Pneumatic robot arranges limbs for MRI 'sweet spot'
By Tamsin Osborne Video: By getting limbs arranged correctly the air powered robot can dramatically improve MRI scans (Footage courtesy Zion Tsz Ho Tse, Imperial College London, UK) A pneumatic robot that positions patients’ limbs inside an MRI scanner allows physicians to exploit a bizarre phenomenon where hard-to-see tendons jump into sharp focus when held at the right angle. That “magic angle” effect happens when a tendon is at 55° to a scanner’s powerful magnetic field and can help with diagnosing tendon injuries. A scanner’s field lines up hydrogen atom nuclei, mostly in water molecules, inside the body. Nuclei are pinged with radio waves to momentarily shove them out of the field’s grip, and they emit their own radio waves as they settle back into place. Those signals reveal their position and are translated into a 3D image. Different tissues can be identified because the density of hydrogen nuclei varies from one to the other. However, dense ordered tissues produce very weak signals, and tendons in particular are hard to see, says Zion Tsz Ho Tse of Imperial College London. This is because the hydrogen nuclei within the tissue prefer to interact with one another rather than the scanner’s magnet. But at the magic angle of 55° to the magnet’s field, interactions between nuclei inside a tendon are cancelled out. They are affected only by the scanner’s magnetic field and produce clear signals. But positioning a person’s limb correctly inside the close confines of an MRI scanner is difficult. The new robot speeds up the process. Building it was a challenge. MRI magnets are strong enough to pull metal furniture across a room, so the robot had to be made of another material. Instead an air supply from outside the machine drives plastic gears inside the robot. “You end up with a motor that is inherently safe and extremely powerful,” says team member Michael Lampérth. The robot supports a person’s limb on a platform that can move in any direction to put a tendon at the magic angle. The robot judges its own position, and even uses image processing software to check whether it has reached the right angle. In initial trials on four healthy volunteers, the researchers found that the device could give signal intensity a 21-fold boost. The device can position the Achilles’ tendon correctly in just a few minutes, compared with up to two hours by hand. Constantinos Mavroidis, director of the Biomedical Mechatronics Laboratory at Northeastern University in Boston, Massachusetts, is impressed by the device. “This is a very efficient solution to a very difficult problem,” he says. “I believe that this device has great clinical potential.” Small versions of the robot can position fingers to better reveal their tendons. In future, devices could apply the magic angle to other tissues, such as nerves. Journal reference: IEEE/ASME Transactions on Mechatronics (DOI: 10.1109/TMECH.2008.924109) More on these topics: