Innovative Technology for MRI Guided Procedures

National Heart, Lung, And Blood Institute

Investigators

Linked publications & trials

Abstract

Background Image guidance is an important tool used for minimally invasive diagnostic and therapeutic procedures in cardiology. Many procedures that required open chest surgery in the past can now be performed percutaneously. Current practice uses X-Ray fluoroscopy for image guidance. This technique provides high spatial and temporal resolution suitable for procedural guidance. However, X-Ray fluoroscopy has some significant drawbacks. Soft tissues (eg. cardiac muscle) are not well visualized on X-Ray images, which hampers the guidance of procedures that require precise tissue localization such as myocardial biopsy. It also exposes the patients and operators to ionizing radiation. Patients with structural heart disease undergo many procedures throughout their lifetime and the cumulative ionizing radiation dose, and ensuing risk of developing cancer, can be substantial. To overcome the limitations of X-Ray guidance, there is a great interest in moving to MRI guidance of procedures. MRI provides superior soft tissue visualization, flexible image contrast, and does not expose the patient to ionizing radiation, but there are other challenges associated with the use of MRI for procedural guidance. In the MRI Technology Program, we are focused on two main challenges: imaging speed and imaging safety. Conventional MRI imaging can take seconds to acquire a single image, which is too slow for procedural guidance which depends on high frame rate imaging (several frames per second). To compensate for this, we acquire undersampled data sets and apply novel reconstruction techniques in real-time to achieve sufficient frame rates. We develop specialized imaging sequences that allow interactive control of imaging parameters, such as image orientation, frame rate and image contrast. Standard catheterization lab procedures rely on long metallic devices (eg. guidewires and catheters) to reach a particular target in the vasculature or heart. These long metallic devices are susceptible to significant heating due to the radiofrequency energy deposited during MRI causing tissue damage. The unavailability of safe and visible devices is a limitation in the field of MRI-guided interventions. We aim to mitigate the device heating problem by developing imaging technologies that deposit less radiofrequency energy in the patient. Progress in fiscal year 2019 We continue to develop MRI technology to enable MRI-guidance of cardiovascular catheterization procedures. We have developed lower energy imaging methods to improve safety of standard commercially available interventional devices during MRI imaging. These methods reduce the radiofrequency duty cycle, thereby limiting deposited energy and metallic device heating during real-time imaging. With this approach, we completed a first-in-human study using one commercial metallic guidewire with a single safe imaging sequence for MRI-guided right heart catheterization. To improve device safety even further, we modified our MRI system to operate at 0.55T while retaining the contemporary hardware capable of real-time imaging. The improved safety profile of the lower field has expanded our use of metallic devices with standard imaging sequences. Since modifying our MRI system, we regularly perform MRI-guided cardiovascular catheterizations with metallic guidewires in patients referred for clinical right heart catheterization. This development of a high-performance low field MRI system can potentially enable more complex procedures with standard metallic devices. We continue our development of real-time spiral imaging with inline image distortion correction. We have advanced these methods to include spiral balanced steady-state free precession imaging for diagnostic cardiovascular imaging. These methods are amenable because they exploit the physical properties at low field. These spiral imaging techniques have been applied to recover image signal at low field and for fast acquisition within the interventional setting. We have also performed comparison studies to validate the accuracy of quantitative cardiac MRI at 0.55T in healthy volunteers and patients with known cardiovascular disease referred for clinical diagnostic cardiac MRI. We participate in the improvement of accessory devices and software for interventional MRI. We have designed and implemented an interactive front end software for real-time flow quantification, which includes the continuous streaming of imaging data and computation using of beat-to-beat cardiac output. Low field MRI also offers significant opportunities for functional lung imaging. Lung MRI is notoriously challenging due to distortions caused by air-tissue interfaces, and low field provides higher quality imaging of lung parenchyma, enabling the application of MRI for measurement of lung function. The MRI Technology program has developed methods to image the lung using low field MRI. This new technology allows comprehensive cardiopulmonary evaluation in the MRI catheterization environment. Our work in MRI guided interventions is done in collaboration with Dr. Robert Lederman and the Laboratory of Cardiovascular Interventions.