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Investigation of Contribution Functions in a Scintillation Camera

Hitachi Central Research Laboratory, Hitachi Ltd., and Hitachi Medical Corporation, Tokyo, Japan

Correspondence: ¹ For reprints contact: Ken Ueda, Hitachi Central Research Laboratory, Hitachi Ltd., 1-280, Higashikoigakubo, Kokubunji-shi, Tokyo, 185, Japan.

ABSTRACT

The statistical spatial resolution and coordinate linearity at the central portion of an image detector are investigated theoretically in relation to the "contributing factor of a photomultiplier (PM) to a positioning signal" and light-guide thickness. The "contributing factor" is the increment of change in the positioning signal corresponding to an infinitessimal change in PM output. The "contribution function to a positioning signal" of a PM group with equal x or y coordinates is defined by this "contributing factor" and expressed as a function of the relative position between the PM group and a scintillation point. When the "contribution function" is determined, the statistical spatial resolution and coordinate linearity can be calculated for a given light-guide thickness.

The ideal contribution functions, which are two-dimensional, minimize the resolving distance when light-guide thickness is in the range of 0.6–0.7 times the PM radius. The contribution functions of the matrix and threshold preamplifier methods, which are approximately one-dimensional, cannot correct the coordinate nonlinearity, which increases as light-guide thickness decreases, although the threshold-preamplifier method improves resolving distance. Modification of the one-dimensional contribution functions—which reduce the nonlinearity to one half or less accompanied by only a slight increase in resolving distance—are proposed. If the acceptable nonlinearity is ±4%, the resolving distance decreases by 26 and 14% over the matrix and threshold-preamplifier methods, respectively, by using the modification for thin light guides.