Two Acceleration Methods Allow for Much Faster MRI Brain Scans

In an article published December 20, 2011, in the journal PLoS ONE, a University of California (UC) Berkeley (USA), physicist and colleagues from the University of Minnesota (Twin Cities [Minneapolis-St. Paul], USA) and Oxford University (UK) described two developments that allow full three-dimensional (3D) brain scans in less than half a second, instead of the typical two to three seconds.

"When we made the first images, it was unbelievable how fast we were going,” said first author Dr. David Feinberg, a physicist and adjunct professor in UC Berkeley's Helen Wills Neuroscience Institute and president of the company Advanced MRI Technologies (Sebastopol, CA, USA). "It was like stepping out of a prop plane into a jet plane. It was that magnitude of difference.”

For neuroscience, in particular, fast scans are vital for capturing the dynamic activity in the brain. "When a functional MRI [fMRI] study of the brain is performed, about 30 to 60 images covering the entire 3D brain are repeated hundreds of times like the frames of a movie but, with fMRI, a 3D movie,” Dr. Feinberg said. "By multiplexing the image acquisition for higher speed, a higher frame rate is achieved for more information in a shorter period of time.”

"The brain is a moving target, so the more refined you can sample this activity, the better understanding we will have of the real dynamics of what's going on here,” added Dr. Marc Raichle, a professor of radiology, neurology, neurobiology, biomedical engineering, and psychology at Washington University in St. Louis (MO, USA), who has followed Dr. Feinberg's work.

Because the technique works on all modern MRI scanners, the impact of the ultrafast imaging technique will be immediate and widespread at research institutions worldwide, according to Dr. Feinberg. In addition to greatly advancing the field of neural imaging, the discovery will have an immediate impact on the Human Connectome Project, funded in 2010 by the US National Institutes of Health (NIH; Bethesda, MD, USA) to map the connections of the human brain through fMRI and structural MRI scans of 1,200 healthy adults.

"At the time we submitted our grant proposal for the Human Connectome Project, we had aspirations of acquiring better quality data from our study participants, so this discovery is a tremendous step in helping us accomplish the goals of the project,” said Dr. David Van Essen, a neurobiologist at Washington University and coleader of the project. "It's vital that we get the highest quality imaging data possible, so we can infer accurately the brain's circuitry--how connections are established, and how they perform.”

The faster scans are made possible by combining two technical improvements devised in the past decade that separately increased scanning speeds two to four times over what was already the fastest MRI technique, echo planar imaging (EPI). Physical limitations of each method prevented further speed improvements, "but together their image accelerations are multiplied,” Dr. Feinberg said. The researchers can now obtain brain scans considerably faster than the time reductions reported in their study and many times faster than the capabilities of current machines.

Nearly 20 years ago, however, a new type of MRI called functional MRI (fMRI) was developed to highlight areas of the brain using oxygen, and thus seemingly engaged in neuronal activity, such as thinking. Utilizing EPI, fMRI vividly differentiates oxygenated blood funneling into working areas of the brain from deoxygenated blood in less active areas.

With EPI, a single pulse of radio waves is used to excite the hydrogen atoms, but the magnetic fields are rapidly reversed several times to elicit about 50 to 100 echoes before the atoms settle down. The multiple echoes provide a high-resolution image of the brain. In 2002, Dr. Feinberg proposed using a sequence of two radio pulses to obtain twice the number of images in the same amount of time. Called simultaneous image refocusing (SIR) EPI, it has proved useful in fMRI and for 3D imaging of neuronal axonal fiber tracks, though the improvement in scanning speed is limited because with a train of more than four times as many echoes, the signal decays and the image resolution decreases.

Another acceleration advance, multiband excitation of several slices using multiple coil detection, was proposed in the United Kingdom at about the same time by Dr. David Larkmann for spinal imaging. The technique was used for fMRI by Dr. Steen Moeller and colleagues at the University of Minnesota .This technique, too, had limitations, mainly because the multiple coils are relatively widely spaced and cannot differentiate very closely spaced images.

The ability to scan the brain in under 400 ms moves fMRI closer to electroencephalography (EEG) for capturing very rapid sequences of events in the brain. The development will affect general fMRI as well as diffusion imaging of axonal fibers in the brain, both of which are needed to achieve the main goal of the Human Connectome Project. Diffusion imaging reveals the axonal fiber networks that are the main nerve connections between areas of the brain, while fMRI shows which areas of the brain are functionally connected, meaning, which areas are active together or sequentially during various activities.

Dr. Raichle, a collaborator in the NIH Human Connectome project, is one of the pioneers of "resting state” MRI, in which brain scans are captured of patients not involved in any specific task. He believes that the ongoing spontaneous activity discovered during such scans will tell the investigators about how the brain remains flexible and maintains a level of homeostatis so that "you know who you are. Being able to sample this ongoing activity at increasing temporal fidelity and precision becomes really important for understanding how the brain is doing this,” Dr. Raichle said.

Related Links:
University of California, Berkeley
University of Minnesota
Oxford University
Human Connectome Project
Washington University in St. Louis