CT Physics

Question 1

Decreasing keV effect on Hounsfield units

Answer 1

Increases HU (more PE effect)

Question 2

Lung window

Answer 2

-400, 1500 width

Question 3

Abdomen window

Answer 3

50, 400 width

Question 4

Bone window

Answer 4

500, 1600 width

Question 5

Brain window

Answer 5

40, 80 width

Question 6

Effect of increasing kVP (CT)

Answer 6

Increase dose, decrease contrast (lower PE)

Question 7

Factors that increase SNR (CT)

Answer 7

Longer rotation time
Decrease pitch
Increase kVp
Larger slice thickness
Larger pixel
Iterative reconstruction

Question 8

Factors that increase spatial resolution (CT)

Answer 8

Decreased focal spot
Sharper filter
Decreased pixel size (FOV/matrix)
Decreased motion

Plain films have high spatial resolution, low contrast resolution compared to CT

Question 9

Contraindications to beta blockers

Answer 9

Severe COPD
Bronchospasm
Asthma on albuterol
Bradycardia
Low BP
2/3rd AV block

Question 10

Contraindication to nitroglycerin

Answer 10

Severe AS
Viagra
HOCM

Question 11

CT dose limits

Answer 11

25 mGy Adult
20 mGy Ped
75 mGy Head

Question 12

Factors that increase dose (CT)

Answer 12

mA (lineear)
kVp (squared)
pitch (linear)
rotation time (linear)

Question 13

Average electron energy relative to kVP

Answer 13

Average keV = 1/2 - 1/3 kVp

Question 14

Types of X rays

Answer 14

1. Bremhstraung
2. Classical (elastic)
3. Compton - loss of outer shell electron. Dominates at higher keV
4. Photoelectric effect - loss of inner shell electron. Outer shell fills in and releases X rays. Increases with higher density giving contrast between tissue types. Predominantly at lower energies

Question 15

Ways to decrease mottle

Answer 15

1. Increase slice thickness
2. Use higher mAs
3. Use soft tissue reconstruction/kernel
4. Increase kVp

Question 16

Mammo mag views

Answer 16

1. Smaller focal spot
2. Less mA
3. Longer exposure time

Question 17

Spatial resolution of different Mammography

Answer 17

1. 15 lp/mm screen film
2. 7 lp/mm digital

Question 18

Typical entrance air kerma (fluoroscopy)

Answer 18

25 mGy/min

Question 19

Factors that increase heel effect

Answer 19

1. Decreased source to image distance
2. Smaller anode angle
3. Larger FOV

Question 20

Iodine kEdge

Answer 20

33 keV