1. Instrumentation in nuclear medicine Flashcards
What is contrast dependent on?
Relative uptake of tracer between the organ of interest and the background.
Spatial resolution
poor
How do radiopharmaceuticals work?
The radionuclide allows detection by emitting gamma - it’s distribution within the body creates image contrast.
The pharmaceutical determines where it will go - so it acts as a tracer within the body for a particular physiological process.
What is Technetium-99m?
The most commonly used radionuclide for diagnostic imaging.
Has a half life of 6 hours.
How is Technetium-99m produced?
Produced by the decay of molybdenum-99 (99Mo) - half of 2.8 days.
How is Molybdenum-99 produced?
By neutron irradiation of 98Mo or extracted from fission products.
How does the Technetium-99m generator work?
- 99Mo is adsorbed on alumina (oxide of aluminium) in a sterilised glass column surrounded by a lead/depleted uranium shield.
- 99Tc is eluted (remove from an adsorbed substance by washing with a solvent) from the alumina column by an ion exchange mechanism when sterile saline is passed through it.
- The resulting eluate is sodium pertechnate (NaTc04).
- This is the starting point for a wide range of Tc-99m labelled radiopharmaceuticals.
How is 99m-Tc prepared from kits?
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How do NaI(Tl) Gamma Cameras work?
- Collimator controls gamma rays reaching scintillator.
- Scintillation crystal converts gamma rays into flashes of light.
- An array of photomultiplier tubes (PMT) transform the light into electrical signals.
- Output’s from PMT’s are converted in to three signals (x, y and x).
X and Y give spatial location of scintillation. Z represents energy deposited in the crystal by the gamma ray. - Z signal passes through a pulse height analyser (PHA) which tests whether the energy of the gamma ray is within expected values.
- If the Z signal has an acceptable value, then a signal is sent instructing the display record that gamma has been detected at positions of X and Y signals.
What is a parallel hole collimator?
- consists of several thousand holes, usually in a hexagonal array.
- The thickness and length of the lead ‘septa’ seperates the holes - determines the properties of the collimator.
What are the two key principles of collimator performance?
Spatial resolution and sensitivity.
There is always a compromise.
High res, low sens for static and SPECT imaging. Low res, high sens for dynamic imaging.
Define resolution.
The ability of the collimator to distinguish between 2 small sources of radioactivity close to each other.
Define sensitivity.
A measure of those gamma rays incident on the collimator which pass through to the detector.
> the higher the sensitivity the greater the count rate recorded.
What is a scintillation crystal?
A thin crystal of sodium iodide, dope with thallium [NaI(Tl)]
What happens when a photon interacts with a crystal?
A photon of visible light is emitted by scintillation.
How do photomultiplier tubes work (PMT’s)?
- A vacuum tube detects light produced in the scintillation.
- Light energy releases electrons from a photocathode which are deposited as a thin layer on the inside of the entrance window.
Inside the tube are a series of electrodes called dynodes, that are held at increasing positive potential to each other by a high voltage supply.
3.The electrons are accelerated from the photocathode to the first dynode by the potential difference and gain kinetic energy.
What is the amount of light received by a particular PMT dependent on?
The inverse square distance from the scintillation to that PMT.
- Tube close to the scintillation receive a larger amount of light than distant.
Each PMT will produce a pulse of electrical charge, what is this dependent on?
The amount of light that is converted to a voltage pulse.
- The maximum value of the voltage pulse (pulse height) is directly proportional to the amount of light received by the PMT.
What is the sum of the pulse heights proportional to?
The total amount of light produced.
Hence the energy absorbed (Z signal) from the gamma photon by the crystal.
What does each trio of X, Y and Z represent?
One scintillation (i.e. one detected gamma ray)
> usually called a ‘count’
Hundred of thousands of counts are required to build up an image.
What are the properties of CZT gamma cameras?
- Improved energy resolution
- Direct conversion of gamma photons to electrons.
- Pixelised detector
- Registered collimation
What are the properties of NaI(Tl) gamma cameras?
- Works in two stages: Gamma photons converted to light photons then converted to electrical signals in PMT.
- Large crystal detector
- Interchangeable collimators
What are semiconductor Radiation Detectors?
When a charged particle enters, and directly ionises the crystal > forms holes and electrons.
This applies a potential difference so a current can flow.
What is a CZT camera head made up of?
Many detector modules each etched into multiple pixel elements.
What is registered collimation?
Every hole in the collimator is aligned with a single, corresponding detector pixel.
Why is registered collimation useful?
Because it simplifies localisation so events are detected within a single tiny pixel, rather than a large pixel by multiple PMTs.
Describe the design of novel cameras.
12 CZT detectors, 360 degree configuration, shape adapted gantry, CT required as standard.
