Evaluation of three contrast correction methods for digital subtraction in dental radiography: an in vitro study

In the present in vitro study we test the efficacy of three most often used contrast correction methods to reduce contrast mismatches that can adversely affect digital subtraction in dental radiography. Five radiographs of the lower molars of a beagle dog with an included reference aluminium wedge were radiographed with different exposure times, captured by a monochrome CCD video camera and digitized. To study only the contrast correction effects, the influences of projective geometry and physical noise from the camera and the analog-to-digital conversion were eliminated by simulating bone loss in digital copies of the five reference radiographic images. Each copy in which loss was simulated was contrast corrected by all methods and subtracted from its original image. Four parameters were defined to test the potential of a method to correct grey level values between two images without corrupting real changes. The ODTF (optical density thickness function) method, which is based on a function relating grey level values of the aluminium wedge image and the corresponding thicknesses of the wedge, induced less contrast correction error than the CDF (cumulative density function) and the LSQA (least square quadratic approximation) methods. Therefore, the ODTF method may detect more subtle changes than the other two methods. The experiments indicate that less error is induced by a contrast correction method which uses a grey level mapping function based on the density distribution in a region representing a reference structure. Moreover, CDF, ODTF, and LSQA functions obtained from the reference structure density distribution may be applied for objective contrast enhancements and for standardization of image quality, while the ODTF function also allows bone change volume estimations.