Perceptibility curve test for digital radiographs before and after application of various image processing algorithms

The aim of the present study was to compare digital radiographs before and after the application of various digital image enhancement methods by means of a perceptibility curve (PC) test.

Radiographs of an aluminium test object containing holes with sizes ranging from 0.5 mm to 1.5 mm were exposed at nine time settings, ranging from 0.05 s to 3.2 s using Digora (Soredex, Helsinki, Finland) storage phosphor system. Original digital images were then enhanced and displayed in colour-coded, contrast- and brightness-enhanced, histogram-equalized and negative modes. Thus, five series of images were created. Eight observers independently evaluated all of the images in the same random order and under the same viewing conditions. The object detail with the lowest perceptible contrast was recorded for each observer and each image type. Modified PCs were plotted according to the mean observer data. Repeated-measures ANOVA was used to compare the PCs and the perceptible number of details in original and enhanced images for each exposure (P=0.05). The Bonferroni/Dunn test was used to identify statistical differences among the groups.

Histogram equalization showed the maximum number of perceptible details in both the lowest optimal exposure time and the widest exposure range (P<0.05). Significant differences were present in low and middle exposure ranges of images enhanced with various modalities (P<0.05), while no difference was obtained among the number of details for different enhancements in the high exposure range (P>0.05).

Histogram-equalization enhancement of Digora images demonstrated subjectively superior image quality, followed by contrast and brightness enhancement.