1. INTRODUCTION
Preventive measures for the early detection of people with possible contamination by SARS-CoV-2, in the verification of thermal changes related to febrile symptoms, are being carried out all over the world. Body temperature screening actions aim to reduce the negative impact on health services and limit the spread of the virus on a large scale in society. Therefore, infrared thermometers in conjunction with the acquisition of thermographic images are being used for rapid assessment [1]. In places with large flow of people, such as airports, shopping malls, companies, military organizations, schools and hospital reception areas, the protocol adopted worldwide to carry out a preliminary approach is based on a temperature body check, as infected people may have fever as one of the symptoms of viral contamination. Thermal imaging cameras for medical use are also being adopted at airports [2], as they can capture reliable values of body temperature on the face up to 1 m away, favoring the possibility of detecting feverish thermal diagnostics, preventing the movement of these passengers in aircraft embarkation and disembarkation spaces and restricting the possibility of transmission.
For an initial assessment of rapid screening, Singapore Standard protocols and technical references, which regulate the parameters of use of medical devices, should be followed. The Standard: SS 582-1: 2020, referring to the specification for thermal imagers for human temperature screening, comprises the necessary requirements for their use and standardizes the thermal screening method [3]. Standard SS 582-2: 2020 presents the specification for thermal imagers for human temperature screening as well as its implementation guidelines [4].
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The technical procedures adopted must comply with the IEC 80601-2-59 [5] standard, this normative standard regulates the basic safety and essential performance of thermal cameras, intended for use in non-invasive individual screening in closed environments, allowing information from portable infrared sensors to be accurate and reproducible. Thermal data fidelity is based on temperature acquisition in accordance with the standard technical protocols established by the International Organization for Standardization, ISO/TR 13154, for Medical Electrical Equipment, which provides technical operational guidelines for the identification of febrile conditions in the use of rapid screening by means of thermography [6].
However, the human temperature is depending on the body site region, higher temperatures are found in the core, while the skin and the extremities are usually colder [7].
The assessment of body temperature requires knowledge of human physiology, and strict following of the correct data acquisition protocols, as to get the most accurate thermal data. Aspects related to the use of medication can favor thermal changes such as the use of analgesics, corticosteroids and nasal decongestants, the ingestion of beverages containing caffeine (coffee / black tea) or beverages with alcohol content, as they cause vasodilation and, therefore, change the measured temperatures [3, 8, 9]. Dehydration can also cause an increase in body temperature; however, smoking, on the contrary, causes peripheral vasoconstriction and a consequent decrease in the temperature of the extremities, invalidating measurements on the wrist and hands. Psychological aspects such as fear, anxiety and stress can also influence screening for peripheral vascular vasoconstriction and consequent temperature decrease [10]. Other variables can also affect the measured temperature, individual factors such as age, physical activity, temporal variables such as the circadian rhythm, which express different temperatures during the day, the menstrual cycle, or pregnancy [8, 11] can cause an elevated body temperature. In these cases, a reported high temperature with IRT (Infrared Thermography) might be mistaken. The spatial variables such as body volume, skin scars, environmental conditions, data acquisition in a non-controlled temperature environment or outdoor (Sun, wind, etc.) can lead to screening errors [10, 12].
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Care should be taken when measuring temperature, with acquisitions in specific regions. The subject must undergo an adaptation period, as the body was under the influence of the outdoor environment temperature. The thermometer must be periodically calibrated by a metrology laboratory (International Temperature Scale 1990 (ITS -90) [13], and it must be positioned perpendicularly to the skin surface.
Environmental conditions must be observed and controlled in the screening room, doors and windows must be closed to avoid drafts that could interfere with the accuracy of the measured temperature. The room temperature must be kept between 21 to 23°C, temperature screening must be away from incident lights on the subject such as lamps or sunlight, and relative humidity maintained between 40 to 75%. The subject should stay far from electrical wires, thus minimizing the interference of infrared energy from external sources, and without metallic objects in the vicinity that could cause reflection to the investigated area [3].
If some of these errors are unavoidable, the standardization of regions is essential in the infrared thermometry exam to reduce systematic errors and make the exam reproducible. The most important aspects regarding temperature acquisition are to be captured perpendicular to the surface of the body, and very close to the skin, usually from 0-80 mm away, as greater inclinations or distances reduce the radiation received by the sensor.
In view of these technical errors, it is observed that the temperature measurements on forehead, wrist and temple are used as reference worldwide in this pandemic period [14].
In many countries, the initial infrared thermometers screening was carried out in the central region of the forehead. However, in the central portion of the skull, no superficial artery passes by. The supratrochlear artery, a terminal branch of the ophthalmic artery, passes in this proximity but deep, through the posterior part of the trochlea of the superior oblique muscle of the eyeball [15]. Its anatomical course is curved, passing through the sides of the glabella, close to the root of the eyebrows, 1 cm on the side of the central point of the frontal region, as shown in Figure 1 (a). The superficial temporal artery may be the most suitable body region for initial rapid screening on the face. This artery originates from the external carotid artery, his path crosses the posterior root of the Zygomatic process of temporal bone, divided into two branches, one frontal and the other parietal [15], being more apparent in the frontal branch, as shown in Figure 1 (b). Temporal artery temperature is close to rectal temperature and therefore accurately reflects a measure of core body temperature [16, 17].
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Thermographic images corroborate the anatomical and physiological information of facial thermal variability and demonstrate the distribution of temperature as a function of its vascularization and according to the path of veins and arteries.
Larger blood vessels have a higher temperature than the small ones, thus the proximity to blood vessels increases local temperature, while terminal portions of vessels with a smaller caliber, such as hands and feet show lower temperature [8, 9].
The temperature scale is mapped by a rainbow color palette, which represents different temperatures. The white color is the highest temperature and the black the coldest one, as shown in the thermographic images Figure 1(b) front view, and Figure 1(d) side view.
The most reliable point would be the trunk, at the upper sternal region – blouse’s neckline; however, this region is normally covered by clothing, and thus this site could cause an embarrassment, therefore preventing a quick tracking.
The temperature in the wrist region, tend to show larger variations. It is also usually cooler than the other sites, and more sensitive to environment changes. Additionally, the dorsal portion of the hand and wrist are warmer on the radial side and cooler on the ulnar side [8, 9]. Thus, the quality of the results is somehow dependent on the skill and experience of the health professional [9, 12]. Technical errors are commonly made when positioning the infrared sensor instead of pointing to the region of the styloid process of the radius bone, where the radial artery is more superficial, they point to the vicinity of the ulnar region, not respecting the distance of the sensor, or making obliques readings Figure 2. illustrates the forearm and hand and the path of the radial artery that runs laterally to the wrist, being a palpable artery medially to the styloid process of the radius bone. It is divided into palmar and dorsal carpal branches, which contribute to the formation of carpal arches and the muscle branches, which supply the anterolateral muscles of the flexor and extensor compartments.
The lack of the anatomy and physiology knowledge of the Covid-19 tracking personnel and absence of technical-scientific consensus on the standard measurement site for IR thermometer screening motivate us to study three regions: temple, forehead and wrist, in white, black and brown volunteers. Additionally, it was also evaluated the hypothesis that different skin tones could show changes in thermal reflectance. it was considered that the differences in the constitution of epidermal pigmentation in relation to the main skin chromophores (melanin and hemoglobin) could eventually be an intervening factor in obtaining the thermal values of the initial screening, according to their ethnical characteristics.
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