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Published online before print December 10, 2004, 10.1148/radiol.2341031950
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(Radiology 2005;234:558-562.)
© RSNA, 2004

Technical Developments

Feasibility of Stent Placement in Carotid Arteries with Real-time MR Imaging Guidance in Pigs1

Lei Feng, MD, PhD, Charles L. Dumoulin, PhD, Stephen Dashnaw, BS, Robert D. Darrow, MS, Robert L. DeLaPaz, MD, Paul L. Bishop, BS and John Pile-Spellman, MD

1 From the Department of Radiology, Columbia University, 177 Fort Washington Ave, MHB 8SK, New York, NY 10032 (L.F., S.D., R.L.D., J.P.S.); and General Electric Global Research Center, Schenectady, NY (C.L.D., R.D.D., P.L.B.). Received December 1, 2003; revision requested February 10, 2004; revision received March 2; accepted March 29. J.P.S. supported by a grant from GE Medical Systems. Address correspondence to J.P.S. (e-mail: jp59@columbia.edu).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and Methods
Results
 Discussion
 REFERENCES
 
All examinations were performed with approval from the institutional animal care and use committee of Columbia University. To assess the feasibility of real-time magnetic resonance (MR) imaging–guided neurovascular intervention in a swine model, the authors placed stents in the carotid arteries of five domestic pigs. Seven-French vascular sheaths were placed in the target carotid arteries via femoral access by using active MR tracking. Ten nitinol stents (8–10 x 20–40 mm) were successfully deployed in the target segments of carotid arteries bilaterally. MR imaging and necropsy findings confirmed stent position. Necropsy revealed no gross vascular injury. Study results demonstrated the feasibility of performing real-time MR imaging–guided neurovascular intervention by using an active-tracking technique in an animal model.

© RSNA, 2004


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and Methods
Results
 Discussion
 REFERENCES
 
Interventional magnetic resonance (MR) imaging offers tremendous opportunities for neurovascular intervention. Imaging blood flow (1–4), the vessel wall (5,6), brain perfusion (7), tissue metabolism (8,9), tissue cell death (10,11), and brain function (12) during interventional procedures will facilitate clinical decision making, particularly in the treatment of acute stroke. Although intravenous and intraarterial thrombolysis has been shown, in randomized clinical trials, to be beneficial for treatment of acute stroke (13,14), only a small percentage of patients who come to the emergency department within the 3-hour time window required for intravenous thrombolysis or within the 6-hour time window required for intraarterial thrombolysis receive this therapy. For most patients who have had an acute stroke and receive medical attention beyond the time window, the risk of hemorrhage outweighs the benefits.

Despite recent advancements in computed tomographic technology (15,16), MR imaging is still considered better for diagnosing stroke because of its time-independent depiction of cell death. The use of perfusion-weighted imaging–diffusion-weighted imaging mismatching to select patients for thrombolysis may help to improve outcomes and reduce the risk of hemorrhage and enable thrombolytic treatment to proceed beyond the 3-hour time window (17–20). Combining diagnosis and treatment applications in one imaging examination will streamline the triage process and save the time required to transport the patient from the imaging unit to the treatment suite, particularly when intraarterial thrombolysis is the treatment of choice.

Carotid artery stent placement is a procedure that is gaining acceptance (21,22). There are several potential advantages to performing carotid artery stent placement with MR imaging guidance. For example, blood flow examination with phase-contrast MR angiography can provide valuable hemodynamic information on carotid artery stenosis (3). Cerebral perfusion examinations, particularly those performed with patient hypoventilation or after acetazolamide administration, can be used to predict the risk of infarction and to evaluate cerebral vascular reserve (23,24). Furthermore, the emerging technology of MR imaging–guided endoluminal gene therapy adds a new dimension to carotid artery stent placement (25,26).

Passive- and active-tracking techniques have been developed for use in interventional MR imaging (27–30). The passive- tracking method involves the use of either a contrast agent–filled catheter (31,32) or the susceptibility artifacts of the guidewire or stent (33–37) to visualize the device on a conventional MR image. Although passive methods require little modification to the MR imaging system hardware, they are acquisition dependent and limited because they yield low temporal resolution. Furthermore, the tip of the device must remain within the imaging plane to be visualized, and this often requires great care.

Compared with passive tracking, active MR tracking offers improved temporal resolution and device conspicuity. Small receiver coils that are incorporated into the body of the device allow the catheter to be identified in real time and in three dimensions (29). Active MR tracking devices have been used in animal models to place catheters in first-order vessels, such as the renal, mesenteric, and coronary arteries (38–40).

