Remote Cherenkov imaging-based quality assurance of a magnetic resonance image-guided radiotherapy system

Time-gated single-pixel imaging of Cherenkov emission from a medical linear accelerator

Cherenkov imaging is an ideal tool for real-time in vivo verification of a radiation therapy dose. Given that radiation is pulsed from a medical linear accelerator (LINAC) together with weak Cherenkov emissions, time-gated high-sensitivity imaging is required for robust measurements. Instead of using an expensive camera system with limited efficiency of detection in each pixel, a single-pixel imaging (SPI) approach that maintains promising sensitivity over the entire spectral band could be used to provide a low-cost and viable alternative. A prototype SPI system was developed and demonstrated here in Cherenkov imaging of LINAC dose delivery to a water tank. Validation experiments were performed using four regular fields and an intensity-modulated radiotherapy (IMRT) delivery plan. The Cherenkov image-based projection percent depth dose curves (pPDDs) were compared to pPDDs simulated by the treatment planning system (TPS), with an overall average error of 0.48, 0.42, 0.65, and 1.08% for the 3, 5, 7, and 9 cm square beams, respectively. The composite image of the IMRT plan achieved a 85.9% pass rate using 3%/3 mm gamma index criteria, in comparing Cherenkov intensity and TPS dose. This study validates the feasibility of applying SPI to the Cherenkov imaging of radiotherapy dose for the first time to our knowledge.

Characterization of Cherenkov imaging parameters and positional constraints on an O-ring linear accelerator

Objective. With the introduction of Cherenkov imaging technology on the Halcyon O-ring linear accelerator platform, we seek to demonstrate the imaging feasibility and optimize camera placement.Approach. Imaging parameters were probed by acquiring triggering data Cherenkov image frames for simplistic beams on the Halcyon and comparing the analyzed metrics with those from the TrueBeam platform. Camera position was analyzed by performing 3D rendering of patient treatment plans for various sites and iterating over camera positions to assess treatment area visibility.Main results. Commercial Cherenkov imaging systems are compatible with the pulse timing of the Halcyon, and this platform design favorably impacts signal to noise in Cherenkov image frames. Additionally, ideal camera placement is treatment site dependent and is always within a biconical zone of visibility centered on the isocenter. Visibility data is provided for four treatment sites, with suggestions for camera placement based on room dimensions. Median visibility values were highest for right breast plans, with values of 80.33% and 68.49% for the front and rear views respectively. Head and neck plans presented with the lowest values at 26.44% and 38.18% respectively.Significance. This work presents the first formal camera positional analysis for Cherenkov imaging on any platform and serves as a template for performing similar work for other irradiation platforms. Additionally, this study confirms the Cherenkov imaging parameters do not need to be changed for optimal imaging on the Halcyon. Lastly, the presented methodology provides a framework which could be further expanded to other optical imaging systems which rely on line of sight visibility to the patient.

Application of Cherenkov radiation in tumor imaging and treatment

Cherenkov radiation (CR) is the characteristic blue glow that is generated during radiotherapy or radioisotope decay. Its distribution and intensity naturally reflect the actual dose and field of radiotherapy and the location of radioisotope imaging agents in vivo. Therefore, CR can represent a potential in situ light source for radiotherapy monitoring and radioisotope-based tumor imaging. When used in combination with new imaging techniques, molecular probes or nanomedicine, CR imaging exhibits unique advantages (accuracy, low cost, convenience and fast) in tumor radiotherapy monitoring and imaging. Furthermore, photosensitive nanomaterials can be used for CR photodynamic therapy, providing new approaches for integrating tumor imaging and treatment. Here the authors review the latest developments in the use of CR in tumor research and discuss current challenges and new directions for future studies.

Direct in-water radiation dose measurements using Cherenkov emission corrected signals from polarization imaging for a clinical radiotherapy application

