Week 5: Matter Interactions, Tube Principles and Tube Age

Question 1

Photoelectric Absorption

Answer 1

Occurs when the photon interact with the inner shell electron. The photon must have slightly greater energy than that of the binding energy of the electron. The incident photons ejects the electron and it is totally absorbed, ionizing the atom. The ejected electron = photoelectron. More likely to occur with lower energy x-rays and in bone more than soft tissue due to its high atomic number. PE effect is radiographically significant because all photons are absorbed = white on image. This is why x-rays are so good at visualizing bone.

Question 2

Compton Scatter

Answer 2

Occurs when x-rays interact with matter and change direction. Scatter is more likely to occur with higher energy x-rays. After the photons scatters it has less energy and will continue to scatter until it is absorbed photoelectrically or until it exits the patient. Scatter photons exiting the patient pose a radiation hazard to the technologist. Scatter impairs image quality because it hits the IR randomly which is unrelated to the patients anatomy.

Question 3

Transmission

Answer 3

X-rays that are transmitted through the patient with no interaction, they create dark areas on the image.

Question 4

Line Focus Principle

Answer 4

Geometric principle applied to x-ray that helps us understand how the angle of the anode affects the size of the created x-ray beam as it trialed towards the patient. This principle tells us that id the anode is angled less than 45 degrees from vertical the effective focal spot will be smaller than the incident actual focal spot, which is what we want. When the anode is angled however there is better heat dissipation due to increased surface area. There must be a balance of spatial resolution and heat dissipation. 12 degrees is that happy medium.

Question 5

Actual Focal Spot

Answer 5

The electrons that strike the focal track.

Question 6

Effective Focal Spot

Answer 6

The beam that is projected towards the patient. We want this as small as possible to maintain detail resolution.

Question 7

The Anode Heel Effect

Answer 7

There is a greater radiation intensity on the cathode side of the tube due to the angled anode. Photons emitted on the anode side are more lily to be absorbed by the anode so there will be more photons exciting at the cathode side. Place thicker part of patient on the cathode side to utilize this affect. Anode heel effect is most noticeable when there is a short SID and IR size is large.