respiratory system. This study provides basic data for modelling of the biological washout effect.

Physics in Medicine & Biology

Page 4, figure 2 (top row) A white rectangular line in figure 2 erroneously surrounds the blue te... more Page 4, figure 2 (top row) A white rectangular line in figure 2 erroneously surrounds the blue text 'C-11, O-15'. Page 4, figure 3(a) A white rectangular line in figure 3(a) erroneously surrounds the text 'optical axis (distance = 44 cm)'.

Physics in Medicine & Biology

In advanced ion therapy, the visualization of the range of incident ions in a patient's body ... more In advanced ion therapy, the visualization of the range of incident ions in a patient's body is important for exploiting the advantages of this type of therapy. It is ideal to use radioactive ion beams for in-beam positron emission tomography (PET) imaging in particle therapy due to the high quality of PET images caused by the high signal-to-noise ratio. We have shown the feasibility of this idea through an in-beam PET study for 11C and 15O ion beams using the dedicated OpenPET system. In this work, we investigate the potential difference between the Bragg peak position and the position of the maximum detected positron-emitting fragments by a PET system for the radioactive beams of 11C and 15O. For this purpose, we measured the depth dose in a water phantom and performed PET scans of an irradiated PMMA phantom for the available beams of 11C and 15O at the Heavy Ion Medical Accelerator in Chiba (HIMAC). Then, we simulated the depth dose profiles in the water phantom and the yield of the positron-emitting fragments in a PMMA phantom for both available beams using the Monte Carlo code PHITS. The positions of the Bragg peak and maximum positron-emitting fragments from the measurements were well reproduced by simulation. The effect of beam energy broadening on the positional differences between two peaks was studied by simulating an irradiated PMMA phantom. The differences in position between the Bragg peak and the maximum positron-emitting fragments increased when the beam energy spread was broadened, although the differences were zero for the ideal mono-energetic beams. Greater differences were observed for 11C ion beams compared to 15O ion beams, although both beams had the same range in water, and the higher energy corresponded to a larger difference. For the known energy spread of the beams, the predicted differences between two peaks from the simulation were consistent with the measured data within submillimetre agreement.

Physics in Medicine and Biology

Heavy ion therapy is a promising cancer therapy technique due to the sharper Bragg peak and small... more Heavy ion therapy is a promising cancer therapy technique due to the sharper Bragg peak and smaller lateral scattering characteristics of heavy ion beams as compared to a proton therapy. Recently, the potential for radioactive ion beam therapy has been investigated in combination with the OpenPET system to improve the accuracy of in vivo beam range verification. However, the characteristics of the radioactive ion beams have not been investigated thoroughly. Optical imaging has been proposed as a novel high-resolution beam range estimation method for heavy ion beams. In this study, high-resolution luminescence imaging and Cerenkov light imaging were performed for the range estimation of radioactive ion beams such as 11C and 15O in the Heavy Ion Medical Accelerator in Chiba (HIMAC) secondary beam line. A polymethyl methacrylate (PMMA) phantom (10.0 × 10.0 × 9.9 cm3) was irradiated by 11C and 15O ion beams. In order to obtain the in-beam luminescence and off-line beam Cerenkov light images, an optical system was used that consisted of a lens and a cooled CCD camera. The Bragg peaks and stopping positions of the 11C and 15O ion beams could be visualized by using the luminescence and Cerenkov light imaging, respectively. The Bragg peaks showed a good correlation with the peak of the luminescence profile with a positional discrepancy of 1 mm and 0.4 mm for the 11C and 15O ion beams, respectively. In conclusion, optical imaging using luminescence and Cerenkov light could be used for the precise range estimation of radioactive ion beams.

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment

Abstract The quality and uniformity of positron emission tomography (PET) images can be improved ... more Abstract The quality and uniformity of positron emission tomography (PET) images can be improved significantly using a PET scanner, which has both time-of-flight (TOF) and depth-of-interaction (DOI) capabilities. We have already developed a DOI dual-end detector using segmented crystal bars by applying a subsurface laser engraving (SSLE) technique, and the DOI was determined by the ratio of the detected light between two readouts using the Anger calculation. In this study, we investigated the influence of the number of DOI segments on the performance of DOI identification and the coincidence timing performance of the detector. The detector consisted of a single lutetium fine silicate (LFS) crystal bar (3 × 3 × 20 mm3) with various numbers of DOI segments that were made by applying the SSLE technique and Hamamatsu silicon photomultiplier (SiPM) modules. The maximum number of DOI segments was six, and the SiPM module included one hundred forty-four 4 mm × 4 mm SiPM readouts. The coincidence resolving time (CRT) of each DOI segment was obtained from the side irradiation of the dual-end detector. All DOI segments of the detector with 2, 3, 4, 5 or 6 DOI segments were clearly identified, and average energy resolutions of 9.8 ± 0.5% and 12.5 ± 1.4% were obtained at the 511 keV photo peak for the detectors with 2 DOI and 6 DOI segments, respectively. The minimum and maximum estimated CRT of 180 ± 6 ps and 236 ± 6 ps were obtained for the detectors with 2 DOI and 6 DOI segments, respectively. Insignificant differences were observed between the CRT values of different segments of one detector. Greater CRT values were obtained for detectors with larger DOIs. The results of this study prove that there is a high potential for segmented crystal bars using the SSLE technique as a good candidate for PET scanners with TOF and DOI capabilities, which can significantly improve the quality of PET images.

