MR imaging of vascular stents: effects of susceptibility, flow, and radiofrequency eddy currents.

PURPOSE: The purpose of this in vitro study was to examine the various sources of artifacts in magnetic resonance (MR) imaging and angiography of vascular stents. MATERIALS AND METHODS: Five low-artifact stents-Wallstent (cobalt alloy), Memotherm (nitinol), Perflex (stainless steel), Passager (tantalum), and Smart (nitinol)-were imaged in a vascular flow phantom, consisting of a thin-walled cellulose vessel model connected to a pump system. The echo time and the angulation of the stents with respect to the direction of the main magnetic field were varied. Spin echo and gradient echo images as well as three-dimensional MR angiograms were obtained to study the effects of flow, magnetic susceptibility, and radiofrequency-induced eddy currents. RESULTS: Susceptibility artifacts were restricted to the stents' direct environment and were mildest at short echo times and with the stents aligned with the main magnetic field. Nitinol stents showed less artifacts than steel stents did. Radiofrequency artifacts obscuring the stent lumen and flow-related lumen displacement were seen in all stents. The extent to which these occurred depended on strut geometry and orientation. CONCLUSIONS: For low-artifact stents, the material the stent is made of is not the only important factor in the process of artifact formation. Susceptibility artifacts, radiofrequency eddy currents and flow-related artifacts all contribute to the image distortion, and are dependent on the geometry and orientation of the struts and on the orientation of the stent in the main magnetic field.

Selective contrast-enhanced MR angiography.

In this study the feasibility of intraarterial contrast administration was investigated. Its use for navigation and treatment evaluation during MR-guided intravascular interventions was explored in phantom and animal experiments. An injection protocol was developed, which accounts for sequence parameters and vessel flow rate. Tracking a bolus of contrast agent was useful to verify the catheter tip position and to assess flow conditions. Compared to intravenous contrast-enhanced magnetic resonance angiography (CE-MRA), selective contrast administration permitted a strongly reduced dose. In two-dimensional (2D) acquisitions overlap of vessels was prevented. Injection and acquisition were easily and accurately synchronized in selective 3D CE-MRA, and a high contrast concentration could be maintained during the entire acquisition. Selective injection is useful in the course of an intervention, to facilitate navigation, provide information on flow conditions, and to evaluate treatment progress repeatedly.

Placement of an inferior vena cava filter in a pig guided by high-resolution MR fluoroscopy at 1.5 T.

Percutaneous placement of an inferior vena cava filter is a means for long-term prevention of pulmonary thromboembolism. In this study we investigated the magnetic resonance (MR) imaging properties of a Nitinol vena cava filter, in various anatomic and angiographic scans, as well as the feasibility of placing this filter under near real-time, high-resolution MR fluoroscopy. We made use of the passive tracking strategy, with on-line image processing and visualization, both in vitro and in a pig. The artifacts provoked by the metallic filter were such that the position and orientation of the filter were well depicted in all scans. Considerable radiofrequency caging obscured the interior of the filter. Our experiments showed that an MR-guided vena cava filter placement, with sufficient temporal and spatial resolution, is possible. Three-dimensional phase contrast MRA allowed direct evaluation of the filter placement procedure, without the use of contrast agent.

Heating around intravascular guidewires by resonating RF waves.

We examined the unwanted radiofrequency (RF) heating of an endovascular guidewire frequently used in interventional magnetic resonance imaging (MRI). A Terumo guidewire was partly immersed in an oblong saline bath to simulate an endovascular intervention. The temperature rise of the guidewire tip during an FFE sequence [average specific absorption rate (SAR) = 3.9 W/kg] was measured with a Luxtron fluoroscopic fiber. Starting from 26 degrees C, the guidewire tip reached temperatures up to 74 degrees C after 30 seconds of scanning. Touching the guidewire may cause sudden heating at the point of contact, which in one instance caused a skin burn. The excessive heating of a linear conductor like the guidewire can only be explained by resonating RF waves. The capricious dependencies of this resonance phenomenon on environmental factors have severe consequences for predictability and safety guidelines.

Facilities for monitoring blood flow during MR-guided diagnostic and therapeutic interventions.

In this paper we describe a hardware and software environment for making available quantitative blood flow data inside and outside the magnetic resonance (MR) scanner room during MR-guided diagnostic and therapeutic interventions. The configuration allows for triggered and nontriggered examinations and provides the interventionalist with updated results within 1 second from data acquisition. The practicality of the setup and its potential for clinical and investigative purposes are demonstrated in vitro and in vivo. J. Magn. Reson. Imaging 1999;10:845-850.

Interventional MR: vascular applications.

