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Biodex Medical Systems, Inc. © 2016 239
Geometry
Geometry Independence means that
the indicated activity does not change
with volume or configuration of the
source material. This test should be
done using a syringe that is normally
used for injections. The following test
assumes injections are done with 3 ml
plastic syringes and that radio
pharmaceutical kits are made in 30 ml
glass vials. If you do not use these,
change the procedure so that your
syringes and vials are tested
throughout the range of volumes
commonly used.
If a significant volume correction results
from these procedures, the tests should
be repeated to verify. Atomlab dose
calibrators have been tested for volume
dependence in Beta measurements
which are expected to be worst case
and the results were 0.13 %/ml.
2
NOTE: There are two methods that can
be used to calculate the results.
• Method one select a normalized
volume and decay correct the activity
readings for the other volumes to the
same time. Divide the calculated
activity for each volume by the actual
reading to get the correction factor for
each volume.
• Method two take the average of all
the volume activity readings. Divide
this average activity by the activity of
each volume to get the correction
factor for each volume. If the test
takes longer than 10 minutes, decay
correct the readings.
Syringe Test (example)
1 In a small vial, mix 2.0 ml of a solu-
tion of Tc-99m with an activity
concentration between 1 and 10
mCi/ml.
2 Set out a second small vial contain-
ing non-radioactive saline solution.
3 Draw 0.5 ml of the Tc-99m solution
into the syringe and assay it.
2. See “Accurate Dose Calibrator Activity Mea-
surement of Y-90 Ibritumomab Tiuxetan,” J. A.
Siegel et al, J Nucl Med, 2004, 45:450-454.
4 Record the volume and activity of
the first assayed sample (Figure 2).
5 Remove the syringe from the cali-
brator, draw an additional 0.5 ml of
non-radioactive saline into the same
syringe (total volume 1.0 ml), and
assay again. Record the volume
and measured activity on the form.
6 Repeat step 5 twice more until you
have assayed 1.5 ml and 2.0 ml vol-
umes and recorded them.
7 Assay the vial used to draw saline
into the syringe. If the measured
activity is greater than 1% of the 0.5
ml syringe assay, Tc99m was lost
during filling. Repeat the procedure.
8 Calculate results:
• Method one - select a
normalized volume and decay
correct the activity readings for
the other volumes to the same
time. Divide the calculated
activity for each volume by the
actual reading to get the
correction factor for each volume.
• Method two - take the average
of all the volume activity readings.
Divide this average activity by the
activity of each volume to get the
correction factor for each volume.
If the test takes longer than 10
minutes, decay correct the
readings.
9 If any correction factors are greater
than 1.05 or less than 0.95, it will be
necessary to make a correction
table that will allow you to convert
from “indicated activity” to “true
activity.” If this is necessary, be sure
to label the table “syringe geometry
dependence”, and note the date of
the test as well as the model num-
ber and serial number of the dose
calibrator.
Vial Test (10 ml) (example)
1 To test the geometry dependence
for a 10 ml glass vial, draw 1.0 ml of
Tc-99m solution (between 1 and 10
mCi/ml) into a syringe and inject it
into the vial. Assay the vial. Record
the volume and activity indicated.
2 Remove the vial from the calibrator
and, using a clean syringe, inject
2.0 or 3.0 ml of non-radioactive
saline, and assay again. Record the
volume and activity indicated on the
form (Figure 2). Repeat the process
until you have assayed a 8.0 ml vol-
ume. The entire process must be
completed within ten (10) minutes,
or, if not, decay-correct the activity.
3 Calculate results:
• Method one - select a
normalized volume and decay
correct the activity readings for
the other volumes to the same
time. Divide the calculated
activity for each volume by the
actual reading to get the
correction factor for each volume.
• Method two - take the average
of all the volume activity readings.
Divide this average activity by the
activity of each volume to get the
correction factor for each volume.
If the test takes longer than 10
minutes, decay correct the
readings.
4 If any correction factors are greater
than 1.05 or less than 0.95, it will be
necessary to make a correction
table that will allow you to convert
from “indicated activity” to “true
activity.” If this is necessary, be sure
to label the table “vial geometry
dependence”, and note the date of
the test and the model number and
serial number of the calibrator.
NOTE: Perform the vial test with the vial
size you commonly use.
NOTE: Other isotopes can be used for
performing Geometry Testing.
January, 2016
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