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 2018 Our approach to device safety has been to limit the radiofrequency duty cycle, and hence limit the deposited energy, when generating real-time images. To achieve this, we have implemented real-time spiral MRI sequences, including inline distortion correction, for procedural guidance. This modified acquisition can be used to enable the use of metallic devices for MRI-guided interventions. We completed a first-in-human study with this sequence to use commercial metallic guidewires for MRI-guided right heart catheterization. The guidewires provided incremental stiffness to the catheter and enhanced catheter shaft visibility, which was valuable for procedural workflow. We have begun exploration other approaches to lower imaging energy for interventional MRI. Real-time spiral sequences are subject to image distortions in the form of gradient hardware inaccuracies and anatomical blurring. Previously, we have implemented inline distortion correction methods that are used during image reconstruction using calibration of the gradient system impulse response function. More recently, we have implemented modifications to our image acquisition to prospectively avoid image distortions. We have developed a pipeline to investigate new susceptibility markers for devices and used this to explore nitinol, stainless steel and iron oxide markers that can be added to commercial devices to improve MRI visibility. In addition, we continue to explore positive contrast visualization methods as an alternative to negative contrast susceptibility imaging. We have explored the properties of a range of commercially available catheters and guidewires using this methodology to determine candidate devices with suitable visualization properties for clinical MRI-guided interventions. We have also participated in the development and testing of accessory devices for MRI guided interventions including MRI hemodynamic recording systems and MRI defibrillators.