Sunday, November 11, 2012

USE OF RADIACTIVE SUBSTANCE


Iodine-131 and phosphorus-32 are also used for therapy. Iodine-131 is used to treat the thyroid for cancers and other abnormal conditions such as hyperthyroidism (over-active thyroid). In a disease called Polycythemia Vera, an excess of red blood cells is produced in the bone marrow. Phosphorus-32 is used to control this excess. 

Radiolabelled Cr is used to assess GFR. Many Radioimmunoassays of hormones are used for diagnosis of endocrine disorders. Fluoro-deoxy glucose (FDG) incorporating F-18 with a half-life of just less than two hours is readily incorporated into the cell without being broken down, and is a good indicator of cell metabolism. Lead-212, with half-life of 10.6 hours, can be attached to monoclonal antibodies for cancer treatment.

A scintillator is a material, which exhibits scintillation—the property of luminescence when excited by ionizing radiation. Luminescent materials, when struck by an incoming particle, absorb its energy and scintillate, i.e., reemit the absorbed energy in the form of light. Sometimes, the excited state is metastable, so the relaxation back out of the excited state is delayed and can lead to delayed fluorescence or phosphorescence, also called after-glow.

A scintillation detector or scintillation counter is obtained when a scintillator is coupled to an electronic light sensor such as a photomultiplier tube (PMT) or a photodiode. PMTs absorb the light emitted by the scintillator and reemit it in the form of electrons via the photoelectric effect.
Scintillation counter measures the radioactivity. The principle types of scintillation detectors found in clinical chemistry laboratory are crystal scintillation detector and the organic liquid scintillation detector. In this process the absorbed energy produces a flash of light, rather than a pulse of current. Crystal scintillation detects gamma emitters like 15Cr, 131I, etc. Liquid scintillation detects beta emitters like tritium, 32P, 14C.

The sensor called scintillator consists of transparent crystal (made up of Na iodide that can absorb gamma ray) or organic liquid (liquid scintillation counting, contains aromatic solvents and fluor a scintillator) that fluoresces when struck by ionizing radiation. A sensitive photomultiplier tube measures the light from crystal. The PMT is attached to amplifier and other electronic equipment to count and quantify the amplitude of the signal.

In liquid scintillation counter beta particles emitted from the sample transfer energy to the solvent molecules, which in turn transfer their energy to the fluors; the excited fluor molecules dissipate the energy by emitting light. In this way, each beta emission (ideally) results in a pulse of light. Scintillation cocktails often contain additives that shift the wavelength of the emitted light to make it more easily detected. These generated light pulses, reach both photomultiplier tube which, are counted.

(Source: Wikipedia) 
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