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Vascular and Interventional Radiology |
1 From the Department of Radiology, University of Regensburg, Klinikum der Universität, Franz-Josef-Strauss-Allee 11, D-93042 Regensburg, Germany. Received March 31, 2000; revision requested May 23; revision received September 12; accepted September 19. Address correspondence to C.M. (e-mail: chmanke@web.de).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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MATERIALS AND METHODS: Thirteen patients with 14 iliac arterial stenoses were examined prospectively. Angioplasty was performed through a femoral sheath by using a conventional 1.5-T MR imaging system. Stents and catheters were visualized on the basis of their artifacts. Nitinol stents were placed with gradient-echo MR imaging guidance. Angioplasty balloons were inflated with gadolinium-based contrast material. Results were evaluated clinically and with both digital subtraction angiography (DSA) and contrast material–enhanced MR angiography.
RESULTS: Ten of 13 patients were treated with technical success by using MR imaging–guided intervention alone. Three patients were treated with additional fluoroscopic guidance, because complications (ie, panic attack, subintimal recanalization, and stent misplacement) occurred with MR guidance. The quality of the postinterventional contrast-enhanced MR angiograms of three of 12 lesions with stents was limited owing to stent-induced signal loss of the lumen. The mean stenosis degree after stent placement was significantly higher at contrast-enhanced MR angiography than at DSA (24.6% vs 6.2%). The mean MR imaging–guided procedure time was 74 minutes.
CONCLUSION: MR imaging–guided stent placement in iliac arteries is feasible in select patients. The presented technique has limitations—that is, long procedure times, lack of real-time monitoring, and stent artifacts—that necessitate further modifications before it can be recommended for clinical use.
Index terms: Angiography, comparative studies, 984.122, 984.12942 • Arteries, grafts and prostheses, 984.1268, 984.1286 • Arteries, stenosis or obstruction, 984.72 • Arteries, transluminal angioplasty, 984.1282, 984.1286 • Magnetic resonance (MR), vascular studies, 984.129412, 984.129416, 984.12942, 984.12943
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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The remaining 13 (42%) patients (seven men, six women; mean age, 63.9 years; age range, 48–75 years) with chronic limb ischemia and focal iliac arterial stenosis were enrolled for MR imaging–guided procedures. Chronic limb ischemia was classified according to Society of Cardiovascular and Interventional Radiology (SCVIR) criteria (9). Twelve patients presented with claudication, and one patient was asymptomatic and had high-grade stenosis of an iliac anastomosis after aortoiliac bypass graft placement for an abdominal aortic aneurysm 3 months previously. Twelve patients had one lesion, and one patient had two stenoses on the same side. All lesions had been diagnosed previously by using digital subtraction angiography (DSA). All patients were informed about the background and procedures of the study and gave written consent. The study protocol was approved by the institutional review board.
MR Imaging
The entire intervention was performed with a 1.5-T MR imaging unit (Magnetom Symphony; Siemens, Erlangen, Germany) by using a circular polarized, phased-array body coil. The short magnet of the imaging unit (1.6-m) allowed sufficient access to the femoral sheath. Coronal contrast material–enhanced three-dimensional fast imaging with steady-state precession MR angiography was used to verify the intraluminal position of the catheter and localize the stenosis (Fig 1) and for postinterventional evaluation. Sequence parameters were 4.7/1.9 (repetition time msec/echo time msec), 25° flip angle, 30-cm field of view, 157 x 256 matrix, 69-mm slab thickness, 46 partitions, 1.5-mm effective section thickness, readout bandwidth of 488 Hz per pixel, and acquisition time of 25 seconds. Twenty milliliters of gadopentetate dimeglumine (0.5 mol/L) (Magnevist; Berlex Laboratories, Wayne, NJ) was injected into a cubital vein with a power injector (Spectris; Medrad, Pittsburgh, Pa) at 2 mL/sec. The imaging delay was 20 seconds. For each contrast-enhanced MR angiographic sequence, 12 maximum intensity projection (MIP) images were reconstructed from the coronal images in 15° steps. The MIP and source images were evaluated.
