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Biodex Medical Systems, Inc. © 2016 237
Linearity
Linearity means the proportionality of
the measurement result to the activity
measured, as determined over the
intended range of use for the dose
calibrator.
This test is done using a vial or syringe
of Tc-99m whose activity is at least as
large as the maximum activity normally
assayed in a prepared radio
pharmaceutical kit, in a unit dosage
syringe, or in a radio pharmaceutical
therapy, whichever is largest.
There are several acceptable methods
for preparing linearity testing. The
difference in these methods is which
reading is used for decay correction of
the activities. The following methods are
all acceptable for determining the
normalized value.
a. Decay correct the test sample from
the first reading and compare each
reading to the decay corrected
reading.
b. For Tc-99 select 30 hours as the
normalized reading, and decay cor-
rect this reading to calculate the
expected activities at the times you
took your readings. Compare the
readings to the calculated activities.
You can change from 30 hours to
another time point and perform your
normalized calculation from that
time point.
The Atomlab 500 automated linear-
ity test uses 30 hours as the default
method.
c. The following decay method
described in detail decay corrects
each reading, then averages these
calculated activities, and then
divides the average by the time cor-
rected reading to determine a
correction factor. If the correction
factor is between certain values, the
calibrator is linear.
Decay Method
1 Zero the Background by pressing
the Zero Background (Bkgnd)
button.
2 Assay the Tc-99m syringe or vial in
the dose calibrator to obtain the
activity. This first assay should be
done in the morning at a convenient
time, for example, 8 A.M.
3 Repeat steps 1 and 2 several times
per day until the displayed activity is
10 microcuries or below the lowest
activity assayed in accordance with
applicable State/NRC regulation or
the condition of your license. One
should collect 2 to 4 readings per
day. Additional readings are accept-
able depending on the isotope
used.
If the display indicates less than 50
µCi, (1.85 MBq), wait 30 seconds
before recording the reading. To get
a faster reading on the Atomlab
100, 100 Plus, 200 and 300, press
the Mo-99 button and then the Tc-
99m button, or adjust the transition
threshold on the Atomlab 400 or 500
controllers. Record the date, time (to
the nearest minute), and activity for
each reading.
NOTE: Some state regulations allow
linearity tests to end at activities greater
than 10 µCi. You may discontinue the
linearity test at the activity which your
state regulations or license permit.
NOTE: Use of a spreadsheet, such as
Microsoft Excel, in steps 4 and 5 will
simplify calculations.
4 Convert the time and date informa-
tion you recorded to hours (t)
elapsed since the first assay.
Decay-correct each displayed activ-
ity measurement (M) using the
equation,
t
----------
T
1
2
M
0
= M
x
(
2)
where T
1/2
= 6.007 h. Record the val-
ues. See linearity form, Figure 1.
Calculate activity correction factors
(cf) by dividing the average of all
decay corrected measurements ( M
0
)
by each M
0
, i.e.,
Average of all M
0
cf
M
=
M
0
(t)
Accept if 0.95 < cf
M
< 1.05. Consider
correcting the displayed activity mea-
surement (M) of radio
pharmaceuticals if a linearity error is
greater than ± 5% and less than ±
10%, i.e., 0.90 < cf
M
< 0.95 or 1.05 <
cf
M
< 1.10. Corrected M = M * cf
M
where subscript M is closest in value
to M in most recent linearity form. For
low activity values, if cf
M
< 0.95 see
the following note about Mo-99
contamination.
NOTE: If there is any Mo-99 contam-
inant in the Tc-99m sample, then long
decay times (>48h) will compromise
the decay linearity test. This will be
apparent with decreasing cf
M
values
for the lowest displayed activity mea-
surements (M). In order to test for
this, prepare another linearity test
sample with an activity of about 1
mCi. Repeat the linearity test. If the
cf
M
values are within an acceptable
range and if the 1st linearity test by
decay produced acceptable cf
M
val-
ues at higher M values, then the dose
calibrator passes. The two linearity
tests should have overlapping activity
values in the acceptable range.
1
Hint:
Recalculate first test ( M
0
) without
including failing values from low (M)
Mo-99 contamination.
5 If cf
M
< 0.90 or > 1.10, contact Bio-
dex technical support and provide
full details, including all linearity
decay data.
Attenuation Tube Method or
Sleeve Test Method
If you decide to use a set of “sleeves” of
various thicknesses or combination of
sleeves to test for linearity, it will be
necessary to first use the Decay
Method to show the calibrator is linear.
Then immediately repeat the test.
Follow the directions furnished with
your lead sleeves (i.e. Lineator
instructions.)
If failure occurs at the low activities,
check for Mo-99 contamination by
repeating the test with 1 mCi. See
NOTE under “Decay Method,” step 4.
NOTE: You can perform the Sleeve
Test procedure first and immediately
perform a traditional Linearity test. This
allows the user to perform both tests
with one dose.
NOTE: Other isotopes can be used to
perform linearity, such as F-18. The
sleeves can only be used with Tc-99.
1. See “Quality Control Testing of Dose Cali-
brators,” R.Y.L. Chu and W. E. Simon, Journal of
Nuclear Medicine Technology, 24, #2, June 1996,
pp 124-128.
January, 2016
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