Radiographic QualityVisibility and Sharpness

By Prof. Stelmark

A primary responsibility of the radiographer is to evaluate radiographic images to determine whether sufficient information exists for a diagnosis. Evaluating radiographic quality requires the radiographer to assess the image for both its visibility of recorded detail (photographic properties) and its sharpness of recorded detail (geometric properties). Radiographic quality is the combination of both the visibility and the sharpness of recorded detail.

Brightness

Photographic Properties (Visibility)

Photographic properties (visibility factors) of recorded detail are determined by the extent to which the structural components of the anatomic area of interest can be seen on the recorded image.

Visibility of the recorded detail is achieved by the proper balance of radiographic density and radiographic contrast.

Radiographic Density

Radiographic density is the amount of overall blackness produced on the image after processing. A radiograph must have sufficient density to visualize the anatomic structures of interest. A radiograph that is too light has insufficient density to visualize the structures of the anatomic part . Conversely, a radiograph that is too dark has excessive density, and the anatomic part cannot be well visualized.

The ability to determine when a radiograph is unacceptable as a result of either insufficient or excessive density requires knowledge of the radiographic factors and clinical experience.

Controlling Factors Influencing Factors

Milliamperage Kilovoltage

Exposure time Distance

Grids

Film-screen speed

Collimation

Anatomic part

Anode heel effect

Reciprocity law

Generator output

Filtration

Film processing

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 radiograph is critical to developing effective problem-solving skills. Factors that directly affect density are identified as controlling factors, whereas factors that indirectly affect density are identified as influencing factors

Exposure Factors and Digital Imaging

The relationship among the exposure factors of mAs, kVp, and SID and their effect on the intensity of radiation reaching the image receptor holds true for digital imaging.

It has been stated that exposure errors ±50% can be adequately adjusted during digital image processing. Exposure errors beyond ±50% can be adjusted, but the quality of the image may be sacrificed and the patient overexposed.

It is important for radiographers to select exposure factors that produce optimal quality images regardless of whether film-screen or digital image receptors are used. Selecting appropriate exposure factors ensures production of a quality image that provides the maximum amount of information needed for diagnosis with the least amount of exposure to the patient.

Excessive Radiation Exposure and Digital Imaging

Although the computer can adjust for exposure errors in digital imaging, routinely using more radiation exposure than needed will increase the radiation dose to the patient unnecessarily.

Radiographic Contrast

Radiographic contrast is a photographic factor that also affects the visibility of recorded detail. Contrast is the degree of difference between adjacent densities. The ability to distinguish between densities enables differences in anatomic tissues to be visualized. When the absorption characteristics of an object differ, the image presents with varying densities. In tissues where the absorption characteristics differ, recorded detail is best visualized when contrast is optimized for the area of interest.

Radiographic contrast is the combined result of two categories: film (image receptor) contrast and subject contrast. Film, or image receptor, contrast is a result of the inherent properties manufactured into the type of film and how it is radiographed (direct exposure or with intensifying screens), along with the processing conditions. Subject contrast is a result of the absorption characteristics of the anatomic tissue radiographed and the level of kilovoltage used.

Subject Contrast Film (Image Receptor) Contrast

Kilovoltage Film type

Tissue composition Direct exposure or intensifying screens

Contrast medium Processing conditions

CONTROLLING FACTOR

Kilovoltage is considered the controlling factor for radiographic contrast (Box 4-6). The quality or penetrating power of the x-ray beam has the most direct effect on controlling the desired level of contrast.

Altering the penetrating power of the x-ray beam affects its absorption and transmission through the anatomic tissue being radiographed. High kilovoltage increases the penetrating power of the x-ray beam and results in less absorption, more transmission, and fewer density differences in the anatomic tissues.

Controlling Factor Influencing Factors

Kilovoltage Grids

Collimation

Object-to-image receptor distance

Anatomic part

Contrast media

Processing

DIGITAL IMAGING

Kilovoltage remains an important exposure factor for providing adequate penetration of the anatomic part. However, altering contrast to best visualize the area of interest occurs during digital image processing and is less dependent on the actual kVp selected. In addition, contrast can be further manipulated once the digital image is displayed on a computer monitor. Influencing factors such as grids, contrast media, and the composition of the anatomic part will still affect the subject contrast created on the digital image. It is important to note that image receptors used in digital imaging are more sensitive to scatter radiation than film-screen receptors.

