INFRARED IMAGING Dr M A Oghabian Medical Physics Group Tehran University of Medical Sciences www.oghabian.net

Thermal imaging is the detection of radiant emission from the skin surface.

black body is an object that absorbs all light that falls on it. No electromagnetic radiation passes through it and none is reflected. Because no light is reflected or transmitted, the object appears black when it is cold. If the black body is hot, these properties make it an ideal source of thermal radiation. At room temperature, black bodies emit infrared light, but as the temperature increases past a few hundred degrees Celsius, black bodies start to emit at visible wavelengths, from red, through orange, yellow, and white before ending up at blue

The color (chromaticity) of black-body radiation depends on the temperature of the black body

Black body spectrum showing spectral energy density at different Tempreture

Temperature measurement of the skin by an IR camera must allow for: 1) Radiation emitted by the skin as a result of its surface temperature T0 and its emissivity (e) 2) Radiation reflected by the skin as a result of the ambient temperature of the surroundings Ta which will be proportional to the reflectance (r) of the skin 3) Radiation transmitted through the skin as a result of the temperature Td at a depth within the body skin is non-transparent for the wavelengths detected, so the transmitted contribution can be neglected.

Signal S resulting from radiation emitted by an object: S = ef(T0) + rf(Ta) ef(T0) is the emitted radiation, which is a function of the object's surface temperature rf(Ta) is the reflected radiation, which is a function of the ambient temperature Since r = 1 - e, then: S = ef(T0) + (1-e)f(Ta)

R(T) = e (T) sAT4 Thermal radiation Stefan-Boltzmann law, states that the total energy radiated per unit surface area of a black body in unit time, (or the emissive power), is directly proportional to the fourth power of the black body's thermodynamic temperature T : R(T) = e (T) sAT4 Where R(T) is the total power radiated into a hemisphere s is Stefan-Boltzmann constant: K is Boltzmann Constant A is the effective radiating area of the body T is the absolute temperature of the radiating surface (Kelvin) For a thermal black body, the emissivity e is unity

آشكارسازهاي حرارتي Thermal Detector تشعشع را از طريق افزايش حرارت درسلولهاي خودش جذب مي كند و مقاومت الكتريكي سلولها يا شارژ سطحي آنها افزايش مي يابد. هر گونه تغيير درمقاومت (حادث شده توسط تغيير درحرارت ) باعث توليد سيگنال الكتريكي يا ولتاژ مشخص مي شود كه بطور الكترونيكي تقويت مي شود. آشكارسازحرارتي Pyroelectric (مثل Barium titanate ) جهت ايـن امـر بكار مـي روند. توان آشكارسازي خــوب درتـشعشـع هاي بـا طـول موج بـلند و عدم نیاز به سرد سازي آشكارساز از مـزايـاي آن است. آشكارساز پـيروالـكـتريك در دوربين Vidicon بكار برده می شوند.

Photon detectors Most thermal imaging systems employ photon detectors These are semiconductor-type devices like: Indium antimonide (InSb) Cadmium Mercury Telluride (CdHgTe) Lead telluride (PbTe)

Types of Photon detectors Photoconductive: The conductivity change due to incident photon flux Voltage change across a resistor is amplified Photovoltaic: Photons within a semiconductor produces a voltage that can be detected without the need for bias supply or load resistor To overcome thermal noise within a detector, cryogenic cooling to -196 'C is used by liquid nitrogen

Detector performance Temperature resolution Angular resolution Field of view Temperature resolution depends on: Efficiency of the optical system Responsivity and noise of the detector Signal-to-noise ratio of the signal processing circuitry

Detector performance Temperature resolution can be expressed in two ways: Noise equivalent temperature difference (NETD): Temperature difference for which the signal-to-noise ratio is unity Minimum resolvable temperature difference (MRTO): The smallest temperature difference that is visible on the display. For Medical thermography: MRTD between 0.1 and 0.3 K Angular resolution is typically 1-3 mrad but can be as small as 0.5 mrad.

