What principle explains the variation of x-ray intensity across the primary beam?

The variation of x-ray intensity across the primary beam is best explained by the anode heel effect. This phenomenon occurs due to the geometry of the x-ray tube, specifically the angle of the anode surface.

When the x-ray beam is produced, more intense radiation is emitted toward the cathode side of the tube than the anode side. This results in a gradient of x-ray intensity across the beam, with stronger exposure on one side. This variation is particularly important when imaging thicker body areas or structures, as it can affect the quality of the resulting images.

The anode heel effect is particularly significant in situations where precise imaging is required, such as in mammography or when imaging parts of the body that are uneven in thickness. Understanding this effect helps technicians and radiologists optimize positioning and settings for better image acquisition.

In contrast, the line-focus principle relates to the effective focal spot size and helps in improving resolution, the transformer law handles the relationship between voltage and current in transformers, and the inverse-square law addresses the intensity of radiation as the distance from the source increases. Each of these aspects plays a role in x-ray physics but does not directly account for the intensity variation across the primary beam as the anode heel effect does.