Color-resolved Cherenkov imaging allows for differential signal detection in blood and melanin content

Abstract

Significance: High-energy x-ray delivery from a linear accelerator results in the production of spectrally continuous broadband Cherenkov light inside tissue. In the absence of attenuation, there is a linear relationship between Cherenkov emission and deposited dose; however, scattering and absorption result in the distortion of this linear relationship. As Cherenkov emission exits the absorption by tissue dominates the observed Cherenkov emission spectrum. Spectroscopic interpretation of this effects may help to better relate Cherenkov emission to ionizing radiation dose delivered during radiotherapy.

Aim: In this study, we examined how color Cherenkov imaging intensity variations are caused by absorption from both melanin and hemoglobin level variations, so that future Cherenkov emission imaging might be corrected for linearity to delivered dose.

Approach: A custom, time-gated, three-channel intensified camera was used to image the red, green, and blue wavelengths of Cherenkov emission from tissue phantoms with synthetic melanin layers and varying blood concentrations. Our hypothesis was that spectroscopic separation of Cherenkov emission would allow for the identification of attenuated signals that varied in response to changes in blood content versus melanin content, because of their different characteristic absorption spectra.

Results: Cherenkov emission scaled with dose linearly in all channels. Absorption in the blue and green channels increased with increasing oxy-hemoglobin in the blood to a greater extent than in the red channel. Melanin was found to absorb with only slight differences between all channels. These spectral differences can be used to derive dose from measured Cherenkov emission.

Conclusions: Color Cherenkov emission imaging may be used to improve the optical measurement and determination of dose delivered in tissues. Calibration for these factors to minimize the influence of the tissue types and skin tones may be possible using color camera system information based upon the linearity of the observed signals.

Keywords: Cerenkov; Cherenkov; luminescence; radiation; radiotherapy.

Fig. 1

(a) Acquisition setup, using C-Dose software for on-site image processing, LINAC for beam delivery and (b) three-channel time-gated intensified cameras for RGB Cherenkov capture (arrow). Reproduced with permission from Alexander et al. (LSA 2021). (c) Detailed schematic of custom camera. (d) Representation of necessary time-gating and synchronization to LINAC pulses.

Fig. 2

(a) Absorption spectra of melanin, Hb, and ${\mathrm{HbO}}_2$ are shown on the same graph with the emission spectra of Cherenkov, with each arbitrarily scaled. In panel (b) the RGB background subtracted (BG sub) sensitivities of the filtered color Cherenkov camera are shown. (c) A TLS was used to calibrate the camera with narrowband monochromatic light, for the data in (b).

Fig. 3

(a) True color images of solutions created in their respective dishes with increasing blood concentration from left to right. (b) Example of single solution dish in blacked-out Petri container. (c) Prepared bovine whole blood and phosphate buffered saline, which were later combined with the Intralipid.

Fig. 4

True color view of epidermal layers created in their respective dishes with increasing melanin concentration from left to right.

Fig. 5

(a) Calibrated reflectance average values; (b) reduced scattering coefficient average values; and (c) absorption coefficient averages, all listed by concentration values in units of mg/ml in the legend.

Fig. 6

(a) White light images of melanin tissue phantoms with varying absorption. Images of the Cherenkov emission images in each of RGB wavelength bands are shown. Concentrations of melanin were 0.0018, 0.0038, 0.0076, 0.0114, 0.019, 0.027, 0.045, and $0.072\mathrm{mg}/\mathrm{ml}$, respectively, left to right, with $600\mathrm{MU}/\min$, 6 MV and 300 MU (b) White-light images of blood phantoms with varying absorption. Images of the Cherenkov emission images in each of RGB wavelength bands are shown. Concentrations of blood were 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, and 3.5%, respectively, left to right, with $600\mathrm{MU}/\min$, 6 MV, and 300 MU.

Fig. 7

Normalized Cherenkov RGB signal intensities versus (a) melanin concentration and (b) blood concentration.

Fig. 8

Visual display of seven phantoms chosen to show slight variation in melanin content at the lowest values. The standard color photograph of the tissue phantoms is shown in Fig. 4 with ambient room lighting, and the Cherenkov color image is shown here in (a) and (b) taken in a darkened room with (a) MeV beams and (b) MV beams. The values depicted above the images are in units of mg/ml to represent biological melanin concentration in each phantom.