Image Characteristics

Image Characteristics


A quality radiographic image accurately represents the anatomic area of interest and its information is well visualized for diagnosis. It is important to identify the characteristics of a radiographic image before the factors that affect its quality can be understood. This chapter focuses on the characteristics of the image displayed on film and the computer monitor.

As stated in Chapter 8, radiographic images can be acquired from different types of image receptors (film-screen or digital). The process of creating the latent image by differential absorption is the same for both types of receptors, but the acquisition, processing, and display vary greatly. Because of the significant differences between film-screen and digital image receptors, image quality is discussed separately for film-screen and digital images. Information about the construction of image receptors and how the image is acquired, processed, and displayed is discussed in detail in subsequent chapters.

Film-Screen Characteristics

The visibility of the anatomic structures and the accuracy of their structural lines recorded determine the overall quality of the radiographic image. For film-screen, visibility of the recorded detail refers to the photographic properties of the image and the geometric properties refer to how accurately the structural lines are recorded. The accuracy of the structural lines is achieved by maximizing the amount of recorded detail and minimizing the amount of distortion. Visibility of the recorded detail is achieved by the proper balance of radiographic density and radiographic contrast (Figure 9-1).


Radiographic film that has been exposed to radiant energy and chemically processed is composed of minute deposits of black metallic silver visualized as density. The varying densities on the processed film represent the attenuation properties of the anatomic part imaged.

Radiographic density is the amount of overall blackness produced on the processed image. A radiograph must have sufficient density to visualize the anatomic structures of interest (Figure 9-2). A radiograph that is too light has insufficient density to visualize the structures of the anatomic part (Figure 9-3). Conversely, a radiograph that is too dark has excessive density, and the anatomic part cannot be well visualized (Figure 9-4). The radiographer must evaluate the overall density on

the image to determine whether it is sufficient to visualize the anatomic area of interest. He or she then decides whether the radiograph is diagnostic or unacceptable.

If a radiograph is deemed unacceptable, the radiographer must determine what factors contributed to the density error. Knowledge about the factors that affect the density on a radiographic image is critical to developing effective problem-solving skills. The factors that affect the density of the image are discussed in Chapter 10, “Radiographic Exposure Technique.”

Optical Density

Density on the printed radiographic image can be quantified and is therefore an objective measure that can be used for comparison. A densitometer is a device used to numerically determine the amount of blackness on the radiograph (i.e., it measures radiographic density).

This device is constructed to emit a constant intensity of light (incident) onto an area of the image and then measure the amount of light transmitted through the area (Figure 9-5). The densitometer determines the amount of light transmitted and calculates a measurement known as optical density (OD).Optical density is a numeric calculation that compares the intensity of light transmitted through an area on the film (It) with the amount of light originally striking (incident) the area (Io). The ratio of these intensities is called transmittance. Math Application 9-1 shows the mathematical formula used to calculate percent transmittance. Because the range of radiographic densities is large, the calculation of radiographic densities is compressed into a logarithmic scale (Table 9-1) for easier management.

As shown in Math Application 9-2, optical density is defined as the logarithm (Log10) of the inverse of transmittance. For example, an area of the image that allows 10% of the original incident light to be transmitted has a transmittance of 1/10 or 0.1. The inverse of transmittance is therefore 10, and the logarithm of 10 (the optical density) is 1. Similarly, an area that allows only 1% of the original incident light through has an optical density of 2.

Notice the relationship between light transmittance and optical density (Table 9-1). When 100% of the light is transmitted, the optical density equals 0. When 50% of the light is transmitted, the optical density is equal to 0.3, and when 25% of the light is transmitted, the optical density equals 0.6. When a logarithmic scale base 10 is used, every 0.3 change in optical density corresponds to a change in the percentage of light transmitted by a factor of 2 (log10 of 2 = 0.3).

Diagnostic Range

Optical densities can range from 0 to 4 OD. However, the diagnostic range of optical densities for general radiography usually falls between 0.5 and 2 OD. This desired range of optical densities is found between the extreme low and high densities produced on the radiograph.

