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Instrumentation 2 > MRI > Flashcards

MRI Flashcards

1
Q

What property does a material need to be considered MR active?

A

Odd mass number

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2
Q

What is it called when a proton is spinning in its own magnetic field?

A

Preccession

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3
Q

When B0 is active (RF is off) are there more parrallel or anti-parallel protons?

A

Parallel (spin-up)

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4
Q

Why does an increase in the magnetic field strength increase SNR?

A

More spin up protons

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5
Q

WHat is the NMV the product of?

A

spin up - spin down

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6
Q

The transverse axis is known as M__?

A

Mxy (mxy , maxymum signal)

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7
Q

What is the resonance frequency for hydrogen in a magnetic field of 5 Tesla

A

212.9 MHz

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8
Q

What affects the magnitude of the flip angle?

A

Amplitude (strength) and duration of RF pulse

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9
Q

What controls the flip angle?

A

The selected pulse sequence

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10
Q

When does saturation start to occur?

A

91+ degrees - 180

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11
Q

When is the signal considered fully saturated?

A

180 degree

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12
Q

WHat is phase coherence?

A

When the NMV is at 90 degrees and there is minimum inhomogenity

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13
Q

What are the two main relaxation processes?

A

T1 Recovery and T2 Decay

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14
Q

What is the fundamental difference between T1 and T2?

A

T1 looks at the amount of magnetisation in the longitudinal plane (as it increases) . It is the NMV moving back to B0. Time taken for 63% of NVM to recover.

T2 is the amount of magnetisation in the trans plane as it decreases. Time taken for protons to dephase

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15
Q

Which decay is faster: T2 or T2*?

A

T2*

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16
Q

If something is appearing black does it typically have a longer or shorter flip angle?

A

Shorter (not pushed as far into transverse plane).

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17
Q

Which Weighting relies on partial saturation?

A

T1

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18
Q

Why is a short TR desirable for T1?

A

Because it will push both water and fat into saturation

19
Q

Will bone appear bright or dark on a Proton density weighting?

A

Dark

20
Q

For T1 does the TR need to be long or short?

A

Short

21
Q

for T2 does the TR need to be long or short?

A

Long and TE is Long too

22
Q

Will a larger bore size improve SNR?

A

no. smaller increases SNR

23
Q

What coil is used to increase homogeneity?

A

Shim coils

24
Q

Is 15ppm good?

A

No we need less than 10 ppm for homogenity

25
Q

Which provides better SNR: Surface coil or volume coil?

A

surface

26
Q

What plane are receiver coils placed in?

A

transverse plane

27
Q

What gives the spatial co-ordinates of anatomy?

A

gradient coils

28
Q

Why does spin echo give better signal?

A

it diminishes the signal loss of T2*

29
Q

What is TE?

A

Time between the 90° excitation pulse and the spin echo (i.e. signal)

30
Q

What is TAU?

A

 Time the NMV takes to dephase after the 90° RF pulse was withdrawn
 Time taken to rephase after the application of the 180° RF pulse

Te=2TAU

31
Q

What does TE control?

A

T2 weighting

32
Q

Which coil is PEG?

A

Y

33
Q

which coil is FEG

A

X

34
Q

is PEG signal recorded when the PEG pulse is on?

A

no

35
Q

Is the FEG signal recorded whilst the FEG is on?

A

yes

36
Q

When is the SSG turned on?

A

during the RF pulse

37
Q

In a chemical shift the higher frequencies are shifted which way?

A

down

38
Q

What factors can affect chemical shift?

A 
Higher B0 strength
Thicker slices (lower gradients and wider bandwidths)
39
Q

Will a short TE reduce chemical shift?

A

no we need a longer TE to allow the fat to dephase

40
Q

We can have a thinner slice to decrease the chance of chemical shift and increase spatial resolution, but what is the trade off?

A

Lower SNR

41
Q

How does partial volume averaging occur?

A

More than one tissue type in a voxel… use a thinner slice to remedy

42
Q

How does zipper artefact occur?

A

Outside RF fields interfering with the scanner

43
Q

3 ways to remedy excitation

A

1/3 slice thickness gap in contiguous slice mode

Interleaving

Optimised pulses with longer TE

Instrumentation 2 (4 decks)

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