Magnetic Resonance Imaging–Guided Balloon Angioplasty of Coarctation o
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《循环学杂志》
the Departments of Congenital Heart Disease and Pediatric Cardiology (J.J.K., P.E., S.Y., D.G., B.P., K.P., H.A.-K., F.B., T.K.) and Cardiology (A.B., E.F., E.N.), Deutsches Herzzentrum Berlin, Berlin, Germany, and Clinical Science Philips (B.S.), Hamburg, Germany.
Abstract
Background— MRI guidance of percutaneous transluminal balloon angioplasty (PTA) of aortic coarctation (CoA) would be desirable for continuous visualization of anatomy and to eliminate x-ray exposure. The aim of this study was (1) to determine the suitability of MRI-controlled PTA using the iron oxide–based contrast medium Resovist (ferucarbotran) for catheter visualization and (2) to subsequently apply this technique in a pilot study with patients with CoA.
Methods and Results— The MRI contrast-to-noise ratio and artifact behavior of Resovist-treated balloon catheters was optimized in in vitro and animal experiments (pigs). In 5 patients, anatomy of the CoA was evaluated before and after intervention with high-resolution respiratory-navigated 3D MRI and multiphase cine MRI. Position monitoring of Resovist-treated catheters was realized with interactive real-time MRI. Aortic pressures were continuously recorded. Conventional catheterization was performed before and after MRI to confirm interventional success. During MRI, catheters filled with 25 μmol of iron particles per milliliter of Resovist produced good signal contrast between catheters and their background anatomy but no image distortion due to susceptibility artifacts. All MRI procedures were performed successfully in the patient study. There was excellent agreement between the diameters of CoA and pressure gradients as measured during MRI and conventional catheterization. In 4 patients, PTA resulted in substantial widening of the CoA and a decrease in pressure gradients. In 1 patient, PTA was ineffective.
Conclusions— The MRI method described represents a potential alternative to conventional x-ray fluoroscopy for catheter-based treatment of patients with CoA.
Key Words: angioplasty balloon coarctation contrast media magnetic resonance imaging
Introduction
Coarctation of the aorta (CoA) is an important congenital heart disease. Depending on the morphology of the stenosis, its hemodynamic severity, the age of the patient, and other associated cardiovascular malformations, CoA may be treated by percutaneous transluminal balloon angioplasty (PTA), endovascular stent placement, or surgery.1–3 However, restenosis or aneurysm formation can occur after interventional or surgical treatment.4–6 Therefore, patients need careful follow-up with repeated cardiac catheterization sessions to evaluate the hemodynamic severity of the stenosis and optionally to reintervene if this is indicated and possible. However, repeated cardiac catheterization under x-ray fluoroscopy is associated with an increased risk of developing solid tumors and infertility, particularly in younger patients.7–11
MRI is a recognized method for the diagnosis and follow-up of patients with CoA.12–14 Its images provide detailed 2D and 3D anatomic and functional information about the site of the CoA and the aortic vessel wall.15–18 The advent of fast imaging techniques, such as interactive real-time MRI (irMRI), makes this method attractive for the guidance of endovascular interventional procedures. irMRI enables continuous visualization of anatomy with good soft tissue contrast and various contrast characteristics.19–21 In addition, this technique is free of exposure to x-ray radiation and iodine-containing contrast media.
Editorial p 1051
Clinical Perspective p 1100
Successful MRI-guided interventional balloon angioplasties were recently reported in several animal studies.22–24 In these studies, passive catheter-tracking techniques were used to guide catheters and monitor balloon inflation. This was based on the use of gadolinium contrast media, which would be safe, but it requires imaging with T1-weighted gradient echo sequences that provide a lower signal-to-noise ratio than T1/T2-weighted real-time steady state free precession (SSFP). In other studies, passive catheter tracking was based on CO2.25,26 However, for balloon dilation of CoA, CO2 cannot be used, owing to the large amounts of gas needed for adequate filling of the dilation balloon and the risk of gas embolism to the brain in the event of balloon rupture.
In the present study, we tested an iron oxide–based contrast medium for MRI monitoring of PTA at the level of the aortic isthmus. Optimal concentrations of iron oxide particles that provided good contrast-to-noise ratio (CNR) between the balloon catheter and the anatomic background were determined in vitro. Next, the feasibility of MRI-guided PTA was assessed in animal experiments. Subsequently, MRI-guided PTA was performed in 5 patients with CoA.
Methods
In Vitro Experiments
The CNR and artifact behavior of the iron-based contrast medium Resovist (ferucarbotran) was tested in vitro. At the commercially available concentration, Resovist contains 500 μmol/mL iron oxide particles (100% Resovist). The contrast medium was diluted with 0.9% saline to concentrations of 1%, 5%, and 10%. The solutions were put into 10-mL plastic tubes that were placed in a saline bath treated with 2.5 mmol/L Gd-DPTA (Resovist, Schering AG, Berlin, Germany; T1=360 ms, T2=280 ms).
The following were determined on transversal irMRI: (1) the extent of susceptibility artifacts, (2) the signal intensity of the tubes and water bath, (3) the CNR between them, and (4) the diameter of the tubes. Detailed MRI sequence parameters are given below.
Animal Study
A total of 4 pigs (weight 18±4 kg) were studied. All studies were performed in accordance with the National Institute of Health guidelines for the care and use of laboratory animals and with the approval of the Committee for Animal Research of our institution. During the induction phase, animals received ketamine 4 mL IM, azaperone 6 mL IM, and etomidate 8 mL IV. For procedures, the animals were given 1.5% isoflurane inhalation to maintain general anesthesia. After completion of the study, the animals were euthanized with sodium pentobarbital (200 mg/kg IV).
For the intervention, commercially available catheters (Tyshak II, NuMed, Ontario, Canada) with a balloon diameter of 20 mm were used. Vascular access was gained by the Seldinger technique in the femoral artery. Catheters were introduced over a long blue sheath (12F, Cook Group Inc, Bloomington, Ind) and positioned at the level of the aortic isthmus. Then, the balloon was gently inflated and the long sheath advanced until it was blocked by the proximal end of the balloon (Figure 1). To passively monitor catheter position under irMRI, the lumen of the catheter and of the blue long sheath was filled with 5% Resovist solution. The desired position of the balloon catheter was confirmed on sagittal and axial irMRI (Figure 2). Finally, a self-made nonmetallic guidewire was advanced through the lumen of the balloon catheter to further stabilize the catheter shaft and avoid back-slipping of the balloon during angioplasty. The guidewire, which consisted of polyetheretherketon (PEEK; C.R. Bard, Inc, Murray Hill, NJ), had a diameter of 0.035 inches, a length of 260 cm, and a round shape. Before use in the experiments, the wire was gas sterilized. Quality testing comprised testing for torque and stress. The guidewire had a premeasured length and was advanced just until the distal port of the balloon catheter. By then, the balloon catheter was fully inflated with 1%, 5%, or 10% Resovist solutions. During inflation, the extent of susceptibility artifacts, the signal intensity of the balloons and the blood pool of the aorta, the CNR between them, and the diameter of the balloon were determined on irMRI (Table). MRI-derived balloon diameters were compared with the diameters given by the manufacture.
CNR and Susceptibility Artifacts With Different Solutions of Resovist
During MRI, aortic pressures were recorded through the liquid-filled introducer sheath and distal port of the balloon catheter. For measurements, the sheath and catheter were connected to a Statham transducer (Statham, Ohmeda, Murray Hill, NJ). Digitized data were amplified, recorded, and transmitted to a display visible to the interventionalist during the procedure.27
Clinical Pilot Study
Interventions were performed in 5 patients (mean age 18.9±12.2 years) with CoA. All patients included in the study were clinically asymptomatic and had weak pulses in the lower extremities (noninvasively measured), pressure differences between the arm and leg (38.4±13.2 mm Hg), and a pressure gradient of 43±7 mm Hg across the aortic isthmus as determined by Doppler echocardiography. Left ventricular dysfunction was not present. Two patients had been treated previously by PTA, and 1 patient had undergone surgical end-to-end anastomosis during early infancy. The other 2 patients had native CoA. Hemodynamic assessment of pressure gradients across the CoA and conventional angiograms of the aorta were performed in the catheterization laboratory before and after the MRI intervention.
