Intravascular MR imaging-guided balloon angioplasty with an MR imaging guide wire: feasibility study in rabbits

PURPOSE: To develop a technique for intravascular magnetic resonance (MR)-guided balloon angioplasty with use of an MR imaging guide wire. MATERIALS AND METHODS: An MR imaging guide wire (0.6-mm loopless antenna) that could be placed within a balloon catheter was manufactured. The guide wire was expected to function as either an MR receiver probe in real-time MR imaging or a guide wire for use with interventional devices. Laparotomy was performed in eight rabbits, and a dilatable stenosis was created at the upper abdominal aorta. Balloon angioplasty, validated at pre- and postoperative MR aortography with renal contrast enhancement was performed by using a 1.5-T MR unit with a fast spoiled gradient-echo pulse sequence, short repetition and echo times, and a rate of three frames per second. RESULTS: During MR tracking, the entire length of the MR imaging guide wire was always visible as a band of high signal intensity. In all cases, the MR imaging guide wires were passed through the aortic stenoses dilated by means of balloon inflation. Before balloon angioplasty, flow in the aorta distal to the stenosis was decreased, which caused mild contrast enhancement in each kidney. After balloon angioplasty, distal flow was restored, resulting in substantial renal enhancement. CONCLUSION: The MR imaging guide wire is a potential tool for use in endovascular interventional MR imaging.     

A polymer-based MR-compatible guidewire: a study to explore new prospects for interventional peripheral magnetic resonance angiography (ipMRA)

PURPOSE: To introduce a newly developed polymer-based and magnetic resonance (MR)-compatible guidewire and to explore its capabilities with respect to interventional peripheral magnetic resonance angiography (ipMRA) in a flow phantom. MATERIALS AND METHODS: The guidewire is based on a polyetheretherketone (PEEK) polymer core, and small iron particles are embedded in its coating. A passive device tracking technique was designed utilizing a susceptibility artifact induced by the wire in images acquired with a balanced steady-state free precession (b-SSFP) sequence using small flip angles. The position of the guidewire tip was determined from image intensity maxima and overlayed onto a roadmap in near real-time. Guidewire tracking and balloon angioplasty of an artificial stenosis were attempted in two configurations of a flow phantom. RESULTS: Successful passive guidewire tracking was performed for all phantom configurations. Robustness and accuracy of the tracking technique were sufficient for phantom studies. A balloon catheter was placed into the stenosis using the guidewire under complete MR guidance, and subsequent balloon angioplasty yielded improved flow conditions. CONCLUSION: The new guidewire is well-suited for clinical application due to an absence of the risk of core fracture and its atraumatic flexible tip. It opens novel prospects for the realization of ipMRA in humans that need to be explored in further studies.     

MR-guided angioplasty of renal artery stenosis in a pig model: a feasibility study

PURPOSE: To test the hypothesis that magnetic resonance (MR) imaging can guide the percutaneous treatment of renal artery stenosis in a pig model. MATERIALS AND METHODS: Ameroid constrictors were surgically placed around six renal arteries in four pigs. After 30-36 days, all stenoses were documented by conventional x-ray aortograms. MR-guided renal angioplasty was attempted for three stenoses. For these pigs, MR angiography was performed with use of contrast-enhanced three-dimensional (3D) techniques. The authors visualized catheters by filling them with dilute 4% gadolinium and imaging with two-dimensional (2D) and 3D MR fast spoiled gradient recalled echo techniques. Under MR guidance, the authors advanced a selective catheter into the affected renal artery and crossed the stenosis with a nitinol guide wire. Angioplasty was performed with a balloon catheter filled with dilute gadolinium. Stenosis and luminal diameter measurements were compared before and after angioplasty. RESULTS: After ameroid constrictor placement, four significant stenoses, one mild stenosis, and one occlusion developed. Under MR guidance, the authors achieved technical success in performing three of three (100%) attempted dilations. After MR-guided angioplasty, the mean reduction in stenosis was 35% and the mean increase in luminal diameter was 1.6 mm. CONCLUSION: Use of MR guidance for the angioplasty of renal artery stenosis in pigs is feasible.     

