The intensity of the x-ray beam from the cathode side of the tube is generally higher because
(A) Soft characteristic radiation emerges from the anode side.
(B) The cathode side is directed to the thickest part of the breast.
(C) The heel effect causes variation in the intensity of the x-ray beam.
(D) The heel effect increases the intensity of the beam at the anode side
The correct answer and explanation is :
The correct answer is (C) The heel effect causes variation in the intensity of the x-ray beam.
Explanation:
The intensity of the x-ray beam is higher on the cathode side of the tube due to the heel effect. The heel effect refers to the phenomenon where the intensity of the x-ray beam is greater on the cathode side and lower on the anode side. This is primarily caused by the geometry of the x-ray tube and the angle of the anode target.
Here’s why the heel effect occurs:
- X-ray tube structure: In a typical x-ray tube, the anode is positioned at an angle to the cathode. The x-ray photons generated by the tube’s target are emitted at various angles. The photons emitted at steeper angles (closer to the target) have to pass through a thicker portion of the anode material before they exit the tube, which results in increased attenuation and lower intensity.
- Anode material: The anode itself is made of a high-density material (like tungsten), which absorbs more of the x-rays emitted from the target. This attenuation effect is more pronounced at the anode side of the tube. As a result, fewer photons emerge from the anode side, leading to a lower intensity of the beam compared to the cathode side.
- Photon absorption: The x-rays produced at the anode side of the tube must travel through the anode material at a greater distance. This increases the likelihood that some of the x-ray photons will be absorbed or scattered by the anode, reducing the overall intensity of the beam on that side.
This phenomenon is taken into account during the design of radiographic equipment, and in some cases, the cathode side of the x-ray tube is directed towards the thicker part of the object being imaged (such as the breast in mammography) to ensure that the x-ray beam has a more uniform intensity across the area of interest.
Other Answer Choices:
- (A) Soft characteristic radiation is not specifically linked to the intensity difference between the cathode and anode sides; it is more about the energy levels of the x-rays produced.
- (B) The cathode side may indeed be directed towards the thicker part of the breast in some imaging systems (such as mammography) to compensate for the variation in beam intensity, but this is not the primary reason for the intensity difference.
- (D) The heel effect actually decreases the intensity on the anode side, not increases it.