Phys Med Biol. 2024 Jul 9. doi: 10.1088/1361-6560/ad6119. Online ahead of print.
ABSTRACT
Objective We propose a nonparametric figure of merit, the contrast equivalent
distance CED, to measure contrast directly from clinical images.
Approach A relative brightness distance δ is calculated by making use of the order
statistic of the pixel values. By multiplying δ with the grey value range R, the mean
brightness distance MBD is obtained. From the MBD, the CED and the distance-to-
noise ratio DNR can be derived. The latter is the ratio of the MBD and a previously
suggested nonparametric measure τ for the noise.
Since the order statistic is independent of the spatial arrangement of the pixel
values, the measures can be obtained directly from clinical images. We apply the new
measures to mammography images of an anthropomorphic phantom and of a phantom
with a step wedge as well as to CT images of a head phantom.
Main results For low-noise images of a step wedge, the MBD is equivalent to the
conventional grey value distance. While this measure permits the evaluation of clinical
images, it is sensitive to noise. Therefore, noise has to be quantified at the same time.
When the ratio σ/τ of the noise standard deviation σ to τ is available, validity limits
for the CED as a measure of contrast can be established. The new figures of merit
can be calculated for entire images as well as on regions of interest (ROI) with an edge
length not smaller than 32 px.
Significance The new figures of merit are suited to quantify the quality of clinical
images without relying on the assumption of a linear, shift-invariant system. They can
be used for any kind of greyscale image, provided the ratio σ/τ can be estimated. This
will hopefully help to achieve the optimisation of image quality vs dose required by
radioprotection laws.
PMID:38981591 | DOI:10.1088/1361-6560/ad6119
