biodex ATOMLAB 950 - Determining Automatic Count Time

NOTE: If there is an existing patient and at some future time another uptake study with a new dose will be
administered, you would go to the Patient Definition screen, select the desired patient, and either select an
existing study or click on <Add Test To Selected Patient>. This allows the addition of a new test to the selected
patient. This can be another uptake study with a new dose, a Schillings test or some other function.
DETERMINING AUTOMATIC COUNT TIME
If automatic counting is selected in the Procedure Definition, the time period for each measurement
becomes a variable. Measurement will continue until a predetermined accuracy is reached as determined
by random counting statistics.
For automatic measurements, statistical uncertainty is user defined (i.e., 1%, 3%, 5%, etc.). The time is
based on the radioactive level of activity being measured, the background level, the type of measurement
being made, and the user selected percent accuracy. The system performs an internal check to ensure the
count was accomplished with an acceptable degree of certainty (i.e., appropriate counting rate, proper
spectrum, etc.). The level of accuracy is also set from the Preferences screen.
With automatic counting selected, each count time for a thyroid uptake measurement is automatically
determined, using counting statistics, by imposing an overall accuracy requirement on the uptake result.
There are four count times which comprise an uptake measurement: Standard, Lab Background, Thyroid
and Patient Background. Each count resulting from a lapsed count time has a statistical uncertainty which
is related to the count. When these four independent counts are combined to yield the uptake value, the
uncertainties must also be combined to yield the total uncertainty of the uptake. This total uncertainty is
preset by the user, in Setup Preferences, when the instrument is installed.
At the start of an uptake procedure, the total uncertainty is partitioned between the four count times in a
rational way which minimizes the count times for all four counts and thereby minimizes the amount of
time the patient must remain immobile. Because the count rate is high, the Standard count time is usually
short (~6 seconds). Time for the Lab Background is determined from the first few seconds of Lab
Background counting. If the count rate is very low, the count time will be short because the net error of
(Std.-Bkgd.) contains only a term proportional to (Bkgd. rate/Std. Rate).
Thyroid count time is also determined during the first few seconds of Thyroid counting. It is calculated
from the approximate Thyroid rate and the Standard count rate in such a way that the Thyroid
measurement uncertainty will be only a fraction of the total target uncertainty. Finally, the Patient
Background is determined during its first few seconds of counting with the requirement of the total
uncertainty satisfying the accuracy goals.
Examples
The following academic numerical examples should provide some idea of the four required measurement
times in actual practice. Each example assumes a user-selected counting accuracy requirement of 98%.
This means that there is a 2% allowed counting error at the 90% confidence level. If you repeat each of the
four measurements and calculate the uptake, nine out of ten results will fall within 2% of the first
measurement.
NOTE: You must select the accuracy requirement in the Uptake Procedure Setup loop. The factory setting is 95%,
if you want 98% as illustrated in this example, you must select it. The counting times get longer with higher
accuracy requirements.
NOTE: The “2%” uncertainty is only with respect to detector counting. If a measurement is “repeated” with
repositioning of the standard or patient, there will be additional geometry setup errors which will add to the
counting errors.
NOTE: The % uncertainty is not an uptake %, but a percentage of the uptake. For example, if the uncertainty is 2%
and the uptake is 50%, then the uncertainty is 2% of 50%, or ±1%, and the uptake range could be 49% to 51%. If
the uptake was 10%, the uptake could be 9.8% to 10.2% (2% of 10% is 0.2%).
CONTENTS
4-5 THYROID UPTAKE