How does blood oxygen level-dependent (BOLD) contrast correlate with oxygen partial pressure (pO2) inside tumors?

Blood oxygen level-dependent (BOLD) contrast-based functional MRI (fMRI) has been reported as a method to assess the evolution of tumor oxygenation after hyperoxic treatments, because of its sensitivity to changes in blood flow and deoxyhemoglobin content. However a number of questions remain: 1) In view of tumor heterogeneity, how good is the correlation between the MR parameters in gradient-echo imaging (signal intensity (SI) or effective transverse relaxation time (T-2*)) and local tumor oxygen partial pressure (pO(2))? 2) Is the magnitude of the change in S1 or T-2* a quantitative marker for variation in PO2? 3) Is initial T-2* a good marker for initial pO(2)? To address these questions, murine tumors were imaged during respiratory challenges at 4.7 Tesla, using fiber-optic microprobes to simultaneously acquire tumor pO(2) and erythrocyte flux. The BOLD signal response (S1 and T-2*) was temporally correlated with changes in pO(2). However, the magnitude of the signal bore no absolute relation to pO(2) across tumors, i.e., a given change in S1 corresponded to a 25 mmHg pO(2) change in one tumor, but to a 100 mmHg change in another. The initial T-2* value did not reliably predict tumor oxygenation at the beginning of the experiment. In conclusion, the major advantages of the technique include noninvasiveness, high spatial resolution, and real-time detection of pO(2) fluctuations. Information afforded by the BOLD imaging technique is qualitative in nature and may be combined with other techniques capable of providing an absolute measure of pO(2).