| photon | <physics> A quantum unit of light energy. (09 Oct 1997) |
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| photon density | The number of counted events recorded in scintigraphy per square centimeter or per square inch of imaged area. Synonym: count density. (05 Mar 2000) |
| photoncia | Any swelling resulting from the intense action of light. Origin: photo-+ G. Onkos, a mass (tumour) (05 Mar 2000) |
| photonosus | Any disease caused by excessive exposure to or unusual intensity of light, or resulting from phototoxicity or photoallergy. Synonym: photopathy. Origin: photo-+ G. Nosos, disease (05 Mar 2000) |
| photons | Discrete concentrations of energy, apparently massless elementary particles, that move at the speed of light. They are the unit or quantum of electromagnetic radiation. Photons are emitted when electrons move from one energy state to another. (12 Dec 1998) |
| absorptiometry, photon | A noninvasive method for quantitating bone mineral content. It is used especially in the diagnosis of osteoporosis and also in measuring bone mineralization in infants. (12 Dec 1998) |
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| single photon emission computed tomography | <radiology> Tomographic imaging of metabolic and physiological functions in tissues, the image being formed by computer synthesis of photons of a single energy emitted by radionuclides administered in suitable form to the patient. The method uses radionuclides which emit a single photon of a given energy. The camera is then rotated 180 or 360 degrees around the patient to capture images at multiple positions along the arc. The computer then reconstructs the transaxial, sagittal, and coronal images from the 3-dimensional distribution of radionuclides in the target area scanned. The advantages of SPECT are that it can be used to observe biochemical and physiological processes as well as the size and volume of the organ. The disadvantage is that, unlike positron emission tomography where the positron-electron annihilation results in the emission of 2 photons at 180 degrees from each other, SPECT requires physical collimation to line up the photons, which results in the loss of available photons and hence degrades the image. Acronym: SPECT (20 Jun 2000) |