Innovative Technology for MRI Guided Procedures

背景:

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. In 2017, we used the lower energy imaging methods (real time gradient echo spiral imaging) to improve safety of standard commercially available interventional devices during MRI 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 on a 1.5T MRI scanner. To improve device safety even further, in 2018 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. The MRI Technology Program has focused on the development of new imaging methods for 0.55T for both real-time procedural guidance and diagnostic cardiopulmonary imaging. Progress in fiscal year 2020: Since modifying our MRI system to operate at 0.55T, we routinely perform MRI-guided cardiovascular catheterization 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 have examined a number of standard interventional devices at 0.55T to assess artifact and RF-induced heating in order to determine those which are suitable for clinical MRI-guidance of cardiovascular procedures. We continue our development of spiral imaging with inline image distortion correction. Overall, spiral imaging 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 advanced these methods to include spiral balanced steady-state free precession imaging for real time imaging, quantification of cardiac function, and quantification of flow. Spiral late gadolinium enhancement imaging and three-dimensional spiral pulmonary imaging methods are under development. In addition, we have explored the application of these methods to improve image quality outside the cardiopulmonary system for structural imaging in the brain. We have performed clinical validation studies to confirm the accuracy of quantitative cardiac MRI measurements. We have demonstrated the accuracy of systolic function and ventricular volumes at 0.55T in healthy volunteers and patients with known cardiovascular disease referred for clinical diagnostic cardiac MRI. We have also demonstrated the validity of 0.55T to assess myocardial scar using late gadolinium enhancement, which is critical for the clinical application of CMR for diagnostic imaging and MRI-guided interventions. In addition to fast imaging methods, we also 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. During a physiological provocation in the catheterization environment, this software allows the continuous streaming of imaging data and continuous computation using of beat-to-beat MRI cardiac output measurement. 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 measurement of function, physiology and tissue characterization to the lung. The MRI Technology program has developed a number of methods to image regional lung function using low field MRI. This new technology allows comprehensive cardiopulmonary evaluation in the MRI catheterization environment. We have also developed high-quality structural lung imaging methods using 0.55T. These methods will apply to a variety of lung disease including lymphangioleiomyomatosis, lung nodules, pulmonary infiltrates, pulmonary fibrosis, bronchiectasis, and COVID-19. We continue to develop reconstruction methods, especially for spiral imaging applications, using the open-source Gadgetron software framework is developed and maintained in NHLBI. We have implemented infrastructure for three-dimensional spiral imaging reconstruction and distortion correction, and implemented infrastructure for gradient waveform streaming in the Gadgetron. In June 2020, a Gadgetron online class was organized through the University of Bordeaux and attended by >200 participants. Staff from the MRI Technology Program participated in the organization and execution of this online training.