Unsharp masking technique as a preprocessing filter for improvement of 3D-CT image of bony structure in the maxillofacial region

Abstract

We evaluated the usefulness of the unsharp masking technique as a preprocessing filter to improve 3D-CT images of bony structure in the maxillofacial region. The effect of the unsharp masking technique with several combinations of mask size and weighting factor on image resolution was investigated using a spatial frequency phantom made of bone-equivalent material. The 3D-CT images were obtained with scans perpendicular to and parallel to the phantom plates. The contrast transfer function (CTF) and the full width at half maximum (FWHM) of each spatial frequency component were measured. The FWHM was expressed as a ratio against the actual thickness of phantom plate. The effect on pseudoforamina was assessed using sliced CT images obtained in clinical bony 3D-CT examinations. The effect of the unsharp masking technique on image quality was also visually evaluated using five clinical fracture cases. CTFs did not change. FWHM ratios of original 3D-CT images were smaller than 1.0, regardless of the scanning direction. Those in scans perpendicular to the phantom plates were not changed by the unsharp masking technique. Those in parallel scanning were increased by mask size and weighting factor. The area of pseudoforamina decreased with increases in mask size and weighting factor. The combination of mask size 3×3 pixels and weighting factor 5 was optimal. Visual evaluation indicated that preprocessing with the unsharp masking technique improved the image quality of the 3D-CT images. The unsharp masking technique is useful as a preprocessing filter to improve the 3D-CT image of bony structure in the maxillofacial region.