Conventional Radiography The Basics

Conventional radiography is still the method most frequently used to initially examine artifacts, victims for forensic examination, and mummified and skeletal remains. Since Rontgen's 1895 discovery, many modifications have changed the design of x-ray equipment. However, the basic principles of x-ray production have remained unchanged. That is, within the x-ray tube, electrons are accelerated from the filament within the negative electrode, or cathode, to the target within the positive electrode, or anode. The interaction between the high-speed electrons and the target material produces x-rays.

Exposure Variables X-Ray Penetration

There are several variables that are manipulated in the production of x-rays. The penetrating ability of the x-ray beam is controlled by the acceleration of the electrons across the tube. An increase in the electron speed will result in shorter-wavelength photons that are more penetrating. Conversely, a stream of slower-moving electrons will produce an x-ray beam consisting of longer-wavelength photons that are less penetrating. The factor on the control panel that adjusts the speed of the electrons is the kilovoltage (kV). When the numerical value is selected, it indicates the maximum kV, or kV peak (kVp), that will be applied to the cathode. This penetrability of the generated x-ray is considered the beam quality.

For each density and thickness of material, there is an optimal kVp setting. For example, on a living patient, the optimal setting would be 55 kVp for a hand and 75 kVp for a lateral skull. Pathology that alters the density of tissue would necessitate compensation in the selection of kVp. For instance, a patient with osteoporosis, a condition that decreases bone density, might require a 5 to 10 kVp reduction to produce an image with acceptable penetration. For mummified remains, 55 kVp has proved to be most suitable for adequate penetration. When viewing a processed radiograph to determine if there is adequate penetration, looking "inside" the bone is considered the best area for an assessment of penetration. It should be possible to see "through" the bone and be able to discern structures such as trabeculae (Figure 2.1A). If it is not possible to see through the bone, the film is considered underpenetrated (Figure 2.1B). If it is possible to see through the bone but the cortex of the bone appeared gray instead of white, the film is considered overpenetrated

Cultural Ray

Figure 2.1A Lateral skull radiograph of the mummy known as George/Fred, taken at 55 kVp and 10 mas. To assess the penetration, look at the teeth. since it is possible to identify all the anatomical features, such as pulp canal, enamel, and dentine, the image is properly penetrated. To assess the density, look at the areas outside of the skull either in front of the teeth or behind the neck.

Figure 2.1B the same projection, but taken at 40 kVp at 20 mAs. Because the mAs value was doubled from the previous exposure, it compensated for the decrease in the kVp, and the resulting films have the same density. However, since the anatomical features of the teeth are not discernable, the penetration at 40 kVp was insufficient and the image can be considered underpenetrated.

Figure 2.1B the same projection, but taken at 40 kVp at 20 mAs. Because the mAs value was doubled from the previous exposure, it compensated for the decrease in the kVp, and the resulting films have the same density. However, since the anatomical features of the teeth are not discernable, the penetration at 40 kVp was insufficient and the image can be considered underpenetrated.

Figure 2.1C another lateral projection, this time taken at 80 kVp at 5 mas. once again, the mas value was adjusted to produce a density similar to the two previous images. The high kVp setting resulted in an image in which all the anatomical features appear as shades of gray and are not easily distinguishable. The image can be considered as overpenetrated.

Figure 2.1C another lateral projection, this time taken at 80 kVp at 5 mas. once again, the mas value was adjusted to produce a density similar to the two previous images. The high kVp setting resulted in an image in which all the anatomical features appear as shades of gray and are not easily distinguishable. The image can be considered as overpenetrated.

(Figure 2.1C). Between the range of 55-80 kVp, the minimum change in kV to produce a noticeable difference in penetration is approximately 5 kV.

Because the kV setting controls the penetrating ability of the x-ray beam, it also controls the visible contrast, the difference between black and white, on the image. Therefore, lower kVp settings are less penetrating and produce images that possess higher contrast: more "black and white" with fewer shades of grays. Conversely, higher kVp settings will generate more penetrating x-rays, resulting in lower-contrast images: more shades of gray and fewer areas of "white."

