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Percutaneous valve replacements are presently being evaluated in clinical trials. As delivery of the valve is catheter based, the safety and efficacy of these procedures may be influenced by the imaging employed. To assist the operator and improve the success of the operation, we have performed transapical aortic valve replacements (AVR) using real-time MRI guidance (rtMRI).
28 swine underwent rtMRI AVR on the beating heart. Stentless bioprostheses mounted on balloon-expandable stents was used. MR imaging (1.5T) was used to identify the critical anatomic landmarks. In addition to anatomic confirmation of adequate placement of the prosthesis, functional assessment of the valve and left ventricle and perfusion was also obtained with MRI. A series of acute feasibility experiments were conducted (n=18) in which the animals were sacrificed after valve placement and MRI assessment. Ten additional animals were allowed to survive and had follow-up MRI scans and confirmatory echocardiography at 1, 3, and 6 months postoperatively.
rtMRI provided superior visualization of the landmarks needed. The time to implantation after apical access was 74±18 seconds. Perfusion scanning demonstrated adequate coronary flow and functional imaging documented preservation of ventricular contractility in all animals following successful deployment. Phase contrast imaging revealed minimal intra or para-valvular leaks. Longer term results demonstrated stability of the implants with preservation of myocardial perfusion and function over time.
rtMRI provides excellent visualization for intraoperative guidance of AVR on the beating heart. Additionally it allows assessment of tissue perfusion and organ function that are not obtainable by conventional imaging alone.
Beating heart valve replacement using either a percutaneous or left ventricular apical access has been reported. (1-9) These minimally invasive approaches reduce trauma and speed recovery for the patient. Compared to the percutaneous transfemoral approach, transapical aortic valve replacement provides a direct and short access to the native valve. It requires general anesthesia, but this approach may be more applicable to a wider range of patients because of the lack of physical anatomic limitations. In addition, the transapical approach permits the potential implantation of a conventional prosthesis that has a known durability and proven success rate.
Typically, the imaging used for percutaneous valve placement is fluoroscopy, which provides 2D visualization with little soft tissue contrast. Contrast agents must be repeatedly injected to determine the location of the aortic annulus and coronary ostia. The patient and physician are exposed to radiation during the intervention. Rapid ventricular pacing to unload the ventricle, which is considered fundamental for the success of the procedure, has precipitated cardiac arrest in some high-risk patients. Moreover, additional imaging and expertise with echocardiography is required for post intervention valvular assessment.
MRI provides better anatomic detail than fluoroscopic or echocardiographic imaging without additional risk of radiation and contrast reaction. As a result of the clarity of MR, image interpretation is easy for anyone familiar with the surgical anatomy. The development of real-time magnetic resonance imaging (rtMRI) allows this imaging modality to provide intraoperative guidance for delivery of prosthetic aortic valves (10, 11). MRI also provides the ability to assess results, such as ventricular and valvular function, and myocardial perfusion, immediately after intervention. To demonstrate how rtMRI can assist the operator and improve the success of the operation, we have performed transapical aortic valve replacements using rtMRI guidance. We report our rtMRI guided transapical aortic valve experiment technique and its mid-term results.
1.5-T Magnetom Espree (Siemens Medical Solutions, Munich, Germany) was used for the intervention. This magnet design, with short (120cm) and wide (70cm) bore, makes surgical access to the patient within the magnet feasible. High-quality images can be obtained at 5-10 frames per second with low latency. In addition to providing standard MR sequences, a fully interactive, rtMRI system connected to the scanner provides a real-time interactive imaging sequencing (12, 13). This system comprises an interactive user interface, an operating room large-screen display, gated pulse sequences, and image reconstruction software. Multiple oblique slices can be obtained in rapid succession and can be simultaneously displayed in a 3D rendering to provide optimal 3D anatomic information. Image contrast, image plane orientations, acquisition speed, 3D rendering, and device tracking can be readily adjusted as needed during scanning. Standard MR sequences are used for pre-operative surgical planning and post-placement evaluation and in between this standard scanning, rtMRI provides feedback on the progress of the procedure.
