Localization of Tumors and Evaluation of Their State of Oxygenation by Phosphorescence Imaging¹

Oxygen-dependent quenching of phosphorescence has been used to image the distribution of oxygen pressure in small tumors and surrounding tissue. Suspensions of cultured 9L cells or small pieces of solid tumors from 9L cells were injected into the surface of the muscle of the hindquarter of rats, and the tumors were grown until they were 0.2–1.0 cm in diameter. The phosphorescent probe for oxygen was injected into the systemic blood, and phosphorescence was imaged with a video camera. Images of the phosphorescence were collected using a series of different delay times after illumination with a light flash (<5-µs width at half-height), and the phosphorescence decay constants (lifetimes) and oxygen pressure were calculated for each pixel of the image arrays. The areas of tissue within the tumors were observed to have increased phosphorescence lifetimes and lower oxygen pressures than the surrounding tissue. Phosphorescence imaging is, therefore, a noninvasive optical method which permits quantitation of the distribution of oxygen in small tumors and also, at least in the 9L tumors, differentiation of tumor from normal tissue.