Perceptual thresholds for differences in CT noise texture

Purpose: The average (f _av) or peak (f _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.

Approach: A model of CT NPS was created based on its f _peak and a half-Gaussian fit (σ) 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 f _peak∕σ-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 f _peak∕σ-space. NPS differences were quantified by the noise texture contrast (C_texture), the integral of the absolute NPS difference.

Results: The two-parameter NPS model was found to be a good representation of various clinical CT reconstructions. Perception thresholds for f _peak alone are 0.2 lp∕cm for body and 0.4 lp∕cm for lung NPSs. For σ, these values are 0.15 and 2 lp∕cm, respectively. Thresholds change if the other parameter also changes. Different NPSs with the same f _peak or f _av can be discriminated. Nonradiologist observers did not need more C_texture than radiologists.

Conclusions: f _peak or f _av 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.