I. Introduction
Flap surgery is a reconstructive procedure that consists of transferring healthy tissue from one part of the body (donor site) to another (recipient site) in order to fill a wound-caused defect, such as a severe burn, a trauma or the removal of cancerous tissue. The transplanted tissue usually comprises skin and fatty tissue, but it may also include muscle tissue. When possible, the flap is transferred maintaining an attachment (pedicle) to the donor area to preserve the original vascular supply. However, certain procedures require a transfer of tissues from a remote part of the body (free flap), which entails disconnecting all vessels supplying and draining blood from the flap and then anastomosing (i.e., reconnecting) them to blood vessels adjacent to the wounded area in order to re-establish blood supply and drainage. The major complication of free flap surgery is the potential occurrence of an arterial or venous occlusion during or after the flap transfer. The visible effect of a vascular occlusion (i.e., the flap changing color to blue or red after an arterial or venous occlusion, respectively) manifests relatively shortly after the blockage, but such effect might not be promptly detected by the clinical staff after surgery, when visual access to the flap is blocked by wound dressing. If sufficiently prolonged, a vascular occlusion can cause temporary or permanent tissue damage (i.e., necrosis) requiring further surgical intervention. Besides periodic visual observation, clinicians currently rely on probing blood flow in the flap with Doppler ultrasound. However, ultrasound provides only a coarse, qualitative assessment of blood perfusion, and it does not allow continuous monitoring after the wound has been dressed.