Question

describe how heel effect and the line focus principle may be applied in chest radiography to optimize image quality

Answer 1

Anode heel effect can be used for  advantage by placing the thinnest portion of the patient under the anode side of the beam and the thickest portion of the patient under the cathode side of the beam.

Line focus principle explains the relationship between the anode surface and the effective focal spot size.

Basic concept

The focal spot is the area of the target upon which the electron beam impinges. The energy of the electrons in the electron beam is mostly converted into heat (approximately 99 percent which is why materials such as Tungsten are used due to their high melting-points) and dissipated uniformly across the focal spot and anode surface. The x-rays produced at the anode comprise of less than one percent of the energy of the electrons in the electron beam.

A large focal spot is therefore useful to protect the tungsten target as the heat accumulates and dissipates within the area of focal spot. However, a small focal spot is required to achieve a good radiographic image quality.

Thus the line focus principle helps resolve this issue by stating that an angulation of the anode surface will result in an apparent decrease in the focal spot size

The apparent focal spot (projected focal spot) size can be determined by the sine of the angle of the anode surface (apparent focal spot size = real focal spot size * sin anode angle). The angle varies as per tube design with a range value of 6 degrees to about 20 degrees.

Advantage of it is The lower the target angle, the smaller the effective focal spot size. The advantage of line- focus principle is that it provides the detail of a small focal spot