Interventional magnetic resonance: realistic prospect or wishful thinking?

MR fluoroscopy in vascular and cardiac interventions (review)

Vascular and cardiac disease remains a leading cause of morbidity and mortality in developed and emerging countries. Vascular and cardiac interventions require extensive fluoroscopic guidance to navigate endovascular catheters. X-ray fluoroscopy is considered the current modality for real time imaging. It provides excellent spatial and temporal resolution, but is limited by exposure of patients and staff to ionizing radiation, poor soft tissue characterization and lack of quantitative physiologic information. MR fluoroscopy has been introduced with substantial progress during the last decade. Clinical and experimental studies performed under MR fluoroscopy have indicated the suitability of this modality for: delivery of ASD closure, aortic valves, and endovascular stents (aortic, carotid, iliac, renal arteries, inferior vena cava). It aids in performing ablation, creation of hepatic shunts and local delivery of therapies. Development of more MR compatible equipment and devices will widen the applications of MR-guided procedures. At post-intervention, MR imaging aids in assessing the efficacy of therapies, success of interventions. It also provides information on vascular flow and cardiac morphology, function, perfusion and viability. MR fluoroscopy has the potential to form the basis for minimally invasive image-guided surgeries that offer improved patient management and cost effectiveness.

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.

Magnetic resonance imaging-guided vascular interventions

Magnetic resonance imaging (MRI), which provides superior soft-tissue imaging and no known harmful effects, has the potential as an alternative modality to guide various medical interventions. This review will focus on MR-guided endovascular interventions and present its current state and future outlook. In the first technical part, enabling technologies such as developments in fast imaging, catheter devices, and visualization techniques are examined. This is followed by a clinical survey that includes proof-of-concept procedures in animals and initial experience in human subjects. In preclinical experiments, MRI has already proven to be valuable. For example, MRI has been used to guide and track targeted cell delivery into or around myocardial infarctions, to guide atrial septal puncture, and to guide the connection of portal and systemic venous circulations. Several investigational MR-guided procedures have already been reported in patients, such as MR-guided cardiac catheterization, invasive imaging of peripheral artery atheromata, selective intraarterial MR angiography, and preliminary angioplasty and stent placement. In addition, MR-assisted transjugular intrahepatic portosystemic shunt procedures in patients have been shown in a novel hybrid double-doughnut x-ray/MRI system. Numerous additional investigational human MR-guided endovascular procedures are now underway in several medical centers around the world. There are also significant hurdles: availability of clinical-grade devices, device-related safety issues, challenges to patient monitoring, and acoustic noise during imaging. The potential of endovascular interventional MRI is great because as a single modality, it combines 3-dimensional anatomic imaging, device localization, hemodynamics, tissue composition, and function.

Passive tracking of catheters and guidewires by contrast-enhanced MR fluoroscopy

Passive MR tracking of catheters and guidewires is usually done by dynamically imaging a single thick slab, subtracting a baseline image, and combining the result with a previously acquired MR angiogram. In the in vitro and in vivo experiments reported here, it is demonstrated that this approach may be greatly simplified by using a suitable intravascular contrast agent. The proposed method, contrast-enhanced MR fluoroscopy, combines tracking and angiography into a single sequence and allows direct visualization of the magnetically prepared parts of catheters and guidewires with respect to the vasculature at a frame rate of about one image per 1.5 seconds. Contrast-enhanced MR fluoroscopy, although still limited in temporal resolution, thus obviates the need for subtraction and overlay techniques and eliminates the sensitivity of tracking to subject motion between acquisitions. Magn Reson Med 45:17-23, 2001.

On-line flow quantification by low-resolution phase-contrast MR imaging and model-based postprocessing

Over the past decade, magnetic resonance (MR) imaging has been developed toward a tool for guiding and evaluating diagnostic and therapeutic interventions. Within the field of vascular MR-guided interventions, MR has potential for providing on-line monitoring of the blood volume flow rate, which is relevant during procedures such as balloon angioplasty and stent placement. We recently reported a hardware and software environment for enabling flow quantification every 8 seconds using nontriggered phase-contrast imaging. In the present study, the objective was to increase temporal resolution further to one evaluation per 4 seconds. We achieve this by lowering spatial resolution to 3 pixels per lumen diameter. The accuracy of the measurements is preserved by applying model-based postprocessing for quantification of the volume flow rate. Phantom and volunteer studies are presented, demonstrating the accuracy of the model-driven approach for the applied short acquisitions. The capabilities of the presented approach are illustrated by the results of several hypercapnia experiments and carotid compression tests performed on healthy volunteers.