Innovative Technology for MRI Guided Procedures

National Heart, Lung, And Blood Institute

Investigators

Linked publications & trials

Abstract

Background Image guided, minimally invasive procedures are an important diagnostic and therapeutic tool in cardiology. Many procedures that required open heart surgery in the past can now be performed percutaneously. The preferred imaging modality is current practice is X-ray flouroscopy. This technique has high spatial resolution and a high frame rate very suitable for real-time image guidance. However, X-ray flouroscopy has come significant drawbacks. The soft tissues (e.g. the cardiac muscle) are not well visualized on X-ray images, which hampers guidance of procedures such as myocardial biopsy. It also exposes the patient the significant doses of ionizing radiation. Many patients with structural heart disease have several procedures in their lives and the cumulative radiation dose, with ensuing risk of developing cancer, can be substantial. As a consequent of the shortcomings of X-ray guided procedures, there is great interest in using MRI as image guidance. MRI provides superior soft tissue contrast and does not expose the patient to radiation risk, but there are other challenges associated with the use of MRI for interventional image guidance. In the Laboratory of Imaging Technology we are focused on the two main challenges; imaging speed and imaging safety, which are, to some extend related. MRI is an inherently slow technique compared to X-ray flouroscopy. It can take seconds to acquire a single image, which is obviously too slow. To compensate for this we acquire undersampled datasets and apply novel reconstruction techniques in real-time to achieve good frame rates. The imaging sequences we use have been modified to allow interactive control of many sequence parameters that control image orientation, frame rate, and contrast. An important aspect of interventional procedures is that long metal devices (wires and catheters) are introduced into the patient to reach a particular target (e.g., in the heart). These long conducting structures can heat up significantly due to the radiofrequency energy deposited by the MRI acquisition. This device heating can cause tissue damage and is an important safety issue associated with current interventional MRI procedures. We aim mitigate this device heating problem (thus enabling the field of interventional MRI) by developing imaging sequences and instruments that deposit less radiofrequency radiation in the patient. Goals/Objectives The specific goals of this project are: 1. Develop low Specific Absorption Rate (SAR) sequences that can be used for real-time imaging. 2. Develop techniques that can visualize passive devices. 3. Develop device heating feedback systems that can regulate SAR in response to device heating. 4. Design new MRI systems that are less prone to device heating. Progess in fiscal year 2016 The key to lowering the device heating is to deposit less radiofrequency energy in the patient while imaging. Our first approach to this was to develop sequences and infrastructure that allow us to run with much lower radiofrequency duty cycle. Our techniques include spiral imaging with long readouts, which can lower the heating significantly. To make the image quality adequate it is necessary to perform corrections for gradient imperfections. We now have such technqiues working in real-time. We have also taken inital steps to develop an MRI system that uses less radio frequency energy. The key to this is lowering the magnetic field. The energy deposited in the patient is proportional to the field strength squared and we propose lowering the field to about 1/3 of the current field strengths, i.e., to use a 0.5T magnet instead of the current 1.5T magnet. In the previous year we have made significant progress on the theoretical foundations of such a system. In particular we have explored what sequences could be used to preserve much of the signal while lowering the field strength. This theoretical work will lead to design specifications for a new system in the coming year. The laboratory has also contributed to the dissemination of new imaging techniques developed at the NIH. In collaboration with Siemens, we have contributed to the development of a works in progress (WIP) package for interventional MRI. This new package includes features developed at the NIH that allow interactive control of imaging contrast and imaging speed. The WIP package works with out Gadgetron (https://gadgetron.github.io) software package.