Cherenkov emission (CE) is a visible blueish light emitted in water mediums irradiated by most radiotherapy treatment beams. However, CE is produced anisotropically which currently imposes a geometrical constraint uncertainty for dose measurements. In this work, polarization imaging is proposed and described as a method enabling precise 2D dose measurements using CE. CE produced in a water tank is imaged from four polarization angles using a camera coupled to a rotating polarizer. Using Malus’ law, the polarized component of CE is isolated and corrected with Monte Carlo calculated CE polar and azimuthal angular distributions. Projected dose measurements resulting from polarization-corrected CE are compared to equivalent radiochromic film measurements. Overall, agreement between polarized corrected CE signal and films measurements is found to be within 3%, for projected percent depth dose (PPDD) and profiles at the different tested energies (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\gamma$$\end{document}γ: 6 and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$18\,\hbox {MV}$$\end{document}18MV, e\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{-}$$\end{document}-: 6 and 18\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\,\hbox {MeV}$$\end{document}MeV). In comparison, raw Cherenkov emission presented deviations up 60% for electron beam PPDDs and 20% for photon beams PPDDs. Finally, a degree of linear polarization between 29% and 47% was measured for CE in comparison to \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$0.2\pm 0.3$$\end{document}0.2±0.3% for scintillation. Hence, polarization imaging is found to be a promising and powerful method for improved radio-luminescent dose measurements with possible extensions to signal separation.

Optical emission-based phantom to verify coincidence of radiotherapy and imaging isocenters on an MR-linac

Purpose

Demonstrate a novel phantom design using a remote camera imaging method capable of concurrently measuring the position of the x‐ray isocenter and the magnetic resonance imaging (MRI) isocenter on an MR‐linac.

Methods

A conical frustum with distinct geometric features was machined out of plastic. The phantom was submerged in a small water tank, and aligned using room lasers on a MRIdian MR‐linac (ViewRay Inc., Cleveland, OH). The phantom physical isocenter was visualized in the MR images and related to the DICOM coordinate isocenter. To view the x‐ray isocenter, an intensified CMOS camera system (DoseOptics LLC., Hanover, NH) was placed at the foot of the treatment couch, and centered such that the optical axis of the camera was coincident with the central axis of the treatment bore. Two or four 8.3mm x 24.1cm beams irradiated the phantom from cardinal directions, producing an optical ring on the conical surface of the phantom. The diameter of the ring, measured at the peak intensity, was compared to the known diameter at the position of irradiation to determine the Z‐direction offset of the beam. A star‐shot method was employed on the front face of the frustum to determine X‐Y alignment of the MV beam. Known shifts were applied to the phantom to establish the sensitivity of the method.

Results

Couch translations, demonstrative of possible isocenter misalignments, on the order of 1mm were detectable for both the radiotherapy and MRI isocenters. Data acquired on the MR‐linac demonstrated an average error of 0.28mm(N=10, R²=0.997, σ=0.37mm) in established Z displacement, and 0.10mm(N=5, σ=0.34mm) in XY directions of the radiotherapy isocenter.

Conclusions

The phantom was capable of measuring both the MRI and radiotherapy treatment isocenters. This method has the potential to be of use in MR‐linac commissioning, and could be streamlined to be valuable in daily constancy checks of isocenter coincidence.

Visual Isocenter Position Enhanced Review (VIPER): a Cherenkov imaging-based solution for MR-linac daily QA

PURPOSE

This study demonstrates a robust Cherenkov imaging-based solution to MR-Linac daily QA, including mechanical-imaging-radiation isocenter coincidence verification.

METHODS

A fully enclosed acrylic cylindrical phantom was designed to be mountable to the existing jig, indexable to the treatment couch. An ABS plastic conical structure was fixed inside the phantom, held in place with 3D-printed spacers, and filled with water allowing for high edge contrast on MR imaging scans. Both a star shot plan and a four-angle sheet beam plan were delivered to the phantom; the former allowed for radiation isocenter localization in the x-z plane (A/P and L/R directions) relative to physical landmarks on the phantom, and the latter allowed for the longitudinal position of the sheet beam to be encoded as a ring of Cherenkov radiation emitted from the phantom, allowing for isocenter localization on the y-axis (S/I directions). A custom software application was developed to perform near-real-time analysis of the data by any clinical user.

RESULTS

Calibration procedures show that linearity between longitudinal position and optical ring diameter is high (R²  > 0.99), and that RMSE is low (0.184 mm). The star shot analysis showed a minimum circle radius of 0.34 mm. The final isocenter coincidence measurements in the lateral, longitudinal, and vertical directions were -0.61 mm, 0.55 mm, and -0.14 mm, respectively, and the total 3D distance coincidence was 0.83 mm, with each of these being below 2 mm tolerance.

CONCLUSION

This novel system provided an efficient, MR safe, all-in-one method for acquisition and near-real-time analysis of isocenter coincidence data. This represents a direct measurement of the 3D isocentricity. The combination of this phantom and the custom analysis application makes this solution readily clinically deployable after the longitudinal analysis of performance consistency.

