System Level Safety for Interventional MRI

Cardiac catheterisation is a common minimally invasive procedure that is used to diagnose and treat many forms of cardiovascular disease. Catheter procedures involve inserting a fine tube or thin wire into a peripheral blood vessel, such as the femoral vein or femoral arteries at the level of the groin, and navigating the tip of the inserted device up to the heart so that diagnostic measurements can be made or therapies can be applied, such as placement of a stent or closure of a hole. Current practice is to use X-ray imaging (Fluoroscopy) to guide the process and ensure correct navigation. Although both efficient and widely available, this approach has some drawbacks: it involves an X-ray radiation dose; X-rays provide very limited contrast for the soft tissues of the heart and other internal structures so is not ideal for navigation guidance and disease assessment; and there are some important measurements that cannot easily be performed, such as quantifying blood flow.

Magnetic Resonance Imaging (MRI) Guided catheter procedures can solve all of the above problems, but currently most of the standard catheter devices and guide wires are not safe for use inside MRI systems as they can cause local heating. Some individual devices are being developed for use with MRI scanners, but these often require compromises on performance to be made to achieve safety.

The incompatibility problem arises because MRI scanners use radio frequency (RF) magnetic fields to excite signals from soft tissues. These RF fields are produced by what is called a transmit coil built into to the main body of the scanner. Interactions, called coupling, between this transmit coil and catheters/guide wires that are electrically conducting may result in local tissue currents with a risk that unacceptable heating may occur. This research project seeks to develop a prototype system with a new type of "Active Transmit Coil" that constantly adjusts itself to ensure that there is no coupling with the catheters so making them systematically safe. If successful this would pave the way for many new catheterisation procedures that combine the power of MRI, to visualize soft tissues in great detail and to make key measurements like blood flow, with the efficacy of using standard catheterisation devices.

Cardiac catheterisation is a minimally invasive procedure that is routinely performed under x-ray fluoroscopic guidance. MRI guided catheterisation is an emerging approach that avoids x-rays, offers superior soft tissue visualisation and physiological measures. Although many catheters and guide wires (collectively "wires") are non-ferrous, they are unsafe for MRI due to risk of radiofrequency (RF) heating caused by coupling to the transit coil, so special devices are used. This approach requires a whole new range of devices to be developed, which is expensive and limits the procedures that can be performed. MR compatibility often entails use of different materials and so far has resulted in inferior performance of the devices themselves. Safe operation can still require reduced RF power, limiting performance of the MRI scanner. System level safety can potentially be achieved using Parallel transmit (PTx) MRI to decouple existing standard catheterisation devices. PTx uses multiple individually controlled RF transmit coils that together produce the required RF magnetic fields. Coupling to each transmit element in the PTx array can be determined by measuring induced currents on the wire using non-contact current sensors. For N transmit channels and M current sensors, there are N-M drive modes available for the array that give zero current at all M sensors (referred to as decoupled modes, DM), and M modes that give maximal readings at these sensors (referred to as coupling modes, CM). We will develop a prototype PTx system that monitors current on the part of a wire that remains outside the patient during catheterisation to constantly maintain DM conditions. The system will interface to standard 1.5T MRI scanners so that standard catheter equipment can be safely used on existing hospital scanners that have suitable medical infrastructure around them. We will test the prototype on anatomically realistic phantoms and in pre-clinical in vivo studies to demonstrate safety.

The sole purpose of this proposal is to achieve impact in the sense of a translation from current research results to a tested prototype combined with IP and know-how that will eventually enable production of a medical device or devices. If successful the resulting device would have a direct impact on patient care by changing the preferred delivery of treatment/diagnosis for some conditions, by improving diagnostic quality, and by reducing x-ray exposure both to patients and occupationally among cardiologists. In addition commercialization of the any such device(s) will have an economic impact through wealth generation and through contributing to the strength of the medical technology industry sector.

Wherever possible, without compromising the above goals, we will seek to publish our work in reputable journals and to present at national and international meetings so that research outputs can impact on those working in related areas.