The Radiographic Image
Course Number: 571
Image Sharpness
Sharpness plays a crucial role in image quality, as it determines the level of detail an imaging system can capture (Figure 11).
Figure 11. Image A is sharpness when compared to B.
Image sharpness is defined by the dimensions of an object’s partially shaded outer region, known as the penumbra. A broader penumbra results in a less sharp image (Figure 12).1,2,5
Figure 12. Shadow casting, umbra and penumbra.
We will only discuss the properties of shadow-casting and the penumbra width related to the x-ray tube and sensor. We will not factor in the patient-related sharpness of the image.
Several factors influence image sharpness. Each factor can be assessed individually while keeping others constant:
The apparent focal spot size: A larger focal spot increases the penumbra, reducing sharpness.1,3,6,11
Source-to-object distance: A greater distance decreases the penumbra, enhancing image sharpness.1,2,4,5,8,11
Object-to-receptor distance: A larger distance increases the penumbra, lowering image sharpness.2-4,7,9,11
X-ray tube motion-related un-sharpness: Movement of the tube during image acquisition enlarges the apparent focal spot, increasing the penumbra and reducing sharpness.2,5,12