Vascular access was gained by the Seldinger technique in the iliac artery. The stenosis was crossed with a standard diagnostic catheter, pressure gradients were measured, and angiograms were performed under x-ray guidance. Then, the catheter was exchanged to a 7F to 10F long blue sheath (Cook) using a long standard metallic guidewire (Amplatz extra-stiff wire guide, 260 cm; Cook). A 10- to 18-mm balloon catheter (Tyshak II, NuMed, Ontario, Canada) was advanced over the wire and positioned with its tip cranial to the stenosis. The metallic guidewire was removed from the body and the patient moved over a sliding tabletop onto an MRI tray and transferred to the MRI laboratory, located &20 meters away from the catheterization laboratory. There, MRI position monitoring of the long blue sheath and the balloon catheter was achieved by injection of a few milliliters of 5% Resovist solution into their lumens. A self-made 0.035-inch PEEK guidewire was advanced just up to the distal port of the balloon catheter, and the long sheath was gently moved forward until it reached the proximal end of the balloon. Finally, the balloon catheter was fully inflated with 5% Resovist solution when the balloon catheter and the long sheath had reached their desired position. At the end of the MRI session, patients were transferred back to the catheterization laboratory to confirm the success of the MRI-guided intervention by conventional angiogram and measurement of pressure gradients.
During MRI, the aortic arch, aortic isthmus, and descending aorta were investigated before and after PTA with high-resolution navigator-gated nearly isotropic MRI (3D-MRI) and multislice-multiphase cine MRI (cine MRI). The maximum diameters of the aortic arch, CoA, and descending aorta (at the level of the diaphragm) were determined on sagittal and axial views of reformatted 3D-MRI images. The aortic vessel wall was evaluated on cine MRI for signs of dissection or aneurysms. Finally, volume rendering was performed from 3D-MRI to view the 3D characteristics of the CoA.
PTA was performed only when conventional angiograms and MRI revealed a circular stenosis of the aortic isthmus with no signs of vascular aneurysm or abnormalities of the aortic arch and therefore no indication for stent placement or surgery. Heparin infusion and mild sedation were performed as clinically indicated.
The study complied with the Declaration of Helsinki and had the constitutional approval of our institution. Informed consent was obtained from all patients or their guardians.
Conventional Cardiac Catheterization
Conventional x-ray angiograms were acquired by biplane projection angiography (Integris, Philips Medical Systems, Best, the Netherlands) with an imaging rate of 12.5 or 25 images per second at 64 kW and injection of radiopaque contrast medium (Ultravist, Schering, Berlin, Germany).
Magnetic Resonance Imaging
All MRI investigations were performed with a 1.5-T Philips scanner (Philips, Intera, release 10). The laboratory was equipped with interventional in-room monitors and an operation console. For assessment of anatomy, cine MRI and a free-breathing respiratory navigator–gated nearly isotropic 3D scan (3D-MRI) with the following sequence parameters were used: respiratory navigator, ECG gating, slice orientation=axial, measured voxel size=2.4x2.4x 3 mm, reconstructed voxel size=1.1x1.1x1.5 mm, field of view=270, matrix=112, repetition time (TR)=3.6 ms, echo time (TE)=1.8 ms, flip angle=100°, SENSE (sensitivity encoding) factor=2.2, T2 preparation pulse (TE=50 ms), fat saturation, gating window=6 mm, number of slices adapted to morphology, acquisition time=145±23 seconds (depending on heart rate, respiratory pattern, and volume size).
Sequence parameters for the cine MRI were as follows: ECG gating, slice thickness=6 mm, measured pixel size=2x2.3 mm, reconstructed pixel size=1.4x1.4 mm, heart phases=25, field of view=350, matrix=176, number of excitations=1, TR=2.8 ms, TE=1.4 ms, flip angle 60°, acquisition time=8 seconds per slice, gating window=6 mm, number of slices adapted to morphology. A real-time interactive SSFP sequence with radial k-space filling was used as the irMRI sequence. Parameters were as follows: TR=3.3 ms, TE=1.6 ms, flip angle=45°, field of view=variable (200 to 350), matrix=144x144, slice thickness=variable (6 to 8 mm), acquisition frame rate=9 frames per second, reconstruction and display rate=online.
Safety Aspects
All patients studied had standard monitoring of vital parameters as required for cardiac catheterization procedures. No metallic guidewires or metallic braided catheters were used, because these are prone to potential heating effects. The nonmetallic guidewire used in the present study had a nontraumatizing tip but was not specifically prepared for MRI tracking. Therefore, it was not advanced beyond the distal end of the balloon catheter, to avoid any potential injury of distally located vascular structures. To ensure uncomplicated management of patients during the MRI intervention, only patients with circular aortic isthmus stenosis and no signs of aneurysm formation or other vessel-wall abnormalities were studied. To minimize the risk of extended vessel-wall dissection due to angioplasty, we used a conservative approach that included (1) selecting a balloon with the size of the median of the sum of the diameters of the transverse arch and the descending aorta at the level of the diaphragm and (2) avoiding pressure inflation over 6 bar.
With the risk of balloon rupture in mind, only Resovist doses below the concentrations licensed for liver examinations in humans were used. Resovist proved to have a very good safety profile in its class of contrast agents, and no significant cardiovascular side effects have been reported.
During the intervention, a fully equipped catheterization laboratory was available as a backup in case of unexpected complications. The catheterization laboratory was located on the same floor in close proximity (20 m) to the MRI laboratory, which would allow immediate transfer of the patient in case of inadvertent events. The medical personal involved in the study were specifically trained in both the management of the conventional catheterization and interventional MRI laboratory.
Calculations and Statistical Analysis
CNR was computed as the signal intensity of the Resovist-treated tubes or balloons minus the signal intensity of the water bath or aortic background, respectively, divided by the standard deviation of the background noise. The paired Student t test with Bonferroni correction for multiple analyses was used to compare CNR of Resovist concentrations and diameters of MRI-derived tube or balloon dimensions with their actual size. Agreement between hemodynamic data and aortic diameters as determined during conventional catheterization and MRI and before and after the intervention were tested with the Bland-Altman test and paired Student t test, respectively. A value of P<0.05 was considered significant. Data are expressed as ±SD where appropriate.
Results
In Vitro Experiments and Animal Study
Concentrations of >5% Resovist, equal to 25 μmol of iron particles per milliliter, produced image distortion at the adjacent surroundings of the tubes or balloons due to susceptibility (Figures 3 and 4). Good CNR between the tube or balloon and the water bath and aortic blood pool, respectively, was noted at a concentration of 5% Resovist. CNR was significantly less when we used a lower concentration of 1% compared with 5% Resovist solution (P<0.01; Table; Figures 3 and 4).
In the animal study, irMRI allowed fast and reliable position monitoring of the balloon catheters. During occlusive inflation of the balloon, the catheter shaft remained in a stable position when splinted with the long blue sheath and the PEEK guidewire. There were no significant differences in the diameter of the 5% Resovist-filled plastic tubes as measured with MRI compared with their actual size (P=0.96) or between the inflated balloon as measured with MRI compared with the sizes when fully inflated as given by the manufacturer (P=0.85).
Clinical Study
MRI-guided PTA of CoA was performed successfully in all patients studied. Volume-rendered 3D-MRI allowed detailed evaluation of the 3D aspect of aortic anatomy before and immediately after the intervention (Figures 5 and 6). The Bland-Altman test showed good agreement between diameters of the aorta as determined by conventional angiograms and reformatted 3D-MRI, with a bias of 0.8±1.1 mm (Figures 6, 7, and 8). Hemodynamic pressure gradients as measured during conventional catheterization and MRI also had excellent agreement, with a bias of –0.2±0.7 mm Hg. Cine MRI revealed no evidence of inadvertent extended vessel-wall dissection before or after the intervention.
irMRI allowed continuous visualization of the CoA during the interventional procedure. The position of the Resovist-filled balloon catheter was easily determined on axial and parasagittal irMRI images (Figure 2). During inflation with 5% Resovist, the balloon was clearly distinguishable from the bright blood pool of the aorta (Figure 2). The long sheath and PEEK wire enabled stable positioning of the shaft of the balloon catheter when placed across the stenosis and prevented backward sliding of the inflated balloons.
PTA was effective in 4 cases, with a substantial decrease in pressure gradient across the CoA and widening of the stenosis (Figures 6 through 8). In 1 case, MRI-guided angioplasty neither decreased the pressure gradient nor measurably increased the diameter of the stenosis owing to elastic recoiling of the aortic wall (patient 3 in Figure 8). In this patient, repeated angioplasty in the catheterization laboratory was performed but was also ineffective. Stent placement was not indicated owing to anatomic restraints, and therefore, the patient was scheduled for elective surgery.
