Topic 2-Production of Radionuclides & QA QC
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Transcript of Topic 2-Production of Radionuclides & QA QC
Production of Radionuclides
• All radionuclides commonly
administered to patients in nuclear
medicine are artificially produced
• Most are produced by cyclotrons,
nuclear reactors, or radionuclide
generators through bombardment or
fission
1. Cyclotrons
• Cyclotrons produce radionuclides by
bombarding stable nuclei with high-
energy charged particles
• Most cyclotron-produced radionuclides
are neutron poor and therefore decay by
positron emission or electron capture
>Specialized hospital-based cyclotrons have been developed to produce positron-emitting radionuclides for positron emission tomography (PET)
>Usually located near the PET imager
because of short half-lives of the
radionuclides produced
2. Nuclear Reactors
• Specialized nuclear reactors used
to produce clinically useful
radionuclides from fission products
or neutron activation of stable
target material
>Uranium-235 fission products can be chemically separated from other fission products.
>Concentration of these “carrier-free”
fission-produced radionuclides is very
high
Neutron Activation
• Neutrons produced by the fission of
uranium in a nuclear reactor can be
used to create radionuclides by
bombarding stable target material
placed in the reactor
>Process involves capture of neutrons by stable nuclei
>Almost all radionuclides produced
by neutron activation decay by
beta-minus particle emission
3. Radionuclide Generators
A generator is a self-contained system housing a parent/daughter mixture in equilibrium.
There must be a method of removing the daughter and leaving the parent behind to regenerate more daughter activity.
It is designed to produce the daughter for some purpose separate from the parent.
Generators produce certain short-lived radioisotopes on-site which cannot be shipped by commercial sources.
To be useful, the parent's half-life must be long compared to the travel time required to transport the generator to recipient.
The typical shelf-life of a Mo/Tc generator is 2 weeks, as is the expiration date.
the process of removing the daughter
from the generator is referred to as
elution;
the solution used to remove the
daughter is called the eluent;
and the solution collected from the generator containing the daughter radioisotope is called the eluate.
• Technetium-99m has been the
most important radionuclide used in
nuclear medicine
• Short half-life (6 hours) makes it
impractical to store even a weekly
supply
• Supply problem overcome by obtaining parent Mo-99, which has a longer half-life (67 hours) and continually produces Tc-99m
• A system for holding the parent in such a way that the daughter can be easily separated for clinical use is called a radionuclide generator
1. Generator output must be sterile and pyrogen-free.
2. The chemical properties of the daughter must be different than those of the parent to permit separation of daughter from parent.
3. Generator should be eluted with 0.9% saline solution and should involve no violent chemical reactions.
4.Daughter isotope for diagnostic studies should be short-lived gamma-emitting nuclides.
5. Inexpensive, effective shielding of
generator, minimizing radiation dose to
those using it.
This is easy to accomplish since lead is very dense and therefore a good attenuator of radiation.
DRY column generator
The saline supply is in a 30-ml bottle/vial for elution
Because saline used never exceeds 20ml, up to 10ml of air follow the saline through the fluid path, effectively drying the column
WET column generator
The saline supply is a 500ml bottle and is an integral part of the generator
Once elution is completed, the fluid path is filled with a saline for the life of the generator and the alumina column is always saturated with 0.9% NaCl solution
Equilibrium
is a condition established in a parent/daughter mixture when both parent and daughter are radioactive and when the daughter’s half-life is shorter than that of the parent.
if the daughter’s half-life exceeds that of the parent, equilibrium will never be reached.
Transient Equilibrium
-is a condition reached when the half-life of the parent is approximately 10 times greater than the half-life of the daughter.
Secular Equilibrium
-if the half-life of the parent is very much longer than that of the daughter (e.g., more than 100 longer).
99Mo/99mTc GENERATOR: PRINCIPLES OF OPERATION
1. Prior to shipping the generator to the
Nuclear Medicine Department, 99Mo
sodium molybdate is immobilized on a
column of alumina (Al2O3; aluminum
oxide).
2. 0.9% saline solution (the eluent) is
passed through the column and Na
pertechnetate, the daughter of 99Mo
decay, is eluted from the column with
high efficiency due to its almost total
lack of affinity for alumina.
3. The pertechnetate is collected in a
shielded, evacuated sterile vial and
must undergo quality control testing,
then must be calibrated prior to use. It
is referred to as the eluate.
99Mo/99mTc GENERATOR
is considered to be the workhorse of all generators and is ideal with no significant limitations
used in almost 80% of nuclear scan performed
Commonly Transported Radioisotopes
*Americium-241= Diagnose thyroid disorders, smoke detectors.
*Cesium-137= Cancer treatment.
*Iodine-125,131= Diagnosis & treatment liver, kidney,heart, lung and brain.
*Technetium-99m=Bone and brain imaging; thyroid and liver studies; localization of brain tumors.
