  Optical coherence tomography (OCT) imaging of the skin is gaining recognition
and is increasingly applied to dermatological research. A key dermatological
parameter inferred from an OCT image is the epidermal (Ep) thickness as a
thickened Ep can be an indicator of a skin disease. Agreement in the literature
on the signal characters of Ep and the subjacent skin layer, the dermis (D), is
evident. Ambiguities of the OCT signal interpretation in the literature is
however seen for the transition region between the Ep and D, which from
histology is known as the dermo-epidermal junction (DEJ); a distinct junction
comprised by the lower surface of a single cell layer in epidermis connected to
an even thinner basement membrane, attached to the underlying dermis. In this
work we investigate the impact of an improved axial and lateral resolution on
the applicability of OCT for imaging of the skin. To this goal, OCT images are
compared produced by a commercial OCT system and by an in-house built ultrahigh
resolution (UHR-) OCT system for dermatology. In 11 healthy volunteers, we
investigate the DEJ signal characteristics. We perform a detailed analysis of
the dark (low) signal band clearly seen for UHR-OCT in the DEJ region where we,
by using a transition function, find the signal transition of axial
sub-resolution character, which can be directly attributed to the exact
location of DEJ, both in normal (thin/hairy) and glabrous (thick) skin. For
selected healthy volunteers, we investigate the dermal papillae and the vellus
hairs and identify distinct features that only UHR-OCT can resolve. Differences
are seen in tracing hairs of diameter below 20mm, and in imaging the dermal
papillae where capillary structures are identified in the hand palm, not
previously reported in OCT studies and specifically for glabrous skin not
reported in any other in vivo optical imaging studies.
