Evaluation of Image Quality on X-ray Films

Evaluation of Image Quality on X-ray Films

Image quality on X-ray films is a crucial factor in accurate diagnosis. Evaluation of image quality includes the following factors:

1. Density:

• Density refers to the darkness of the film, determined by the amount of light that passes through it.

• Density is measured using the optical density (D) scale, with the formula: D = log10(I0/It), where I0 is the intensity of light entering the film and It is the intensity of light passing through the film.

• X-ray films used in dentistry have a density range from 0.39 (very light) to 2 (very dark).

Factors affecting density:

• Directly proportional: mA, exposure time, developing time.

• Directly proportional to the square: kVp.

• Inversely proportional: Thickness and density of tissues.

• Inversely proportional to the square: Distance from the focal spot to the film.

2. Contrast:

• Contrast refers to the difference in density between different areas on the film.

• kVp is the primary factor determining contrast.

• Low kVp results in high contrast, whereas high kVp results in low contrast.

3. Unsharpness:

• Unsharpness refers to image distortion caused by geometric, motion, and intensifying screen factors.

Factors causing unsharpness:

• Geometric unsharpness:

• Focal spot size.

• Distance from the focal spot to the film.

• Distance from the object being imaged to the film.

• Motion unsharpness: Movement of the patient, film, or cone during exposure.

• Screen unsharpness: Use of intensifying screens.

4. Image size distortion:

• Image size distortion refers to the magnification of the image on the film compared to the actual size of the object being imaged.

• This magnification is due to the diverging nature of the X-ray beam.

• The greater the distance from the focal spot to the film, the less the image is magnified.

• The smaller the distance from the object being imaged to the film, the less the image is magnified.

5. Image shape distortion:

• Image shape distortion refers to non-uniform magnification in different parts of the object being imaged.

• When the central beam is perpendicular to the film but the object is not parallel to the film, the image will be shortened.

• When the central beam is perpendicular to the object but the film is not parallel to the object, the image will be elongated.

Principles for creating good images:

• The smaller the focal spot, the better.

• The greater the distance from the focal spot to the film, the sharper the image.

• The smaller the distance from the object being imaged to the film, the sharper the image.

• The central beam should be perpendicular to both the object and the film.

Conclusion:

Understanding the factors that influence image quality helps healthcare professionals adjust technical parameters to obtain sharp, accurate images, facilitating effective disease diagnosis.