HomeStretch CRACK Physics CT Flashcards
what kind of technology allows gantry to rotate?
slip ring technology
HVL of soft tissue in a human in:
conventional radiography
CT
Mammo
3 cm
3-4 cm
1 cm
Slice thickness on CT is determined by -
Single slice CT
Multi slice CT
collimation
width of detector rows (aka detector element aperture width)
Equation for pitch
what is a pitch > 1 all mean?
Pitch = table movment / beam width
pitch > 1 = table moves fast, creates gap, and decreases dose
What is the relationship between HU and X-ray attenuation?
when HU increases by 10 HU, e-ray attenuation increases by 1%
How does HU change with keV?
decrease keV =
positive HU values become more positive
negative HU values become more negative
Level and Window for
Brain
Lung
Abdomen
Bone
- Brain = L40 W80
- Lung = L-400 W1500
- Abdomen= L50 W400
- Bone= L500 W1600
- remember you want the level at the area of interest. . .
increase kV in CT does what to dose, contrast and noise?
increase dose and decrease noise and contrast
CT dose increase from mA
from kVp
proportional
square of kV
kVp for CT angio study?
80 - 100 kVp
over beaming decreases with more/less detector rows and thinner/wider beam?
more detector rows
wider beam
increasing pitch does what to over ranging?
increased over ranging!
What are all the things that will increase SNR?
Higher mA (4x mA to increase SNR by 2x)
Longer rotation time
higher kVp
larger slice thickness
larger pixel size
decreased pitch
Contrast resolution - is it good on CT?
how about spatial resolution?
excellent! (remember you can tell difference between white and grey matter. . .)
spatial resolution . . not so good - Plain radiograph is superior!
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?
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!
Cardiac CT Beta-blocker dosage and antidotes
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
Cardiac CT med nitro doses
0.8-1.2 mg glycerol nitrate
5mg isosorbide dinitrate
ACR limits of dose, average doses and effective doses
Abd CT adult
Abd CT peds (5 yo)
Head CT
- 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)
what dose dose temporary hair loss occur? how much radiation is a typical head CT?
temp hair loss = 3 Gy
CT head = 50 mGy (need 50 head CTs).
Pitch equation
Pitch = table movement / Beam width
Pixel size equation
Pixel size = FOV / Matrix
weighted CTDI equation
weighted CTDI = 1/3 central + 2/3 peripheral (in mGy)
equation for Volume CTDI
Volume CTDI = weighted CTDI / Pitch
volume CTDI is over or underestimated in peds patients? how about it the morbidly obese?
underestimated in peds
overestimated in morbidly obese
