Report from SCMR: 1.5T versus 3T coronary artery test produces a draw

February 4, 2005

A head-to-head clinical comparison of 3T and 1.5T MR imaging indicates that higher field strength produces no better than equivalent results for the diagnosis of coronary artery disease.

A head-to-head clinical comparison of 3T and 1.5T MR imaging indicates that higher field strength produces no better than equivalent results for the diagnosis of coronary artery disease.

A study of 18 patients conducted by Dr. Torsten Sommer, cardiovascular imaging unit director at the University of Bonn in Germany, found that the standard Navigator-gated, 3D gradient-echo sequences performed at 3T produced specificity and sensitivity rates of 82% and 89%, respectively, for the detection of coronary stenoses. Sequences optimized for 1.5T imaging generated specificity and sensitivity rates of 82% and 88%.

Rather than producing an expected doubling of the 1.5T scanner's signal-to-noise ratio, the 3T platform actually generated 30% higher SNR, according to Dr. Matthias Hackenbroch, a radiology resident who presented the paper in January at the 2005 Society for Cardiovascular Magnetic Resonance in San Francisco. Doubling magnetic induction raised the contrast-to-noise ratio 22%.

Also speaking at the SCMR meeting, Dr. Matthias Stuber blamed the disappointing results on 3T's problems with specific absorption rate (SAR) and B0 and B1 inhomogeneity problems. Suboptimal SAR and B0 homogeneity compromise the quality of steady-state free precession imaging. Some studies have shown that up to a 100% difference in B1 homogeneity can be observed across the field-of-view, he said. This degrades the performance of pulse sequences that use high flip angles, such as T2 Prep, an important sequence for contrast-enhanced coronary MR angiography.

Help is on the way from strategies also discussed at the SCMR meeting. New 3T shimming algorithms developed by physicist Michael Schär at the University of Zurich in Switzerland compensate for B0 inhomogeneity, resulting in notable image quality improvements, Stuber said.

New adiabatic T2 Prep pulses have been equally adept at substantially reducing artifacts associated with B1 inhomogeneity, he said. Stuber, an associate professor of radiology at Johns Hopkins University, displayed images from his lab that characterized right coronary artery segments as long as 12 cm and arterial branching on the right and left coronary arteries that cannot be observed with convention T2 Prep sequences.

Resolution will rise even further as massively parallel imaging techniques, such as those developed by Dr. Daniel K. Sodickson, director of biomedical imaging research at Beth Israel Deaconess Hospital in Boston, are commercialized. High-resolution coronary artery flow images can now be acquired at 0.7 x 0.7 mm with a temporal resolution as fast as 15 msec. Under experimental conditions, 17 pixels have been acquired across a cross-section of an RCA, a more than fourfold improvement over conventional techniques.

Another two years may be needed to port all of the cardiac sequence from the 1.5T to the 3T platforms, but Dr. Gerald Pohost, chief of cardiovascular medicine at the University of Southern California, predicts that the theoretical advantages of 3T will be realized.

"Three-T is the next frontier," he said.

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