A bedsore occurs when sustained pressure of bedclothes against the skin causes local obstruction to the blood supply, resulting in damage and potential loss of the affected tissue. Bedsores affect approximately 20% of hospitalised patients. The cost in both human suffering and financial terms is high; in the US the care of pressure ulcers costs US$ 3 to US$ 5 billion annually, whilst in the UK this figure is currently £300 million, estimated soon to reach £500 million. Both suffering and cost of treatment may be greatly reduced when the ulcer and any changes resulting from therapy can be accurately and reliably measured. Regular objective measurement enables medical staff to accurately assess the progress of wound healing and to improve the treatment strategy. Any measuring technique should preferably be non-invasive so as to avoid damage, infection and pain.

Measurement Techniques
The current clinical method is to place a transparent acetate sheet on the wound and count the number of its bounding squares; this provides a measure of area. Volume is estimated by filling the wound with saline; the volume dispensed from the syringe equals the wound volume. These methods are inaccurate, unreliable and involve contact with the open wound. The non-invasive technique of laser scanning requires a still object during the scan period, which cannot be guaranteed for dynamic living objects like human bodies. Photogrammetry enables recording of objects in a split second and is widely used for non-invasive, quantitative and precise analysis of parts of the human body. For some applications, conventional stereo matching algorithms have their limitations with respect to reliability of match and accuracy. In recent years multi-camera configurations have been proposed to solve some of these limitations for medical applications.

• A multi-camera system has been developed at the Swiss Federal Institute of Technology in Switzerland for 3D face-feature extraction and reconstruction.
  
• The Aristotle University of Thessaloniki University in Greece has developed a video camera system for the extraction of 3D surface models of the human back.
  
• The Delft University of Technology in The Netherlands has developed a three-camera system armed with structured light for the measurement of the shape and volume of wounds, to support bedsore analysis.

• human skin is relatively uniform and featureless, so measurement of points on its surface is difficult or impossible
  
• a wound surface is wet, resulting in specular reflections which disturb the Lambertian condition necessary for intensity-based matching and thus reducing its reliability
  
• reliable results require an approximation of match positions in the order of a few pixels
  
• intensity-based methods cannot altogether cope with images of differing radiometric and geometric properties.

Frank van den Heuvel is an assistant professor at the Delft Institute of Earth Observation and Space Systems (DEOS) of the Faculty of Aerospace Engineering of the Delft University of Technology, The Netherlands. Dr van den Heuvel holds a PhD in photogrammetry and his research interests lie in architectural, industrial and medical photogrammetry.

Ali Azizi is associate professor at the Faculty of Engineering, University of Tehran. He holds a PhD in digital photogrammetry. His research interests are in close-range and satellite photogrammetry.