Lecture 1 Flashcards
What are the 4 steps of radionuclide imaging principles?
- Label small amounts of molecules with a radionuclide
- Administer the radiotracer to the biological system
- Detect the signal of the radionuclide decay
- Covert the signal into a meaningful image
Name two photon emitting radiotracers that are used in general nuclear medicine and their uses
99m- Tc. Used for myocardial perfusion
123-I. Used for Thyroid cancer
Name three positron emitting radiotracers which are used for PET scans and their application
15 O. Cerebral blood flow
11 C. Tumour protein synthesis
18F. Glucose metabolism
What is a metastable state?
A temporary stable state- an excited state that remains for a measureable period of time before de-excitation
What is alpha?
Heavy particle- 2n, 2p. Travels straight but coulomb interaction with tissue stops them within a few um of travel. Imaging is not possible
What is beta? How far can it travel in tissue? Is it useful for imaging?
Lighter particle ( electrons and positrons) Interaction with Coulomb force but do not travel straight due to being small. Their average range in tissue is 1-5mm. In vivo imaging is possible but positrons and electrons annihilate and create two gamma photons (511keV each)
What is gamma?
EM radiation- longest range. It interacts with matter via the photoelectric and Compton scattering effects. Their energy helps them escape human tissue
What is an isomeric transition? Why is it useful?
Occurs when an excited nucleus has a long-lived metastable state. Considered as a separate decay. We count the final state as a third radionuclide ( e.g. parent to daughter to granddaughter). It is useful to separate out the particulate emission stage ( parent to daughter) from the pure photo emission stage ( daughter de-excitation) in the decay chain.
What is the equation for activity (rate of decay)?
What is the Si unit of activity?
dN/dt= - λN
λ= probability of decay N= number of atoms
SI unit= Becquerel (Bq)
Describe the photoelectric effect?
What is the probability of PE absorption?
All of the energy from an incident photon is transferred to an electron. The electron is ejected with an energy equal to the incident photon energy minus the binding energy of the electron.
probability is proportional to Z^3/E^3
Describe Compton scatter.
What affects the probability of Compton scatter
An incident photon collides with an outer shell electron ( free electron). The electron is ejected and the incident photon is scattered with a loss in energy.
The probability of Compton scattering increases within photon energy and material electron density. It doesn’t vary much with Z.
Describe three different types of gamma camera and their uses
Triple head- 3 heads at 60 degrees. optimised for tomography of brain or body
Dual head opposed- Two heads opposite each other. Used for whole body scanning, tomography and general purpose.
Dual head cardiac- two heads at 90 degrees. Optimised for cardiac imaging.
What is the function of the collimator?
Gamma photons cannot be focussed like light photons.. It controls the direction by eliminating photons that are not incident in the preferred direction. There is attenuation of photons within the collimator. It is designed to maximise the number of useful photons in terms of energy and direction.
What are the three things that a collimator can be described in terms of?
- Spatial resolution
- Sensitivity- counts detected per unit of activity in the patient
- Energy range- septal thickness/ depth determines the energy of the gammas that can be attenuated by the collimator
What is the purpose of the collimator septa? What are the effects of increasing the septal thickness?
The septa must be thick enough to stop the radiation with the energy of the imaged isotope. Increasing the septal thickness means an increase in the imaging energy and decreases the sensitivity.
what does the collimator hole depth describe?
What are the effects of increasing the collimator hole depth?
The hole depth defines the solid angle field of view of each hole. Increasing the hole depth causes no change in imaging energy, decreases sensitivity and increases resolution.
What are the effects of decreasing the collimator hole size diameter?
Smaller holes means more holes which means more area is taken up by lead septa. Decreasing the hole size causes no change in the imaging energy, it decreases sensitivity and increases resolution.
Define spatial resolution
FWHM- Two points that can be resolved as separate entities.
Define geometric resolution
The resolution of the collimator combined with the intrinsic resolution of the detector
Define system resolution and the equation
No matter how good a detector, the image quality will always be limited by the collimator which will always be poorer.
Rs= sqrt ( Intrinsic resolution)^2 + (geometric resolution) ^2
Describe the four types of collimators
- Parallel hole- most common
- Diverging/ fish tail- Used to increase the FOV with small cameras, reduces image size, distorts the image
- Converging ( fan or cone beam)- Magnifies to improve spatial resolution, generally used for tomography.
- Pinhole- Produces a magnified and inverted image. Used for small organs e.g. thyroid.
Describe the 4 stages of events that happen within a scintillation detector
- ABSORPTION- Gamma ray goes into the crystal and interacts by Compton or ideally PE to eject an electron.
- EXCITATION- the electron is excited to the conduction band ( can move through the crystal)
- RELAXATION- the electron relaxes back to the valence band
- EMISSION- Electron hole pairs created, resulting in the emission of visible light by the crystal.
Name 6 characteristics of an ideal scintillator detector
- high efficiency
- No scatter
- High conversion of photons to light
- Linear conversion- light yield proportional to deposited energy
- Crystal transparent to light
- Short scintillation period
Why are NaI crystals doped with Thallium iodide?
Name 5 characteristics of a Thallium activated sodium iodide scintillator
To achieve efficient scintillation at room temperatures. it also puts emitted light in visible light frequency
- High density
- High atomic number
- 20-30 light photons per keV
- wavelength of emitted light is reasonably suited to PMT’s
- Fragile- protect from physical and thermal shock
How are gamma camera crystal grown
Grown in a furnace- sodium iodide is melted at a high temperature and gradually cooled to form a crystal over a couple fo months
What does the light guide do?
Forms a couple between the crystal and PMT’s
High refractive index
Minimise variation in light collection along crystal
Why is optical coupling done and how?
Air/ solid interface between scintillation crystal and PMT photocathode would cause total internal refractions.
Silicon grease or oil of a similar optical index to the crystal and photocathode is used as a light guide between the surfaces.
What is a PMT? what does it do?
basically a photocathode and a vacuum tube with dynodes. It converts visible light into a voltage pulse
