Amide Proton Transfer (APT) Imaging of Brain Tumors at 7 T: The Role of Tissue Water T1-Relaxation Properties

Purpose: To provide insight into the effect of water T-1 relaxation (T-1wat) on amide proton transfer (APT) contrast in tumors. Three different metrics of APT contrast-magnetization transfer ratio (MTRRex), relaxation-compensated MTRRex (AREX), and traditional asymmetry (MTRasym)-were compared in normal and tumor tissues in a variety of intracranial tumors at 7 Tesla (T). Methods: Six consented intracranial tumor patients were scanned using a low-power, three-dimensional (3D) APT imaging sequence. MTRRex and MTRasym were calculated in the region of 3 to 4 ppm. AREX was calculated by T1wat correction of MTRRex. Tumor tissue masks, which classify different tumor tissues, were drawn by an experienced neuroradiologist. ROI-averaged tumor tissue analysis was done for MTRRex, AREX, and MTRasym. Results: MTRRex and MTRasym were slightly elevated in tumor associated structures. Both metrics were positively correlated to T1wat. The correlation coefficient (R) was determined to be 0.88 (P < 0.05) and 0.92 (P << 0.05) for MTRRex and MTRasym, respectively. After T1wat correction (R = - 0.21, P = 0.69), no difference between normal and tumor tissues was found for AREX. Conclusions: The strong correlation of MTRRex and MTRasym with T1wat and the absence thereof in AREX suggests that much of APT contrast in tumors for the low-power, 3D-acquisition scheme at 7 T originates from the inherent tissue water T-1-relaxation properties. (C) 2016 International Society for Magnetic Resonance in Medicine