Head and neck radiotherapy on the MR linac: a multicenter planning challenge amongst MRIdian platform users

Purpose

Purpose of this study is to evaluate plan quality on the MRIdian (Viewray Inc., Oakwood Village, OH, USA) system for head and neck cancer (HNC) through comparison of planning approaches of several centers.

Methods

A total of 14 planners using the MRIdian planning system participated in this treatment challenge, centrally organized by ViewRay, for one contoured case of oropharyngeal carcinoma with standard constraints for organs at risk (OAR). Homogeneity, conformity, sparing of OARs, and other parameters were evaluated according to The International Commission on Radiation Units and Measurements (ICRU) recommendations anonymously, and then compared between centers. Differences amongst centers were assessed by means of Wilcoxon test. Each plan had to fulfil hard constraints based on dose–volume histogram (DVH) parameters and delivery time. A plan quality metric (PQM) was evaluated. The PQM was defined as the sum of 16 submetrics characterizing different DVH goals.

Results

For most dose parameters the median score of all centers was higher than the threshold that results in an ideal score. Six participants achieved the maximum number of points for the OAR dose parameters, and none had an unacceptable performance on any of the metrics. Each planner was able to achieve all the requirements except for one which exceeded delivery time. The number of segments correlated to improved PQM and inversely correlated to brainstem D_0.1cc and to Planning Target Volume1 (PTV) D_0.1cc. Total planning experience inversely correlated to spinal canal dose.

Conclusion

Magnetic Resonance Image (MRI) linac-based planning for HNC is already feasible with good quality. Generally, an increased number of segments and increasing planning experience are able to provide better results regarding planning quality without significantly prolonging overall treatment time.

Supplementary Information

The online version of this article (10.1007/s00066-021-01771-8) contains supplementary material, which is available to authorized users.

In-line MRI-LINAC depth dose measurements using an in-house plastic scintillation dosimeter

Plastic scintillation dosimeters (PSDs) have many properties that make them desirable for relative dosimetry with MRI-LINACs. An in-house PSD, Farmer ionisation chamber and Gafchromic EBT3 film were used to measure central axis percentage depth dose distributions (PDDs) at the Australian MRI-LINAC Mean errors were calculated between each detector's responses, where the in-house PSD was on average within 0.7% of the Farmer chamber and 1.4% of film, while the Farmer chamber and film were on average within 1.1% of each other. However, the PSD systematically over-estimated the dose as depth increased, approaching a maximum overestimation of the order of 3.5% for the smallest field size measured. This trend was statistically insignificant for all other field sizes measured; further investigation is required to determine the source of this effect. The calculated values of mean absolute error are comparable to the those of trusted dosimeters reported in the literature. These mean absolute errors, and the ubiquity of desirable dosimetric qualities inherent to PSDs suggest that PSDs in general are accurate for relative dosimetry with the MRI-LINAC. Further investigation is required into the source of the reported systematic trends dependent on field-size and depth of measurement.

Detective quantum efficiency of intensified CMOS cameras for Cherenkov imaging in radiotherapy

In this study the metric of detective quantum efficiency (DQE) was applied to Cherenkov imaging systems for the first time, and results were compared for different detector hardware, gain levels and with imaging processing for noise suppression. Intensified complementary metal oxide semiconductor cameras using different image intensifier designs (Gen3 and Gen2+) were used to image Cherenkov emission from a tissue phantom in order to measure the modulation transfer function (MTF) and noise power spectrum (NPS) of the systems. These parameters were used to calculate the DQE for varying acquisition settings and image processing steps. MTF curves indicated that the Gen3 system had superior contrast transfer and spatial resolution than the Gen2+ system, with [Formula: see text] values of 0.52 mm-1 and 0.31 mm-1, respectively. With median filtering for noise suppression, these values decreased to 0.50 mm-1 and 0.26 mm-1. The maximum NPS values for the Gen3 and Gen2+ systems at high gain were 1.3 × 106 mm2 and 9.1 × 104 mm2 respectively, representing a 14x decrease in noise power for the Gen2+ system. Both systems exhibited increased NPS intensity with increasing gain, while median filtering lowered the NPS. The DQE of each system increased with increasing gain, and at the maximum gain levels the Gen3 system had a low-frequency DQE of 0.31%, while the Gen2+ system had a value of 1.44%. However, at a higher frequency of 0.4 mm-1, these values became 0.54% and 0.03%. Filtering improved DQE for the Gen3 system and reduced DQE for the Gen2+ system and had a mix of detrimental and beneficial qualitative effects by decreasing the spatial resolution and sharpness but also substantially lowering noise. This methodology for DQE measurement allowed for quantitative comparison between Cherenkov imaging cameras and improvements to their sensitivity, and yielded the first formal assessment of Cherenkov image formation efficiency.