Since the intervention is performed in the MRI scanner, the instruments and materials must be compatible with the magnetic field. Commercially available 21-25mm stentless bioprostheses (Toronto SPV, St. Jude Medical, Minneapolis, MN, or Freestyle aortic root bioprosthesis, Medtronic, Inc Minneapolis, MN) were selected based on the aortic valve size scanned with MRI. These bioprosthetic valves were mounted on an analogous-sized, platinum-iridium stent (Cheatham Platinum, NuMed, Hopkinton, NY). Small austenitic stainless steel fragments (0.2mm) were welded on the side of the stent between the bioprosthetic valve commissures. This paramagnetic marker is visible as a dark signal in the MRI and is used to indicate the orientation of the valve/stent. The bioprosthesis and stent were then circumferentially compressed over two-stage balloon-tipped catheters (NuMed, 25-30mm OD, 50mm long). Figure 1 shows a Medtronic valve mounted on NuMed platinum iridium stent and its MR image. A central guide wire allowed tracking of the balloon-tipped catheter antegrade across the native aortic valve.
The delivery device consists of a straight plastic rod, outside of which is a sheath protecting the bioprosthetic valve before it is deployed. The length of the delivery device is 60cm. The diameter of the delivery device is 9.5mm and fits into a 10mm trocar. The inner rod has a 6.35mm central channel for the guidewire directed balloon-tipped catheter. At the distal end of the inner rod, a locking system is used to prevent relative motion between the catheter and the rod itself. The translation and rotation of the rod directly relate to the translation and rotation of the balloon catheter. The inner rod and the sheath are relatively tight with each other; a small rubber gasket is used to prevent blood leakage from the central channel.
All experiments were performed under protocols approved by the National Institutes of Health Animal Care and Use committee. After induction, twenty eight domestic pigs were intubated and anesthetized. Standard MR sequences were performed to obtain the orientation of the heart, evaluate ventricular and valve function, locate the native valve annulus and the origin of the coronary arteries. Using standard titanium surgical instruments via a 6-cm subxiphoid incision, the pericardium was opened and the apex of the heart was exposed. Two concentric purse strings were placed around the apex, through which a 10-mm trocar (Ethicon Surgical Inc, Somerville, NJ) was inserted into the left ventricle (LV). Typical time to complete this part of the procedure was 15-20 minutes.
Pre-scanning also allows setting up scan planes to be used for real-time imaging during valve implantation and follow-up myocardial perfusion and aortic flow imaging. Three imaging planes were prescribed for real-time imaging during implantation. Two of these planes were positioned to provide long-axis views of the LV, showing the right coronary artery and left anterior descending coronary artery origins, respectively. The other plane provided an axial view of the aortic valve. The coronary ostia and aortic annulus location were digitally marked. These digital marks remained visible at all times in the 3D rendering and were used for anatomic reference.
The surgeon views the real-time imaging on a projection screen while manipulating the deployment device within the animal in the magnet. Three snapshots of the rtMRI with multiple image planes are shown in Figure 2.
A guide wire is advanced through the trocar across the native aortic valve, after which the prosthetic valve and delivery system are advanced through the trocar. During implantation, the axial slice was shifted as needed to visualize the device and guide proper orientation of commissures with the help of the passive marker. The long-axis views were interactively modified to show the path of the delivery device, while keeping the coronary origins in view. The surgeon is in contact with the scanner operator by means of headphones and a microphone (Magnacoustics, Atlantic Beach, NY) to request changes in the imaging planes as needed.
Once in place, the balloon is partially inflated by using normal saline mixed 100:1 with an MR contrast agent Gd-DTPA (Magnavist, Berlex Inc, Montville, NJ), the position is re-confirmed to be ideal and the balloon is then fully expanded and the valve/stent deployed while monitoring with the rtMRI. The balloon is then deflated; the catheter and guide wire are removed through the trocar. Ventricular function is immediately assessed with the real-time imaging.
During the procedure, the animals were monitored with ECG, oxygen saturation, end-tidal CO2, systemic and left ventricular blood pressure, and arterial blood gas analysis.
