Petr Bruza

Cherenkov imaging in the potential roles of radiotherapy QA and delivery

Cherenkov emission has a direct proportionality to the deposited dose at the local level, and capture of these emitted light signals allows visualization of real time maps of dose in vivo. Mapping the Cherenkov signals through water tanks illustrates how 3D Cherenkov can be achieved, either as 2D plus time, or 3D in static imaging. Imaging Cherenkov from patients shows how signals can be acquired which map out radiation dose in real time. The signals are affected by several factors, each of which will take some calibration to resolve, yet intrinsically the signal is shown to be a linear reporter of dose delivered. Development of calibration methodologies is ongoing in both research and development work.

TU-AB-BRA-12: Quality Assurance of An Integrated Magnetic Resonance Image Guided Adaptive Radiotherapy Machine Using Cherenkov Imaging

PURPOSE To investigate the viability of using Cherenkov imaging as a fast and robust method for quality assurance tests in the presence of a magnetic field, where other instruments can be limited. METHODS Water tank measurements were acquired from a clinically utilized adaptive magnetic resonance image guided radiation therapy (MR-IGRT) machine with three multileaf-collimator equipped 60Co sources. Cherenkov imaging used an intensified charge coupled device (ICCD) camera placed 3.5m from the treatment isocenter, looking down the bore of the 0.35T MRI into a water tank. Images were post-processed to make quantitative comparison between Cherenkov light intensity with both film and treatment planning system predictions, in terms of percent depth dose curves as well as lateral beam profile measurements. A TG-119 commissioning test plan (C4: C-Shape) was imaged in real-time at 6.33 frames per second to investigate the temporal and spatial resolution of the Cherenkov imaging technique. RESULTS A .33mm/pixel Cherenkov image resolution was achieved across 1024×1024 pixels in this setup. Analysis of the Cherenkov image of a 10.5×10.5cm treatment beam in the water tank successfully measured the beam width at the depth of maximum dose within 1.2% of the film measurement at the same point. The percent depth dose curve for the same beam was on average within 2% of ionization chamber measurements for corresponding depths between 3-100mm. Cherenkov video of the TG-119 test plan provided qualitative agreement with the treatment planning system dose predictions, and a novel temporal verification of the treatment. CONCLUSIONS Cherenkov imaging was successfully used to make QA measurements of percent depth dose curves and cross beam profiles of MRI-IGRT radiotherapy machines after only several seconds of beam-on time and data capture; both curves were extracted from the same data set. Video-rate imaging of a dynamic treatment plan provided new information regarding temporal dose deposition. This study has been funded by NIH grants R21EB17559 and R01CA109558, as well as Norris Cotton Cancer Center Pilot funding.

Time-gated scintillator imaging for real-time optical surface dosimetry in total skin electron therapy

The purpose of this study was to measure surface dose by remote time-gated imaging of plastic scintillators. A novel technique for time-gated, intensified camera imaging of scintillator emission was demonstrated, and key parameters influencing the signal were analyzed, including distance, angle and thickness. A set of scintillator samples was calibrated by using thermo-luminescence detector response as reference. Examples of use in total skin electron therapy are described. The data showed excellent room light rejection (signal-to-noise ratio of scintillation SNR  ≈  470), ideal scintillation dose response linearity, and 2% dose rate error. Individual sample scintillation response varied by 7% due to sample preparation. Inverse square distance dependence correction and lens throughput error (8% per meter) correction were needed. At scintillator-to-source angle and observation angle  <50°, the radiant energy fluence error was smaller than 1%. The achieved standard error of the scintillator cumulative dose measurement compared to the TLD dose was 5%. The results from this proof-of-concept study documented the first use of small scintillator targets for remote surface dosimetry in ambient room lighting. The measured dose accuracy renders our method to be comparable to thermo-luminescent detector dosimetry, with the ultimate realization of accuracy likely to be better than shown here. Once optimized, this approach to remote dosimetry may substantially reduce the time and effort required for surface dosimetry.

Cherenkov imaging for Total Skin Electron Therapy (TSET)

Total Skin Electron Therapy (TSET) utilizes high-energy electrons to treat cancers on the entire body surface. The otherwise invisible radiation beam can be observed via the optical Cherenkov photons emitted from interaction between the high-energy electron beam and tissue. Using a specialized camera-system, the Cherenkov emission can thus be used to evaluate the dose uniformity on the surface of the patient in real-time. Each patient was also monitored during TSET via in-vivo detectors (IVD) in nine locations. Patients undergoing TSET in various conditions (whole body and half body) were imaged and analyzed, and the viability of the system to provide clinical feedback was established.

