Swine flu and thermal imaging

Published in The Hindu on May 21, 2009

thermal imaging
The biggest limitation of thermal imaging is that it takes 48 to 72 hours for a person infected with H1N1 to show any symptoms.

If screening baggage using X-rays has become the order of the day in airports across the world, many airports have now installed thermal imaging scanners to screen travellers who are likely to be infected with swine flu.

Since one of the symptoms of Influenza A(H1N1) infection is fever, an instrument that can be used for screening people with fever may be an ideal way to identify such cases.

The thermal imaging scanner, first introduced during the SARS outbreak, has become the first choice for many countries to screen people with fever, possibly due to H1N1 infection.

The principle

It works on the principle that any object above the non-zero temperature radiates electromagnetic waves of different wavelengths, including infrared. The thermal imaging scanner picks up the IR emitted by a body. In other words, thermal imaging scanner picks up IR emitted by a body, much like an ordinary camera picks up visible light reflected by a body.

The scanner picks up IR in the 8-14 micrometre range, which corresponds to a room temperature of 25-30 degrees Celsius. Though the normal body temperature is 37 degrees Celsius, the tail of the bell shaped curve formed by human body temperature will be seen at the 8-14 micrometre window of the scanner.

“All the organs of a person are not at the same temperature. For instance, eyes are the coldest and groin is the hottest region in a human. The stomach becomes warmer when digestion takes place,” said Prof. C.V. Krishnamurthy, Principal Project Officer, Centre for Non-Destructive Evaluation, Mechanical Engineering Department, IIT, Madras. “So what we get to see is a thermal map of a human body.” Different temperatures are assigned different colours. “The scale can be chosen by us,” he said. When a person with fever is scanned, the IR emitted by that person will be higher and the tail of the bell shaped curve will be elevated. Hence the same part of the body of a person with fever would show up brighter. A scanner looks for such changes.

The biggest advantage

The biggest advantage is that screening can be done quickly and does not require any physical contact with a person. With the possibility of studying temperature differences of even 0.1-0.2 degrees Celsius, thermal imaging can easily identify people with fever.

There are several limitations, though. For one, it can only identify people with fever; it cannot tell if the fever is due to swine flu. Since clothing can alter the thermal image produced, only the exposed parts of the body can reveal the real body temperature.

By far the biggest limitation is that it takes 48 to 72 hours for a person infected with H1N1 to show any symptoms. That means an infected person will not have fever for the first 2-3 days, and hence will be missed by thermal imaging scanners.

But an infected person can spread the virus even before symptoms show up. The incubation period for H1N1 is 24-48 hours. That is the reason why the WHO does not support the concept of screening travellers to prevent the spread of the infection.

It states: “Although identifying signs and symptoms of influenza in travellers can help track the path of the outbreak, it will not reduce the spread of influenza, as the virus can be transmitted from person to person before the onset of the symptoms.”