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Experimental Studies |
1 From the Department of Radiology, Johns Hopkins University School of Medicine, Outpatient Center Rm 4243, 601 N Caroline St, Baltimore MD 21287-0845. Received September 21, 1999; revision requested November 1; final revision received February 4, 2000; accepted February 22. Supported in part by National Institutes of Health grant number R29HL57483, the Whitaker Foundation, and Surgi-Vision. Address correspondence to X.Y. (e-mail: xyang@mri.jhu.edu).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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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.
Index terms: Animals • Interventional procedures, experimental studies, 943.1282, 943.7229 • Magnetic resonance (MR), contrast enhancement, 943.12943, 961.12943 • Magnetic resonance (MR), guidance, 943.1282 • Magnetic resonance (MR), vascular studies, 81.129412, 81.12942, 81.12943, 943.129412, 943.12942, 943.12943
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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Other investigators (5–7) developed an active tip-tracking technique in which the position of an interventional instrument is determined from the MR signal received by a miniature radio-frequency coil attached to its tip. Active tip-tracking provides robust determination of the position of the device tip and offers higher tracking speeds. However, since only the tip of the device can be located, possible kinks in the body of the device cannot be observed during active tip tracking. In addition, the miniature radio-frequency coils may have several limitations, such as reduced maneuverability of an interventional device and reduced suitability for use in the tracking of microcatheters and guide wires at MR imaging.
Recently, an alternate approach to intravascular MR imaging that involves the insertion of an MR loopless antenna receiver into the vessel has been developed (8). This development offers potential applications in intravascular MR imaging of examinations and treatments, for example, (a) MR imaging of vessels, (ie, acquisition of high-spatial-resolution MR angiograms, observation of the cross-sectional images of stenotic arteries, and quantitative and qualitative analyses of atherosclerotic plaques [9]) and (b) real-time MR imaging, which may be an alternative to x-ray fluoroscopy in the guidance of endovascular interventions (10).
Findings of one recent study (11) demonstrated the usefulness of the loopless antenna in monitoring the balloon dilation process in an experimental aortic stenosis. However, in this study, the antenna and balloon catheter were placed parallel to each other in the vessel. This placement required the use of two vascular access sites and the passage of two devices (ie, balloon catheter and antenna) through the stenosis, which increased the risk of technical failure and complications, such as dissection of the vessel wall. Usually, interventional devices such as a balloon catheter have a central channel for use in the placement of a guide wire or in the administration of contrast material. This observation inspired us to explore the possibility of placing a thin loopless antenna in an interventional device; this design would require only one vascular access site.
The objective of the present study was to develop a technique of intravascular MR imaging–guided balloon angioplasty by using a thin loopless antenna, which was expected to function not only as an MR imaging guide wire at real-time MR imaging but also as a guide for the endovascular interventional procedures.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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Devices
Loopless antenna MR imaging guide wire.—We produced a 75-cm-long loopless antenna consisting of a soft conducting wire that was an inner conductor from a 50-ohm, 0.6-mm, coaxial cable with a polyester jacket (Pico-Coax, Axon Cable, Norwood, Mass); the wire was extended to fit in a 40-cm-long balloon catheter. The inner and outer conductors of the coaxial cable were made of silver-plated copper alloy with a fluoroethylene polymer as the dielectric. The proximal end of the coaxial cable was connected through a matching tuning-decoupling circuit to the MR imager. The conducting wire was 9 cm in length and 0.4 mm in diameter; the coaxial cable was 66 cm in length and 0.6 mm (2.1 F) in diameter.
Since this design had a simple structure, it was relatively easy to construct an antenna with a very small diameter, which allowed it to be passed directly through the aortic stenosis or directly inserted into the central channels of the interventional catheters (Fig 1). Thus, the 0.6-mm antenna was expected to function as both an MR receiver probe and a conventional guide wire; it was called an MR imaging guide wire.
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Balloon catheter.—A 5-F, 40-cm-long balloon catheter was used (Meditech, Boston Scientific, Watertown, Mass). The balloon portion was 4 cm in length and 6 mm in diameter, with a burst pressure of 15 atm (1.5 x 106 Pa). There were two alloy (tantalum) rings at the proximal and distal ends of the balloon. The balloon catheter had been tested at MR imaging and was proved to be MR compatible, as it did not produce substantial artifacts.
