Thyroid Uptake Process

1. After measuring the radioactive iodine pill in the dose calibrator it is placed into a clear plastic tube (LT). The tube is then places into a thyroid neck phantom (RT)



2. The thyroid neck phantom is then placed under a probe and the pill is counted for 1 minute. There are several issues that must be addressed at this point to assure the correct counting method is being applied
   1. The probe must always maintain the same distance - from phantom to the patient's neck. If the distance varies what do you think might might happen? Consider the inverse square law
   2. Usually most clinics use a distance between 20 and 25 cm
3. The counting of the pill determines the zero hour counts for the pill. This value will be saved and then decayed to a 24 hour value
1. The patient swallows the pill and is instructed to return 24 hours later
2. Some clinics also count the patient's neck at 4 to 6 hours to obtain additional uptake data



4. At 24-hours post the patient's neck is counted (in the area of the thyroid). Several key points must be applied to assure that the application is being done correctly
   1. Distance between the patient's neck and the probe must be the same as the distance between the probe and thyroid neck phantom (taken at 0 hours)
      1. The bar extended from the probe shows the distance on the scale B and its mark on the scale C
      2. The bar is then extended to that distance where A should be touching the neck
      3. The neck is then counted for 1 minute
   2. The 0 hour count must be decayed to the time in which the patient's neck at 24 hours, however, if there is 15 or more minutes from 24 hours then the correct decay time must be applied
   3. As noted in the previous lecture the following formula is applied


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Images were obtained from the following website
http://www.mcg.edu/radscape/NuclearMedicine/ThyroidPhantom/neck_phantom.htm (This site appears to no longer be active)