Interventional MR imaging represents worthwhile investment

If radiologists could design the perfect modality for guiding interventional procedures, the resulting technology would undoubtedly produce high-quality images without exposing patients to any ionizing radiation. So given the widespread availability of MRI, why are so many interventions still performed in the angiography suite?

The absence of ionizing radiation is one of the main arguments in favor of interventional MRI. However, even fervent supporters of this technology appreciate the many practical difficulties involved in providing real-time MR image guidance. Despite this, the potential benefits suggest that today's interventional toy could still become the routine tool of tomorrow.

MRI was used to guide placement of active catheter in aorta of pig (left). Embolization was subsequently performed in upper right renal parenchyma. (Provided by M. Bock)

Speakers at Sunday's special focus session explained exactly why interventional MRI is well worth the effort.

The term "interventional MRI" covers a broad spectrum of procedures, ranging from straightforward biopsies to more complex cardiovascular repairs. To reflect this, the session covered a wide variety of clinical applications, with an emphasis on the future.

Following the increased availability of MR mammography, some imaging clinics now use MRI to guide the biopsy of suspicious lesions in the breast. Providing MR-based breast imaging or even screening without this interventional capacity simply does not make sense, said Prof. Dr. Arno Bücker, who chaired the session.

"If you perform MRI and you see a lesion on the images that was not visible on x-ray mammography, you need a way to localize and biopsy that lesion under MRI guidance," said Bücker, a professor of radiology at Saarland University Hospital in Homburg, Germany.

Interest in MR-guided endovascular interventions is growing, too. MRI has been used to guide puncture in the bile duct and renal system during percutaneous cholangiography with drainage and percutaneous nephrostomy. The modality is then used to control localization of the catheter and guidewire.

MRI can visualize both the vessel wall and surrounding tissue when imaging vessels with atherosclerotic plaque. Animal experiments have shown almost all cardiovascular interventions to be feasible, including the catheterization and stenting of coronary arteries, according to Prof. Dr. Gabrielle Krombach, professor of radiology at Aachen University Hospital in Germany.

MR-guided intra-arterial injection (Gd-DTPA) through catheter positioned in celiac trunk with subsequent follow-up. In contrast to conventional fluoroscopy, full 3D data sets were acquired. (Provided by M. Bock)

Emerging ablative techniques that use focused ultrasound may benefit from MR guidance as well. In addition to guiding the therapeutic probe, MRI can also visualize the effects of tissue heating in real-time, improving safety and efficacy. No other imaging modality offers this dual benefit.

In spite of these innovations, many routine image-guided interventions are currently performed under x-ray fluoroscopy, especially vascular procedures. One reason is the lack of MR-compatible instruments. Making instruments that can be used safely inside an operational MR system means more than just selecting nonmagnetic materials, said Michael Bock, Ph.D., head of the interventional MRI research group at the German cancer research center (DKFZ) in Heidelberg.

"For example, if you think about a guidewire made from Nitinol, that will be nonmagnetic, but it is also electrically conducting. As soon as you introduce it into the patient, it will act as an antenna," he said. "When you perform an imaging sequence, the wire will absorb radiofrequency power and transfer this heat to the patient."

The development of truly MR-compatible instruments is a priority, and this message should be taken to manufacturers, Bock said. First and foremost, the wider availability of interventional MRI would have real benefits to young children, especially those undergoing repeated interventions or procedures lasting 30 minutes or more. Procedures performed under MR guidance should be safer for interventional radiologists too.

MRI offers the option of 3D visualization, unlike fluoroscopy, which is limited to 2D views. Functional information on blood flow or oxygenation, for example, can be acquired, too.

Switching to MR guidance would avoid the risk of radiation skin burns to patients during transjugular intrahepatic portosystemic shunt (TIPS) placement. This liver interventional procedure involves a high cumulative x-ray dose. The ability of MRI to visualize soft tissue could also be helpful during embolization procedures in the liver.

"It is technically more demanding to do these kinds of things under MRI guidance, but once you have the infrastructure set up, you realise that it is much easier to locate the target volume, put the catheter in, and perform the procedure. You have the 3D control, you see the soft tissues, you see the blood vessels that lead to the lesion," Bock said.

MR-guided cardiovascular interventions are by no means easy, either. Realistically, these techniques will have a future only if applications can be found that are impossible using any other modality, Krombach said. The introduction of molecular imaging into the interventional arena may lead to the development of such "killer applications." For example, MRI could be used during angioplasty to check that antirestenosis drugs were actually reaching the vessel wall. Conventional angiography is unable to show this kind of detail.

"Another point is that most interventional MRI procedures are done with a 1.5T system. If we could do them at 3T, we would have a better signal-to-noise ratio. This may be promising for interventions combined with molecular imaging," she said.

The special focus session contained a considerable amount of information that is not available in textbooks or published articles, Bücker said.

"This is something that is brand new, it is a hot topic, and it is going to be the future," he said.