Path:Index:Hardware
Once a radiopharmaceutical has been administered, it
is necessary to detect the gamma ray emissions in order to attain the functional
information. The instrument used in Nuclear Medicine for the detection
of gamma rays is known as the Gamma camera. The components making
up the gamma camera are the collimator, detector crystal, photomultiplier
tube array, position logic circuits, and the data analysis computer.
The purpose of each is briefly described below.
1. Camera Collimator
The first object that an emitted gamma photon encounters
after exiting the body is the collimator. The collimator is a pattern of
holes through gamma ray absorbing material, usually lead or tungsten, that
allows the projection of the gamma ray image onto the detector crystal.
The collimator achieves this by only allowing those gamma rays traveling
along certain directions to reach the detector; this ensures that
the position on the detector accurately depicts the originating location
of the gamma ray.
Click here to see a top and side
view of the collimator.
2. Scintillation Detector
In order to detect the gamma photon we use scintillation
detectors. A Thallium-activated Sodium Iodide [NaI(Tl)] detector
crystal is generally used in Gamma cameras. This is due to this crystal's
optimal detection efficiency for the gamma ray energies of radionuclide
emission common to Nuclear Medicine. A detector crystal may be circular or rectangular. It
is typically 3/8" thick and has dimensions of 30-50 cm.
A gamma ray photon interacts with the detector by means
of the Photoelectric Effect or Compton Scattering with the iodide ions
of the crystal. This interaction causes the release of electrons
which in turn interact with the crystal lattice to produce light, in a
process known as scintillation.
Click here to see a diagram of the
Photoelectric Effect.
Click here to see a diagram of Compton
Scattering.
Click here to see a diagram of Scintillation.
3. Photomultiplier Tubes
Only a very small amount of light is given off from the scintillation
detector. Therefore, photomultiplier tubes are attached to the back
of the crystal. At the face of a photomultipler tube (PMT) is a photocathode
which, when stimulated by light photons, ejects electrons. The PMT
is an instrument that detects and amplifies the electrons that are produced
by the photocathode. For every 7 to 10 photons incident on the photocathode,
only one electron is generated. This electron from the cathode is
focused on a dynode which absorbs this electron and re-emits many more
electrons (usually 6 to 10). These new electrons are focused on the
next dynode and the process is repeated over and over in an array of dynodes.
At the base of the photomultiplier tube is an anode which attracts the
final large cluster of electrons and converts them into an electrical pulse.
Click here to see a diagram of a single
photomultiplier tube.
Each gamma camera has several photomultiplier tubes arranged
in a geometrical array. The typical camera has 37 to 91 PMT's.
A Photomultiplier Tube Array
4. Position Circuitry
The position logic circuits immediately follow the photomultiplier
tube array and they receive the electrical impulses from the tubes in the
summing matrix circuit (SMC). This allows the position circuits to
determine where each scintillation event occurred in the detector crystal.
Click here to see a diagram
of the position circuitry.
5. Data Analysis Computer
Finally, in order to deal with the incoming projection data
and to process it into a readable image of the 3D spatial distribution
of activity within the patient, a processing computer is used. The
computer may use various different methods to reconstruct an image, such
as filtered back projection or iterative reconstruction, both of which
are further described in this tutorial.
