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Medical Physics: Imaging w/ Non-Ionising Radiation > MRI: Sequences > Flashcards

MRI: Sequences Flashcards

1
Q

What are the T1 and T2 of Grey Matter?

A

T1 - 1000ms

T2 - 100ms

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2
Q

What are the T1 and T2 of White Matter?

A

T1 - 600ms

T2 - 90ms

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3
Q

What are the T1 and T2 of CSF?

A

T1 - 3000ms

T2 - 600ms

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4
Q

What are the T1 and T2 of Liver?

A

T1 - 600ms

T2 - 30ms

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5
Q

What are the T1 and T2 of Muscle?

A

T1 - 1100ms

T2 - 50ms

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6
Q

What are the T1 and T2 of Kidney?

A

T1 - 800ms

T2 -50ms

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7
Q

What causes the changes in T1 and T2 between tissues?

A

Different chemical compositions and physical properties.

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8
Q

How is the echo time (TE) measured?

A

Time elapsed between pulse and centre of Acquisition

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9
Q

How is the repetition time measured?

A

Time between the same RF pulse on two adjacent sequences.

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10
Q

What type of contrast comes from changing the echo time?

A

T2

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11
Q

What type of contrast comes from changing the repetition time?

A

T1

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12
Q

Describe briefly how a gradient echo sequence produces a signal.

A

RF pulse knocks M into the x-y plane (might not be complete, could be at any angle).
Negative readout gradient causes the signal to dephase due to inhomogeneities in B field.
Gradient reversed, and signals recombine after the same amount of time, causing an increase in the signal strength which is recorded.

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13
Q

How does altering the TE affect the signal from a gradient echo image?

A

Longer TE gives less signal due to dephasing of magnetisation in the x-y plane.

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14
Q

What causes the dephasing that leads to T2 relaxation?

A

Spatial variations in magnetic field - caused by differences in the magnetic properties of the tissue.
Temporal variation in the magnetic field - caused by molecular motion and influenced by tissue composition.

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15
Q

Describe briefly how a spin echo sequence produces a signal.

A

Rf pulse knocks M into the x-y plane (full 90deg)
Phase encoding gradient applied, dephasing of signal begins.
Second RF pulse (twice as long) applied half way between initial rf pulse and acquisition time (TE/2), flips signal by 180deg.
Signals recombine after the same amount of time (TE/2), causing an increase in the signal strength which is recorded.

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16
Q

What are the advantages of spin-echo imaging over gradient echo imaging?

A

More signal

Some dephasing reversed

17
Q

What is the spin-spin relaxation mechanism for a spin-echo imaging sequence?

A

T2

18
Q

What is the spin-spin relaxation mechanism for a gradient-echo imaging sequence?

A

T2*

19
Q

Why does a gradient echo sequence have less signal than a spin echo sequence?

A

T2* is shorter than T2 so signal decays quicker.

20
Q

What is the disadvantage of a spin-echo sequence over a gradient-echo sequence?

A

Minimum echo time is longer so sequences take longer to complete.

21
Q

What sort of TRs and TEs do we want for a T1-weighted scan?

A

Short TR, Short TE

TR < 750ms, TE < 40ms

22
Q

What sort of TRs and TEs do we want for a T2-weighted scan?

A

Long TR, Long TE

TR > 1500ms, TE > 75ms

23
Q

What sort of TRs and TEs do we want for a PD-weighted scan?

A

Long TR, Short TE

TR > 1500ms, TE < 40ms

24
Q

What sort of flip angle and TE do we want for a T1-weighted scan?

A

Large flip angle, Short TE

a > 50, TE < 15ms

25
Q

What sort of flip angle and TE do we want for a T2-weighted scan?

A

Small flip angle, Long TE

a < 40, TE > 30ms

26
Q

What sort of flip angle and TE do we want for a PD-weighted scan?

A

Small flip angle, Short TE

a < 40, TE < 15ms

27
Q

Name three types of imaging that use pre-pulses to alter contrast.

A

Fat Saturation - removes fat from the signal with fat-specific rf pulse.
Inversion recovery - measures T1, or removes some component if inversion delay corresponds to the time required for the signal to reach the x-y plane.
Perfusion imaging - sensitive to blood flow.

28
Q

How does echo-planar imaging reduce scan times?

A

Maps all of k-space in one acquisition, instead of line by line.

Medical Physics: Imaging w/ Non-Ionising Radiation (9 decks)

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