Thermal Imaging

What is Thermal Imaging?

Thermal imaging, also known as clinical thermography, is capable of providing non-contact, in vivo diagnostic information, much like other medical imaging techniques. The technology is based upon the fact that every object with a temperature above absolute zero (-273 ^oC) emits electromagnetic radiation, the wavelength distribution of which is a function of temperature.

Hence, by detecting this emitted radiation, it is possible to map small variations of the body's surface temperature and identify thermal abnormalities that accompany various physiological conditions. Being a passive technique, (i.e. without external sources of radiation) thermography is non-invasive and therefore intrinsically harmless.

Thermograms of the face, heart (normal and ischemic), and hand

How Does Thermal Imaging Work?

Planck's Formula for black body radiation describes the distribution of spectral radiance emitted from a heated body. Plotting this distribution against wavelength produces a series of blackbody curves, with a separate curve for each temperature The graph on the right illustrates that the blackbody curve has a maximum that is shifted to shorter wavelengths (Wien's Displacement Law) and increases in output with rising temperature (Stefan-Boltzmann's Law).

Blackbody curves of the sun's temperature and body temperature

Hence, while the sun's output peaks in the visible region, the peak output for body temperature (and therefore, the optimum bandwidth for detection) occurs in the electromagnetic spectrum's mid-infrared region, at between 9 - 10 µm.

For medical imaging applications this wavelength region is also advantageous because skin's emissivity is high (> 98 %), reducing the possibility of spurious results due to reflected infrared radiation.

Detection in the mid-infrared range dictates use of specialized optical and detector technology. At the leading edge of this trend are systems based on Quantum Well Infrared Photodetectors (GaAs). Such systems offer a thermal sensitivity of 20 mK at 30^oC and focal plane arrays of over 75,000 pixels without the requirement of liquid nitrogen cooling.

Finally, PC-based image storage allows for hundreds of sequential thermal images to be recorded each second. Such temporal resolution allows for measurements based upon the time dependence of skin temperature, known as dynamic area telethermometry.

How Can Thermal Imaging Be Used?

Applications of thermography include breast cancer screening, intraoperative surgery, dentistry, vascular disorders, musculoskeletal injury, and dermatology among others.

Thermal image acquisition of the face and resulting thermogram.

Research at NRC-IBD includes investigating the diagnostic potential of thermal and near infrared imaging techniques for detection of maxillary sinusitis. The image on the right shows a thermal camera imaging a control subject whose head is held steady in a headrest. The resultant thermogram is displayed below.

The thermological aspect of this study aims to test the hypothesis that inflammation or infection of maxillary sinuses, which are located in each cheek, results in localized temperature increases on the surface of the face. Either thermal imaging or near infrared imaging may yield diagnostically relevant information in its own right; however, clear synergies exist between the two that, if exploited, could lead to greater diagnostic power than either alone.

To achieve this fusion of modalities, accurate multimodal registration of corresponding pixels is required to enable direct comparison of relevant anatomical areas.

Thermal imaging is also being used as a complementary technique to other infrared and optical modalities including studies to detect regional variations in heart oxygenation for the intraoperative identification of poorly perfused areas; thermal response to dermatological irritation; and intraoperative assessment of skin flap viability during reconstructive surgery.

Related Information

Read more about Thermal Imaging:

• Diffuse Reflectance Spectroscopy
• Non-linear Optical Microscopic Imaging
• Fluorescence Imaging
• Frequency Modulated Spectroscopy
• MRI Studies
• Bioluminescence Imaging
• Molecular Imaging - Spectroscopy
• Optical Coherence Tomography
• Raman Spectroscopy
• Speckle Imaging
• High Throughput Diagnostics
• Diagnostic Technologies

Related Programs:

• Medical Photonics

Institutes:

• NRC Institute for Biodiagnostics