After placement of the valve, the trocar was removed and the apex closed with the purse string sutures. Post-placement images were acquired to confirm the positions of the prostheses and the valvular and heart function. In addition to anatomic confirmation of adequate placement of the prosthetic valve in relation to the aortic annulus and the coronary arteries, functional assessment of the valve and left ventricle was also obtained with MR imaging. Gated CINE MR was used to assess mitral valve function and myocardial function. Phase contrast CINE MR imaging was used to identify flow through the new valve as well as intra-valvular or para-valvular regurgitation. An MR first-pass perfusion scan (14) was performed during intravenous injection of Gd-DTPA contrast agent to confirm that myocardial blood flow was intact to all segments of the myocardium.
A series of acute feasibility experiments were conducted (n=18) in which the animals were sacrificed after valve placement and MRI assessment. Ten additional animals were allowed to survive for long-term follow up. At 1 and 3 months postoperatively, follow-up MRI scans and transthoracic echocardiography were acquired while at 6 months postoperatively MRI scans and confirmatory 2D and 3D transesophageal echocardiography were acquired. Retrospectively gated CINE MR, phase contrast CINE MR, and MR first-pass perfusion scaning during intravenous injection of Gd-DTPA contrast agent were repeated at those time points to confirm the position of the prostheses and the valvular and heart function.
rtMRI provided superior visualization of the landmarks needed to implant the aortic valve prostheses compared to fluoroscopy or echocardiography. The passive marker on the stent is a superb indicator of the valve orientation in MRI at the time of placement and can be used to determine if the valve migrates over time. The time to implantation after the apical access was obtained to deployment of the valve was 74 ± 18 seconds. This includes the time to correctly orient the prosthetic valve with respect to the native annulus and the coronary ostia. The average procedure duration from skin incision to closure including scanning was less than forty minutes.
Retrospectively gated cine MRI revealed excellent myocardial function after valve implantation in both long- and short-axis views (snap shot is shown in figure3, column1, row1 and row2). The phase-contrast CINE MR images (figure 3, column 1, row 3) show the through-plane blood velocity at both systole and diastole. These images confirm good systolic flow with excellent valve leaflet opening and no evidence of turbulence, diastolic regurgitant flow, or paravalvular leak. First-pass perfusion studies demonstrated adequacy of myocardial blood flow after valve placement in all animals following successful deployment. A first-pass perfusion scan at the base of the heart after valve implantation is shown in figure 3, column 1, row 4. These perfusion results confirm adequacy of blood flow at the tissue level, indicating proper valve positioning with respect to the coronary ostia. Echocardiographic results confirmed the MRI findings and further documented the stability of prostheses position and function over time. Necropsy of all these animals confirmed the deployed prosthetic valve location with respect to the aortic annulus and the mitral valve, and the commissures did not obstruct the coronary ostia. The observed distances between the commissural marker and a coronary ostia were 4mm or greater. These necropsy findings verified the MRI and echocardiography results.
Among ten animals allowed to survive, seven animals survived for 6 months. Three animals died between one and two weeks postoperative due to respiratory complications. The seven long term survivors were brought back for echocardiography and MRI scan 1, 3, and 6 months post procedure respectively. Figure 3 shows long term follow-up MRI scan for a pig at 1, 3, and 6 months, with regard to anatomic positioning, phase contrast and perfusion. In the follow-up studies, perfusion scanning demonstrated adequate coronary flow and functional imaging documented preservation of ventricular contractility. Phase contrast imaging revealed minimal intra or para-valvular leaks. The ventricular and valvular parameters of seven animals based on 1, 3, and 6 month follow-up echocardiography and MRI scan are presented in Table 1. ANOVA was performed to determine the significance of the differences between the 1, 3, and 6 month data. The p-values are also shown in Table 1. From these p-values we can conclude that there is no significant difference among the ventricular and valvular parameters during the 6 months post procedure. The degree of insufficiency of the aortic valve and the mitral valve over 6 months post procedure were measured and shown in Figure 4. Longer term results demonstrated stability of the implants without migration or change in position and with preservation of myocardial perfusion and function over time. The histopathology reports of mid-term (6 month) verified that all the implanted aortic stent and valve assemblies were in place in the aortic root. The stents were properly seated in firm apposition to the aortic wall from the level of the prosthetic leaflets to the distal commissures tips. The prosthetic device leaflets remained cusp shaped and in the closed position exhibited good coaptation of the free edges. These necropsy findings correlated with both the MRI and echocardiography results. Two animals grew from 50kg to 200kg during the 6 month period of follow up; and the aortas partially outgrew the original implanted aortic stents. At necropsy after 6 months in these animals, some focal gaps were observed between the device and the native aortic wall in the area of the native aortic leaflet. These gaps allowed for perivalvular regurgitation which was noted on MRI and echocardiography prior to necropsy.
