SINUSOIDAL MODULATION ANALYSIS FOR OPTICAL-SYSTEM MTF MEASUREMENTS

The modulation transfer function (MTF) is a commonly used metric for d efining the spatial resolution characteristics of imaging systems. Whi le the MTF is defined in terms of how an imaging system demodulates th e amplitude of a sinusoidal input, this approach has not been in gener al use to measure MTFs in the medical imaging community because produc ing sinusoidal x-ray patterns is technically difficult. However, for o ptical systems such as charge coupled devices (CCD), which are rapidly becoming a part of many medical digital imaging systems, the direct m easurement of modulation at discrete spatial frequencies using a sinus oidal test pattern is practical. A commercially available optical test pattern containing spatial frequencies ranging from 0.375 cycles/mm t o 80 cycles/mm was used to determine the MTF of a CCD-based optical sy stem. These results were compared with the angulated slit method of Fu jita [H. Fujita, D. Tsia, T. Itoh, K Doi, J. Morishita, K. Ueda, and A Ohtsuka, ''A simple method for determining the modulation transfer fu nction in digital radiography,'' IEEE Trans. Medical Imaging 11, 34-39 (1992)]. The use of a semiautomated profile iterated reconstruction t echnique (PIRT) is introduced, where the shift factor between successi ve pixel rows (due to angulation) is optimized iteratively by least-sq uares error analysis rather than by hand measurement of the slit angle . PIRT was used to find the slit angle for the Fujita technique and to find the sine-pattern angle for the sine-pattern technique. Computer simulation of PIRT for the case of the slit image (a line spread funct ion) demonstrated that it produced a more accurate angle determination than ''hand'' measurement, and there is a significant difference betw een the errors in the two techniques (Wilcoxon Signed Rank Test, p < 0 .001). The sine-pattern method and the Fujita slit method produced com parable MTF curves for the CCD camera evaluated. (C) 1996 American Ass ociation of Physicists in Medicine.