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 of the object’s temperature. 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 skin temperature changes, i.e., tissues that heat the skin.
This technique began to be used more intensively in the 1990s, following the progress of technology in 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 skin’s surface and displays it as a map of the body in colors or gray tones on the monitor. Temperatures of interest in 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 thermogram changes 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, i.e., their dysfunction. The thermogram is an indicator of further diagnostics, which confirms the change and the possibility to monitor the effect of the therapy.
Blood flow is under the influence of the sympathetic nervous system. You can monitor body warmth with an accuracy of 0.01 C, 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. With 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 an obvious inflammatory process that shows a tendon, joint, capsular, or muscular disease.
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 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 detector’s sensitivity is sufficient.
Infrared camera sensor system
Considering that the human temperature goes up to 310 Kelvin, we get that the emission’s maximum wavelength 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 is due to the assumption that a black body is 1%, ie. 0.3 Kelvin. The body’s total radiation energy in a cold environment is up to 1 kW.
Cooled infrared detectors. The cooled detectors are in vacuum containers and cooled to liquid nitrogen’s temperature. 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 separate the body’s radiation.
Cooled infrared detectors
The cooled detectors are in vacuum containers and cooled to liquid nitrogen’s temperature. 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 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 microbolometer 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 cooled detectors’ resolution, but their price is significantly lower.
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.
The human has a larger dynamic range for grayscale, but sometimes it is a 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.
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 with 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 patient’s camera. 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 eliminates the “one-way” signal. The detector’s alternating signal is amplified, corrected, and, along with the filter, the bandpass (the chopper’s frequency determines medium-frequency) is fed to the display system. The gain is usually 120 dB.
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 for the sensor to work, and thermoelectric cooling is used; the camera’s weight 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 kg and 5 seconds to collect 23.5 kg.
The camera’s spatial resolution 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 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 a 10 x “True color” palette or grayscale 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 artifacts are due to reflection that depends on external radiation sources and skin pigmentation. A body exposed to radiation in the infrared light partly reflects that radiation.
Illustration of thermograph operation
To better understand the results of the measurement with an infrared camera, we will show several examples shown on the website that describe the camera “Thermomed Med2000” in detail.
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, leading to changes in the tissues (vascular and neural changes). The right foot temperature 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 why significant median nerve dysfunction.
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.
Female breast thermographs show the convenience of diagnosis. The thermogram shows a normal finding on the upper left panel, 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 about the right.
Other tests confirmed two shots showing carcinogenic changes.
Today, there is an understanding that other methods can diagnose breast cancer changes earlier. That is why the application of more invasive techniques is recommended, but with a better result for patients.
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