Chapter 7 - Principles of Exposure and Image Quality Flashcards
Exposure Time (Seconds)
*Controls radiographic density
*Controls quantity of x-rays produced
*Controlled by adjusting the timer in x-ray circuit
*Controls duration of exposure
*Quantity of exposure is directly proportional to exposure time
Kilovolts (kVp)
*Controls radiographic contrast
*Controls x-ray penetration
*Controls the quantity and quality of the x-ray beam
*Increased kVp results in increased quantity of photons
*Increased kVp results in increased penetration of the body part
Milliamperes (mA)
*Controls radiographic density
*Controls quantity of x-rays produced
*Controlled by adjusting the mA
*Quantity of exposure is directly proportional to mA
Source-Image Receptor Distance (SID)
*Affects the density and intensity of the x-ray beam
*Quantity of exposure is inversely proportional to the square of the distance
principle factors that affect x-ray quantity
milliamperage-seconds (mAs), kilovoltage (kVp), source-image receptor distance (SID), and filtration
factors that affect x-ray quality
kVp and filtration
prime factors of exposure
milliamperage (mA), exposure time (S), kVp, and SID
mA
Exposure is directly proportional to mA; that is, if the mA doubles, the quantity of exposure also doubles.
Exposure Time
Like the mA described earlier, the quantity of exposure is also directly proportional to the exposure time. The dose to the patient is also directly proportional; for example, if the exposure time is doubled, the dose to the patient is doubled.
kVp
When kVp is increased, density is increased; however, mAs is the primary controller of density. Unlike the effects of mA, exposure time, or mAs, changes in exposure are not directly proportional to kVp. The kVp is never doubled owing to the fact that doubling of the kVp would result in four times more photons being emitted! Conversely, the kVp would never be halved owing to the fact that four times fewer photons would result.
Contrast
The contrast of the image is directly affected by kVp. High kVp produces a low-contrast image and low kVp produces a high-contrast image.
inverse square law
The relationship between the SID and the intensity of the beam is expressed by the inverse square law, which states that the intensity is inversely proportional to the square of the distance.
four primary factors that directly affect how the x-ray image looks:
density, contrast, distortion, and spatial resolution
Properties
Density and contrast are considered photographic properties, and distortion and recorded detail are considered geometric properties.
Density
Density is a photographic property that refers to the overall blackness or darkness of the radiographic image. The greater the quantity of exposure, the darker the image will be. An image that is too dark is said to be over-exposed, and one that is too light is underexposed. Density is primarily controlled by varying the mAs, usually by increasing or decreasing the exposure time.
window level
The brightness (density) on the viewing monitor is adjusted by a control called the window level.
radiographic contrast
Contrast is a photographic property defined as the difference in radiographic density between adjacent portions of the image.
penetrometer
It is a solid piece of aluminum with steps of varying thickness. A penetrometer is often referred to as step-wedge because of its shape. A radiographic image of a penetrometer is a gray scale that shows the amount of penetration of each step. It simulates the different densities that would be seen on a patient’s radiograph.
short-scale contrast
This would be considered high contrast; high contrast is also called short-scale contrast because the range of densities is short.
long-scale contrast
This would be considered low contrast; low contrast is called long-scale contrast because the range of densities is long.
Contrast
Contrast is directly influenced by the presence of fog and collimation.
window width
The contrast on the viewing monitor is adjusted by a control called the window width.
Distortion
Distortion is a geometric property and refers to differences between the actual subject and its radiographic image. Because the subject is three-dimensional and the image is flat (two-dimensional), all radiographic images have some degree of distortion. Distortion is unequal magnification of different portions of the same object.
Size distortion
Size distortion is always in the form of magnification enlargement.
Shape distortion
Shape distortion is the result of unequal magnification of the actual shape of the structure.
Magnification
when the SID is great and the OID is minimal, there is little magnification distortion. The object and its image are almost the same size. As the OID is increased, the magnification increases and distortion of the part occurs
least shape distortion
The least shape distortion occurs when the plane of the subject is parallel to the plane of the IR and the central ray (CR) is perpendicular to both (Fig. 7.11). Angulation of the part in relation to the IR, or angulation of the x-ray beam, produces shape distortion (Figs. 7.12 and 7.13). For these reasons, effort is made to position the patient so that the object of clinical interest is as parallel to the IR as possible and to minimize the need for tube angulation. Even when the x-ray beam is directed perpendicular to the IR, only the CR is truly perpendicular. Therefore the least distortion occurs at the center of the image. Structures at the outer edges of the radiograph will exhibit some degree of distortion, especially when the IR is large. For this reason, the object of primary clinical interest is usually placed in the center of the field.
shape distortion
foreshortening and elongation
Distortion
Distortion is primarily controlled by the OID, SID, CR angle, part position, and IR position.
Spatial resolution
Spatial resolution is also a geometric property. Before digital imaging, it was referred to as recorded detail. Spatial resolution refers to the sharpness of the image, and is more casually referred to as resolution, sharpness, definition, or simply detail. It is the edge sharpness of all portions of the image that determines whether the image appears sharp or blurred.
factors that affect spatial resolution
patient motion, OID, SID, and the focal spot.
geometric factors
The geometric factors that control the formation of the image are SID, OID, and focal spot size.
Involuntary motion
Involuntary motion involves movements over which the patient has no control, such as tremors, peristalsis, and heartbeats.
Voluntary motion
Voluntary motion is normally controllable, although certain patients may be unable to control them (e.g., unconscious patients or small babies who cannot hold their breath for a few seconds; patients who are in severe pain; or those who are unable to cooperate).
first step in avoiding motion
Effective communications with both adults and children are key in avoiding motion.
principal means of controlling involuntary motion
The principal means of controlling involuntary motion is to use a short exposure time.
Quantum mottle
It occurs when the imaging system does not record the anatomic densities, usually because of lack of photons. Quantum mottle will occur when either the mAs or the kVp is set too low. This results in a blotchy, grainy, or noisy image (Fig. 7.21). The result is decreased spatial resolution.
