Intrinsic plane sensitivity: A new parameter most suitable for evaluation and comparison of moving and/or stationary scanners

Abstract

Author's unified theory of the moving and stationary scanners showed that the stationary detector head of a moving scanner constitutes merely a “potential scanner”, that a “functioning scanner” gets generated only when the head has described an area, and that the potential scanner is quite distinctly different from the functioning scanner in field properties. Lack of proper appreciation of this distinction has led to some continuing confusion which this paper seeks to clarify. Point sensitivity S of a functioning scanner (moving or stationary) is now completely defined as “counts per unit counting time per microcurie”—counting time being the duration for which counts are collected from a point source. For a stationary scanner, counting time is the scanning (exposure) time, but for a moving scanner it is the time taken to describe an area equal only to that enclosed within the contour enveloping peripheral holes on the collimator face of stationary detector head—called its “collecting area”. Plane sensitivity of any scanning system S a is re-defined as equal to the photopeak counts recorded per unit counting time from an infinite plane source placed normally to the central axis and having a uniform surface activity concentration of 1 μCi/cm 2. Plane sensitivity per unit effective collecting area of the system is proposed to be recognised as a new parameter: “Intrinsic Plane Sensitivity S ai ”. It is found that S ai equals S, and is distance independent; S ai for a moving scanner equals that for its stationary detector head. Of two functioning scanners the one having lower S ( = S ai ) but higher S a —because of much larger scan area—should, on the conventional criterion of S a , be a better instrument but would produce worse quality scans! This fallacy gets resolved when S ai replaces S a as the comparing criterion. Therefore S ai may, with advantage, be used to evaluate/compare moving and/or stationary scanners by both the producers and the users including the ICRU. Experimental evidence is discussed and is found to support the theory.