Measurement of radiation isocenter is a fundamental part of commissioning and quality assurance (QA) for a linear accelerator (linac). In this work we present an automated procedure for the analysis of the stars-shots employed in the radiation isocenter determination. Once the star-shot film has been developed and digitized, the resulting image is analyzed by scanning concentric circles centered around the intersection of the lasers that had been previously marked on the film. The center and the radius of the minimum circle intersecting the central rays are determined with an accuracy and precision better than 1% of the pixel size. The procedure is applied to the position and size determination of the radiation isocenter by means of the analysis of star-shots, placed in different planes with respect to the gantry, couch and collimator rotation axes.

In this paper, the authors review the field of parallax error (PE) minimization in positron emission tomography (PET) imaging systems by using depth of interaction (DOI) capable concepts. The review includes apparatus as well as an overview of various methods described in the literature. It also discusses potential advantages gained via these approaches, as discussed with reference to various metrics and tasks, particularly in the improvement of spatial resolution (SR) performance. Furthermore, the authors emphasize limitations encountered in the context of DOI decoding, which can be a considerable pitfall depending on the task of interest.

A new X-ray imaging detector based on a 2D-THCOBRA micropatterned structure using a simple position readout is proposed. It consists of a hybrid device that combines the properties of a THGEM and a 2D-MHSP in a single structure, having two charge multiplication stages reaching the demanded gains for the use of charge division readout methods.For position determination, the new 10×10 cm 2 2D-THCOBRA structure uses two orthogonal resistive lines located at the end of the anode and top electrodes. The charge signal pulses collected at the end of each resistive line are digitized and processed in order to determine the center-ofgravity of the electron avalanche distribution.The detector uses a preamplification stage performed by a THGEM, followed by the 2D-THCOBRA, being operated in Ne/5%CH 4 at atmospheric pressure. A characterization of the detector in terms of charge gain was made as a function of the voltage applied to the electrodes of the 2D-THCOBRA structure. The energy resolution of the system was also measured, envisaging Energy Weighting Technique (EWT) applications.The system characterization in terms of spatial resolution is presented together with imaging examples to evaluate its performance in X-ray imaging applications.

The Silicon Photomultiplier (SiPM) generates noise due to thermal excitation, producing dark counts. This may be a critical performance drawback of the SiPM for low light level applications, despite its otherwise promising features such as high internal gain and quantum efficiency, low power consumption and insensitivity to magnetic fields. The dark count rate is highly dependent on temperature and bias voltage, increasing with both. Dark count pulses are similar to single photon interactions and introduce complex challenges in low light level measurements. In applications where the pulse integration time is dependent on the decay time of a slow scintillation counter, the longer the signal integration the higher the probability of dark counts during the integration period, and this can be critical for detecting very low number of photons in that period. For higher light levels, the effects of dark counts can be reduced by setting an appropriate threshold. This work presents Monte Carlo simulation techniques to evaluate the impact of dark counts into position detection algorithms, in low light level imaging applications where multiple SiPMs are used to detect position of interaction.

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