A Phosphorescence Imaging System for Monitoring of Oxygen Distribution in Rat Liver under Ischemia and Reperfusion
Sam Hong-Yi Huang,
Oxygen-dependent quenching of phosphorescence has been validated as a useful and essentially noninvasive optical method for measuring oxygen both in vitro and in vivo. The capability of imaging oxygen distribution based on phosphorescence quenching provides a highly useful tool for investigation of oxygen delivery in tissues at the microvascular level. We demonstrated the implementation of a phosphorescence imaging system incorporating an intensified, gated, monochrome CCD camera which equips a red sensitive Gen III cathode and power supply. By selecting of the delay time sequence for phosphorescence imaging in accordance with the quenching constant and lifetime at zero oxygen of used phosphor, a series of phosphorescence intensity images corresponding to the oxygen pressures within the region of interest would be acquired and digitized. The phosphorescence lifetimes were converted form the digitized intensity levels pixel by pixel upon the assumption of single exponential decay of phosphorescence. Thus, the oxygen distribution of interested area could be calculated from the phosphorescence lifetime map in virtue of well-defined Stern-Volmer equation. Two new phosphors of Generation 2 polyglutamic Pd-porphyrin-dendrimers were used in present study, which have high water solubility and intend to stay in blood circulation without extravasating through the vascular membrane. A Pd-meso-tetra-(4-carboxyphenyl) porphyrin based phosphor termed Oxyphor G2 was used in testing of the instrument. The changes of hepatic oxygen pressure under the portal triad clamping model (PTC model) of ischemia and subsequent reperfusion were real-time monitored by phosphorescence imaging. The images indicated a transient recovery of hepatic oxygen level during the reperfusion following the PTC ligation. The lack of complete restoration of oxygen level has been implicated to the similar pattern of hepatic blood flow observed during reperfusion in previous reports. The other phosphor Oxyphor G2, derived from Pd-meso-tetra-(4-carboxyphenyl) tetrabenzoporphyrin, was recognized by its near infrared spectral characteristic. The calibration of Oxyphor G2 illustrated the independence of quenching constant to pH in the physiological range (6.4 to 7.8) and as such to make it a suitable phosphor for oxygen measurements, especially for those at depth in tissue.