Amide proton transfer (APT) imaging of brain tumors at 7 T: The role of tissue water T1-Relaxation properties

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PurposeTo provide insight into the effect of water T1 relaxation (T1wat) 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).MethodsSix 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.ResultsMTRRex 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.ConclusionsThe 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 T1-relaxation properties. Magn Reson Med 77:1525–1532, 2017. © 2016 International Society for Magnetic Resonance in Medicine

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