Field Dependence Study of Commercial Gd Chelates With SQUID Detection

The contrast agents (CAs), which can enhance the image contrast by changing the relaxation times in the target locations, are widely used in clinical high-field magnetic resonance imaging (MRI). However, the commercial clinical CAs have attracted little attention in ultralow field (ULF) MRI, whose measurement field is four orders of magnitude lower than that of the clinical high-field counterpart. In this paper, we measure the relaxivity levels of three gadolinium (Gd)-based commercial CAs (Gd-DTPA, Gd-EOB-DTPA, and Gd-DTPA-BMA) with our four-channel magnetically unshielded ULF-MRI system using low-T c superconducting quantum interference devices as detectors. The system has a 0.65-T permanent-magnet pair as prepolarization field B p and applies a static B 0 field of 129 uT. We compare the longitudinal relaxivity levels of the CAs at B p and B 0 field strengths, as well as the reported values at 1.5 T. The results show that the relaxivity levels of three commercial CAs at 129 uT are all about twice those at high fields, which demonstrates one particular advantage of ULF-MRI.