CT

Question 1

what is the importance of filter

Answer 1

• added thickness: made of copper, aluminium
• removes low energy x-rays that do not contribute to image info but do increase patient dose
• loss of lower energy photos from a polychromatic beam causes
  *narrowing of xray beam - creating a more monochromatic beam
  *change in shape of beam spectrum with filtration: beam hardening

Question 2

why is a bow tie shaped filter used

Answer 2

• lateral edges of body are thinner than the center = xray beam less attenuated
• bow tie shape filter compensates for this by attenuating the lateral edges of the beam more than the center

Question 3

what is the use of a pre-patient collimator

Answer 3

• lower radiation dose to patient
• restricts scatter from outside of desired slice

Question 4

what is the use of a post patient collimator

Answer 4

suppress scatter radiation (act as a grid)

Question 5

use of a slip ring

Answer 5

• to keep scanner charged without tangling the cords
• electromechanical device
• allows transmission of power and electrical signals from a stationary to a rotating structure

Question 6

requirement of a detector

Answer 6

• high accuracy
• wide dynamic range
• high stability
• high efficiency
• fast response
• narrow gaps between active elements
• low cost
• small physical size

Question 7

properties of a solid state detector

Answer 7

• high detection efficiency
• high geometrical efficiency
• small physical size of detector elements

Question 8

formula to calculate pitch

Answer 8

distance per rotation/ slice thickness

Question 9

3 phases of CT image formation

Answer 9

1. scan and data acquisition
2. image reconstruction
3. digital/analog conversion and display control

Question 10

lower kv will

Answer 10

• give more contrast
• produce more noise
• decrease radiation dose