MR juggles obstacles, advantages in liver RFA guidance

MR offers decided advantages as a tool for image guidance in radiofrequency ablation of liver tumors, but it also presents significant problems that must be overcome before its use becomes widespread, according to a pair of presentations March 5 at the European Congress of Radiology.

MR is the only imaging modality that can estimate heat or energy loads during ablation procedures. This is a more accurate indication of tissue ablation area than the perfusion strategies in current use, presenters said. Some tumors are visible only with MR.

Most RFA imaging guidance is currently provided by ultrasound and CT. Ultrasound can provide real-time guidance but is less precise than CT, said Chrit W.T. Moonen of the University Victor Segalen in Bordeaux, France. Conversely, CT is more precise but fails to provide real-time guidance.

Turning to MR offers more precise targeting and monitoring of the ablation process but introduces other problems: RF probes and generators introduce artifacts, and turning on the generator during a procedure can actually eliminate the image altogether, said Dr. Christoph D. Becker of the University Hospital in Geneva.

Different strategies have been tried to overcome these obstacles. One involves alternating between RFA and image acquisition, but it is difficult to monitor temperature-related signal changes, Becker said. Another approach is to image during RFA with both internal and external frequency filtering of the generator output. This still leaves some artifacts, however.

Becker mentioned published data (Eur Radiol 2004;14(1):31-37) on six patients, most of whom had relatively small tumors where the images were obtained on a 0.23T open scanner. Acquiring temperature readings, however, requires a high-field (1.5T) scanner.

Even when temperature readings become available, other issues remain. Switching between MR and other modalities can be difficult in a busy setting, tying up two imaging teams, Becker said. Artifacts from the probes and from patient motion remain a problem. Patient motion artifacts can be minimized by using anesthesia but may be more significant under conscious sedation.

Becker suggested several improvements that would increase the value of MR for guiding RF liver tumor ablation:

• faster navigation methods for targeting tumors

• better interventional capabilities in high-field MR scanners, including patient access and sequences

• better MR compatibility of RF generators and probes

• better reduction of motion artifacts to avoid misregistration while monitoring the procedure, especially for those who want to use conscious sedation in lieu of anesthesia