thermography showing animals and humans
Figure 1: Thermographic image of an animal (left) and human footprints in the dark. The middle panel shows the color distribution in relation to the absolute temperature of the radiating object.

A thermograph is a device that uses a camera that is sensitive to light in the infrared part of the spectrum and thus enables the display of the distribution of radiation sources of heated bodies. We will illustrate the thermographic picture with two examples (Fig. 1).

Thermographic images show infrared radiation originating in both cases from a source that is warmer than the environment. The middle panel in the picture shows the color distribution in relation to the temperature of the object. It is important to mention that the ambient temperature should be sufficiently different from the subject (in the cases shown below 20 C).

Medical digital infrared thermal imaging

Medical Digital Infrared Thermal Imaging (DITI) is a non-invasive diagnostic technique that enables visualization and quantification of changes in skin temperature, ie tissues that heat the skin.

This technique began to be used more intensively in the 1990s, following the progress of technology in the field of infrared cameras and the advancement of computer signal processing from infrared cameras. DITI can be more easily understood as an infrared scanner that measures the radiation coming from the surface of the skin, and displays it as a map of the body in colors or gray tones on the monitor. Temperatures that are of interest for displaying the human body are around 36 ± 10 Celsius.

Visual representation of skin temperature is called “Thermogram”. A thermogram is a spectrum of colors that indicates an increase or decrease in the body’s infrared radiation. Given the symmetry of radiation of a healthy organism and the known distributions of skin temperature that characterize a healthy organism, the changes on the thermogram indicate the existence of some pathology.

How a thermogram works

The basic changes indicated by the thermograph originate from the vascular, muscular, neural skeletal systems, ie their dysfunction. The thermogram is an indicator of further diagnosis which confirms the change, but at the same time the possibility of monitoring the effect of therapy.

Blood flow is under the influence of the sympathetic nervous system. Body warmth can be monitored with an accuracy of 0.01 C, which is sufficient for precise monitoring of surface blood flow.

Neuro-thermography refers to monitoring the skin’s blood flow and locating local changes in certain places on the body. In peripheral nerve injuries, temperature changes of approximately 1.5 C occur, and the magnitude of the changes indicates the severity of the injury.

Rheumatological processes are clearly seen as “hot areas”. This is a very clear inflammatory process that shows whether it is a tendon, joint, capsular, or muscular disease.

Thermographic system

The basic parts of a thermographic system are an infrared camera with a scanning system and a computer that displays a map with a low-resolution temperature of the skin that radiates because it is warmer than the environment.

The thermographic camera has an infrared radiation detector and is connected to a scanning system. Usually, the range of registered temperatures is limited to 298-308 K, so a resolution in the range of about 0.5 K is obtained. Almost all clinical changes that need to be detected are in the range of part of the Kelvin degree, which means that the sensitivity of the detector is sufficient.

Infrared camera sensor system

diagram sun man radiation
Figure 2: Radiation spectra of the human body and the sun

Considering that the human temperature goes up to 310 Kelvin, we get that the maximum wavelength of the emission is around 10 µm. Radiation flux was determined by Stefan-Boltzmann’s law W = σT4. This is true for a black body. W is the flux expressed in W / cm2. Measurement uncertainty due to the assumption that it is a black body is 1%, ie. 0.3 Kelvin. The total radiation energy of the body in a cold environment is up to 1 kW.

diagram 2
Figure 3: Radiation diagram of infrared sensors in the range of interest for thermography

Cooled infrared detectors. The cooled detectors are in vacuum containers and are cooled to the temperature of liquid nitrogen. This increases the sensitivity because their temperature is significantly lower than the subject being recorded. These sensors at room temperature behave like light detectors would not be able to record because the “background” of its own radiation would be too large to be able to separate the body’s radiation.

Cooled infrared detectors

The cooled detectors are in vacuum containers and are cooled to the temperature of liquid nitrogen. This increases the sensitivity because their temperature is significantly lower than the subject being recorded. These sensors at room temperature behave like light detectors would not be able to record because the “background” of its own radiation would be too large to be able to separate the body’s radiation.

The disadvantage of cooled cameras is the complexity of application and price, and the advantage is the image quality (compared to uncooled detectors). The materials used for the detectors are: indium antimonide (InSb), indium arsenide (InAs), cadmium-mercury-telluride (CdHgTe), lead sulfide (PbS), lead selenide (PbSe), and the like.

Uncooled infrared detectors

Uncooled thermal cameras use sensors that work at ambient temperature, and measure changes in resistance, voltage or current caused by thermal radiation. Pyroelectric materials and microbolometric technology are most commonly used. Uncooled detectors give more stable results if their temperature is controlled. The resolution of uncooled detectors is lower than the resolution of cooled detectors, but their price is significantly lower.

slika termografske kamere
Figure 4: Monochromatic image (gray tones) and polychromatic image obtained by associating colors with individual gray tones

Images from infrared cameras are monochrome because the cameras are made with one type of sensor (Fig. 4, left panel). Color cameras are more complicated and have to use more sensors. Chromatic changes in the infrared spectrum do not have the same significance as changes in colors in the domain of visible light. In some cases, monochrome cameras are used to display in color (Fig. 4 right panel), which meant that certain gray tones were assigned a color from the palette to be selected.

