Infrared thermography (IRT) for the assessment of microvascular skin blood flow in a specialist connective tissue disease unit
Aim. To establish standardised infrared thermography (IRT) within a specialist connective tissue disease unit, assessing its utility: • for the evaluation of Raynaud’s Phenomenon (RP) in clinical rheumatology and research • for the detection of active localised scleroderma (LS) lesions in paediatric patients and to develop improvements in IRT quality assurance for these medical applications. Methods. For the evaluation of RP, a protocol for cold challenge of the feet was developed and validated. IRT was applied with hand cold challenge for the assessment of response to oral vasodilator therapies in two large randomised pilot studies. An infrared thermometer technique was developed, validated against IRT, and subsequently used for the assessment of peripheral vasospasm in a twin study into the heritability of RP. The utility of inspection of thermograms for detecting clinically active LS lesions was established. A protocol was developed incorporating photography, IRT and laser Doppler flowmetry (LDF) for LS assessment, and the normal range of temperature and LD blood flow across several body sites was established in adults and children. The utility of the protocol for assessing LS activity in children was investigated. To develop quality assurance of thermography, the author contributed to the specification and validation of blackbody medical temperature reference sources, and published guidelines for procuring and commissioning a medical thermal imager. Results. Healthy controls had a higher mean toe temperature than RP patients (at baseline 29.2 ± 1.5ºC v 24.8 ± 1.5ºC [mean ± SD], p<0.01; t-test). IRT demonstrated improved finger rewarming 10 minutes after cold challenge in primary RP patients 11 treated with fluoxetine compared with those treated with nifedipine (58.8% v 43.1%, p=0.03; t-test). IRT showed no such improvement in finger rewarming over nifedipine in patients treated with losartan. In a hospital setting, an infrared thermometer technique performed similarly to IRT with cold challenge for the detection of RP: the sensitivity of IRT was 83%, whereas for the infrared thermometer it was 89%. The specificity of both instruments was 84%. In a population setting using the infrared thermometer both baseline finger temperature and rewarming after ten minutes were significantly lower for RP subjects than for controls (for baseline: 28.3ºC v 30.0ºC, p<0.01, t-test; for rewarming: 4.6ºC v 5.3ºC, p<0.05, t-test). Infrared thermometer measurements in monozygotic and dizygotic twin pairs revealed a heritability of 65% for baseline finger temperature, 35% for fall after cold challenge, and 24% for rewarming over ten minutes. In the larger of two published studies on the inspection of thermograms for detecting clinically active LS, sensitivity was 92%, and specificity was 68%. In lesions imaged within 2 years of onset, sensitivity was 81% and specificity 88%. Validation of a protocol combining IRT and LDF measurements revealed that, in adult controls, the mean temperature difference between the two sides of the body was less than 0.5ºC at all body regions. Mean differences in contralateral LD flux were less than 40% at all body sites. Variability in LD and IRT readings due to experimental factors was acceptably small in comparison to the physiological differences recorded. Applying the protocol in children with LS, the median relative increase in LD blood flow in clinically active lesions (compared with blood flow in contralateral unaffected skin) was 89% (range -69% to +449%), whereas the median flow increase in clinically inactive lesions was 11% (range -46% to +302%), p<0.001. Using IRT, the median temperature difference between clinically active lesions and contralateral unaffected skin was 0.5ºC (range -0.1ºC to +4.1ºC), whereas the median temperature difference for clinically inactive plaques was 0.3ºC (range -1.9ºC to +2.7ºC), p=0.024. 12 In hand cold challenge measurements made at the Royal Free Hospital, application of the medical blackbody temperature reference sources reduced the overall uncertainty in temperature readings by a factor of about 4, from typically ±2°C to ±0.5°C. Conclusion. IRT or infrared thermometer data on skin temperature before and after cold challenge affords RP studies an important element of objectivity. RP detected in a population setting exhibits milder vasospasm than RP recruited from hospital patients, and thus the results of research performed at specialist centres may not be translatable to community settings. Inspection of thermograms is an effective method for the detection of clinically active LS, although LDF performed better than IRT using a protocol reliant on objective measurements from small regions of interest. IRT and infrared thermometry were generally less effective at discriminating between healthy and diseased subjects in situations where the temperature difference between groups was small (<2°C). The introduction of temperature reference sources, which reduce uncertainty in radiometric measurements to the order of 0.5°C, would increase the utility of IRT in settings where the temperature change associated with disease is small.
Thermography, Infrared imaging, Scleroderma (Disease)
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