Extremity Scanners and ‘Moving’ MRI

MRI scanners rely on superconducting magnets to produce images of the body. However, cooling these magnets to the necessary temperatures involves immersing them in liquid helium baths, a solution both expensive (because, well, it’s helium) and somewhat ungainly.

Now, the Center’s Jerry Ackerman and colleagues are fine-tuning an approach that enables portable MRI by sidestepping this need. Instead of cooling the magnet by submerging it in liquid helium, the new technology, initially developed by the company Superconducting Systems, works by replacing the liquid helium bath with a cryogenic (ultralow temperature) refrigerator.

Conventional scanners already use a similar technology, called cryocooling, but only to meet a relatively small part of the scanners’ cooling needs. “The advance with the Superconducting Systems technology,” Ackerman says, “was coming up with a practical way to provide all of the cooling with a cryocooler.” So rather than relying on a rare and therefore expensive element—helium—cooling of the magnet is powered almost entirely by readily available and comparatively cheap electricity.

As a demonstration of the clinical potential of the technology, Ackerman and colleagues built an “extremity scanner” designed for imaging of patients’ arms and legs. This compact scanner is roughly four feet wide and maybe two feet deep and includes a central bore for the actual imaging and two “dummy” bores added for ergonomic reasons—so the patient can comfortably put both legs in bores rather than inserting only one leg, leaving the other to sit awkwardly outside the scanner.

The extremity scanner is by no means the only potential application of the technology, though. With its compactness, its relatively low purchase cost, its small footprint, and the relatively low expense of cooling the magnet, the Superconducting Systems technology recommends itself for a host of different uses.

Ackerman continues: “The technology doesn’t necessarily allow you to do things you’ve never done before. What it does do is enable you to perform the same tasks you could in a conventional scanner but less expensively and more efficiently. By extension, you may be able to provide imaging services, including high-quality head scans, in places where this previously hasn’t been possible—for example, in resource-limited areas of the developing world.

“Even in places where MRI is already available, the technology provides a less expensive alternative. And because most of your body remains outside the scanner, it’s less intimidating for people with claustrophobia or others who simply don’t like going into a scanner.”

Taking a Stroll With MRI

Not having a large bath of helium suggests another application for the portable MRI technology. In an “interesting, somewhat crazy concept,” Ackerman says, he and colleagues are now pursuing development of a moving MRI for applications requiring scanning while the subject is in motion.

He points to vestibular physiology as the primary area of interest in pursuing the technology. Here, scientists study inner ear organs that enable people to sense rotational changes in the head—acting almost like the linear accelerometers behind airbags in cars or rocket guidance systems. Currently, they will put a subject on a moving platform and use infrared sensors, EEG and self-report questionnaires to try to ascertain what is happening in the brain during the movement.

Ackerman understood that functional MRI could provide much of the information the scientists were seeking, if only they could overcome one major obstacle.

With stationary MRI scanners, any sigificant movements, such as those a subject might make while undergoing physiological testing of the vestibular system, will result in severe motion artifacts in the image—that is, distortions similar to blurring in photography when the subject moves while the camera shutter is open.

To eliminate these effects, Ackerman proposed a new type of moving MRI using the technology behind the extremity scanner to move the magnet along with the head. The liquid helium-free magnet makes this possible. He and colleagues are now in the beginning stages of developing the technology and hope to be able to scale it to humans within a few years.

The proposed system suggests a host of exciting applications. “Once you can use MRI to study a subject moving around, you can imagine learning how the brain moves with respect to the skull,” Ackerman says. “My hope is that this will point the way to mapping the mechanical properties of brain tissue in living organisms. This could help with a range of applications, including developing models of traumatic brain injury.”

Above: an illustration of the proposed “moving MRI” system. In the photo at the top of the story: The prototype extremity scanner operates at 1.5 Tesla. The next-generation extremity scanner will run at several fields between 0.5 and 1 Tesla.