Human body thermography
Figure 5: Changing the appearance of the same thermography by choosing different pallets

The human has a larger dynamic range for grayscale, but sometimes it is color perception when colors are used. This technique is called “density slicing”. The color palette selection allows different displays on the DITI system display. In this way, it is possible to visualize the part of interest much better. All transformations are the basic assignment of individual colors to isotherms. 

Scanning system

The scanning system ensures that the degree of heating of individual points on the body is recorded. The optical system that is part of the scanning system is made of silicone lenses that have good anti-reflection. 120 and 200 lenses are used.

Lenses with 120 are more suitable for routine examinations, due to the more convenient distance of the camera from the patient. For measurements at distances less than 1.5 m, a lens with 200 is used. As in photography, rings (macrothermography) are used to move the focal length.

The system has a switching device (chopper) that allows the reference signal to be in phase with the measuring signal and thus eliminates the “one-way” signal. The alternating signal from the detector is amplified, corrected and with the filter the bandwidth of the band (the average frequency is determined by the frequency of the chopper) is brought to the display system. The gain is usually 120 dB.

Thermographic camera
Figure 6: One example of using an infrared camera to diagnose changes in the chest. The left panel shows the monitor (display), and the right panel the person being photographed and the thermographic camera scanning and sending signals to the computer system that forms the image in colors assigned based on the selected palette, and isothermal lines recorded by the camera.

The image shows one of the medical infrared cameras (Meditherm, Med2000) used for diagnostics. Here are some of the basic features as an example: 1) the camera must be cooled in order for the sensor to work and thermoelectric cooling is used; the weight of the camera is about 2 kg and the size is 14 cm x 43 cm x 11 cm. The operating temperature of the camera is between 100 C and 370 Celsius.

The camera collects information using a resolution of 244 x 193 = 47 thousand pixels (kp). The scan speed is 8 seconds to collect 47 kp, and 5 seconds to collect 23.5 kp.

The spatial resolution of the camera is 0.4 mm if the camera is at a distance of 15 cm, and it is reduced to 1 mm when the camera is at a distance of 40 cm from the radiation source. The camera can register signals that are in the range of 283 to 313 K, with a resolution of 0.01 K. The field of view at acquisition is 300 x 22.50. The display system allows observation of 10 x “True color” palette or gray scale with 16 levels.

The display allows monitoring of 3 x 16 levels of isotherms. The dynamic range is 24 bits, with a temperature step of 0.1 to 2 degrees K. The image is recorded in TIFF format with a maximum of 95 kB.

This system also allows several types of analog processing: 1) signal level analysis; 2) thermal amplitude analysis; 3) measuring the video signal and determining the mean, maximum and minimum values on the observed part of the body; 4) area analysis; 5) analysis of individual isothermal areas; 6) analysis of thermal profiles; and 7) selecting the isotherm and displaying it as a profile of that line.

It is important to know that there are artifacts due to reflection that depends on external radiation sources but also on skin pigmentation. A body exposed to radiation in the range of infrared light partly reflects that radiation.

Illustration of thermograph operation

In order to better understand the results of the infrared camera measurement, we will show several examples that are shown on the website that describes the “Thermomed Med2000” camera in detail.

hand thermography
Figure 7: Thermographs of foot pain syndrome (left panel) and carpal tunnel syndrome (right panel)

The painful foot syndrome, shown in Figure 7 (left panel) is a consequence of a calcaneum fracture that was not treated in the best way, so it led to changes in the tissues (vascular and neural changes). The temperature of the right foot is 3.70 C lower than the temperature of the left foot. Figure 7 on the right panel shows thermography made by a so-called cold strs test in which the hand is cooled and then recorded. The picture indicates carpal tunnel syndrome, which is the reason for significant median nerve dysfunction.

knee thermography
Figure 8: Frontal, medial and lateral thermography of the “painful” knee

Figure 8 is a thermogram of a person’s right knee after knee surgery. This surgery is accompanied by severe pain during the rehabilitation period, and thermography shows changes in blood flow and indicates the place that is the focus of pain.

Thermography of female breasts
Figure 9: Thermography of female breasts

Female breast thermographs show the convenience of diagnosis. On the upper left panel, the thermogram shows a normal finding, and the others indicate changes. Red lines on the lower left panel indicate a fibrocystic finding. The fibrocystic finding shows strong blood flow to the left breast in relation to the right.

Two shots showing carcinogenic changes were confirmed by other tests.

Today, there is an understanding that other methods can diagnose changes in the breast earlier, and that is why the application of more invasive techniques is recommended, but with a better result for patients.

Literature:

1. Bronzino, J. et al., (Eds.), Handbook of Biomedical Engineering, CRC Press and MIT Press, NY, 1995.
2. Cobbold, RSC, Transducers for biomedical measurments: Principles and applications, J Wiley, NY, 1974.
3. Macovski, A., Medical Imaging systems, Prentice Hall, NJ, 1983.
4. Webster, J., Medical instrumentation, application and design, II ed, Houghton Mifflin Company, Boston, Toronto, 1992.

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