We propose the use of a new contrast mechanism, Off Resonance Contrast Angiography (ORCA), to facilitate MR-guided endovascular procedures. ORCA relies on the observation that a paramagnetic contrast agent (such as gadolinium-DTPA) induces a frequency shift due to the bulk magnetic susceptibility effect of gadolinium. In essence, ORCA """"""""tunes in"""""""" the desired structures while """"""""tuning out"""""""" signals from other tissues. The method shows great promise for visualization and real-time tracking of angiographic catheters. ORCA is particularly attractive for MR-guided endovascular procedures for several reasons: (1) standard angiographic catheters containing low concentrations of gadolinium chelate are depicted with high conspicuity on projection images;(2) it enables real-time catheter tracking using a sliding window radial acquisition without undersampling artifact;(3) background tissues can be completely suppressed even after repeated infusions of contrast agent. In this proposal, we will evaluate BMS-induced frequency shifts using """"""""off the shelf"""""""" contrast agents, develop novel catheter designs and coatings that exploit the ORCA effect, and implement an ultra-fast MRI pulse sequence that incorporates real-time frequency adjustment for catheter tracking.
Our aims are as follows:
Specific Aim 1 : Parameterize the ORCA effect and determine the lowest concentration of contrast agent that allows adequate visibility of an MRI compatible catheter at 1.5 Tesla. Tasks involve: (1) systematically varying the concentrations of two MRI contrast agents, gadolinium-DTPA and ferumoxytol, to determine which holds the most promise for improving the conspicuity of catheters for interventional MRI, (2) designing radiofrequency pulses with properties that optimize catheter-to-background contrast.
Specific Aim 2 : Maximize catheter conspicuity using off-resonance imaging and modified catheter designs. Tasks involve: (1) measuring the resonance frequency of spins within and surrounding the lumen of a nylon catheter containing a dilute solution of paramagnetic contrast agent;(2) applying spherically-symmetrical frequency-shifting coatings to the surface of the guidewire in order to minimize orientation-dependence of the frequency shift and catheter conspicuity Specific Aim 3: Compare the utility of off-resonance and T1-dependent methods for passive catheter tracking using an in vivo model. Tasks will involve: (1) measurement of visualized catheter length and spatial fidelity using high frame-rate off-resonance and T1-based methods. A static, high-resolution 3D trueFISP acquisition will be the reference standard;(2) demonstrate capability for facilitating renal artery catheterization, comparing outcomes between the techniques using efficacy and safety as endpoints.
We have developed a novel magnetic resonance technology that could have broad applications to the diagnosis and therapy of vascular disorders. This study will result in a better understanding of the basic contrast mechanisms and will lead to improved treatment strategies for renovascular hypertension using MR-guided interventional procedures.