Discussion
This study is, to the best of our knowledge, the first report about successful MRI-guided PTA in patients with CoA. The major findings of this study are that (1) balloon catheters filled with 5% Resovist solution produce good CNR to the aortic blood pool in MRI, which enables well-controlled angioplasty, and (2) interventional MRI has been shown to be an alternative technique to conventional fluoroscopy for guiding PTA in initial clinical experience.
Successful MRI-controlled balloon angioplasties of the iliac and renal artery and aorta have been described recently in animal studies.22–24 In these studies, balloon catheters filled with gadolinium contrast medium were visualized with T1-weighted gradient echo sequences to exploit the T1 effect of gadolinium. However, real-time turbo field echo sequences have smaller signal-to-noise ratios than T2/T1-weighted SSFP and therefore produce anatomic images of lesser contrast.28 This is because gadolinium reduces T1 and T2 relaxation time, so that the relevant changes in the T1/T2 ratio are significantly less than with T1 changes alone. In the present study, we used the T2 effect of an iron oxide–based contrast medium to visualize angioplasty balloons. Use of an iron oxide–based contrast medium was recently reported for successful passive catheter tracking with real-time SSFP.29 However, the present results show that iron oxide particles can produce marked susceptibility artifacts when used at high concentrations and in larger volumes, such as in angioplasty balloons (Figure 4). Therefore, we diluted Resovist with 0.9% saline solution to concentrations that yielded good CNR to the anatomic background owing to only slight local susceptibility and that did not distort adjacent anatomic structures.
Some patients with CoA have to undergo more than 1 catheterization session in their lifetime. The exponential effect of repeated exposure to x-ray radiation, particularly in the young, can cause an increased risk of solid tumors, among other risks.7–11 Interventional MRI techniques would therefore be beneficial for these patients because they eliminate x-ray exposure. A further invaluable advantage of MRI over x-ray angiography is its capability to provide continuous imaging of soft tissue anatomy throughout the intervention. High-resolution whole-heart 3D scans and cine MRI techniques provide important insight into the 3D course of the CoA and into aspects of the vessel-wall morphology of the aorta.16–18 This information is an a priori advantage over biplane computed tomography (CT) or MRI acquisition. Reconstructed 3D or dyna-CT rotational angiograms provide good 3D reconstruction, but if not gated, they can be problematic at the level of the aortic isthmus because of motion artifacts, and they are associated with substantial exposure to x-ray radiation.30 Good knowledge of the 3D characteristics of the stenosis or vessel-wall morphology is invaluable for planning the optimal treatment strategy. It is the basis for making the decision whether PTA, stent placement, or surgery should be performed and allows potential complications such as vessel-wall abnormalities to be assessed.1–3,17
Several studies have reported evaluation of the function and anatomy of CoA using velocity-encoded cine MRI and contrast-enhanced magnetic resonance angiography.15,31 Assessment of pressure gradients across a vascular stenosis is limited when velocity-encoded cine MRI is used because of spin dephasing, which accompanies turbulent blood flow.32 Quantification of collateral blood flow was proposed as an alternative method to determine the functional significance of a CoA33,34; however, there is no knowledge of the degree to which collateral blood flow diminishes immediately after intervention. Therefore, we relied on invasive pressure measurements to determine the success of the interventional procedure. The accuracy of measuring invasive pressures during MRI was shown previously.27
Gadolinium contrast media change the T1 relaxation time of the blood, which can potentially affect image quality of the T2/T1-weighted real-time SSFP as used in the present study. Therefore, in this study, we used isotropic whole-heart 3D imaging to evaluate the anatomy and morphology before and after the intervention instead of contrast media–enhanced magnetic resonance angiography. The whole-heart 3D scans acquired had a comparable level of image resolution to magnetic resonance angiography images.18,35 Our results show that 3D-MRI–derived diameters of the aortic arch, CoA, and descending aorta had excellent agreement with diameters derived from conventional magnetic resonance angiograms (Figure 8). In addition, the acquisition of 3D-MR images was shown to be largely operator independent and quite reproducible and may serve as an alternative in interventional MRI application for the assessment of cardiovascular structures.18,36
Study Limitations
In the present study, no metallic guidewires were used, but instead, a blue long sheath and PEEK guidewire were used to stabilize the balloon catheter during PTA. This setup might be difficult to control and time-consuming under MRI in complex anatomy. Therefore, the development and assessment of MRI-compatible and -trackable guidewires with good torque characteristics must be subject to future research.37,38
In addition, bioelectrically safe catheter-tracking methods that enable automated slice tracking and tip detection would be desirable to extend the clinical application of interventional MRI to more complex procedures and to allow good catheter control in tortuous anatomy. Currently, such techniques are at the experimental stage and need further evaluation before being applied in humans.20,39–42
For the time being, the interventionalist, who is steering guidewires, catheters, and sheaths, cannot conduct the imaging source alone, as is possible under fluoroscopy. During irMRI, only 2D visualization of anatomic slices is possible. Close communication with the technician within the operation room is mandatory and must be adapted to the noise level of the scanner.
MRI has proved to be a valid tool for assessment of aortic dissection or aneurysmal formation.17 Conventional x-ray angiograms often show discrete signs of aortic dissection at the immediate proximity of the stenosis after effective dilation of a CoA. In the present study, MRI revealed no evidence of inadvertent extended dissection. However, regional dissecting tears were also not noted. MRI techniques that allow for improved assessment of the morphology of the aortic vessel wall before and after balloon dilation should be the subject of future research. The MRI technique described in the present study was successfully applied in 5 patients; however, further validation of this technique in a larger number of patients is needed.
Conclusions
The findings of the present study demonstrate that Resovist-treated balloon catheters are well visualized during MRI. The interventional MRI method described represents a potential alternative to conventional x-ray fluoroscopy for catheter-based treatment of patients with CoA.
Acknowledgments
We thank Dr H. Vogler, Schering, Berlin, Germany, and A.M. Gale (editorial) for their kind support. This work was supported in part by the Competence Network for Congenital Heart Defects, funded by the German Federal Ministry of Education and Research (BMBF, FKZ01G10210) and the Deutsche Forschungsgemeinschaft (DFG, KU1329/3-1).
Disclosures
None.
References
Zabal C, Attie F, Rosas M, Buendia-Hernandez A, Garcia-Montes JA. The adult patient with native coarctation of the aorta: balloon angioplasty or primary stenting Heart. 2003; 89: 77–83.
Ewert P, Abdul-Khaliq H, Peters B, Nagdyman N, Schubert S, Lange PE. Transcatheter therapy of long extreme subatretic aortic coarctations with covered stents. Catheter Cardiovasc Interv. 2004; 63: 236–239.
Pedra CA, Fontes VF, Esteves CA, Pilla CB, Braga SL, Pedra SR, Santana MV, Silva MA, Almeida T, Sousa JE. Stenting vs. balloon angioplasty for discrete unoperated coarctation of the aorta in adolescents and adults. Catheter Cardiovasc Interv. 2005; 64: 495–506.
Rao PS, Galal O, Smith PA, Wilson AD. Five- to nine-year follow-up results of balloon angioplasty of native aortic coarctation in infants and children. J Am Coll Cardiol. 1996; 27: 462–470.
Suarez de Lezo J, Pan M, Romero M, Medina A, Segura J, Lafuente M, Pavlovic D, Hernandez E, Melian F, Espada J. Immediate and follow-up findings after stent treatment for severe coarctation of aorta. Am J Cardiol. 1999; 83: 400–406.
Di Filippo S, Sassolas F, Bozio A. Long-term results after surgery of coarctation of the aorta in neonates and children [in French]. Arch Mal Coeur Vaiss. 1997; 90: 1723–1728.
Modan B, Keinan L, Blumstein T, Sadetzki S. Cancer following cardiac catheterization in childhood. Int J Epidemiol. 2000; 29: 424–428.
Frush DP, Donnelly LF, Rosen NS. Computed tomography and radiation risks: what pediatric health care providers should know. Pediatrics. 2003; 112: 951–957.
McFadden SL, Mooney RB, Shepherd PH. X-ray dose and associated risks from radiofrequency catheter ablation procedures. Br J Radiol. 2002; 75: 253–265.
