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Instrumentation: Gamma Camera - Coggle Diagram
Instrumentation: Gamma Camera
Scintillation Creystal
a single crystal of thallium activated
sodium iodide, NaI(Tl)
When γ-ray strikes the crystal, it loses energy through photoelectric
Compton interactions with the crystal.
electrons ejected by these interactions lose energy in a short distance
exciting the scintillation molecules
Intensity of the light is proportional to the energy of the incident γ–ray
The time for the excited states to
return to equilibrium, is 230 ns for NaI(Tl)
10^4-10^5 γ-rays per second can be recorded
accurately
NaI(Tl) at 140 keV has a high value, 2.22
cm^-1
90% of the γ-rays that strike the scintillation crystal are absorbed
in a 1 cm thickness
13% of the energy deposited in the crystal
Emitted as visible light
Pulse Height Analyzer
The "z signal' is sent to a pulse-height analyzer (PHA)
Compares the z-signal to a threshold value
If the z-signal is below this threshold, it is rejected as having
originated from a γ-ray that has been Compton-scattered
99mTc scan corresponds to that produced by a γ-ray with energy 140 keV
Collimators
Reduces the contribution of γ-rays
that have been Compton-scattered in tissue
Reduce image CNR
Most common geometry is a parallel-hole
collimator,
only γ-rays traveling at angles close to 90° are detected
constructed from thin strips of
lead
The normal pattern of the lead strips is a hexagonally
based "honeycomb" geometry
Two general classes of collimators, referred to
as high resolution (HR) and high sensitivity (HS)
for imaging small organs close to the surface of the body
for imaging very small organ, primarily for thyroid and parathyroid imaging
to image a structure larger than the size of the detector
Coupled Photomultiplier Tubes
The light photons emitted by the crystal will be detected
by hexagonal PMTs
Light emitted by the scintillation
crystal is converted into a current
every electron striking a dynode, a significantly greater number of electrons is emitted.
the property that the distance from the center of one PMT to that of each neighboring PMT is the same
Anger Position Network
Produces four output signals, X+, X-, Y+, and Y-
Produce relative
magnitudes
Produces signs of which define the location of the scintillation
event in the crystal