In addition to the quality (kVp) of the beam, there are variables that influence the quantity of x-rays produced. Manipulation of the quantity of x-rays is accomplished by manipulating two complementary factors, milliamperage and time. Milliamperage (mA) determines the quantity of electrons that will be available at the filament within the cathode. Time, usually in seconds (s), determines the duration of the exposure. Together, these factors combine to produce the milliamperage-seconds (mAs) and influence the overall "blackness," or density, on the processed film. The region on the processed film to assess adequate density is the area around the part or object of interest. That area should be sufficiently "black" so that when the film is held up to the light, you can't see your fingers placed about 10 in. (25 cm) behind the film. A film that lacks sufficient density, or "blackness," is termed underexposed (Figure 2.2A). Conversely, if the film is too dense, it will obscure the part of interest and be termed overexposed (Figure 2.2B). In order to see a visible difference in density on a processed film, the mAs value either must be increased or decreased by a minimum of 50%.

Figure 2.2A A lateral projection of the skull taken at 55 kVp at 5 mAs. From the previous image, Figure 2.1A, it was determined that the kVp selected provided sufficient penetration. However, because the soft tissue structure of the nose was clearly demonstrated and the area in front of the nose was "gray" and not "black," the image must be considered underexposed.

Figure 2.2B This lateral projection was taken at 55 kVp at 20 mAs. One again, the satisfactory kVp was selected; however, the mAs value is so high that the entire image is dark and should be considered overexposed.

Exposure

Figure 2.3 The characteristic curve: graphical representation of the relationship between the intensity of the radiation exposure and the resulting density on the processed film.

Exposure

Figure 2.3 The characteristic curve: graphical representation of the relationship between the intensity of the radiation exposure and the resulting density on the processed film.

The study of the relationship between the intensity of the radiation exposure and the resulting density, or blackness, on the film is known as sensitometry (Bushong 2008b). Although a thorough understanding of this complex topic is beyond the scope of this text, it will serve as a reference point later in the discussion of digital radiography. The graphical representation of sensitometry is known as a characteristic curve, or a Hurter and Driffield (H & D) curve (Figure 2.3). The graph is divided into three sections: the toe, straight line, and shoulder. Film exposures in the region of the toe would be considered grossly underexposed, whereas in the shoulder the effect would be extremely overexposed. The acceptable exposure would be in a narrow portion of the straight line region where smaller changes in mAs result in more noticeable differences in film density. The slope, or gradient, of the straight line section determines the film's maximum contrast or latitude. A film with a steep slope would have inherently higher contrast, and as the slope decreases, there would be more latitude, lower contrast, and more shades of gray on the processed film.

Focal Spot

The area of the anode or target that is bombarded by the electron stream is known as the focal spot. The dimension of this area becomes significant when taking into consideration the interactions between the principal exposure factors, kVp and mAs. Only approximately 1% of the energy of the electron stream is converted to x-ray, and the remainder is lost as heat. The heat generated at the focal spot in the anode is directly related to three factors: the quantity (mA) of electrons available at the filament, the duration of the exposures, and speed of the electrons (kVp) applied to the filament. A fourth factor, the current waveform, is determined by the type of voltage fluctuation and may be identified simply as single phase, three phase, or high frequency. Single-phase current represents the greatest voltage fluctuation; it is the least efficient means of x-ray production and affects heat production the least, whereas high-frequency generation results in the least voltage variation and is the most efficient x-ray production during an exposure, but contributes significantly to anode heating.

Since overheating of the anode reduces x-ray tube life, tremendous engineering efforts over the past century have been invested in designs to dissipate heat. The simplest and most basic design, the stationary anode supplied with single-phase current, was commonly found in dental and some portable units with lower kVp and mA outputs. These units generally had larger focal spots (1.0 mm/2.0 mm) and were the least expensive, but required longer exposure times to deliver the necessary amount of radiation. X-ray procedures that require very short exposure times necessitated equipment with high-frequency generators capable of higher kVp and mA settings and a smaller focal spot (0.5 mm/1.0 mm) embedded in a rotating anode to dissipate the tremendous heat produced. Most x-ray equipment found in hospital imaging centers, including modern portable units, is powered by high-frequency generators. In order to minimize damaging the x-ray tube, Manufacturers provide charts that indicates maximum kVp and mAs settings related to anode cooling times.