Bacher K, Bogaert E, Lapere R, De Wolf D, Thierens H. Patient-specific dose and radiation risk estimation in pediatric cardiac catheterization. Circulation. 2005; 111: 83–89.
Brenner D, Elliston C, Hall E, Berdon W. Estimated risks of radiation-induced fatal cancer from pediatric CT. AJR Am J Roentgenol. 2001; 176: 289–296.
Soler R, Rodriguez E, Requejo I, Fernandez R, Raposo I. Magnetic resonance imaging of congenital abnormalities of the thoracic aorta. Eur Radiol. 1998; 8: 540–546.
Konen E, Merchant N. Coarctation of the aorta before and after correction: the role of cardiovascular MRI. AJR. 2004; 182: 1333–1339.
Bogaert J, Kuzo R, Dymarkowski S, Janssen L, Celis I, Budts W, Gewillig M. Follow-up of patients with previous treatment for coarctation of the thoracic aorta: comparison between contrast-enhanced MR angiography and fast spin-echo MR imaging. Eur Radiol. 2000; 10: 1847–1854.
Godart F, Labrot G, Devos P, McFadden E, Rey C, Beregi JP. Coarctation of the aorta: comparison of aortic dimensions between conventional MR imaging, 3D MR angiography, and conventional angiography. Eur Radiol. 2002; 12: 2034–2039.
Riquelme C, Laissy JP, Menegazzo D, Debray MP, Cinqualbre A, Langlois J, Schouman-Claeys E. MR imaging of coarctation of the aorta and its postoperative complications in adults: assessment with spin-echo and cine-MR imaging. Magn Reson Imaging. 1999; 17: 37–46.
Pereles FS, McCarthy RM, Baskaran V, Carr JC, Kapoor V, Krupinski EA, Finn JP. Thoracic aortic dissection and aneurysm: evaluation with nonenhanced true FISP MR angiography in less than 4 minutes. Radiology. 2002; 223: 270–274.
Sorensen TS, Korperich H, Greil GF, Eichhorn J, Barth P, Meyer H, Pedersen EM, Beerbaum P. Operator-independent isotropic three-dimensional magnetic resonance imaging for morphology in congenital heart disease: a validation study. Circulation. 2004; 110: 163–169.
Lardo AC. Real-time magnetic resonance imaging: diagnostic and interventional applications. Pediatr Cardiol. 2000; 21: 80–98.
Kuehne T, Weiss S, Brinkert F, Weil J, Yilmaz S, Schmitt B, Ewert P, Lange P, Gutberlet M. Catheter visualization with resonant markers at MR imaging-guided deployment of endovascular stents in swine. Radiology. 2004; 233: 774–780.
Kuehne T, Yilmaz S, Meinus C, Moore P, Saeed M, Weber O, Higgins CB, Blank T, Elsaesser E, Schnackenburg B, Ewert P, Lange PE, Nagel E. Magnetic resonance imaging-guided transcatheter implantation of a prosthetic valve in aortic valve position: feasibility study in swine. J Am Coll Cardiol. 2004; 44: 2247–2249.
Buecker A, Adam GB, Neuerburg JM, Kinzel S, Glowinski A, Schaeffter T, Rasche V, van Vaals JJ, Guenther RW. Simultaneous real-time visualization of the catheter tip and vascular anatomy for MR-guided PTA of iliac arteries in an animal model. J Magn Reson Imaging. 2002; 16: 201–208.
Omary RA, Frayne R, Unal O, Warner T, Korosec FR, Mistretta CA, Strother CM, Grist TM. MR-guided angioplasty of renal artery stenosis in a pig model: a feasibility study. J Vasc Interv Radiol. 2000; 11: 373–381.
Godart F, Beregi JP, Nicol L, Occelli B, Vincentelli A, Daanen V, Rey C, Rousseau J. MR-guided balloon angioplasty of stenosed aorta: in vivo evaluation using near-standard instruments and a passive tracking technique. J Magn Reson Imaging. 2000; 12: 639–644.
Miquel ME, Hegde S, Muthurangu V, Corcoran BJ, Keevil SF, Hill DL, Razavi RS. Visualization and tracking of an inflatable balloon catheter using SSFP in a flow phantom and in the heart and great vessels of patients. Magn Reson Med. 2004; 51: 988–995.
Razavi R, Hill DL, Keevil SF, Miquel ME, Muthurangu V, Hegde S, Rhode K, Barnett M, van Vaals J, Hawkes DJ, Baker E. Cardiac catheterisation guided by MRI in children and adults with congenital heart disease. Lancet. 2003; 362: 1877–1882.
Kuehne T, Yilmaz S, Steendijk P, Moore P, Groenink M, Saaed M, Weber O, Higgins CB, Ewert P, Fleck E, Nagel E, Schulze-Neick I, Lange P. Magnetic resonance imaging analysis of right ventricular pressure-volume loops: in vivo validation and clinical application in patients with pulmonary hypertension. Circulation. 2004; 110: 2010–2016.
Kuehne T, Saeed M, Higgins CB, Gleason K, Krombach GA, Weber OM, Martin AJ, Turner D, Teitel D, Moore P. Endovascular stents in pulmonary valve and artery in swine: feasibility study of MR imaging-guided deployment and postinterventional assessment. Radiology. 2003; 226: 475–481.
Wacker FK, Reither K, Ebert W, Wendt M, Lewin JS, Wolf KJ. MR image-guided endovascular procedures with the ultrasmall superparamagnetic iron oxide SH U 555 C as an intravascular contrast agent: study in pigs. Radiology. 2003; 226: 459–464.
Rigattieri S, Ghini AS, Silvestri P, Tommasino A, Ferraiuolo G, Palamara A, Loschiavo P. A randomized comparison between rotational and standard coronary angiography. Minerva Cardioangiol. 2005; 53: 1–6.
Chernoff DM, Derugin N, Rajasinghe HA, Hanley FL, Higgins CB, Gooding CA. Measurement of collateral blood flow in a porcine model of aortic coarctation by velocity-encoded cine MRI. J Magn Reson Imaging. 1997; 7: 557–563.
Stahlberg F, Sondergaard L, Thomsen C, Henriksen O. Quantification of complex flow using MR phase imaging: a study of parameters influencing the phase/velocity relation. Magn Reson Imaging. 1992; 10: 13–23.
Rupprecht T, Nitz W, Wagner M, Kreissler P, Rascher W, Hofbeck M. Determination of the pressure gradient in children with coarctation of the aorta by low-field magnetic resonance imaging. Pediatr Cardiol. 2002; 23: 127–131.
Steffens JC, Bourne MW, Sakuma H, O’Sullivan M, Higgins CB. Quantification of collateral blood flow in coarctation of the aorta by velocity encoded cine magnetic resonance imaging. Circulation. 1994; 90: 937–943.
Weber OM, Martin AJ, Higgins CB. Whole-heart steady-state free precession coronary artery magnetic resonance angiography. Magn Reson Med. 2003; 50: 1223–1228.
Nagel E, Bornstedt A, Schnackenburg B, Hug J, Oswald H, Fleck E. Optimization of realtime adaptive navigator correction for 3D magnetic resonance coronary angiography. Magn Reson Med. 1999; 42: 408–411.
Buecker A, Spuentrup E, Schmitz-Rode T, Kinzel S, Pfeffer J, Hohl C, van Vaals JJ, Gunther RW. Use of a nonmetallic guide wire for magnetic resonance-guided coronary artery catheterization. Invest Radiol. 2004; 39: 656–660.
Yang X, Atalar E. Intravascular MR imaging-guided balloon angioplasty with an MR imaging guide wire: feasibility study in rabbits. Radiology. 2000; 217: 501–506.
Quick HH, Kuehl H, Kaiser G, Hornscheidt D, Mikolajczyk KP, Aker S, Debatin JF, Ladd ME. Interventional MRA using actively visualized catheters, TrueFISP, and real-time image fusion. Magn Reson Med. 2003; 49: 129–137.
Weiss S, Vernickel P, Schaeffter T, Schulz V, Gleich B. Transmission line for improved RF safety of interventional devices. Magn Reson Med. 2005; 54: 182–189.
Vernickel P, Schulz V, Weiss S, Gleich B. A safe transmission line for MRI. IEEE Trans Biomed Eng. 2005; 52: 1094–1102.
