The principle of Photodynamic Therapy
Photodynamic Therapy (PDT) is a minimally
invasive treatment that is used in various medical and cosmetic
fields. It is based on the interaction of a pharmaceutical active -
the so-called photosensitiser - and visible light. The molecule
itself is harmless to the human body, however, at the precise place
where it gets activated by normal light of a suitable colour, a photochemical process
is induced that leads to the desired effect that might be the destruction
of the target tissue (such as in permanent hair removal) or the
attenuation of a metabolic cycle (such as in the treatment of acne).
The active is delivered to the target tissue structure by various routes of administration, in the case of dermatology mainly using a cream. The activating light is mostly produced by a laser, a filtered lamp or light emitting diodes (LED).
The principle of PDT using a topic photosensitiser in dermatology is schematically illustrated in the drawing below.
The principle of Fluorescence Photodiagnostics
Fluorescence can be
exploited in certain circumstances for diagnostic purposes. For example, the intrinsic fluorescence -
the so-called autofluorescence - can be used to discover certain
tissue anormalities such as cancer cells or to monitor certain metabolic
statuses. Another application in PDT takes advantage from the fact that most photosensitisers fluoresce when excited
at a suitable wavelength for non-invasive in vivo and
in vitro pharmacokinetics.
In the clinical practise, the instrumentations used are divided into imaging devices with full spatial, but restricted spectral resolution (e.g. for drug localisation) and in single point measuring spectrometers (e.g. for pharmacokinetics). The use of a fluorescence imaging device is illustrated in the picture below showing the distribution of a photosensitiser in a psoriatic plaque indicated by red fluorescence.