Cable tie.—To create the experimental stenosis, a 12-cm-long and 2.5-mm-wide plastic (polyethylene) cable tie (Baynesville Electronics, Baltimore, Md) was bound around the exposed upper abdominal aorta of the rabbit. This created a 5-mm-long stenosis, with a reduction of approximately 80%–90% of the cross-sectional lumen when the cable tie was completely tightened. The percentage, or (area of stenotic lumen ÷ area of normal lumen) x 100%, of the experimental stenosis was measured and calculated by observing the cross section of a 6-mm homemade phantom with the same stenosis that was created by completely tightening the same cable tie. In the phantom, we also proved the capability of 100% release of the cable tightness; we fully inflated the balloon and achieved 100% dilatation of the stenosis. Since we always performed the same procedure by completely tightening the same cable tie and by fully inflating the same balloon in each experiment, the 80%–90% reduction of the target aortic lumens and 100% opening of the stenoses were identical in the 14 experiments. The cable tie was tightened in a direction opposite the usual direction so that we could easily slide it open as the balloon inflated. The cable tie was also MR compatible.
Surgical Procedure in Animals
Fourteen male New Zealand White rabbits (Robinson Services, Clemmons, NC; weight, 3.5–4.5 kg) were used. The first group of six rabbits was used for technical refinement, which included surgery, insertion of the balloon catheter and MR imaging guide wire, and development of the MR imaging protocol in which different pulse sequences for different MR techniques were tested. The remaining eight rabbits were used for feasibility investigation of intravascular MR imaging–guided balloon angioplasty, which was validated at MR aortography and renal perfusion MR imaging. The animals were treated according to the Principles of Laboratory Animal Care of the National Society for Medical Research and the Guide for the Care and Use of Laboratory Animals (12). The animal care and use committee at our institution approved the experimental protocol.
The details about general anesthesia and surgery for creation of the aortic stenosis in rabbits are described elsewhere (11). An ear vein was cannulated, which permitted either administration of pentobarbital for maintenance of anesthesia or injection of contrast agent for MR angiography and perfusion MR imaging. Then, 0.3 mL of heparin (1,000 U/mL) was intravenously infused through the ear vein. Nonferromagnetic electrodes were attached to the limbs for surface electrocardiography. Both electrocardiographic and blood pressure signals were used to monitor the condition of the animal during the experiment. At completion of the experiments, the animals were euthanized by means of intravenous injection of 100 mg of pentobarbital per kilogram of body weight.
MR Imaging Techniques
Intravascular MR imaging–guided balloon angioplasty.—All experiments were performed with a Signa LX 1.5-T cardiac MR unit (GE Medical Systems, Milwaukee, Wis) with a maximum gradient of 40 mT · m-1 and a slew rate of 150 T · m-1 · sec-1. The animals were placed in a supine position in the unit and were aligned with the main magnetic field. The MR imaging guide wire was used in the transmit-receive mode. To record the entire procedure of intravascular MR imaging–guided balloon angioplasty (which included tracking the MR imaging guide wire to locate the stenosis, tracking the balloon catheter to target the stenosis, and monitoring balloon dilation of the stenosis), we used a fast spoiled gradient-echo pulse sequence (5.0/1.4 [repetition time msec/echo time msec]), a 62.5-kHz bandwidth, a rectangular 24 x 12-cm field of view, a 256 x 128 matrix, and no section selection. We acquired a total of 240 images, with 60 images acquired during each of the following 20-second procedures: MR imaging guide wire tracking, balloon catheter tracking, balloon inflation and deflation, and balloon and antenna withdrawal. With this MR protocol, we performed intravascular real-time MR imaging at a rate of three frames per second.
MR aortography and renal MR perfusion imaging.—Two-dimensional contrast-enhanced coronal MR aortography with renal perfusion MR imaging was performed with manual injection of 1 mL of gadopentetate dimeglumine (Magnevist; Berlex Laboratories, Wayne, NJ; 469 mg/mL) into the ear vein at rate of approximately 0.5 mL/sec. We used a cardiac phased-array coil (GE Medical Systems), a fast spoiled gradient-echo pulse sequence (6.3/1.7), 60° flip angle, 31.2-kHz bandwidth, 24-cm field of view, 256 x 160 matrix, 10-mm section thickness (to depict both kidneys and the aorta), one frame per second, and a total imaging time of 2 minutes. After injection of the contrast agent, a 3-mL saline bolus was injected to ensure rapid delivery of the entire dose into the vein.
