MRI provides unparalleled soft-tissue contrast, but its drawback is its temporal resolution. MR imaging can take up to 20 minutes or longer. Several developments could accelerate and simplify cardiac MR to make it more competitive with cardiac CT.
Speed vs. resolution
“We want to try to accelerate the scan to capture an even motion of the heart within a shorter period of time,” says Hiroyuki Fujita, founder and CEO of the Cleveland-based Quality Electrodynamics, which develops RF coil technology.
A high-channel count RF coil is needed to support parallel imaging in cardiac applications. Researchers have experimented with 128-channel RF coils, which potentially allow more information to be acquired simultaneously, thereby shortening the scan time.
GE Global Research experimented with a 128-channel body MRI with a flexible high-density receiver-coil array comprised of two 64-coil “clamshells.” The configuration enabled significantly higher acceleration factors for parallel imaging and improved SNR, which theoretically would lead to improved sensitivity.
Researchers from Massachusetts General Hospital in Boston tested a 128-channel receive-only cardiac coil for highly accelerated cardiac MRI at 3T. The in vivo measurements with the 128-channel coil resulted in SNR gains compared to a 24-channel coil. The 128- and 32-channel coils showed similar SNR in the heart. The ability of the 128-channel coil to accelerate cardiac imaging was demonstrated in four volunteers using acceleration factors up to seven-fold.
While the increase in speed may be minimal between 32- and 128-channel coils, the 128-channel model allows acceleration over larger volumes and multiple different body areas, according to Daniel K. Sodickson, MD, PhD, vice chair for research in the department of radiology at New York University Langone Medical Center in New York City. “Without changing out of the coil, you can get ultrafast acquisition and comprehensive imaging of the heart and then ultrafast comprehensive imaging of the peripheral arterial system,” he says.
The next step is to maximize coil fittings to the anatomy. “Well-designed coil arrays are critical for parallel acquisition,” says Stephen Riederer, PhD, director of the MR lab at Mayo Clinic in Rochester, Minn. “They have the potential to allow better superior and inferior coverage.” Riederer and colleagues designed a coil array with longer, narrower elements in the anterior-posterior positions, which “increased spatial resolution dramatically.”
The simplicity factor
Simplicity is another cardiac MR challenge. Cardiac MR requires planning each scan plane separately, which can take up to an hour for a full study. Sodickson and colleagues are developing coils and image reconstruction techniques that can accelerate imaging by at least a factor of eight.
“Rather than image each coronary artery one after the other, with enough speed we can essentially press a ‘go’ button and scan from the top of heart to the bottom of heart in one breath-hold. In the next breath-hold, wall motion would be imaged for functional data, then perfusion in a breath-hold, then delayed enhancement in a breath-hold.”
1.5T vs. 3T and beyond
When parallel imaging is used with a 1.5T scanner, scan time shortens but images are often poor for cardiovascular applications. At 3T, the scanner has more SNR, so the SNR sacrificed to speed doesn’t influence image quality as much as on a 1.5T system. At 3T, scan time is faster and image quality is better, says Fujita.
The next step is to combine high-Tesla scanners with multi-channel coils and optimized protocols into one package with workflow optimization, says Fujita, adding that this reality is a few years away. At some point, however, adding more coils will not increase speed. To that end, researchers are experimenting with acquiring pre-compressed images. Called “compressed sensing,” this technique could increase speed without adding coils.
“I’m not a believer in the either/or philosophy of MR or CT,” says Sodickson. “MR is not necessarily best positioned to replace CT entirely, but it is getting faster and simpler and has rich information content.”