Memoirs of a Postgrad has got a great write-up of a new low-power MRI machine, the technology that does most of the structural and functional brain scans. Even the smaller MRI machines need huge electromagnets, but this new technology uses magnets thirty thousand times weaker to image the brain.
In a standard MRI machine, a strong magnetic field is used to align the proton in each of the hydrogen atoms before using an RF pulse to knock them out of alignment. As they snap back into alignment with the magnetic field, they emit a signal which can be detected and used to create a 3D image. In the new version, the very small magnetic field isn’t enough to align the protons, so a short duration (1 second) magnetic pulse of slightly higher magnitude (30 millitesla).
The resulting signals are very small, so an array of highly sensitive magnetometers are used (so-called superconducting quantum interference devices, or SQUIDS). A hugely important additional advantage of using these SQUIDS is that they are also used in the MEG (magnetoencephalography) imaging technique. This potential for MRI and MEG using the same machine raises the intriguing possibility of producing simultaneous structural images (using the MRI) and brain activation maps (using the MEG).
Unfortunately, the use of SQUIDs dashes any hopes of making the machines much smaller.
The SQUID sensors need to be extremely cold (working at approximately -170 degrees C) and so are usually bathed in liquid nitrogen, meaning a huge insulated tank sits atop the scan area.
IEEE Spectrum magazine has an article with some images from the new type of scanner, which look pretty fuzzy at the moment, but apparently can better distinguish tumours in the brain and will undoubtedly become clearer as new software is developed.