Simultaneous real-time visualization of the catheter tip and vascular anatomy for MR-guided PTA of iliac arteries in an animal model

PURPOSE: To examine the feasibility of simultaneous MR real-time active tip tracking and near real-time depiction of the vascular anatomy for percutaneous angioplasty of iliac arteries under MR guidance. MATERIALS AND METHODS: Nine surgically created stenoses of external iliac arteries in pigs were dilated with MR-compatible balloon catheters (Cordis, Roden, The Netherlands). These catheters were equipped with a microcoil for active tracking of the catheter tip with an in-plane update rate of 10 positions per second. The procedures were performed on an interventional 1.5 T Gyroscan ACS-NT scanner (Philips, Best, The Netherlands). Real-time calculation of images acquired by radial k-space filling was performed on a specially designed backprojector exploiting the sliding window reconstruction technique (Philips Research Laboratories, Hamburg, Germany). The image update rate was 20 frames per second using a radial gradient-echo technique (TR = 12 msec, TE = 3.3 msec, 300 radials). MR angiography and X-ray digital subtraction angiography on the X-ray system positioned in-line next to the interventional MR system served as control for the angioplasty results. RESULTS: Real-time guidance and positioning of the balloon catheters was possible. The actual position of the catheter tip was indicated in the MR images without any time delay for the reconstruction of the anatomical MR images, which were updated with a rate of 20 frames per second. This yielded a combination of a roadmap and fluoroscopy image, in which the catheter position and the anatomical background image both were continuously updated in real time. Six out of nine stenoses were successfully dilated. The effects of the angioplasty could be visualized by the real-time MR technique, as was proven by X-ray digital subtraction angiography. CONCLUSION: Active tip tracking simultaneous with MRI of the vascular anatomy-both in real time-is possible with the applied technique, enabling MR-guided percutaneous dilatation (PTA) of iliac arteries.     

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.     

MR-guided intravascular catheter manipulation: feasibility of both active and passive tracking in experimental study and initial clinical applications

PURPOSE: To evaluate clinically the feasibility and usefulness of MR-guided intravascular procedures with either active or passive tracking. METHODS: With an active MR tacking system and a 0.2 Tesla open MR imager, real-time and biplane displays of positions of a receive-only coil at the tip of catheters were obtained. For passive tracking, 4 Fr catheters with suitable susceptibility for passive tracking were used. Passive tracking with a 1.5 Tesla MR imager could be performed with parameters of TR 15-50 msec, TE 2.2-2.7 msec, and FA 30 degrees. The temperature of each tracking catheter was monitored in vitro. Clinical trials were performed on six patients (three each). The catheter was introduced to the superior mesenteric artery (SMA) under MR guidance by active or passive tracking to perform MR imaging during arterial portography. RESULTS: The temperature increased minimally. In humans, the SMA and celiac artery could be easily introduced on active tracking. MR-guided manipulation of catheters by active or passive tracking in the model and in dogs' vessels was very successful. Introduction of the catheter into the SMA was successful in two cases of active tracking and in all cases of passive tracking. CONCLUSION: The MR-guided intravascular manipulation of catheters by active or passive tracking may be a clinically feasible method.     

Application of MR-guided intravascular procedures by passive tracking utilizing the magnetic susceptibility effect: a preliminary report]

In MR-guided intravascular procedures, the position of catheters must be confirmed on near-real-time MR images. One way of monitoring this is by passive tracking utilizing the magnetic susceptibility effect. A catheter with a stainless steel braid had suitable visibility when tracked by magnetic susceptibility on fast GRE images, although the direction of the static magnetic field affected the apparent width of the catheter. Passive tracking with a 1.5T MRI unit was performed in a patient at one image/2 seconds with fast GRE. The catheter could be introduced to the SMA and celiac artery from the aorta with passive tracking.     

MR imaging- versus conventional X-ray fluoroscopy-guided renal angioplasty in swine: prospective randomized comparison

PURPOSE: To test the hypothesis that the technical success rates, complication rates, and procedural times for magnetic resonance (MR) imaging-guided percutaneous transluminal angioplasty (PTA) and conventional (x-ray) fluoroscopy-guided PTA for treatment of renal artery stenosis are similar. MATERIALS AND METHODS: The study was animal care and use committee approved. After surgically inducing bilateral renal artery stenosis in 11 swine, the authors performed baseline digital subtraction angiography. They transferred each animal to a 1.5-T MR imaging unit and randomly decided which artery would be treated with MR-guided PTA. With MR imaging guidance, angioplastic devices were tracked by using active and passive techniques. Vascular depiction was achieved by using catheter-directed MR angiography. Stenotic vessels were dilated by using 5-6-mm-diameter balloon catheters. PTA was then performed in the contralateral artery by using conventional fluoroscopy-guided techniques. With the intention to treat, the authors compared the technical success (residual stenosis < 50%) rates, complication rates, and procedural times for each guidance method. They compared technical successes and complications by using the McNemar test and procedural times by using a paired t test, with P < .05 indicating a significant difference. RESULTS: The authors successfully dilated nine (82%) of 11 renal arteries with MR guidance and all 11 arteries (100%) with conventional fluoroscopic guidance. The difference was not significant (P = .5). Complications occurred in three (27%) arteries with MR guidance and in one (9%) artery with fluoroscopic guidance, with no significant differences (P = .5). The mean MR-guided PTA procedural time was 46 minutes longer than the fluoroscopy-guided PTA procedural time; this difference was significant (P = .01). CONCLUSION: In a small cohort of swine, the authors did not observe a significant difference between MR imaging- and conventional fluoroscopy-guided renal artery PTA in terms of success and complication rates. However, no evidence of similarity between the techniques should be assumed. Procedural times differed significantly.     

