Thermography

Thermography records the infrared heat given off by the skin and shows it as a map of surface temperature. A camera detects wavelengths of 7 to 14 micrometres, converts them to temperature, and displays them on a colour scale. No radiation, contrast or contact is used.

Skin temperature reflects blood flow, sweating and the metabolism of tissue just under the skin. An area that is warmer or cooler than the matching area on the opposite side can signal inflammation, vascular disease or nerve dysfunction.

Modern cameras detect differences of 0.03 to 0.05 degrees Celsius and have a resolution of 320 by 240 pixels or higher. The examination takes 10 to 20 minutes, after the patient has been left in a controlled room.

Thermography shows function, not anatomy. It cannot show deep lesions, and it cannot tell the cause of a temperature change.

It has no proven place in the diagnosis or screening of breast cancer. Its sensitivity is much lower than mammography, and it must never replace mammography, ultrasound or MRI.

Its main uses are the study of skin blood flow and the monitoring of inflammatory and vascular disease, and as a research tool.

Indications.

• Raynaud phenomenon: confirmation of abnormal rewarming of the fingers after cold challenge, and differentiation of primary and secondary disease.
• Peripheral vascular disease and diabetic foot: detection of areas of reduced perfusion or raised temperature before ulcer formation, and monitoring of foot temperature.
• Complex regional pain syndrome: asymmetry in limb temperature (over 1 degree Celsius) as supporting evidence.
• Inflammatory arthritis and soft tissue inflammation: heat at affected joints and monitoring of response.
• Peripheral nerve injury and sympathetic nerve dysfunction: changes in skin temperature along the nerve territory.
• Assessment of burn depth, flap viability and perforator mapping before reconstructive surgery.
• Fever screening at borders (limited, with low accuracy for infection).
• Monitoring of treatment in clinical research.
• Thermography has no accepted role as a test for breast cancer.

Contraindications and precautions.

• No physical contraindications. The test is safe in pregnancy and in children.
• The examination room should be at 20 to 24 degrees Celsius, free of draughts, with stable humidity.
• The patient rests for 15 to 20 minutes with exposed skin to reach thermal balance.
• Creams, lotions, powder, deodorant and cosmetics are not applied to the area.
• Exercise, hot baths, massage, sunbathing, physiotherapy and sauna are avoided for 2 to 4 hours.
• Caffeine, nicotine and alcohol are avoided for several hours, because they change skin blood flow.
• Vasoactive drugs, such as vasodilators, beta blockers and nitrates, are recorded.
• Hair, dressings and clothing hide the skin and should be removed from the area.
• The patient must not be febrile or very anxious, since this changes skin temperature.

Preparation and technique.

The patient undresses and sits or stands in the room for 15 to 20 minutes. Medicines, symptoms and the site of interest are recorded.

The camera is placed 1 to 2 metres from the patient with a set angle, distance and background, and the emissivity of skin (0.98) is entered. Views are taken from the front, back and sides. Several images are taken for comparison.

For Raynaud phenomenon, a baseline image is followed by a cold challenge (hands in water at 20 degrees Celsius for 1 minute), and then images for 15 to 20 minutes to measure rewarming.

For the diabetic foot, both soles are imaged, and the temperature of matching points on the two feet is compared.

Images are analysed with regions of interest, temperature profiles, and the difference between sides.

Normal appearances.

Skin temperature is symmetrical, and the distribution is stable. The trunk is warmest, and the hands, feet and nose are cooler. Mean skin temperature is about 32 to 35 degrees Celsius in the trunk and 28 to 32 degrees Celsius in the extremities in a room at 22 degrees Celsius.

A difference of under 0.5 to 1.0 degree Celsius between matching points on the left and right sides is normal. Veins near the surface may appear as warm linear structures.

Interpretation.

• Asymmetry: a difference between sides of over 1.0 degree Celsius is abnormal, and over 2.2 degrees Celsius in the feet of diabetics signals a risk of ulceration.
• Hot area: inflammation, infection, increased blood flow, tumour, venous disease or recent injury.
• Cold area: reduced blood flow from arterial disease, vasospasm, sympathetic dysfunction, oedema or nerve injury.
• Raynaud phenomenon: delayed rewarming after cold challenge (over 15 to 20 minutes), with cold fingers, suggests abnormal vasospasm. A clear cut off between fingers and palm is typical of secondary disease.
• Peripheral arterial disease: cold feet or toes, with a cold segment along the limb.
• Complex regional pain syndrome: persistent asymmetry (warmer or colder than the opposite limb) with a pattern that does not follow one nerve.
• Inflammatory arthritis: increased temperature over the joints.
• Neuropathy: temperature change in a nerve distribution.
• Surgical use: warm spots mark perforating vessels.
• Findings are described as hot, cold, symmetrical or asymmetrical, with the temperature difference stated. They always need clinical and imaging correlation.

Critical values and findings needing immediate action.

• Thermography rarely shows an emergency by itself.
• A cold, pulseless limb with pain: urgent vascular assessment, since this is acute limb ischaemia.
• A hot, swollen, red area with fever: infection or deep vein thrombosis, and urgent assessment.
• Hot foot in a diabetic patient with pain or swelling: suspected infection or Charcot foot, and urgent referral.
• Digital ischaemia with a cold, dusky digit: urgent treatment to prevent gangrene.

Limitations and artefacts.

• Thermography measures surface temperature only. It does not show deep tissue or give a diagnosis.
• Sensitivity and specificity are low and variable, and there is no firm cut off that works for all patients.
• Room temperature, air flow, humidity, time of day, exercise, food, drugs and menstrual cycle change results.
• Skin emissivity, hair, scars, tattoos, moisture and creams cause errors, and reflections from nearby warm objects produce false hot spots.
• Camera calibration, distance and angle need control, and results are not comparable between cameras.
• Training for operators and readers is variable, and no widely accepted reading method exists.
• Sympathetic activity, anxiety and fever change skin blood flow.
• Thermography has not been shown to reduce deaths from breast cancer. Use in its place delays diagnosis.
• Commercial claims for screening without proof are misleading and may cause harm.
• Cost and limited access.

Special groups.

• Children and pregnant women: safe, since no radiation is used.
• Patients with diabetes: foot temperature monitoring at home is helpful for ulcer prevention.
• Patients with Raynaud phenomenon: cold challenge procedures need a controlled room.
• Patients with scleroderma or lupus: used for digital perfusion and response to treatment.
• Patients with burns: used for depth estimation in some centres.
• Regions: thermography is available in few places and is used chiefly for research. Clinical examination, pulse examination, Doppler ultrasound and ankle brachial index are the practical tools for vascular disease, and mammography, ultrasound and clinical examination for the breast.

Follow up tests.

• Doppler ultrasound of arteries and veins.
• Ankle brachial and toe brachial index.
• Nailfold capillaroscopy for Raynaud phenomenon.
• Antinuclear antibody and other autoantibodies.
• CT or MR angiography.
• Plain radiographs and MRI for joint and bone disease.
• Nerve conduction studies.
• Mammography, breast ultrasound and breast MRI for any breast concern.
• Referral to rheumatology, vascular surgery, podiatry or pain medicine.