Lesson 1, Topic 1
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Principles of Exposure and Image Quality

April 11, 2024

Principles of Exposure and Image Quality

Learning Objective: Examine the principles of exposure and image quality.

This section explains the prime factors of radiographic exposure and their radiographic effects. In addition, it introduces the primary factors of radiographic quality and the principal methods for controlling them.

Prime Factors of Radiographic Exposure

Learning Objective: Explain the prime factors of radiographic exposure.

Exposure is a broad term used to describe the x-rays that the patient is exposed to, the amount of x-rays in the primary beam, and the amount of x-rays that reach the IR. The x-ray beam is often described in terms of quantity and quality. The principal factors that affect x-ray quantity are mAs, kVp, SID, and filtration. The factors that affect x-ray quality are kVp and filtration. Note that kVp and filtration affect both quantity and quality. The quantity and quality of the x-ray beam are controlled by four prime factors, which are under the operator’s direct control. The prime factors are milliamperage (mA), exposure time (S), kVp, and SID.

Milliamperage, Exposure Time, and Milliampere-Seconds

As previously discussed, changes in mA affect the rate of exposure—that is, the number of photons produced per second during an exposure. An increase in mA increases the quantity of exposure, and a decrease in mA decreases the quantity of exposure. Exposure time controls the exposure to the IR and determines the length of the exposure. The number of mAs is the product of mA and exposure time (mA × time = mAs). Exposure is directly proportional to mAs; therefore, if mAs doubles, the quantity of exposure also doubles. The dose given to the patient is also directly proportional to the mAs.

Kilovoltage

The kVp controls both the quality and quantity of the x-ray beam. As the kVp increases, the energy of the photons in the beam increases, changing the quality. As the photon energy increases, the penetrating ability of the photons increases. kVp also affects the quantity of exposure to the IR. When the kVp is increased, the electrons from the filament reach the anode with more energy. More interactions then occur in the anode, and more x-rays are emitted.
The contrast of the image is directly affected by the kVp. A high kVp produces a low-contrast image, and a low kVp produces a high-contrast image. Low-contrast images have many shades of gray, which high-contrast images have fewer shades of gray.

Source-Image Receptor Distance

The distance between the tube target and the IR is the SID. Because the x-ray beam diverges, forming the shape of a cone, the photons spread farther apart as the distance from the target increases; thus, the SID affects the intensity of the x-ray beam and the quantity of x-rays. The inverse square law expresses the relationship between SID and intensity, which states that the intensity is inversely proportional to the square of the distance (FIGURE 37.19). The inverse square law is expressed mathematically as a formula:The contrast of the image is directly affected by the kVp. A high kVp produces a low-contrast image, and a low kVp produces a high-contrast image. Low-contrast images have many shades of gray, which high-contrast images have fewer shades of gray.

In this formula, I represents radiation intensity, and D represents SID. For all SID calculations, the distance is always squared. As the distance increases, the intensity decreases and vice versa. For example, if the distance were doubled, the intensity would decrease to one-fourth of the original intensity. If the distance were reduced by 50%, the intensity would increase by four times. In FIGURE 37.20, suppose D1 is 40 inches, and D2 is 80 inches, or twice as great. If the original intensity at D1 had a value of 100mR, the intensity at D2 would be 25 mR. This value is determined by using the inverse square law formula as follows:

Insert values:

Square distances:

Cross multiply:

FIGURE 37.19  The Inverse Square Law.The radiation intensity at a given distance from a point source is inversely proportional to the square of the distance. From Adler AM, Carlton RR: Introduction to radiologic and imaging sciences and patient care, ed 7, St. Louis, 2020, Saunders.

FIGURE 37.20  SID and Radiation Intensity.Source-to-image receptor distance affects both maximum field size and radiation intensity. At 40 inches, the field size is four times as large, and the x-ray intensity is four times less than at 20 inches. From Johnston J, Fauber TL: Essentials of radiographic physics and imaging, ed 3, St. Louis, 2020, Mosby.

Divide by x:

It should be evident that the four prime factors are important technical elements in the production of an x-ray image. If the mA, exposure time, mAs, kVp, and SID are not set correctly, the body part will not have the correct contrast on the final image.

37.4

Critical Thinking Application

Stephanie has a request for a chest x-ray on Mr. Hawkins. When she meets and assesses Mr. Hawkins, it is evident that Mr. Hawkins must remain seated in his wheelchair for his chest x-ray. A chest x-ray is typically performed at 72 inches, using 4 mAs. However, due to the patient’s condition, the distance will be 60 inches. Using the inverse square law, how will Stephanie determine the mAs at 60 inches?

Prime Factors of Radiographic Exposure

Milliamperes (mA)

• Controls quantity of x-rays produced
• Quantity of exposure is directly proportional to mA

Exposure Time (Seconds)

• Controls quantity of x-ray produced
• Controls duration of the exposure
• Quantity of exposure is directly proportional to the exposure time

Kilovolts (kVp)

• Controls x-ray penetration
• Controls the quantity and quality of the x-ray beam
• Increased kVp results in an increased quantity of photons
• Increased kVp results in increased penetration of the body part

Source-to-Image Receptor Distance (SID)

• Affects the intensity of the x-ray beam
• Quantity of exposure is inversely proportional to the square of the distance
• Each dimension of the radiation field is proportional to the SID. Therefore, the field area is proportional to the square of the SID, and the radiation intensity is inversely proportional to the square of the SID.

