Fast spin-echo imaging at 4.7T becomes feasible

Producing high-resolution images of the brain at magnetic fields higher than 3T has challenged MR researchers for years. The oft-cited hurdle - radio-frequency-related inconsistencies - is no longer a concern, according to British investigators.

University College London researchers assessed 10 healthy subjects using a fast spin-echo (FSE) sequence optimized to work in a 4.7T environment. They generated brain images of acceptable quality in short time and within power deposition safety limits.

Researchers optimized the number of echoes, the repetition time (TR), and the time spent acquiring each echo (TE) to conform with U.K. safety guidelines (4 watts/Kg in the head) for specific absorption rate (SAR). They obtained uniform T2-weighted contrast images, despite significant RF nonuniformity, by using a four-channel "birdcage" RF coil that allowed less sensitivity in the central coil region.

The investigators successfully scanned all 10 subjects using the FSE sequence. They acquired images with submillimeter in-plane resolution, high signal-to-noise ratio, and good structural contrast covering large brain regions in clinically acceptable acquisition times. They published their findings in the September issue of the British Journal of Radiology.

Optimizing MR imaging sequences at high fields is complicated, said coauthor Dr. Roger Ordidge, a professor of medicine and applied physics at UCL. But the results pay off. Mastering FSE and other sequences at fields higher than 3T would enable clinicians to examine the brain in fine detail and thus better monitor disease progression and therapies.

"Past ideas about an upper limit to field strength for human images are no longer relevant," said Dr. A. Gregory Sorensen, director of the Center for Biomarkers in Imaging at Massachusetts General Hospital.

In light of the study, and considering that the FDA has deemed field strengths up to 8T harmless, Sorensen expected additional exploration into very high field MR imaging to soon follow suit in the U.S.

The British study has shortcomings, however. In order to be clinically relevant, neuroimaging needs angiography, T1-weighted, echo-planar, and other imaging sequences that the authors did not fully explore, Sorensen said.

The study has implications for MR-related technical developments as well as for clinical applications, he said. Multichannel coils add a big boost in SNR, sometimes as much as double. A 3T magnet could match a 6T-equivalent SNR with a birdcage coil like the one used in the UCL study.

Premier eight-channel 3T systems used in routine neuroradiology today will not only outperform a 4.7T single channel system in raw SNR, but will have all the advanced applications clinicians need, he said.

"Patients and clinicians can use this paper to recognize that 4.7T and higher fields are safe, meaning that 3T is fully safe and ready to use now. With additional engineering, these higher field systems, including some of the 7T systems that have been shipped, will be ready for further research," Sorensen said.

For more information from the Diagnostic Imaging online archives:

• Use of best imaging options maximizes readability

• Regulators pose potential threat to high-field MR

• FDA loosens reins on investigational MR

• Ultrahigh-field MR seeks new details at 7T and 9.4T