MRI Physics

Question 1

T1 in different tissues

Answer 1

Spin-Lattice
Water (slowest)
Muscle
Fat (fastest due to long chains have complex thermally-induced flexions and rotations)

Question 2

T2 in different tissues

Answer 2

Spin-Spin - density related:
Water (slowest)
Fat
Muscle (fastest)

Question 3

How TE affects T2 contrast

Answer 3

Question 4

T1 Weighted sequence

Answer 4

TR - weight long enough the spins have dephased and the transverse vector is net zero

If you apply an RF pulse AGAIN
- The net longitudinal vector that was T1 recovery will be flipped 90 degrees

So fat would have :
- relaxed more, have a higher net longitudinal vector
- when flipped 90 degrees, have a higher magnitude transverse vector

• Use a short TE to negate T2 so now you’re measuring T1 differences in the transverse plane!

Question 5

T1 weighted scan

Answer 5

Negate T2 differences: SHORT TE

Maximise T1 contrast: SHORT TR

Question 6

Proton density scan

Answer 6

Negates
T1 differences - LONG TR
T2 differences - VERY SHORT TE (before they begin dephasing)

(Water + Fat have a higher proton density)

Question 7

T2 Weighted Scan

Answer 7

Negate T1 differences - LONG TR
Maximise T2 contrast - Short TE (but allowing some dephasement)

Question 8

T1 and T2 recovery

Answer 8

Question 9

T2 Decay Vs T1 signal gain

Answer 9

T2 happens A LOT FASTER

Question 10

Long TR value range

Answer 10

1500-2000’s

Question 11

Long TE value range

Answer 11

80-160ms

Short is 10’s

Question 12

Lamour Frequency = B0 x Gyromagnetic ratio

Answer 12

Question 13

Slice selection using RF

Answer 13

• Apply a gradient along B0 creating a gradient of precessional frequencies
• Apply an RF pulse of a bandwidth for your slice

Question 14

Slice selection by changing the strength of Bo

Question 15

Increase slice thickness by increasing bandwidth

Question 16

Decrease slice thickness by increasing field strength

Answer 16

Higher max Bo - greater range, so a set bandwidth of RF would resonate with a narrower plane.

Question 17

With a slice thickness, there’s a gradient of B0, which creates a slice phase.

Answer 17

Question 18

So after RF pulse, apply a rephasing gradient.

Equal and opposite gradient to Z axis

Slice is now completely in phase

Answer 18

Slice selection sequence

1. 90 degree flip
2. Rephasing flip/dip
3. 180 flip to account for T2* (free induction decay)

Question 19

You need multiple TEs to infer what’s happening to net magnetisation over time.

And receiver coil has a bandwidth as well.

Answer 19

Visualising T2* correction

So you need to factor this in when using multiple TEs

Question 20

Frequency encoding gradient applied along x-axis AT THE TIME OF READ OUT

So frequency differs depending on x-axis location

Answer 20

But if you apply frequency encoding gradient, they would lose phase and transverse magnetisation vector / signal.

So you apply an equal and opposite frequency encoding gradient prior to read out.

So more IN PHASE despite DIFFERENT FREQUENCIES during readout.

Question 21

You can now sample at multiple TEs during frequency-encoded readout.

Answer 21

Every read-out is a numerical number for the whole image. Frequencies and their amplitudes can be teased out to give you x.

Question 22

The Fourier transformation converts this time-based data set into a frequency-based data set.

Answer 22

Also the more you sample, the more frequencies you can tease out.