Methods of detecting radiation

A wide variety of instrumentation has been devised for measuring radiation in its many forms. For measuring radiation used in diagnostic and therapeutic procedures, three principal types of apparatus need to be considered: gas-filled detectors, crystal-based scintillation detectors, and liquid scintillation counters. Gas-filled detectors operate on the principle that ionizing radiation interacting with gases such as hydrogen, neon, or argon produce an electric current proportional to the degree of ionization of the contained gas. The two principal applications of gas-filled detectors are dose calibration and survey monitoring of the laboratory environment. Dose calibrators are used to precisely measure amounts of radioactive material contained in radiopharmaceuticals prior to administration to patients. They are comprised of a well, into which the syringe or dose container can be inserted, and a scale calibrated in microcuries or millicuries selectable and specific for a variety of the most commonly used radionuclides (Fig 1). Geiger-Mueller counters are highly sensitive but imprecise monitors that measure relative radiation from any g and some ß emitting radionuclides, expressed in counts per unit time (Fig 2). They are mainly used for locating and documenting the approximate level of radiation around the laboratory environment.

Scintillation detectors depend on certain crystals that emit light when exposed to ionizing radiation. When ionizing radiation interacts with matter the absorbed energy is usually converted to heat. Some substances emit a portion of this excess energy as visible or ultraviolet light and are known as scintillators. The brightness of each flash is proportional to the amount of energy deposited in the crystal, which is proportional to the energy of the incident photon. Thus, both the number and energy of incident photons can be recorded. Scintillation detectors consist of a scintillation crystal and a photomultiplier tube. Sodium iodide (NaI) is the most commonly used scintillation crystal. It has the highest conversion efficiency, about 13% of the energy deposited in the crystal is emitted as visible light. The performance of NaI crystals is maximized by adding a small amount of ionic thallium.

Gamma Cameras
Gamma cameras are the workhorses of nuclear medicine, providing the great majority of scintigraphic images in both human and veterinary medicine. The gamma camera operates on the same principle as the scintillation counter, but is considerably more complex in construction and operation. The essential elements are a multihole, interchangeable collimator attached to the front of the camera, the flat 6- to 12-mm thick sodium iodide crystal measuring up to 50 cm in diameter, backed by a geometric array of 19 to 91 photomultiplier tubes optically coupled to the crystal by “light pipes” of various designs. This assembly is housed in a thick lead container that prevents stray radiation from striking the crystal and is supported by a gantry system that allows at least moderate maneuverability of the heavy camera head. The remaining essential components comprise a logic circuit for determining the position of photon strikes (scintillations) in the crystal, a pulse height analyzer for selection and discrimination of the ap-propriate radionuclide(s), timing circuits, and various means of displaying the image, ranging from a simple cathode ray display (“p-scope”) to a high-resolution microdot imager. The image output from the logic circuit can also be sent to a dedicated computer system for further image processing and analysis. The reason the gamma camera has dominated scintigraphic imaging is the ingenious method by which the position of photon strikes in the crystal are so accurately determined by the logic circuit, despite the small number and relatively large diameters of the photomultiplier tubes. When a scintillation event occurs in the crystal the output from all photomultiplier tubes is recorded simultaneously as a single pulse on an X-Y coordinate system. Determination of the X-Y position of the strike is independent of photomultiplier size and number because all tubes contribute to the signal. A third signal (the Z-pulse) representing the sum of energy deposited in the crystal with each strike is also generated and fed through the pulse-height analyzer. Only those coordinates corresponding to strike energies falling close to the photopeak (within the photopeak “window”) are accepted for display. A single dot of light is then gen-erated on the recording system at the X-Y coordinate corresponding to the same relative position on the camera face. The same X, Y, and Z signals are available to attached computer systems.