Victor J. Wright

Real-Time Magnetic Resonance Imaging–Guided Stenting of Aortic Coarctation With Commercially Available Catheter Devices in Swine

Background—Real-time MR imaging (rtMRI) is now technically capable of guiding catheter-based cardiovascular interventions. Compared with x-ray, rtMRI offers superior tissue imaging in any orientation without ionizing radiation. Translation to clinical trials has awaited the availability of clinical-grade catheter devices that are both MRI visible and safe. We report a preclinical safety and feasibility study of rtMRI-guided stenting in a porcine model of aortic coarctation using only commercially available catheter devices. Method and Results—Coarctation stenting was performed wholly under rtMRI guidance in 13 swine. rtMRI permitted procedure planning, device tracking, and accurate stent deployment. “Active” guidewires, incorporating MRI antennas, improved device visualization compared with unmodified “passive” nitinol guidewires and shortened procedure time (26±11 versus 106±42 minutes; P=0.008). Follow-up catheterization and necropsy showed accurate stent deployment, durable gradient reduction, and appropriate neointimal formation. MRI immediately identified aortic rupture when oversized devices were tested. Conclusions—This experience demonstrates preclinical safety and feasibility of rtMRI-guided aortic coarctation stenting using commercially available catheter devices. Patients may benefit from rtMRI in the future because of combined device and tissue imaging, freedom from ionizing radiation, and the ability to identify serious complications promptly.

Real-Time Magnetic Resonance Imaging–Guided Endovascular Recanalization of Chronic Total Arterial Occlusion in a Swine Model

Background— Endovascular recanalization (guidewire traversal) of peripheral artery chronic total occlusion (CTO) can be challenging. X-ray angiography resolves CTO poorly. Virtually “blind” device advancement during x-ray–guided interventions can lead to procedure failure, perforation, and hemorrhage. Alternatively, MRI may delineate the artery within the occluded segment to enhance procedural safety and success. We hypothesized that real-time MRI (rtMRI)–guided CTO recanalization can be accomplished in an animal model. Methods and Results— Carotid artery CTO was created by balloon injury in 19 lipid-overfed swine. After 6 to 8 weeks, 2 underwent direct necropsy analysis for histology, 3 underwent primary x-ray–guided CTO recanalization attempts, and the remaining 14 underwent rtMRI-guided recanalization attempts in a 1.5-T interventional MRI system. Real-time MRI intervention used custom CTO catheters and guidewires that incorporated MRI receiver antennae to enhance device visibility. The mean length of the occluded segments was 13.3±1.6 cm. The rtMRI-guided CTO recanalization was successful in 11 of 14 swine and in only 1 of 3 swine with the use of x-ray alone. After unsuccessful rtMRI (n=3), x-ray–guided attempts were also unsuccessful. Conclusions— Recanalization of long CTO is entirely feasible with the use of rtMRI guidance. Low-profile clinical-grade devices will be required to translate this experience to humans.

Invasive Human Magnetic Resonance Imaging: Feasibility During Revascularization in a Combined XMR Suite

We tested the feasibility and safety of invasive magnetic resonance imaging (MRI) during peripheral angioplasty. Real‐time MRI can image soft tissue and may potentially guide therapeutic procedures without ionizing radiation or nephrotoxic contrast. MRI‐guided diagnostic catheterization has been described recently, but safe and conspicuous catheter devices are not widely available. An active guidewire, which serves as an MRI receiver antenna, might be useful to guide catheterization or even to image atheroma. We describe a combined interventional suite offering both X‐ray fluoroscopy and real‐time MRI. We used a 0.030″ active guidewire receiver coil for invasive MRI after X‐ray lesion traversal in patients undergoing percutaneous iliofemoral artery revascularization. Intravascular MRI was compared with noninvasive MRI, X‐ray angiography, and intravascular ultrasound (IVUS). Seven eligible patients consented to participate, but three were excluded because of lengthy revascularization procedures. Four remaining patients safely underwent combined X‐ray fluoroscopy and real‐time magnetic resonance imaging (XMR) transport, continuous monitoring, and all imaging modalities. There was no device dislodgment, contamination or evidence of heating. The intravascular MRI coil was well visualized except at the tip, but did not provide superior mural imaging compared with IVUS. Therefore, because an adequate safety and workflow experience was obtained, enrollment was terminated after only four subjects. Invasive MRI is feasible and apparently safe during peripheral angioplasty. Patients can safely be transported and monitored in an XMR interventional suite. An active quarter‐wavelength guidewire coil does not provide superior imaging compared with IVUS, but provides satisfactory guidewire visualization. These tools may prove useful for advanced therapeutic procedures in the future. Catheter Cardiovasc Interv 2005;64:265–274. Published 2005 Wiley‐Liss, Inc.

