brated for photon energies in the same manner as the well counter using
the 662-keV g-ray energy of^137 Cs, and then discriminator settings are set
for the 364-keV g-ray of^131 I. Attenuation of photons in the thyroid tissues
reduces the overall detection efficiency of the probe.
Photons scattered in the thyroid gland by Compton scattering may reach
and interact in the detector because they originate in the field of view and
are not stopped by the collimator thickness. These scattered photons,
however, are excluded from the total measured counts by selecting the
appropriate lower and upper discriminator settings on the PHA for the
364-keV g-ray of^131 I.
Thyroid Uptake Measurement
In the thyroid uptake test, a^131 I-NaI capsule containing about 10 to 15mCi
(0.37–0.55 MBq) of^131 I is measured in a lucite thyroid phantom at a fixed
distance using the thyroid probe and the settings for 364-keV photons of
(^131) I. The thickness and composition of the lucite phantom are equivalent to
those of the patient’s neck. This count is considered as the standard count.
The capsule is then administered to the patient orally, and the thyroid count
is obtained at the same distance as the standard count 24 hr after adminis-
tration. The room background count is taken to subtract from the standard
count, and the thigh count is taken as background to subtract from the
thyroid count. The thyroid uptake is then calculated as follows:
(8.9)
where Ais the thyroid count,Bis the thigh count,Cis the standard count
corrected for 24-hr decay, and Dis the room background. Identical proce-
dures are employed with^123 I-NaI using ~300mCi (11.1 MBq). At times, the
6-hr thyroid uptake also is determined depending upon the clinical judge-
ment of the physicians.
Questions
- Describe the mechanism of g-ray interaction in the NaI(Tl) detector. In
g-ray counting, why is NaI(Tl) commonly chosen as the detector? - (a) Describe the operation of a photomultiplier (PM) tube.
(b) What is the typical high voltage applied to the PM tube?
(c) What are the photocathodes commonly made of?
(d) How many photoelectrons are emitted from the photocathode for
each keV of photon energy? - (a) Ideally, a photopeak should appear as a line in a g-ray spectrum.
Indicate different factors that contribute to the broadening of the
photopeak.
% uptake=()− ×
()−
AB
CD
100
Questions 105