Physics in Medicine & Biology

Depth of interaction (DOI) information is indispensable to improving the sensitivity and spatial ... more Depth of interaction (DOI) information is indispensable to improving the sensitivity and spatial resolution of positron emission tomography (PET) systems, especially for small field-of-view PET such as small animal PET and human brain PET. We have already developed a series of X'tal cube detectors for isotropic spatial resolution and we obtained the best isotropic resolution of 0.77 mm for detectors with six-sided readout. However, it is still challenging to apply the detector for PET systems due to the high cost of six-sided readout electronics and carrying out segmentation of a monolithic cubic scintillator in three dimensions using the subsurface laser engraving (SSLE) technique. In this work, we propose a more practical X'tal cube with a two-sided readout detector, which is made of crystal bars segmented in the height direction only by using the SSLE technique. We developed two types of prototype detectors with a 3 mm cubic segment and a 1.5 mm cubic segment by using 3 × 3 × 20 mm3 and 1.5 × 1.5 × 20 mm3 crystal bars segmented into 7 and 13 DOI segments, respectively, using the SSLE technique. First, the performance of the detector, composed of one crystal bar with different DOI segments and two thorough silicon via (TSV) multi-pixel photon counters (MPPCs) as readout at both ends of the crystal bar, were evaluated in order to demonstrate the capability of the segmented crystal bars as a DOI detector. Then, performance evaluation was carried out for a 4 × 4 crystal array of 3 × 3 × 20 mm3 with 7 DOI segments and an 8 × 8 crystal array of 1.5 × 1.5 × 20 mm3 with 13 DOI segments. Each readout included a 4 × 4 channel of the 3 × 3 mm2 active area of the TSV MPPCs. The three-dimensional position maps of the detectors were obtained by the Anger-type calculation. All the segments in the 4 × 4 array were identified very clearly when there was air between the crystal bars, as each crystal bar was coupled to one channel of the MPPCs; however, it was necessary to optimize optical conditions between crystal bars for the 8 × 8 array because of light sharing between crystal bars coupled to one channel of the MPPCs. The optimization was performed for the 8 × 8 array by inserting reflectors fully or partially between the crystal bars and the best crystal identification performance was obtained with the partial reflectors between the crystal bars. The mean energy resolutions at the 511 keV photo peak for the 4 × 4 array with air between the crystal bars and for the 8 × 8 array with partial reflectors between the crystal bars were 10.1% ± 0.3% and 10.8% ± 0.8%, respectively. Timing resolutions of 783 ± 36 ps and 1.14 ± 0.22 ns were obtained for the detectors composed of the 4 × 4 array and the 8 × 8 array with partial reflectors, respectively. These values correspond to single photon timing resolutions. Practical X'tal cubes with 3 mm and 1.5 mm DOI resolutions and two-sided readout were developed.

Physics in Medicine and Biology

Dose monitoring and range verification are important tools in carbon ion therapy. For their imple... more Dose monitoring and range verification are important tools in carbon ion therapy. For their implementation, positron emission tomography (PET) can be used to image the beta+\beta^{+}beta+-activation of tissue during treatment. Predictions of these beta+\beta^{+}beta+-activity distributions are usually obtained from Monte Carlo simulations, which demands high computational time and thus limits the applicability of this technique in clinical scenario. Nevertheless, it is desirable to explore faster approaches able to give such a prediction, since only its comparison with the measured distributions allows a definite assessment of potential range deviations from the planned treatment. For the first time, we present an approach to perform deconvolution from PET data in carbon ion therapy and reconstruct the dose. A filtering method is used to predict positron emitter profiles from dose profiles in short time. In order to reverse the convolution and estimate a dose distribution from a positron emitter distribution, we apply an evolutionary algorithm. Filters are obtained from either a library or are created in advance for a specific problem, assuming that a prediction of the positron emitter distribution is available. To perform the latter method and find the best filter for a specific problem, we use another evolutionary algorithm, hence optimizing the filter on-the-fly for the given treatment scheme. The application of our method is shown for dose and positron emitter distributions in homogeneous phantoms using simulated and newly measured online PET data. Carbon ion ranges can be predicted within 2 mm and the shape of the dose distribution is reconstructed with an overall promising agreement.