A prostate MR image
obtained using
Siemens Healthineers’ Body
30/60 surface coil

MR coils: the pursuit of a better signal-to-noise ratio

October 10, 2016
by Lauren Dubinsky, Senior Reporter
The signal-to-noise ratio (SNR) of MR imaging is intrinsically weak and limits the speed of the exam and the resolution of the images. Researchers at the University of Houston (UH) have developed a high-temperature superconducting coil called the cryo-coil that boosts the SNR by a factor of two to three compared to conventional coils.

“For small coil sizes at high-field MRI, when the coil noise is the governing source of noise, it has long been recognized that cooling the receiving coils or using high critical temperature superconductors (HTS) materials reduce this noise contribution and therefore can significantly increase the SNR,” says Jarek Wosik, a principal investigator at the Texas Center for Superconductivity at UH.

The improved SNR from the cryo-coil allows physicians to see details that cannot be seen when conventional coils are used. Those details don’t show up even when multiple images are acquired using conventional coils. For initial studies, the cryo-coil has been optimized for rat brain imaging. At the International Symposium of Magnetic Resonance in Medicine’s annual meeting in May, the researchers reported that they demonstrated an isotropic resolution of 34 microns in rat brain imaging.

The cryo-coil has been developed for preclinical MR imaging, but it’s also intended for clinical imaging. Since clinical applications involve imaging a larger area, an array of small coils needs to be used. The research in animal models produces useful information for improving diagnosis and treatment of human diseases and disorders. This work has the potential to benefit clinical MR through highquality imaging and by shortening the time the patients are in the scanner, according to Wosik.

The researchers have already built two prototypes, which have been installed at the University of Texas Medical School and are collecting data. UH’s spinoff company, Cryo-sensors, is in the process of writing Small Business Innovation Research proposals for the cryocoil, and Wosik expects a commercial coil to be available within one to two years. A few companies have introduced cryogen-free magnets to the market, which could provide a much easier way to integrate cryocoils into MR. MR Solutions offers a 7T preclinical, cryogen-free MR system, and Time Medical Systems is offering a similar scanner.

Another way to improve SNR
Since poor coil placement results in a large loss in SNR, a coil that fits perfectly on top of the patient’s anatomy is ideal. Researchers from the University of California, Berkeley, have found a way to create light, flexible MR coils with screen printing technology. The coils can be individually designed with the screen printer to fit patients of different sizes, including infants and toddlers. They can also be custom made for individual patients.

The coils can be wrapped tightly around the patient’s body, which both increases the sensitivity of the exam and provides clearer images. SNR can be improved with the use of contrast agents and higher-field scanners, but better coils often result in more significant gains. The technology used to develop the coils is the same that’s used for printing designs on T-shirts. Since this technology is becoming more widely used, this new way of designing coils could be made practical for mass manufacturing.

One of the challenges with MR is that the exams can sometimes span more than an hour and patients must remain still the entire time. That’s an especially difficult task for pediatric patients, who are often anesthetized.

MR scans take a long time because of the technology’s low sensitivity, but one way to increase the sensitivity is to use high-density MR coil arrays. They can both increase the strength of the received MR signal and accelerate the acquisition of the signal to achieve clearer images. The receiver coils on the market are not matched to each patient’s body and are sometimes heavy, inflexible and uncomfortable. The researchers at UC Berkeley have developed a prototype of a blanket with the coils inside that can be wrapped around an infant.

They have also partnered with a pediatric radiologist at Lucile Packard Children’s Hospital, Dr. Shreyas Vasanawala, and GE Healthcare, to bring the new coil to clinical practice. The research was funded by the National Institute of Biomedical Imaging and Bioengineering, which is part of the National Institutes of Health.

Non-invasive prostate coils
The endorectal coils used for prostate MR exams are uncomfortable and can also make the procedure longer and more costly. Noninvasive, flexible coils may be a better option. In January, the FDA cleared Siemens Healthineers’ noninvasive SEEit prostate MR imaging solution, which include the Body 30 and Body 60 coils and new software. It can perform a routine prostate exam in 10 minutes without the use of an endorectal coil.

It’s designed to be used on the MAGNETOM Aera 1.5T and MAGNETOM Skyra 3T MR systems. The Direct RF and high-density coil technology, Tim 4G, combined with its RESOLVE diffusion technology, provide enough SNR and resolution to perform the exams with only the surface coil. “Because we are using a higher-density coil, we’re able to pick up more signal,” says John Metellus, product manager at Siemens. “We get a 20 percent gain in signal in the prostate area by using these higher capacity coils so you don’t have to use an endorectal coil.”

The endorectal coils have less density – one or two channels – so they have to be invasive. But the Body 30 is a 30-element body coil with 30 amplifiers built into it and the Body 60 is two Body 30s. Siemens is not the only company that offers a noninvasive coil for prostate exams. In June 2015, ScanMed introduced its noninvasive prostate/pelvic MR coil, ProCure.

It’s a wearable MR coil that places multiple antennae elements as close as possible to the prostate and pelvis. It can also assist a biopsy if the radiologist detects a suspicious lesion. The latest version of the American College of Radiology’s Prostate Imaging Reporting and Data System (PI-RADS) supports the use of higher-density coils for noninvasive prostate MR exams. “They feel that the image quality is credible enough to that of the invasive solution using the endorectal coil,” says Metellus. “It’s up to the physician whether they want to use the endorectal coil or not. More people are starting to use it noninvasively with the higher-density body coils.”