Radiographic Geometric Factors

Learning Objective: Examine the radiographic geometric factors.

Two primary factors directly affect how the x-ray image looks: distortion and spatial resolution. An understanding of these factors is essential to the discussion and evaluation of x-ray images. Each factor is influenced and controlled differently. Knowledge of these concepts enables the radiographer to identify the nature of problems that relate to image quality and to solve these problems effectively.

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 distortion. Distortion is an unequal magnification of different portions of the same object. Radiographic distortion may be categorized by whether it primarily affects the size of the object or its shape. Size distortion is always in the form of magnification enlargement. Shape distortion is the result of unequal magnification of the actual shape of the structure.

FIGURE 37.21  OID and Size Distortion.When the object is near the image receptor, the object and its image are nearly the same size. When the OID is increased, it causes more magnification than a short OID. The image in (A) is larger than (B) because the object is farther from the IR. From Johnston J, Fauber TL: Essentials of radiographic physics and imaging, ed 3, St. Louis, 2020, Mosby.

Size Distortion

Size distortion occurs when the part is magnified. Magnification is a function of the relationship between the SID and the distance between the subject and the IR. This distance is the object-to-image receptor distance (OID) . As shown in FIGURE 37.21, there is little magnification distortion when the SID is great, and the OID is minimal. The object and its image are almost the same size.
As the OID is increased, the magnification increases, and distortion of the part occurs. Radiographic images can never be smaller than the actual size of the body. All images are magnified slightly because the body part is always above the IR. The goal is to keep the magnification as low as possible to prevent size distortion. Size distortion will occur when either the OID increases or the SID decreases from the standard positioning. Therefore, size distortion is controlled by positioning the body part as close as possible to the IR and using the longest practical SID.

Shape Distortion

Shape distortion, as mentioned previously, is the result of unequal magnification. The least amount of shape distortion occurs when the plane of the subject is parallel to the plane of the IR, and the CR is perpendicular to both. Angulation of the part in relation to the IR, or angulation of the x-ray beam, produces shape distortion. For these reasons, an effort should be made to position the patient so that the object of clinical interest is as parallel to the IR as possible and to minimize tube angulation.
Shape distortion displaces the projected image of an object from its actual position. It can be projected either shorter or longer; this is known as foreshortening and elongation. Foreshortening projects the part so it appears shorter than it actually is. This usually occurs when the body part is not correctly aligned. Elongation projects the object so it appears longer than it actually is. This distortion occurs when either the IR or the x-ray tube is not correctly aligned with the part. Distortion is primarily controlled by the OID, SID, CR angle, part position, and IR position.

Spatial Resolution

Spatial resolution refers to the sharpness of the image and is also referred to as resolution, sharpness, or detail. The edge sharpness of all portions of the image determines whether the image appears sharp or blurred. When the resolution is optimal, the edge sharpness of structures in the image is crisp. Poor resolution tends to appear fuzzy or unclear. Several key factors that affect spatial resolution are patient motion, OID, SID, and focal spot size.
To understand how spatial resolution is maintained or improved, the terms umbra and penumbra are used. Umbra is the actual anatomic area, body part, or structure shown in the radiographic image. The penumbra describes the unsharp edges of the umbra or body part. All body parts will have some unsharpness at the edges of the image. The goal is to reduce penumbra as much as possible. Penumbra is also referred to as blur or geometric unsharpness. When the OID decreases, the penumbra decreases, prompting greater spatial resolution. When the SID increases, the magnification and penumbra decrease, producing greater recorded detail; thus, there is also magnification of the penumbra whenever there is magnification of the image.

TABLE 37.3

Effects of Image Geometry on Spatial Resolution

FactorDirection of ChangeEffect on Resolution
Focal spot sizeDecreaseIncrease
Object-to-image receptor distance (OID)DecreaseIncrease
source-to-Image receptor distance (SID)IncreaseIncrease

For this reason, magnification results in image unsharpness. These relationships are summarized in TABLE 37.3. Because only the OID and SID affect magnification, the shortest OID and the longest SID possible should be used.

Motion

Learning Objective: Explain the impact of motion on image quality.

Any movement during radiography will cause blurring of the image, reducing spatial resolution. This applies to patient motion, of course, and the movement of the IR or the x-ray tube. Patient motion may be categorized as either voluntary or involuntary. Voluntary motion usually is controllable, although some patients may be unable to control them. Involuntary motion involves movements over which the patient has no control, such as tremors, peristalsis, or heartbeats. The principal means of controlling involuntary motion is to use a shorter exposure time.

37.5

Critical Thinking Application

Stephanie has an x-ray request for a hand x-ray on Mrs. Roberts. When she meets and assesses Mrs. Roberts, it is evident that Mrs. Roberts has Parkinson disease, which causes tremors. How can Stephanie adjust her technical factors based on the patient’s condition?

Quantum Mottle

Learning Objective: Define quantum mottle.

Quantum mottle is a term used to describe the situation in which a grainy or mottled (spotty) image is created. Quantum mottle occurs when either the mAs or the kVp is set too low. The result is a blotchy image with decreased spatial resolution. Spatial resolution is primarily controlled by the OID, SID, focal spot, motion, and quantum mottle.

37.6

Critical Thinking Application

Stephanie has just completed imaging of Ms. Johnson’s abdomen. Upon evaluation of the image, quantum mottle is visible, and it is evident that Stephanie will need to repeat the imaging process. What adjustments can Stephanie make to the technical factors to eliminate quantum mottle on the repeated image?