Dermoscopic images can be acquired with the dermoscope in direct contact with the patient's skin or not. Contact dermoscopes have a glass plate which contacts the skin via a liquid interface (immersion media). Some vendors include a millimeter measurement scale which is etched or imprinted onto the glass contact plate. Resultant images include the scale as shown in Figure TTTT.1-1. This scale can be used to calibrate measurement tools in display software.
An alternative way to support distance measurements is when the vendor encodes the Pixel Spacing (0028,0030) attribute with the physical distance between the center of adjacent pixels as defined in 10.7.1.3 Pixel Spacing Value Order and Valid Values in PS3.3 . If Pixel Spacing contains values then measurements tools in the display software do not need to calibrate against an object of known size (e.g., millimeter measurement scale) to be able to provide a distance measurement. Pixel Spacing can be geometrically calculated when there is a known source-to-object distance as would occur with contact dermoscopy. Some non-contact dermoscopes also have fixed distance lens cones which also make it possible to geometrically calculate pixel spacing. It is difficult to accurately calculate pixel spacing when the source-to-object distance is not fixed.