Image-guided and -monitored renal artery stenting using only MRI

PURPOSE: To demonstrate the ability of a unique interventional MR system to be used safely and effectively as the only imaging modality for all phases of MR-guided stent-supported angioplasty. MATERIALS AND METHODS: An experimental disease model of renal stenosis was created in six pigs. An interventional MR system, which employed previously reported tools for real-time catheter tracking with automated scan-plane positioning, adaptive image parameters, and radial true-FISP imaging with steady-state precession (True-FISP) imaging coupled with a high-speed reconstruction technique, was then used to guide all phases of the intervention, including: guidewire and catheter insertion, stent deployment, and confirmation of therapeutic success. Pre- and postprocedural X-ray imaging was used as a gold standard to validate the experimental results. RESULTS: All of the stent-supported angioplasty interventions were a technical success and were performed without complications. The average postoperative residual stenosis was 14.9%. The image guidance enabled the stents to be deployed with an accuracy of 0.98 +/- 0.69 mm. Additionally, using this interventional MRI system to guide renal artery stenting significantly reduces the procedure time, as compared to using X-ray fluoroscopy. CONCLUSION: This study has clearly demonstrated the first successful treatment of renal artery stenting in an experimental animal model solely under MRI guidance and monitoring.     

Invited. Visualization of MR-compatible catheters by electrically induced local field inhomogeneities: Evaluation in vivo

The purpose of this study was to assess the feasibility of a newly developed field inhomogeneity catheter for interventional MRI in vivo. Different prototypes of a field inhomogeneity catheter (pigtail and multipurpose configuration, balloon catheters) were investigated in pigs. The catheters were introduced in Seldinger technique via the femoral vessels over a guidewire on an interventional MR system (Philips Gyroscan NT combined with a C-arm fluoroscopy unit [Philips BV 212]). Catheters were placed in veins and arteries. The catheter position was controlled by a fast gradient-echo sequence (turbo field echo [TFE]). Catheters were introduced over a guidewire without complications in all cases. Using the field inhomogeneity concept, catheters were easily visualized in the inferior vena cava and the aorta by the fast gradient-echo technique on MR in all cases. Although aortic branches were successful cannulated, the catheters were not well displayed by the TFE technique due to the complex and tortuous anatomy. All animals survived the experiments without complications. MR-guided visualization of a field inhomogeneity catheter is a simple concept that can be realized on each MR scanner and may allow intravascular MR-guided interventions in future.     

High-resolution real-time spiral MRI for guiding vascular interventions in a rabbit model at 1.5 T

PURPOSE: To study the feasibility of a combined high spatial and temporal resolution real-time spiral MRI sequence for guiding coronary-sized vascular interventions. MATERIALS AND METHODS: Eight New Zealand White rabbits (four normal and four with a surgically-created stenosis in the abdominal aorta) were studied. A real-time interactive spiral MRI sequence combining 1.1 x 1.1 mm(2) in-plane resolution and 189-msec total image acquisition time was used to image all phases of an interventional procedure (i.e., guidewire placement, balloon angioplasty, and stenting) in the rabbit aorta using coronary-sized devices on a 1.5 T MRI system. RESULTS: Real-time spiral MRI identified all rabbit aortic stenoses and provided high-temporal-resolution visualization of guide-wires crossing the stenoses in all animals. Angioplasty balloon dilatation and deployment of coronary-sized copper stents in the rabbit aorta were also successfully imaged by real-time spiral MRI. CONCLUSION: Combining high spatial and temporal resolution with spiral MRI allows real-time MR-guided vascular intervention using coronary-sized devices in a rabbit model. This is a promising approach for guiding coronary interventions.     

