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

Purpose: To provide insight into the effect of water T relaxation (T ) on amide proton transfer (APT) contrast in tumors. Three different metrics of APT contrast-magnetization transfer ratio (MTR ), relaxation-compensated MTR (AREX), and traditional asymmetry (MTR )-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 and MTR were calculated in the region of 3 to 4 ppm. AREX was calculated by T correction of MTR . Tumor tissue masks, which classify different tumor tissues, were drawn by an experienced neuroradiologist. ROI-averaged tumor tissue analysis was done for MTR , AREX, and MTR .

Results: MTR and MTR were slightly elevated in tumor-associated structures. Both metrics were positively correlated to T . The correlation coefficient (R) was determined to be 0.88 (P < 0.05) and 0.92 (P << 0.05) for MTR and MTR , respectively. After T correction (R = -0.21, P = 0.69), no difference between normal and tumor tissues was found for AREX.

Conclusions: The strong correlation of MTR and MTR with T 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 -relaxation properties. Magn Reson Med 77:1525-1532, 2017. © 2016 International Society for Magnetic Resonance in Medicine.