CT Basics (1-2)

Question 1

LAC (Radiation Physics)

Answer 1

Linear Attenuation Coeffiecient: fraction of radiation attenuated (absorbed/scattered) per cm

Question 2

CT numbers (Hounsfield Units)

Answer 2

Unit to which scanner converts LAC of all tissues within scan field

Question 3

Large CT numbers are generated by…

Answer 3

…materials (e.g., bone/metal) that absorb a large fraction of the x-ray beam (and appear bright)

Question 4

Small (even negative) CT numbers are generated by…

Answer 4

…materials (e.g., air/fat) that attenuate very little radiation (and appear dark)

Question 5

3 conditions of x-ray production (which occur within a vacuum-sealed x-ray tube)

Answer 5

Source of Electrons
Means of rapid acceleration of electrons
Means of rapid deceleration of electrons (releasing electron energy)

Question 6

Source of electrons

Answer 6

Cathode filament

Question 7

Rotating Anode

Answer 7

Target of electron stream

Question 8

Rotating anode where accelerated electrons collide and release energy as heat and x-rays

Answer 8

Tungsten target

Question 9

Means of accomodating significant heat generated by electron collisions within x-ray tube (3)

Answer 9

Thicker anodes
Faster rotation
Unique tungsten alloys with higher heat capacities and faster cooling rates

Question 10

Photon “quantity” aka “Intensity”

Answer 10

Number of photons in x-ray beam
Directly proportional to radiation dose and detector exposure

Question 11

X-ray “Quality”

Answer 11

“Energy” of x-ray photons in the beam
A.k.a. “Hardness,” “Penetrability”

Question 12

Polychromatic

Answer 12

aka Polyenergetic
x-ray beam’s photons have different energies

Question 13

The unit keV describes:

Answer 13

Average energy of photons in the x-ray beam

Question 14

3 Prime (adjustable) exposure factors that control the energy & intensity of the x-ray beam

Answer 14

Milliamperage (mA)
Rotation Time (sec)
Kilovoltage peak (kVp)

Question 15

mA

Answer 15

“milliamperage”
Measurement for tube current
Controls number of electrons flowing through x-ray tube per second

Question 16

Intensity (quantity), patient dose, detector exposure are all directly proportional to (1)

Answer 16

mA

Question 17

ADM

Answer 17

Automatic Dose Modulation
mA varies throughout scan as density of tissue varies

Question 18

Rotation Time

Answer 18

Total time required for one full cycle of x-ray tube around patient

Question 19

mAs

Answer 19

Rotation time x milliamperes
Carries a reciprocal relationship to time (mAs/s=mA)

Question 20

kVp (3)

Answer 20

“Kilovoltage peak”
Voltage of the x-ray tube
“tube potential”

Question 21

Result of increased kVp (5)

Answer 21

Higher energy photons (“beam quality”) More total photons in x-ray beam (“beam quantity”)
More “intense” beam, more Pt dose, more photons to detector
Higher % photons penetrating Pt to reach image receptor
More even penetration of patient loss of image contrast

Question 22

kVp modification required for increased part thickness/density

Answer 22

Increase

Question 23

Aluminum filtration effect (3)

Answer 23

Removes mostly low energy photons from beam (they would add dose to patient, but not reach detectors)
Decreases intensity
Increases avg beam energy

Question 24

Bowtie Filter

Answer 24

Shapes intensity of beam to match shape of patient (reducing intensity on outer edges of Pt)

Question 25

Bowtie Filter

Answer 25

Shapes intensity of beam to match shape of patient (reducing intensity on outer edges of Pt)

Question 26

Utility of CT warm-up (3)

Answer 26

Slowly heats anode to avoid:
Cracking anode disk
Becoming unstable in rotation
Rendering tube useless

Question 27

CT warm-up

Answer 27

Using low-mA, low-kVp to slowly increase anote temp