Perceptual thresholds for differences in CT noise texture

The average ($$) or peak ($$) noise power spectrum (NPS) frequency is often used as a one-parameter descriptor of the CT noise texture. Our study develops a more complete two-parameter model of the CT NPS and investigates the sensitivity of human observers to changes in it.

A model of CT NPS was created based on its $$ and a half-Gaussian fit ($$) to the downslope. Two-alternative forced-choice staircase studies were used to determine perceptual thresholds for noise texture, defined as parameter differences with a predetermined level of discrimination performance (80% correct). Five imaging scientist observers performed the forced-choice studies for eight directions in the $$-space, for two reference NPSs (corresponding to body and lung kernels). The experiment was repeated with 32 radiologists, each evaluating a single direction in the $$-space. NPS differences were quantified by the noise texture contrast ($$), the integral of the absolute NPS difference.

The two-parameter NPS model was found to be a good representation of various clinical CT reconstructions. Perception thresholds for $$ alone are $$ for body and $$ for lung NPSs. For $$, these values are 0.15 and $$, respectively. Thresholds change if the other parameter also changes. Different NPSs with the same $$ or $$ can be discriminated. Nonradiologist observers did not need more $$ than radiologists.

$$ or $$ is insufficient to describe noise texture completely. The discrimination of noise texture changes depending on its frequency content. Radiologists do not discriminate noise texture changes better than nonradiologists.