3T breast MR heightens speed and spatial detail

April 15, 2005

The higher signal of 3T MR imaging generally gives radiologists a choice: Ramp up the resolution or reduce scanning times. But what happens when both are needed? Results of breast MR performed at 3T suggest that a workable balance is possible.

The higher signal of 3T MR imaging generally gives radiologists a choice: Ramp up the resolution or reduce scanning times. But what happens when both are needed? Results of breast MR performed at 3T suggest that a workable balance is possible.

Breast imaging can put great demands on MR. On one hand, radiologists need to obtain very high in-plane and through-plane spatial resolution. But they must also work quickly to avoid masking the signal from enhancing tumors, said Prof. Christiane Kuhl, a professor of radiology at the University of Bonn, Germany.

"These diverging demands are difficult to compromise with any given technology, including 3T MRI," she said.

Signal gain from 3T breast MR will presumably translate into improved specificity, Kuhl said. Images acquired with much higher spatial resolution should show the details of subtle lesions such as very small spicular cancers. Characterizing lesions less than 5 mm remains difficult even if the voxel size is only a few millimeters.

Scanning at 3T should also make fat suppression easier because the higher signal leads to broader separation of fat and water resonance frequencies. In practice, however, this can be difficult to achieve. Fat suppression causes fewer difficulties when used for unilateral sagittal scanning, as it is easier to attain a homogeneous magnetic field across the smaller field-of-view, Kuhl said. She prefers to use a bilateral sagittal approach, scanning each breast separately with a small FOV, but has yet to decide on an optimum protocol.

"There are so many possibilities that I think it will be some time before I have worked out the best use of this extra signal," Kohl said.

Dr. Mitch Schnall, chief of MRI at the University of Pennsylvania Medical Center, uses the signal boost from 3T to maximize contrast-to-noise. Differences between enhancing and nonenhancing tissue become clearer at higher field strengths because the tissue T1 times become longer, while the relaxivity of gadolinium remains fairly constant, he said.

Schnall is also trying to gain the maximum resolution from his 3T scanner within diagnostically applicable temporal limits. Working with a 1.5-minute time frame, he has attained in-plane resolution of 200 microns (0.2 mm) of both breasts.

"The pixels are only twice as big as those you would get on digital mammography, which is fairly spectacular," he said. "Now the challenge is to see whether that information actually makes a difference to the way we diagnose and care for patients. I know the images are better. What I don't know is whether better images translate into better patient care."

Schnall's research on 3T breast MR includes three populations: diagnosed breast cancer patients, women with abnormal screening results scheduled for biopsy, and high-risk subjects. The use of contrast in the studies rules out a head-to-head comparison of 1.5T and 3T because recruits would have to be scanned on two separate days.

"To get them in once is a challenge; to get them in on two days is almost impossible," Schnall said.

Kuhl agrees that interscanner studies are logistically difficult with breast MRI, but she is managing to accumulate comparison data. Forty women with biopsy-proven lesions have now undergone breast MRI at 1.5T and 3T at Bonn.

The head-to-head data so far indicate that breast MRI at 3T reveals more vessels than at 1.5T, and some lesions are correctly characterized only at 3T. Given that these additional findings were seen in patients already diagnosed with a larger cancer at 1.5T, however, the clinical relevance of high-field scanning remains to be seen, Kuhl said.

"As with MR angiography, we are seeing more details, which can't be bad, but we don't know if the extra information that we find will actually change patient management," she said.