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(a)(trans), 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(a)(trans) were coregistered, a region of interest (ROI) that included the tumor and its surround was selected, and the two estimates of K(a)(trans) 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(a)(trans) 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. Magn Reson Med 66: 1422-1431, 2011. (C) 2011 Wiley Periodicals, Inc.