Fully MR-guided hepatic artery catheterization for selective drug delivery: a feasibility study in pigs.

PURPOSE: To demonstrate the feasibility of hepatic catheterization for selective delivery of therapeutic agents using a clinical MRI scanner for real-time image guidance. MATERIALS AND METHODS: Experiments were performed in three domestic pigs (70-80 kg) using a clinical 1.5-T MR scanner. After abdominal three-dimensional contrast-enhanced MR angiography (3D-CE-MRA) was performed, endovascular devices with susceptibility markers were tracked with passive tracking techniques. Catheters were maneuvered into the primary and secondary hepatic arteries. Selective catheterization was verified using selective time-resolved CE angiography. Paramagnetic microspheres were administered to a different region for each liver. The resulting biodistributions were investigated using MR images. RESULTS: Successful selective hepatic catheterization was repeatedly demonstrated using passive tracking techniques. 3D-CE-MRA significantly aided the interventional procedure by showing the vascular anatomy, and maximum-intensity projections (MIPs) were used as roadmaps during the interventions. In all cases, microspheres were successfully delivered to the selected regions. The catheters were visualized at a maximum frame rate of five frames per second, allowing a good depiction of the devices and a reliable catheterization of the hepatic arteries. CONCLUSION: Fully MR-guided real-time navigation of endovascular devices permits complex procedures such as selective intra-arterial delivery of therapeutic agents to parts of the liver.

Adaptive subtraction as an aid in MR-guided placement of catheters and guidewires.

PURPOSE: To demonstrate the utility of mask subtraction optimization in magnetic resonance (MR)-guided placement of catheters and guidewires. MATERIALS AND METHODS: MR-guided positioning of magnetically prepared catheters and guidewires was done by dynamically imaging a single thick slab at two frames per second. Selective visualization of the prepared parts of the devices was achieved by the use of a conventional baseline subtraction technique and by the use of an adaptive subtraction technique. In the latter, the best reference image is automatically selected from a fixed or a sliding subset of hitherto acquired dynamic images. The efficacy of both approaches was compared by tracking experiments in a flow phantom and in the aortoiliac arteries of a pig. RESULTS: Baseline subtraction produced adequate visualization of paramagnetic markers in the absence of subject motion and for fixed scan conditions. The sensitivity to subject motion and interactive modification of the scan parameters was greatly reduced by using adaptive subtraction. Adaptive subtraction images, other than conventional subtraction images, appeared to be insensitive to slow periodic motion, e.g., respiratory motion, and were only transiently affected by gross subject motion and interactive alterations of the scan parameters. CONCLUSION: Adaptive subtraction is superior to baseline subtraction for guiding the manipulation of catheters and guidewires in the presence of gross and periodic subject motion and whenever scan parameters are modified in the course of a procedure.

Resolution-insensitive velocity and flow rate measurement in low-background phase-contrast MRA.

Magnetic resonance (MR) phase-contrast (PC) flow measurements are degraded by partial volume errors when the spatial resolution is low, in particular when a large difference in signal magnitude exists between the fluid and the surrounding material. The latter is often the case in phantom studies and may be encountered when flow is measured in prosthetic vessel segments (such as shunts, grafts, and bypasses) and in contrast-enhanced blood. This paper presents a new method that is designed to measure flow in vessels of circular cross-section with Poiseuille flow and negligible background signal arising from static material around the lumen. The method calculates the average flow velocity directly from the original complex image data by integrating the signal in oppositely velocity-sensitized PC images. The radius is calculated from the summed signal modulus. The method allows accurate and resolution-insensitive measurements of the average flow velocity to be obtained in both cross-sectional and in-plane acquisitions. It is not critical to any of the assumed conditions. The validity and capabilities of the proposed technique are demonstrated by in vitro experiments.