Image analysis based on an optical model of the skin
for detection of early signs of melanoma
Funded by EPSRC, grant number GR/M53035
This research is concerned with the characterisation of
pigmented skin lesions to help with early diagnosis of malignant melanoma, a skin cancer.
Out group has developed a novel image analysis method which uses
physics-based modelling of optical properties of the skin. The method
computes parametric maps characterising skin structure and composition. The images
show histological quantities in the skin, such as concentration
of pigment melanin, concentration of blood and thickness of collagenous
tissue. They also show whether melanin is present in the dermis - such
presence is a very sensitive indicator of melanoma.
Skin structure
The skin consists of a number of layers with distinct function and distinct optical
properties. White light shone onto the skin penetrates superficial skin layers and
whilst some of it is absorbed, much is remitted back and can be registered by a camera.
Model of tissue colouration - normal skin
The key to the interpretation of image colours in terms of the underlying histological
parameters is a model of tissue colouration which provides a cross-reference between
the colour and the histology. This model is constructed by computing the spectral
composition of light remitted from the skin given parameters specifying its structure
and optical properties. This step needs to be carried out only once. As the mapping
between the colours and the parameters is unique for the skin, each colour
corresponds to one specific set of histological parameters.
Expressed as a fragment of a pseudocode, the process of building of the model of
colouration can be described as follows:
given
incident light
absorption coefficients of melanin and blood
scatter coefficient of the papillary dermis
scatter coefficient and thickness of the reticular dermis
spectral response functions for the red, green blue and nir primaries
for all valid concentrations of epidermal melanin
for all valid concentrations of dermal blood
for all valid thicknesses of papillary dermis
compute
remitted light spectrum
colour vector [r g b nir]
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This forward process computes explicitly tissue colour given a set of histological
parameters. As the mapping between the histological parameters and the primaries is
unique for the skin, the inverse mapping is possible: from the tissue colour to its
histological parameters:
[r g b nir] <-> [ melanin haemoglobin papillary_dermis ]
The quantities [ melanin haemoglobin papillary_dermis ] are then used to
construct parametric maps.
The model of normal skin colouration is representative of all the normal skins,
irrespective of racial origin, age or gender. The structure remains the same,
and the only differences are in the magnitudes of the parameters.
Abnormal skin colouration
The model above has been constructed for skin which has a normal structure.
Skin colouration associated with abnormal conditions does not necessarily have
to conform to this model.
