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MRI > Image Production > Flashcards

Image Production Flashcards

1
Q

What is Heisenberg’s Uncertainty principle?

A

we cannot know the exact direction of a particle’s spin at any point in time

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2
Q

What two factors determine the Larmor frequency?

A

Gyromagnetic ratio and magnetic field strength (B0)

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3
Q

Why is protium used in MRI?

A

protium is an isotope of hydrogen. Hydrogen is very abundant in the human body, and because its solitary proton gives it a relatively large magnetic moment.

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4
Q

What is the gyromagnetic ratio of hydrogen?

A

42.58 Mhz/T

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5
Q

What are the results of resonance?

A

all magnetic moments move to the same position, with the same precessional path around B0.

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6
Q

What is gradient rise time, and what is it measured in?

A

The time it takes for the gradient to reach its full amplitude. Measure in microseconds or milliseconds

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7
Q

What is slew rate and how is it measured?

A

Acceleration of the gradient magnetic field to its maximum amplitude. Measure in in Tesla/meter/second or millitesla/meter/millisecond

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8
Q

What is duty cycle?

A

The time the gradients are on during one TR period

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9
Q

How is SAR measured?

A

Watts per kilogram

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10
Q

What is the primary biological effects of radiofrequency fields?

A

Tissue heating and thermal heating

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11
Q

FDA limits for main magnetic field

A

4 Tesla for entire population

8 Tesla for those over 1 month old

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12
Q

What temperature are the cryogens cooled to?

A

4 kelvin

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13
Q

define ferromagnetic

A

iron and iron-like substances that can generate a relatively strong magnetic field

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14
Q

define paramagnetic

A

Have a very weak magnetic field.

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15
Q

define diamagnetic

A

Have no magnetic field.

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16
Q

T1 relaxation time

A

the recovery of 63% longitudinal magnetization of a specific tissue’s original magnetization.

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17
Q

T2 relaxation time

A

the time it takes for 63% of the transverse magnetization to decay, or the time it takes the spins to de-phase to 37% of their original value

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18
Q

Spin-spin relaxation

A

The return of the transverse magnetization to its

equilibrium value, zero (T2 decay)

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19
Q

spin-lattice relaxation

A

The return of the longitudinal magnetization to its

equilibrium value along the Z axis (T1 recovery)

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20
Q

Fat has a ____ t1 relaxation time and a ____ t2 relaxation time

A

Short & short

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21
Q

Water has a ____ t1 relaxation time and a _____ t2 relaxation time

A

Long & long

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22
Q

Hydrogen in fat recovers _____ along the longitudinal axis than water and loses transvers magnetization _____ than water

A

Faster & faster

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23
Q

T1 weighted imaging uses a ____ TR and a _____ TE

A

Short & short

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24
Q

T2 weighted imaging uses a _____ TR and a _____ TE

A

Long & long

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25
Q

Proton Density imaging uses a ____ TR and a _____ TE

A

Long TR, Short TE

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26
Q 
Increasing the TR will
\_\_\_\_\_ SNR
\_\_\_\_\_ scan time
\_\_\_\_\_ t1 weighting
\_\_\_\_\_ available number of slices
A

Increase SNR
Increase scan time
Reduce t1 weighting
Increase slice #

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27
Q 
Decreasing the TR will:
\_\_\_\_\_ SNR
\_\_\_\_\_ scan time
\_\_\_\_\_ t1 weighting
\_\_\_\_\_ the available # of slices
A

Lower SNR
Reduce scan time
Increase t1 weighting
Reduce the # of slices

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28
Q

TR range for a t1 weighted image

A

350-700 ms

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29
Q

TR range for a PD weighted image

A

1500-3000 ms

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30
Q

Tr range for a t2 weighted image

A

2000-6000 ms

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31
Q

TE range for a t1 weighted image

A

10-30 ms

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32
Q

TE range for a PD weighted image

A

10-30 ms

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33
Q

Define superparamagnetic

A

Demonstrate positive magnetic susceptibilities stronger than paramagnetic substances and weaker than ferromagnetic substances. Often send as t2 contrast agents

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34
Q

TE range for a t2 weighted image

A

70-120 ms

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35
Q

How can you reduce scan time?

A 
Reduce TR
Reduce NSA/NE
Increase parallel imaging factor
Utilize RecFOV (rectangular FOV)
Utilize half-Fourier/halfscan
Use coarsest matrix possible
36
Q

When increasing NEX/NSA, SNR increases by what rate?

A

The square root of the %increase

(2 NEX to 6, 300% increase, 3 times, square root of 3, 1.73 times more SNR

37
Q

How can you increase SNR?

