Thermography
Principle
Thermography, or digital infra red thermal imaging (DITI), is a type of infra red (IR) imaging.
Thermographic cameras detect IR radiation (roughly 0.9-14µm) and produce images
of the radiating body. Since infrared radiation is emitted by all objects based
on their temperature, according to the laws of (black body) electromagnetic
radiation (see Stefan-Boltzmann Law and Wien’s displacement law). Thermography makes it
possible to "see" one's environment with or without visible illumination.
The amount of radiation emitted by an object increases with temperature. Therefore
thermography allows one to see variations in temperature, hence the name. An
infrared scanning device is used to convert infrared radiation emitted from the
object surface into electrical impulses that are visualized in color on a
monitor. This visual image graphically maps the surface temperature and is
referred to as a thermogram. With a thermographic camera warm objects stand out
well against cooler backgrounds. Humans and other warm-blooded animals become
easily visible against the environment day or night, hence historically its
extensive use can be ascribed to military and security services.
Medical DITI is a noninvasive diagnostic technique
that allows the examiner to visualize and quantify changes in skin surface
temperature. Since there is a high degree of thermal symmetry in the normal
body, subtle abnormal temperature asymmetry's can be easily identified.
Medical DITI's major clinical value is in its high
sensitivity to pathology in the vascular, muscular, neural and skeletal systems
and as such can contribute to the pathogenesis and diagnosis made by the
clinician. Attractive is its completely non-invasive nature and the use of a
body-generated signal.
Application
Fig. 1 A left
sural muscle injury
Clinical uses for DITI include:
·
To define the extent of a lesion of which a diagnosis
has previously been made;
·
To localize an abnormal area not previously
identified, so further diagnostic tests can be performed;
·
To detect early lesions before they are clinically
evident;
·
To monitor the healing process before the patient is
returned to work or training.
Skin blood flow is under the control of the sympathetic
nervous system. In healthy people there is a symmetrical dermal pattern which
is consistent and reproducible for any individual. This is recorded in precise
detail with a temperature sensitivity of
Regular Imaging applications are e.g. early
diagnostics of breast cancer and superficial neuro-musculo-skeletal
examinations). Further, applications in rheumatology and dermatology and sports
medicine.
Fig.
2 Functional near-IR optical topography a). Child seated for a PC screen. b) channel
configuration with emitting and receiving channels. c) Time series of relative changes in [oxy-Hb]
(red line) and [deoxy-Hb] (green line) at channel 11 over the occipital cortex
and at channel 23 over the frontal cortex of a 2-month-old infant. Black and
gray bars indicate the test and control trials, respectively. d) response to
visual stimulation. Epoch-averaged images of [oxy-Hb] over the occipital and
frontal cortex of a 4-month-old infant (S6) are illustrated at 2-s intervals.
Scale is relative [oxy-Hb] change from an arbitrary zero baseline. From ref. 1.
A
limitation of the latter is the restricted depth of imaging. Yet, there are
techniques to measure Deep Venous Thrombosis.
Some
other physiological activities can also be monitored with thermographic
imaging, for instance brain activity, especially in infants as an alternative
of the expensive fMRI technique. Actually
it is an application in the field of spectroscopy,
more precisely a multichannel near-IR optical topography measuring time courses
of the levels of oxy-Hb (780 nm) and deoxy-Hb
(830 nm) (Fig. 2.
Literature
1) http://www.pnas.org/cgi/reprint/100/19/10722
2) http://www.meditherm.com/therm_page1.htm