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PIONEERING LOW-COST MEDICAL SCANNER DEVELOPED IN THE UK

July 10, 2000

A new type of medical imaging camera which is much less expensive than its conventional counterpart has been developed by researchers funded by the Engineering and Physical Sciences Research Council. The imaging method - positron emission tomography - is becoming increasingly important in the diagnosis of many diseases, especially cancer.Clinical trials of the new detector are scheduled to start shortly and a consortium of companies and cancer charities has been established to investigate the new system's commercial potential.The new camera has been developed by Professor Robert Ott, of the Institute of Cancer Research in collaboration with the Rutherford Appleton Laboratory.In PET imaging a radioactive chemical - termed a radioisotope - is attached to a molecule to form a 'radiopharmaceutical' which targets a specific organ, such as the heart or brain, or a particular biochemical activity in the body, such as glucose metabolism. When the isotope undergoes radioactive decay it releases a particle called a positron. Positrons cannot easily be detected directly because they are extremely short-lived and usually travel less than a millimetre through tissue. However, once emitted the positron very quickly collides with an electron in its surroundings, causing the release of a short burst of energy in the form of two gamma rays. These can be detected, enabling an image of the distribution of the radiopharmaceutical to be produced.One important radiopharmaceutical mimics glucose. As it enters the cell it begins to be metabolised in the normal way. However, its chemistry is subtly different from glucose and as a result it becomes jammed in the metabolic pathway, causing it to accumulate. This accumulation can be detected by a PET scanner. It has been discovered that cancer cells are extremely glucose greedy - they metabolise glucose far more rapidly than healthy cells. In this way PET imaging can be used to detect cancers which might otherwise remain invisible.A big problem is the expense of the scanning equipment, with a machine costing up to £1.7 million. This is largely due to the complexity of the detection system.The gamma rays emerging from the body strike a detector, consisting of a special type of material called a scintillator. When the gamma ray strikes the scintillator a flash of light is emitted which is amplified by a device called a photomultiplier, turning the light signal into an electronic signal. Even to image a relatively small area of the body many detectors are required. "You might need many thousands of these," says Professor Ott. "They are expensive, and the bigger the camera the more elements you require and the more expensive it becomes."Professor Ott has successfully adapted technology used in particle physics laboratories to develop an equally effective but less expensive system for detecting gamma rays.The new system dispenses with the need for a large number of photomultipliers. It is based on a chamber containing a flat grid of criss-crossed wires, travelling up-and-down and from side-to-side, spaced 2 mm apart. Over the wires is a transparent pane of scintillator made from many small tiles. The chamber is filled with a special vapour which reacts to light.During the scanning process, the gamma rays emerging from the patient's body strike the scintillator. This creates a burst of ultraviolet light at that particular point, which in turn stimulates the photosensitive gas to release an electron. This event is detected by the crossed wires at the given position.The research team, based at the Royal Marsden Hospital in Surrey, has built two of the novel detectors, each nearly three times the size of existing conventional cameras, and at a much lower cost.- ENDS -

Engineering and Physical Sciences Research Council (EPSRC)