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Imaging was started at an in-room console, and MR images were displayed on a liquid crystal display monitor adjacent to the magnet (In-Room MR Console; Siemens). Vessel diameters and artifact sizes were measured by using the standard software on the MR unit (NUMARIS 3.5a11b; Siemens).
Interventional Procedure
By using the Seldinger technique, a 7-F sheath (Cordis, Miami, Fla) was inserted blindly into the ipsilateral femoral artery through a nitinol guide wire (Terumo, Tokyo, Japan). MR imaging–guided puncture and guide-wire insertion—with an MR imaging–compatible needle, a nitinol wire (Cope; Cook, Bloomington, Ind), and a micropuncture introducer set (Cook)—were necessary in an obese patient (patient 13) with a pulseless femoral artery. The nitinol guide wire was exchanged for a nonbraided 5-F multipurpose or pigtail angiographic catheter with platinum markers (Cook). The intraluminal position was assumed when there was brisk blood flow from the catheter. Intraluminal crossing of the stenosis with the catheter was then verified at initial contrast-enhanced MR angiography as a linear signal void on the source images. The small artifacts of the platinum markers, apparent as small signal voids, enhanced the visibility of the catheter (Fig 2). Initial contrast-enhanced MR angiography also depicted the coordinates of the stenosis along the z direction and the anatomic landmarks adjacent to the stenosis.
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A self-expanding nitinol stent (Memotherm; Bard-Angiomed, Karlsruhe, Germany) was inserted. A stent size that would cover the entire lesion plus a safety margin of at least 1 cm on both sides was selected to avoid incomplete coverage of the atheromatous lesion, which is a known risk factor for restenosis (10). An expanded stent diameter at least 1 mm larger than the estimated normal arterial lumen diameter at contrast-enhanced MR angiography was chosen. The stent mounted in the delivery system was identified as an artifact similar to those of biopsy needles. Advancement of the stent delivery system was performed with MR imaging guidance: After each catheter advancement step (2–3 cm), FLASH images were obtained in modified coronal or sagittal planes. Anatomic landmarks—that is, the aortic bifurcation, vertebral bodies, and acetabulum—and the known coordinates of the stenosis along the z direction depicted on the initial MR angiograms were used to localize the stenosis and the desired stent position (Fig 2). To avoid artifacts, the devices were moved only between consecutive acquisitions.
Monitoring of the stent release was performed with MR imaging because we knew from experience that the nitinol stent we used tends to migrate distally at the beginning of deployment. Stent release could be seen as a broadening of the artifact (mean increase, 2.4 mm; range, 1–6 mm) progressing from the distal to the proximal end of the stent. An additional bending of the flexible stent usually was observed after expansion owing to absence of support from the outer stent catheter.
All stents were dilated after placement by using commercially available angioplasty catheters with a 4-cm balloon length (Blue Max; Meditech/Boston Scientific, Watertown, Mass). Balloon diameters are summarized in the Table. The two platinum markers of the angioplasty catheter were insufficient for identification with the FLASH sequence. The balloons were inflated by manually injecting 0.05 mol/L of gadopentetate dimeglumine with a 10-mL plastic syringe. After partial inflation with a small volume of diluted gadolinium-based contrast material, the angioplasty catheter balloon could be visualized as a 4-cm-long signal void. This balloon artifact was placed within the stenosis marked by the stent and inflated completely.
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In cases of complications and/or ambiguous device visualization with MR imaging guidance, stent placement had to be continued with fluoroscopic guidance in a nearby angiography suite. The entire procedure was conducted with continuous electrocardiographic monitoring. Before stent placement, all patients received intravenously 10,000 U of heparin, 2 g of cefazolin, and 900 mg of lysinmono(acetylsalicylate) (equivalent to 500 mg of aspirin). Following the procedure, patients received 300 mg of aspirin a day indefinitely.
