It is also called a scintillation camera or Anger camera and it is used to image gamma radiation emitting radioisotopes, a technique known as scintigraphy. The applications of scintigraphy include early drug development and nuclear medical imaging to view and analyze images of the human body or the distribution of medically injected, inhaled, or ingested radionuclides emitting gamma rays.
A gamma camera consists of one or more flat crystal planes (or detectors) optically couple to an array of photomultiplier tubes, the assembly is known as a “head”, mount on a gantry. The gantry is connect to a computer system that both controls the operation of the camera as well as acquisition and storage of acquired images. The construction of a gamma camera is sometimes known as a compartmental radiation construction. The system accumulates events, or counts, of gamma photons that are absorb by the crystal in the camera. Usually, a large flat crystal of sodium iodide with thallium doping in a light-sealed housing is use.
the crystal scintillates in response to incident gamma radiation. When a gamma photon leaves the patient, it knocks an electron loose from an iodine atom in the crystal, and a faint flush of light is produce when the dislocated electron again finds a minimal energy state. The initial phenomenon of the excited electron is similar to the photoelectric effect and the Compton effect and after the flash of light is produce, it is detect
o Photomultiplier tubes (PMTs) behind the crystal detect the fluorescent flashes and a computer sums the counts.
- Gamma camera uses sets of vacuum tube photomultipliers (PMT).
- Each tube has an exposed face of about 7.6 cm in diameter and the tubes are arrange in hexagon configurations, behind the absorbing crystal.
- Electronic circuit connecting the photodetectors is wire so as to reflect the relative coincidence of light fluorescence as sensed by the members of the hexagon detector array.
- All the PMTs simultaneously detect the (presumed) same flash of light to varying degrees, depending on their position from the actual individual event.
- Thus, the spatial location of each single flash of fluorescence is reflect as a pattern of voltages within the interconnecting circuit array.
- The location of the interaction between the gamma-ray and the crystal can determine by processing the voltage signals from the photomultipliers; in simple terms, the location can find by weighting the position of each photomultiplier tube by the strength of its signal and then calculating a mean position from the weighted positions.
- The total sum of the voltages from each photomultiplier is proportional to the energy of the gamma-ray interaction, thus allowing discrimination between different isotopes or between scattered and direct photons
- In order to obtain spatial information about the gamma-ray emissions from an imaging subject, a method of correlating the detected photons with their point of origin is require.
- Conventional method is to place a collimator over the detection crystal/PMT array.
- The collimator consists of a thick sheet of lead, typically 1-3 inches thick, with thousands of adjacent holes through it.
- Collimator attenuates most of the incident photons and thus greatly limits the sensitivity of the camera system.
- Scintigraphy is the use of gamma cameras to capture emitted radiation from internal radioisotopes to create two-dimensional images.
- SPECT (single-photon emission computed tomography) imaging, as use in nuclear cardiac stress testing, is perform using gamma cameras.