Parathyroid Case 1 Answers

1) What radiopharmaceuticals have been used? Describe the rationale for this imaging process

The parathyroid imaging, in this case, has been performed with technetium-99m labelled MIBI (hexakis-2-methoxyisobutyl isonitrile). MIBI is a large cationic lipophilic complex. The thyroid imaging has been done with technetium-99m pertechnectate on a different day, although iodine can be used with similar results.

MIBI is taken up into parathyroid tissue more than thyroid tissue and washes out slowly from parathyroid tissue. Normal parathyroid glands are normally 30-40mg and their differential uptake of sestamibi is usually too small to be seen

This is the so-called dual phase MIBI-wash out technique. Early and late images are obtained over a three hour period, here at 15 minutes, with delayed images at 3 hours. It is compared visually against the thyroid image

2) What other imaging protocols are in common use?

Thallium imaging has fallen by the wayside for several reasons. Firstly, MIBI-based parathyroid imaging is more sensitive and secondly, MIBI is lower dose than thallium.

The major argument is now between washout studies and subtraction studies. Washout studies are simpler in that they can be done in a single patient visit and give good results. However, it may be associated with higher numbers of false positives and false negatives Subtraction studies simple digitally subtract the thyroid image from the MIBI image to reveal the underlying adenoma. Subtraction studies can be done on the same day but this does not leave time for it to be combined with a washout study. A two-day combined washout and subtraction protocol can be performed. There is on-going argument as to which is more sensitive and specific.

3) Explain A

There is an area of high uptake, best seen on the thyroid imaging but isointense with the thyroid tissue on the MIBI images. 99% of raised areas of uptake in the thyroid are benign adenomas.

4) Any evidence of a parathyroid adenoma?

No

5) What is the sensitivity of the test?

Dual phase sestamibi has sensitivity for adenomas of around 95-98% but overall for primary hyperparathyroidism is around 80-88%. Sensitivity is essentially based on the size of the abnormal gland, they need to enlarge to 350mg (7x normal) before being visible on scintigraphy. Small abnormal glands may therefore be missed. This is particularly true where there is multiglandular disease, where the sensitivity for the test can drop as low as 55%.

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The text is entirely the opinion of the author and does not necessarily reflect that of RUH NHS Trust or the Bristol Radiology Training Scheme. Website content devised by Paul McCoubrie.