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C HAPTE R 1 Terminology, Positioning, and Imaging Principles
4. Upside-down focused grid
1
Each grid is labeled to indicate the side that must be positioned to face the x-ray tube. The lead strips are tilted or focused to allow the x-ray beam to pass through unimpeded (if the SID is within the focal range and the grid is correctly placed). If the grid is positioned upside-down, the image will show severe cutoff (Fig. 1-131).
CR
Summary of Contrast Factors Selection of the appropriate kV is
a balance between optimal image contrast and lowest possible patient dose. A general rule states that the highest kV and the lowest mAs that yield sufficient diagnostic information should be used on each radiographic examination.* Close collimation and correct use of grids also ensure that the processed radiographic image displays optimal contrast.
SPATIAL RESOLUTION Spatial resolution is defined as the recorded sharpness of structures on the image. Resolution on a radiographic image is demonstrated by the clarity or sharpness of fine structural lines and borders of tissues or structures on the image. Resolution is also known as detail, recorded detail, image sharpness, or definition. Resolution of film-screen images generally is measured and expressed as line pairs per millimeter (lp/mm), in which a line pair is seen as a single line and an interspace of equal width. The higher the line pair measure, the greater is the resolution; it is typically 5 to 6 lp/mm for general imaging. Lack of visible sharpness or resolution is known as blur or unsharpness.
Correctly centered grid
Upside-down grid (results in grid cutoff or decreased density on both sides of image)
Fig. 1-131 Upside-down grid cutoff.
Large focal spot
Small focal spot
Controlling Factors
The optimal radiograph displays a sharp image, as listed under “Evaluation Criteria” for each position in this text. Resolution with film-screen imaging is controlled by geometric factors, the filmscreen system, and motion. Geometric Factors Geometric factors that control or influence resolution consist of focal spot size, SID, and object image receptor distance (OID). The effect of OID is explained and illustrated in Fig. 1-137. The use of the small focal spot results in less geometric unsharpness (see Fig. 1-132). To illustrate, a point source is used commonly as the source of x-rays in the x-ray tube; however, the actual source of x-rays is an area on the anode known as the focal spot. Most x-ray tubes exhibit dual focus; that is, they have two focal spots: large and small. Use of the small focal spot results in less unsharpness of the image, or an image with a decreased penumbra. A penumbra refers to the unsharp edges of objects in the projected image. However, even with the use of the small focal spot, some penumbra is present. Film-Screen System With film-screen imaging systems, the filmscreen speed used for an examination affects the detail shown on the resultant film. A faster film-screen system allows shorter exposure times, which are helpful in preventing patient motion and reducing dose; however, the image is less sharp than when a slower system is used.
*Bushong S: Radiologic science for technologists, ed 9, St. Louis, 2009, Mosby.
Larger focal spot (more penumbra)
Small focal spot (less penumbra)
Fig. 1-132 Effect of focal spot size.
