Innovative Technology for MRI Guided Procedures

National Heart, Lung, And Blood Institute

Investigators

Linked publications, trials & patents

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 patient to significant doses of 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 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 Laboratory of Imaging Technology, 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 2017 Our approach to improve device safety has been to generate images with much lower radiofrequency duty cycle while still obtaining sufficient image quality. We use spiral imaging with long readouts and inline GPU-aided image reconstruction using the our open-source reconstruction framework Gadgetron (https://gadgetron.github.io). To maintain image quality, we also perform inline distortion corrections for gradient imperfections and have developed an inline spiral image deblurring framework. The computationally intensive algorithm for image deblurring is conventionally performed during post-processing, but we have implemented this algorithm for real-time imaging with adaptation to changing slice orientation during an MRI-guided intervention. In collaboration with the Cardiovascular Intervention Program (Robert Lederman), we have used low energy imaging for right heart catheterization with a standard commercial metallic guidewire in patients. We have performed extensive safety evaluations and have shown that it is safe and feasible to use some conductive metallic devices in the MRI scanner with low energy imaging. We have also performed further work on the visualization of metallic devices during MRI-guided procedures by isolating their specific metallic signal. These techniques were used during the patient studies. We have made progress on the real-time measurement of blood flow within the interventional environment using spiral image acquisitions and on improving spiral image contrast using new balanced steady state free precession acquisitions. The laboratory also continues to disseminate new imaging techniques developed at the NIH to other research groups and through our collaboration with Siemens Medical Solutions.