A gamma camera is a device 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 analyse images of the human body of the distribution of medically injected, inhaled, or ingested radionuclides emitting gamma rays. Nuclear medicine is a branch of Medicine and Medical imaging that uses the nuclear properties of matter in diagnosis and therapy A radionuclide is an Atom with an unstable nucleus, which is a nucleus characterized by excess energy which is available to be imparted either to a newly-created Gamma rays (denoted as &gamma) are a form of Electromagnetic radiation or light emission of frequencies produced by sub-atomic particle interactions
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Construction
A gamma camera consists of one or more flat crystal planes or, detectors, optically coupled to an array of photomultiplier tubes, the assembly is known as a "head", mounted on a gantry. The gantry is connected to a computer system that both controls the operation of the camera as well as acquisition and storage of acquired images.
The system accumulates events, or counts, of gamma photons that are absorbed by the crystal in the camera. Gamma rays (denoted as &gamma) are a form of Electromagnetic radiation or light emission of frequencies produced by sub-atomic particle interactions In Physics, the photon is the Elementary particle responsible for electromagnetic phenomena Usually a large flat crystal of sodium iodide with thallium doping in a light-sealed housing is used. The highly efficient capture method of this combination for detecting gamma rays was discovered by noted physicist Robert Hofstadter in 1948 [1]). Robert Hofstadter ( February 5, 1915 &ndash November 17, 1990) was the winner of the 1961 Nobel Prize in Physics "for his
The crystal scintillates in response to incident gamma radiation. Scintillation is a flash of Light produced in a transparent material by an Ionization event When a gamma photon leaves the patient (who has been injected with a radioactive pharmaceutical), it knocks an electron loose from an iodine atom in the crystal, and a faint flash of light is produced when the dislocated electron again finds a minimal energy state. Radiopharmacology is the study and preparation of radiopharmaceuticals, which are radioactive Pharmaceuticals Radiopharmaceuticals are used in the field The initial phenomenon of the excited electron is similar to the photoelectric effect and (particularly with gamma rays) the Compton effect. Introduction When a Metallic surface is exposed to Electromagnetic radiation above a certain threshold Frequency, the light is absorbed and Electrons The Compton shift formula Klein-Nishina formulaCompton used a combination of three fundamental formulas representing the various aspects of classical and modern physics combining After the flash of light is produced, it is detected. Photomultiplier tubes (PMTs) behind the crystal detect the fluorescent flashes (events) and a computer sums the counts. Photomultiplier tubes ( photomultipliers or PMT s for short members of the class of Vacuum tubes and more specifically Phototubes are extremely The computer reconstructs and displays a two dimensional image of the relative spatial count density on a monitor. This reconstructed image reflects the distribution and relative concentration of radioactive tracer elements present in the organs and tissues imaged.
Signal processing
Hal Anger developed the first gamma camera in 1957. Hal Oscar Anger ( May 20, 1920 &mdash October 31, 2005) was an American electrical engineer and biophysicist at Year 1957 ( MCMLVII) was a Common year starting on Tuesday (link displays the 1957 Gregorian calendar) His original design, frequently called the Anger camera, is still widely used today. The Anger camera uses sets of vacuum tube photomultipliers. This article is about the electronic device not an evacuated pipe used for experiments in Free-fall. Generally each tube has an exposed face of about 3 inches in diameter and the tubes are arranged in hexagon configurations, behind the absorbing crystal. The electronic circuit connecting the photodetectors is wired 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 reflected as a pattern of voltages within the interconnecting circuit array.
The location of the interaction between the gamma ray and the crystal can be determined by processing the voltage signals from the photomultipliers; in simple terms, the location can be found 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.
Spatial resolution
In order to obtain spatial information about the gamma emissions from an imaging subject (e. Gamma (uppercase &Gamma, lowercase γ Γάμμα is the third letter of the Greek alphabet. g. a person's heart muscle cells which have absorbed an intravenous injected radioactive, usually thallium-201 or technetium-99m, medicinal imaging agent) a method of correlating the detected photons with their point of origin is required.
