Physics Flashcards
Signature Peak Rh
23
Signature Peak Mo
20
Signature Peak Silver
25
Signature Peak Tungsten
70
K edge of Iodine
33
K edge of Barium
37
Smaller focal spot in mammo will have what effect on motion artifact?
increased artifact (longer exposure time)
What causes “ring artifact” on CT?
Seen in 3rd generation CT when there are defective detector elements.
What is the basic idea behind a fast spin echo sequence?
Multiple refocusing pulses are used to sample multiple lines of k-space after 1 exitation pulse.
In a spin echo sequence, what effect with shortening the echo time have?
decreases T2 weighting
Effective radiation dose of chest radiograph
0.1 mSv
effective radiation dose mammography
0.4 mSv
Effective radiation dose CT head
2 mSv
effective radiation dose CT Chest
7 mSv
effective radiation dose CT Abd pelv w/o and with and without
10 mSv and 20 mSv
mSv and cancer risk according to FDA…
10 mSv increases fatal cancer risk by 1/2000
overall fatal cancer death is 400/2000 (20%)
adding 10 mSv changes fatal cancer to 401/2000
What is the symbol for bulk magnetic moment?
M
What direction is the z axis/ longitudinal?
crainiocaudal
larmor frequency equation
(42. 6 MHz/T) x B
synonyms: precession freq, resonance freq
T2 is which relaxation?
transverse
T1 is which relaxation?
longitudinal
T1 = time required for ____% recovery of Mz?
63%
T2= _____% remaining spin-spin, transverse mag
37%
Fat is ___ on T1 and ____ on T2
short
think of short and fat winnie the pooh
free induction decay is given by a rate given by which relaxation time?
T2*
Free induction decay is synomym for T2* decay
Which relaxation time is the shortest between T1, T2, and T2*
T2*
slowest T1
PD weighting has ___TR and ___ TE?
Long TR and short TE
high signal
low contrast
all the tissues are participating ALL hydrogens
Type 1 chemical shift artifact
frequency-encoding gradient causes water and fat in a single voxel to map onto different pixels
difference gets bigger with higher telsa
how does higher bandwidth affect chemical shift?
increased bandwidth decreases the appearance of type 1 chemical shift artifact
(you are assigning more Hz per pixel)
How to do calculate how long it takes to fill out K-space?
repetitions x # slices x TR x # phase enchoding steps
synonyms for repitions+ # signal averages (NSA) and # of excitations (NEX)
- for turbo spin echo you divide by the turbo factor
- Single-shot fast spine echo (SSFSE)= all rows of k-space in one TR (pretending that all the echos are the same) good for fetal MRI
BLADE
faster or slower?
when is it good?
slower
but good with motion because you are sampling the center of K-space multiple times
half fourier methods compare to full k-space sampling in what way?
lower SNR
faster aquisition
How do you increase SNR?
increase static magnetic field (b0)
increase voxel size
3.
increase reciever bandwidth
Three types of fat suppression?
Inversion recovery
chemical fat suppression (chemical shift spectral sensitive fat suppresion)
DIXON
pros and cons of chemical fat suppression
pros: unaffect tissue contrast
works with gad
cons: fails with field inhomogeneity (based on chemical shift)- happens with large or oddly shaped anatomy
saturation pulse increases aquisition time
DIXON fat suppression
you take in and out of phase and subtract to figure out fat
pros: works with gad
cons: susceptible to B-field inhomogeneity, better than chemical fat suppression
longer TR needed in and out phase aquired together
Inversion recovery fat suppression pros and cons
pros: reobust to b-field inhomogeneity
cons: also suppresses gad
Features of main black blood methods.
(T2 spin echo, inversion recovery, double inversion recovery)
T2 Spin echo- blood isn’t pitch black
Inversion recovery- dark blood
Double inversion recovery- bright myocardium, dark blood
specific absorption rate (SAR) in MRI is…
watts (W) per kilogram (kg)
1 W/kg -> 1 deg C change/ hr
to decrease SAR- decrease number of RF pulses, lengthen out TR
SAR limit is 4 W/kg over 15 min interval
“Effective mA or mAs” = ?
The term “effective mA” or “effective mAs” is used in helical CT scanning and is the mAs/pitch. As the pitch increases with all other settings remaining constant, the number of x-ray photons contributing to the slice data will decrease (effective mAs). The effective mAs determines the dose to the slice (CTDIvol) and signal to noise.
What effects SAR?
What’s the max SAR you can give someone?
SAR= Specific absorption rate
SAR = duty rate x magnet strength2 x alpha<strong>2</strong> (flip angle)
max SAR you can give someone is 4 W/kg in the body, 1.5 W everyone else
What is the primary source of acoutstic noise in MRI?
Magnetic field gradients.
What sequence is most likely to bother your ears?
DWI
(EPI/echo planar imaging)
