Radiopharmacy Flashcards
What is radiopharmacy?
- Use of radioactivity in clinical diagnosis and treatment
Preparation and use of appropriate radiopharmaceuticals
A radiopharmaceutical is…
“A radiopharmaceutical is a radioactive diagnositc or therapeutic pharmaceutical that targets a specific organ or system by virtue of its molecular design, and that is used for imaging, in vitro testing or treatment”
Basic physics of atomic structure and radioactivity
- Some isotopes are more stable than others
- As an unstable isotope breaks down, it emits radioactivity
- Isotope stability is really the stability of the nucleus
What governs the stability of the nucleus?
○ Relative amount of neutrons and protons
○ Nucleonic binding energy
○ Mass defect
Magic numbers
2, 8 20, 28, 50, 82, 126
Full shells are more stable than non full shells
Isotopes with magics numbers of either protons or neutrons have a higher nuclear binding energy and are therefore more stable than other, non-magic isotopes
Examples of doubly magic nuclides
○ 4He is one of the most stable and abundant nuclei in the universe
208Pb is the heaviest stable nuclide
For non magic isotopes
○ Even numbers of nucleons promotes stability
High neutron: proton ratio promotes instability
The mass defect can be used to
Assess the stability of an isotope
How can the mass defect be used?
○ The actual total mass of a nucleus is always less than the theoretical total mass of a nucleus
○ By Einstein’s equation, mass and energy are interconvertible
This calculated energy is the nuclear binding energy
How to separate nucleons completely
need to input energy equivalent to the nuclear binding energy
○ Higher binding energies = greater isotope stability
○ Approc 6-9 Me V per single nucleon
* Over 3000 isotopes have been discovered or man-made - most are unstable
Unstable isotopes will revert to more stable isotopes by emitting radioactivity
Gamma
Form - photon
Range - Air: 1km Water: 1m
External exposure
Moderate contamination
Diagnostic use
Beta
Form - Charged particle
Range - Air: 5m Water: 5mm
No external exposure
High contamination
Therapeutic use
Alpha
Form - Charged particle
Range - Air: 1mm Water: <1mm
No external exposure
High contamination
Therapeutic use
What is an alpha particle
Helium nucleus
No electrons so a 2+ charge
Occurs if the nucleus is very heavy
Daughter nuclide has - atomic number 2 less than the parent, and mass number 4 less than the parent
Beta minus particle
Electron so a 1- charge
What is beta minus radioactivity
Occurs if the nucleus is “neutron rich”
Neutron breaks down to a proton, an electron and an anti-neutrino (without mass or charge, to conserve energy)
Daughter nuclide has atomic number 1 more than the parent and mass number the same as the parent
Beta plus particle
Anti-electron = positron = 1+ charge
What is beta plus radioactivity
Occurs if the nucleus is relatively “neutron poor”
but need an energy diff of >1.02MeV to happen
Proton -> Neutron, positron and neutrino
Daughter nuclide has atomic number 1 less than the parent and mass number the same as the parent
A positron will
collide with an electron (anti particles)
Mutual annihilation
Two 511 keV photons moving in opposite directions
Gamma rays and X rays
Electro-magnetic radiation
No mass
No charge
Photons of high energy
Some overlap in electro-magnetic character, therefore distinguished by source:
gamma rays from the nucleus
X rays from electrons outside the nucleus
Gamma rays generally have
Gamma rays generally have frequences >10^18 Hx, wavelengths <10^-11 m and energies >100keV
X rays generally have
Frequences 10&15 to 10^8
Wavelengths 10^-8 to 10^-11m
and energies >120eV to 120keV
Gamma rays are generated from
Generated from isomeric transition
Occurs if the nucleus is metastable (m)
* in an excited state above its ground state =isomeric state
* release of gamma energy allows nuclide to transform to its ground state
* no particle release
Daughter nuclide has atomic and mass number the same as parent
Two ways X ray ionisation occurs
Generation from electron capture and Bemsstrahlung
How are X rays generated from electron capture
Occurs if the nucleus is relatively neutron poor
Usually energy difference of <1.02MeV
