1. Field of the Invention
The present invention generally relates to nuclear medicine devices, in particular the calibration of scintillation cameras to correct for non-uniformities in energy distribution as a function of position across the camera field of view.
2. Description of the Background Art
Nuclear medicine is a unique medical specialty wherein radiation is used to acquire images that show the function and anatomy of organs, bones or tissues of the body. Radiopharmaceuticals are introduced into the body, either by injection or ingestion, and are attracted to specific organs, bones or tissues of interest. Such radiopharmaceuticals produce gamma photon emissions that emanate from the body. One or more detectors are used to detect the emitted gamma photons, and the information collected from the detector(s) is processed to calculate the position of origin of the emitted photon from the source (i.e., the body organ or tissue under study). The accumulation of a large number of emitted gamma photons allows an image of the organ or tissue under study to be displayed.
The detector or camera of the “Anger” type is generally comprised of a scintillation crystal and an array of photo-multiplier tubes. The radiation photons interact with the scintillation crystal, which in turn produces light events or flashes of multiple light photons. The light from the crystal is detected by the array of photomultiplier tubes, which outputs a signal proportional to the energy of the incident gamma. The signal is then processed by a computer and stored as image data in a memory, which data is in turn translated into an image for viewing on a display monitor or printed out as a hard copy.
However, inaccuracies are introduced into the camera imaging due to the manufacturing process as well as the physical characteristics of the detecting apparatus. The major distortions are linearity errors, or spatial distortion, and energy non-uniformity across the face of the crystal. Spatial distortion generally arises from the fact that the geometric coordinates of light events occurring either at the edges of or between adjacent photomultiplier tubes in a photodetector array will be computed differently than the coordinates of events occurring directly over the center of a photomultiplier tube, due to the physical limitations of the photomultiplier tube. Therefore, when a radiation source interacts with the scintillation crystal at one x-y coordinate, it will appear in the image as a different x-y coordinate. Linearity correction methods to remedy such distortions, such as disclosed in U.S. Pat. No. 3,745,345 to Muehllehner, (incorporated herein by reference) generally are known in the art. Similarly, energy non-uniformity corrections also are generally known in the art, as disclosed in U.S. Pat. No. 4,323,977 to Arseneau (incorporated herein by reference).
Uniformity distortions are present when the sensitivity of the detector to the energy of incident photons varies across the face of the crystal depending on its coordinate position, as opposed to energy variations in the radiation source. Addressing non-uniformity during manufacture would not be practicable and would drive up cost. However, uniformity is one of the most important specifications of gamma cameras and is used as a major indicator of image quality.
During scanning of a radiation source, there may be emissions or scattered events from unwanted sources, which if recorded can negatively affect the precision of the resulting image. Energy windows have been employed to prevent the recordation of energy events outside the window, thereby accepting only radiation emissions of a specified energy, which are added to the image data being accumulated. However, due to non-uniformities across the scintillation crystal, emissions with appropriate energy may appear to fall outside the energy window and so will not be recorded by the apparatus, thereby introducing image deterioration.
Current methods that attempt to correct non-uniformity, such as the so-called gradient procedure, improve uniformity but at the expense of linearity. Often, as uniformity is improved, the linearity distortion increases as well. Current methods do not adequately address the relationship between energy correction and linearity correction. If uniformity is corrected before linearity correction, uniformity correction cannot be achieved in totality. Consequently there exists a need in the art for achieving total uniformity while not compromising the linearity correction.