4 Producing Quality Radiographs

Objectives Define the key words. Evaluate a radiographic image identifying the basic requirements of acceptability. Differentiate between radiolucent and radiopaque areas on a dental radiograph. Define radiographic density and contrast. Differentiate between subject contrast and film contrast.

Objectives List the factors that influence magnification and distortion. List the geometric factors that affect image sharpness. Summarize the factors affecting the radiographic image. Describe how mA, kVp, and exposure time affect image density.

Objectives Discuss how kVp affects image contrast. Explain target-surface, object-image receptor, and target-image receptor distances. Demonstrate the practical use of the inverse square law.

Key Words Contrast Crystal Definition Density Distortion Exposure chart Exposure factors Exposure time

Key Words Extraoral radiography Film contrast Focal spot Geometric factors Grid Intensifying screen Intraoral radiography Inverse square law

Key Words Kilovoltage peak (kVp) Long-scale contrast Magnification Milliampere (mA) Motion Object-image receptor distance Penumbra

Key Words Position indicating device (PID) Radiographic contrast Radiolucent Radiopaque Sharpness Short-scale contrast

Key Words Subject contrast Target-image receptor distance Target-object distance Target-surface distance

Introduction Each patient presents with a unique set of characteristics for which a customized approach to exposure settings is needed. The dental radiographer has an ethical responsibility to produce the highest diagnostic quality radiographs for patients who agree to be exposed to ionizing radiation.

Introduction To consistently produce diagnostic quality radiographs at the lowest possible radiation dose, the dental radiographer needs to understand the inter-relationships of the components of the dental x-ray machine.

Introduction Three basic requirements for an acceptable diagnostic radiograph: All parts of the structures recorded must be imaged as close to their natural shapes and sizes as the patient’s oral anatomy will permit. Distortion and superimposition of structures should be at a minimum.

Introduction Three basic requirements for an acceptable diagnostic radiograph: The area examined must be imaged completely, with enough surrounding tissue to distinguish between the structures. The radiograph should be free of errors and show proper density, contrast, and definition.

Figure 4-1 An acceptable diagnostic radiograph.

Terminology Radiolucent Radiopaque Density Contrast Sharpness Short-scale contrast Long-scale contrast Sharpness

Figure 4-2 Radiographic density Figure 4-2 Radiographic density. Radiograph (A) is underexposed and appears too light (less dense). Radiograph (B) is overexposed and appears too dark (more dense).

Figure 4-3 Penetrometer tests demonstrate radiographically that a longer contrast scale results from the use of 100 kilovolt exposures. Dental radiographs exposed at 100 kVp have long-scale contrast. Radiographs exposed at 60 kVp have short-scale contrast.

Figure 4-4 Radiographic contrast Figure 4-4 Radiographic contrast. Radiograph (A) exposed at 60 kVp, has high contrast. Radiograph (B) exposed at 90 kVp, has low contrast.

Shadow Casting A radiograph is a two-dimensional image of three-dimensional objects. Therefore, it is necessary to apply the rules for creating a shadow image to produce a quality radiographic image.

Rules for Shadow Casting Small focal spot — to reduce the size of the penumbra (partial shadow around the objects of interest) resulting in a sharper image and slightly less magnification Long target-object distance — to reduce the penumbra and magnification Short object-film distance — to reduce penumbra and magnification

Rules for Shadow Casting Parallel relationship between object and film — to prevent distortion of the image Perpendicular relationship between central ray of x-ray beam and the object and film — to prevent distortion of the image

Factors Affecting the Radiographic Image Radiographic contrast Subject kVp Scatter radiation Film/digital sensor type Exposure Processing

Table 4-1 Summary of Factors Influencing Radiographic Image Contrast

Factors Affecting the Radiographic Image Sharpness/Definition Focal spot size Target-image receptor distance Object-image receptor distance Motion Screen thickness Screen-film contact Crystal/pixel size of intraoral image receptors

Table 4-2 Summary of Factors Influencing Radiographic Image Sharpness

Figure 4-5 Using a small focal spot on the target a long target-image receptor distance, and a short object-image receptor distance will result in a sharp image.

Figure 4-6 Large focal spot on the target and long object-film distance results in more penumbra and therefore loss of image sharpness.

Figure 4-7 Movement of the tube head Figure 4-7 Movement of the tube head. Motion, even slight, of the tube head will effectively create a larger surface area of the focal spot, resulting in penumbra.

Figure 4-8 Large focal spot on the target and short target-image receptor distance results in more penumbra and loss of image sharpness.

Figure 4-9 Blurry, unsharp image caused by movement of the patient, the film, or the tube head.

