Image resolution and exposure time of digital radiographs affects fractal dimension of periapical bone

The aim of this study was to evaluate the effect of exposure time and image resolution on fractal dimension (FD) of periapical bone on images obtained using a storage phosphor plate (SPP) system.

Periapical images of premolar and molar teeth on both sides of ten dry human mandibles were obtained with Digora Optime (Soredex Corp., Helsinki, Finland) SPP system. The SPPs were exposed with three exposure times (0.05, 0.12, and 0.30 s) and scanned immediately after exposure with high and super resolutions. FD was calculated using public domain software (ImageJ with FracLac plug-in) on two non-overlapping region of interest (ROIs) on premolar and molar periapical bone areas of each radiograph using differential box-counting method. The ROIs on corresponding images were of the same size and position. FDs were compared using two-way ANOVA and Tukey-Kramer multiple comparison tests (p = 0.05).

Images obtained with super resolution scans gave significantly higher FD values compared to high-resolution scanning for all exposures (p < 0.0001). FD values were decreasing with increase in exposure time for both resolutions (p < 0.0001). The highest FD was found for images with super resolution and shortest exposure time, which exhibited the highest noise.

FD analysis seems not so robust method as it was believed previously. It shows significant changes with image resolution and exposure time.

Exposure time and scanning resolution of SPPs should be carefully chosen when evaluating the change in FD of alveolar bone for various bone disorders.