The times required for the following procedures were noted: initial and postinterventional contrast-enhanced MR angiography (scout view acquisition to end of MIP image reconstruction), stent placement (end of initial MIP image reconstruction to removal of the stent catheter), balloon dilation (removal of the stent catheter to removal of the balloon catheter), and entire imaging-guided procedure (initial scout view acquisition to last MIP image reconstruction). Differences in procedure time were calculated by performing the nonparametric Mann-Whitney rank sum test. Primary technical success was assumed when MR imaging–guided angioplasty alone resulted in residual stenosis of less than 30% of the diameter of the nonstenotic vessel segment at DSA.
The DSA images and contrast-enhanced MR angiograms were evaluated prospectively and in a nonblinded manner by two experienced vascular radiologists (C.M., J.L.). A final decision was made by means of consensus. Maximum stenosis degree was measured on the digital subtraction and contrast-enhanced MIP MR angiograms by using a graded jeweler’s eyepiece. The image quality of the postinterventional contrast-enhanced MR angiograms (ie, source and MIP images) was scored on a three-grade scale: 0 for nondiagnostic; 1 for fair quality, assessment of stent patency possible; and 2 for good quality. The residual stenosis degree on the contrast-enhanced MR angiograms obtained before and after intervention was compared with that on the DSA images. The differences were calculated by performing the paired Student t test. For all statistical tests, a P value of .05 indicated statistical significance. Treadmill and ankle-brachial pressure index tests were performed before and 1–3 days after stent placement. Initial clinical improvement was graded on the SCVIR scale (9).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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Stent misplacement occurred in patient 7. The stent was placed superior to an external iliac arterial stenosis. This misplacement occurred because the patient moved after initial contrast-enhanced MR angiography and the anatomic landmarks were not considered by the operators. Balloon angioplasty of this stenosis resulted in flow-limiting dissection. Postinterventional contrast-enhanced MR angiography depicted the stent misplacement as a signal intensity reduction of the artery superior to the lesion. The angioplasty-induced dissection was depicted as a subintimal contrast material deposit, but the flow-limiting effect was underestimated at MR angiography. This failure was treated successfully with additional fluoroscopy-guided stent placement.
The artifacts used to visualize the stents and balloons at two-dimensional FLASH imaging generally appeared larger than the nominal sizes of the devices. The mean difference between artifact length and nominal stent length was 13.3 mm (range, 12–15 mm). The unreleased stent with a 2-mm outer diameter produced an average artifact diameter of 7.1 mm (range, 5–10 mm). After complete inflation with diluted gadolinium-based contrast material, the diameter of the balloon artifact was 17 mm (mean range, 13–21 mm), which on average was 9 mm (range, 4–14 mm) larger than the nominal balloon diameter. The mean artifact length of the fully inflated 4-cm-long balloon was 6.8 cm (range, 6.0–7.9 cm).
The degrees of residual stenosis on the MR angiograms and DSA images and the quality of the postinterventional MR angiograms are summarized in the Table. All vessel segments with stents showed a moderate signal loss at postinterventional MR angiography (Fig 3). The evaluation of vessel segments with stents on postinterventional MIP images was limited in three of 12 treated lesions because of stent-induced signal loss of the lumen and high background signal intensity following contrast material injection for initial MR angiography. However, the patency of the segments with stents could be confirmed on the source images obtained in these three patients.
Fourteen stenotic vessel segments were evaluated before intervention with contrast-enhanced MR angiography and DSA. The mean preinterventional stenosis degree was significantly higher at MR angiography than at DSA (81.79% vs 70.71%, P = .001). Twelve vessel segments with stents were evaluated with contrast-enhanced MR angiography and DSA. The mean residual stenosis was significantly higher on the MIP MR angiograms than on the DSA images (24.58% vs 6.25%, P = .015). A lumen reduction of 30% or greater was seen within the stent at MIP MR angiography in three external iliac arteries with stents, whereas DSA depicted no significant stenoses. A common feature of these three lesions was a small vessel diameter (6 and 7 mm). At MR angiography, a bandlike artifact was observed at the cranial end of four stents.
