Why does increasing source–image distance (SID) improve spatial resolution?

Increasing the source–image distance (SID) improves spatial resolution because it reduces geometric unsharpness and magnification. As the X-ray source is moved further from the patient, the X-ray beam becomes less divergent at the level of the object, producing a sharper projection on the detector.

Increasing SID improves spatial resolution by reducing beam divergence, geometric unsharpness, and image magnification.

Understanding the physics

When the X-ray source is closer to the patient (short SID), the beam diverges more strongly. This increased divergence causes greater geometric magnification and increases the size of the penumbra, resulting in a blurrier image.

As SID increases, the X-ray beam becomes more parallel by the time it reaches the patient. This reduces the difference in projection between rays originating from different parts of the focal spot, thereby reducing the size of the penumbra.

The effect of SID on image sharpness can be understood from the geometric unsharpness relationship:

U_g = (F × OID) / SOD

where (source–object distance) increases as SID increases (assuming object position remains constant). Because geometric unsharpness is inversely proportional to SOD, increasing SID reduces geometric blur.

In addition to reducing blur, increasing SID also reduces magnification, making the projected image more faithful to the true size of the object.

Where this matters clinically

Increasing SID is a simple and effective way to improve image sharpness in radiography. For example, chest radiographs are typically performed with a long SID (e.g. 180 cm) to minimise magnification of the heart and improve spatial resolution.

However, increasing SID also reduces the intensity of the X-ray beam reaching the detector due to the inverse square law:

I ∝ 1 / d²

where is the distance from the X-ray source. As a result, increasing SID requires an increase in exposure (typically mAs) to maintain adequate image quality.

Radiographic technique therefore balances improved spatial resolution against the need to maintain sufficient detector signal and manage radiation dose.