There are a number of different radiation detection devices. The scintillation counter is by far the most widely used in nuclear medicine.
Scintillation counters consist of a detector system and a processing display unit. The detector system is made up of a sodium iodide crystal coupled to a photomultiplier tube. When gamma ray photons strike the crystal, flashes of blue-violet light or scintillation occurs. The crystal is transparent to light and it’s enclosed in a light tight container. A powder is used to reflect light out, only through the crystal area adjacent to the photomultiplier tube. Once the flashes of light reach the surface of the photomultiplier tube, electrons are released.
These electrons become amplified in the photomultiplier tube and are then transmitted through a preamplifier to the main unit only to be amplified further. Now the electrons are ready to be processed and displayed. The light emitted from the output signal from the detector unit is proportional to the energy released inside the crystal by way of the gamma photon.
Collimators play an integral part in detector systems. Collimators are designed so that the detector can only see photons in a specific area inside a patient while rejecting others from outside this area. The wide angle collimator is most commonly used and each type is designed for a specific purpose. When counts from a large field of view are needed the wide angle is used.
Parallel collimators are used with camera systems which also view a large area but are concerned mainly with the distribution of the radioactive isotope.
Focusing collimators are used with scanning devices which view a small area as in organ imaging. The collimator with more holes has what is called increased resolution, with decreased sensitivity and the opposite is true for a collimator with less holes.
The electrical pulses of electrons are directed to the processing unit from the detector system. The spectrometer is used to sort the spectrum of gamma energies and accept or reject ones of specific pulse height. The window must be adjusted to reject all pulses above and below certain energy levels. A window is the range of energy of an isotope. In the case of technetium, its energy range is 140Kev. The energy range is set up for 20% of the isotope, one will allow 10% above and 10% below the energy range. This will help to reduce the counts from scattered radiation.
From the spectrometer the information is passed on to be displayed as counts on a scaler. A scaler measures the amount of radioactivity from within the source.
Scanners are designed to produce two-dimensional pictures of the distribution of the radioactive isotope in an organ. Organ scanning is achieved by a systematic movement of a scintillation detection assembly with a focusing collimator, going back and forth across the organ of interest. These rectilinear scanners are now almost obsolete. The gamma camera is the number one imaging device that can produce an image without moving the detector unit. The camera has the ability to “see” certain organs in their entirety. A brief description of the Gamma Scintillation Camera and the Multi-crystal Camera follows.:
The three types of Collimators used in Nuclear Medicine
(figure available in print form)
Diagram of a Scintillation counter in simplified form
(figure available in print form)