Computed Cerenkov luminescence yields for radionuclides used in biology and medicine

Cerenkov luminescence imaging is an emerging biomedical imaging modality that takes advantage of the optical Cerenkov photons emitted following the decay of radionuclides in dielectric media such as tissue. Cerenkov radiation potentially allows many biomedically-relevant radionuclides, including all positron-emitting radionuclides, to be imaged in vivo using sensitive CCD cameras. Cerenkov luminescence may also provide a means to deliver light deep inside tissue over a sustained period of time using targeted radiotracers. This light could be used for photoactivation, including photorelease of therapeutics, photodynamic therapy and photochemical internalization. Essential to assessing the feasibility of these concepts, and the design of instrumentation designed for detecting Cerenkov radiation, is an understanding of the light yield of different radionuclides in tissue. This is complicated by the dependence of the light yield on refractive index and the volume of the sample being interrogated. Using Monte Carlo simulations, in conjunction with step-wise use of the Frank-Tamm equation, we studied forty-seven different radionuclides and show that Cerenkov light yields in tissue can be as high as a few tens of photons per nuclear decay for a wavelength range of 400-800 nm. The dependency on refractive index and source volume is explored, and an expression for the scaling factor necessary to compute the Cerenkov yield in any arbitrary spectral band is given. This data will be of broad utility in guiding the application of Cerenkov radiation emitted from biomedical radionuclides.