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Image guidance using in-beam real-time magnetic resonance (MR) imaging is expected to improve the targeting accuracy of proton therapy for moving tumors, by reducing treatment margins, detecting interfractional and intrafractional anatomical changes and enabling beam gating. The aim of this study is to quantitatively characterize the static magnetic field and image quality of a 0.22T open MR scanner that has been integrated with a static proton research beamline. The magnetic field and image quality studies are performed using high-precision magnetometry and standardized diagnostic image quality assessment protocols, respectively. The magnetic field homogeneity was found to be typical of the scanner used (98ppm). Operation of the beamline magnets changed the central resonance frequency and magnetic field homogeneity by a maximum of 16Hz and 3ppm, respectively. It was shown that the in-beam MR scanner features sufficient image quality and influences of simultaneous irradiation on the images are restricted to a small sequence-dependent image translation (0.1–0.7mm) and a minor reduction in signal-to-noise ratio (1.3%–5.6%). Nevertheless, specific measures have to be taken to minimize these effects in order to achieve accurate and reproducible imaging which is required for a future clinical application of MR integrated proton therapy.

IEEE transactions on radiation and plasma medical sciences : ITRPFI 5(5), 619-629 (2021). doi:10.1109/TRPMS.2020.3030483, Published by IEEE, New York, NY

Monolithic crystals are examined as an alternative to segmented scintillator arrays in positron emission tomography (PET). Monoliths provide good energy, timing and spatial resolution including intrinsic depth of interaction (DOI) encoding. DOI allows reducing parallax errors (radial astigmatism) at off-center positions within a PET ring. We present a novel DOI-estimation approach based on the supervised machine learning algorithm gradient tree boosting (GTB). GTB builds predictive regression models based on sequential binary comparisons (decision trees). GTB models have been shown to be implementable in FPGA if the memory requirement fits the available resources. We propose two optimization scenarios for the best possible positioning performance: One restricting the available memory to enable a future FPGA implementation and one without any restrictions. The positioning performance of the GTB models is compared with a DOI estimation method based on a single DOI observable (SO) comparable to other methods presented in literature. For a 12 mm high monolith, we achieve an averaged spatial resolution of 2.15 mm and 2.12 mm FWHM for SO and GTB models, respectively. In contrast to SO models, GTB models show a nearly uniform positioning performance over the whole crystal depth., The accepted version of the article; DOI included; added one citation; results kept unchanged

For a first characterization, we used the two KETEK-PM3325-WB SiPMs each equipped with a 3x3x5 mm${}^3$ LYSO scintillation crystal provided with the PETsys TOFPET2 ASIC Evaluation Kit. We changed the lower of two discriminator thresholds (D_T1) in the timing branch from vth_t1 = 5 - 30. The overvoltage was varied in a range of 1.25 V - 7.25 V. The ambient temperature was kept at 16{\deg}C. For all measurements, we performed an energy calibration including a correction for saturation. We evaluated the energy resolution, the coincidence resolving time (CRT) and the coincidence rate. At an overvoltage of 6 V, we obtained an energy resolution of about 10% FWHM, a CRT of approximately 210 ps FWHM and 400 ps FWTM, the coincidence rate showed only small variations of about 5%. To investigate the influence of the ambient temperature, it was varied between 12{\deg}C - 20{\deg}C. At 12{\deg}C and an overvoltage of 6.5 V, a CRT of approx. 195 ps FWHM and an energy resolution of about 9.5% FWHM could be measured. Observed satellite peaks in the time difference spectra were investigated in more detail. We could show that the location of the satellite peaks is correlated with a programmable delay element in the trigger circuit., Comment: This paper is under review with IEEE TRPMS. It has been presented in a talk at the PSMR 2018. This version of the manuscript was submitted as revision 2 to TRPMS after incrporating the comments of the reviewers. Only minor textchanges were made. Results, values and figures did not change

IEEE transactions on radiation and plasma medical sciences : ITRPFI 2(5), 411-421 (2018). doi:10.1109/TRPMS.2018.2837738, Published by IEEE, New York, NY