What determines spatial resolution in radiography?

Spatial resolution in radiography describes the ability of an imaging system to distinguish small, closely spaced structures as separate objects. High spatial resolution allows fine anatomical detail to be seen clearly, while poor spatial resolution causes structures to appear blurred.

Spatial resolution in radiography is determined mainly by focal spot size, imaging geometry, detector pixel size, and motion during the exposure.

Understanding the physics

Spatial resolution depends on how accurately the imaging system preserves the spatial location of X-ray photons detected at the image receptor. Several physical processes can blur this information and therefore reduce image sharpness.

One important factor is the size of the focal spot in the X-ray tube. X-rays are produced from a finite area on the anode rather than from a perfect point source. Because photons originate from slightly different locations within the focal spot, edges in the object produce a small region of partial shadow at the detector known as the penumbra. Larger focal spots produce greater penumbra and therefore reduce spatial resolution.

The degree of geometric blur caused by the focal spot is described by the geometric unsharpness equation:

U_g = (F × OID) / SOD

where U_g is geometric unsharpness, is focal spot size, is object–image distance, and is source–object distance.

This relationship shows that geometric blur increases when the focal spot is larger, when the object is further from the detector, or when the X-ray tube is closer to the patient.

Diagram showing geometric unsharpness in radiography and how focal spot size, source distance, and object distance affect penumbra. — This diagram illustrates geometric unsharpness (penumbra) in projection radiography and how it can be reduced. Geometric unsharpness arises because the X-ray focal spot has a finite size, causing edges of the projected image to appear blurred. The magnitude of geometric unsharpness is described by the relationship 𝑈𝑔 = 𝐹⋅𝑂𝐼𝐷/𝑆𝑂𝐷 , where F is focal spot size, OID is object-to-image distance, and SOD is source-to-object distance. Geometric unsharpness can be minimised by reducing focal spot size, decreasing the object-to-detector distance, and increasing the source-to-object distance (which effectively increases the source-to-image distance).

Detector characteristics also influence spatial resolution. Digital radiography images are composed of pixels, and the size of these pixels determines how finely the image can be sampled. Smaller pixels allow finer spatial detail to be recorded.

Detector design can also influence resolution. In indirect detectors, visible light generated in the scintillator may spread slightly before reaching the photodiodes, which can reduce spatial resolution. Structured scintillators such as cesium iodide reduce this effect by guiding light toward the detector elements.

Finally, motion during the exposure can blur the image. If the patient or anatomical structures move while the exposure is being made, the detected signal is spread over a larger region of the detector, reducing image sharpness.

These factors together determine how well the radiographic system can reproduce fine detail.

Where this matters clinically

Spatial resolution is critical when imaging structures where small details must be clearly visualised, such as subtle fractures, fine cortical bone detail, or microcalcifications in mammography.

Radiographic technique can influence spatial resolution. For example, using a smaller focal spot, minimising object–detector distance, and reducing exposure time to limit motion can all improve image sharpness.

However, improving spatial resolution often requires balancing other considerations such as tube loading limits and image noise, so imaging protocols must optimise overall image quality rather than a single parameter.