The mean ankle-brachial pressure index in 10 patients treated with MR imaging guidance alone improved from 0.71 to 0.93. The change in limb status in these 10 patients was graded according to SCVIR guidelines as +3, or normalized, in seven patients; +2, or moderate, in one patient; +1, or minor, in one patient; and 0, or no change, in one patient. The two patients with minor or no changes underwent iliac angioplasty either to improve inflow before surgical bypass graft placement for femoral occlusion (patient 11) or to prevent bypass occlusion owing to asymptomatic high-grade anastomotic stenosis after aortoiliac bypass surgery (patient 10). In addition to the complications of subintimal recanalization and stent misplacement described previously, there were three cases of minor groin hematoma that did not require surgery and one common femoral artery pseudoaneurysm, which thrombosed spontaneously.
The mean MR imaging–guided procedure time was 73.7 minutes (range, 47–122 minutes). Pre- and postinterventional contrast-enhanced MR angiography required a mean of 22 minutes (range, 9–38 minutes) in the 11 patients in whom stents were placed with MR guidance. For the 12 stenoses treated with MR guidance, stent placement required a mean of 38 minutes (range, 13–74 minutes) and balloon dilation required a mean of 14 minutes (range, 3–27 minutes). Most of the procedure time for stent placement and balloon dilation was spent finding an adequate MR imaging plane and making adjustments due to patient movement and stent insertion.
There was a steep learning curve in performing MR imaging–guided stent placement for the 12 stenoses in the 11 patients treated with MR guidance. The average MR imaging–guided procedure time decreased significantly from 91 minutes (range, 79–122 minutes) for the first five patients to 60 minutes (range, 47–72 minutes) for the last six patients (P < .01). This decreased time was due to significantly faster stent placement in the last six patients (P < .01). There were no significant changes in procedure time for contrast-enhanced MR angiography and balloon dilation.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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There are two approaches for device localization with MR imaging: Active techniques—that is, tip tracking and profiling—allow real-time imaging, with small coils integrated in the catheters or guide wires on a previously acquired road-map image (16). These techniques require special devices and dedicated hardware. Passive visualization, based on susceptibility artifacts, is a valuable alternative (17). With passive techniques, device visualization is possible with any newer generation MR unit. Select conventional angiographic materials can be used following in vitro evaluation of visibility and safety. The main advantages of these materials are familiar handling, predictable results, and excellent suitability for x-ray fluoroscopy if complications occur.
Our preliminary experience indicates that angioplasty with MR imaging–guided stent placement for simple iliac arterial stenosis is feasible with passive instrument visualization and conventional angiographic material. However, the low primary success rate of 77% and the limited evaluation of vessel segments with stents indicate some major limitations of the described MR imaging technique. Owing to lack of real-time imaging capability, the guide wire had to be advanced without monitoring. This resulted in one case of subintimal crossing of a stenosis with the guide wire and the catheter, as depicted at MR imaging. Real-time monitoring of catheter and wire advancement with a higher frame rate and susceptibility markers on the catheters and wires might have prevented this complication. However, subintimal recanalization of arteriosclerotic lesions can occur also with fluoroscopic guidance, and subintimal angioplasty is described as a feasible recanalization technique (18).