The radiation exposure to the film-screen image receptor primarily determines the amount of optical density created on the film after processing. The intensity of radiation exposure, or exposure intensity, is a measurement of the amount and energy of the x-rays reaching an area of the film. When all other factors remain the same, increasing the exposure intensity increases the optical density.

In film-screen imaging the optical densities created on the processed radiograph cannot be altered. As a result, choosing the proper exposure intensity to create an appropriate range of optical densities (or diagnostic densities) during film-screen imaging is critical to producing a good-quality radiographic image. Diagnostic densities must be present in a radiographic image to visualize the anatomic area of interest.

Radiographic Contrast

Producing a radiographic image with diagnostic densities is important to visualize the anatomic area of interest. To differentiate among the anatomic tissues, there must be density differences. Density differences are a result of the tissues’ differential absorption of the x-ray photons.

Density differences refer to an image’s radiographic contrast. Radiographic contrast affects the visibility of the structural lines that make up the recorded image.

Radiographic contrast is the degree of difference or ratio between adjacent densities. The ability to distinguish between densities enables differences in anatomic tissues to be visualized. An image that has a diagnostic density but no differences in densities appears as a homogeneous object (Figure 9-6). This appearance indicates that the absorption characteristics of the object are equal. When the absorption characteristics of an object differ, the image presents with varying densities (Figure 9-7). It is because of these density differences (i.e., radiographic contrast) that the anatomic tissues are easily differentiated. Tissues that attenuate the x-ray beam equally are more difficult to visualize because the densities are too similar to differentiate.

Unlike density, which is easily measurable, contrast is a more complex characteristic. Evaluating radiographic quality in terms of contrast is more subjective (it is affected by individual preferences). The level of radiographic contrast desired in an image is determined by the composition of the anatomic tissue to be radiographed and the amount of information needed to visualize the tissue for an accurate diagnosis. For example, the level of contrast desired in a chest radiograph is different from that required in a radiograph of an extremity.

Radiographic images are typically described by their scale of contrast or the range of densities visible. A radiograph with few densities but great differences among them is said to have high contrast. This is also described as short-scale contrast (Figure 9-8). A radiograph with a large number of densities but little differences among them is said to have low contrast. This is also described as long-scale contrast (Figure 9-9).

Radiographic contrast is the combined result of multiple factors associated with the anatomic structure, quality of the radiation, and the capabilities of the film. Subject contrast refers to the absorption characteristics of the anatomic tissue radiographed along with the quality of the x-ray beam. Differences in tissue thickness, density, and effective atomic number contribute to subject contrast. For example, the chest is composed of tissues that vary greatly in x-ray lucency, such as the air-filled lungs, heart, and the bony thorax. This anatomic region creates high-subject contrast because the tissues attenuate the x-ray beam very differently (Figure 9-10). When the chest is imaged, great differences in densities are recorded for the varying tissues (Figure 9-11). The abdomen on the other hand, is composed of tissues that attenuate the x-ray beam similarly and therefore is considered to be a region of low-subject contrast (Figure 9-12). The densities representing the organs in the abdomen are more similar (Figure 9-13). It is therefore difficult to distinguish the stomach from the kidneys.

As discussed in Chapter 8, the quality of the x-ray beam also affects its attenuation in tissues, which in turn alters subject contrast. Increasing the penetrating power of the x-ray beam decreases attenuation, reduces absorption, and increases x-ray transmission; as a result the density differences recorded in the radiographic image change. Figure 9-14 shows a chest radiographed using a low and a high penetrating x-ray beam.

The radiographer must evaluate the composition of the anatomic structure to be radiographed and determine the factors to manipulate to produce the desired level of radiographic contrast. Achieving the desired level of contrast that best visualizes the anatomic area of interest maximizes the amount of information visible for a diagnosis.

The inherent response of the film (receptor contrast) also affects radiographic contrast. Film can be manufactured to display different levels of contrast, such as medium or high. In addition, the quality of chemical processing affects the contrast displayed on the radiograph.

Stay updated, free articles. Join our Telegram channel

Feb 27, 2016 | Posted by in GENERAL RADIOLOGY | Comments Off on Image Characteristics

Full access? Get Clinical Tree

Get Clinical Tree app for offline access