gamma cameras
DESCRIPTION
Class notes on the fundamental technology of Anger (Gamma) camera. Given at Lehigh Valley Hospital Nuclear Medicin Technologist training program, Allentown, PATRANSCRIPT
Contact Information
• David S. Graff PhD
• http://GraffNucMed.blogspot.com
Some photons pass through the scintillatorsome drop all their energysome scatter out and deposit some energy
Deposited energy is converted to visible light by scintilatorAmount of light proportional to deposited energy
Visible light is converted into electrons at surface of photomultiplier tubeNumber of electrons is proportional to amount of light
Electrons pulled to positively charged plateKnock more electrons off plateThese electrons pulled towards next plate with more positive voltageLarge number of electrons land on final plate, generate pulseSize of pulse proportional to number of electrons
A scintillator converts the energy into light.
This light is turned into an electic pulse by the PMT.
The height of the electric pulse is proportional to the total light
A scintillator converts the energy into light.
This light is turned into an electic pulse by the PMT.
The height of the electric pulse is proportional to the total light
The total light collected by all PMTs yields the energy of the incoming photon.
The position of the photon is the weighted sum of the signal from the different PMTs.
Weights must be precisely tuned.
A scintillator converts the energy into light.
This light is turned into an electic pulse by the PMT.
The height of the electric pulse is proportional to the total light
The total light collected by all PMTs yields the energy of the incoming photon.
The position of the photon is the weighted sum of the signal from the different PMTs.
Weights of each PMT must be precisely tuned.
Miscorrections in position formula can lead to PMTs visible in flood image
Technologist responsible for examining daily flood for echos of PMTs
Collimators
For a camera to work, we need to be able to reconstruct where the gamma ray came from.
Gamma rays can’t be focussed
A pinhole collimator only lets in photons that pass through the small
hole. The rest are thrown away.
How collimators workMost photons are
blocked by collimator
Photons emitted in every direction
Radioactive tracer only goes to
certain tissues
Only photons parallel to
collimator pass through
Detector sees blurred projection of tracer in patient
Collimators are defined by the following characteristics:
Col
limat
or le
ngth
l
Hole Diameterd
Septal Thicknessa
A scintillator converts the energy into light.
This light is turned into an electic pulse by the PMT.
The height of the electric pulse is proportional to the total light
The total light collected by all PMTs yields the energy of the incoming photon.
The position of the photon is the weighted sum of the signal from the different PMTs.
Weights of each PMT must be precisely tuned.
Miscorrections in position formula can lead to PMTs visible in flood image
Technologist responsible for examining daily flood for echos of PMTsCollimators absorb nearly all photons.
Pinhole collimators can magnify, useful for small objects.
Parallel collimators are most common, only let in photons coming from single direction