Device safety and visualisation in MRI-guided cardiac catheterisation

Project summary: Interventional cardiac MRI (‘iCMR’) is an alternative to standard x-ray fluoroscopy, that has the benefits of eliminating radiation dose, allowing visibility of soft tissues and performing accurate blood flow measurements. A challenge for interventional MRI is that guide-wires and braided catheters commonly used for interventional procedures are metallic and cause tissue heating by focusing the radiofrequency (RF) fields produced by the scanner within the patient’s body. At St.Thomas’ we have produced a prototype ‘active decoupled’ RF system to be used within a normal MRI scanner for iCMR. This alternative RF system is capable of eliminating induced currents, and can also be used to improve visualisation of interventional devices, another challenge for iCMR.

The aim of this project is to develop this technology for ultimate use within humans by solving two specific challenges: (i) development of new current sensor technology, and (ii) devising rapid MRI methods for enhanced device visualisation.

Project description:  MRI guidance for cardiac interventions usually requires the use of exclusively non-metallic instruments (guidewires and catheters, referred to as ‘wires’ for simplicity) in order to avoid radiofrequency heating. Such devices are being developed commercially and they can offer suitable performance for some procedures, however they often lack the mechanical stability and stiffness that is available from standard metal wires used in x-ray fluoroscopy. In addition, although MRI offers excellent visualisation of soft tissues, it is often a challenge to visualise these (usually plastic or fibreglass) devices using MRI.

A radically different approach pioneered within BMEIS is to replace the built-in “body coil” of the MRI scanner with an on-body surface transmit array of multiple elements; the approach is known as parallel transmission (pTx). By measuring induced currents on inserted wires in real time it is possible to control the array to cancel out induced currents and hence produce safe MRI conditions for imaging. It is also possible to excite wires preferentially (using very low power), providing a promising route for intraprocedural device visualisation.  

Currently we have a prototype system built for performing these procedures at 1.5T and have performed in vivo cardiac MRI procedures on sheep to demonstrate its safe operation. However a workable clinicalcapability requires solving key challenges outlined below; this project aims to address these areas and will link in with further grant funding to produce first in man testing.

Objective 1: Current sensing

‘Active decoupling’ of induced currents requires continuous monitoring; for cardiac interventions one way to do this is to include physical sensors at the wire insertion point. These current sensors can double as transmitters for device visualisation. The first objective of the project will be to investigate miniaturised sensors that are capable of this dual function. Unlike bulky prototype designs, the demonstration sensor would be of a design that could in principle be used for human studies, as evaluated by the clinical partner in this study.

Objective 2: high frame rate real-time wire visualisation using AI

Wire excitation using pTx to directly couple to wires has been shown to be a viable method for visualisation, however current frame-rates (approximately 0.5fps) are lower than required (5-10fps). The student will investigate use of fast MR methods, coupled with real-time image reconstruction and AI based whole-wire segmentation to generate overlays for anatomical images. 

Objective 3: Clinical workflow 
Demonstrations of the technology have so far been limited to simple experimental scenarios, but have not explored operation under clinical conditions. The student will work with Dr. Pushparajah (clinical co-supervisor) to perform realistic imaging tests using purpose built (existing) phantoms, to identify and resolve bottlenecks.