Scatter Radiation and Digital Imaging

Digital imaging receptors are more sensitive to scatter radiation than film-screen receptors. Efforts must be routinely made to limit the amount of scatter radiation reaching the digital image receptor.

Radiographic Variables Density Contrast

↑ mAs ↑ 0

↓ mAs ↓ 0

↑ kVp ↑ ↓

↓ kVp ↓ ↑

↑ Grid ratio ↓ ↑

↓ Grid ratio ↑ ↓

↑ Film-screen speed ↑ 0

↓ Film-screen speed ↓ 0

↑ Collimation ↓ ↑

↓ Collimation ↑ ↓

Analog System

Radiographic Variables Density Contrast

↑ mAs 0 0

↓ mAs 0 0

↑ kVp 0 ↓

↓ kVp 0 ↑

↑ Grid ratio 0 ↑

↓ Grid ratio 0 ↓

↑ Film-screen speed 0 0

↓ Film-screen speed 0 0

↑ Collimation 0 ↑

↓ Collimation 0 ↓

Digital System

Geometric Properties (Sharpness)

The geometric properties of a film-screen image refer to the sharpness of structural lines recorded in the radiographic image. A radiographic image cannot be an exact reconstruction of the anatomic structure. Some information is always lost during the process of image formation. It is the radiographer's responsibility to minimize the amount of information lost by accurately manipulating the factors that affect the sharpness of the recorded image. Optimal geometric quality is achieved by maximizing the amount of recorded detail and minimizing the amount of image distortion

Recorded Detail

Recorded detail refers to the distinctness or sharpness of the structural lines that make up the recorded image. The ability of a radiographic image to demonstrate sharp lines will determine the quality of the recorded detail. The imaging process makes it impossible to produce a radiographic image without some degree of unsharpness. A radiographic image that has a greater amount of recorded detail will minimize the amount of unsharpness of the anatomic structural lines. The amount of recorded detail is controlled by minimizing geometric unsharpness and receptor unsharpness and by eliminating motion unsharpness.

GEOMETRIC UNSHARPNESS

The amount of geometric unsharpness is a result of the relationship among the size of the focal spot, SID, and OID

IMAGE RECEPTOR UNSHARPNESS

The type of device used to record the image also affects the amount of unsharpness recorded in the image. In conventional radiography, various intensifying screen-film combinations have created a complex system of image receptors. Variations in the construction and composition of the intensifying screen combined with different types of radiographic film affect not only the photographic properties of the image but also its geometric properties.

MOTION UNSHARPNESS

Motion unsharpness has the most detrimental effect on the recorded detail of the radiographic image. Motion of the tube, part, or image receptor causes a profound decrease in recorded detail. Motion must not just be decreased; it must be eliminated.

Distortion

Distortion results from the radiographic misrepresentation of either the size (magnification) or shape of the anatomic part. When the image is distorted, recorded detail is also reduced.

SIZE DISTORTION (MAGNIFICATION)

The term size distortion/magnification refers to an increase in the object's image size compared with its true, or actual, size. Radiographic images of objects are always magnified in terms of the true object size. The distances used (SID and OID) play an important role in minimizing the amount of size distortion of the radiographic image.

SHAPE DISTORTION

In addition to size distortion, objects that are being imaged can also be misrepresented radiographically by distortion of their shape. Shape distortion can appear in two different ways radiographically: elongation or foreshortening. Elongation refers to images of objects that appear longer than the true objects. Foreshortening refers to images that appear shorter than the true objects

Radiographic Variables

Density Contrast Recorded Detail Distortion

↑mAs ↑ 0 0 0

↓mAs ↓ 0 0 0

↑kVp ↑ ↓ 0 0

↓kVp ↓ ↑ 0 0

↑SID ↓ 0 ↑ ↓

↓SID ↑ 0 ↓ ↑

↑OID† ↓ ↑ ↓ ↑

↓OID ↑ ↓ ↑ ↓

↑Grid ratio ↓ ↑ 0 0

↓Grid ratio ↑ ↓ 0 0

↑Film-screen speed ↑ 0 ↓ 0

↓Film-screen speed ↓ 0 ↑ 0

↑Collimation ↓ ↑ 0 0

↓Collimation ↑ ↓ 0 0

↑Focal spot size 0 0 ↓ 0

↓Focal spot size 0 0 ↑ 0

↑Central ray angle ↓ 0 ↓ ↑