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Physics Flashcards

1
Q

Signature Peak Rh

A

23

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2
Q

Signature Peak Mo

A

20

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3
Q

Signature Peak Silver

A

25

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4
Q

Signature Peak Tungsten

A

70

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5
Q

K edge of Iodine

A

33

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6
Q

K edge of Barium

A

37

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7
Q

Smaller focal spot in mammo will have what effect on motion artifact?

A

increased artifact (longer exposure time)

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8
Q

What causes “ring artifact” on CT?

A

Seen in 3rd generation CT when there are defective detector elements.

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9
Q

What is the basic idea behind a fast spin echo sequence?

A

Multiple refocusing pulses are used to sample multiple lines of k-space after 1 exitation pulse.

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10
Q

In a spin echo sequence, what effect with shortening the echo time have?

A

decreases T2 weighting

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11
Q

Effective radiation dose of chest radiograph

A

0.1 mSv

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12
Q

effective radiation dose mammography

A

0.4 mSv

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13
Q

Effective radiation dose CT head

A

2 mSv

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14
Q

effective radiation dose CT Chest

A

7 mSv

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15
Q

effective radiation dose CT Abd pelv w/o and with and without

A

10 mSv and 20 mSv

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16
Q

mSv and cancer risk according to FDA…

A

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

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17
Q

What is the symbol for bulk magnetic moment?

A

M

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18
Q

What direction is the z axis/ longitudinal?

A

crainiocaudal

19
Q

larmor frequency equation

A

(42. 6 MHz/T) x B
synonyms: precession freq, resonance freq

20
Q

T2 is which relaxation?

A

transverse

21
Q

T1 is which relaxation?

A

longitudinal

22
Q

T1 = time required for ____% recovery of Mz?

A

63%

23
Q

T2= _____% remaining spin-spin, transverse mag

A

37%

24
Q

Fat is ___ on T1 and ____ on T2

A

short

think of short and fat winnie the pooh

25
Q

free induction decay is given by a rate given by which relaxation time?

A

T2*

Free induction decay is synomym for T2* decay

26
Q

Which relaxation time is the shortest between T1, T2, and T2*

A

T2*

slowest T1

27
Q

PD weighting has ___TR and ___ TE?

A

Long TR and short TE

high signal

low contrast

all the tissues are participating ALL hydrogens

28
Q

Type 1 chemical shift artifact

A

frequency-encoding gradient causes water and fat in a single voxel to map onto different pixels

difference gets bigger with higher telsa

29
Q

how does higher bandwidth affect chemical shift?

A

increased bandwidth decreases the appearance of type 1 chemical shift artifact

(you are assigning more Hz per pixel)

30
Q

How to do calculate how long it takes to fill out K-space?

A

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
31
Q
A
32
Q

BLADE

faster or slower?

when is it good?

A

slower

but good with motion because you are sampling the center of K-space multiple times

33
Q

half fourier methods compare to full k-space sampling in what way?

A

lower SNR

faster aquisition

34
Q

How do you increase SNR?

A

increase static magnetic field (b0)

increase voxel size

3.

increase reciever bandwidth

35
Q

Three types of fat suppression?

A

Inversion recovery

chemical fat suppression (chemical shift spectral sensitive fat suppresion)

DIXON

36
Q

pros and cons of chemical fat suppression

A

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

37
Q

DIXON fat suppression

A

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

38
Q

Inversion recovery fat suppression pros and cons

A

pros: reobust to b-field inhomogeneity
cons: also suppresses gad

39
Q

Features of main black blood methods.

(T2 spin echo, inversion recovery, double inversion recovery)

A

T2 Spin echo- blood isn’t pitch black

Inversion recovery- dark blood

Double inversion recovery- bright myocardium, dark blood

40
Q

specific absorption rate (SAR) in MRI is…

A

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

41
Q

“Effective mA or mAs” = ?

A

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.

42
Q

What effects SAR?

What’s the max SAR you can give someone?

A

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

43
Q

What is the primary source of acoutstic noise in MRI?

A

Magnetic field gradients.

44
Q

What sequence is most likely to bother your ears?

A

DWI

(EPI/echo planar imaging)

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