MRI’s Expanding Frontiers: Scanning More, Scanning Faster

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grappa.jpg - GRAPPA-MRI
GRAPPA reconstructions of head images using acceleration factors R=2, R=3, and R=4 and 24 ACS lines. A total of 12 receiver coils were used to acquire these data. Note that as the acceleration factor increases, the noise enhancement increases (compare R=2 and R=3), and residual aliasing artifacts start to appear (R=4).
Source: (J Magn Reson Imaging 2012;36:55-72) / Wiley

Numerous recent advancements in MR technology and technique are offering promising prospects for research and clinical use. The growth of PET/MR, parallel imaging and whole-body MRI means the modality is more versatile and efficient, but what does that mean for patient care?

Reaching the crest of the MRI wave

Hybridity is a beautiful thing, as it creates products that offer the best of both worlds. PET/MR scanners blend two imaging technologies into one, illustrating the many benefits that come from melding medical modalities.

Researchers at Washington University in St. Louis installed one of the first PET/MR scanners in the U.S. at an affiliating hospital in 2012. Unlike some other PET/MR scanners, this one is unique in that it simultaneously performs the two types of scans. This benefits both patients and referring physicians, says Bob McKinstry, MD, PhD, director of the Center for Clinical Imaging Research and Washington University School of Medicine radiology and pediatrics professor.

“PET/MR is a win for the patient and a win for the referring physician,” says McKinstry. Simultaneous PET/MR provides a 40 percent time reduction over separately acquired scans. Referring physicians often have the difficult task of scheduling multiple scans, particularly for pediatric patients. Children generally need sedation, and available anesthesiologists can be hard to find. PET/MR scanners help alleviate this and other pediatric patient issues by reducing radiation exposure, simplifying scheduling, and curtailing serial sedation.

Not only do PET/MR scanners profit patients and physicians, but they improve research methods as well. The hybrid modality allows for more creative and complex clinical research, according to McKinstry.

“This marriage goes beyond convenience and efficiency,” he explains. “MR provides superior soft tissue contrast. Simultaneous acquisition improves anatomical registration, which ultimately provides a better look at the cancer.”

Most experience thus far with PET/MR at the Washington University School of Medicine has been in screening the pelvis for cervical cancer. In the future, McKinstry hopes the combination will look at other malignancies. The duo could sub-characterize parts of tumors, providing a deeper level of characterization in areas like the brain, breast, and pelvis.

PET/MR hybrids also could eventually become the standard imaging technique for head and neck cancers. McKinstry points to brain cancer and cardiac imaging as two key directions in which the technology is headed next.

The number of scanners currently installed in the U.S. is relatively modest, with about 10 to 12 available nationwide, says McKinstry. While they’ve been acquired largely for research in the U.S., their adoption has been much more rapid in Europe where they are used for clinical purposes on a daily basis.

In the U.S., FDA approval for PET/MR scanners in 2011 came quickly after their advent. Traditional research is still needed to convince physicians of its clinical merits compared with other modalities.

“We are just at the start of seeing what PET/MRI scanners can do. The wave is just beginning to crest,” says McKinstry

Picking up the pace

While hybrid imaging looks to improve physiological insights, parallel imaging (PI) aims to make MRI scanners more efficient. PI makes quicker image acquisition possible, offering numerous new MR applications for cardiothoracic, abdominal, cardiovascular, and renal imaging. The speed boost is the result of collecting a reduced amount of data in k-space (the MR data space) and using an array of receiver coils to fill in missing data. This enables shortened breath-hold times for patients, and thus fewer motion-corrupted exams.

“Parallel imaging makes possible a huge jump in clinical practice, where imaging can be extended to body parts that could not be imaged without significant reductions in imaging times,” says Vikas Gulani, MD, PhD, of the Case Western Reserve University in Cleveland.

Gulani, and colleagues published a thorough review of PI in the July 2012 issue of the Journal of Magnetic Resonance Imaging. The amount of data collected in k-space is reduced, the authors explain, and thus the “undersampled” data are gathered more quickly. This undersampling, however, leads to aliasing, or repeated representations of the image.

This is where parallel imaging algorithms come into play. They reconstruct artifact-free images either directly in the image space by employing the sensitivity