2 types of Imaging System -1با استفاده از آشكارسازهاي مجزا ( discrete detector) ميدان شئي توسط سيستم مكانيكي، الكترونيكي، يا اپتيكي اسكن مي شود تا تمام سطح شئـي روي سطح ديتكتورها ( صفحه تصوير ) پــوشـانـده شود -2ديتكتورهاي تصويري ( Imaging detectors) استفاده مي شود كه در آنها بدون سيستم اسكن كردن، تصوير IR به يك تصوير نوراني تبديل مي شود مانند: فــتـوگــرامـيـك IR phosphors تيوب مبدل تصوير كريستال مايع تيوب هاي ويديكون پيروالكتريك.

Imaging System using discrete detector The scene is viewed by an optical system capable of transmitting and focusing IR radiation Scanning systems might employ configurations of lenses. rotating prisms, rocking mirrors or rotating multisided mirror drums. A high value of IR refractive index is advantageous in lens design (materials that have high refractive indices tend to have low transmittances) Germanium and Silicon are used frequently for IR optics. Germanium has a refractive index of about 4 and a transmittance of 47%.

Thermovision Camera

جاروب کردن سطح شئـي روی discrete detector

Pyroelectric imaging systems using Imaging detectors Using Ferromagnetic crystals such as Barium Titanate and Triglycine Sulphate (TGS) Developed as a pyroelectric vidicon camera tubes When exposed to radiation, behave as capacitors on which electrical charge appears Usually, no scanning devices is used A germanium IR transmitting lens focuses the image onto a thin disc of TGS pyroelectric material This disc is an electrically conducting layer The disc is scanned in a TV raster by the electron beam

Specification of Pyroelectic Camera The sensor does not respond to a steady flux of radiation. Image quality is inferior to photon-detector scanning systems This systems is not able to record clinical temperature patterns with an Resolution better than 0.3-0.4 "C. Do not require cryogenic cooling They are relatively inexpensive and cheaper than photon-detector

Pyroelectric Vidicon Camera

LIQUID-CRYSTAL THERMOGRAPHY Thermochromic liquid crystals are a class of compound that exhibit color-temperature sensitivity. Major disadvantage: It can alter the temperature pattern it is being used to measure

MICROWAVE THERMOGRAPHY Emission of IR radiation from tissue at 30°C is most common at wavelengths around 10 mm. Surface probes which are sensitive to 1.3 GHz (23 cm wavelength) or 3.3 GHz (9.1 cm wavelength) are used in Microwave Thermography Since body tissue is partially transparent at this wavelength, the method can be used to estimate body temperatures at depths of a few cm

MICROWAVE THERMOGRAPHY the intensity of radiation at this Microwave wavelength (10 cm) is 10-8 smaller than that of IR (10 mm) wavelength radiation Suitably designed microwave radiometers placed in contact with the skin surface can be used to detect Microwave radiation.

Physiological Factors Blood perfusion has an important role in maintaining deep-body temperature The rate of blood flow (Venous networks) in the skin is the factor that has the main influence on the internal conductance of the body temp. The higher the blood flow, then the greater is the rates of transfer of metabolic heat from the tissues to the skin Also, the surface temperature distribution of a person depends upon age, sex and fatness, metabolism, and topography

Clinical Thermography application ( a ) The assessment of inflammatory conditions such as rheumatoid arthritis. (b) Vascular-disorder studies including (i) the assessment of deep vein thrombosis, (ii) the localization of varicosities. (iii) the investigation of vascular disturbance syndromes, and (iv) the assessment of arterial disease. (c) Metabolic studies. (d) The assessment of pain and trauma. ( e ) Oncological investigations

INTERPRETATION: There are no significant thermal asymmetries seen in the breasts, and no indication of any neovascularity. The slight vascular patterns seen in both breasts do not appear suspicious but should be monitored for change. This study should be archived and compared with a repeat study in annual testing.

Early detection saves lives

Inflammatory Cancer Fibrocystic Fibrocystic Ductal Carcinoma