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HomeStretch/Numbers > HomeStretch CRACK Physics CT > Flashcards

HomeStretch CRACK Physics CT Flashcards

1
Q

what kind of technology allows gantry to rotate?

A

slip ring technology

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2
Q

HVL of soft tissue in a human in:

conventional radiography

CT

Mammo

A

3 cm

3-4 cm

1 cm

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3
Q

Slice thickness on CT is determined by -

Single slice CT

Multi slice CT

A

collimation

detector rows (ala detector element aperture width)

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4
Q

Equation for pitch

what is a pitch > 1 all mean?

A

Pitch = table movment / beam width

pitch > 1 = table moves fast, creates gap, and decreases dose

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5
Q

What is the relationship between HU and X-ray attenuation?

A

when HU increases by 10 HU, e-ray attenuation increases by 1%

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6
Q

How does HU change with keV?

A

decrease keV =

positive HU values become more positive

negative HU values become more negative

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7
Q

Level and Window for

Brain

Lung

Abdomen

Bone

A
  • Brain = L40 W80
  • Lung = L-400 W1500
  • Abdomen= L50 W400
  • Bone= L500 W1600
    • remember you want the level at the area of interest. . .
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8
Q

increase kV in CT does what to dose, contrast and noise?

A

increase dose and decrease noise and contrast

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9
Q

CT dose increase from mA

from kVp

A

proportional

square of kV

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10
Q

kVp for CT angio study?

A

80 - 100 kVp

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11
Q

over beaming decreases with more/less detector rows and thinner/wider beam?

A

more detector rows

wider beam

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12
Q

increasing pitch does what to over ranging?

A

increased over ranging!

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13
Q

What are all the things that will increase SNR?

A

Higher mA (4x mA to increase SNR by 2x)

Longer rotation time

higher kVp

larger slice thickness

larger pixel size

decreased pitch

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14
Q

Contrast resolution - is it good on CT?

how about spatial resolution?

A

excellent! (remember you can tell difference between white and grey matter. . .)

spatial resolution . . not so good - Plain radiograph is superior!

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15
Q

Trivia!

What kind of x-rays are used with CT?

What is the matrix size for CT, each pixel is?

If you turn down mAs, what happens to your images?

A

Highly filtered, High kV (average = 75 keV)

matrix is 512 x 512, with each pixel representing 4096 possible shades of gray (12bits) 212=4096

Less mA = more noise!

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16
Q

Cardiac CT Beta-blocker dosage and antidotes

A

metoprolol 2.5mg IV

Antidotes:

atropine 0.5 mg IV up to X3

Glucagon = 50 micrograms/kg IV loading dose followed by infusion of 1-15mg/h

17
Q

Cardiac CT med nitro doses

A

0.8-1.2 mg glycerol nitrate

5mg isosorbide dinitrate

18
Q

ACR limits of dose, average doses and effective doses

Abd CT adult

Abd CT peds (5 yo)

Head CT

A
  • Abd CT adult = 25 mGy ; 18 mGy (effective dose 8-11 mSv)
  • Abd CT peds (5 yo) = 20 mGy ; 15 mGy
  • Head CT = 75 mGy (effective dose 1-2 mSv)
19
Q

what dose dose temporary hair loss occur? how much radiation is a typical head CT?

A

temp hair loss = 3 Gy

CT head = 50 mGy (need 50 head CTs).

20
Q

Pitch equation

A

Pitch = table movement / Beam width

21
Q

Pixel size equation

A

Pixel size = FOV / Matrix

22
Q

weighted CTDI equation

A

weighted CTDI = 1/3 central + 2/3 peripheral (in mGy)

23
Q

equation for Volume CTDI

A

Volume CTDI = weighted CTDI / Pitch

24
Q

volume CTDI is over or underestimated in peds patients? how about it the morbidly obese?

A

underestimated in peds

overestimated in morbidly obese

25
Q

CTDI is reported in what units?

What about DLP?

What about absorbed dose?

How about effective dose?

A

CTDI is reported in mGy

mGy X cm

Absorbed dose also reported in G

effective dose is reported in mSv

26
Q

Dose Leng Product equation

A

DLP = CTDIvol X length of scan in cm

(reported in mGy X cm)

27
Q

Effective dose equation. What units is it reported in?

A

Effective dose = (constant k) X DLP

reported in mSv!

28
Q

Factors effective dose

doubling mA

doubling kVp

doubling pitch

doubling rotation time

A
  • doubling mA = double CTDI, DLP and Effective dose
  • doubling kVp = quaduple
  • doubling pitch = half
  • doubling rotation time = double
29
Q

Risk of radiation induced cancer per Sv for

child, adult and someone older than 50

A

Child = 15% / Sv

Adult = 5% / Sv

1/10 that for someone older than 50

30
Q

Dose nomenclature summary

CTDIw -> CTDIvol -> DLP -> Effective dose

A
  • CTDIw
    • mGy
    • adjusted for pitch =
  • CTDIvol
    • mGy
    • adjusted for distance scanned =
  • DLP
    • mGy X cm
    • adjusted for organ sensitivity =
  • Effective dose
    • =mSv
31
Q

Dose of 1 Chest CT is equal to how many PA + lateral chest x-rays?

What is the embyo dose in a CT A/P?

A

100

30mGy

32
Q

Individual dose monitoring is mandated if the occupational exposure is favored to be greater than ___ of the annual dose limit of ____

A

10% of the annual dose limit of 500rem

HomeStretch/Numbers (24 decks)

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