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Gamma Cameras David S. Graff PhD

Gamma Cameras

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

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Gamma CamerasDavid S. Graff PhD

Contact Information

• David S. Graff PhD

[email protected]

• http://GraffNucMed.blogspot.com

Review of Scintillation cameras

How a gamma camera works

Artifacts Collimators

Review of Scintillation cameras

How a gamma camera works

Artifacts Collimators

A Gamma Camera is a giant souped-up scintillation camera

A Gamma Camera is a giant souped-up scintillation camera

ScintillatorPhotomultiplier Tube

PMT

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

Photon deposits energy in scintillatorSize 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 Scintillation cameras

How a gamma camera works

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 Scintillation cameras

How a gamma camera works

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 Scintillation cameras

How a gamma camera works

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 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

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 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

Scatter can reduce contrast

Idealized Gamma-Ray Spectrum

in NaI

Energy

Counts per

Energy Interval

Eo

theoretical

Actual

Too wide a window adds scattered photons and reduces contrast