Gamma Camera (OCR A Level Physics) : Revision Note

Gamma Camera Components

• The progress of a medical tracer around the body can be detected using a gamma camera

• Images obtained by a gamma camera can be used for diagnosing issues in specific organs

• A gamma camera is comprised of four major components:

  • Collimator

  • Scintillator

  • Photomultiplier tubes

  • Computer and display

Structure of the Gamma Camera

A gamma camera detects the gamma rays emitted by a radioactive tracer in the body using a large scintillator crystal connected to an array of photomultipliers

Collimator

• Images of slices of the body can be taken to show the position of the gamma-emitting radioactive tracers

• Once injected with a tracer, the patient lays stationary in a tube surrounded by a ring of detectors

• When gamma rays are emitted, they may be absorbed by thin lead tubes known as collimators

• Collimators are the key to producing the sharpest and highest resolution images 

  • Photons moving parallel to the collimator will not be absorbed, which means only these photons reach the scintillator crystal

  • Photons moving in any other direction will be absorbed, as excluding scattered photons allows for sharper images to be produced

  • The narrower and longer the collimators, the more gamma rays that are absorbed and hence, the more electrons that will be produced

  • This improves the image quality as more electrons contributing to the electrical pulse output will increase the resolution of the image

The Collimator

The collimator ensures high resolution images are produced by only allowing photons travelling parallel to the lead plates to pass through

Scintillator

• When the gamma-ray (γ-ray) photon is incident on a crystal scintillator, an electron in the crystal is excited to a higher energy state

  • As the excited electron travels through the crystal, it excites more electrons

  • When the excited electrons move back down to their original state, the lost energy is transmitted as visible light photons

The Scintillator

The scintillator crystal converts the energy from gamma photons into visible light photons

Photomultiplier Tubes

• The photons produced by the scintillator are very faint

• Hence, they need to be converted to an electrical signal and amplified by a photomultiplier tube

• When photons from the scintillator reach the photomultiplier, electrons are released from a photocathode

• The liberated electrons accelerate through a series of dynodes, each at a progressively higher potential difference, before reaching an anode at the end of the tube

• Energy gained by the acceleration of the electrons triggers the release of more electrons at each dynode, resulting in a stronger electrical signal 

A photomultiplier tube

A photomultiplier detects the faint flashes of light from the scintillator, converts them into voltage pulses, and amplifies the signals

Image formation on a computer

• The signals produced by the photomultiplier tubes are used to produce an image using the electrical signals from the detectors

• The tracers will emit lots of γ rays simultaneously, and the computers will use this information to create an image

• The more photons from a particular point, the more tracer that is present in the tissue being studied, and this will appear as a bright point on the image

• An image of the tracer concentration in the tissue can be created by processing the arrival times of the gamma-ray photons

Diagnosis Using a Gamma Camera

• Gamma camera imaging can be used for diagnosing issues in multiple organs

• When imaging a patient using a gamma camera, a gamma emitter, usually technetium-99m, is used as the radioactive tracer

  • The 'm' stands for metastable which means its nucleus stays in a high-energy state for extended periods

• Tc-99m loses energy by the emission of a gamma photon with an energy of exactly 140 keV

• When Tc-99m decays through gamma emission (with a half-life of approximately 6 hours), it becomes Tc-99, which is stable and has a half-life of 210,000 years

• Technetium-99m is used because:

  • Its short half-life means it stays around long enough to be imaged but reduces harm to the patient.

  • Its chemical properties enable a small quantity to be incorporated into several tissues, so it can be used to image several organs at once