Nuclear Medicine Questions and Answers Graded A+
20% photopeak window:
Symmetric energy window centered and extending from 10% below to 10% above the photopeak energy
Photomultiplier tube electron multiplication
For every 5 scintillation photons impacting the photocathode, one electron is...
Nuclear Medicine Questions and
Answers Graded A+
20% photopeak window: - answer Symmetric energy window centered and
extending from 10% below to 10% above the photopeak energy
Photomultiplier tube electron multiplication - answer For every 5 scintillation photons
impacting the photocathode, one electron is emitted.
For each accelerated electron that impacts the dynode → approximately 5 electrons will
be emitted.
Total "electron multiplication" or gain achieved by the PMT is equal to 5N, where N =
the # of dynodes
SPECT detector configuration: - answer Dual detector systems for cardiac imaging
→ typically used in a 90° configuration, sometimes called the "L-mode."
For both gated myocardial perfusion and gated blood pool SPECT → the most
commonly used arc for data acquisition is -45° RAO view down to the +135V LPO view.
Reconstruction artifacts → start to become apparent for tilt angles > 10° for myocardial
perfusion data acquired from -45° RAO view to +135° LPO
Whole body tomograms (e.g. liver SPECT) → more commonly use 2 detectors
configured at 180° opposed angles, sometimes called "H-mode."
PET spatial resolution - answer Spatial resolution ~ 4-6 mm for whole-body imaging
~2.2 mm for PET cameras for brain imaging (e.g. high resolution research tomograph,
HRRT)
~1-2 mm for application-specific designs dedicated to the breast and prostate.
3D mode (septa retracted) → ↑ coincidence efficiency by about a factor of 5 in
comparison to a 2D acquisition (septa extended)
At the expense of ↑ randoms, ↑ scattered coincidences, and ↑ system dead-time.
Radionuclide half-lives: C11, N13, O15, F18, Ga68, Rb82 - answer 11C → 20.4 min
13N → 9.96 min
15O → 123 sec
18F → 110 min
68Ga → 68.3 min
82Rb → 78 sec
F18 energy - answer 18F → emits a positron with energy of 0.635 MeV → travels a
maximum of 2.4 mm before annihilating with an electron to produce two 511 keV
photons that travel away in opposite directions.
Typically 370-555 MBq (10-15 mCi) of 18F-FDG for adults
PET study usually ~1 hour after FDG administration.
,Gamma emitter characteristic energy: Tc-99, Thallium-201, Indium-111, Gallium-68 -
answer Tc-99m:
Half-life = 6 hours
Single gamma ray from Tc-99m has characteristic energy of 140 keV.
Thallium-201: 167 keV, 135 keV, 70-80 keV
Indium-111: 245 keV, 171 keV
Gallium-68: 400 keV, 300 keV, 185 keV, 93 keV
Radionuclide generators: Mo99-Tc99m - answer 99Mo-99mTc ("moly") generator
A stored quantity of 99Mo decays with a half-life of 65.9 hours → decays into the
radioisotope 99mTc with half-life of 6.02 hours.
Essentially all 99mTc used clinically on a daily basis is obtained from moly generators.
Moly generator is typically purchased weekly → provides usable amounts of 99mTc
each day.
Radionuclide Generators→ follow Parent-Daughter Equilibrium
99Mo-99mTc ("moly") generator
Maximal activity of 99mTc is obtained about 23 hours after the previous elution of the
generator.
Because of the decay of 99Mo → the obtainable maximal 99mTc activity decreases with
each subsequent day.
After about a week, a moly generator no longer produces sufficient 99mTc to satisfy
clinical needs for a day→ the old generator is replaced with a new one.
By federal regulation→ a maximum breakthrough of 0.15 microcuries of 99Mo per
millicurie of 99mTc is allowed
Area checks: - answer Restricted area
Surveys of ambient radiation levels must be performed daily
Wipe tests must be performed weekly
Unrestricted area
Radiation surveys are required weekly.
Wipe tests are required monthly
Transport index of radioactive package: - answer TI = exposure rate in mR/hr
(milliroentgen/hr) measured at 1 meter from the surface of the package.
Moly breakthrough mechanism: - answer The 140 keV gamma rays from 99mTc →
cannot penetrate through the Pb shield
But the 740 keV emission of 99Mo → can penetrate the shield
If the dose calibrator registers the presence of activity from the shielded solution, 99Mo
must be present.
By federal regulation→ a maximum breakthrough of 0.15 microcuries of 99Mo per
millicurie of 99mTc is allowed
Dose calibrator accuracy testing: - answer Accuracy testing→ testing required after
installation and repair, and on an annual basis thereafter using Dose Calibrator
Accuracy
, Uses one or more NIST-traceable radioisotope calibration standards.
Cs-137 source (662 keV; T1/2 ~30 years) or other long-lived source is typically used for
accuracy testing.
Ideally the dose calibrator would also be tested at low and high energies
→ Co-57 (122 keV, T1/2 ~282 days) and Co-60 (1.33 MeV; T1/2 ~5.3 years) are useful
for this purpose.
Dose calibrator: constancy, linearity, and geometry - answer Constancy test
commonly performed using a long-lived source such as Cs-137.
Inter-day readings should agree within + 5%.
Dose calibrator linearity → should be verified at installation, after repair, and on a
quarterly basis otherwise
Using a linear fit across all measurement points → should verify that each measurement
point is within + 5% of the expected response.
Dose Calibrator Geometry
If the dose calibrator response using a particular source configuration exceeds + 5% of
the reference configuration, a correction factor must be applied to all readings obtained
using that particular source/activity geometry.
Survey meter QC: - answer For routine QC on a daily basis→ the zero-level of the
survey meter must be verified in an area with a very low background rate, and then the
constancy of the survey meter must be tested with a long-lived radioactive "check
source."
The inter-day constancy measurement must be within + 5% → otherwise the survey
meter may need to be recalibrated.
May not use survey instruments in the difference between the indicated exposure rate
and the calculated exposure rate is > 20%
Well-counter QC - answer The efficiency and energy resolution of the well counter
should be evaluated at installation, annually, and after repair.
Background reading and daily constancy (sensitivity) test should also be performed, and
inter-day constancy readings should agree to within + 5%
Suspected malfunction of a well-counter (or other counting system) → can be evaluated
using a Chi-Squared Test → determines if the counting system is behaving as predicted
by statistical theory.
Thyroid uptake probe QC - answer The energy resolution of the thyroid probe should
be evaluated annually, and the probe efficiency should be tested at regular intervals
depending on the frequency of use and the manufacturer's recommendations.
Daily tests:
Daily verification of the pulse height analyzer (PHA) window setting
Constancy test using a long-lived check source
Intra-operative probe QC - answer Should be tested for constancy prior to each use
using a dedicated long-lived check source → should be within + 5% of the expected
reading.
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