Abstract
The use of positron-emitters offers significant advantages in scintigraphy. Several different types of positron cameras have been constructed and studied by Anger. The positron camera presently in use has a high sensitivity, good resolution, and tomographic effect, but disadvantages of this camera, in addition to its complicated electronic circuits, are that it overloads easily and many accidental coincidences arise, because a collimator is not employed. For the purpose of improving these disadvantages, a positron camera with a focused collimator has been constructed for trial. It consists of an image detector with the Anger camera, a coincidence detector with a sodium iodide crystal of 3-inch diameter by 3-inch thickness, a focused collimator, and electronic circuits consisting of a fast coincidence circuit, a constant fraction timing single channel analyzer, and others. Distance from the face of the focused collimator to the coincidence detector, called the focal distance, is 60 cm. An effective diameter of the coincidence detector can be varied by a small collimator. When the effective diameter is 2.5 cm, the overall resolution is independent of the distance from the face of the focused collimator to the active source, and is nearly equal to the intrinsic resolution of the Anger camera. The sensitivity of the positron camera decreases to about 1/4 by attaching the focused collimator, while the counting rates in the image detector decrease to about 1/40. This gives the same effect as the 10-fold increase in sensitivity, because the positron camera will not overload even when the amount of positron emitters administered to a patient is increased about 40 times. The usefulness of this positron camera was ascertained by its use in animals and human patients. Especially in clinical examples, it makes local diagnosis possible because of the high resolution. however, the disadvantage of this positron camera is that the field of view is very small. In order to im-prove this disadvantage, a focused collimator with a focal length of 150 cm has been constructed. This instrument will be used for routine clinical studies in the near future.
