Objective.Interventional radiology procedures can expose patients and physicians to high doses of radiation. Virtual fluoroscopy based on fan-beams helps to reduce this level of irradiation. Here, we demonstrate the benefits of using fan-beams obtained by curved-slit collimation rather than the straight-slit collimation previously proposed.Approach.We study spiral-slits that allow for a localization procedure identical to that with straight-slits. We show that the localization error depends on the shape of the slit and we bound this error. This bound can be used to optimize the slit geometry according to different performance criteria.Main results.Several curved-slit geometries are studied, and their relative performance is evaluated through numerical experiments and preliminary tests on x-ray equipment. These experiments show a systematic improvement in performance with the proposed curved-slit geometries compared with the straight-slit. In particular, a curved-slit geometry with a linear spiral part and an exponential part reduces localization error and solves the problem of the blind central zone with the straight-slit geometry.Significance.We have developed a framework for analyzing the amplification of localization errors in a rotating collimator with symmetrical spiral-slits, as well as criteria for measuring this amplification and bounds indicating their optimality. This framework can be reused to identify or compare slits under conditions of use different from those in our study.

Synchrotron microbeam radiotherapy (MRT), which has entered the clinical transfer phase, requires the development of appropriate quality assurance (QA) tools due to very high dose rates and spatial hyperfractionation. A microstrip plastic scintillating detector system with associated modules was proposed in the context of real-time MRT QA. A prototype of such a system with 105 scintillating microstrips was developed and tested under MRT conditions. The signal obtained from each microstrip when irradiated was reproducible, linear with the dose, and independent of both the dose rate and the beam energy. The detector prototype was capable of measuring an entire 52-microbeam field in real time and exhibited outstanding radiation hardness. It could withstand more than 100 kGy absorbed dose, which is at least ten times higher than the doses reported in the literature for plastic scintillators before deterioration. The potential of this detector system in MRT QA was demonstrated in this study.

To meet the needs of modelling small-sized detectors of orthovoltage X-rays, for spatially fractionated radiotherapy, we propose a model to determine responses of the detectors in cases of highly heterogeneous cavity. The model was applied to a detector whose cavity features a sparse high-Z perovskite scintillation layer embedded in an epoxy resin. Irradiations were carried out using the Small Animal Radiation Research Platform. Local photon and electron spectra were computed by the SpekPy toolbox and Penelope Monte Carlo simulations, respectively. The detector model considers that X-ray photons mainly interact with medium surrounding the scintillation layer, and that the output of the detector mainly results from secondary electrons in this layer. Results from model computations are compared with measurement data, showing differences less than 16% for beam energies in the 60 kVp -220 kVp range.

The integration of Single-Photon Avalanche Diodes (SPADs) in CMOS Fully Depleted Silicon-On-Insulator (FD-SOI) technology under a buried oxide (BOX) layer and a silicon film containing transistors makes it possible to realize a 3D SPAD at the chip level. In our study, a nanostructurated layer created by an optimized arrangement of Shallow Trench Isolation (STI) above the photosensitive zone generates constructive interferences and consequently an increase in the light sensitivity in the frontside illumination. A simulation methodology is presented that couples electrical and optical data in order to optimize the STI trenches (size and period) and to estimate the Photon Detection Probability (PDP) gain. Then, a test chip was designed, manufactured, and characterized, demonstrating the PDP improvement due to the STI nanostructuring while maintaining a comparable Dark Count Rate (DCR).

We present the development study of a reliable and low-power actuator for microfluidics-based active glasses. The adaptive part of the lens implements two liquids of a specific refractive index separated by a thin membrane, the modification of their relative volumes allowing adaptive optical power corrections. The proposed actuator is connected to an adaptive lens by microchannels since it is intended to be installed in the temple of the glasses. The actuation is based on the electrostatic displacement of a thin film, which changes the relative volumes of two cavities filled with these liquids. The metalized film is placed slack with an "S-shape" between two electrodes biased with the actuation voltage. Very compact actuator prototypes have been developed and characterized. Power corrections ranging from +0D to +3D can be achieved via liquid volume displacement as low as 120 µL and with a power consumption of a few mW. The prototypes show good reliability without any significant change in their operation after more than 1 million actuations. For RD purposes, we have replaced some electrodes with transparent windows. With this setup, we show experimental results on the interplay between the performances and the film folding inside the actuator.