Structural Cherenkov luminescence imaging with Hadamard-patterned field illumination (Conference Presentation)

Cherenkov-excited luminescence scanned imaging (CELSI) has been proposed for radiation-dose determination in medical physics due to its high spatial-resolution over centimeters of tissue. However, dense line-scanning illumination in typical CELSI is time-cost owing to the mechanical movement of the leaves in multi leaf collimator (MLC), resulting into increased radiation exposure. As a result, a scanningless Cherenkov luminescence imaging modality is herein proposed through structuring epi-illumination with MLC-based Hadamard-patterns, which utilizes a reduced radiation does by limiting illumination patterns, extremely shortening the sampling process. In order to effectively reconstruct unknowns from the resultant underdetermined linear system with sparse samplings, a compressed sensing-based reconstruction methodology with l1-norm regularization is adopted. Numerical and phantom experiments show that the proposed methodology achieves the same image quality as the traditional CELSI does.

Algorithm development for intrafraction radiotherapy beam edge verification from Cherenkov imaging

Abstract. Imaging of Cherenkov light emission from patient tissue during fractionated radiotherapy has been shown to be a possible way to visualize beam delivery in real time. If this tool is advanced as a delivery verification methodology, then a sequence of image processing steps must be established to maximize accurate recovery of beam edges. This was analyzed and developed here, focusing on the noise characteristics and representative images from both phantoms and patients undergoing whole breast radiotherapy. The processing included temporally integrating video data into a single, composite summary image at each control point. Each image stack was also median filtered for denoising and ultimately thresholded into a binary image, and morphologic small hole removal was used. These processed images were used for day-to-day comparison computation, and either the Dice coefficient or the mean distance to conformity values can be used to analyze them. Systematic position shifts of the phantom up to 5 mm approached the observed variation values of the patient data. This processing algorithm can be used to analyze the variations seen in patients being treated concurrently with daily Cherenkov imaging to quantify the day-to-day disparities in delivery as a quality audit system for position/beam verification.

Cherenkov-excited luminescence sheet imaging (CELSI) tomographic reconstruction

A tomographic reconstruction algorithm for Cherenkov-excited luminescence scanned imaging (CELSI) is proposed and demonstrated for the first time, to reconstruct distributions of luminescent source. Coupled continuous wave (CW) diffusion equations are used to model luminescent photon propagation in biological tissues. The CELSI reconstruction was achieved by minimizing the difference between measured and computed data based on Tikhonov regularization technique. The feasibility and effectiveness of the algorithm were tested with numerical simulations on noisy data. In addition, comparisons between conventional diffuse optical fluorescence tomography (DOFT) and CELSI were also performed. Contrast-detail analysis was also used to evaluate the imaging performance of CELSI.

TH‐AB‐209‐04: 3D Light Sheet Luminescence Imaging with Cherenkov Radiation

PURPOSE To recover a three-dimensional density distribution of luminescent molecular probes located several centimeters deep within a highly scattering tissue. METHODS We developed a novel sheet beam Cherenkov-excited luminescence scanned imaging (CELSI) methodology. The sample was irradiated by a horizontally oriented, vertically scanned 6 MV X-ray sheet beam (200mm × 5mm, 0.2mm vertical step) from a radiotherapy linear accelerator. The resulting Cherenkov light emission - and thus luminescent probe excitation - occurred exclusively along the irradiation plane due to a short diffusion path of secondary particles and Cherenkov photons. Cherenkov-excited luminescence was detected orthogonally to the sheet beam by gated, intensified charge coupled device camera. Analogously to light sheet microscopy, a series of luminescence images was taken for varied axial positions (depths) of the Cherenkov light sheet in sample. Knowledge of the excitation plane position allowed a 3D image stack deconvolution and depth-variant attenuation correction. The 3D image post-processing yielded a true spatial density distribution of luminescent molecules in highly scattering tissue. RESULTS We recovered a three-dimensional shape and position of 400 µL lesion-mimicking phantom tubes containing 25 µM solution of PtG4 molecular probe from 3 centimeter deep tissue-like media. The high sensitivity of CELSI also allowed resolving 100 micron capillaries of test solution. Functional information of partial oxygen pressure at the site of PtG4 molecular probe was recovered from luminescence lifetime CELSI. Finally, in-vivo sheet beam CELSI localized milimeter-sized PtG4-labelled tumor phantoms in multiple biological objects (hairless mice) from single scan. CONCLUSION Presented sheet beam CELSI technique greatly extended the useful depth range of luminescence molecular imaging. More importantly, the light sheet microscopy approach was successfully adapted to CELSI, providing means to recover a completely attenuation-corrected 3D image of luminescent probe distribution. Gated CELSI acquisition yielded functional information of a spatially resolved oxygen concentration map of deep lying targets. This work was supported by NIH research grant R01CA109558 and R21EB017559, as well as by Pilot Grant Funds from the Norris Cotton Cancer Center.