Weiss S, Kuehne T, Brinkert F, Krombach G, Katoh M, Schaeffter T, Guenther RW, Buecker A. In vivo safe catheter visualization and slice tracking using an optically detunable resonant marker. Magn Reson Med. 2004; 52: 860–868.(Julia J. Krueger, MD; Peter Ewert, MD; S)
Abstract
Background— MRI guidance of percutaneous transluminal balloon angioplasty (PTA) of aortic coarctation (CoA) would be desirable for continuous visualization of anatomy and to eliminate x-ray exposure. The aim of this study was (1) to determine the suitability of MRI-controlled PTA using the iron oxide–based contrast medium Resovist (ferucarbotran) for catheter visualization and (2) to subsequently apply this technique in a pilot study with patients with CoA.
Methods and Results— The MRI contrast-to-noise ratio and artifact behavior of Resovist-treated balloon catheters was optimized in in vitro and animal experiments (pigs). In 5 patients, anatomy of the CoA was evaluated before and after intervention with high-resolution respiratory-navigated 3D MRI and multiphase cine MRI. Position monitoring of Resovist-treated catheters was realized with interactive real-time MRI. Aortic pressures were continuously recorded. Conventional catheterization was performed before and after MRI to confirm interventional success. During MRI, catheters filled with 25 μmol of iron particles per milliliter of Resovist produced good signal contrast between catheters and their background anatomy but no image distortion due to susceptibility artifacts. All MRI procedures were performed successfully in the patient study. There was excellent agreement between the diameters of CoA and pressure gradients as measured during MRI and conventional catheterization. In 4 patients, PTA resulted in substantial widening of the CoA and a decrease in pressure gradients. In 1 patient, PTA was ineffective.
Conclusions— The MRI method described represents a potential alternative to conventional x-ray fluoroscopy for catheter-based treatment of patients with CoA.
Key Words: angioplasty balloon coarctation contrast media magnetic resonance imaging
Introduction
Coarctation of the aorta (CoA) is an important congenital heart disease. Depending on the morphology of the stenosis, its hemodynamic severity, the age of the patient, and other associated cardiovascular malformations, CoA may be treated by percutaneous transluminal balloon angioplasty (PTA), endovascular stent placement, or surgery.1–3 However, restenosis or aneurysm formation can occur after interventional or surgical treatment.4–6 Therefore, patients need careful follow-up with repeated cardiac catheterization sessions to evaluate the hemodynamic severity of the stenosis and optionally to reintervene if this is indicated and possible. However, repeated cardiac catheterization under x-ray fluoroscopy is associated with an increased risk of developing solid tumors and infertility, particularly in younger patients.7–11
MRI is a recognized method for the diagnosis and follow-up of patients with CoA.12–14 Its images provide detailed 2D and 3D anatomic and functional information about the site of the CoA and the aortic vessel wall.15–18 The advent of fast imaging techniques, such as interactive real-time MRI (irMRI), makes this method attractive for the guidance of endovascular interventional procedures. irMRI enables continuous visualization of anatomy with good soft tissue contrast and various contrast characteristics.19–21 In addition, this technique is free of exposure to x-ray radiation and iodine-containing contrast media.
Editorial p 1051
Clinical Perspective p 1100
Successful MRI-guided interventional balloon angioplasties were recently reported in several animal studies.22–24 In these studies, passive catheter-tracking techniques were used to guide catheters and monitor balloon inflation. This was based on the use of gadolinium contrast media, which would be safe, but it requires imaging with T1-weighted gradient echo sequences that provide a lower signal-to-noise ratio than T1/T2-weighted real-time steady state free precession (SSFP). In other studies, passive catheter tracking was based on CO2.25,26 However, for balloon dilation of CoA, CO2 cannot be used, owing to the large amounts of gas needed for adequate filling of the dilation balloon and the risk of gas embolism to the brain in the event of balloon rupture.
In the present study, we tested an iron oxide–based contrast medium for MRI monitoring of PTA at the level of the aortic isthmus. Optimal concentrations of iron oxide particles that provided good contrast-to-noise ratio (CNR) between the balloon catheter and the anatomic background were determined in vitro. Next, the feasibility of MRI-guided PTA was assessed in animal experiments. Subsequently, MRI-guided PTA was performed in 5 patients with CoA.
Methods
In Vitro Experiments
The CNR and artifact behavior of the iron-based contrast medium Resovist (ferucarbotran) was tested in vitro. At the commercially available concentration, Resovist contains 500 μmol/mL iron oxide particles (100% Resovist). The contrast medium was diluted with 0.9% saline to concentrations of 1%, 5%, and 10%. The solutions were put into 10-mL plastic tubes that were placed in a saline bath treated with 2.5 mmol/L Gd-DPTA (Resovist, Schering AG, Berlin, Germany; T1=360 ms, T2=280 ms).
The following were determined on transversal irMRI: (1) the extent of susceptibility artifacts, (2) the signal intensity of the tubes and water bath, (3) the CNR between them, and (4) the diameter of the tubes. Detailed MRI sequence parameters are given below.
Animal Study
A total of 4 pigs (weight 18±4 kg) were studied. All studies were performed in accordance with the National Institute of Health guidelines for the care and use of laboratory animals and with the approval of the Committee for Animal Research of our institution. During the induction phase, animals received ketamine 4 mL IM, azaperone 6 mL IM, and etomidate 8 mL IV. For procedures, the animals were given 1.5% isoflurane inhalation to maintain general anesthesia. After completion of the study, the animals were euthanized with sodium pentobarbital (200 mg/kg IV).
For the intervention, commercially available catheters (Tyshak II, NuMed, Ontario, Canada) with a balloon diameter of 20 mm were used. Vascular access was gained by the Seldinger technique in the femoral artery. Catheters were introduced over a long blue sheath (12F, Cook Group Inc, Bloomington, Ind) and positioned at the level of the aortic isthmus. Then, the balloon was gently inflated and the long sheath advanced until it was blocked by the proximal end of the balloon (Figure 1). To passively monitor catheter position under irMRI, the lumen of the catheter and of the blue long sheath was filled with 5% Resovist solution. The desired position of the balloon catheter was confirmed on sagittal and axial irMRI (Figure 2). Finally, a self-made nonmetallic guidewire was advanced through the lumen of the balloon catheter to further stabilize the catheter shaft and avoid back-slipping of the balloon during angioplasty. The guidewire, which consisted of polyetheretherketon (PEEK; C.R. Bard, Inc, Murray Hill, NJ), had a diameter of 0.035 inches, a length of 260 cm, and a round shape. Before use in the experiments, the wire was gas sterilized. Quality testing comprised testing for torque and stress. The guidewire had a premeasured length and was advanced just until the distal port of the balloon catheter. By then, the balloon catheter was fully inflated with 1%, 5%, or 10% Resovist solutions. During inflation, the extent of susceptibility artifacts, the signal intensity of the balloons and the blood pool of the aorta, the CNR between them, and the diameter of the balloon were determined on irMRI (Table). MRI-derived balloon diameters were compared with the diameters given by the manufacture.
CNR and Susceptibility Artifacts With Different Solutions of Resovist
During MRI, aortic pressures were recorded through the liquid-filled introducer sheath and distal port of the balloon catheter. For measurements, the sheath and catheter were connected to a Statham transducer (Statham, Ohmeda, Murray Hill, NJ). Digitized data were amplified, recorded, and transmitted to a display visible to the interventionalist during the procedure.27
Clinical Pilot Study
Interventions were performed in 5 patients (mean age 18.9±12.2 years) with CoA. All patients included in the study were clinically asymptomatic and had weak pulses in the lower extremities (noninvasively measured), pressure differences between the arm and leg (38.4±13.2 mm Hg), and a pressure gradient of 43±7 mm Hg across the aortic isthmus as determined by Doppler echocardiography. Left ventricular dysfunction was not present. Two patients had been treated previously by PTA, and 1 patient had undergone surgical end-to-end anastomosis during early infancy. The other 2 patients had native CoA. Hemodynamic assessment of pressure gradients across the CoA and conventional angiograms of the aorta were performed in the catheterization laboratory before and after the MRI intervention.