Objective . Patient-specific Quality Assurance (QA) measurements are of key importance in radiotherapy for safe and efficient treatment delivery and allow early detection of clinically relevant errors. Such QA processes remain challenging to implement for complex Intensity Modulated Radiation Therapy (IMRT) radiotherapy fields delivered using a multileaf collimator (MLC) which often feature small open segments and raise QA issues similar to those encountered in small field dosimetry. Recently, detectors based on long scintillating fibers have been proposed to measure a few parallel projections of the irradiation field with good performance for small field dosimetry. The purpose of this work is to develop and validate a novel approach to reconstruct MLC-shaped small irradiation fields from six projections. Approach . The proposed field reconstruction method uses a limited number of geometric parameters to model the irradiation field. These parameters are iteratively estimated with a steepest descent algorithm. The reconstruction method was first validated on simulated data. Real data were measured with a water-equivalent slab phantom equipped with a detector made of 6 scintillating-fiber ribbons placed at 1 m from the source. A radiochromic film was used to acquire a reference measurement of a first dose distribution in the slab phantom at the same source-to-detector distance and the treatment planning system (TPS) provided the reference for another dose distribution. In addition, simulated errors introduced on the delivered dose, field location and field shape were used to evaluate the ability of the proposed method to efficiently identify a deviation between the planned and delivered treatments. Main results . For a first small IMRT segment, 3%/3 mm, 2%/2 mm and 2%/1 mm gamma analysis conducted between the reconstructed dose distribution and the dose measured with radiochromic film exhibited pass rates of 100%, 99.9% and 95.7%, respectively. For a second and smaller IMRT segment, the same gamma analysis performed between the reconstructed dose distribution and the reference provided by the TPS showed pass rates of 100%, 99.4% and 92.6% for the 3%/3 mm, 2%/2 mm and 2%/1 mm gamma criteria, respectively. Gamma analysis of the simulated treatment delivery errors showed the ability of the reconstruction algorithm to detect a 3% deviation between the planned and delivered doses, as well as shifts lower than 7 mm and 3 mm when considering an individual leaf and a whole field shift, respectively. Significance . The proposed method allows accurate tomographic reconstruction of IMRT segments by processing projections measured with six scintillating-fiber ribbons and is suitable for water-equivalent real-time small IMRT segments QA.

AbstractPurposeTo develop a novel instrument for real‐time quality assurance (QA) procedures in radiotherapy. The system implements a scintillation‐based phantom and associated signal acquisition and processing modules and aims to monitor two‐dimensional (2D) dose distributions of small fields.Materials and methodsFor the proposed phantom, we have designed and realized a prototype implementing six high‐resolution tissue‐equivalent scintillating fiber ribbons stacked with in‐plane 30° rotated orientations from each other. Each ribbon output is coupled to a silicon photodiode linear array (with an element pitch of 400 μm) to detect scintillating signal, which represents the projected irradiation profile perpendicular to the ribbon's orientation. For the system providing six acquired projected dose profiles at different orientations, we have developed a two‐step signal processing method to perform 2D dose reconstruction. The first step is to determine irradiation field geometry parameters using a tomographic geometry approach, and the second one is to perform specific penumbra estimation. The QA system prototype has been tested on a Novalis TrueBeam STX with a 6‐MV photon beam for small elliptic fields defined by 5‐ and 10‐mm cone collimators and for 10 × 10‐ and 20 × 10‐mm2 rectangular fields defined by the micro‐multileaf collimator. Gamma index analysis using EBT3 films as reference has been carried out with tight 2%‐dose‐difference (DD)/700‐μm‐distance‐to‐agreement (DTA) as well as 1%‐DD/1‐mm‐DTA criteria for evaluating the system performances. The testing also includes an evaluation of the proposed two‐step field reconstruction method in comparison with two conventional methods: filtered back projection (FBP) and simultaneous iterative reconstruction technique (SIRT).ResultsThe reconstructed 2D dose distributions have gamma index pass rates higher than 95% for all the tested configurations as compared with EBT3 film measurements with both 2%‐DD/700‐μm‐DTA and 1%‐DD/1‐mm criteria. 2D global gamma analysis shows that the two‐step and FBP radiation field reconstruction methods systematically outperform the SIRT approach. Moreover, higher gamma index success rates are obtained with the two‐step method than with FBP in the case of the fields defined with the stereotactic cones.ConclusionsThe proposed small‐field QA system makes a use of six water‐equivalent scintillating detectors (fiber ribbons) to acquire dose distribution. The developed two‐step signal processing method performs tomographic 2D dose reconstruction. A system prototype has been built and tested using hospital facilities with small rectangular and elliptic fields. Testing results show 2D reconstructed dose distributions with high accuracy and resolution. Such a system could potentially be an alternative approach to film dosimetry for small‐field QA, which is still widely used as reference in clinical practice.

