The concordance of MRI and quantitative autoradiography estimates of the transvascular transfer rate constant of albumin in a rat brain tumor model.

The apparent forward transfer constant, K transa, for albumin was measured in 9L cerebral tumors in 15 rats. An MRI study using gadolinium-labeled bovine serum albumin was followed by terminal quantitative autoradiography (QAR) using radioiodinated serum albumin. Look-Locker MRI estimates of T(1) followed gadolinium-labeled bovine serum albumin blood and tissue concentration. QAR and MRI maps of K transa were coregistered, a region of interest (ROI) that included the tumor and its surround was selected, and the two estimates of K transa from the ROI on QAR and MRI maps were compared by either mean per animal ROI or on pixel-by-pixel data using a generalized estimating equation. An ROI analysis showed a moderate correlation between the two measures (r = 0.57, P = 0.026); pixel-by-pixel generalized estimating equation analysis concurred (r = 0.54, P < 0.0001). The estimates of QAR with MRI of last time points (e.g., 25 min) showed a moderate correlation (ROI r = 0.55, P < 0.035; generalized estimating equation r = 0.58, P < 0.0001). Differences between the QAR and MRI estimates of K transa did not differ from zero, but the MRI 25-min estimate was significantly lower than the QAR estimate. Thus, noninvasive MRI estimates of vascular permeability can serve as a surrogate for QAR measures.