X-rays


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Related to X-rays: Gamma rays, MRI, ultraviolet

x-rays

 [eks´rāz]
high-energy electromagnetic radiation produced by the collision of a beam of electrons with a metal target in an x-ray tube; the penetrability and hardness of the x-rays increase with the voltage applied to the tube, which controls the speed with which the electrons strike the target. Called also roentgen rays.



For diagnostic radiography, tube voltages in the range 80 to 120 kilovolts peak (kVp) are normally used. For radiation therapy, voltages in the 1 to 2 megavolt range are used for most treatment. Accelerating electrons to speeds high enough to produce megavoltage x-rays requires a linear accelerator (lineac). Kilovoltage lower than 80 kVp is often used for the extremities, 25 to 30 kVp is used for mammography, and up to 150 kVp can be used for chest imaging.

The x-ray exposure is proportional to the tube current (milliamperage) and also to the exposure time. In diagnostic radiography, the tube voltage and current and exposure time are selected to produce a high-quality radiograph with the correct contrast and film density. In radiation therapy, these exposure factors are selected to deliver a precisely calculated radiation dose to the tumor. The total dose is usually fractionated so that tumor cells can be oxygenated as surrounding cells die; this increases the sensitivity of the cells to radiation.

Body tissues and other substances are classified according to the degree to which they allow the passage of x-rays (their radiolucency) or absorb x-rays (their radiopacity). Gases are very radiolucent; fatty tissue is moderately radiolucent. Compounds containing high-atomic-weight elements, such as barium and iodine, are very radiopaque; bone and deposits of calcium salts are moderately radiopaque. Water; muscle, skin, blood, and cartilage and other connective tissue; and cholesterol and uric acid stones have intermediate density.
X-ray Contrast Media. A contrast medium is a substance introduced into a structure in order to increase the radiographic contrast with surrounding tissues. The radiopaque contrast media include a variety of organic iodine compounds and the insoluble salt barium sulfate. Radiolucent contrast media are gases such as air, oxygen, or carbon dioxide.



Barium is used for gastrointestinal studies. Water-soluble, iodinated contrast media excreted by the kidneys are used for many procedures, including all types of angiography and for intravenous and retrograde urography; the most commonly used are diatrizoate and iothalamate. Those excreted by the liver are used for oral or intravenous cholangiography or cholecystography. Oily iodinated media are used for lymphangiography, bronchography, and myelography.

All iodinated contrast media can cause reactions, which may range from the common reactions of mild flushing and a feeling of warmth and nausea and vomiting to rare life-threatening reactions requiring immediate aggressive therapy. The cause of these reactions may be allergy; however, this is disputed.

A double contrast study uses both a radiopaque and a radiolucent contrast medium; for example, the walls of the stomach or intestine are coated with barium and the lumen is filled with air. The resulting radiographs clearly show the pattern of mucosal ridges.
Standard stationary anode x-ray tube; diagram in longitudinal section. From Dorland's, 2000.
Simple radiograph. A, X-ray machine; B, patient; and C, x-ray film. From Malarkey and McMorrow, 1996.

X-rays

short-wavelength electromagnetic ionizing radiation, which can penetrate the body structures to varying degrees. Discovered in 1895 by Wilmhelm Röntgen, a German physicist (the very first X-ray photograph showed the bones of his wife's hand, with her wedding ring). Used in general, and in sport, to produce photographic images of body structures, notably bones (to detect fractures), also heart and lungs. See also radiography, radiology.

X-rays

beam of X-rays is directed at a body part, passing through tissues, to impinge on a photographic film; different tissue densities are reflected in the negative film image (i.e. plain radiograph), where highly ray-permeable, less dense tissues (e.g. soft tissues) show as a dark grey image and less ray-permeable, dense tissues (e.g. bone) or materials (e.g. metal or glass) show as a white image on the film plate; air (e.g. ulcer or abscess) shows as a black image (see radiograph; Table 1)
Table 1: Common radiographic projections of the foot and ankle
ProjectionVisualization
Foot projections
Dorsiplantar (DP) projection or anteroposterior (AP) viewWeight-bearing with the beam directed at 15° to the frontal plane, to eliminate distortion due to the angulation of the metatarsals and centred on the metatarsal shafts
It is used to visualize the phalanges, metatarsophalangeal joints, the metatarsals and the midfoot
Lateromedial oblique projectionWeight-bearing with the beam angled at 45° to the lateral side of the sagittal plane and centred on the forefoot; or non-weight-bearing, with the beam vertical and foot everted so that the plantar surface is at 45° to the ground surface
It is used to visualize the phalanges, metatarsals, metatarsocuneiform joints and sesamoids, but tends to give an elongated image of bony architecture
Mediolateral oblique projectionWeight-bearing with the beam angled between 25 and 45° to the medial side of the sagittal plane and centred on the forefoot
It is used to visualize the first ray and associated structures, but tends to give an elongated image of bony architecture
Lateral projectionWeight-bearing or non-weight-bearing, with the beam angled at 90° to the lateral aspect of the foot and centred on the mid- or hindfoot
It is used to visualize the profile of the whole foot, but obscures the midtarsal joint, due to superimposition of local structures
Digital projectionThe lateromedial oblique projection is useful to visualize subungual exostoses, especially when the hallux (or affected toe) is raised up on a pad
Sesamoid projection or skyline projectionWeight-bearing, with the metatarsophalangeal joints dorsiflexed to 45° and the beam angled to be parallel to the ground surface on the sagittal plane, and centred on the plantar aspect of the forefoot
It is used to visualize the relationship of the sesamoids with the head of the first metatarsal
Tarsal and ankle projections
Anteroposterior viewWeight-bearing with the beam angled at 90° to the frontal plane and the beam centred on the ankle joint
Used to visualize the ankle mortise and the trochlear surface of the talus
Axial calcaneal projectionWeight-bearing with the beam angled at 45° to the posterior aspect of the sagittal plane with the beam centred on the hindfoot
It is used to visualize calcaneal trauma
Harris-Beath projectionSimilar to the axial calcaneal projection, but the patient is positioned as if making a ski-jump, that is, weight-bearing with the foot dorsiflexed at the ankle and the beam angled at 45° to the posterior aspect of the sagittal plane with the beam centred on the ankle
It is used to visualize the subtalar joint where talar fusions are suspected
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