In the current study, the stents usually were placed according to the z coordinates of the stenosis and the anatomic landmarks derived at initial contrast-enhanced MR angiography. This is similar to the fluoroscopy-guided procedure, in which the stent is placed with the help of bone landmarks or an external ruler. However, these z coordinates are unreliable if the patient moves after initial MR angiography. Therefore, MR imaging of anatomic landmarks is necessary to verify the correct stent position. In one patient, stent misplacement occurred because this additional information was not taken into account. The intervention had to be modified to a fluoroscopy-guided procedure in one patient owing to a panic attack after sheath insertion, although this patient had undergone diagnostic contrast-enhanced MR angiography uneventfully the day before. Patients must lie still in the narrow bore of the magnet for about 1 hour or longer, which is unacceptable for some patients. Open 1.0-T MR systems that offer improved patient comfort and acceptable image quality are being developed. Although the imaging-guided procedure time decreased during the course of our study, it has to be reduced further to improve patient comfort, and thus allow more complex interventions, and treatment of complications. Most of the imaging-guided procedure time was spent finding an adequate MR imaging plane and making adjustments due to patient movement and stent insertion. Bücker et al (8) made a similar observation in their swine model. Time-consuming adjustment of the imaging plane is a typical problem in cross-sectional imaging for vascular intervention, and this limitation is not influenced greatly by higher frame rates. Improved experience and strategies to adjust to the appropriate plane might reduce future procedure times.
The nitinol stent used in our study has been reported to show no ferromagnetic properties of the deployment device, allow good visibility of the stent before deployment, and allow acceptable visualization of the stent lumen after deployment in vitro (19). However, our study results demonstrated that contrast-enhanced MR angiographic evaluation is unreliable after stent placement because of bandlike artifacts at the cranial stent end, signal loss within the stent lumen, and artificial lumen narrowing. The bandlike artifacts are due to local magnetic field alterations that result in misregistration in the readout direction (20). Vessel segments with stents displayed a significantly higher degree of residual stenosis on contrast-enhanced MR angiograms than on DSA images and at least a moderate signal intensity reduction within the stent lumen. The signal intensity reduction within the stent lumen is due to shielding of the spins from the radio-frequency pulses (21). These effects depend on the composition, design, and orientation of the stent (15,19,22–25).
Stents made of surgical steel are known to produce large artifacts, whereas tantalum stents are known to induce minor artifacts. Nitinol is known to be nonmagnetic and induce moderate artifacts. Lenhart et al (22) found a wide range of artificial narrowing and stent-induced signal loss within the lumen that were based on the design of nine nitinol stents and stent grafts. Schürmann et al (25) compared two nitinol stents in an animal model. They found that the stent made of a knitted nitinol wire (ZA-stent; Cook) induced substantially fewer artifacts than did the stent made of a laser-cut slotted tube (Memotherm). Link and co-workers (15) reported similar limitations in the contrast-enhanced MR angiographic follow-up of patients after placement of stents of various designs. With the increasing use of contrast-enhanced MR angiography for vascular disease evaluation, further studies are required to better understand the effect of stent design on the MR appearances and to facilitate development of appropriate stents. Indirect criteria for patency, such as cine phase-contrast MR blood flow (26) and intraarterial blood pressure (27) measurements, can be used when contrast-enhanced MR angiography yields ambiguous results after stent placement.
General limitations of the presented technique are the artifact size of the devices and the limited spatial resolution of the two-dimensional FLASH sequence, which was, nevertheless, sufficient for successful stent placement in focal stenoses of straight iliac arteries. However, more complex interventional procedures, such as transjugular intrahepatic portocaval shunt placement and renal arterial stent placement, require precise stent placement, which currently can be achieved only by using fluoroscopy.