This article presents a study of Single Photon Avalanche Diodes (SPAD) implemented in 28 nm Fully Depleted Silicon-On-Insulator (FD-SOI) CMOS technology based on transient TCAD simulations and Dark Count Rate (DCR) measurements. The integration of SPAD in this technology is currently being studied. This work allows for a better understanding of the mechanism behind the quite high DCR measured at relative low excess bias voltages with the initial FD-SOI SPAD design (asymptotic to 500 Hz/mu m(2) at 5% excess bias voltage). In this study, a TCAD transient simulation methodology is introduced to better understand SPAD behavior during the avalanche process. TCAD simulations revealed that Shallow Trench Isolation (STI) structures within the active area have a negative effect on avalanche quenching, because of slower carrier evacuation with possible occurrence of secondary avalanches in series. Based on this analysis and on previous optimization works, we propose a new architecture of the FD-SOI SPAD combining several modifications to achieve a lower DCR (asymptotic to 20 Hz/mu m(2) at 5% excess bias voltage measured with passive quenching).

This article presents a study of Single Photon Avalanche Diodes (SPAD) implemented in 28nm Fully Depleted Silicon-On-Insulator (FD-SOI) CMOS technology based on transient TCAD simulations. The integration of SPAD in this technology is currently being studied. This work allows for a better understanding of the mechanism behind the quite high Dark Count Rate (DCR) measured at relative low excess bias voltages with the previous FD-SOI SPAD design. In this study, TCAD transient simulation methodology is introduced to better understand SPAD behavior during the avalanche process. TCAD simulations revealed that Shallow Trench Isolation (STI) structures in the active area have a negative effect on avalanche quenching, because of slower carrier evacuation with possible occurrence of secondary avalanches in series. Based on this analysis, we propose a new SPAD architecture to achieve a lower DCR.

A physically-based model of the Buried Multiple Junction (BMJ) detector has been established with consideration of the device operation in reach-through (RT) conditions. In such a condition, the breakdown voltage of one junction can shift depending on its adjacent junction;s bias voltage. The modeling approach consists in detecting RT conditions, determining depletion limits, electrostatic potential barrier height and thermionic emission current. It allows computations of the detector;s dark currents and spectral responses to monochromatic light illumination. Computations of the detector;s static characteristics have been compared with TCAD simulations and they are in good agreement, which is a model validation. The proposed model explains and determines the detector;s breakdown voltage shift behavior, as well as its changes of spectral responses depending on bias voltages.

Quality assurance of small dose fields shaped with a multileaf collimator is crucial in x-ray radiotherapy. Tissue-equivalent scintillating fibers (SciFi) can measure a sinogram of the dose field at the detector plane sampled with a few angles and open up the possibility of real-time quality assurance. We investigate the use of such detectors for estimating dose fields obtained with multileaf collimators (MLC). We propose a forward model of the tomographic projection based on geometrical parameters of the MLC and the beam dose. A least squares minimization is then used to identify these parameters. We validate our approach on real data acquired with a prototype SciFi detector measuring six projections of a field generated with a 6 MeV linear accelerator and shaped by nine pairs of leaves. We observe a good fit with the reference gafchromic film.

Purpose or Objective Beam characterization during commissioning and periodic QA verifications for SRS treatment equipment relies nowadays on time consuming procedures. To address this problem, we propose the use of the SciFi detector, a real-time 2D detector (initially developed for the LHCb experiment at CERN) for commissionning and QA of SRS systems.

