Sinusoidal modulation analysis for optical system MTF measurements

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Abstract

The modulation transfer function (MTF) is a commonly used metric for defining the spatial resolution characteristics of imaging systems. While the MTF is defined in terms of how an imaging system demodulates the amplitude of a sinusoidal input, this approach has not been in general use to measure MTFs in the medical imaging community because producing sinusoidal x-ray patterns is technically difficult. However, for optical systems such as charge coupled devices (CCD), which are rapidly becoming a part of many medical digital imaging systems, the direct measurement of modulation at discrete spatial frequencies using a sinusoidal test pattern is practical. A commercially available optical test pattern containing spatial frequencies ranging from 0.375 cycles/mm to 80 cycles/mm was used to determine the MTF of a CCD-bused optical system. These results were compared with the angulated slit method of Fujita [H. Fujita, D. Tsia, T. Itoh, K. Doi, J. Morishita, K. Ueda, and A Ohtsuka, 'A simple method for determining the modulation transfer function in digital radiography,' IEEE Trans. Medical Imaging 11, 34-39 (1992)]. The use of a semiautomated profile iterated reconstruction technique (PIRT) is introduced, where the shift factor between successive pixel rows (due to angulation) is optimized iteratively by least-squares 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 function) demonstrated that it produced a more accurate angle determination than 'hand' measurement, and there is a significant difference between the errors in the two techniques (Wilcoxon Signed Rank Test, p<0.001). The sine-pattern method and the Fujita slit method produced comparable MTF curves for the CCD camera evaluated.

Original languageEnglish (US)
Pages (from-to)1955-1963
Number of pages9
JournalMedical Physics
Volume23
Issue number12
DOIs
StatePublished - 1996

Keywords

  • charge coupled device
  • line spread function
  • modulation transfer function
  • sinusoid

ASJC Scopus subject areas

  • Biophysics

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