Vascular access was gained by the Seldinger technique in the iliac artery. The stenosis was crossed with a standard diagnostic catheter, pressure gradients were measured, and angiograms were performed under x-ray guidance. Then, the catheter was exchanged to a 7F to 10F long blue sheath (Cook) using a long standard metallic guidewire (Amplatz extra-stiff wire guide, 260 cm; Cook). A 10- to 18-mm balloon catheter (Tyshak II, NuMed, Ontario, Canada) was advanced over the wire and positioned with its tip cranial to the stenosis. The metallic guidewire was removed from the body and the patient moved over a sliding tabletop onto an MRI tray and transferred to the MRI laboratory, located &20 meters away from the catheterization laboratory. There, MRI position monitoring of the long blue sheath and the balloon catheter was achieved by injection of a few milliliters of 5% Resovist solution into their lumens. A self-made 0.035-inch PEEK guidewire was advanced just up to the distal port of the balloon catheter, and the long sheath was gently moved forward until it reached the proximal end of the balloon. Finally, the balloon catheter was fully inflated with 5% Resovist solution when the balloon catheter and the long sheath had reached their desired position. At the end of the MRI session, patients were transferred back to the catheterization laboratory to confirm the success of the MRI-guided intervention by conventional angiogram and measurement of pressure gradients.
During MRI, the aortic arch, aortic isthmus, and descending aorta were investigated before and after PTA with high-resolution navigator-gated nearly isotropic MRI (3D-MRI) and multislice-multiphase cine MRI (cine MRI). The maximum diameters of the aortic arch, CoA, and descending aorta (at the level of the diaphragm) were determined on sagittal and axial views of reformatted 3D-MRI images. The aortic vessel wall was evaluated on cine MRI for signs of dissection or aneurysms. Finally, volume rendering was performed from 3D-MRI to view the 3D characteristics of the CoA.
PTA was performed only when conventional angiograms and MRI revealed a circular stenosis of the aortic isthmus with no signs of vascular aneurysm or abnormalities of the aortic arch and therefore no indication for stent placement or surgery. Heparin infusion and mild sedation were performed as clinically indicated.
The study complied with the Declaration of Helsinki and had the constitutional approval of our institution. Informed consent was obtained from all patients or their guardians.
Conventional Cardiac Catheterization
Conventional x-ray angiograms were acquired by biplane projection angiography (Integris, Philips Medical Systems, Best, the Netherlands) with an imaging rate of 12.5 or 25 images per second at 64 kW and injection of radiopaque contrast medium (Ultravist, Schering, Berlin, Germany).
Magnetic Resonance Imaging
All MRI investigations were performed with a 1.5-T Philips scanner (Philips, Intera, release 10). The laboratory was equipped with interventional in-room monitors and an operation console. For assessment of anatomy, cine MRI and a free-breathing respiratory navigator–gated nearly isotropic 3D scan (3D-MRI) with the following sequence parameters were used: respiratory navigator, ECG gating, slice orientation=axial, measured voxel size=2.4x2.4x 3 mm, reconstructed voxel size=1.1x1.1x1.5 mm, field of view=270, matrix=112, repetition time (TR)=3.6 ms, echo time (TE)=1.8 ms, flip angle=100°, SENSE (sensitivity encoding) factor=2.2, T2 preparation pulse (TE=50 ms), fat saturation, gating window=6 mm, number of slices adapted to morphology, acquisition time=145±23 seconds (depending on heart rate, respiratory pattern, and volume size).
Sequence parameters for the cine MRI were as follows: ECG gating, slice thickness=6 mm, measured pixel size=2x2.3 mm, reconstructed pixel size=1.4x1.4 mm, heart phases=25, field of view=350, matrix=176, number of excitations=1, TR=2.8 ms, TE=1.4 ms, flip angle 60°, acquisition time=8 seconds per slice, gating window=6 mm, number of slices adapted to morphology. A real-time interactive SSFP sequence with radial k-space filling was used as the irMRI sequence. Parameters were as follows: TR=3.3 ms, TE=1.6 ms, flip angle=45°, field of view=variable (200 to 350), matrix=144x144, slice thickness=variable (6 to 8 mm), acquisition frame rate=9 frames per second, reconstruction and display rate=online.
Safety Aspects
All patients studied had standard monitoring of vital parameters as required for cardiac catheterization procedures. No metallic guidewires or metallic braided catheters were used, because these are prone to potential heating effects. The nonmetallic guidewire used in the present study had a nontraumatizing tip but was not specifically prepared for MRI tracking. Therefore, it was not advanced beyond the distal end of the balloon catheter, to avoid any potential injury of distally located vascular structures. To ensure uncomplicated management of patients during the MRI intervention, only patients with circular aortic isthmus stenosis and no signs of aneurysm formation or other vessel-wall abnormalities were studied. To minimize the risk of extended vessel-wall dissection due to angioplasty, we used a conservative approach that included (1) selecting a balloon with the size of the median of the sum of the diameters of the transverse arch and the descending aorta at the level of the diaphragm and (2) avoiding pressure inflation over 6 bar.
With the risk of balloon rupture in mind, only Resovist doses below the concentrations licensed for liver examinations in humans were used. Resovist proved to have a very good safety profile in its class of contrast agents, and no significant cardiovascular side effects have been reported.
During the intervention, a fully equipped catheterization laboratory was available as a backup in case of unexpected complications. The catheterization laboratory was located on the same floor in close proximity (20 m) to the MRI laboratory, which would allow immediate transfer of the patient in case of inadvertent events. The medical personal involved in the study were specifically trained in both the management of the conventional catheterization and interventional MRI laboratory.
Calculations and Statistical Analysis
CNR was computed as the signal intensity of the Resovist-treated tubes or balloons minus the signal intensity of the water bath or aortic background, respectively, divided by the standard deviation of the background noise. The paired Student t test with Bonferroni correction for multiple analyses was used to compare CNR of Resovist concentrations and diameters of MRI-derived tube or balloon dimensions with their actual size. Agreement between hemodynamic data and aortic diameters as determined during conventional catheterization and MRI and before and after the intervention were tested with the Bland-Altman test and paired Student t test, respectively. A value of P<0.05 was considered significant. Data are expressed as ±SD where appropriate.
Results
In Vitro Experiments and Animal Study
Concentrations of >5% Resovist, equal to 25 μmol of iron particles per milliliter, produced image distortion at the adjacent surroundings of the tubes or balloons due to susceptibility (Figures 3 and 4). Good CNR between the tube or balloon and the water bath and aortic blood pool, respectively, was noted at a concentration of 5% Resovist. CNR was significantly less when we used a lower concentration of 1% compared with 5% Resovist solution (P<0.01; Table; Figures 3 and 4).
In the animal study, irMRI allowed fast and reliable position monitoring of the balloon catheters. During occlusive inflation of the balloon, the catheter shaft remained in a stable position when splinted with the long blue sheath and the PEEK guidewire. There were no significant differences in the diameter of the 5% Resovist-filled plastic tubes as measured with MRI compared with their actual size (P=0.96) or between the inflated balloon as measured with MRI compared with the sizes when fully inflated as given by the manufacturer (P=0.85).
Clinical Study
MRI-guided PTA of CoA was performed successfully in all patients studied. Volume-rendered 3D-MRI allowed detailed evaluation of the 3D aspect of aortic anatomy before and immediately after the intervention (Figures 5 and 6). The Bland-Altman test showed good agreement between diameters of the aorta as determined by conventional angiograms and reformatted 3D-MRI, with a bias of 0.8±1.1 mm (Figures 6, 7, and 8). Hemodynamic pressure gradients as measured during conventional catheterization and MRI also had excellent agreement, with a bias of –0.2±0.7 mm Hg. Cine MRI revealed no evidence of inadvertent extended vessel-wall dissection before or after the intervention.
irMRI allowed continuous visualization of the CoA during the interventional procedure. The position of the Resovist-filled balloon catheter was easily determined on axial and parasagittal irMRI images (Figure 2). During inflation with 5% Resovist, the balloon was clearly distinguishable from the bright blood pool of the aorta (Figure 2). The long sheath and PEEK wire enabled stable positioning of the shaft of the balloon catheter when placed across the stenosis and prevented backward sliding of the inflated balloons.
PTA was effective in 4 cases, with a substantial decrease in pressure gradient across the CoA and widening of the stenosis (Figures 6 through 8). In 1 case, MRI-guided angioplasty neither decreased the pressure gradient nor measurably increased the diameter of the stenosis owing to elastic recoiling of the aortic wall (patient 3 in Figure 8). In this patient, repeated angioplasty in the catheterization laboratory was performed but was also ineffective. Stent placement was not indicated owing to anatomic restraints, and therefore, the patient was scheduled for elective surgery.