Magnetic resonance-guided percutaneous angioplasty of femoral and popliteal artery stenoses using real-time imaging and intra-arterial contrast-enhanced magnetic resonance angiography

OBJECTIVE: The aim of this study was to demonstrate the possibility of performing magnetic resonance (MR)-guided interventional therapy for femoral and popliteal artery stenoses with commercially available materials supported by MR real-time imaging and intra-arterial MR angiography. MATERIALS AND METHODS: A total of 15 patients suffering from symptomatic arterial occlusive disease of the lower limbs with 19 stenoses were included. Interventional intra-arterial digital subtraction angiography was performed before and after angioplasty on each patient as standard of reference. MR images were acquired on a 1.5-T MR scanner. A fast-low-angle shot (FLASH) 3D sequence was applied for a contrast enhanced MR-angiography (ceMRA). A total of 5 mL of diluted gadodiamide was injected via the arterial access. Maximum intensity projections (MIPs) were used as roadmaps and localizers for the interactive positioning of a continuously running 2D-FLASH sequence with a temporal solution of 2 images/second. The lesion was crossed by a balloon-catheter, which was mounted on a guidewire. The visibility was provided by the radiopaque markers on the balloon and was improved by injection of 1 mL of gadolinium into the balloon. Postinterventional control was performed by intra-arterial MR angiography and catheter angiography. RESULTS: Stenoses were localized by intra-arterial MR angiography. The guidewire/balloon combination was visible, and the balloon was placed correctly to cover the entire stenoses. Balloon dilation reduced the degree of stenosis by approximately 57% on average. No complications were observed. CONCLUSION: MR-guided balloon dilation of femoral and popliteal artery stenoses supported by real-time MR imaging and intra-arterial MR angiography is feasible with commercially available materials.     

Active MR tracking on a 0.2 Tesla MR imager.

An active MR tracking system was implemented on a 0.2 Tesla open MRI system. Interventional devices with receive-only microcoils at their tips were developed and investigated on the scanner. Microcoils having a diameter of about 1 mm and 20 turns were found to provide sufficient signal-to-noise ratios for stable tracking. Positional accuracy and precision were found to be acceptable under practical conditions. Simulation of MR-guided biopsy using biplane images with tracking was performed in a gelatin phantom and dog livers. Successful tracking of catheters with integrated microcoils was also demonstrated in the aorta and IVC of live dogs.     

MR-Guided PTA in Experimental Bilateral Rabbit Renal Artery Stenosis and MR Angiography Follow-Up Versus Histomorphometry

Purpose: To assess in vivo 1) MR-guided percutaneous transluminal renal angioplasty (PTRA) in experimental bilateral rabbit renal artery stenosis (RAS); 2) postprocedural follow-up by gadolinium-enhanced MR angiography versus histomorphometry. Methods: Fifteen male NZW rabbits of mean weight 4.0 kg (range 3.5–4.2 kg) underwent bilateral RAS induction by combined overdilation–deendothelialization with a gadolinium-filled balloon, passively MR-guided by the artifact of a 0.014-inch guidewire. After 4 weeks the rabbits were randomized into two groups: group A (n = 8) underwent right-sided PTRA for treatment of RAS, group B (n = 7) underwent left-sided PTRA. After another 4 weeks the rabbits were killed to assess by histomorphometry recurrent stenosis and contralateral induction injury stenosis lesions. Each step was preceded by gadolinium-enhanced three-dimensional MR angiography, and the cortex-to-aorta (C/A) signal intensity ratio was calculated. Results: RAS induction was successful in all cases. Fourteen arteries developed restenosis and 13 only initial stenosis. MR-guided PTRAs were feasible in 22 arteries (73%). For a successful catheterization of the ostium (20 arteries, 66% success rate), 10–25 steps were required. Five to eight steps were required for balloon localization and inflation for each PTRA. The restenosis effect was reflected by a 16% (12%–27%) decrease in C/A values on MR angiograms (p < 0.05). Conclusion: MR guidance and MR angiography represent a feasible, less invasive alternative for performing and assessing experimental PTRA in RAS.     

Magnetic resonance-guided vascular catheterization: feasibility using a passive tracking technique at 0.2 Telsa in a pig model.

The objective of this study was to demonstrate in an animal model the feasibility of a passive tracking technique for catheter visualization of magnetic resonance (MR)-guided endovascular procedures. All experiments were performed in a 0.2 Tesla open MR system. Susceptibility-based catheters and guide wires were introduced into the aorta and were advanced selectively into the splenic and renal arteries under MR guidance. Based on a previously acquired contrast-enhanced magnetic resonance angiography (MRA) data set, the catheter positioning was performed by using a single-slice true fast imaging with steady state precession (FISP) sequence with a frame rate of 1.3 seconds. Contrast-enhanced MRA was performed in all animals. All catheters were advanced without complications into the aorta and were introduced into the proximal parts of the right renal and splenic arteries under MR guidance. Catheter manipulations were more difficult in the distal parts of these vessels due to the more complex anatomy. Passive catheter tracking is a valuable and technically robust alternative to active tracking methods, because it does not require additional hardware and, thus, can be implemented and used easily with any open MR imaging system. J. Magn. Reson. Imaging 1999;10:841-844.     

