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The Radiographic Image

Course Number: 571

Image Sharpness

Sharpness is considered a major factor in determining image quality as it is the factor that determines the amount of detail an imaging system can reproduce (Figure 11).


Figure 11. Image A is sharpness when compared to B.

Sharpness is defined by the dimensions of the partially shaded outer region or the penumbra of an object. The wider the penumbra, the less sharp the image (Figure 12).1,2,5

Shadow casting, umbra and penumbra

Figure 12. Shadow casting, umbra and penumbra.

We will only discuss the properties of shadow-casting and the width of the penumbra that are related to the x-ray tube and sensor. We will not factor in patient-related sharpness of the image.

The sharpness of the image depends on multiple factors. We will consider one variable at a time, keeping the other variables fixed:

  1. The apparent focal spot size: The larger is the size of the apparent focal spot, the larger is the penumbra, resulting in a less sharp image.1,3,6,11

  2. Source-to-object distance: The greater is the source-to-object distance, the smaller is the penumbra, resulting in a sharper image.1,2,4,5,8,11

  3. Object-to-receptor distance: The greater is the object-to-receptor distance, the larger is the penumbra, resulting in a less sharp image.2-4,9,7,11

  4. X-ray tube motion-related un-sharpness: If the tube moves when the x-ray image is being taken, the apparent focal spot size will become larger resulting in a larger penumbra and a less sharp image.2,5,12