Discussion
This study is, to the best of our knowledge, the first report about successful MRI-guided PTA in patients with CoA. The major findings of this study are that (1) balloon catheters filled with 5% Resovist solution produce good CNR to the aortic blood pool in MRI, which enables well-controlled angioplasty, and (2) interventional MRI has been shown to be an alternative technique to conventional fluoroscopy for guiding PTA in initial clinical experience.
Successful MRI-controlled balloon angioplasties of the iliac and renal artery and aorta have been described recently in animal studies.22–24 In these studies, balloon catheters filled with gadolinium contrast medium were visualized with T1-weighted gradient echo sequences to exploit the T1 effect of gadolinium. However, real-time turbo field echo sequences have smaller signal-to-noise ratios than T2/T1-weighted SSFP and therefore produce anatomic images of lesser contrast.28 This is because gadolinium reduces T1 and T2 relaxation time, so that the relevant changes in the T1/T2 ratio are significantly less than with T1 changes alone. In the present study, we used the T2 effect of an iron oxide–based contrast medium to visualize angioplasty balloons. Use of an iron oxide–based contrast medium was recently reported for successful passive catheter tracking with real-time SSFP.29 However, the present results show that iron oxide particles can produce marked susceptibility artifacts when used at high concentrations and in larger volumes, such as in angioplasty balloons (Figure 4). Therefore, we diluted Resovist with 0.9% saline solution to concentrations that yielded good CNR to the anatomic background owing to only slight local susceptibility and that did not distort adjacent anatomic structures.
Some patients with CoA have to undergo more than 1 catheterization session in their lifetime. The exponential effect of repeated exposure to x-ray radiation, particularly in the young, can cause an increased risk of solid tumors, among other risks.7–11 Interventional MRI techniques would therefore be beneficial for these patients because they eliminate x-ray exposure. A further invaluable advantage of MRI over x-ray angiography is its capability to provide continuous imaging of soft tissue anatomy throughout the intervention. High-resolution whole-heart 3D scans and cine MRI techniques provide important insight into the 3D course of the CoA and into aspects of the vessel-wall morphology of the aorta.16–18 This information is an a priori advantage over biplane computed tomography (CT) or MRI acquisition. Reconstructed 3D or dyna-CT rotational angiograms provide good 3D reconstruction, but if not gated, they can be problematic at the level of the aortic isthmus because of motion artifacts, and they are associated with substantial exposure to x-ray radiation.30 Good knowledge of the 3D characteristics of the stenosis or vessel-wall morphology is invaluable for planning the optimal treatment strategy. It is the basis for making the decision whether PTA, stent placement, or surgery should be performed and allows potential complications such as vessel-wall abnormalities to be assessed.1–3,17
Several studies have reported evaluation of the function and anatomy of CoA using velocity-encoded cine MRI and contrast-enhanced magnetic resonance angiography.15,31 Assessment of pressure gradients across a vascular stenosis is limited when velocity-encoded cine MRI is used because of spin dephasing, which accompanies turbulent blood flow.32 Quantification of collateral blood flow was proposed as an alternative method to determine the functional significance of a CoA33,34; however, there is no knowledge of the degree to which collateral blood flow diminishes immediately after intervention. Therefore, we relied on invasive pressure measurements to determine the success of the interventional procedure. The accuracy of measuring invasive pressures during MRI was shown previously.27
Gadolinium contrast media change the T1 relaxation time of the blood, which can potentially affect image quality of the T2/T1-weighted real-time SSFP as used in the present study. Therefore, in this study, we used isotropic whole-heart 3D imaging to evaluate the anatomy and morphology before and after the intervention instead of contrast media–enhanced magnetic resonance angiography. The whole-heart 3D scans acquired had a comparable level of image resolution to magnetic resonance angiography images.18,35 Our results show that 3D-MRI–derived diameters of the aortic arch, CoA, and descending aorta had excellent agreement with diameters derived from conventional magnetic resonance angiograms (Figure 8). In addition, the acquisition of 3D-MR images was shown to be largely operator independent and quite reproducible and may serve as an alternative in interventional MRI application for the assessment of cardiovascular structures.18,36
Study Limitations
In the present study, no metallic guidewires were used, but instead, a blue long sheath and PEEK guidewire were used to stabilize the balloon catheter during PTA. This setup might be difficult to control and time-consuming under MRI in complex anatomy. Therefore, the development and assessment of MRI-compatible and -trackable guidewires with good torque characteristics must be subject to future research.37,38
In addition, bioelectrically safe catheter-tracking methods that enable automated slice tracking and tip detection would be desirable to extend the clinical application of interventional MRI to more complex procedures and to allow good catheter control in tortuous anatomy. Currently, such techniques are at the experimental stage and need further evaluation before being applied in humans.20,39–42
For the time being, the interventionalist, who is steering guidewires, catheters, and sheaths, cannot conduct the imaging source alone, as is possible under fluoroscopy. During irMRI, only 2D visualization of anatomic slices is possible. Close communication with the technician within the operation room is mandatory and must be adapted to the noise level of the scanner.
MRI has proved to be a valid tool for assessment of aortic dissection or aneurysmal formation.17 Conventional x-ray angiograms often show discrete signs of aortic dissection at the immediate proximity of the stenosis after effective dilation of a CoA. In the present study, MRI revealed no evidence of inadvertent extended dissection. However, regional dissecting tears were also not noted. MRI techniques that allow for improved assessment of the morphology of the aortic vessel wall before and after balloon dilation should be the subject of future research. The MRI technique described in the present study was successfully applied in 5 patients; however, further validation of this technique in a larger number of patients is needed.
Conclusions
The findings of the present study demonstrate that Resovist-treated balloon catheters are well visualized during MRI. The interventional MRI method described represents a potential alternative to conventional x-ray fluoroscopy for catheter-based treatment of patients with CoA.
Acknowledgments
We thank Dr H. Vogler, Schering, Berlin, Germany, and A.M. Gale (editorial) for their kind support. This work was supported in part by the Competence Network for Congenital Heart Defects, funded by the German Federal Ministry of Education and Research (BMBF, FKZ01G10210) and the Deutsche Forschungsgemeinschaft (DFG, KU1329/3-1).
Disclosures
None.
References
Zabal C, Attie F, Rosas M, Buendia-Hernandez A, Garcia-Montes JA. The adult patient with native coarctation of the aorta: balloon angioplasty or primary stenting Heart. 2003; 89: 77–83.
Ewert P, Abdul-Khaliq H, Peters B, Nagdyman N, Schubert S, Lange PE. Transcatheter therapy of long extreme subatretic aortic coarctations with covered stents. Catheter Cardiovasc Interv. 2004; 63: 236–239.
Pedra CA, Fontes VF, Esteves CA, Pilla CB, Braga SL, Pedra SR, Santana MV, Silva MA, Almeida T, Sousa JE. Stenting vs. balloon angioplasty for discrete unoperated coarctation of the aorta in adolescents and adults. Catheter Cardiovasc Interv. 2005; 64: 495–506.
Rao PS, Galal O, Smith PA, Wilson AD. Five- to nine-year follow-up results of balloon angioplasty of native aortic coarctation in infants and children. J Am Coll Cardiol. 1996; 27: 462–470.
Suarez de Lezo J, Pan M, Romero M, Medina A, Segura J, Lafuente M, Pavlovic D, Hernandez E, Melian F, Espada J. Immediate and follow-up findings after stent treatment for severe coarctation of aorta. Am J Cardiol. 1999; 83: 400–406.
Di Filippo S, Sassolas F, Bozio A. Long-term results after surgery of coarctation of the aorta in neonates and children [in French]. Arch Mal Coeur Vaiss. 1997; 90: 1723–1728.
Modan B, Keinan L, Blumstein T, Sadetzki S. Cancer following cardiac catheterization in childhood. Int J Epidemiol. 2000; 29: 424–428.
Frush DP, Donnelly LF, Rosen NS. Computed tomography and radiation risks: what pediatric health care providers should know. Pediatrics. 2003; 112: 951–957.
McFadden SL, Mooney RB, Shepherd PH. X-ray dose and associated risks from radiofrequency catheter ablation procedures. Br J Radiol. 2002; 75: 253–265.
Bacher K, Bogaert E, Lapere R, De Wolf D, Thierens H. Patient-specific dose and radiation risk estimation in pediatric cardiac catheterization. Circulation. 2005; 111: 83–89.
