Section 7 Flashcards
Gamma rays and xrays are both
high energy electromagnetic radiation (photons)
What is the difference between xrays and gamma rays?
Xrays produced by electrons in atoms
Gamma produced by nucleus
What are parallels of DR & NM?
Detecting nonvisible radiation; see visible structures in body & give diagnostic info abt patients
How is NM different from DR?
NM has source in the patient and uses gamma rays
Xrays are an __ source and show __
External source and show anatomy
Gamma rays are ___ source and show ___
Internal source and show biofunctionality
What is the most common isotope used in NM procedures?
Technicium 99
Gamma energy is determined by
the radioisotope, as the energy of the decay is the same - there is no kVp adjustment
Radiopharmaceutical is a
radioactive isotope attached to a pharmaceutical molecule
Why do we need a detector for gamma rays?
To measure how much gamma radiation; spatial position of source & energy of the radiation
Since gamma rays cannot be switched of, what factors must we consider?
- Low Dose Rate
- Small signals from scintillator
Photomultiplier tube uses
photoelectric effect to turn light into electrical signal
Describe the key properties of NM detectors
PMT, PMT correlation and scintillator
Describe the gamma camera
Detect scintillations (flashes of light) produced when gamma rays, resulting from radioactive decay of single photon emitting radioisotopes, interact with a sodium iodide crystal at the front of the camera.
Collimator acts as a lens for our camera - enables the parallel rays to be detected and non-parallel rays to be absorbed
Describe the PMT correlation
- Gamma rays hit the scintillator
- Multiple visible photons are produced
- Nearby PMT get high signal and distant PMT have a weak signal
- SPL works out where gamma rays hits scintillator
SPL =
Spatial Position Logic
A collimator enables the
gamma source location to be determined
What are the collimator walls called?
Septa and are thin as possible to minimise absorption
Small Collimator holes =
High resolution
Large collimator holes=
Low resolution
Larger septa depth =
High resolution
Smaller septa depth =
Low resolution
Increase in resolution means __ sensitivity?
decrease in sensitivity
Pulse Height Analysis is where
we reject scattered photons
What are the deteriments of image quality?
- Spatial resolution
- Contrast resolution
-Sensitivity - Motion
Spatial resolution
Better SR means finer details of an object but we have low dose rate and have a short scan time
Contrast in NM comes from
functional distribution of radioactive material
Detection of scattered radiation
may decrease contrast - use PHA for energy discrimination
Sensitivity is how
much radioactivity required for the production of an image - expressed in keps/MBq
Decrease in sensitivity =
increase in radioactivity and dose
Gating (switching) can be used
to measure only at specific points in a cycle
SPL measures
where the gamma ray hit the scintillator/detector
What does the height of the PMT pulse indicate?
Greater the height indicates how close gamma absorption was
PMT detects
gamma rays
PMT correlation works out
where they hit the detector
Collimator works out
which direction gamma rays came from and we make sure they came in a straight line
