Measurement of arterial input function of 17O water tracer in rat carotid artery by using a region-defined (REDE) implanted vascular RF coil

A method of determining arterial input function (AIF) by continuously detecting the ¹⁷O MR signal changes of ¹⁷O-labeled water tracer in the rat carotid artery using a region-defined (REDE) implanted vascular RF coil at 9.4 Tesla is reported. This coil has a compact physical size of 1 mm inner diameter, 3 mm outer diameter and 11 mm in length. It can be readily implanted into the rat neck and wrapped around the rat carotid artery for achieving adequate MR detection sensitivity for determining AIF with minimal surgical trauma. Water phantom and in vivo MR experiments were conducted for validating the coil's performance. A signal-to-noise ratio of ~20:1 was achieved for the ¹⁷O signal acquired from naturally abundant H₂ ¹⁷O in a small amount of blood (~7 μl) inside the rat carotid artery with an acquisition time of 11 s. The REDE RF coil design electromagnetically isolates the rat carotid artery from surrounding tissues and ensures that the MR signal detected by the RF coil is only attributable to the artery blood. It also minimizes the electromagnetic coupling between the implanted RF coil and a head surface coil tuned at the same operating frequency (two-coil configuration). This configuration allowed simultaneous measurements of dynamic changes of ¹⁷O MR signal of the H₂ ¹⁷O tracer in both rat carotid artery and brain. Compared to most contemporary MR approaches, the REDE implanted RF provides a simple, accurate, and promising solution for determination of AIF in small experimental animals.