Percutaneous valve placement has been under development for many years. The development of this real-time MRI-guided technique for valve delivery may circumvent many of the problems associated with delivering a valve through a relatively small catheter (8
). The ability to obtain direct access to the heart opens up opportunities for the placement of more durable, well-tested devices compared to those available through catheter access, since the port is bigger, the distances are smaller, and the force transduction from the surgeon’s hand to the delivery device is more direct. The large port access and real-time visualization of the surgical field also facilitates the development of methods to remove sections of calcified or otherwise compromised valves.
The distinct advantage of MRI for guiding cardiac surgery is that the surgeon can see “through” the blood, and thus all of the morphological landmarks for positioning the device are visible. The short magnet makes it possible to have high-performance, real-time MRI available while the prosthetic valve is being manipulated under image guidance. In addition to aortic valve replacements, other target applications for real-time MRI-guided cardiac surgery include mitral, pulmonary, and tricuspid valve replacements or repairs (17
Although the current stent-valve is made of FDA-approved components, it may have to be modified to improve long-term fixation pending long-term testing for stent-valve migration. Additional MR-visible markers may be beneficial to ensure that the valve is positioned correctly with respect to the location of the coronary arteries.
The ability to measure cardiac function online is also an advantage to performing the surgery within the MR scanner. Myocardial perfusion, blood flow, and valve leaflet motion are all available for the surgeon to observe in the working heart. For revascularization procedures, instant feedback on the impact of the procedure on perfusion is available in addition to anatomic data. For valve repairs, real-time feedback on leaflet function can be used to tailor the repair accordingly. Presently, surgical repairs are done on flaccid, empty hearts, and the ultimate effect of the correction is not apparent until the heart is refilled and beating.
The major limitation of this study is that it entailed a set of acute experiments that were designed to test the feasibility of valve delivery. Many questions remain unanswered, such as those regarding the long-term stability and functionality of the valve, and the response of the heart to the procedure. We are currently designing chronic experiments that we hope will answer important questions about the prevalence of ischemia in the heart and brain, LV aneurysm, and thrombosis in the postoperative period.
Although we have demonstrated here that real-time MRI is an excellent method for guiding valve placement, a number of developments need to occur before this procedure can be evaluated in clinical trials. First, the delivery device must be optimized so that it will have the smallest impact on the ventricle while retaining the ability to deliver a standard valve. The method used to secure the valve needs to be improved to guarantee that the valve will not migrate. Also, a procedure must be developed to extract the calcified diseased valve before the new valve is placed.
Real-time MRI provided better image quality than all competing imaging methods for directing minimally invasive surgery, including x-ray fluoroscopy/angiography (in which some anatomical structures are not visible) and echocardiography (in which the FOV is small and the images are of low quality). The MRI-guided surgery also allowed direct functional assessments to be made prior to, during, and immediately after valve implantation. The combination of real-time noninvasive, noncontrast imaging that can provide both anatomic details and functional assessments will enable the use of minimally invasive approaches that may provide patients with a less morbid and more durable solution to structural heart disease.