The carotid arteries in pigs are similar in branching order to and slightly larger than the middle cerebral arteries in humans (41). In addition, these vessels in pigs can be catheterized with active MR tracking techniques (42). Thus, the purpose of this study was to assess the feasibility of real-time MR imaging–guided neurovascular intervention by placing stents in carotid arteries in a swine model.


   Materials and Methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results
 Discussion
 REFERENCES
 
Animals and Procedures
All examinations were performed with the approval of the institutional animal care and use committee of Columbia University. Real-time MR imaging–guided stent placement in carotid arteries was performed in five of six 18–25-kg female domestic pigs (Animal Biotech Industries, Danboro, Pa). The carotid arteries were catheterized with active-tracking devices, as previously described (42). The first pig was used to optimize the intraarterial contrast agent–enhanced MR angiographic imaging parameters. These parameters were then used to image the five animals with implanted stents.

The procedures were performed by using an MR imaging–compatible anesthesia machine (Anestiva/5; Datex Ohmeda, Madison, Wis) to administer isoflurane (Henry Schein Pharmaceutical, Long Island, NY) to the pigs for general anesthesia. Heart rate, respiration rate, invasive arterial blood pressure, oxygen saturation, end-tidal CO2 level, and concentration of exhaled agent were monitored in the pigs throughout the procedure by using the monitors on the anesthesia machine and the Millennia Vital Signs Monitoring System (In vivo Research, Orlando, Fla). The body temperature of the animals was monitored with an oral mercury thermometer and was recorded every 15–20 minutes.

Percutaneous access to the right femoral artery was gained on the MR imaging table by using a micropuncture kit. A 9-F vascular sheath was inserted into the right femoral artery over a 0.035-inch guidewire (Terumo; Boston Scientific, Natick, Mass). The animals were intravenously injected with 5000 IU of heparin at the beginning of the study. Additional doses of heparin were administered before stent placement according to the time that had elapsed since the beginning of the procedure. No radiographic equipment was used during the entire stent placement procedure.

The MR intervention suite was conveniently set up to enable the operators (L.F., J.P.S.) to manipulate the catheters while looking at the active-tracking MR images. The anesthesia machine and the monitoring system generated information on the cardiac and respiratory functions of the animals that was sufficient for the operators to manage the care of acutely ill patients in an MR imaging suite. Although the MR unit (TwinSpeed; GE Medical Systems, Milwaukee, Wis) was equipped with an in-room control keyboard, this was cumbersome to use because of the limited space on the MR imaging table. Instead, the operators used a headphone with a built-in microphone to relay MR pulse sequence and parameter instructions to the MR technologist (S.D.) in the control room; this protocol was easier to use.

The length of the 90-cm 7-F sheath was marked on the 5-F active-tracking catheter before the catheter was used by one of the two operators (L.F. or J.P.S.) to cannulate the target carotid artery with real-time MR imaging guidance. Because domestic pigs do not have cervical internal carotid arteries (41), the catheter tip was placed at the level of the thyroid cartilage to aid in placing landmarks and performing postmortem dissection. After the catheter position was confirmed with intraarterial contrast-enhanced (with gadolinium diethylenetriaminepentaacetic acid; Sigma, Grand Island, NY) MR angiography (42), the 90-cm 7-F extension sheath was advanced over the 5-F catheter into the target carotid artery so that the tip of the 7-F sheath matched the tip of the 5-F catheter. The 5-F active-tracking catheter and the guidewire were removed, and the 7-F sheath was filled with 5% gadolinium diethylenetriaminepentaacetic acid.

The position of the contrast agent–filled 7-F sheath was confirmed by using a three-dimensional fast spoiled gradient-recalled acquisition (FSPGR) MR imaging pulse sequence and the following parameters: 4.3/minimum (repetition time msec/echo time msec), 30° flip angle, 62.5-KHz bandwidth, 28 x 28-cm field of view, 1.4-mm section thickness with 0 skip, 256 x 128 matrix, and one signal acquired. The operators (L.F. and J.P.S.) compared the active-tracking and the three-dimensional FSPGR MR images to assess the concordance of the catheter positions.

