Purpose: Cherenkov imaging has demonstrated its usefulness for real-time beam monitoring and incident detection during X-ray and electron radiotherapy, as well as for remote in vivo surface dosimetry. In the latter case, sub-surface tissue composition introduces approximately 30% error in Cherenkov-dose accuracy. Here we introduce the concept of in vivo color Cherenkov imaging and investigate the potential for correcting these tissue composition factors, and explore novel application of seamless intra-fraction tissue oxygenation monitoring.
Methods: A three-channel intensified CMOS camera was developed using a relay lens system and a pair of dichroic beamsplitters, which split the incoming image onto three separate intensifiers. The intensified RGB channels were read by three CMOS sensors and combined into 24-bit color image following spatial and color calibration. Tissue-mimicking phantoms containing water, 1% intralipid and blood were imaged during 6MV beam irradiation to study the effects of tissue composition (0-3.5% blood) and blood oxygenation (adjusted by glucose oxidase-catalase assay) on the color of Cherenkov emission. Lastly, in vivo color Cherenkov images of three right-sided breast patients were acquired for one treatment fraction each.
Results: Oxygenated blood in tissue phantom images produced a visibly redder hue compared with deoxygenated blood due to decreased attenuation of red wavelengths by oxyhemoglobin. Variation in blood concentration produced a blue-red linear correlation (R2=0.96) between blood concentration and the derived luminance parameter x in the CIE xyY color space. Cherenkov emission from patients exhibited pink color, and the luminance parameter was found to be in correlation with fibroglandular vs. fat tissue composition.
Conclusion: This first investigation of color Cherenkov imaging has uncovered the natural relationship between tissue composition and the color of emitted Cherenkov light. The correlation between blood content and color will be translated into future work for in vivo sensing of variations in fat and fibroglandular content in irradiated tissues.
Funding Support, Disclosures, and Conflict of Interest: Hardware and software support for this project was provided by DoseOptics LLC, a company manufacturing Cherenkov imaging cameras. B. Pogue is the President and P. Bruza is an employee of DoseOptics LLC. D. Alexander reports receiving consulting fees from DoseOptics LLC. Intellectual Property application is pending.