Passive tracking and system interfaces for interventional MRI

Abstract
One of the major pre-requisite of MR-guided interventions is the visualization and localization of interventional devices relative to the vascular system and surrounding tissues. A number of approaches have been proposed for device visualization in an MR environment that are categorized into active, passive and semi-active techniques. In spite of considerable growth of these techniques, there is a scope to investigate and develop new pulse sequences and system interfaces for these techniques to make clinically feasible. In this thesis fast and novel passive tracking approaches to efficiently visualize and track the interventional guidewire tip using positive contrast are provided and highly miniaturized active micro-coils with built-in pre-amplifier are introduced.
In passive tracking techniques, the paramagnetic markers are utilized having susceptibility differences with respect to surrounding water protons. When this marker is placed in a uniform main magnetic field B0, it cause significant field inhomogeneities leading to dual effect of local frequency shifts and local gradient induction in marker's immediate surrounding.
The first method introduced in this thesis exploit the local frequency shifts induced by paramagnetic marker to create positive contrast. The binomial spatial-spectral (SPSP) excitation pulse that selectively excites the spectral component of the off-resonant tissues in the close proximity of the marker material and within the selected spatial region is incorporated into a temporally efficient steady state free precession (SSFP)-echo sequence. In this work, the principle of off-resonance excitation of tissues using SPSP pulse is explained theoretically. The in-vitro experiments demonstrate the proper localization and feasibility of paramagnetic marker detection mounted on the interventional guidewire tip. The analysis of the signal and localization characteristics of the SPSP pulse is done theoretically and verified experimentally.
The next novel positive contrast method for passive localization and visualization of paramagnetic susceptibility markers is based on an echo-dephased SSFP sequence. Gradients dephase any signal by ±π at the centered echo-time (TE=TR/2) and induce a total dephasing of ±π per pixel within TR. This ensures that background tissues do not contribute to signal formation and thus appear dark. However, within the close vicinity of the paramagnetic marker, local gradient fields compensate for the intrinsic dephasing to form an echo. Conceptual issues of gradient compensation and its visualization characteristics are analyzed. The flow phantom experiments concluded that reliable tracking of the interventional guidewire is feasible using echo-dephased SSFP. Finally utilization of echo-dephased SSFP for positive contrast visualization of SPIO-labeled islet cells is demonstrated both in vitro and in vivo.
A novel and fast approach for passive real-time tracking of interventional devices using paramagnetic markers termed Projection Reconstruction Imaging with Echo-Dephasing (PRIDE) is also presented in this thesis. PRIDE is based on the acquisition of echo-dephased projections along all three physical axes. Dephasing is preferably set to 4π within each projection ensuring that background tissues do not contribute to signal formation and thus appear heavily suppressed. However, within the close vicinity of the paramagnetic marker, local gradient fields compensate for the intrinsic dephasing to form an echo. Successful localization of the paramagnetic marker with PRIDE is demonstrated in-vitro and in-vivo in the presence of different types of off-resonance (air/tissue interfaces, main magnetic field inhomogeneities, etc). In order to utilize the PRIDE sequence for vascular interventional applications, it was interleaved with balanced steady-state free precession (bSSFP) to provide positional updates to the imaged slice using a dedicated real-time feedback link. Active slice positioning with PRIDE is demonstrated in vitro requiring approximately 20 ms for the positional update to the imaging sequence comparable to existing active tracking methods.
The active tracking method of device visualization utilizes locally sensitive transmit/receive coils for localization and visualization of its tip. The localization of the coil is achieved in few milliseconds through acquisition of one dimensional (1D) projections along all three spatial directions (X, Y and Z). However, the dimensions of these coils are bulky in nature thereby increasing the size of normal interventional devices (for e.g. catheter). In order to overcome this disadvantage in the last part of this thesis, highly miniaturized micro coils with built-in amplifier are introduced and experimentally tested.