CT Basics (1-2) Flashcards

1
Q

LAC (Radiation Physics)

A

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

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2
Q

CT numbers (Hounsfield Units)

A

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

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3
Q

Large CT numbers are generated by…

A

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

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4
Q

Small (even negative) CT numbers are generated by…

A

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

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5
Q

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

A

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

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6
Q

Source of electrons

A

Cathode filament

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7
Q

Rotating Anode

A

Target of electron stream

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8
Q

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

A

Tungsten target

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9
Q

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

A

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

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10
Q

Photon “quantity” aka “Intensity”

A

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

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11
Q

X-ray “Quality”

A

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

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12
Q

Polychromatic

A

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

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13
Q

The unit keV describes:

A

Average energy of photons in the x-ray beam

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14
Q

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

A

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

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15
Q

mA

A

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

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16
Q

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

17
Q

ADM

A

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

18
Q

Rotation Time

A

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

19
Q

mAs

A

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

20
Q

kVp (3)

A

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

21
Q

Result of increased kVp (5)

A

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

22
Q

kVp modification required for increased part thickness/density

23
Q

Aluminum filtration effect (3)

A

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

24
Q

Bowtie Filter

A

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

25
Bowtie Filter
Shapes intensity of beam to match shape of patient (reducing intensity on outer edges of Pt)
26
Utility of CT warm-up (3)
Slowly heats anode to avoid: Cracking anode disk Becoming unstable in rotation Rendering tube useless
27
CT warm-up
Using low-mA, low-kVp to slowly increase anote temp