Three strategies for visualisation of MR-dedicated guidewires and catheters have been proposed, namely active tracking, the technique of locally induced field inhomogeneity and passive susceptibility-based tracking. In this article the pros and cons of these techniques are discussed, including the development of MR-dedicated guidewires and catheters, scan techniques, post-processing tools, and display facilities for MR tracking. Finally, some of the results obtained with MR tracking are discussed.

MR-guided balloon angioplasty: in vitro demonstration of the potential of MRI for guiding, monitoring, and evaluating endovascular interventions.

The purpose of this study was to demonstrate the potential of MRI for guiding, monitoring, and evaluating endovascular interventions. This was done by investigating the feasibility of MR-guided balloon angioplasty in a stenosed vessel model. Catheters and guidewires were prepared for susceptibility-based MR visualization by incorporating paramagnetic markers into their walls. Near real-time monitoring (up to 1 image/sec) of the interventional procedure was achieved by using a dynamic two-dimensional gradient-echo technique. Devices were localized by on-the-fly subtraction of a baseline image from consecutive dynamic images and by merging the subtraction images with a previously acquired road map. All steps involved in balloon angioplasty, from the introduction and placement of a guidewire to the positioning of a catheter across the stenosis, inflation of the balloon, and dilatation of the stenosis could adequately be monitored with MR fluoroscopy. The beneficial effect of dilatation could be substantiated by a reduction of stenosis-related hypointensities and hyperintensities in the posttreatment MR angiogram as compared to the pretreatment angiogram and by a posttreatment increase of the volumetric flow rate.

Pressure or flow recordings for the surveillance of hemodialysis grafts.

Venous pressures (VP) measured by the dialysis machine are widely used for access surveillance and have significantly improved outcomes. VP reflect the resistance in the venous outflow tract, which will rise in the presence of stenosis. Low graft flow caused by high graft resistance predicts thrombosis. In this study we investigated whether high VP coincides with low graft flow (measured by ultrasound dilution technique). Of 70 forearm bridge grafts in 42 chronic hemodialysis patients, 31 had an angiographically proven outflow stenosis. VP at 200 ml/min blood flow (VP200), total graft resistance and venous outflow resistance were higher whereas graft flow was lower in patients with venous outflow tract stenosis as compared to patients without stenosis. Diagnostic power of the tests for identifying patients with venous stenosis showed no important differences. However, arterial inflow resistance, which is not reflected in VP measurements, represented a substantial and, more importantly, a highly variable percentage of total graft resistance. As a result graft flow showed no correlation with VP measurements. In conclusion, although patients with venous outflow stenosis may be identified accurately using venous pressure assessments, graft flow measurements seem to be more suitable for selecting patients at risk for thrombosis.

Renal artery stenosis: detection and quantification with spiral CT angiography versus optimized digital subtraction angiography.

PURPOSE: To compare spiral computed tomographic (CT) angiography with optimized digital subtraction angiography (DSA) for accurate detection and quantification of renal artery stenosis. MATERIALS AND METHODS: In 71 consecutive patients with possible renovascular hypertension, spiral CT angiography was performed of the renal arteries before DSA. Optimized DSA (performed with projection angles calculated from axial spiral CT source images) was used as the standard. Two independent observers evaluated spinal CT angiograms for the presence and grade of renal artery stenosis. Sensitivity, specificity, and interobserver variability were calculated. RESULTS: With spiral CT angiography, all 166 renal arteries and accessory arteries were identified correctly by both reviewers. Overall sensitivity and specificity for assessment of stenoses of grade 0 (none), grade 1 (1%-49%), grade 2 (51%-99%), and grade 3 (occlusion) were 97% and 100%, 92% and 98%, 96% and 96%, and 100% and 100%, respectively. Agreement was strong between spiral CT angiographic and DSA findings in assessment of all grades of stenosis (kappa coefficient, 0.9 and 0.9, respectively) for the two observers. CONCLUSION: Spiral CT angiography enabled accurate assessment of renal artery stenosis in patients with possible renovascular hypertension and may assist in selecting patients for interventional treatment.

Pulmonary embolism after thrombolysis of hemodialysis grafts.

The aim of this study was to determine the incidence of pulmonary embolization occurring after mechanical or pharmacomechanical percutaneous intravascular thrombolysis in 23 patients with occluded hemodialysis grafts. In all patients, pulmonary perfusion scintigraphy was performed before and immediately after thrombolysis. In eight (35%) of the patients, there was evidence of pulmonary embolism resulting from the interventional procedure; however, in only one was there clinical symptoms. It is concluded that there is substantial risk of pulmonary embolism in patients undergoing percutaneous intravascular thrombolysis for an occluded hemodialysis graft.