Gamma Cameras

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Education

Published on November 18, 2008

Author: david.s.graff

Source: slideshare.net

Description

Class notes on the fundamental technology of Anger (Gamma) camera. Given at Lehigh Valley Hospital Nuclear Medicin Technologist training program, Allentown, PA

Gamma Cameras David S. Graff PhD

Contact Information • David S. Graff PhD • David.S.Graff@gMail.com • http://GraffNucMed.blogspot.com

Review of How a gamma Scintillation camera works cameras Artifacts Collimators

Review of How a gamma Scintillation camera works cameras Artifacts Collimators

A Gamma Camera is a giant souped-up scintillation camera

A Gamma Camera is a giant souped-up scintillation camera Photomultiplier Tube Scintillator PMT

Some photons pass through the scintillator some drop all their energy some scatter out and deposit some energy

Deposited energy is converted to visible light by scintilator Amount of light proportional to deposited energy

Visible light is converted into electrons at surface of photomultiplier tube Number of electrons is proportional to amount of light

Electrons pulled to positively charged plate Knock more electrons off plate These electrons pulled towards next plate with more positive voltage Large number of electrons land on final plate, generate pulse Size of pulse proportional to number of electrons

Photon deposits energy in scintillator Size of pulse from PMT proportional to deposited energy.

A gamma camera is simply a single large scintillator crystal covered with photomultiplier tubes.

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

Review of How a gamma Scintillation camera works cameras Artifacts Collimators

Anger circuit: Sum of all outputs measures total light = energy of incoming photon: Z

Anger circuit: Weigted sum of all outputs measures position: X+, X- X-

Anger circuit: Weigted sum of all outputs measures position: X+, X- X+

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.

Review of How a gamma Scintillation camera works cameras Artifacts Collimators

What if the gain on a PMT is too high?

Poor positioning can lead to non-linearities

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

Review of How a gamma Scintillation camera works cameras Artifacts Collimators

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

Pinhole Collimator

A pinhole collimator only lets in photons that pass through the small hole. The rest are thrown away.

Pinhole collimators can magnify a small object by moving close.

Parallel collimator

Collimators only pass rays in a small set of angles

How collimators work Most photons are blocked by collimator Only photons Radioactive tracer parallel to only goes to collimator pass certain tissues through Detector sees Photons emitted blurred projection in every direction of tracer in patient

Collimators are defined by the following characteristics: Collimator length l Septal Thickness Hole Diameter a d

Acceptance angle is d/l

Bigger d, wider angle

Shorter l, wider angle

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 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

Scatter can reduce contrast

Idealized Gamma-Ray Spectrum in NaI theoretical Counts per Energy Interval Actual Energy Eo

Too wide a window adds scattered photons and reduces contrast

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