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)
The thyroid neck phantom is then placed under a probe and counted for 1 to 3 minutes. There are several issues that must be addressed at this point to assure that the correct counting method is being applied
The probe must always maintain the same same distance - from phantom to the patient's neck. If the distances vary what do you think might might happen? Consider the inverse square law.
Usually most clinics use a distance between 20 and 25 cm
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
The patient is swallows the pill and is instructed to return 24 hours later
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
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)
The amount of time counted should be same at 0 and 24 hours
The 0 hour count must be decayed to the time in which the patient's neck was counted (usually somewhere around 24 hours
As noted in the previous lecture the following formula is applied