Purpose For determining small‐field profile and output factor during stereotactic radiotherapy quality assurance (QA) procedures, we propose a novel system based on the scintillating fiber (SciFi) detector with output image acquisition and processing to allow real‐time monitoring of profile and output factor. Materials and methods The employed detector is a SciFi detector made of tissue‐equivalent scintillating plastic fibers arranged in 6‐layer fiber ribbons with a fiber pitch of 275 μm in each layer. The scintillating signal at the detector output is acquired by a sCMOS (scientific complementary metal–oxide–semiconductor) camera and represents the projected field profile along the fibers axis. An iterative reconstruction method of the field from its projected profile based on a priori knowledge of some features of the radiation field defined by the stereotactic cones is suggested. The detector with implemented data processing has been tested in clinical conditions, for determining beam profiles and output factors, using cone collimators of different sizes from 4 to 15 mm diameter. The detector under test was placed at 1.4 cm depth and 98.6 cm source to surface distance (SSD) in a water‐equivalent phantom and irradiated by a 6 MV photon beam. Results The reconstructed field profiles obtained from the detector are coherent with data from EBT3 radiochromic films, with differences within ±0.32 mm for both the FWHM and the penumbra region. For real‐time determination of the field output factor, the measured data are also in good agreement with data independently determined by the French Institute for Radiological Protection and Nuclear Safety (IRSN) based on radiochromic films and thermoluminescent 1 × 1 mm 2 micro‐cubes dosimeters (TLD). The differences are within ±1.6% for all the tested cone sizes. Conclusions We propose and have tested a SciFi plastic scintillating detector with an optimized signal processing method to characterize small fields defined by cone collimators. It allows the determination of key field parameters such as full width at half maximum (FWHM) and field output factors. The results are consistent with those independently measured using TLD and radiochromic films. As the SciFi detector does not require a correction factor, it is in line with the International Atomic Energy Agency (IAEA) and the American Association of Physicists in Medicine (AAPM) TRS‐483 recommendations, and can be suitable for online QA of small radiation fields used in photon beam radiotherapy, and is compatible with MRI‐LINAC.

We propose an analytical model for the CMOS Buried Multiple Junction (BMJ) detector exhibiting breakdown voltage shift depending on adjacent junction's bias. The device's singular behavior has been observed when two adjacent junctions are in reach-through (RT) condition. The breakdown current has been identified to be predominated by thermionic emission. The proposed model determines, for a given BMJ structure with uniform or Gaussian doping distributions under bias conditions, whether two adjacent junctions are in RT condition. In this case, it calculates the merged depletion limits, electric field and electrostatic potential profile. The potential barrier height of each merged depletion region can then be extracted and the thermionic current be computed. Model computations have been compared with TCAD simulations and measurements on the BMJ detector. Good agreements have been observed for different structures in different bias conditions at different temperatures.

This article presents an optimization of Single Photon Avalanche Diodes (SPAD) implemented in CMOS 28nm Fully Depleted Silicon-On-Insulator technology. With the standard process and design rules, first attempt of SPAD cells exhibited high Dark Count Rate (DCR) at low excess voltage, attributed mainly to band-to-band mechanism associated with field-enhanced trap assisted tunneling effects. In this study, we propose the modification of the diode junction profile to increase the breakdown voltage, as well as the modification of the SPAD architecture (Shallow Trench Isolation, STI layout). The obtained results with the optimized SPAD confirm significant lower DCR achievement allowing higher excess bias voltages.

This article focusses on Single Photon Avalanche Diodes (SPAD) integrated in CMOS UTBB FDSOI (Ultra-Thin Body and Box Fully Depleted Silicon-On-Insulator technology), as an original approach for natively 3D SPAD pixels. In parallel to the optimization of the SPAD performances, some design issues relative to body-biasing effects are discussed in this paper. The associated electronics placed on top of the SPAD is constrained: the well layer below the box must be a P-type. Thus, only regular threshold voltage NMOS and low threshold voltage PMOS transistors can be used. The SPAD avalanche events affect the electronics through body-biasing effects, which can be advantageously exploited for an indirect sensing of the SPAD activity. Two simple indirect sensing cells are then studied. Firstly, a voltage divider realized with two transistors in series (PFET and NFET operating as active resistances) is simulated and measured to demonstrate its ability to detect avalanches. Secondly, an even simpler cell is studied, as it consists of only one NMOS transistor configured as an equivalent capacitive bridge (gate and box capacitances). Finally, the advantages and drawbacks from a design point of view are addressed.