x-ray
Definition
X rays are a type of radiation used in imaging and therapy that uses short wavelength energy beams capable of penetrating most substances except heavy metals
Electromagnetic radiation of wavelengths ranging between 5.0 × 10?6 and 5.0 × 10?4 ?m X-rays are produced when fast-moving electrons with sufficient energy strike a target. Most of the electron energy is converted to heat, but a very minute amount —less than 1 per cent —is converted to x-rays Spectrum of Electromagnetic Radiations. X-rays resemble visible light rays very closely but have the distinguishing feature that their wave lengths are very short
Feature of x-ray : • No mass • No charge • Travel at the speed of light • Short wave length • High energy • Source of x-ray from the electron shells x-ray production : •
X-rays are generated by an x-ray tube, a vacuum tube that uses a high voltage to accelerate electrons released by a hot cathode to a high velocity. The high velocity electrons collide with a metal target, the anode, creating the x-rays.
In medical x-ray tubes the target is usually tungsten the insert is evacuated from air because : • Prevent oxidation of the cathode filament • Decrease resistance against electron movement
Advantage of oil in x-ray tube : • Shock absorber • Heat absorber
Two different processes give rise to radiation of X-ray frequency. In one process radiation is emitted by the high-speed electrons themselves as they are slowed or even stopped in passing near the positively charged nuclei of the anode material. This radiation is often called brehmsstrahlung [Ger.,=braking radiation]. In a second process radiation is emitted by the electrons of the anode atoms when incoming electrons from the cathode knock electrons near the nuclei out of orbit and they are replaced by other electrons from outer orbits. The spectrum of frequencies given off with any particular anode material thus consists of a continuous range of frequencies emitted in the first process, and superimposed on it a number of sharp peaks of intensity corresponding to discrete frequencies at which X rays are emitted in the second process. The sharp peaks constitute the X-ray line spectrum for the anode material and will differ for different materials. X-radiation (composed of X-rays) is a form of electromagnetic radiation. X-rays have a wavelength in the range of 10 to 0.001 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz (30 × 1015 Hz to 30 × 1018 Hz) and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays. In many languages, X-radiation is called R?ntgen radiation after one of its first investigators, Wilhelm Conrad R?ntgen. X-rays, or roentgen rays, are electromagnetic waves in which periodically variable electric and magnetic fields are perpendicular to each other and to the direction of propagation. Thus they are identical in nature with visible light and all the other types of radiation that constitute the electromagnetic spectrum. In general, x-rays are generated as the result of energy transitions of atomic electrons caused by the bombardment of a material of high atomic weight by high-energy electrons. The range of x-rays in the electromagnetic spectrum, as excited in x-ray tubes by the bombardment of anode targets by cathode electrons under a high accelerating potential, overlaps the ultraviolet range on the order of 100 nanometers on the long-wavelength side, and the shortest-wavelength limit moves downward as voltages increase. An accelerating potential of 109 volts, now readily generated, produces a wavelength of 10?15 m (10?6 nm). An average wavelength used in research is 0.1 nm, or about 1/6000 the wavelength of yellow light. See also X-ray tube. X rays pass easily through air and soft tissue of the body. When they encounter more dense material, such as a tumor, bone, or a metal fragment, they are stopped. Diagnostic x rays are performed by positioning the part of the body to be examined between a focused beam of x rays and a plate containing film. This process is painless. The greater the density of the material that the x rays pass through, the more rays are absorbed. Thus bone absorbs more x rays than muscle or fat, and tumors may absorb more x rays than surrounding tissue. The x rays that pass through the body strike the photographic plate and interact with silver molecules on the surface of the film. Once the film plates have been processed, dense material such as bone shows up as white, while softer tissue shows up as shades of gray, and airspaces look black. A radiologist, who is a physician trained to interpret diagnostic x rays, examines the pictures and reports to the doctor who ordered the tests. Plain film x rays normally take only a few minutes to perform and can be done in a hospital, radiological center, clinic, doctor s or dentist s office, or at bedside with a portable x-ray machine. • X ray, invisible, highly penetrating electromagnetic radiation of much shorter wavelength (higher frequency) than visible light. The wavelength range for X rays is from about 10?8 m to about 10?11 m, or from less than a billionth of an inch to less than a trillionth of an inch; the corresponding frequency range is from about 3 × 1016 Hz to about 3 × 1019 Hz (1 Hz = 1 cps). • X rays are also produced in a highly evacuated glass bulb, called an X-ray tube, that contains essentially two electrodes—an anode made of platinum, tungsten, or another heavy metal of high melting point, and a cathode. When a high voltage is applied between the electrodes, streams of electrons (cathode rays) are accelerated from the cathode to the anode and produce X rays as they strike the anode
The gray (Gy) which has units of (J/kg), is the SI unit of absorbed dose which is the amount of radiation required to deposit 1 joule of energy in 1 kilogram of any kind of matter.
The rad is the (obsolete) corresponding traditional unit, equal to 0.01 J deposited per kg. 100 rad = 1 Gy. The equivalent dose is the measure of the biological effect of radiation on human tissue. For X-rays it is equal to the absorbed dose.
The sievert (Sv) is the SI unit of equivalent dose, which for X-rays is equal to the gray (Gy).
The rem is the traditional unit of equivalent dose. For X-rays it is equal to the rad or J of energy deposited per kg. 1 sievert = 100 rem
Definition Computed tomography (CT) an imaging method that uses x-rays to create cross-sectional pictures of the body. Alternative Names CAT scan; Computed axial tomography (CAT) scan What is computed tomography? Computed tomography, commonly known as a CT scan, uses X-rays and computers to produce images of a cross-section of the body. The patient must lie as still as possible as the table moves through the large, donut-shaped scanning device. Movement could blur In conventional x-rays, a beam of energy is aimed at the body part being studied. A plate behind the body part captures the variations of the energy beam after it passes through skin, bone, muscle, and other tissue. While much information can be obtained from a regular x-ray, specific detail about internal organs and other structures is not available With computed tomography scan (also called CT or CAT scan), the x-ray beam moves in a circle around the body. This allows for many different views of the same organ or structure, and provides much greater detail. The x-ray information is sent to a computer which interprets the x-ray data and displays it in two-dimensional form on a monitor How the Test is Performed You will be asked to lie on a narrow table that slides into the center of the CT scanner. Depending on the study being done, you may need to lie on your stomach, back, or side. Once inside the scanner, the machine s x-ray beam rotates around you. (Modern "spiral" scanners can perform the exam in one continuous motion.)