Intelligent IoT-based large-scale inverse planning system considering postmodulation factors

The model and algorithm of intensity-modulated radiotherapy (IMRT) are updated increasingly quickly, but the hardware upgrade of primary hospitals often lags behind. The new generation of intelligent precise radiotherapy platforms provides users with intelligent medical consortium services using big data, artificial intelligence and industrial Internet of Things technology. This technology can ensure that under the real-time guidance of a professional medical consortium, primary hospitals can realize rapid large-scale reverse planning design and can more accurately consider many factors of postprocessing. Although large-scale healthcare systems, such as volumetric-modulated arc therapy and other accurate radiotherapy technologies, have developed rapidly, the development of step-and-shoot-mode IMRT technology is still very important for developing countries. For software, in addition to the conformity of the dose distribution, the modulation speed, convenience and stability of the later dose delivery should also be considered in inverse planning. Therefore, this paper analyzes the main problems in conventional IMRT inverse planning, including the smoothing of the fluence map, the selection of the gantry angle and the dose leakage of tongue–groove effects. To address these issues, a novel Intelligent IoT-based large-scale inverse planning strategy with the key factors of the postmodulation is developed, and a detailed flow chart is also provided. The scheme consists of two steps. The first step is to obtain a relatively optimal combination of gantry angles by considering the dose distribution requirements and constraints and the modulation requirements and constraints. The second step is to optimize the intensity map, to smooth the map based on prior knowledge according to the determined angles, and to obtain the final modulation scheme according to the relevant objectives and constraints of the map decomposition (leaf sequencing). In an experiment, we calculate and validate the clinical head and neck case. Because of the special gantry angle selection, the angle combination is optimized from the initial equivalent distribution to adapt to the target area and protect the nontarget area. The value of the objective function varies greatly after the optimization, especially in the target area, and the target value decreases by approximately 10%. On this basis, we smooth the fluence map by a partial differential equation with prior knowledge and a minimization of the total number of monitor units. It is also shown from the objective function value that the target value is essentially unchanged for the target area, while for the nontarget area, the value decreases by 16%, which is very impressive.

Scintillation imaging as a high‐resolution, remote, versatile 2D detection system for MR‐linac quality assurance

PURPOSE

To demonstrate the potential benefits of remote camera-based scintillation imaging for routine quality assurance (QA) measurements for magnetic resonance guided radiotherapy (MRgRT) linear accelerators.

METHODS

A wall-mounted CMOS camera with a time-synchronized intensifier was used to image photons produced from a scintillation screen in response to dose deposition from a 6 MV FFF x-ray beam produced by a 0.35 T MR-linac. The oblique angle of the field of view was corrected using a projective transform from a checkerboard calibration target. Output sensitivity and constancy was measured using the scintillator and benchmarked against an A28 ion chamber. Field cross-plane and in-plane profiles were measured for field sizes ranging from 1.68 × 1.66 cm² to 20.02 × 19.92 cm² with both scintillation imaging and using an IC profiler. Multileaf collimator (MLC) shifts were introduced to test sensitivity of the scintillation imaging system to small spatial deviations. A picket fence test and star-shot were delivered to both the scintillator and EBT3 film to compare accuracy in measuring MLC positions and isocenter size.

RESULTS

The scintillation imaging system showed comparable sensitivity and linearity to the ion chamber in response to changes in machine output down to 0.5 MU (R²  = 0.99). Cross-plane profiles show strong agreement with defined field sizes using full width half maximum (FWHM) measurement of <2 mm for field sizes below 15 cm, but the oblique viewing angle was the limiting factor in accuracy of in-plane profile widths. However, the system provided high-resolution profiles in both directions for constancy measurements. Small shifts in the field position down to 0.5 mm were detectable with <0.1 mm accuracy. Multileaf collimator positions as measured with both scintillation imaging and EBT3 film were measured within ± 1 mm tolerance and both detection systems produced similar isocenter sizes from the star-shot analysis (0.81 and 0.83 mm radii).

