Patent Number: 048521426
Section: summary

BACKGROUND OF THE INVENTION This invention relates to an alloy filter for providing improved contrast resolution in a camera for recording gamma rays, e.g. a planar or tomographic camera. The invention disclosed and claimed herein was first disclosed in Disclosure Document No. 150133 filed at the U.S. Patent and Trade Mark Office on May 9th 1986. Tomography is an increasingly widely used technique for providing images of internal organs. In this technique, a patient is fed a pharmaceutical substance labelled with a radionuclide, which substance collects preferentially in the organ to be examined, for example the liver or the lung. The radionuclide emits gamma rays from which an image is formed by a gamma-ray camera. This usually comprises a flat scintillating crystal, usually of sodium iodide, behind a collimator in the form of a perforated lead plate or the like. The sodium iodide crystal scintillates whenever it receives a gamma ray having a predetermined minimum energy, and the scintillations are detected by a battery of photomultipliers behind the crystal. Signals from the photomultipliers are then digitised and an image of the organ being examined is constructed from the digitised signals, generally over an exposure time of a few minutes. DESCRIPTION OF THE PRIOR ART To prevent lower energy emissions from being falsely registered, for example if two such emissions pass through the collimator together, it has been proposed by Pillay, Shapiro and Cox (Eur J Nucl Med (1986) 12:293-295) to interpose an alloy filter behind the collimator. The above paper describes the improved images obtained using a brass filter with a thickness of 0.7 mm. The purpose of the filter is to attenuate considerably the relatively low energy emissions while allowing the passage of higher energy emissions thereby improving image resolution. SUMMARY OF THE INVENTION It is an object of the present invention to provide a filter for a gamma camera which improves image resolution still further. This object is achieved in accordance with the present invention in that there is provided a filter for a gamma camera comprising a plate made from a homogeneous alloy the essential ingredients of which are 85 to 95 wt. % cadmium and 5 to 15 wt. % copper. The preferred ranges of contents are about 88 to about 92 wt. % cadmium and about 8 to about 12 wt. % copper, more preferably about 90 wt. % cadmium and 10 wt. % copper. The alloy may also comprise up to about 3 wt. % of incidental impurities, typically iron, antimony and/or nickel.