The conventional method is to place a collimator over the detection crystal/PMT array. A collimator is a device that narrows a beam of particles or waves The collimator consists of a thick sheet of lead, typically 1-3 inches thick, with thousands of adjacent holes through it. Characteristics Lead has a dull luster and is a dense, Ductile, very soft highly The individual holes limit photons which can be detected by the crystal to a cone; the point of the cone is at the midline center of any given hole and extends from the collimator surface outward. However, the collimator is also one of the sources of blurring within the image; lead does not totally attenuate incident gamma photons, there can be some crosstalk between holes. In Electronics, the term crosstalk ( XT) refers to any phenomenon by which a signal transmitted on one circuit or channel of a Transmission system
Unlike a lens, as used in visible light cameras, the collimator attenuates most (>99%) of incident photons and thus greatly limits the sensitivity of the camera system. Large amounts of radiation must be present so as to provide enough exposure for the camera system to detect sufficient scintillation dots to form a picture.
Other methods of image localization (pinhole, rotating slat collimator with CZT (Gagnon & Matthews) and others) have been proposed and tested; however, none have entered widespread routine clinical use. A' pinhole camera' is a very simple Camera with no lens and a single very small Aperture. Cadmium zinc telluride, (CdZnTe or CZT, is (as the name indicates a compound of Cadmium, Zinc and Tellurium or more strictly speaking an alloy
The best current camera system designs can differentiate two separate point sources of gamma photons located a minimum of 1. 8 cm apart, at 5 cm away from the camera face. Spatial resolution decreases rapidly at increasing distances from the camera face. This limits the spatial accuracy of the computer image: it is a fuzzy image made up of many dots of detected but not precisely located scintillation. This is a major limitation for heart muscle imaging systems; the thickest normal heart muscle in the left ventricle is about 1. 2 cm and most of the left ventricle muscle is about 0. 8 cm, always moving and much of it beyond 5 cm from the collimator face. To help compensate, better imaging systems limit scintillation counting to a portion of the heart contraction cycle, called gating, however this further limits system sensitivity.
Scanning and use in PET
SPECT (single photon emission computed tomograpy) imaging, as used in nuclear cardiac stress testing, is performed using gamma cameras, usually one, two or three detectors or heads, are slowly rotated around the patient's torso. Single photon emission computed tomography (SPECT or less commonly SPET is a Nuclear medicine tomographic imaging technique using Gamma rays. A cardiac stress test is a Medical test that indirectly reflects arterial Blood flow to the Heart during Physical exercise
Multi-headed gamma cameras can also be used for Positron emission tomography scanning, provided that their hardware and software can be configured to detect 'coincidences' (near simultaneous events on 2 different heads). Positron emission tomography ( PET) is a Nuclear medicine imaging technique which produces a three-dimensional image or map of functional processes in the Gamma camera PET is markedly inferior to PET imaging with a purpose designed PET scanner, as the scintillator crystal has poor sensitivity for the high-energy annihilation photons, and the detector area is significantly smaller. However, given the low cost of a gamma camera and its additional flexibility compared to a dedicated PET scanner, this technique is useful where the expense and resource implications of a PET scanner cannot be justified.
See also
References
- H. Nuclear medicine is a branch of Medicine and Medical imaging that uses the nuclear properties of matter in diagnosis and therapy Anger. A new instrument for mapping gamma-ray emitters. Biology and Medicine Quarterly Report UCRL, 1957, 3653: 38. (University of California Radiation Laboratory, Berkeley)
- Anger HO. Scintillation camera with multichannel collimators. J Nucl Med 1964 Jul;65:515-31. PMID 14216630
- PF Sharp, et al, Practical Nuclear Medicine, IRL Press, oxford
- D. Gagnon, C. G. Matthews, US Patent #6,359,279 & #6,552,349
- Physics in nuclear medicine, third edition, Cherry, Sorenson, Phelps
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