Imaging System, Prosthetic Valve and Devices
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
). 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). 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.
Figure 1 Bioprosthesis mounted on a platinum iridium stent. A stainless steel marker welded on the side of the stent between the commissures (a). The marker is visible as a dark signal in the MRI and indicates the orientation of the prosthesis (b). The short axis (more ...)
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 .
Real-time MRI provided superior visualization for implanting the aortic valve prosthesis. Snapshots show multiple image planes displayed at their relative 3D position. The 3D rendering provide 3D anatomic information.
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.