CONCLUSIONS

Remote scintillation imaging of a two-dimensional screen provided a rapid, versatile, MR-compatible solution to many routine quality assurance procedures including output constancy, profile flatness and symmetry constancy, MLC position verification and isocenter size. This method is high-resolution, does not require post-irradiation readout, and provides simple, instantaneous data acquisition. Full automation of the readout and processing could make this a very simple but effective QA tool, and is adaptable to all medical accelerators.

Medical physics challenges in clinical MR-guided radiotherapy

The integration of magnetic resonance imaging (MRI) for guidance in external beam radiotherapy has faced significant research and development efforts in recent years. The current availability of linear accelerators with an embedded MRI unit, providing volumetric imaging at excellent soft tissue contrast, is expected to provide novel possibilities in the implementation of image-guided adaptive radiotherapy (IGART) protocols. This study reviews open medical physics issues in MR-guided radiotherapy (MRgRT) implementation, with a focus on current approaches and on the potential for innovation in IGART.Daily imaging in MRgRT provides the ability to visualize the static anatomy, to capture internal tumor motion and to extract quantitative image features for treatment verification and monitoring. Those capabilities enable the use of treatment adaptation, with potential benefits in terms of personalized medicine. The use of online MRI requires dedicated efforts to perform accurate dose measurements and calculations, due to the presence of magnetic fields. Likewise, MRgRT requires dedicated quality assurance (QA) protocols for safe clinical implementation.Reaction to anatomical changes in MRgRT, as visualized on daily images, demands for treatment adaptation concepts, with stringent requirements in terms of fast and accurate validation before the treatment fraction can be delivered. This entails specific challenges in terms of treatment workflow optimization, QA, and verification of the expected delivered dose while the patient is in treatment position. Those challenges require specialized medical physics developments towards the aim of fully exploiting MRI capabilities. Conversely, the use of MRgRT allows for higher confidence in tumor targeting and organs-at-risk (OAR) sparing.The systematic use of MRgRT brings the possibility of leveraging IGART methods for the optimization of tumor targeting and quantitative treatment verification. Although several challenges exist, the intrinsic benefits of MRgRT will provide a deeper understanding of dose delivery effects on an individual basis, with the potential for further treatment personalization.

Optical imaging method to quantify spatial dose variation due to the electron return effect in an MR-linac

PURPOSE

Treatment planning systems (TPSs) for MR-linacs must employ Monte Carlo-based simulations of dose deposition to model the effects of the primary magnetic field on dose. However, the accuracy of these simulations, especially for areas of tissue-air interfaces where the electron return effect (ERE) is expected, is difficult to validate due to physical constraints and magnetic field compatibility of available detectors. This study employs a novel dosimetric method based on remotely captured, real-time optical Cherenkov and scintillation imaging to visualize and quantify the ERE.

METHODS

An intensified CMOS camera was used to image two phantoms with designed ERE cavities. Phantom A was a 40 cm × 10 cm × 10 cm clear acrylic block drilled with five holes of increasing diameters (0.5, 1, 2, 3, 4 cm). Phantom B was a clear acrylic block (25 cm × 20 cm × 5 cm) with three cavities of increasing diameter (3, 2, 1 cm) split into two halves in the transverse plane to accommodate radiochromic film. Both phantoms were imaged while being irradiated by 6 MV flattening filter free (FFF) beams within a MRIdian Viewray (Viewray, Cleveland, OH) MR-linac (0.34 T primary field). Phantom A was imaged while being irradiated by 6 MV FFF beams on a conventional linac (TrueBeam, Varian Medical Systems, San Jose, CA) to serve as a control. Images were post processed in Matlab (Mathworks Inc., Natick, MA) and compared to TPS dose volumes.

RESULTS

Control imaging of Phantom A without the presence of a magnetic field supports the validity of the optical image data to a depth of 6 cm. In the presence of the magnetic field, the optical data shows deviations from the commissioned TPS dose in both intensity and localization. The largest air cavity examined (3 cm) indicated the largest dose differences, which were above 20% at some locations. Experiments with Phantom B illustrated similar agreement between optical and film dosimetry comparisons with TPS data in areas not affected by ERE.

CONCLUSION

There are some appreciable differences in dose intensity and spatial dose distribution observed between the novel experimental data set and the dose models produced by the current clinically implemented MR-IGRT TPS.

