Radiopharma Flashcards
What are the properties for radiopharmaceuticals?
Must be chemically pure, non-toxic, needs to have a short half life (reduce radiation dose & do enough damage to affected area)
What is radio nuclidic purity
The radio nuclidic purity is defined as the fraction of the total radioactivity in the sample that is in the form of the required radionuclide
What is the specific activity?
The specific activity determines the mass of a compound that is introduced for a given dose.
The mass should be low; thus specific activity should be high
What is the physical half life
Physical half-life should be within seconds to days - must consider patient dose risk, and sufficient time for dose to prepare
What is the ideal emission
Gamma = energy of 50 – 600 KeV are suitable, lower
energies will not get transmitted through the body
What are the benefits of commercial generators?
Sterilised, automated, and large size, and impurity levels are low
What is a disadvantage of commercial generators?
99Mo “breakthrough” which
is partial elution of the 99Mo parent.
* The amount of breakthrough needs to be
kept to a minimum for patient safety
reasons
Describe the features of fission using molybdenum
Has a higher specific activity
* The amount of alumina required is
determined by the amount of
stable carrier
- Small volumes of saline required
for elution, which is great for bolus
injections
What do dry generators have
Modern generators, typically “dry cell” construction using an evacuated vial to draw saline from a second vial through the ion- exchange column.
What is a radionuclide generator?
A radionuclide generator is a device containing a radionuclide that decays to form the required daughter product
How is the daughter nuclide separated in radionuclide generator?
The daughter can be separated by a physical process
– gaseous daughter
– solubility
What are the characteristics of radiation from cyclotrons?
Cyclotron products have high photon/particle
emission ratios obtained in EC decay and Beta plus
Difference between cyclotrons and nuclear reactor
Cyclotrons produce smaller quantities of radionuclides than nuclear reactors, thus are more expensive to operate
In cyclotrons, the products are carrier free because
Addition of positive ions changes the atomic
number, thus products are usually carrier free
and have high specific activity
Why does it undergo beta plus emission (cyclotron radiation)?
+ve charge is given to the nucleus, therefore
the product lies below the line of stability and
tend to decay by EC or Beta Plus
Why are beta plus used for PET scans?
Beta plus are used for PET studies. Most have short
T1/2 values - need to have ready access to
cyclotron
Explain how a cyclotron works with negative ions
When the –ve charged particles reach the
outermost orbit, they are passed through a thin
carbon foil which strips off the electrons creating +ve ions. The interaction of the magnetic field with the
+ve ion bends towards the target.
Can 2 beams be extracted in the same time in cyclotron? (Yes)
Two beams can be extracted at the same time,
by positioning a carbon stripping foil part way
into the path of the beam, so that only a portion
of the beam is extracted
Explain the cyclotron design
1.Uniform magnetic field
makes the particles orbit
2. Electric field keeps speeding them up
3. As they speed, orbital radius increases
4. They shoot out of the cyclotron at high speeds
What does the e field do every half orbit?
Direction of the e-field changes every half orbit to keep particles accelerating across the gap
Explain the operation of the cyclotron
- Particles are generated in bursts by the ion source
- High frequency AC voltage is applied across the dees.
- particles are injected into the gap and accelerated
towards one of the dees, the particles follow a circular path as they are in a magnetic field - particles are accelerated across the gap and continue on a circular path in the opposite direction; with the particles gaining energy each time they cross the gap
- When the particles reach the maximum radius, they may be directed onto a target placed
directly in the beam path, or may be extracted from the cyclotron and deflected by a negatively
charged plate onto an external target (most common)
Explain briefly (n,gamma) neutron activation.
A target nucleus captures a neutron and is converted into a product nucleus; this is in an excited state. The nucleus immediately
undergoes de-excitation (prompt decay) to the ground state by emitting
a gamma ray.
In neutron gamma activation, the target and product are ______
The target and product represent
different isotopes of the same chemical
element
What is neutron activation
When neutrons strike a target, some of the neutrons are captured by nuclei of the
target atom - either releasing a gamma ray or a proton
What are the characteristics of fission radionuclides?
Fission products may be carrier free: Radionuclides
can be produced with high specific activity
- Lack of specificity
- Fission products always have an excess of neutrons hence undergo decay by beta minus emissions
until a stable nuclide is reached
Gamma decay typically follows as daughter nucleus
reaches stable state
What are the fission products produced?
The fission process produces medically important radionuclides (131I and 99Mo) directly as fission products
* Fission of 235U leads to one fragment with a mass
number in the range 85 to 105 and another
fragment with a mass number in the range 130 to
150
What is the purpose of coolant
To remove heat from the nuclear reactor core and transfer it to electrical generators and the environment.
What is the purpose of the control rods
- Strong neutron absorbers
- Used to expose or shield the fuel cells
- Fuel cells and control rods are carefully positioned to control the chain reaction
- If removed, there could be a potential meltdown (e.g. Chernobyl)
- Insertion of additional control rods results in
excessive absorption of neutrons and terminates the chain reaction (shut down)
What is the role of moderator
- Slows down the energetic fission neutrons
- Slow (thermal) neutrons are more efficient initiators of additional fission events
- Uses “heavy water” (D2O) or graphite
What is the purpose of fuel rods?
Fuel Cells
– Contain the fissionable material (generally U-235)
Why is 235 U fissile?
Most important reaction:
235U + n → 236U* promptly undergoes fission releasing additional neutrons
– This means 235U is fissile. Results in a controlled, self-sustaining nuclear chain reaction
What does carrier-free mean
A preparation of a radioactive isotope which is ‘free’ from stable isotopes of the element in question.
No stable isotope present
- Has the highest specific activity
- Fission products (once separated) provide highest carrier free product
Name all the artificial radioisotopes
Reactor produced: Fission Particles & Neutron Activation
Accelerator Produced: Cyclotron
Radionuclide generators:
99Mo–99mTc generator