Intravascular MR Imaging–guided Balloon Angioplasty
By using a coaxial technique (ie, placing the MR imaging guide wire into the 5-F balloon catheter), we first tracked the MR imaging guide wire through the introducer into the upper abdominal aorta until the aortic stenosis was located. The entire tracking process was performed with intravascular real-time MR imaging by using the same MR imaging guide wire. Then, the balloon catheter and the MR imaging guide wire were delivered and positioned into the stenosis. The central point of the balloon and the most sensitive region of the MR imaging guide wire (ie, point between the extended inner conductor and coaxial cable body) were adjusted and centered just at the level of the target stenosis so that high-spatial-resolution MR images of the target vessel could be obtained.
The balloon was inflated by manually injecting 2 mL of contrast medium with a 20-mL plastic syringe. The contrast medium used consisted of a 6% solution of the prior contrast medium (28 mg/mL) diluted with saline. The 6% concentration was previously found to be optimal for use with the fast spoiled gradient-echo pulse sequence (11). The pressure for manual inflation of the balloon was approximately 8–9 atm (8.1–9.1 x 105 Pa). Balloon inflation was started 4-6 seconds after MR imaging began. Inflation was maintained by means of manual control for 10 seconds and was terminated 4–6 seconds before the completion of MR imaging. Then, postoperative MR aortography and renal perfusion MR imaging were performed by using 1 mL of the contrast medium and the same MR protocols as those used at preoperative MR aortography and renal perfusion MR imaging. The interval between pre- and postoperative MR angiography and renal perfusion MR imaging was approximately 30–40 minutes, which allowed for near-complete emptying of the kidneys.
In this study, we performed balloon angioplasty and recorded the results after each acquisition. We evaluated the appearance of the MR imaging guide wire, the course of the guide wire into the aortic stenosis, and balloon dilation of the stenosis and than compared changes in aortic flow and renal contrast enhancement before and after MR imaging–guided balloon angioplasty.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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The MR imaging guide wire passed through the aortic stenosis in all 14 cases. The aortic stenosis were clearly depicted at MR aortography and were always visible during tracking of the MR imaging guide wire (Fig 2). In two cases, the MR imaging guide wire traveled into the aortic branches, such as the renal arteries, during the first attempt. However, the MR imaging guide wire was removed, and after two additional attempts, it reached the stenoses. Motion artifacts from either the aorta or MR imaging guide wire were not notable primarily because of the fast imaging acquisition technique that we used and the high heart rate of rabbits, which reduced the pulsatility of the blood flow.
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In the group of eight rabbits used in the feasibility investigation, two-dimensional contrast-enhanced MR aortography also clearly demonstrated decreased flow in the aorta distal to the stenoses before balloon angioplasty, whereas aortic distal flow was restored after balloon angioplasty (Fig 3). The experimental aortic stenoses resulted in only mild contrast enhancement in the cortex of both kidneys at preoperative renal perfusion MR imaging. After balloon angioplasty, contrast enhancement immediately increased and quickly expanded in the entire kidney (Fig 3). Time to peak contrast enhancement in the kidneys before balloon angioplasty was longer than 2 minutes, whereas time to peak enhancement after balloon angioplasty was in the range of 30–40 seconds in our eight experiments. One milliliter of intravenously administered contrast material was enough for detection of the aortic stenosis and examination of renal perfusion in the same imaging plane.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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A characteristic of the intravascular MR loopless antenna (MR imaging guide wire) used in our study is that its entire length in the field of view can be depicted during MR tracking. This is different from passively or actively tracked MR imaging guide wires with which only the signals from the paramagnetic rings or miniature radio-frequency coil can be depicted (6,13). The MR imaging parameters used in this study enabled us to see the MR imaging guide wire and surrounding tissues, such as blood, as a 5–6-mm bright band and to clearly outline the stenosis of the 6-mm vessel. One group (10) reported that the loopless antenna with the highest signal intensity stands at the central line of this bright band, and adjustment of the radio-frequency power applied to the antenna can change the thickness of this bright band.
Other characteristics of the loopless antenna include the following: (a) high sensitivity to the MR signal along its entire length; (b) sensitivity that is inversely proportional to the distance it; (c) production of very high signal around it when it is used as a transmitter-receiver probe; and (d) ability to be used in the creation of projection image, since the antenna localizes the MR signal around itself and does not require section selection (10).