Optical imaging provides rapid verification of static small beams, radiosurgery, and VMAT plans with millimeter resolution

PURPOSE

We demonstrate the feasibility of optical imaging as a quality assurance tool for static small beamlets, and pretreatment verification tool for radiosurgery and volumetric-modulated arc therapy (VMAT) plans.

METHODS

Small static beams and clinical VMAT plans were simulated in a treatment planning system (TPS) and delivered to a cylindrical tank filled with water-based liquid scintillator. Emission was imaged using a blue-sensitive, intensified CMOS camera time-gated to the linac pulses. For static beams, percentage depth and cross beam profiles of projected intensity distribution were compared to TPS data. Two-dimensional (2D) gamma analysis was performed on all clinical plans, and the technique was tested for sensitivity against common errors (multileaf collimator position, gantry angle) by inducing deliberate errors in the VMAT plans control points. The technique's detection limits for spatial resolution and the smallest number of control points that could be imaged reliably were also tested. The sensitivity to common delivery errors was also compared against a commercial 2.5D diode array dosimeter.

RESULTS

A spatial resolution of 1 mm was achieved with our imaging setup. The optical projected percentage depth intensity profiles agreed to within 2% relative to the TPS data for small static square beams (5, 10, and 50 mm² ). For projected cross beam profiles, a gamma pass rate >99% was achieved for a 3%/1 mm criteria. All clinical plans passed the 3%/3 mm criteria with >95% passing rate. A static 5 mm beam with 20 Monitor Units could be measured with an average percent difference of 5.5 ± 3% relative to the TPS. The technique was sensitive to multileaf collimator errors down to 1 mm and gantry angle errors of 1°.

CONCLUSIONS

Optical imaging provides ample spatial resolution for imaging small beams. The ability to faithfully image down to 20 MU of 5 mm, 6 MV beamlets prove the ability to perform quality assurance for each control point within dynamic plans. The technique is sensitive to small offset errors in gantry angles and multileaf collimator (MLC) leaf positions, and at certain scenario, it exhibits higher sensitivity than a commercial 2.5D diode array.

Space-variant deconvolution of Cerenkov light images acquired from a curved surface

PURPOSE

Cerenkov photons are generated by high-energy radiation used in external beam radiation therapy (EBRT). This study expands upon the Cerenkov light dosimetry formula previously developed to relate an image of Cerenkov photons to the primary beam fluence. Extension of this formulation allows for deconvolution to be performed on images acquired from curved geometries.

METHODS

The integral equation, which represented the formation of Cerenkov photon image from an incident high-energy photon beam, was expanded to allow for space-variance of the convolution kernel called as the Cerenkov scatter function (CSF). The GAMOS (Geant4-based Architecture for Medicine-Oriented Simulations) Monte Carlo (MC) particle simulation software was used to obtain the CSF for different incident beam angles. The image of a curved surface was first projected to a flat plane by using a perspective correction method. Then, the planar image was partitioned into small segments (or blocks), where a CSF corresponding to a specific beam incident angle was applied for deconvolution. The block size and the margin around the block were optimized by studying the effects of those parameters on the deconvolution accuracy for a test image. We evaluated three deconvolution techniques: Richardson-Lucy, Blind, and Total Variation minimization (TV/L2) algorithms, to select the most accurate method for the current applications.

RESULTS

Analysis of deconvolution algorithms showed that the TV/L2 method provided the most accurate solution to the deconvolution problem for Cerenkov imaging. Optimization of space-variant deconvolution parameters showed that including a margin that is at least 42.9% of the image width provided the most accurate product image. There was no optimal size for the deconvolution area and should be chosen based on the presence of unique CSF kernels within an image. Space-variant deconvolution improved measured field size in Cerenkov photon images by 7.37%, as compared with 1.74% by space-invariant deconvolution. Space-variant deconvolution improved measured penumbra by 99.3%, as compared with 76.7% by space-invariant deconvolution. Space-variant deconvolution introduced artifacts in flat regions of the beam. Artifacts were avoided through selective space-variant deconvolution in only the penumbra region.

CONCLUSIONS

Primary photon fluence distributions of a curved surface can be obtained by using space-variant deconvolution methods in Cerenkov light dosimetry. The TV/L2 algorithm is the best method for deconvolution of Cerenkov photon images from an open-field beam derived from either a flat or curved surface. The partition size chosen for space-variant deconvolution should be at least six times the full width at half maximum (FWHM) of the corresponding scatter kernel used in deconvolution. Space-variant deconvolution is necessary if the incident beam angle difference is larger than 6 ∘ between regions of an image.