Percutaneous transfemoral aortic valve replacement has been reported in a highly selected cohort of patients (2). The percutaneous technique requires excessive minimization of the prosthesis, which still remains bulky and could result in device embolization, and is difficult to traverse undersized, calcified or tortuous iliofemoral vessels (15). Transapical aortic valve implantation is a relatively easy, safe and straightforward direct technique, which allows excellent alignment between the prosthesis and the aortic root. Patients with symptomatic, severe aortic stenosis and high-surgical risk are more suitable for transapical approach (16, 17).
According to the literature, in both the percutaneous transfemoral and transapical approach, positioning of the prosthesis is mostly aided by 2D M-mode transesophageal echocardiography and high-resolution fluoroscopy (7). Fluoroscopy provides little soft tissue contrast. Additionally what weakly visible landmarks that the calcification of the stenotic valve provides are lost or distorted following the mandatory balloon valvuloplasty done prior to valve placement. Contrast agents must be frequently injected to determine the location of aortic annulus and coronary ostia. The patient and physician are exposed to radiation during the intervention. Echocardiography is dependent on ultrasonic beam reflection and therefore is subject to impaired imaging caused by limited acoustic windows. This is particularly true for the aortic valve when considering the impact of calcification and/or prosthesis shadowing. Fusion imaging combining computerized tomography and fluoroscopy has been proposed to overcome these issues but are not real time, are highly dependent on stable patient registration and significantly increase the radiation exposure. Clinical results of both transfemoral and transapical approaches for percutaneous aortic valve replacement have reported success with these techniques. Thus far these procedures have carried a significant morbidity and mortality. MACCE have been seen in 32% to 65% of the patients (15). Peri-procedural mortality has ranged from 10% to 23% (18, 19). While some of these outcomes are due to the comorbidities of the high risk patients being treated, many can be attributed to imaging related problems including coronary obstruction, myocardial infarction, device misalignment, and valve embolization. In fact, valve malposition or migration are two of the top three reasons for procedural failure (20). Other frequently reported limitations that are imaging related include landmark loss after balloon valvuloplasty, perivalvular leak due to inability to determine whether the stent is adequately opposed to the aorta and aortic annulus, and fatal arrhythmias due to the rapid ventricular pacing required to provide time for valve placement. These difficulties should be decreased by improving the imaging employed to perform the procedure.
rtMRI provides excellent visualization for intraoperative guidance of aortic valve replacement on the beating heart. It provides better image quality and a complete view of the entire chest more than other competing imaging methods, such as fluoroscopy/angiography, in which some anatomical structures are not visible, and echocardiography, in which the field of view is small. MRI-guided surgery also allowed direct functional assessments to be made prior to, during, and immediately after valve implantation that are not obtainable by conventional imaging alone (21).
Expansion of rtMRI guidance to facilitate other types of cardiac surgical procedures, including mitral, pulmonary, and tricuspid valve replacements or repairs, should be considered to minimize trauma and enhance patient benefit.
The authors are supported through the Intramural Research Program of the National Heart, Lung, and Blood Institute, NIH, DHHS.
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This paper was presented at 2009 Western Thoracic Surgical Association meeting.
Keith A. Horvath, Cardiothoracic Surgery Research Program (CSRP), National Heart, Lung and Blood Institute (NHLBI), National Institutes of Health (NIH)
Dumitru Mazilu, Biomechanical Engineer, CSRP, NHLBI, NIH.
Michael A. Guttman, Laboratory of Cardiac Energetics, NHLBI, NIH.
Arthur Zetts, Laboratory of Animal Medicine and Surgery, NHLBI, NIH.
Timothy Hunt, Laboratory of Animal Medicine and Surgery, NHLBI, NIH.
Ming Li, Biomechanical Section Chief, CSRP, NHLBI, NIH.