Monitoring Photodynamic Oxygen Consumption by Endogenous Oxygen Contrast MRI.

Photodynamic oxygen consumption was measured by changes in spin-lattice relaxation time (T1 ) in aqueous solution in a clinical GE scanner at 1.5 Tesla. Similar measurements were attempted in excised laryngeal and thyroid tissues that were infused with Rose Bengal. First, T1 was measured as a function of dissolved oxygen in argon and in oxygen pre-saturated water samples that were opened to the atmosphere in a series of steps allowing air to diffuse into or out of solution; for both argon and oxygen saturated water solutions, stepwise air re-equilibration resulted in a return to air-saturated water T1 . Secondly, T1 was measured as a function of time under type II photooxidative conditions in aqueous solution. Under type II photooxidative conditions, a 492 ± 53 ms increase in T1 was measured following 300 seconds of visible light illumination of aqueous solutions containing the photosensitizer Rose Bengal (2.5 × 10-6 M) and the singlet oxygen trap methionine (0.0012 M). The 492 ± 53 ms increase in T1 corresponded to consumption of all the measurable dissolved oxygen (˜ 0.1 mg O2 in 15.0 mL of H2 O) during photooxidation of methionine in air saturated water. This rapid oxygen consumption, indicated by an increase in T1 , is due to irreversible trapping of photogenerated singlet oxygen by methionine. Thirdly, an increase in T1 was observed in Rose Bengal infused normal laryngeal tissue, and in normal and cancerous thyroid tissue samples following 20 minutes of exposure to visible light. An increase in T1 was not observed after 40 minutes of illumination which suggests that the increases in T1 observed after 20 minutes were not due to water uptake, but rather to photoconsumption of interstitial dissolved oxygen.