Medical infrared thermal imaging, also known as thermal imaging or thermography, is a valuable noninvasive diagnostic imaging technique increasingly used in clinical practice. Medical infrared thermal imaging involves the recording of cutaneous thermal patterns generated by the emission of heat; these patterns are recorded in the form of a color map that is then interpreted.1–5 Medical infrared thermal imaging relies on sympathetic nervous system control of skin blood flow to reflect sympathetic nervous system response to dysfunction. There is no direct conduction of heat from deeper structures to the skin for heating or cooling.1,2,6–13 In this regard, MITI may be used as an adjunctive imaging technique to aid in the interpretation of physical examination findings, as a screening test to help guide therapeutic management, and as a means to assess long-term response to treatment.1–4,12,14–21 In humans, MITI has been successfully used for evaluation of burn patients22 and assessment of breast cancer,22,23 vascular disorders,7,10,13,24 scrotal varicocele,25 pneumothorax,26 radiculopathies,27 intervertebral disk disease,28,29 Chiari malformation and syringomyelia,30,31 pulpal blood flow to teeth,32 and joint disease.17,20,33–38 In veterinary medicine, it has been used in lameness evaluations to detect ligamentous, osseous, muscular, articular, and neurologic injuries in horses, cats, dogs, and raccoons.3,4,18,19,39–43
Hyperthyroidism is a multisystemic disease defined as an excessive production and secretion of thyroid hormones. In humans, the most common cause for hyperthyroidism is Graves’ disease, which is associated with an autoimmune disease process.44,45 Toxic nodular goiter affects geriatric humans; in this condition, one or more hyperfunctioning adenomatous nodules are present in the thyroid gland. Toxic nodular goiter closely resembles hyperthyroidism in cats46; however, the true etiopathogenesis of feline hyperthyroidism remains unknown. Thyroid carcinoma has been documented in 1% to 3% of hyperthyroid cats47–51 and is the primary cause of hyperthyroidism in dogs. Excessive circulating concentrations of thyroid hormones can increase metabolic rate and sensitivity to catecholamines, both of which cause cardiovascular and metabolic abnormalities.47,52–55 These abnormalities can perpetuate multisystemic organ dysfunction.
Hyperthyroidism develops in cats > 6 years old, with a mean age of onset of 12.8 years.52,56 There is no known breed or sex predilection, unlike in humans, among whom females are 4.5 times as likely as males to develop hyperthyroidism.57–59 In cats, the most common clinical sign is polyphagia with concurrent weight loss.52,56 Approximately 10% of cats will have signs of depression, weakness, progressive weight loss, and anorexia, and often these signs are a consequence of concurrent nonthyroidal illness. In humans, this nonthyroidal illness is described as apathetic hyperthyroidism.56 Clinical signs and physical examination findings of cats with hyperthyroidism may include weight loss, polyphagia, hyperactivity, tachycardia, polyuria, polydipsia, cardiac murmurs, vomiting, diarrhea, increased fecal volume, anorexia, polypnea, dyspnea, muscle weakness or tremors, and congestive heart failure. Poor coat quality (matting and shedding) have been described as well. Palpably enlarged thyroid gland lobes are evident in approximately 90% of hyperthyroid cats,47,49,52,60 although normally sized thyroid glands have also been reported.61
The clinical signs of hyperthyroidism in cats are considered nonspecific, and a variety of disease processes may possibly result in any combination of these signs. For most cats, establishing a diagnosis of hyperthyroidism is achieved by detection of clinical signs consistent with the disease, the presence of a palpably enlarged thyroid gland, and high serum thyroxine concentration (> 4.0 mg/dL).47,52 Other diagnostic techniques that may be used to evaluate a cat for hyperthyroidism include the triiodothyronine suppression test, thyrotropin-releasing hormone stimulation test, nuclear scintigraphy, thoracic radiography, thoracic or cervical CT, and cervical ultrasonography.47,52,61 The assessment of basal serum free thyroxine concentration by equilibrium dialysis, in combination with serum total thyroxine measurement, may aid in the diagnosis of hyperthyroidism in cats with mild disease, for which previous total thyroxine concentrations had remained within the reference interval.47 Nuclear scintigraphy with technetium Tc 99m compounds and the use of a gamma camera is very effective in identifying hyperfunctioning thyroid glands as well as locating possible ectopic thyroid tissue47,50,61; however, nuclear scintigraphy is expensive, requires patient quarantine, and necessitates regulatory controls to limit radiation exposure to both patient and operator.
Medical infrared thermal imaging may have potential as a noninvasive screening test for hyperthyroidism in cats. Studies62,63 involving MITI in humans with thyroid gland disease have revealed that this imaging technique is less expensive than scintigraphy (infrared cameras cost $8,000 to $18,000), noninvasive, and safe. Because MITI does not emit radiation, the procedure is not under legal regulation and does not require isolation of patients. The purpose of the study reported here was to determine the usefulness of MITI as a screening tool for hyperthyroidism in cats, evaluate the need for hair clipping over the ventral aspect of the neck to achieve optimal images, and determine whether there is a change in thermal patterns at 1 and 3 months after radioactive sodium iodide I 131 treatment.
Medical infrared thermal imaging
Med 2000 IRIS, Meditherm Inc, Beaufort, NC.
CVIPtools, Computer Vision and Image Processing Laboratory, Department of Electrical and Computer Engineering, School of Engineering, Southern Illinois University, Edwardsville, Ill.
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