Cherenkov-excited luminescence scanned imaging using scanned beam differencing and iterative deconvolution in dynamic plan radiation delivery in a human breast phantom geometry

PURPOSE

The purpose of this study was to demonstrate high resolution optical luminescence sensing, referred to as Cherenkov excited luminescence scanning imaging (CELSI), could be achieved during a standard dynamic treatment plan for a whole breast radiotherapy geometry.

METHODS

The treatment plan beams induce Cherenkov light within tissue, and this excitation projects through the beam trajectory across the medium, inducing luminescence where there can be molecular reporter. Broad beams generally produce higher signal but low spatial resolution, yet for dynamic plans the scanning of the multileaf collimator allows for a beam-narrowing strategy by recursively temporal differencing each of the Cherenkov images and associated luminescence images. Then reconstruction from each of these size-reduced beamlets defined by the differenced Cherenkov images provides a well-conditioned matrix inversion, where the spatial frequencies are limited by the higher signal-to-noise ratio beamlets. A built-in stepwise convergence relies on stepwise beam size reduction, which is associated with a widening of the bandwidth of Cherenkov spatial frequency and resultant increase in spatial resolution. For the phantom experiments, europium nanoparticles were used as luminescent probes and embedded at depths ranging from 3 to 8 mm. An intensity modulated radiotherapy (IMRT) plan was used to test this.

RESULTS

The Cherenkov images spatially guided where the luminescence was measured from, providing high lateral resolution, and iterative reconstruction convergence showed that optimization of the initial and stopping beamlet widths could be achieved with 15 and 4.5 mm, respectively, using a luminescence imaging frame rate of 5/s. With the IMRT breast plan, the original lateral resolution was improved 2X, that is, 0.08-0.24 mm for target depths of 3-8 mm. In comparison, a dynamic wedge (DW) plan showed an inferior image fidelity, with relative contrast recovery decreasing from 0.86 to 0.79. The methodology was applied to a three-dimensional dataset to reconstruct Cherenkov excited luminescence intensity distributions showing volumetric recovery of a 0.5 mm diameter object composed of 0.5 μM luminescent microbeads.

CONCLUSIONS

High resolution CELSI was achieved with a clinical breast external beam radiotherapy (EBRT) plan. It is anticipated that this method can allow visualization and localization for luminescence/fluorescence tagged vasculature, lymph nodes, or superficial tagged regions with most dynamic treatment plans.

Cherenkov imaging for linac beam shape analysis as a remote electronic quality assessment verification tool

PURPOSE

A remote imaging system tracking Cherenkov emission was analyzed to verify that the linear accelerator (linac) beam shape could be quantitatively measured at the irradiation surface for Quality Audit (QA).

METHODS

The Cherenkov camera recorded 2D dose images delivered on a solid acrylonitrile butadiene styrene (ABS) plastic phantom surface for a range of square beam sizes, and 6 MV photons. Imaging was done at source to surface distance (SSD) of 100 cm and compared to GaF film images and linac light fields of the same beam sizes, ranging over 5 × 5 cm² up to 20 × 20 cm² . Line profiles of each field were compared in both X and Y jaw directions. Each measurement was repeated on two different Clinac2100 machines. An interreader comparison of the beam width interpretation was completed using procedures commonly employed for beam to light field coincidence verification. Cherenkov measurements are also done for beams of complex treatment plan and isocenter QA.

RESULTS

The Cherenkov image widths matched with the measured GaF images and light field images, with accuracy in the range of ±1 mm standard deviation. The differences between the measurements were minor and within tolerance of geometrical requirement of standard linac QA procedures conducted by human setup verification, which had a similar error range. The measurement made by the remote imaging system allowed for beam shape extraction of radiation fields at the SSD location of the beam.

CONCLUSIONS

The proposed Cherenkov image acquisition system provides a valid way to remotely confirm radiation field sizes and provides similar information to that obtained from the linac light field or GaF film estimates of the beam size. The major benefit of this approach is that with a fixed installation of the camera, testing could be done completely under software control with automated image analysis, potentially simplifying conventional QA procedures with appropriate calibration of boundary definitions, and the natural extension to capturing dynamic treatment beamlets at SSD could have future value, such as verification of beam plans with complex beam shapes, like IMRT or "star-shot" QA for the isocenter.