What causes the intensity of the x-ray beam to be generally higher from the cathode side of the tube?

The intensity of the x-ray beam is generally higher from the cathode side of the tube primarily due to the heel effect. This phenomenon occurs because of the geometry of the x-ray tube and the characteristics of the anode. In an x-ray tube, the anode is angled, which influences how the x-ray photons are emitted.

As the x-rays travel through the anode, some of them are attenuated or absorbed, particularly those that are emitted at angles that are not directly perpendicular to the anode surface. This results in a greater intensity of x-rays being produced from the cathode side of the tube, where the path through the anode is shorter as compared to the x-rays generated near the anode side. Consequently, this setup creates an uneven distribution of x-ray intensity across the beam, with the cathode side having a higher concentration of useful x-ray photons for imaging.

Understanding the heel effect is crucial for positioning patients and setting exposure parameters in mammography, as it allows radiographers to optimize image quality by taking advantage of the varying intensity across the beam.