The use of conductive guide wires in an MR environment remains a major safety concern. A simple way to avoid this problem is to use a nonmetallic guide wire. Unfortunately, the steerability and flexibility of this wire is inferior to that of modern nitinol wires (28). When Nitz et al (29) evaluated nitinol guide wires with different sequence parameters in vitro and in a volunteer, no heating was observed with the use of gradient echos and low flip angles (
30°). The sequence parameters used in our clinical study were selected accordingly to prevent hazards to the patients and investigators. Nevertheless, the use of conducting wires in MR imaging is hazardous: The selection of inadequate imaging parameters may cause injuries to patients and investigators owing to exposure to high radio-frequency energy. Use of fast, or turbo, spin-echo and gradient-echo sequences with higher flip angles especially may result in burns due to excessive heating (30). Besides these safety concerns, further criteria for the sequences used in this study were high sensitivity to susceptibility artifacts and reasonable spatial resolution. The described concerns resulted in our use of a gradient-echo sequence with a relatively low temporal resolution of 2.4 seconds per frame. This frame rate was sufficient to monitor stent and balloon catheter placement. A higher frame rate would be desirable for more complex interventions, such as recanalization of complex stenoses, occlusion treatment, and selective catheterization. Experimental MR-guided vascular interventions with faster image aquisition and display have been reported in several studies (2,7,8,17). These techniques were not applicable for the MR unit in this study, but we are currently developing sequences that allow near real-time imaging with a frame rate of more than one image per second.
Experience with MR imaging–guided stent placement is very limited. To our knowledge, the first MR-monitored deployment of a nitinol stent graft in vitro was reported in 1996 and involved spin-echo imaging (31). Results of a second study (8) demonstrated the feasibility of MR-guided iliac arterial placement of a self-expanding nitinol stent in a swine model by using a short-magnet 1.5-T system similar to the unit used in the present study and a fast gradient-echo sequence. Kee et al (7) recently reported on MR-guided transjugular intrahepatic portocaval shunt placement in a swine model by using a technique and material similar to those used in our study: passive visualization, nitinol guide wire and stent, and inflation of the angioplasty balloon with gadolinium-based contrast material. Successful portal puncture, stent placement, and balloon dilation were achieved by performing echo-planar imaging at a rate of 3.5 frames per second, with a time lag of about 0.8 seconds to visualize instrument changes on the monitor. The stent was depicted as a signal void, and patency was demonstrated at contrast-enhanced MR portography.
To our knowledge, there are no other reported experiences of either the MR imaging or the MR-guided treatment of angioplasty-induced complications. Although we observed subintimal recanalization of a stenosis, stent misplacement, and angioplasty-induced dissection at MR angiography, all patients underwent DSA to evaluate the technical results and the postinterventional MR angiographic findings and to exclude peripheral embolic complications. Evaluation of peripheral arteries after iliac stent placement is not possible with the described technique, but it may be feasible with further technical developments, such as stepping-table MR angiography and dedicated extremity coils (32).
In conclusion, MR imaging–guided stent placement for simple iliac arterial stenoses is feasible with newer generation diagnostic MR units, passive instrument visualization, and select conventional angiographic material. The major limitations of the presented technique are long procedure times, lack of real-time monitoring for guide wire and catheter manipulation, and limited postinterventional MR evaluation owing to stent-related artifacts. Further technical developments, such as open high-field-strength MR units, devices with susceptibility markers, stents with reduced artifacts, and faster pulse sequences, and further evaluation are necessary before the procedure can be recommended for routine clinical use. However, this study may represent the beginnings of a new imaging technique for percutaneous vascular therapy.
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FOOTNOTES |
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Author contributions: Guarantors of integrity of entire study, C.M., W.R.N., J.L.; study concepts, C.M., S.F., J.L., W.R.N.; study design, C.M., B.D., W.R.N., M.V.; literature research, C.M., W.R.N., M.L.; clinical studies, C.M., B.D., M.L., W.R.N., J.L.; data acquisition, C.M., J.L., B.D., M.V.; data analysis/interpretation, C.M., J.L., M.L., M.S.; statistical analysis, C.M., M.L.; manuscript preparation, C.M., W.R.N., M.L.; manuscript definition of intellectual content, J.L., W.R.N., S.F.; manuscript editing, C.M., W.R.N.; manuscript revision/review, M.S., M.L., M.V., B.D.; manuscript final version approval, all authors.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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