Imaging assessment of photosensitizer emission induced by radionuclide-derived Cherenkov radiation using charge-coupled device optical imaging and long-pass filters

• Published in: World journal of radiology 2023-11, Vol.15 (11), p.315-323
• Main Authors: Aung, Winn, Tsuji, Atsushi B, Rikiyama, Kazuaki, Nishikido, Fumihiko, Obara, Satoshi, Higashi, Tatsuya
• Format: Article
• Language: English
• Subjects: Basic Study
• ISSN: 1949-8470; 1949-8470
• DOI: 10.4329/wjr.v15.i11.315

Radionuclides produce Cherenkov radiation (CR), which can potentially activate photosensitizers (PSs) in phototherapy. Several groups have studied Cherenkov energy transfer to PSs using optical imaging; however, cost-effectively identifying whether PSs are excited by radionuclide-derived CR and detecting fluorescence emission from excited PSs remain a challenge. Many laboratories face the need for expensive dedicated equipment. To cost-effectively confirm whether PSs are excited by radionuclide-derived CR and distinguish fluorescence emission from excited PSs. The absorbance and fluorescence spectra of PSs were measured using a microplate reader and fluorescence spectrometer to examine the photo-physical properties of PSs. To mitigate the need for expensive dedicated equipment and achieve the aim of the study, we developed a method that utilizes a charge-coupled device optical imaging system and appropriate long-pass filters of different wavelengths (manual sequential application of long-pass filters of 515, 580, 645, 700, 750, and 800 nm). Tetrakis (4-carboxyphenyl) porphyrin (TCPP) was utilized as a model PS. Different doses of copper-64 ( CuCl ) (4, 2, and 1 mCi) were used as CR-producing radionuclides. Imaging and data acquisition were performed 0.5 h after sample preparation. Differential image analysis was conducted by using ImageJ software (National Institutes of Health) to visually evaluate TCPP fluorescence. The maximum absorbance of TCPP was at 390-430 nm, and the emission peak was at 670 nm. The CR and CR-induced TCPP emissions were observed using the optical imaging system and the high-transmittance long-pass filters described above. The emission spectra of TCPP with a peak in the 645-700 nm window were obtained by calculation and subtraction based on the serial signal intensity (total flux) difference between CuCl + TCPP and CuCl . Moreover, the differential fluorescence images of TCPP were obtained by subtracting the CuCl image from the CuCl + TCPP image. The experimental results considering different CuCl doses showed a dose-dependent trend. These results demonstrate that a bioluminescence imaging device coupled with different long-pass filters and subtraction image processing can confirm the emission spectra and differential fluorescence images of CR-induced TCPP. This simple method identifies the PS fluorescence emission generated by radionuclide-derived CR and can contribute to accelerating the development of Cherenkov energy transfer ima