Research News: 2002 - 2003 - 2004

Evolving Applications on the Interventional X/MR System
Authors:

• David Saloner, PhD
• William P. Dillon, MD
• Chris Dowd, MD
• Roy L. Gordon, MD
• Van Halbach, MD
• Randall Higashida, MD
• Alastair Martin, PhD
• Maythem Saeed, PhD
• Kendrick Shunk, MD
• Oliver Weber, PhD
• Mark Wilson, MD
• Charles B. Higgins, MD

Investigators are pursuing a broad range of studies using the unique capabilities of the Interventional X/MR system in the Department of Radiology at UCSF. In routine clinical use, these two systems function independently as a state-of-the-art MRI system (Philips Medical Systems, Intera 1.5T MRI) and an X-ray angiography suite (Philips Medical Systems, Integris V5000 angiography system) in adjoining rooms. In research mode, the doors separating these two systems are opened and the patient table can be positioned either in the MRI scanner or in the angiography suite. Patients can thus be safely and quickly moved between the two modalities, providing the capability to monitor the effects of endovascular interventions on the surrounding soft tissue and on the end organ.

The combined X/MR system has the potential not only to provide important information about the underlying physiological mechanisms that follow interventional treatments, but also to make it possible to reduce the technical challenges of many of these interventions. The placement of devices in specific locations can often be challenging, given the absence of clear information about the surrounding soft tissue and the difficulty of navigating through three- dimensional space using projection images alone. The possible benefit of using the combined X/MR system in the repair of atrial septal defects, for example, is currently under investigation in an animal model where X-ray guidance is used to advance a catheter to the site of interest. MR guidance is then used to deploy a device that occludes the hole in the septum. The three- dimensional capabilities of MRI aids in monitoring the intervention to ensure that one half of the device is deployed on either side of the septum, providing satisfactory occlusion of the hole. While focused on endovascular applications, research on the Interventional X/MR system is directed at different organ systems, including neuro, body, and cardiac. Examples of three ongoing studies are presented below.

Balloon Test Occlusion of the Carotid Artery
Patients with intracranial aneurysms are at substantial risk of devastating injury if the aneurysm bleeds. In many instances aneurysms can be treated by packing the aneurysmal space with balloons or coils, thereby excluding the aneurysm from the parent vessel from which it originates. However, in other cases, such as unfavorable aneurysm geometry, that procedure is not effective. If the territory supplied by the parent vessel has collateral supply, an alternative is to occlude the parent artery. This is the situation when the aneurysm arises from an internal carotid artery where the contralateral carotid artery and the vertebral arteries can serve as sources of collateral flow. Before sacrificing the parent artery, it is necessary to determine whether the patient has sufficient collateral circulation to tolerate arterial sacrifice or whether a surgical bypass (such as an EC/IC procedure which shunts flow from the external to the internal carotid artery) must be performed. To test this, a temporary occlusion of the carotid artery is achieved by endovascular inflation of a balloon in the affected artery. Typically, these patients are monitored for neurological performance over a 30 to 60-minute examination period to determine their tolerance for loss of this vessel.
Neurological evaluations are relatively qualitative and it would be valuable to have a quantitative measure of the extent to which occlusion of a specific artery impacts on neurological function. MR may be a valuable tool for detecting subclinical ischemia during temporary occlusion of the carotid artery. We have been performing baseline perfusion, diffusion, post- contrast turbo-FLAIR, and flow measurements prior to occlusion and then repeating these measures during balloon inflation (Figure 2). Conventional diffusion weighted scans (b=1000 s/mm2) have not demonstrated perceptible changes during inflation, an effect that was unlikely considering the short time scales associated with test occlusions. Post-contrast turbo-FLAIR acquisitions, however, have demonstrated pial enhancement in brain parenchyma distal to the occluded carotid artery in some instances. First pass perfusion imaging further allows quantification of perfusion delays associated with collateral perfusion as well as elucidating hemispherical differences in blood volume and flow levels. The combination of these acquisitions may eventually provide additional parameters from which an improved determination of tolerance to carotid occlusion may be assessed.

Renal Artery Embolization
Renal artery embolization is often employed in the treatment of tumors of the kidney. While catheter angiography provides an effective means of delivering the embolic agents, MR imaging has potential advantages in identifying the extent to which the agent localizes in specific regions in the kidney. To evaluate this concept, we are investigating the use of microspheres that have been impregnated with Gadolinium, which provides a clearly identifiable signal on MR images. The size of the microspheres can be selected to match the caliber of the intra-renal arteries and thereby control the depth of penetration at which embolization occurs in the renal parenchyma. We have found that the correlation of microsphere distribution with microsphere size is better appreciated on MR imaging than on catheter angiography. MR also provides a means to determine the rate of deposition of the microspheres on a region-specific basis throughout the three-dimensional geometry of the kidney. We are also investigating whether the use of the high-speed acquisition mode of the MR scanner, which can provide real-time image acquisition at a rate of up to 10 fps, is an effective method for monitoring delivery of the microspheres. In this mode, MRI clearly demonstrates the flow of the agent in the renal arteries and is a sensitive means of detecting reflux from the renal artery.

Coronary Stenting
The coronary arteries represent a substantial challenge for MR due to their small caliber and the degree to which they move during cardiac and respiratory motion. Accordingly, stenting of the coronaries demands a combination of high spatial and temporal resolution that only recently has become achievable. The use of MR to guide coronary stenting has some substantial benefits that will only be enhanced with the continued development of drug eluting stent designs. Notably, MR may be able to depict regions of significant atherosclerosis independent of whether or not the plaque has created substantial narrowing of the lumen. Such information may provide a more appropriate target for placement of stents that attempt not only to improve vessel patency, but also to administer localized drug therapy.

Coronary stent placement has been performed in an animal model (Figure 4). MR fluoroscopy was achieved using an interactive viewer with a true temporal resolution of 5 fps and a reconstruction rate of 10 fps. Three scan plane geometries corresponding to the aortic arch, the plane of the aortic valve and the circumflex artery were stored and used to subsequently monitor the procedure. It was possible to guide a catheter to the plane of the aortic valve and position it at the ostium of the left coronary artery (frame B). Positioning was confirmed by puffs of dilute Gd solutions, whose path was visualized on T1-weighted real-time acquisitions. A guidewire was then introduced in the circumflex artery and visualized via its stainless steel tip, which produced a relatively strong artifact (frames C, D). The nitinol body of this guidewire could not be visualized in this case so the passage of the tip was visualized in orthogonal planes along the coronary artery to assure its position prior to stent deployment. This study affirmed the possibility of using real-time MR scanning to deliver stents even to the most difficult anatomical locations, but reinforced the need for substantial improvements in optimizing the MR visibility of endovascular devices.

Discussion
The combined X/MR suite provides an interesting new opportunity to probe physiological responses following interventional procedures. In particular, physiological measures such as volume flow and tissue perfusion as well as changes in the soft tissue signal can now be evaluated in the interval immediately following intervention. The combined system provides the potential to use MR to establish quantitative measures for determining whether a treatment has achieved its goal or whether additional interventions are needed.
The capabilities of X/MR will expand as investigators develop devices that are better suited to the requirements of the different imaging environments. Indeed, a number of devices are being evaluated where the catheters and stents used in the treatment procedure perform double service in that they are also used as probes to boost the detected signal.
The X/MR facility provides new and exciting challenges to radiologists and basic scientists in the Radiology Department. It fosters increased research collaboration with colleagues in a number of other clinical services, and offers hope for effective new endovascular approaches to a variety of disease processes.