Statistical limitations in computed tomography

A number of apparent errors have been encountered in various published analyses of the limitations imposed by statistical fluctuations of photon numbers on the measurement of X-ray attenuation coefficients with computed tomography. An attempt is made to remedy these errors by re-deriving the expression for the variance in measured attenuation coefficients given by Brooks and Di Chiro (1976, Med. Phys. 3, 237). Small but important discrepancies in other published work are brought to light by this calculation. (author)