EFFECTS OF HYPEROXIA ON T-2(ASTERISK) AND RESONANCE FREQUENCY WEIGHTED MAGNETIC-RESONANCE IMAGES OF RODENT TUMORS

Experiments were performed to determine whether T-2() and resonance f requency weighted MR images are sensitive to effects of hyperoxia on m odel tumors. Hyperoxia can increase tumor oxygen tension and thus affe ct T-2() and/or the average resonance frequency within each image vox el due to the paramagnetism of oxygen itself or through modulation of the oxidation state of hemoglobin. Alternatively, changes in T-2 duri ng hyperoxia may reflect changes in tumor water content due to changes in systemic blood pressure. Mammary adenocarcinomas implanted in the flanks of rats were studied. Imaging sequences were preceded by two 90 degrees pulses separated by an evolution period of 50 or 75 ms and fo llowed by a crusher gradient to eliminate transverse magnetization. Th is pulse sequence produced images which were sensitized to both T-2 a nd the average resonance frequency of each voxel. Images were produced at 2 T using a gradient echo imaging method with a TR of 3s. Images o btained during inhalation of air and 100% O-2 were compared. Significa nt increases in image intensity were observed in most tumors during hy peroxia, particularly at the tumor center. The increase was accentuate d when the evolution period was increased and greatly reduced when a 1 80 degrees refocusing pulse was placed at the center of the evolution period. These results suggest that hyperoxia reduces local magnetic su sceptibility gradients leading to an increase in T-2 or causes a shif t in resonance frequency. The magnitude of this change may be a functi on of the rate at which oxygen is delivered to and metabolized by tumo rs and may also reflect tumor oxygen tension under normoxic conditions . Therefore, response to hyperoxia may provide useful contrast in MR i mages which could aid identification and characterization of tumors an d provide an indirect measure of tumor blood flow and oxygenation.