Accounting for anatomical noise in search-capable model observers for planar nuclear imaging

Abstract

Model observers intended to predict the diagnostic performance of human observers should account for the effects of both quantum and anatomical noise. We compared the abilities of several visual-search (VS) and scanning Hotelling-type models to account for anatomical noise in a localization receiver operating characteristic (LROC) study involving simulated nuclear medicine images. Our VS observer invoked a two-stage process of search and analysis. The images featured lesions in the prostate and pelvic lymph nodes. Lesion contrast and the geometric resolution and sensitivity of the imaging collimator were the study variables. A set of anthropomorphic mathematical phantoms was imaged with an analytic projector based on eight parallel-hole collimators with different sensitivity and resolution properties. The LROC study was conducted with human observers and the channelized nonprewhitening, channelized Hotelling (CH) and VS model observers. The CH observer was applied in a "background-known-statistically" protocol while the VS observer performed a quasi-background-known-exactly task. Both of these models were applied with and without internal noise in the decision variables. A perceptual search threshold was also tested with the VS observer. The model observers without inefficiencies failed to mimic the average performance trend for the humans. The CH and VS observers with internal noise matched the humans primarily at low collimator sensitivities. With both internal noise and the search threshold, the VS observer attained quantitative agreement with the human observers. Computational efficiency is an important advantage of the VS observer.

Keywords: collimators; image quality; internal noise; model observers; nuclear medicine; planar imaging; scanning observers; visual search.

Fig. 1

Example greyscale intensity distributions for the normal and abnormal focal points identified by the VS observer in an image set.

Fig. 2

Sample study images obtained for a pair of high-contrast lesion cases: (a) a prostate lesion; (b) a lymph-node lesion. For both cases the first row consists of images from collimators C1 to C4 and the bottom row C5 to C8. The lesion position for each case is indicated by the white arrow in the first image.

Fig. 3

Model-observer and average human-observer performance as a function of collimator and lesion contrast. The model observers were applied without inefficiencies. (a) Model-observer performances with the low-contrast lesions; (b) human and model-observer performances with the high-contrast lesions. Uncertainties for the model observers did not exceed $\pm 0.02$.

Fig. 4

Effects of internal noise on CH observer performance with the high-contrast lesions. The noise parameter $\gamma$ varied from 0 to 2.0, with $\gamma =0$ indicating no noise. Also shown is average human performance. The uncertainties in model-observer performance were $\pm$0.03.

Fig. 5

Individual effects of internal noise and search thresholding on VS observer performance with the high-contrast lesions. (a) Effects of internal noise; the noise parameter $\gamma$ varied from 0 to 2.0, with $\gamma =0$ indicating no noise. (b) Threshold effects; the threshold parameter $\beta$ varied from 0.2 to 0.8. The uncertainties in model-observer performance were $\pm 0.03$.

Fig. 6

Effects of applying the VS observer with multiple inefficiencies. Average human performance is compared with the performance from the VS observer corresponding to $\gamma =1.5$ and $\beta =0.6$. The uncertainties in model-observer performance were $\pm 0.03$.