Computed Radiography[ edit ] We have previously described the process of photostimulable luminescence which is exploited in Computed Radiography, where the absorption of radiation causes electrons to become trapped at intermediate energy levels. Phosphors having this property are therefore referred to as Storage Phosphors. A latent X-ray image can be recorded using a plate coated with crystals of barium fluorohalide compounds which contain trace amounts of europium.
Principles[ edit ] A material can emit light either through incandescencewhere all atoms radiate, or by luminescencewhere only a small fraction of atoms, called emission centers or luminescence centers, emit light.
In inorganic phosphors, these inhomogeneities in the crystal structure are created usually by addition of a trace amount of dopantsimpurities called activators.
In rare cases dislocations or other crystal defects can play the role of the impurity. The wavelength emitted by the emission center is dependent on the atom itself and on the surrounding crystal structure.
The scintillation process in inorganic materials is due to the electronic band structure found in the crystals. An incoming particle can excite an electron from the valence band to either the conduction band or the exciton band located just below the conduction band and separated from the valence band by an energy gap.
This leaves an associated hole behind, in the valence band. Impurities create electronic levels in the forbidden gap. The excitons are loosely bound electron—hole pairs that wander through the crystal lattice until they are captured as a whole by impurity centers.
The latter then rapidly de-excite by emitting scintillation light fast component. In case of inorganic scintillatorsthe activator impurities are typically chosen so that the emitted light is in the visible range or near-UVwhere photomultipliers are effective.
The holes associated with electrons in the conduction band are independent from the latter. Those holes and electrons are captured successively by impurity centers exciting certain metastable states not accessible to the excitons.
The delayed de-excitation of those metastable impurity states, slowed down by reliance on the low-probability forbidden mechanismagain results in light emission slow component. Phosphor degradation[ edit ] Many phosphors tend to lose efficiency gradually by several mechanisms.
The activators can undergo change of valence usually oxidationthe crystal lattice degrades, atoms — often the activators — diffuse through the material, the surface undergoes chemical reactions with the environment with consequent loss of efficiency or buildup of a layer absorbing either the exciting or the radiated energy, etc.
The degradation of electroluminescent devices depends on frequency of driving current, the luminance level, and temperature; moisture impairs phosphor lifetime very noticeably as well. Harder, high-melting, water-insoluble materials display lower tendency to lose luminescence under operation. Three mechanisms are involved; absorption of oxygen atoms into oxygen vacancies on the crystal surface, diffusion of Eu II along the conductive layer, and electron transfer from Eu II to adsorbed oxygen atoms, leading to formation of Eu III with corresponding loss of emissivity.
Eu phosphors under electron bombardment in presence of oxygen form a non-phosphorescent layer on the surface, where electron—hole pairs recombine nonradiatively via surface states. Mn, used in AC thin-film electroluminescent ACTFEL devices degrades mainly due to formation of deep-level trapsby reaction of water molecules with the dopant; the traps act as centers for nonradiative recombination.
The traps also damage the crystal lattice.
|Pelvis Phantom (Digital Photospot )||Conventional x-ray radiography produces images of anatomy that are shadowgrams based on x-ray absorption.|
|RADIOLOGY BOOKS||The figure shows the K-shell at the bottom with an occupancy of 2 electrons and with the greatest binding energy. Above it is the L-shell with an occupancy of 8 electrons and a lower binding energy - and so on for the other shells.|
|Basic Physics of Digital Radiography/The Image Receptor - Wikibooks, open books for an open world||Angiograhpy is an X-ray examination with radio-opaque contrast medium in the vascular system to image the configuration of vascular circulation.|
Phosphor aging leads to decreased brightness and elevated threshold voltage. Electron-stimulated reactions of the surface are directly correlated to loss of brightness. The electrons dissociate impurities in the environment, the reactive oxygen species then attack the surface and form carbon monoxide and carbon dioxide with traces of carbonand nonradiative zinc oxide and zinc sulfate on the surface; the reactive hydrogen removes sulfur from the surface as hydrogen sulfideforming nonradiative layer of metallic zinc.
Sulfur can be also removed as sulfur oxides. The reduced metal can be observed as a visible darkening of the phosphor layer.Angiography Systems Pei-Jan Paul Lin Department of Radiology Beth Israel Deaconess Medical Center and Harvard Medical School X-ray Photons Electrons Output Phosphor Electrons Light Photons Thin Aluminum Layer Fluorescent Screen Input Phosphor (a) Fluorescent screens are made of Cesium Iodide (CsI) which has a.
DIGITAL SUBTRACTION ANGIOGRAPHY (DSA). Angiograhpy is an X-ray examination with radio-opaque contrast medium in the vascular system to image the configuration of vascular circulation.
Digital Subtraction Angiography (DSA) CHAPTER 8 TABLE OF CONTENTS Diagnostic Radiology Physics: a Handbook for Teachers and Students –chapter 8, 3. IAEA Ratio of Luminanceat the output phosphor to the Incident X-ray Air Kerma Rateat the input phosphor typically cd.m-2/µGy.s-1 FLUOROSCOPIC . Imaging of inner structures: nowadays there is a request for noninvasive techniques in diagnostics, and hence invasive ones are restricted to presurgical examinations and interventional radiology; projection: reduction of dimensionality; 2D maximum intensity projection (MIP): a 3D image (x/y/z) projected into the x/y plane by assignign the maximum intensity that can be found along the z axis.
The output phosphor of the x-ray image intensifier, which typically is called P20, is a fluorescent compound made of silver-activated zinc-cadmium sulfide (ZnCdS:Ag). The emission spectrum of P20 is at a maximum around nm (green light). REVIEW ARTICLE Scintillation detectors for x-rays Martin Nikl Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnicka 10, photodetector at the output of which an electrical signal is available for further processing.
Scintillation conversion mechanism thin layer of phosphor is used to convert an x-ray image.