K(1)-shell electron “captured” by the nucleus
Electron combines with a proton, producing a neutron and a neutriono
Characteristric X-rays are given off when the empty K(1)-shell space is filled by a more energetic electron
Daughter nuclide has atomic number 1 less than the parent and mass number the same as the parent
How are X rays generated from Bemsstrahlung
- Occurs as a secondary radiation effect with beta radiation
- electrons are deflected by the mass and charge of the nucleus
- electrons lose speed = bremsen (braking)
- excess energy is released as X rays (Strahlung)
intensity of Bremsstrahlung radiation is proportional to the energy of the beta particles and the atomic number of the material through which the beta particles are passing
Alpha vs beta vs gamma table
Lecutre notes
Effect of radiation on the body
- “Ionising radiation”
- will lose energy through collision as it goes through body tissue
- will cause localised heating
will create ion pairs as it goes through body tissue
Amount of alpha / beta / gamma per cm
Alpha particles: 20,000 to 30,000 ion pairs per cm
- Beta particles: few hundred ion pairs per cm
Gamma and X rays: few ion pairs per cm
Tissue damage from within and outside
- Tissue damage from within the body
- a real problem with alpha and beta radiation
- less of an issue with gamma and X rays (but don’t ignore)
- Tissue damage from outside the body
- less of a problem with alpha and beta radiation
more of an issue with gamma and X rays
Mathematics of radioactivity
ln Nt = ln N0 - k.t
Half life
Time for half the radioactive atoms to decay
Half life equation
t1/2 = 0.693 / k
Combine half life with other equation gives
ln Nt = ln N0 - 0.693. t/t1/2
Important for radiopharmaceutical preparation
Important for assessment of radioactive dose and damage
Absorbed radioactive dose
is a measure of how much energy from the radiation has been deposited in the body
SI unit = Gray (Gy) = 1J/kg
usually use milli-Gy or micro-Gy
SI unite = Sievert (Sv) = 1J/Kg
Same units as Gy
Units of radioactive dose
Different types of radioactivity cause different biological effects e.g. alpha particles cause more dammage than gamma rays
Equivalent dose
Absorbed dose x Radiation weighting factor
Radiation type - Energy range - radiation weighting factor
Look at table
Specific gamma ray constant
Relates the energy of the gamma ray and the distance between the source and the tissue to the effective dose received
Increases with increasing gamma ray energy
Decreases with distance between source and target
Typical radiation dose
Average annual = 2.5 mSv per year in the UK
Acute doses
1 Sv causes nausea
2-5Sv causes hair loss, haemorrhaging, death
Cyclotron
- Charged particles accelerated in a circular path
- Particles attain high energies, up to GeV
- Particles collide with “target” stable atoms
- Nuclear reactions occur:
- incident particle imparts energy to nucleus, then leaves
incident particle is completely absorbed by the nucleus
- incident particle imparts energy to nucleus, then leaves
Cyclotron daughter nuclide
- Daughter nuclide is now radioactive and may decay by particle emission or gamma / X rays emission
may get many nuclides, depending on the incident energy - Daughter nuclide is relatively neutron-deficient
decays by β+ emission or electron capture
Cyclotron examples
111In
Yttrium daughters
Production of radionuclides
Nuclear reactor
Nuclear reactor (1)
Nuclear fission i.e. splitting
“Fissile” heavy elements inserted into reactor core
Nucleus bombarded with neutrons, absorbs nuetrons and undergoes fission, releasing two “half size” nuclides, nucleons and energy
Daughter nuclides are usually neutron rich decay by Beta emission
Nuclear reactor (2)
Neutron capture, followed by gamma ray emission
* Produces isotope of the same element
Daughter nuclides are neutron-rich
* Decay by Beta emission
Production of radiopharmaceuticals
Generator
* More convenient, can be used in a hospital environment
* Parent radionuclide has a long half life
* Daughter radionuclide has a short half life
* Quantity of daughter increases as quantity of parent decreases
* Chemical nature of the daughter must be different to that of the parent to allow separation
