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The physical basis for empirical rules used to determine equivalent fields for phantom scatter
Author(s) -
McDermott Patrick N.
Publication year - 1998
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.598420
Subject(s) - perimeter , rectangle , square (algebra) , mathematics , imaging phantom , field (mathematics) , equivalence (formal languages) , mathematical analysis , basis (linear algebra) , geometry , calculus (dental) , optics , physics , discrete mathematics , pure mathematics , medicine , dentistry
Simple empirical rules are widely employed to determine the size of the square radiation field which is equivalent to either a rectangular field or a circular field with respect to phantom scatter. For a rectangular field, the area divided by the perimeter of the rectangle is equated to the area divided by the perimeter of the equivalent square. For circular fields, the area of the circle is sometimes equated to the area of the equivalent square. These empirical rules are very simple and remarkably accurate. They have been in daily use for 30 years, yet there is little discussion in the literature of their fundamental physical basis. An equation is derived for the equivalence of rectangular fields which has the form of the area over perimeter result multiplied by a “correction” factor. This equation is based on fundamental physical principles and it agrees better, within its domain of applicability, with published data than the area over perimeter rule. It is also shown that what appear to be different arbitrary rules for circles and rectangles both emerge from the same underlying physical analysis.