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.