Active MR tracking using micro coils for both transmit and receive

Introduction Active MR tracking of devices employs small receive coils with restricted spatial sensitivity [1]. When the Signal-to-Noise Ratio (SNR) of the detected MR signals is high, the accuracy and precision of tracking are well suited to a wide variety of applications. Non-idealities of the tracking device and the MR tracking environment, however, can restrict MR tracking rate and robustness. Some of the challenges to signal quality in MR tracking arise from the construction of the device. For example, some or all of the MR signals detected by MR tracking coils in catheters arise from regions outside of each solenoid coil. This results in less efficient detection (i.e. lower SNR) and a complicated phase sensitivity profile. The sensitivity profile of a small solenoid coil is further complicated by: a) the orientation of the coil within the static magnetic field of the MR system, b) magnetic susceptibility differences between the catheter and its surroundings, and c) the orientation of the coil with respect to the applied magnetic field gradients that are used in MR tracking pulse sequences. Unfortunately, no a-priori knowledge of the orientation of the coil can be used if a fully robust device tracking system is desired [2]. Robust MR tracking under low SNR conditions is also made difficult by coupling of unwanted MR signals into the MR tracking coil. This coupling is usually inconsequential in high SNR cases, but can result in large rolling baseline artifacts that make identification of the small tracking peaks difficult. We have developed a system which reduces these errors by using the tracking coils for both transmitting and receiving the RF signal. With this approach, only spins near the tracking coils are excited, and the potential for coupling of MR signals from other locations is greatly reduced. In addition, phase cancelation of the MR signal caused by the dipole sensitivity profile of the small coil is virtually eliminated since the same coil now performs both excitation and reception. These improvements in line shape allow for a more accurate determination of the location of each tracking coil.