Introduction to Imaging Devices
Medical imaging predates the recording of an electrocardiogram (ECG) by about 8 years. X-rays were discovered on November 8, 1895, by Wilhelm Conrad Röntgen. Over the next 7 weeks, Röntgen conducted additional research and published his findings. He did not apply for any patent protection on his discovery. He was awarded the Nobel Prize for Physics in 1901.
Many of the eras top scientists, including the Curies and Thomas Edison, got involved with x-rays. Progress was slow for many years, and some scientists developed x-ray applications that were later abandoned (such as acne and tonsillitis treatment regimens, as well as the commercial viewing of feet to check the fit of shoes). After World War II, mobile vans were equipped with x-ray units for tuberculosis screenings.
In the late 1940s in Japan, sound navigation and ranging (sonar), used to detect underwater objects, was adapted for viewing internal organs and structures. Sonar arrived in the United States during the early 1950s, mostly as a research method; it did not achieve wide clinical use until the late 1960s. The 1980s brought color ultrasound, and three-dimensional ultrasonography was developed in the 1990s.
In the 1970s, computed tomography (CT) scanning was introduced, but its widespread use was hindered by high costs and low reliability. As electronics advanced, so did CT, to the point that it is now rare for a hospital to not have at least one CT unit.
The 1980s brought the introduction of nuclear magnetic resonance imaging, soon called magnetic resonance imaging (MRI) to avoid negative associations with the word nuclear. Early problems associated with motion artifacts and long image-acquisition times have been overcome, and MRI is now a primary diagnostic tool for physicians. Today, imaging technologies such as CT and positron-emission tomography are even being combined.
X-rays are high-frequency electromagnetic radiation at wavelengths so short (generally 0.01 to 0.1 nm) that they are often thought of as particles (photons). Their energy is measured in electron volts. A clinically useful radiograph requires a minimum of 45 kiloelectron volts. As electrons from a source (filament) are accelerated and strike the target (anode), two types of x-rays are produced, along with heat. General radiation, also known as braking radiation and bremsstrahlung, is generated when the accelerated electron passes near the nucleus of an atom in the target, where it is deflected and decelerated. When this happens, a small amount of x-radiation (1%) is emitted; the other 99% of the output is heat. Characteristic radiation is produced when the accelerated electron, passing near the nucleus of an atom in the target, collides with and ejects an electron from one of the inner rings of the atom. An electron from one of the outer rings will move to replace the ejected electron, emitting an x-ray photon. The intensity of the x-ray beam is the number of photons in the beam multiplied by the energy of each photon (both general and characteristic). Intensity is measured as roentgens per minute.
X-Ray Tubes and Housing
Those working in the field should never replace an x-ray tube, as this calls for considerable skill and specialized equipment. The tube housing, however, can be replaced in the field. It contains a new tube that has already been aligned and a jacket filled with oil or (less commonly) another coolant. All tubes contain three basic parts: an enclosure (generally, borosilicate glass, but metallic or ceramic for some high-power tubes); a tungsten-alloy filament; and the anode or target, usually made of tungsten. Most tubes will have one filament for each focal track on the anode, a focusing cup or cathode to direct the electrons moving from the filament to the target, and an anode, with one or more focal tracks machined in it, rotating at up to 20,000 rpm.
The various parts are assembled in the enclosure, which is vacuum sealed. The material used to seal the enclosure must have the same expansion characteristics as the enclosure itself so that the vacuum will not break when the tube is heated. If the enclosure is not properly evacuated, it is called a gassy tube; there can be internal arcing and inconsistent output. Sometimes, this can be corrected through a process called seasoning, which is performed when the tube is installed or is reactivated after being unused for a long period.
Filament voltages range from 2.5 to 15 volts, and each filament in the tube can have a different voltage. The filament current range is generally 3 to 6 amperes. The filaments are left on as long as the unit has power. Since the filaments are kept warm, they release some electrons, which are confined by the cathode. This focus cup also aims the released electrons at the target when the system is generating x-rays.
The anode is a disk about 10 cm in diameter and 2 cm or less in thickness. The disk is surfaced with a tungsten alloy over a copper base. Copper is used for its heat-conduction properties. There will be one or more angled focal surfaces or tracks on the disk where the electrons accelerated from the filament hit, to generate the x-ray. In all but the simplest tubes (mostly dental x-ray units), the anode is rotated. The rotating anode allows heat to be distributed over the entire target. This also prevents the focal surfaces from being distorted by the heat, keeping the focal spot consistent and increasing the life of the tube. The bearings on the anode are sealed and self-lubricating; they cannot be worked on in the field.
The tube is placed into a housing, and connections are made from the filaments and anode to the exterior connection on the housing. In most cases, the housing is then filled with oil that acts as both an electrical isolator and a thermal conductor to move the heat away from the tube. The tube housing is cooled using conduction in low-use application, convection (with a fan) in higher-use settings, and cooling systems in systems with very high use. Tubes are rated in heat units, and will shut down if their ratings are exceeded. Tube housings with fans need to be cleaned on a regular basis to ensure good cooling.
Fluid levels in the tube housings that require cooling systems should also be checked regularly. Fluid levels can be checked at the window on one side of the tube housing where the x-ray beam exits. Several recent studies have indicated that coolant oil loses its ability to conduct heat after prolonged use; this may contribute to shortened tube life.
An indication of this problem in the tube is sputtering (nonlinear output). Heat-unit problems are rare for film studies but common for fluoroscopic studies, so the procedure in progress when the tube started to sputter should be documented.
The focal spot of a tube is a function of the geometry of the target, the target material and its texture, the sizes of the filament and the focus cup, and any wobble that may occur in the anode as it rotates. Focal spots are stated in mm (typically 0.3, 0.5, 1, or 1.5 mm). Focal spots are measured using a pinhole camera or a star pattern. With a pinhole camera, the image is measured and divided by the amplification factor. The star pattern requires the use of more mathematics, and it is sometimes difficult to establish where the lines blur. These measurements are made at the factory and by the physicist (during validation testing). The focal spots size, as built, is listed on the tube housing and serves as a reference point for future validations.
As the tube ages, the focal spot will increase in size. Some of this growth can be compensated for by radiologic technologists, who may change the voltage, current, and time settings. Traditional thinking holds that the smaller the focal spot is, the better the resolution for small objects becomes. The consistency of the x-rays also affects resolution, however. This is evident when the edges of objects are not sharp, but blended. The use of filters and grids can increase sharpness and decrease the blurred/blended area (also called the edge gradient). The person testing a new tube upon its installation should document not only the size and shape of the focal spot, but also any tilt, blurred edges, or uneven radiation. A hard copy of this information should be kept on file as long as the tube is in service.
Focal-spot geometry produces an uneven beam of x-radiation. In what is called the heel effect, there is more energy at one end of the field than at the other. Ideally, aligning the tube so that the radiation caused by the heel effect is directed toward the thickest part of the object being studied helps maintain the density and contrast of the image. If complaints start suddenly about density and contrast problems, check to be sure that the tube has not been rotated.
In future articles, grids, filters, power supplies, and video systems will be covered. 24×7
David Harrington, PhD, director of staff development and training at Technology in Medicine (TiM), Holliston, Mass, is a member of 24×7s editorial advisory board.
Carl Genereux is a TiM account manager, Marlborough Hospital, Marlborough, Mass.
1. An x-ray beam is made up of _____________.
a) general radiation
c) characteristic radiation
d) a and c
2. As an x-ray tube ages, the focal spot ____________________.
3. Resolution is dependent upon _________________.
Answers: 1-d, 2-a, 3-a