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 (MTR _Rex), relaxation-compensated MTR _Rex (AREX), and traditional asymmetry (MTR _asym)—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. MTR _Rex and MTR _asym were calculated in the region of 3 to 4 ppm. AREX was calculated by T _1wat correction of MTR _Rex. Tumor tissue masks, which classify different tumor tissues, were drawn by an experienced neuroradiologist. ROI-averaged tumor tissue analysis was done for MTR _Rex, AREX, and MTR _asym. Results: MTR _Rex and MTR _asym were slightly elevated in tumor-associated structures. Both metrics were positively correlated to T _1wat. The correlation coefficient (R) was determined to be 0.88 (P < 0.05) and 0.92 (P << 0.05) for MTR _Rex and MTR _asym, respectively. After T _1wat correction (R = −0.21, P = 0.69), no difference between normal and tumor tissues was found for AREX. Conclusions: The strong correlation of MTR _Rex and MTR _asym with T _1wat 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. Magn Reson Med 77:1525–1532, 2017.