Stent Placement
Because there is no stent specifically designed for MR imaging–guided deployment, we used 8–10 x 20–40-mm self-expanding nitinol stents (Smart Stent; Cordis, Miami, Fla), which are routinely placed in carotid arteries with radiographic guidance. Although nitinol stents are MR imaging compatible, the stainless steel mandrel of the delivery system precluded the direct visualization of the devices with MR imaging before they were deployed. Therefore, before the stent delivery system was inserted, it was measured to the length of the 7-F sheath and then advanced into the target carotid artery inside the sheath to match the sheath and the artery tip to tip. The 7-F sheath was pulled back to uncover the stent delivery system, and the stent was deployed into the target carotid artery. Consequently, the stent position matched the position of the tip of the 5-F active-tracking catheter. The left carotid artery stent was always deployed first, before the right stent was deployed.

Stent Evaluation
After removal of the stent delivery system, post–stent placement coronal two-dimensional FSPGR MR imaging of the neck was performed with 84/1.9, a 90° flip angle, a 20 x 20-cm field of view, a 3-mm section thickness, a 256 x 256 matrix, and three acquisitions. The operators compared the positions of the stent on these two-dimensional FSPGR MR images with the positions of the contrast agent–filled catheter on the three-dimensional FSPGR MR images. After both carotid stents were deployed, the animals were euthanized by means of barbiturate overdose. To confirm the stent position relative to the thyroid cartilage, as shown on the two-dimensional FSPGR images, and to identify possible vascular dissection and perforation, one of the operators (L.F. or J.P.S.) performed necropsy on all animals by making a midline incision.


   Results
TOP
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results
Discussion
 REFERENCES
 
Use of the active-tracking system enabled placement of catheters in the target segments of the carotid arteries within 5 minutes after the insertion of the catheter at the femoral artery access site. The position of the contrast agent–filled 7-F sheath in the carotid arteries was revealed by using a 16-second three-dimensional FSPGR sequence (Fig 1). The operators (L.F., J.P.S.) agreed that the catheter position was the same as the active-tracking catheter position on all occasions.



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  		Figure 1. Guiding catheter in left common carotid artery of a pig. Coronal maximal intensity projection three-dimensional FSPGR MR image of swine neck obtained by using very short repetition and echo times (4.3/minimum, 30° flip angle, 62.5-KHz bandwidth, 28 x 28-cm field of view, 1.4-mm section thickness with no intersection gap, 256 x 128 matrix, one signal acquired) clearly depicts 90-cm 7-F vascular sheath filled with 5% gadolinium diethylenetriaminepentaacetic acid.

 
A total of 10 stents were placed bilaterally in the carotid arteries. The stents were seen in the target segment of the common carotid arteries in all animals on the coronal post–stent placement FSPGR MR images. The nitinol stents were well visualized owing to susceptibility artifacts; even the struts were seen (Fig 2, A). Bilateral stent placement in the common carotid arteries was completed in all five animals, and necropsy findings confirmed the positions of all 10 stents (Fig 2, B). Upon opening the carotid sheath, we observed shiny white adventitia along the entire length of the bilateral common carotid arteries, without evidence of mural hematoma formation or visible perforation, in all animals.



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  		Figure 2. Bilateral stent placement in carotid arteries of a pig. A, Coronal two-dimensional FSPGR MR image (84/1.9, 90° flip angle, 20 x 20-cm field of view, 3-mm section thickness, 256 x 256 matrix, three acquisitions) with heavy T1 weighting shows hyperintense blood in lumen of bilateral carotid arteries. The stents (arrows) are identified owing to the susceptibility artifacts caused by the nitinol struts. Inferior margin of left stent is intentionally matched with superior margin of right stent. B, Neck tissue specimen obtained at postmortem dissection shows the stents (arrows) in the same positions as shown in A. The stents, which are 8 mm in diameter, are not fully expanded because the common carotid arteries are only 5 mm in diameter. Both common carotid arteries have shiny adventitia without evidence of perforation or mural hematoma formation.

 

   Discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results
 Discussion
REFERENCES
 
The results of this animal study demonstrate that it is feasible to perform neurovascular interventions with real-time MR imaging guidance by using a conventional clinical MR imaging unit. Hybrid radiographic and MR imaging systems have been developed so that limited x-ray fluoroscopy can be used to supplement inadequate MR imaging guidance (43,44). Although these dedicated systems currently are available only in major academic centers, they are useful research tools for helping to establish interventional MR imaging in the clinical arena and increase public awareness of the potential of interventional MR imaging. However, owing to their limited use and availability, it is unlikely that these systems will have a major clinical effect.