The tip of the antenna is loopless and is essentially a dipole. Since the antenna has limited space for the placement of a tuning and coupling component at its distal (neck) portion (between the tip of the inner conductor wire and long coaxial cable), this MR imaging guide wire is tuned and matched at its proximal end (at the interface between the coaxial cable and surface coil input of the imager); this end remains outside the vessels and does not have performance degradation. This design enables us to make the loopless antenna with a very small diameter. Currently, the thinnest MR imaging guide wire has a diameter of 0.6 mm and can be easily built in our laboratory.
Cardiovascular MR imaging techniques provide high-spatial-resolution images of the vessel wall and atherosclerotic plaques (15,16) that are not possible with X-ray fluoroscopic techniques. Intravascular MR technology provides us with the opportunity to combine high-spatial-resolution MR imaging with interventional procedures in a single setting. The intravascular MR loopless antenna was shown to be useful in the acquisition of high-spatial-resolution MR images of the aortic wall (9) and in the performance of intravascular MR fluoroscopy (10,11).
In the present study, we produced a thin (0.6-mm) loopless antenna that enabled us not only to generate intravascular real-time MR images but also to locate the stenosis and guide the balloon catheter into the stenosis. These overall results suggest that the loopless antenna may be used as an MR imaging guide wire, functioning either as an intravascular MR receiver probe in the acquisition of high-spatial-resolution MR images and at MR fluoroscopy or as a conventional guide wire for endovascular interventions performed with MR imaging guidance. Indeed, for reasons of safety and for operational purposes (such as torque control, subselective placement, and negotiation of hard atherosclerotic lesions), an MR imaging guide wire with both imaging and guidance functions is required.
This experiment was performed by using an animal model with a short stenosis in a straight vessel. To negotiate complex, tortuous, and eccentric atherosclerotic stenoses, the MR imaging guide wire must be modified before clinical use. Various properties of the MR imaging guide wire—including its ability to be torqued, resistance to kinking, lubricity, floppiness, preset or shapeable curvature, stiffness, bluntness, and taper—need to be made similar to those of standard conventional guide wires.
An important concern about active MR tracking is the heating caused by the radio-frequency conductive parts of the devices from either the miniature coils attached to the conventional guide-wire tips or from the entire coaxial cable body of the loopless antenna. To our knowledge, there are few data available on the safety of the intravascular MR technique. Some investigators (6) have found that when the active tracking guide wires are used in the transmit/receive mode, no temperature rise can be detected. Others (17) tested a field-inhomogeneity catheter that was equipped with a current induction wire. There was no proof of electrically induced damage in the vessel wall. Recently, we also evaluated the local thermal effect of the 0.6-mm MR imaging guide wire and found no evidence of radio-frequency–induced thermal injury during intravascular MR imaging of a normal rabbit aorta (18). However, the biologic and operative safety of this intravascular MR technology needs to be extensively evaluated and confirmed.
Functional MR pre- and posttherapeutic interventions are a current reality, and the online monitoring of the effectiveness of such interventions for guidance of the next therapeutic step promises more effective patient care. The combination of MR angiography, perfusion MR imaging, and MR imaging–guided interventions will refine the method of comprehensive management of organ ischemia with the use of a single modality in a single sitting. MR angiography can be used to assess the angioplastic site itself, while perfusion MR imaging can be used to validate the physiologic response to angioplasty.
In conclusion, we demonstrated an alternative approach to balloon angioplasty with the use of an MR imaging guide wire. The MR imaging guide wire offers the potential to function as either an MR receiver probe for real-time MR imaging or as a conventional guide wire for use in endovascular interventional procedures.
Practical application: Cardiovascular MR technology provides high-spatial-resolution images of the vessel (including the vessel wall), multiple diagnostic evaluations of organ function and morphology, and imaging in multiple planes without risk of ionizing radiation. Combined with MR angiography and functional MR imaging, the development of intravascular MR imaging–guided interventions is an important step toward future online comprehensive management of cardiovascular atherosclerotic diseases with MR technology. This potential should facilitate clinical acceptance and, therefore, more extensive clinical use of MR methods to improve the diagnosis and treatment of cardiovascular disorders.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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Author contributions: Guarantors of integrity of entire study, X.Y., E.A.; study concepts and design, X.Y., E.A.; definition of intellectual content, X.Y., E.A.; literature research, X.Y., E.A.; experimental studies, X.Y., E.A.; data acquisition and analysis, X.Y., E.A.; manuscript preparation, editing, and review, X.Y., E.A.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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