6. Low radiation dose
7. Safe
8. Convenient
9. Cost-effective
QC program is especially important in
two main areas:
Instrumentation
Radiopharmaceutical preparation
Instrumentation:
Well counters
Dose calibrators
Thyroid probes
Gamma camera
QC for Gamma Camera
Spatial resolution Weekly
Uniformity Daily (before first patient)
Image linearity Weekly
Energy resolution Annually
Count rate response Annually
Sensitivity Annually
Collimator integrity Annually or when suspicious of damage
Formatter performance Annually
Whole-body accessory Annually
Window setting For each patient
Radiopharmacy
Generator and radionuclide purity
Radiochemical labeling
Sterility
Operation and routine QA
• Energy discrimination windows
must be adjusted to center them on
the photopeak or photopeaks of the
desired radionuclide
Operation and routine QA (cont.)
• Uniformity of the camera should be assessed daily and after each repair
• May be made intrinsically by using a Tc-99m point source
• Images must contain enough counts that quantum mottle does not mask uniformity defects
• Uniformity test will reveal most
malfunctions of a scintillation
camera
Other QA
• Spatial resolution and spatial
linearity should be assessed at
least weekly
• Efficiency of each camera head
should be measured periodically
• Complete evaluation at least annually
– Include multienergy spatial registration and count-rate performance
Peaking
Counting Rate
Field Uniformity
Spatial Resolution
Spatial Linearity
Sensitivity
QC for Gamma Camera
Gamma Camera Quality Control
QC Procedure Frequency
Peaking Daily & before each new radionuclide used
Counting rate limits Daily
Field uniformity Daily, after repair
Spatial resolution Weekly, after repair
Spatial linearity Weekly, after repair
Sensitivity Quarterly
• Energy discrimination windows
must be adjusted to center them
on the photopeak or photopeaks
of the desired radionuclide.
QC Procedures for Gamma Camera
Peaking
• “Peaking” may be done manually
by adjusting the energy window
settings while viewing the spectrum
or automatically by the camera
• Should be peaked before first use
each day and before imaging a
different radionuclide
• Small source used to
peak camera; radiation
emitted by the patient
would have a large scatter
component.
• Sensitivity of a gamma camera generally decreases with increased amounts of activity.
• During the high activity the detector is “paralyzed” & cannot count.
• Dead Time is the system’s inability to count
QC Procedures for Gamma Camera
Counting Rate Limits
Field Uniformity
• Refers to the gamma camera’s ability to detect a uniform source of radiation
• Uniformity depends on the uniform response of the NaI crystal & the PMTs.
QC Procedures for Gamma Camera
Intrinsic Uniformity Flood
A. Field Uniformity Flood B. Non-uniform Flood
Intrinsic Uniformity Flood
A, B, C. Damage PMT D. Cracked Crystal
• Uniformity can be:
intrinsically (w/o collimator)
extrinsically (w/ collimator)
• May be made intrinsically by using a Tc-99m point source
• Edge packing – is the phenomenon that can show up as a bright rim activity around the perimeter of the flood.
Extrinsic uniformity
• Two common radionuclide sources used:
• acrylic plastic (Plexiglas) container filled w/ H2O 1-10mCi Tc-99m
• solid-sealed 10 mCi cobalt-57 sheet
• Extrinsic can be evaluate/assess the defects of the collimator
Extrinsic Uniformity Flood
Damaged Collimator
• Images must contain enough counts that quantum mottle does not mask uniformity defects
• Uniformity test will reveal most
malfunctions of a scintillation camera
• Spatial resolution – gamma camera’s ability to reproduce small details of a radioactive distribution.
• Required to be performed a minimum of once a week on every imaging system
• Spatial Linearity – gamma camera’s ability to produce a linear image w/ straight lines corresponding to the same straight lines of the bar pattern.
• Performed to determine the gamma camera detector’s ability to detect the ionizing event that occur in the NaI crystal.
• Events recorded as counts per minute are calculated and expressed as cpm per microcurie of acitivity present
• Performed biannualy
Sensitivity
• NRC requires that all photo-emitting sealed sources containing 100 µCi or more be tested for leakage biannualy.
• USNRC stated that any sealed sources with more than 0.005 µCi of removable activity per test must immediately be removed, properly stored, and reported to the NRC.
• NRC requires that all sealed sources be inventoried and surveyed quarterly for radiation exposure.
Sealed Radioactive Source
• Two common generators used:
• Molybdenum-99
• Technetium-99m
• QC is essential on the Technetium eluent each time generator is eluted, to ensure that the eluent does not contain any contaminants or impurities including radionuclide impurity of Mo-99, molybdate, chemical impurity of Al+3, alumina, or radiochemical impurity of hydrolyzed reduced technetium(HR-Tc).
Radionuclide Generator
Radiation Protection
The basis for all radiation protection
activities is the supposition that radiation
is harmful and that the smaller the
radiation doses we receive, the smaller
are the risks.
Justification- no practice involving exposures to radiation should be adopted unless it produces sufficient benefit
Optimization- individual doses, number of people exposed and occurrence of exposures should be kept ALARA
Individual dose and risk limits- the exposure of individuals from the combination of all relevant practices is subject to dose limits
Cardinal Principles
Shielding: If you have a thick shield between yourself and the radioactive material, more of the radiation will be absorbed by the thick shield, and you will be exposed to less rays.
Time: Minimizing the time spent with the radioactive source will also reduce radiation risks.
Distance:
The farther you are from the radioactive source,the lower your exposure. Thus reducing the probable risks.