Figure 4-10 Screen thickness Figure 4-10 Screen thickness. X-ray A strikes a crystal far from the film and the divergent light exposes a wide area of the film resulting in unsharpness. X-ray B strikes a crystal close to the film, resulting in less divergence of the light that exposes the film and therefore a sharper image. The thicker the screen, the less sharp the image.

Factors Affecting the Radiographic Image Magnification/enlargement is mostly influenced by the target-object distance and the object-image receptor distance. The target-object distance is determined by the length of the PID.

Factors Affecting the Radiographic Image Distortion is the result of unequal magnification of different parts of the same object. Distortion results when the image receptor is not parallel to the object (Figure 4-13) and/or when the central ray of the x-ray beam is not perpendicular to the object and the plane of the image receptor (Figure 4-14)

Figure 4-11 Magnification. Comparison of 8-in. (20. 5-cm) and 16-in Figure 4-11 Magnification. Comparison of 8-in. (20.5-cm) and 16-in. (41-cm) target-object and target-image receptor distances. The image is magnified (enlarged) when these distances are shortened. (Courtesy of Dentsply Rinn)

Figure 4-13 Object and film are not parallel, resulting in distortion.

Figure 4-14 Central ray of x-ray beam is not perpendicular to the objects and image receptor, resulting in distortion and over-lapping of object A and object B. Note that object A is magnified larger than object B because object A is a greater distance from the image receptor than object B.

Table 4-3 Effect of Varying Exposure Factors on Image Density

Effects of Varying the Exposure Factors Variations in milliamperage (mA) Variations in exposure time Milliampere-seconds (mAs) Variations in kilovoltage (kVp)

Effects of Variations in Distances The operator must take into account several distances to produce the ideal diagnostic quality image: The distance between the x-ray source (at the focal spot on the target) and the surface of the patient’s skin The distance between the object to be x-rayed (usually the teeth) and the image receptor

Effects of Variations in Distances The operator must take into account several distances to produce the ideal diagnostic quality image: The distance between the x-ray source and the recording plane of the image receptor the terms target-surface distance, object-image receptor distance, target-object distance, and target-image receptor distance are used.

Figure 4-15 Distances. Relationship among target, skin surface, object (tooth), and image receptor distance

Effects of Variations in Distances Target-surface distance Object-image receptor distance Target-image receptor distance

Figure 4-16 Object-image receptor distance Figure 4-16 Object-image receptor distance. This placement of the image receptor places the crown of the tooth closer to the receptor than the root.

Figure 4-17 Inverse square law Figure 4-17 Inverse square law. Relationship of distance (D) to the area covered by x-rays emitted from the x-ray tube. X-rays emerging from the tube travel in straight lines and diverge from each other. The areas covered by the x-rays at any two points are proportional to each other as the square of the distances measured from the source of radiation.

Inverse Square Law Inverse square law states that the intensity of radiation varies inversely as the square of the distance from its source. Inverse square law may be written as:

Inverse Square Law Where: I1 is the original intensity I2 is the new intensity D1 is the original distance D2 is the new distance

Exposure Charts Exposure charts, available commercially or custom made by the practice, should be posted at the x-ray unit control panel for easy reference. These charts show at a glance how much exposure time is required for a film of any given speed or a digital sensor when used with all possible combinations of exposure time, milliamperage, and peak kilovoltage.

Review: Chapter Summary An acceptable diagnostic radiograph must show the areas of interest completely and with minimum distortion and maximum sharpness. When evaluating a radiographic image the oral health care professional should utilize appropriate scientific terminology.

Review: Chapter Summary The dental radiographer must have a working knowledge of the factors that affect the radiographic image.

Recall: Study Questions General Chapter Review

Reflect: Case Study You have just been hired to work in a new oral healthcare facility. Prior to providing patient services, you are asked to help develop exposure settings and equipment recommendations for the practice. The equipment and image receptor manufacturers’ suggestions are as follows: F Speed Film; 8-in. (20.5 cm) PID; 85 kVp

Reflect: Case Study

Reflect: Case Study You recommend that the facility replace the 8-in. (20.5 cm) PID with a 16-in. (41 cm) PID. Develop a new exposure chart for using the new 16-in. (41 cm) PID. You recommend using a kVp setting of 70 when exposing radiographs for the purpose of detecting caries. Develop a new exposure chart for 70 kVp.

Reflect: Case Study You recommend using a kVp setting of 90 when exposing radiographs for the purpose of evaluating supporting bone and periodontal disease. Develop a new exposure chart for 90 kVp.

Relate: Laboratory Application Proceed to Chapter 4, Laboratory Application, to complete this activity.