MR-guided catheter navigation of the intracranial subarachnoid space

Percutaneous intraspinal navigation (PIN) is a new minimally invasive approach to the CNS. The authors studied the utility of MR-guided intracranial navigation following access to the subarachnoid compartment via PIN. The passive tracking technique was employed to visualize devices during intracranial navigation. Under steady-state free precession (SSFP) MR-guidance a microcatheter-microguidewire was successfully navigated to multiple brain foci in two cadavers. SSFP MR fluoroscopy possesses adequate contrast and temporal resolution to allow MR-guided intracranial navigation.     

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.     

Thermal laser-assisted angioplasty of renal artery stenosis for renovascular hypertension

Percutaneous transluminal laser-assisted angioplasty of a renal artery stenosis was performed in a 16-year-old woman with renovascular hypertension. The stenotic portion of the renal artery was predilated by delivering Nd-YAG laser energy to the terminal tip of a laser catheter. Although the luminal diameter did not increase sufficiently with laser angioplasty alone, it allowed passage of the balloon catheter and subsequent successful balloon angioplasty. Immediately after dilatation, the patient's blood pressure fell to normal, and plasma renin activity decreased. There were no serious complications. Thermal laser angioplasty seems to be an effective adjunct technique for the treatment of severe renal artery stenosis which does not allow initial passage of a balloon catheter.     

Stenoses of vascular anastomoses after hepatic transplantation: treatment with balloon angioplasty.

Vascular complications after liver transplantation include occlusion or stenosis at the sites of anastomosis in the hepatic artery, portal vein, and vena cava. From our experience with more than 600 liver transplants, vascular stenoses have been identified in 10 patients and treated by balloon angioplasty in nine. Three patients with hepatic artery stenosis and deteriorating graft function were treated by balloon angioplasty with a coaxial technique. A specially designed catheter facilitated a successful femoral artery approach. Portal vein stenoses in three patients resulted in portal hypertension. These were treated by balloon dilatation via transhepatic catheterization of the portal vein. Stenoses of the suprahepatic caval anastomosis were dilated in three patients with severe lower limb edema. Technical success was achieved in all three cases of hepatic artery stenosis with improvement in graft function. Recurrent stenoses in two patients were successfully treated with repeated dilatations. Portal hypertension resolved in two of three patients after portal venoplasty. Dilatation of a caval stenosis resulted in the resolution of leg edema in all three cases. Repeated dilatation was required in one case. No reduction in the portal venous pressure gradient occurred after venoplasty in one case, and an ultimately fatal caval thrombosis developed in one patient with caval stenosis before venoplasty could be performed. Our experience suggests that balloon angioplasty of arterial and venous stenoses complicating hepatic transplantation carries little risk and is a useful procedure for the treatment of these problems.     

MR-guided intravascular procedures: real-time parameter control and automated slice positioning with active tracking coils

PURPOSE: To implement and optimize a real-time pulse sequence and user interface to perform intravascular interventions using active catheter tracking. MATERIALS AND METHODS: In magnetic resonance (MR)-guided interventions, small radio-frequency coils can be used to rapidly determine the device position (active tracking). In this work, active catheter tracking was combined with a dedicated real-time pulse sequence and user interface. The pulse sequence offered the imaging contrasts fast low angle shot (FLASH), true Fast imaging with steady state precession (TrueFISP), and projection MR digital subtraction angiography (MR-DSA), which could be selected by the radiologist from within the scanner room at any time during the intervention. Automatic slice positioning was added to the real-time pulse sequence so that the location of the tracking coils defined the image slice position and orientation. The technique was assessed in phantoms and animal experiments. RESULTS: At a reaction time of 24 msec and a frame rate of three images per second, the movement of an active intravascular catheter could be monitored in the aorta and the renal arteries of a pig. With interactive contrast and orientation changes, the renal vasculature could be assessed by a fully MR-guided catheterization in less than 10 minutes. CONCLUSION: With carefully designed active catheters, a dedicated user interface, and an optimized pulse sequence intravascular interventions can successfully be performed by a single operator from within the MR scanner room.