A dual detector system for thyroid uptake measurement

An account is given of past efforts to standardise thyroid uptake measurement techniques so as to afford reasonable accuracy and permit intercomparison of results. Particular reference is made to the work of the International Atomic Energy Agency in this field. Recommendations contained in the report of the panel of consultants set up by the Agency resulted in a general improvement in the accuracy of measurement accompanied by a reduction in the dose to the patient. However, even when the I.A.E.A. panel's recommended procedures are employed, relatively large measurement errors may still arise mainly through failure to take account of the varying response of a front directed detector to activity at different depths in the neck. The response to extrathyroidal activity is shown to be relatively small when well designed detectors are used. It is adequate to apply a correction derived from a phantom, without carrying out a separate measurement on each subject. A technique of measurement is described using two opposed detectors. This allows uptake determinations, with either 131I or 132I, to be made quickly and conveniently, whilst taking acount of all the main sources of error. It is shown that such a system greatly reduces errors due to variation in the depth distribution of the activity in the neck. At the same time, as used here, it does not lead to an enhanced response to extrathyroidal activity.