The size of the focal spot ultimately determines the size of the object that can be visualized on the processed image. The smaller the focal spot, the sharper the image. Simply stated, a structure smaller than the focal spot size will not be clearly demonstrated. In medical imaging, mammography units have fractional or microfocal spots, less than 1 mm, typically 0.1 to 0.3 mm, to enable visualization of microcalcifications in the breast tissue. However, the smaller the focal spot size, the faster the anode will heat up. Therefore, focal spot size should be taken into consideration once the objectives of the study have been determined.

Source-to-Image Distance

The distance between the x-ray source and the image receptor also affects the exposure settings. Referred to technically by several terms such as the source-to-image receptor distance (SID), target-to-aim distance (TFD), and focal aim distance (FFD), this distance is based on the physical principle that x-ray photons diverge from the source of production. Because x-ray and visible light are both forms of electromagnetic (EM) radiation, the dispersal characteristics of both are identical and obey the inverse square law. If the SID is doubled, the intensity of the radiation would be reduced by 1/4. To compensate for the reduction of radiation, the quantity, or mAs, would have to be increased by a factor of 4. Therefore, the compensation procedure is known as the direct square law. For example, a satisfactory image is obtained using 10 mAs at a 100 cm SID. If the same object was radiographed at an SID of 200 cm, the quantity of radiation must be increased to 40 mAs to produce an image of satisfactory density.

Image Distortion

The appearance of the object on the image is affected by the SID and focal spot size (fss). Radiographs were originally known as shadowgrams or shadowgraphs because they resembled shadows cast by light. As previously mentioned, the dispersal rate of both forms of EM are identical and can therefore be demonstrated using visible light. To demonstrate this concept, position a light 40 in. (100 cm) from a wall. The wall represents the image receptor or the film. Place an object, for example your hand, 4 in. (10 cm) from the wall in the path of the light, and the shadow of your hand will be cast on the wall. Around the margin of the shadow is a "fuzzy" region known as the penumbra. As your hand moves closer to the wall, the shadow and the penumbra will reduce in size or be less magnified. If you move your hand further from the wall, the shadow and penumbra will get larger or more magnified. Therefore, in order to minimize magnification and penumbra, the object should be closest to the film, or more precisely, the object-to-image receptor distance (OID) should be as small as possible. However, unless the object is flat, the parts farther away from the film will be magnified more than those closest to the film. This unequal magnification is termed distortion. Parts of the object that are not in the same plane as the film will appear distorted on the processed image. Magnification and distortion are two related disadvantages of conventional radiography. Consequently, to minimize magnification and distortion, always place the part of interest closest and in the same plane as the film. If there are several parts of interest, additional exposures should be taken to ensure that each region or area of interest is closest and parallel to the film. Since it is extremely difficult to get all body parts, foreign bodies, and/or artifacts parallel to the film, it is fundamentally impossible to totally eliminate distortion. If the target structure of interest is successfully positioned parallel to the film, the actual size of the object can only be determined if the distance between the object and the film is known. Therefore, it is extremely difficult to determine the actual size of an object from a radiograph.

Beam Collimation

As previously indicated, x-rays diverge from the source at the same angle as visible light. A visible light source superimposed over the path of the x-ray beam and projected onto the subject will indicate the area of the subject that will be irradiated. When adjustable lead shutters are added to the visible light projection device, termed a collimator, it is possible to linearly shape the area that will be irradiated. There are three principal benefits to collimation. First, it greatly reduces scatter radiation, which degrades the image and reduces contrast. Second, it decreases radiation exposure to the patient and operator. Third, it allows the x-ray beam to be precisely centered and limits the area irradiated to only the area or part of interest.

Was this article helpful?

0 0

Post a comment