Known to quantum physicists as a 'Squid', the device lies at the heart of a magnetometer, an instrument that builds up an image of the brain at work. Squid magnetometry is likely to prove a more powerful diagnostic tool than conventional imaging, which provides only a static picture of brain tissue.
'We are aiming to build up an image of where the current is flowing,' says Dr Steven Swithenby, director of the biomagnetism group at the Open University, where his multidisciplinary team of scientists is developing the instruments and software for equipment that he believes will have a major impact on clinical research. 'Doctors and medical scientists have never before had a tool that looks at activity and not just structure,' he says.
Squid is the acronym for superconducting quantum interference device, and it measures magnetic flux or field extremely accurately at ultra-low levels, such as the level reached when a group of neurons in the brain is triggered.
The device is made of a ring of superconducting material, usually niobium metal, a few millimetres wide, with a slice of insulator, a few atoms thick, sandwiched into the loop. When an electric current is applied to this superconductor, the flowing current generates a magnetic field around the wire loop.
Inside the superconducting loop this magnetic field is extremely sensitive to any changes in magnetism. If a change in magnetic field is detected, the current flow in the Squid changes to re-adjust the field strength to counter the external force.
The effect of the superconductivity and the restriction of the insulator is that the flowing current and the magnetic field are quantised: the changes in current and magnetism can occur only in discrete steps or quanta. These spurts are detected by the magnetometer as pulses of current which are converted to a voltage, allowing the number of quantum jumps to be counted.
The instrumentation that Dr Swithenby's team is developing has 37 Squids attached at points around the head or other parts of the body. From the 37 sets of readings, a three-dimensional 'magnetic map' of brain or neural activity can be built up.
But there is still the problem of interpreting the information. 'We can look at what is going on in the head, but it takes a lot of mathematics to unscramble the whole mess so that we can make a sensible image,' Dr Swithenby says. Much of the Open University group's effort has gone into developing the methods of analysis.
Squids pose other practical difficulties. Since superconductivity requires temperatures close to absolute zero - 273 degrees below the freezing point of water - the Squid and its pick-up coils have to be immersed in liquid helium at 4 degrees above absolute zero. Non-metallic Dewar flasks keep the system 'magnetic free'.
The magnetic field generated by the brain in response to an external stimulus, and measured by the Squid, is about 100 million times weaker than the Earth's magnetic field, and a million times weaker than the magnetic fields around overhead power cables. Extreme precautions have to be taken to prevent interference from magnetic fields generated by electric motors, lifts and road traffic, as well as power lines.
One approach is to create a 'magnetic-free' environment around the Squid, its pick-up coil and the patient, by isolating them in a room lined with steel or aluminium.
A less expensive and more practical approach, used at the Open University, is to couple the Squid to another device known as a gradiometer. In effect the gradiometer is a matched pair of (non-superconducting) magnetometers placed between the Squid and patient's head. One of the pair measures the external magnetic field outside the brain, the other measures the total field, including the contribution from the brain, and the difference between the two is measured by the Squid.
One of the most exciting potential applications for Squid magnetometers is monitoring the effectiveness of drugs for treating people suffering from strokes and Alzheimer's disease. Other potential applications are identifying the point of genesis of epilepsy and seeing if there is any loss of activity in the brain associated with clinical depression.
The so-called high-temperature superconductors - metal oxides that can work at temperatures up to 100 degrees above absolute zero - are the next stage in the development of workable machines. Until recently there appeared to be little hope of carrying out this application beyond a few degrees close to absolute zero, because the intrinsic motion of the atoms in the superconductor causes 'noise' which swamps out the Squid detector.
However, Dr Swithenby says these practical problems have now been overcome. 'In three or four years' time, who knows what Squids will be made of?'
Squids are not new: they were first postulated by the theoretical physicist Anthony Leggett at the University of Illinois in the early Eighties. They have also been harnessed for other uses. The National Physical Laboratory at Teddington, west London, uses them for setting standards for laser radiation, voltage and electrical resistance. They can also be used for accurate measurement of very low temperatures.
But it is only now that the technology has reached a stage where this quirk of quantum theory can be passed from the physicist to the medical scientist.Reuse content