Focus film distance

Focus film distance (FFD), also known as source image distance (SID), is a term used in radiology and radiography to describe the distance between the x-ray source and the x-ray film or detector. This distance is crucial in determining the scale of the projected image of the patient's body onto the film or detector.

Overview

The focus film distance is an important factor in the production of medical imaging. It affects the magnification, spatial resolution, and dose of radiation received by the patient. The FFD is typically measured in millimeters (mm) and can vary depending on the type of examination being performed.

Factors Influencing Focus Film Distance

The FFD can be influenced by several factors, including:

• Patient size: Larger patients may require a greater FFD to ensure the entire area of interest is captured on the film or detector.
• Type of examination: Certain types of examinations, such as mammography, may require a smaller FFD to achieve higher resolution images.
• Equipment design: The design of the x-ray machine can also influence the FFD. Some machines have a fixed FFD, while others allow for adjustment.

Impact on Image Quality

The FFD has a direct impact on the quality of the radiographic image. A larger FFD can result in less magnification and distortion of the image, but may also result in a lower spatial resolution. Conversely, a smaller FFD can result in higher resolution images, but with greater magnification and potential for distortion.

Safety Considerations

The FFD also plays a role in patient safety. A larger FFD can result in a lower dose of radiation received by the patient, as the intensity of the x-ray beam decreases with distance. However, care must be taken to ensure the FFD is not so large that it compromises the quality of the image.

See Also

• Radiology
• Radiography
• Medical imaging
• X-ray

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