Brenner D, Elliston C, Hall E, Berdon W. Estimated risks of radiation-induced fatal cancer from pediatric CT. AJR Am J Roentgenol. 2001; 176: 289–296.
Soler R, Rodriguez E, Requejo I, Fernandez R, Raposo I. Magnetic resonance imaging of congenital abnormalities of the thoracic aorta. Eur Radiol. 1998; 8: 540–546.
Konen E, Merchant N. Coarctation of the aorta before and after correction: the role of cardiovascular MRI. AJR. 2004; 182: 1333–1339.
Bogaert J, Kuzo R, Dymarkowski S, Janssen L, Celis I, Budts W, Gewillig M. Follow-up of patients with previous treatment for coarctation of the thoracic aorta: comparison between contrast-enhanced MR angiography and fast spin-echo MR imaging. Eur Radiol. 2000; 10: 1847–1854.
Godart F, Labrot G, Devos P, McFadden E, Rey C, Beregi JP. Coarctation of the aorta: comparison of aortic dimensions between conventional MR imaging, 3D MR angiography, and conventional angiography. Eur Radiol. 2002; 12: 2034–2039.
Riquelme C, Laissy JP, Menegazzo D, Debray MP, Cinqualbre A, Langlois J, Schouman-Claeys E. MR imaging of coarctation of the aorta and its postoperative complications in adults: assessment with spin-echo and cine-MR imaging. Magn Reson Imaging. 1999; 17: 37–46.
Pereles FS, McCarthy RM, Baskaran V, Carr JC, Kapoor V, Krupinski EA, Finn JP. Thoracic aortic dissection and aneurysm: evaluation with nonenhanced true FISP MR angiography in less than 4 minutes. Radiology. 2002; 223: 270–274.
Sorensen TS, Korperich H, Greil GF, Eichhorn J, Barth P, Meyer H, Pedersen EM, Beerbaum P. Operator-independent isotropic three-dimensional magnetic resonance imaging for morphology in congenital heart disease: a validation study. Circulation. 2004; 110: 163–169.
Lardo AC. Real-time magnetic resonance imaging: diagnostic and interventional applications. Pediatr Cardiol. 2000; 21: 80–98.
Kuehne T, Weiss S, Brinkert F, Weil J, Yilmaz S, Schmitt B, Ewert P, Lange P, Gutberlet M. Catheter visualization with resonant markers at MR imaging-guided deployment of endovascular stents in swine. Radiology. 2004; 233: 774–780.
Kuehne T, Yilmaz S, Meinus C, Moore P, Saeed M, Weber O, Higgins CB, Blank T, Elsaesser E, Schnackenburg B, Ewert P, Lange PE, Nagel E. Magnetic resonance imaging-guided transcatheter implantation of a prosthetic valve in aortic valve position: feasibility study in swine. J Am Coll Cardiol. 2004; 44: 2247–2249.
Buecker A, Adam GB, Neuerburg JM, Kinzel S, Glowinski A, Schaeffter T, Rasche V, van Vaals JJ, Guenther RW. Simultaneous real-time visualization of the catheter tip and vascular anatomy for MR-guided PTA of iliac arteries in an animal model. J Magn Reson Imaging. 2002; 16: 201–208.
Omary RA, Frayne R, Unal O, Warner T, Korosec FR, Mistretta CA, Strother CM, Grist TM. MR-guided angioplasty of renal artery stenosis in a pig model: a feasibility study. J Vasc Interv Radiol. 2000; 11: 373–381.
Godart F, Beregi JP, Nicol L, Occelli B, Vincentelli A, Daanen V, Rey C, Rousseau J. MR-guided balloon angioplasty of stenosed aorta: in vivo evaluation using near-standard instruments and a passive tracking technique. J Magn Reson Imaging. 2000; 12: 639–644.
Miquel ME, Hegde S, Muthurangu V, Corcoran BJ, Keevil SF, Hill DL, Razavi RS. Visualization and tracking of an inflatable balloon catheter using SSFP in a flow phantom and in the heart and great vessels of patients. Magn Reson Med. 2004; 51: 988–995.
Razavi R, Hill DL, Keevil SF, Miquel ME, Muthurangu V, Hegde S, Rhode K, Barnett M, van Vaals J, Hawkes DJ, Baker E. Cardiac catheterisation guided by MRI in children and adults with congenital heart disease. Lancet. 2003; 362: 1877–1882.
Kuehne T, Yilmaz S, Steendijk P, Moore P, Groenink M, Saaed M, Weber O, Higgins CB, Ewert P, Fleck E, Nagel E, Schulze-Neick I, Lange P. Magnetic resonance imaging analysis of right ventricular pressure-volume loops: in vivo validation and clinical application in patients with pulmonary hypertension. Circulation. 2004; 110: 2010–2016.
Kuehne T, Saeed M, Higgins CB, Gleason K, Krombach GA, Weber OM, Martin AJ, Turner D, Teitel D, Moore P. Endovascular stents in pulmonary valve and artery in swine: feasibility study of MR imaging-guided deployment and postinterventional assessment. Radiology. 2003; 226: 475–481.
Wacker FK, Reither K, Ebert W, Wendt M, Lewin JS, Wolf KJ. MR image-guided endovascular procedures with the ultrasmall superparamagnetic iron oxide SH U 555 C as an intravascular contrast agent: study in pigs. Radiology. 2003; 226: 459–464.
Rigattieri S, Ghini AS, Silvestri P, Tommasino A, Ferraiuolo G, Palamara A, Loschiavo P. A randomized comparison between rotational and standard coronary angiography. Minerva Cardioangiol. 2005; 53: 1–6.
Chernoff DM, Derugin N, Rajasinghe HA, Hanley FL, Higgins CB, Gooding CA. Measurement of collateral blood flow in a porcine model of aortic coarctation by velocity-encoded cine MRI. J Magn Reson Imaging. 1997; 7: 557–563.
Stahlberg F, Sondergaard L, Thomsen C, Henriksen O. Quantification of complex flow using MR phase imaging: a study of parameters influencing the phase/velocity relation. Magn Reson Imaging. 1992; 10: 13–23.
Rupprecht T, Nitz W, Wagner M, Kreissler P, Rascher W, Hofbeck M. Determination of the pressure gradient in children with coarctation of the aorta by low-field magnetic resonance imaging. Pediatr Cardiol. 2002; 23: 127–131.
Steffens JC, Bourne MW, Sakuma H, O’Sullivan M, Higgins CB. Quantification of collateral blood flow in coarctation of the aorta by velocity encoded cine magnetic resonance imaging. Circulation. 1994; 90: 937–943.
Weber OM, Martin AJ, Higgins CB. Whole-heart steady-state free precession coronary artery magnetic resonance angiography. Magn Reson Med. 2003; 50: 1223–1228.
Nagel E, Bornstedt A, Schnackenburg B, Hug J, Oswald H, Fleck E. Optimization of realtime adaptive navigator correction for 3D magnetic resonance coronary angiography. Magn Reson Med. 1999; 42: 408–411.
Buecker A, Spuentrup E, Schmitz-Rode T, Kinzel S, Pfeffer J, Hohl C, van Vaals JJ, Gunther RW. Use of a nonmetallic guide wire for magnetic resonance-guided coronary artery catheterization. Invest Radiol. 2004; 39: 656–660.
Yang X, Atalar E. Intravascular MR imaging-guided balloon angioplasty with an MR imaging guide wire: feasibility study in rabbits. Radiology. 2000; 217: 501–506.
Quick HH, Kuehl H, Kaiser G, Hornscheidt D, Mikolajczyk KP, Aker S, Debatin JF, Ladd ME. Interventional MRA using actively visualized catheters, TrueFISP, and real-time image fusion. Magn Reson Med. 2003; 49: 129–137.
Weiss S, Vernickel P, Schaeffter T, Schulz V, Gleich B. Transmission line for improved RF safety of interventional devices. Magn Reson Med. 2005; 54: 182–189.
Vernickel P, Schulz V, Weiss S, Gleich B. A safe transmission line for MRI. IEEE Trans Biomed Eng. 2005; 52: 1094–1102.
Weiss S, Kuehne T, Brinkert F, Krombach G, Katoh M, Schaeffter T, Guenther RW, Buecker A. In vivo safe catheter visualization and slice tracking using an optically detunable resonant marker. Magn Reson Med. 2004; 52: 860–868.(Julia J. Krueger, MD; Peter Ewert, MD; S)