Real-time MRI guided atrial septal puncture and balloon septostomy in swine

Cardiac perforation during atrial septal puncture (ASP) might be avoided by improved image guidance. X‐ray fluoroscopy (XRF), which guides ASP, visualizes tissue poorly and does not convey depth information. Ultrasound is limited by device shadows and constrained imaging windows. Alternatively, real‐time MRI (rtMRI) provides excellent tissue contrast in any orientation and may enable ASP and balloon atrial septostomy (BAS) in swine. Custom MRI catheters incorporated “active” (receiver antenna) and “passive” (iron or gadolinium) elements. Wholly rtMRI‐guided transfemoral ASP and BAS were performed in 10 swine in a 1.5T interventional suite. Hemodynamic results were measured with catheters and velocity encoded MRI. Successful ASP was performed in all 10 animals. Necropsy confirmed septostomy confined within the fossa ovalis in all. BAS was successful in 9/10 animals. Antenna failure in a re‐used needle led to inadvertent vena cava tear prior to BAS in 1 animal. ASP in the same animal was easily performed using a new needle. rtMRI illustrated clear device‐tissue‐lumen relationships in multiple orientations, and facilitated simple ASP and BAS. The mean procedure time was 19 ± 10 minutes. Septostomy achieved a mean left to right shunt ratio of 1.3:1 in these healthy animals. Interactive rtMRI permits rapid transcatheter ASP and BAS in swine. Further technical development may enable novel applications. Published 2006 Wiley‐Liss, Inc.

Measurement of Skeletal Muscle Perfusion During Postischemic Reactive Hyperemia Using Contrast-Enhanced MRI With a Step-Input Function

The regional distribution of skeletal muscle blood flow was measured during postischemic reactive hyperemia using Gd‐DTPA contrast‐enhanced (CE) MRI. The release of an occlusive thigh cuff was used to deliver a step‐input of contrast concentration that was coincident with the onset of reactive hyperemia. A first‐order tracer kinetic equation was used to estimate the unidirectional influx constant, Ki (ml/100 g/min), and the distribution volume of Gd‐DTPA in the tissue, ve, from T1‐weighted images acquired with saturation recovery (SR) steady‐state free precession (SSFP) and spoiled gradient‐echo (SPGR) protocols. The capillary permeability surface (PS) area increased significantly during reactive hyperemia, which facilitated rapid extraction of Gd‐DTPA during the first pass. Regional muscle group studies from 11 normal volunteers yielded blood flow (Ki) values of 108.3 ± 34.1 ml/100 g/min in the gastrocnemius, 184.3 ± 41.3 ml/100 g/min in the soleus, and 122.4 ± 34.4 ml/100 g/min in the tibialis anterior. The distribution volumes (ve) in the corresponding muscle groups were respectively 8.3% ± 2.1%, 9.3% ± 1.9%, and 7.9% ± 1.8% from the kinetic model, and 8.8% ± 2.4%, 9.1% ± 1.9%, and 7.2% ± 1.4% from tissue relaxometry studies. Bulk blood flow studies in the same volunteers using phase‐contrast velocimetry (popliteal artery) yielded significantly lower flow values, but with a correlation coefficient R2 = 0.62 and P = 0.004. Magn Reson Med 54:289–298, 2005. Published 2005 Wiley‐Liss, Inc.

