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Example of the definition of periphery and center for an axial slice of the left lung for calculation of the penetration ratio. (a) Shows the mask as fits onto the SPECT data. (b) Shows the distribution of fractional radiuses over the mask indicated by the grayscale. The center with a fractional radius of zero is displayed with black inside the lung. (c) Shows the resulting division of the mask in central zone, shown in white, and the peripheral zone, shown in gray.
