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Simulations of Breast Tissue Argue in Favor of 7 T Breast Scans, Researchers Say

No two women have identical breast tissue, so MRIs intended to detect and monitor cancer shouldn’t treat them all the same.

More informative cancer detection is possible with stronger magnetic fields that, unfortunately, increase the risk of tissue heating during a screening. Without a way to prove that a new MRI protocol is safe for all women, clinical MRIs haven’t been able to keep pace with the latest advances in MRI research.

Purdue University researchers have simulated how more than 20 different breast tissue ratios respond to heat given off by MRIs at 7 T, a higher field strength than is presently available in hospitals. The simulations, they say, demonstrate that cutting-edge MRI techniques meet safety limits—as defined by regulatory entities including the FDA—and should herald clinical trials for real-life use.

Furthermore, the knowledge of how much radiofrequency energy each breast tissue ratio can handle could be harnessed into new techniques that target the heat produced from an MRI directly at tumors—providing physicians with another weapon against cancer.

Despite the limitations of clinical field strengths, a yearly MRI screening is still recommended for women with higher than average risk of breast cancer because it’s more sensitive than a standard mammogram.

“We’re starting to develop techniques at high field strengths that could immediately monitor how tumors respond to treatment. So we don’t want tissue heating concerns to stand in the way of improving such a powerful tool,” says Joseph Rispoli, PhD, an assistant professor of biomedical engineering at Purdue.

The density of a woman’s breast determines how much radiofrequency energy from an MRI the breast will absorb in the form of heat, defined as the specific absorption rate (SAR). The more fibroglandular tissue in the breast compared with fat tissue, the higher the breast density and SAR.

But not all women have the same ratio of fibroglandular to fat tissue. This makes it harder for researchers to show that a potentially life-saving MRI technique is safe for every woman, even though the risk of overheated tissue is generally low.

“In hospitals, MRI field strengths are currently up to 3 T,” Rispoli says. “Many techniques would be far superior at 7 T. This would come at a fivefold increase in SAR, but also double the MRI sensitivity.”

The researchers demonstrated through their simulations that a fivefold increase could still stay within FDA limits for most breast tissue ratios, even at 7 T.

To create these simulations, the team faced several public health hurdles, the first being that there are far fewer computer models, or phantoms, of the female body than the male body. Researchers can test their techniques on computational phantoms, typically generated from MRI or CT image sets, before the techniques are clinically approved for use on humans. For example, Japan’s National Institute of Information and Communications Technology developed the “Hanako” model to represent the average Japanese woman, and the Swiss Foundation for Research on Information Technologies in Society developed “Ella” for a typical Caucasian woman.

Other existing phantoms lie behind a paywall; all were developed in a standing or upward position even though a woman would be lying downward for an MRI, and none have been successfully combined with breast phantoms to accurately predict SAR.

Purdue researchers fused 36 breast phantoms at various densities, as classified by the ACR’s BI-RADS atlas, with the full-body Hanako and Ella models. They then simulated the behavior of each fused phantom in response to MRI coils at 7 T.

The simulations will help other researchers tailor their techniques to each woman’s unique breast tissue ratio; for example, power should have some limitations for those with more fibroglandular tissue.

“We want to facilitate the most cutting-edge of breast MRI techniques at any site in the world,” Rispoli says. “Ultimately, a woman will be able to go in, have a fast low-power anatomical MRI scan, and then the computer could quickly simulate on the fly what the SAR would be in that patient.”

Published findings are featured in the journal Magnetic Resonance in Medicine, and software code for the simulations is available to the scientific community via Github.

— Source: Purdue University