Real-time MRI-guided right heart catheterization in adults using passive catheters

AIMS Real-time MRI creates images with superb tissue contrast that may enable radiation-free catheterization. Simple procedures are the first step towards novel interventional procedures. We aim to perform comprehensive transfemoral diagnostic right heart catheterization in an unselected cohort of patients entirely using MRI guidance. METHODS AND RESULTS We performed X-ray and MRI-guided transfemoral right heart catheterization in consecutive patients undergoing clinical cardiac catheterization. We sampled both cavae and both pulmonary arteries. We compared success rate, time to perform key steps, and catheter visibility among X-ray and MRI procedures using air-filled or gadolinium-filled balloon-tipped catheters. Sixteen subjects (four with shunt, nine with coronary artery disease, three with other) underwent paired X-ray and MRI catheterization. Complete guidewire-free catheterization was possible in 15 of 16 under both. MRI using gadolinium-filled balloons was at least as successful as X-ray in all procedure steps, more successful than MRI using air-filled balloons, and better than both in entering the left pulmonary artery. Total catheterization time and individual procedure steps required approximately the same amount of time irrespective of image guidance modality. Catheter conspicuity was best under X-ray and next-best using gadolinium-filled MRI balloons. CONCLUSION In this early experience, comprehensive transfemoral right heart catheterization appears feasible using only MRI for imaging guidance. Gadolinium-filled balloon catheters were more conspicuous than air-filled ones. Further workflow and device enhancement are necessary for clinical adoption.

Bright-blood T(2)-weighted MRI has high diagnostic accuracy for myocardial hemorrhage in myocardial infarction: a preclinical validation study in swine.

Background— Myocardial hemorrhage after myocardial infarction (MI) usually goes undetected. We investigated the diagnostic accuracy of bright-blood T2-weighted cardiac MRI for myocardial hemorrhage in experimental MI. Methods and Results— MI was created in swine by occluding the left anterior descending (n=10) or circumflex (n=5) coronary arteries for 90 minutes followed by reperfusion for ⩽3 days (n=2), 10 days (n=7), or 60 days (n=6). MRI was performed at 1.5 T, using bright-blood T2-prepared steady-state free-precession, T2* and early (1 minute) and late (10–15 minutes) gadolinium enhancement (EGE, LGE, respectively) MRI. Left ventricular sections and histology were assessed for hemorrhage by an experienced cardiac pathologist blinded to the MRI data. Hypointense regions on T2-weighted and contrast-enhanced MRI were independently determined by 3 cardiologists experienced in MRI who were also blinded to the pathology results. Eighty ventricular pathological sections were matched with MRI (n=68 for EGE MRI). All sections with evidence of MI (n=63, 79%) also exhibited hyperintense zones consistent with edema on T2-weighted MRI and infarct on LGE MRI. Myocardial hemorrhage occurred in 49 left ventricular sections (61%) and corresponded with signal voids on 48 T2-weighted (98%) and 26 LGE-MRI (53%). Alternatively, signal voids occurred in the absence of hemorrhage in 3 T2-weighted (90% specificity) and 5 LGE MRI (84% specificity). On EGE MRI, 27 of 43 cases of early microvascular obstruction corresponded with hemorrhage (63% sensitivity), whereas 5 of 25 defects occurred in the absence of hemorrhage (80% specificity). The positive and negative predictive values for pathological evidence of hemorrhage were 94% and 96% for T2-weighted, 84% and 55% for LGE MRI, and 85% and 56% for EGE MRI. Conclusions— Bright-blood T2-weighted MRI has high diagnostic accuracy for myocardial hemorrhage.

Transthoracic delivery of large devices into the left ventricle through the right ventricle and interventricular septum: preclinical feasibility

BackgroundWe aim to deliver large appliances into the left ventricle through the right ventricle and across the interventricular septum. This transthoracic access route exploits immediate recoil of the septum, and lower transmyocardial pressure gradient across the right versus left ventricular free wall. The route may enhance safety and allow subxiphoid rather than intercostal traversal.MethodsThe entire procedure was performed under real-time CMR guidance. An “active” CMR needle crossed the chest, right ventricular free wall, and then the interventricular septum to deliver a guidewire then used to deliver an 18Fr introducer. Afterwards, the right ventricular free wall was closed with a nitinol occluder. Immediate closure and late healing of the unrepaired septum and free wall were assessed by oximetry, angiography, CMR, and necropsy up to four weeks afterwards.ResultsThe procedure was successful in 9 of 11 pigs. One failed because of refractory ventricular fibrillation upon needle entry, and the other because of inadequate guidewire support. In all ten attempts, the right ventricular free wall was closed without hemopericardium. There was neither immediate nor late shunt on oximetry, X-ray angiography, or CMR. The interventricular septal tract fibrosed completely. Transventricular trajectories planned on human CT scans suggest comparable intracavitary working space and less acute entry angles than a conventional atrial transseptal approach.ConclusionLarge closed-chest access ports can be introduced across the right ventricular free wall and interventricular septum into the left ventricle. The septum recoils immediately and heals completely without repair. A nitinol occluder immediately seals the right ventricular wall. The entry angle is more favorable to introduce, for example, prosthetic mitral valves than a conventional atrial transseptal approach.