The current study was performed entirely with a conventional MR imaging system, without any radiographic equipment. The improved visualization of the catheters and guidewires with use of the active-tracking system enabled sufficient guidance for the catheterization of distal and difficult vessels, negating the need for radiographic backup. Because MR imaging can depict not only the flow characteristics within the vessel lumen but also the vessel wall and the surrounding tissues, in the future, interventional MR imaging may be superior to conventional fluoroscopy at revealing potential complications of the given intervention and thus facilitate the improved safety of the procedure. The described active-tracking system is designed to work with only minor hardware modifications to the clinical magnets currently used in most hospitals. Therefore, the technology can be rapidly disseminated, and this availability may enable treatment of the more than 700 000 patients who have strokes each year (45).

Patients with neurovascular diseases often have altered mental status and require general anesthesia or deep sedation with propofol for interventional procedures. For this reason, we used general anesthesia in this animal study. In addition, we worked closely with anesthesiologists to identify potential anesthesia-related problems and incorporated their suggestions to improve the setup of our MR imaging intervention suite. The availability of MR imaging–compatible anesthesia machines enables one to address difficult respiratory and cardiovascular issues in the MR imaging suite, and MR imaging intervention will become even safer as anesthesiologists gain more experience sedating and anesthetizing critically ill patients during MR imaging examinations.

This study was limited by the lack of direct visualization of stent deployment. The catheter-matching technique used in this study cannot be applied to patients, in whom direct visualization of the stent deployment process is necessary. Although the unambiguous identification of the catheter and the wire tip and the rapid real-time update of device position greatly facilitated the catheterization of target vessels, it is likely that both active and passive tracking techniques will be required to visualize the devices optimally in clinical practice. Once access to the target vessel is gained by using active tracking—this is usually the most difficult part of the procedure—the deployment of interventional devices requires not only identification of the device location but also visualization of the device’s shape, motion, and relationship with the vessel wall.

Passive tracking performed by using contrast agent–filled devices or the susceptibility artifacts caused by the devices should have important roles in interventional MR imaging (27,31,37,46–48). In this study, we found that active-tracking coils functioned in some braided catheters despite susceptibility artifacts; this finding suggests that with careful design and material selection, combined active and passive tracking devices can be developed. Collaboration with device manufacturers and pharmaceutical companies is required to develop U.S. Food and Drug Administration–approved MR imaging–guided interventional devices for clinical trials.

Nonetheless, it is hoped that the experiments described herein will set the stage for future studies of acute stroke treatment with real-time MR imaging guidance, which, when perfected, may represent a major clinical advancement. A large number of patients have ischemic strokes each year (45). The ability to monitor tissue status with MR imaging has tremendous advantages for the large number of patients who are currently prohibited from stroke treatment (18,19,49–53). In contrast, cardiac intervention is a more challenging environment for interventional MR imaging applications owing to the motion of the beating heart. In addition, little is gained by performing cardiac intervention with MR imaging guidance rather than with x-ray methods (37,39,54), except in pediatric patients, in whom radiation exposure is a major concern.

Tumor embolization may be another area of advancement in interventional MR imaging, and the benefits of this modality in this setting require further evaluation (43,55). Acute stroke treatment with real-time MR imaging guidance, by potentially enabling increased numbers of patients to be treated with thrombolysis (56–60), offers huge social and economic benefits. More attention should be directed and more resources should be allocated toward the development of real-time MR imaging–guided stroke treatments.


   ACKNOWLEDGMENTS
 
We are grateful for the editorial assistance provided by Michael Baytion, MD, and Guarav Gupta, MD, and for the guidance of Philip O. Alderson, MD.


   FOOTNOTES
 
Abbreviation: FSPGR = fast spoiled gradient-recalled acquisition

See also the other article by Feng et al in this issue.

C.L.D. is the inventor of the MR tracking method described in this article, which is owned by General Electric Global Research Center. R.D.D. is one of the patent holders of this MR tracking method.

Author contributions: Guarantors of integrity of entire study, J.P.S., C.L.D.; study concepts, L.F., J.P.S., C.L.D.; study design, L.F., J.P.S., C.L.D., R.D.D.; literature research, L.F., R.L.D.; experimental studies, L.F., J.P.S., S.D., P.L.B.; data acquisition, L.F., S.D., P.L.B.; data analysis/interpretation, L.F., J.P.S., R.L.D., S.D.; manuscript preparation, editing, and revision/review, L.F., C.L.D., R.L.D., J.P.S.; manuscript definition of intellectual content, L.F., C.L.D., J.P.S.; manuscript final version approval, L.F., J.P.S.


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