THERMOLOGY: THE EQUIPMENT

The first generation of electronic infrared cameras were bulky electro-mechanical-optical devices with revolving prisms and tilting mirrors run by synchronized electric motors that scanned a two-dimensional image across a single-element detector one point at a time. The first models required twenty minutes to completely scan a single image. These were analog instruments and the images produced were non-quantitative. However, they heralded a totally new means of imaging what had previously been invisible; thermal energy and with it a new world of information. Unlike visible light, infrared energy is evaluated directly from a source rather than by reflection. There are no colors inherent to infrared, just energy levels that can be quantified in meaningful units, quantum or degrees Celsius. 

The very large costs involved in developing this technology were borne by military intelligence and applied to surveillance from orbiting satellites. The first non-military application of this technology was for medicine and the first medical application was to evaluate breast cancer (1). While infrared imaging has been slow to integrate into common application in Medicine, infrared imaging has widely applied into many scientific, engineering and industrial applications. These demanding technical applications of infrared imaging have spurred the development of solid-state cameras with focal plane array detectors; essentially tens of thousands of individual infrared detectors on a chip. These cameras have provided great improvements in equipment reliability, frame rate and spatial resolution. 

As detector technology has improved, it is possible to place more of these detector elements into the focal plane array for higher spatial resolution and improve the sensitivity of those detector elements for higher thermal resolution (2). There are a variety of infrared cameras available to meet the many applications that range from industrial preventive maintenance to medical diagnostic imaging and research.

Infrared imaging systems intended for medical application are regulated by the US Food and Drug Administration under Title 21, Parts 800-898 of the Code of Federal Regulations. The ‘510(k)’ provision of this regulation only requires the infrared imaging system be “substantially equivalent to devices legally marketed in interstate commerce prior to the May 28, 1976 enactment of the Medical Devices Amendment to the Federal Food, Drug and Cosmetic Act.” The rate of technological developments of infrared imaging systems since that time has been at least as great as the developments of the personal computer. Consider how much more powerful is the contemporary personal computer than was available in 1976 and you can appreciate the relatively low technologic standards by which infrared imaging systems can be legally sold as a class one medical device.

The infrared imaging systems that are used in medical applications today fall into two categories: thermal imagers and radiometric infrared cameras.

Thermal imagers typically produce a non-calibrated analog or non-quantified digital output signal based upon the level of infrared energy emitted by the body. In order to obtain a temperature measurement of any point within the image, the signal output of the thermal imager must be associated to a calibrated temperature reference (blackbody). This temperature reference is, usually, external hardware that enables an offset adjustment to the imaging software running on a computer. The imaging software must be scaled to a calibration equation that interpolates the temperature values along a mathematical curve that is fit between the levels of the calibrated temperature reference sources. This interpolation process must be done continuously since thermal imagers have no built-in thermal drift compensation which causes the temperature calibration scale level to “float” over changing conditions in and around the thermal imager (2). Thermal imagers are speculative tools for any type of quantitative application that will become increasingly unreliable between calibrations due to thermal drift of their detectors but are marketed for medical application as they are significantly less costly.

Radiometric infrared cameras operate in a manner that is markedly different from infrared imagers.  Radiometric infrared cameras perform the quantitative thermal measurements of a patient's emitted infrared energy within the firmware of the camera by continuously calculating digital temperature measurements through a large thermal span and not just scaled to two externally calibrated temperature standards.  The radiometric infrared cameras also provide for thermal drift compensation that guarantees the temperature measurements are stable and accurate at all points within the camera operating temperature range (i.e. the values do not “float” as with non-calibrated thermal imaging cameras). Radiometric infrared cameras should be considered quantitative instruments as they produce digital temperature measurements internally and output this data to the computer rather than requiring the computer’s software to perform an interpolation of the temperature values (2).

 BACK 

Sources

1. Lawson RN: Thermography-A new tool in the investigation of breast cancer. Can Serv Med J. 1957;13:517-518. 

2. Dereniak E.L. and Boreman G.D. (1996) Infrared detectors and systems. John Wiley & Sons, New York, pp. 526- 538.