I n clinical thermography the amount of heat energy received by the detector is interpreted in terms of a distribution of skin temperature but it also depends on the emissivity of the surface. I t has been concluded previously that the emissivity in the range 2-5 pm for skin at normal incidence is about 0.98, and that variations are not likely to represent a difference in apparent temperature of more than f 0 . 5 ' ~. However, theoretical considerations are presented for the variat'ion of emissivity with the angle a t which the surface is viewed. These indicate a significant fall in emissivity as the angle to the normal is increased beyond go", corresponding to a reduction of 4"c or more in apparent surface temperature. Thus it would be possible for a ' hot spot ' associated with significant pathology to remain undetected on a surface viewed obliquely. Examples of this obliquity effect in clinical and experimental thermographs are demonstrated.

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A case of parenchymal and intraparotid lymph node tuberculosis is reported in a young Asian male. The diagnosis was suggested pre-operatively by the contrast enhanced CT (CECT) appearance. There were no systemic symptoms of tuberculosis and this is a characteristic feature of the 100 cases reported in the literature. The presence of thick walled rim enhancing lesions with a central lucency on CECT should suggest the diagnosis. Filling defects with or without thin walls are non-specific findings and are seen in tumours and other inflammatory processes. In an appropriate clinical setting, thick walled round rim enhancing lesions with a central lucency are characteristic of tuberculosis.

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