Calcification-related absorption in thyroid scintigraphy

The enhanced absorption of X-rays in calcified structures is a basic prerequisite for performing conventional bone radiography. On the other hand, nothing has been reported on possible absorption effects of 'calcifications' as frequent findings in thyroid nodules or in the sternal bone / sternoclavicular joints. This may be caused by the general opinion, that the high energy of 99mTc-photons (140 keV) do not make visible absorptions effects very likely.

To prove possible absorption effects of calcifications on thyroid scintigraphy experimentally, effects of calcium absorbers were tested on a technetium flood phantom. Furthermore, absorption effects of various calcifications (discs of calcium sulphate and calcium carbonate with varying thickness) on normal thyroid tissue and autonomous nodules were simulated in a thyroid phantom. CT 130 kV-images of 46 consecutive patients were checked for presence of retrosternal or retroclavicular growth of the thyroid gland and to measure the extent and density of the sternal bone and calcified intrathyroidal nodules. In addition, clinical cases are presented in which a possible absorption by calcifications seems to be likely.

Bony structures in front of the thyroid gland or calcified intrathyroidal nodules could be seen on CT in 24/46 patients. The mean averaged density was 219 Houndsfield units (SD: 89 HU). The quantitative measurements using a 99mTc-flood source showed a mean absorption of 4.9%. In a thyroid phantom, absorption effects were visible only in 3/20 positions of the calcium discs over the thyroid phantom. Focal effects could be better detected in situations of only moderate uptake of the surrounding tissue. A dependence of absorption and chemistry (sulphate, carbonate) could not be found.

Visible absorption effects caused by sternal bone or thyroid calcifications are seldom but potentially able to diminish the visible uptake and should be taken into account when interpreting thyroid scintigrams.