Chapter 7 - Principles of Exposure and Image Quality

Question 1

Exposure Time (Seconds)

Answer 1

*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

Question 2

Kilovolts (kVp)

Answer 2

*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

Question 3

Milliamperes (mA)

Answer 3

*Controls radiographic density
*Controls quantity of x-rays produced
*Controlled by adjusting the mA
*Quantity of exposure is directly proportional to mA

Question 4

Source-Image Receptor Distance (SID)

Answer 4

*Affects the density and intensity of the x-ray beam
*Quantity of exposure is inversely proportional to the square of the distance

Question 5

principle factors that affect x-ray quantity

Answer 5

milliamperage-seconds (mAs), kilovoltage (kVp), source-image receptor distance (SID), and filtration

Question 6

factors that affect x-ray quality

Answer 6

kVp and filtration

Question 7

prime factors of exposure

Answer 7

milliamperage (mA), exposure time (S), kVp, and SID

Question 8

mA

Answer 8

Exposure is directly proportional to mA; that is, if the mA doubles, the quantity of exposure also doubles.

Question 9

Exposure Time

Answer 9

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.

Question 10

kVp

Answer 10

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.

Question 11

Contrast

Answer 11

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.

Question 12

inverse square law

Answer 12

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.

Question 13

four primary factors that directly affect how the x-ray image looks:

Answer 13

density, contrast, distortion, and spatial resolution

Question 14

Properties

Answer 14

Density and contrast are considered photographic properties, and distortion and recorded detail are considered geometric properties.

Question 15

Density

Answer 15

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.

Question 16

window level

Answer 16

The brightness (density) on the viewing monitor is adjusted by a control called the window level.

Question 17

radiographic contrast

Answer 17

Contrast is a photographic property defined as the difference in radiographic density between adjacent portions of the image.

Question 18

penetrometer

Answer 18

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.

Question 19

short-scale contrast

Answer 19

This would be considered high contrast; high contrast is also called short-scale contrast because the range of densities is short.

Question 20

long-scale contrast

Answer 20

This would be considered low contrast; low contrast is called long-scale contrast because the range of densities is long.

Question 21

Contrast

Answer 21

Contrast is directly influenced by the presence of fog and collimation.

Question 22

window width

Answer 22

The contrast on the viewing monitor is adjusted by a control called the window width.

Question 23

Distortion

Answer 23

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.

Question 24

Size distortion

Answer 24

Size distortion is always in the form of magnification enlargement.

Question 25

Shape distortion

Answer 25

Shape distortion is the result of unequal magnification of the actual shape of the structure.

Question 26

Magnification

Answer 26

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

Question 27

least shape distortion

Answer 27

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.

Question 28

shape distortion

Answer 28

foreshortening and elongation

Question 29

Distortion

Answer 29

Distortion is primarily controlled by the OID, SID, CR angle, part position, and IR position.

Question 30

Spatial resolution

Answer 30

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.

Question 31

factors that affect spatial resolution

Answer 31

patient motion, OID, SID, and the focal spot.

Question 32

geometric factors

Answer 32

The geometric factors that control the formation of the image are SID, OID, and focal spot size.

Question 33

Involuntary motion

Answer 33

Involuntary motion involves movements over which the patient has no control, such as tremors, peristalsis, and heartbeats.

Question 34

Voluntary motion

Answer 34

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).

Question 35

first step in avoiding motion

Answer 35

Effective communications with both adults and children are key in avoiding motion.

Question 36

principal means of controlling involuntary motion

Answer 36

The principal means of controlling involuntary motion is to use a short exposure time.

Question 37

Quantum mottle

Answer 37

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.