Philips Research Technology Backgrounder

Philips develops new technology to treat heart arrhythmias via MRI-guided minimally invasive intervention

Scientists at Philips Research in Hamburg, Germany, have developed new catheters and guidewires that for the first time allow the use of MRI scanners during minimally invasive surgery, particularly in the area of cardiology. Minimally invasive procedures reduce trauma, thereby minimizing damage to healthy tissue and the amount of pain relieving medication required, which is better for the patient and shortens recovery times. For many minimally invasive interventions, particularly those that involve the insertion of a catheter through the patient’s vascular system into the heart, surgeons need information from medical scanners to view what is happening inside the patient.

The new catheter technology that has been developed by Philips Research overcomes the sensitivity of many existing catheters to the strong radio frequency fields encountered in MRI scanners, which currently renders them unusable with MRI. Catheters that are electrically compatible with MRI equipment promise to make it significantly easier for heart surgeons to carry out delicate procedures and will thereby improve patient care.

Today, most catheter tracking is done using X-ray equipment. In fact, one in every two procedures to open obstructed heart arteries via catheterization is guided by a Philips X-ray system. However, because of its ability to provide detailed 3D images of soft tissue and fluid-filled organs such as the heart and vascular system, MRI has the potential to provide even more information on catheter positioning, making it easier to identify exactly where the catheter is in relation to these structures. In some cases, MRI guided catheterization will allow the simultaneous imaging of diseased tissue and the catheter, so that surgeons will know precisely when they have the catheter correctly positioned. An additional benefit of MRI is that it avoids the use of ionizing radiation.

Catheterization is used for a range of procedures in cardiology including balloon dilatation coupled with stent insertion, electrophysiology and local drug delivery (for example, for tumor treatment). Especially in electrophysiology and drug delivery applications, MRI’s functional and molecular imaging capabilities (its ability to image physiological processes) will add the possibility of real-time therapy monitoring. However, the conventional catheters currently employed in these procedures are not approved for use with MRI.

An electrical challenge
When used in conjunction with MRI scanners, the problem with many existing catheters is that they contain electrical leads. For example, catheters used to treat atrial fibrillation and other types of cardiac arrhythmia (irregular heartbeat) must be able to transmit ECG signals and catheter position signals from inside the heart via catheter leads to the outside. Catheter leads are also used to transmit signals to the heart for pacing purposes (similar to an implanted pacemaker) and for ablation of cardiac tissue (the destruction of abnormal tissue). Ablation therapy is used to break the loops of abnormal electrical activity in the heart that often cause an irregular heart beat.

The problem is that the leads needed to carry these various signals through the catheter act like a radio antenna, absorbing energy from the electro-magnetic pulses emitted by the MRI scanner. This disturbs correct operation of the catheter and of the scanner. RF pulses from the scanner can even cause potentially dangerous local heating in the catheter, especially at its tip.

The scientists at Philips Research have overcome these electrical problems in a number of ways. To stop the catheter leads absorbing energy from the MRI scanner’s RF field, they have electrically divided the catheter leads up into a number of shorter sections. Each of these sections still acts like a radio antenna, but the shorter length of each antenna means that they are vastly de-tuned from the MRI scanner’s radio frequency. As a result, much less energy is absorbed and local heating is reduced to insignificant levels. To maintain signal transmission between the sections, each section is coupled to the next by an ultra-miniature transformer.

To prevent heating in the leads that transmit ECG-signals from inside the heart, Philips researchers have developed special leads equipped with a highly resistive nano-coating. This avoids the pick-up of dangerous RF energy, while still allowing the ECG signal to be transmitted to the externally connected electro-cardiograph with diagnostic quality.

In addition to catheters, Philips Research has helped in the development of MRI-compatible guide wires. Guide wires are indispensable tools for most catheterizations, allowing surgical devices such as balloon catheters and stents to be accurately positioned and manipulated. Normal guide wires are metallic and would therefore be prone to dangerous tip heating if used in MRI-guided procedures. To overcome this problem, Philips Research developed a fiber-reinforced composite guide wire.

“Development of these new MRI-compatible catheter and guide wire technologies is another example of how taking a total system approach can enable new interventional techniques,” says Dr. Falko Busse, Director of Philips Research Hamburg. “It depends on having a fundamental grasp of the physics involved and the way it causes the scanner and catheter to interact with one another.”