Real-Time MR Imaging-guided Laser Atrial Septal Puncture in Swine

PURPOSE The authors performed this study to report their initial preclinical experience with real-time magnetic resonance (MR) imaging-guided atrial septal puncture by using a MR imaging-conspicuous blunt laser catheter that perforates only when energized. MATERIALS AND METHODS The authors customized a 0.9-mm clinical excimer laser catheter with a receiver coil to impart MR imaging visibility at 1.5 T. Seven swine underwent laser transseptal puncture under real-time MR imaging. MR imaging signal-to-noise ratio profiles of the device were obtained in vitro. Tissue traversal force was tested with a calibrated meter. Position was corroborated with pressure measurements, oximetry, angiography, and necropsy. Intentional non-target perforation simulated serious complication. RESULTS Embedded MR imaging antennae accurately reflected the position of the laser catheter tip and profile in vitro and in vivo. Despite having an increased profile from the microcoil, the 0.9-mm laser catheter traversed in vitro targets with similar force (0.22 N +/- 0.03) compared with the unmodified laser. Laser puncture of the atrial septum was successful and accurate in all animals. The laser was activated an average of 3.8 seconds +/- 0.4 before traversal. There were no sequelae after 6 hours of observation. Necropsy revealed 0.9-mm holes in the fossa ovalis in all animals. Intentional perforation of the aorta and atrial free wall was evident immediately. CONCLUSIONS MR imaging-guided laser puncture of the interatrial septum is feasible in swine and offers controlled delivery of perforation energy by using an otherwise blunt catheter. Instantaneous soft tissue imaging provides immediate feedback on safety.

Bright-Blood T2-Weighted MRI Has High Diagnostic Accuracy for Myocardial Hemorrhage in Myocardial InfarctionClinical Perspective

Background— Myocardial hemorrhage after myocardial infarction (MI) usually goes undetected. We investigated the diagnostic accuracy of bright-blood T2-weighted cardiac MRI for myocardial hemorrhage in experimental MI. Methods and Results— MI was created in swine by occluding the left anterior descending (n=10) or circumflex (n=5) coronary arteries for 90 minutes followed by reperfusion for ⩽3 days (n=2), 10 days (n=7), or 60 days (n=6). MRI was performed at 1.5 T, using bright-blood T2-prepared steady-state free-precession, T2* and early (1 minute) and late (10–15 minutes) gadolinium enhancement (EGE, LGE, respectively) MRI. Left ventricular sections and histology were assessed for hemorrhage by an experienced cardiac pathologist blinded to the MRI data. Hypointense regions on T2-weighted and contrast-enhanced MRI were independently determined by 3 cardiologists experienced in MRI who were also blinded to the pathology results. Eighty ventricular pathological sections were matched with MRI (n=68 for EGE MRI). All sections with evidence of MI (n=63, 79%) also exhibited hyperintense zones consistent with edema on T2-weighted MRI and infarct on LGE MRI. Myocardial hemorrhage occurred in 49 left ventricular sections (61%) and corresponded with signal voids on 48 T2-weighted (98%) and 26 LGE-MRI (53%). Alternatively, signal voids occurred in the absence of hemorrhage in 3 T2-weighted (90% specificity) and 5 LGE MRI (84% specificity). On EGE MRI, 27 of 43 cases of early microvascular obstruction corresponded with hemorrhage (63% sensitivity), whereas 5 of 25 defects occurred in the absence of hemorrhage (80% specificity). The positive and negative predictive values for pathological evidence of hemorrhage were 94% and 96% for T2-weighted, 84% and 55% for LGE MRI, and 85% and 56% for EGE MRI. Conclusions— Bright-blood T2-weighted MRI has high diagnostic accuracy for myocardial hemorrhage.