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A 10 MV x‐ray zero‐area tissue‐maximum ratio expression constructed by taking into account depth and off‐axis beam quality change
Author(s) -
Iwasaki Akira
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 - 10.1118/1.598419
Subject(s) - zero (linguistics) , expression (computer science) , kerma , mathematics , function (biology) , dimension (graph theory) , attenuation , convolution (computer science) , quality (philosophy) , physics , optics , geometry , dosimetry , nuclear medicine , quantum mechanics , combinatorics , computer science , medicine , philosophy , linguistics , evolutionary biology , machine learning , artificial neural network , biology , programming language
A 10 MV x‐ray zero‐area TMR (tissue‐maximum ratio) expression was constructed using an attenuation coefficient expression which is a function of depth ( z ) and off‐axis distance ( r ). The zero‐area TMR expression can take into account depth and off‐axis beam quality change. The construction is based on a method of convolving one‐dimension forward and backward primary dose‐spread functions with the primary water collision kerma. Additional information required to construct the zero‐area TMR expression for a given fanline at an off‐axis distance of r is the zero‐area TMR value at z = 0[ D s ( r ) ] and the v ( r ) and w ( r ) values in the backward primary dose‐spread function. It was found that the depth of peak zero‐area TMR[ z peak ( r ) ] decreases with increasing r and that for large depths, the zero‐area TMR expression changes significantly as a function of r . Obtaining an accurate expression for zero‐area TMR is important for accurate construction of the three‐dimensional (3D) forward and backward primary dose‐spread functions needed for a 3D convolution method.