A 
Increase NEX/NSA
Increase FOV
Decrease phase matrix
Increase TR
Increase slice thickness
Reduce bandwidth
Reduce ETL (echo train length)
38
Q

Spin Echo scan time formula

A

TR X phase matrix x NEX

39
Q

Fast spin echo scan time formula

A

(TR x phase matrix x NEX)/ETL

40
Q

2D gradient echo scan time formula

A

TR x phase matrix x NEX

41
Q

3D gradient echo sequence time formula

A

TR x phase matrix x NEX x #slices

42
Q

How is pixel size determined?

A

Field of view / matrix

43
Q

What is gradient moment nulling?

A

Flow compensation. Compensated for first order motion (protons moving with constant velocity) and slow moving vessels

44
Q

Ernst angle?

A

Optimal flip angle that yields the maximum signal for a particular spin in e least amount of time

45
Q

Diffusion Tensor

A

Displays the mobility of water molecules in all three coordinates

46
Q

At 1 Tesla the difference in precessional frequencies between fat and water is ____ hz

A

147

47
Q

For Inversion Recovery, the null point is _____ the T1

A

0.69 times

48
Q

Does gadolinium pass thru the intact blood-brain barrier?

A

No, only if the barrier is disrupted

49
Q

On gradient echo sequences, what controls T1 weighting?

A

Flip angle. Larger flip angle produces more T1 weighting

50
Q

The strength of the magnetic field is determined by the _________ of the current

A

Amount

51
Q

The strength of the earth’s magnetic field is…

A

0.5 gauss

52
Q

Permanent magnets generally have a field strength of

A

0.06 to 0.35 Tesla

53
Q

Resistive magnets generally have a field strength up to

A

0.3 Tesla

54
Q

When substances have an ______ they are said to be MR active

A

Uneven mass number

Not atomic number

55
Q

A radio wave is a/an _________ electromagnetic field

A

Oscillating

56
Q

If the flip able is doubled the RF power is… it

A

Increased 4 times

57
Q

The FID signal can be changed from a plot of signal over time into a split of signal over frequency known as…

A

NMR spectrum

58
Q

Fourier transform occurs in the …

A

Array processor

59
Q

The t2 time is about ______ in fat

A

50 msec

60
Q

Water has a t2 time of about…

A

200 msec

61
Q

Fat has a T1 recovery of about

A

150 msec (short)

62
Q

Water has a T1 recovery of about

A

2000 msec (long)

63
Q

The purpose of the gradient coils is to produce a magnetic field that

A

varies in intensity in a given direction

64
Q

In a spin echo sequence the slice selection gradient is applied during…

A

The 90 degree RF pulse and the 180 degree RF pulse

65
Q

With each repitition, the phase encoding gradient will be applied with a different…

A

Amplitude and/or polarity

66
Q

Which operator selectable acquisition parameters affect SNR?

A

All of them

67
Q

RF shielding can be accomplished by lining the scan room with

A

Copper

68
Q

A short TI suppresses …

A

Suppresses fat

69
Q

A long TI suppresses…

A

Water

70
Q

At an intermediate TE, the differences between _____ will be demonstrated

A

Gray and white matter

71
Q

Echoes encoded with high amplitude phase encoding steps are weighted more for ______ and reside in the ______ of k-space.

A

Spatial resolution, or edge detail

Outer edges

72
Q

Does number of frequency encodings affect scan time?

A

No

73
Q

Scan time formula for 2d acquisition SE or GRE

A

TR x Ny (# of phase encodings) x NSA

74
Q

Scan time formula for FSE

A

TR x Ny (# of phase encodings) x NSA / ETL

75
Q

3D scan time formula

A

TR x Ny (# of phase encodings) x NSA x NSl (# of slices)

76
Q

Time of flight MRA sequences use ______ gradient echo acquisitions

A

T1 (spoiled or incoherent)

77
Q

Phase contrast MRA sequences use _______ gradient echo acquisitions

A

T2 (steady state or coherent)

78
Q

2D TOF is optimal for ______ flow in _____ vessels

A

Slow

Large

79
Q

3D TOF is optimal for _____ flow in ______ vessels

A

Fast

Small

80
Q

_________ flip angles are used with 3d TOF than 2D TOF

A

Lower

81
Q

The lower the field strength the ________ the T1 time

A

Lower

82
Q

Zero fill reduces scan time at the expense of ______

A

Spatial resolution

83
Q

Cost of gradient moment nulling

A

Increase minimum TE, signal dephasing is increased, vascular signal can be decreased

84
Q

In GRE reducing the flip angle has the same effect as…

A

Increasing the TR

85
Q

Vascular signal is increased when slices are acquired _______ the flow

A

against

86
Q

Routine weight-based contrast administration is typically __________

A

0.2ml/kg

MRI (8 decks)

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