Image formation 2: Basic Pulse Sequence

Question 1

What is it difficult to reconstruct?

Answer 1

Artefact free images from projections

Question 2

What is technically difficult to measure?

Answer 2

MR signal immediately after a 90 degree pulse

- can be overcome by collecting a spine echo (SE)

Question 3

What is pulse sequence?

Answer 3

Succession of commands that the computer gives the scanner to obtain desired signal, hence image

Question 4

What is spin echo?

Answer 4

Regeneration of spin phase information apparently lost during decay of FID
– Insensitive to magnetic field B0 inhomogeneity, but longer scan times

Question 5

What is Gradient echo?

Answer 5

1. Applying external dephasing gradient field across the specimen or tissue
   - calibrated change in local magnetic field and hence alter resonance frequencies slightly across the specimen
   - Accelerated dephasing of FID

Question 6

What is the first step in the formation of a spin echo?

Answer 6

90˚ pulse is applied along +x’ which rotates the longitudinal magnetization MS into the transverse plane along y’ axis

Question 7

What is the second step in the formation of a spin echo?

Answer 7

180˚ RF pulse is applied again along the postive x’ axis

-This reverses the direction of the magnetisation so now it is along the -y’ axis

Question 8

What is the third step in the formation of a spin echo?

Answer 8

The spins continue to move in the same direction as they were travelling before the 180˚ pulse

Question 9

What is the fourth step in the formation of a spin echo?

Answer 9

There is a time point (the ‘spin echo’) where the isochromats rephase and the magnetisation of the entire spin ensemble reaches a maximum again

Question 10

What is the fifth step in the formation of a spin echo?

Answer 10

The centre of the spin echo (where the signal will reach a maximum) occurs at a time referred to as the “echo time” or TE of the sequence – The sampling or acquisition window will be centred around this time point • Finally the isochromats begin to dephase again

Question 11

What happens after the spin . echo has formed?

Answer 11

The magnetisation will continue to precess and therefore dephase

Question 12

What is the basis of spin echo Imaging?

Answer 12

Combining the effect of a field gradient (to localise the signal) with the concept of refocusing transverse magnetisation

Question 13

What is Spin Echo?

Answer 13

Add another pulse to generate an ‘‘echo’’ that occurs later in time and happens to reverse the T2* effects to leave us with T2 only

Question 14

Why do we rotate the protons with a 180-degree pulse?

Answer 14

They continue precessing in the same direction but will refocus along the -y axis

Question 15

What is the spin dephasing effect due to?

Answer 15

Dispersion in the resonance frequency produced

Question 16

What is the rephase lobe on the gradient needed?

Answer 16

Counteract the dephasing of spins during the application of pulse

Question 17

What is the k-space sampling?

Answer 17

1. Apply a 90 degree RF pulse (produces an FID)
2. Apply large, negative phase encode gradient along y direction
3. Apply a positive dephase gradient along the x axis
4. At a time TE/2 after the 90 degree pulse w eapply a 180 degree pulse
5. Apply a readout gradient and start measuring the echo signal

Question 18

What will produce different degrees of contrast between tissues?

Answer 18

Different combinations of TR and TE

Question 19

What will changing TR change the contrast between?

Answer 19

Tissues with different T1 relaxation times

Question 20

What will changing TE change the contrast between?

Answer 20

Tissues with different T2 values

Question 21

What does T1 weighting have?

Answer 21

1. Short TE
2. Short TR
   - PD weighting

Question 22

What does T2 have?

Answer 22

1. Long TE
2. long TR
   - PD weighting aswell

Question 23

What generally produces no signal?

Answer 23

1. Long TE

2. Short TR

Question 24

What does fluids have?

Answer 24

1. Long T1

2. Long T2

Question 25

What does water-vased tissues have?

Answer 25

1. Mid-range T1 | 2. Short T2

Question 26

What does Fat-based tissue have?

Answer 26

1. Short T1 | 2. Short T2

Question 27

What happens in PD-weighted images?

Answer 27

Tissues with highest PD are brighter

Question 28

What happens in T2-weighted images?

Answer 28

Tissues with longest T2 are brighter | -Takes longer for transverse magnetisation to decay

Question 29

What happens in T1-weighted imageS?

Answer 29

Tissues with longest T1 are darker | -Takes longer for longitudinal magnetisation to recover

Question 30

What are the spin echo tissue signal intensity?

Answer 30

``` • Short TE - Grey matter is white - White matter is darker - CSF bright - The contrast is from PD • Longer TE - GM still white - WM is still darker - Contrast greater than short TE - CSF is very bright - The contrast is from PD and T2 ``` • A very long TE - WM and GM are black - Only CSF remains

Question 31

What is Dual echo sequence?

Answer 31

Include images with different weightings and/or echo times and are used to obtain both proton density and T2 weighted images without increasing the measurement time

Question 32

What do different compartment in tissues have?

Answer 32

Different relaxation characteristics

Question 33

What can be detected by fitting the transverse magnetisation decay curve (TMDC)?

Answer 33

Multi-exponential relaxation

Question 34

How is a gradient echo formed?

Answer 34

First dephasing and subsequently rephasing the FID so that all isochromat magnetisation vectors are back in phase with one another at the centre of gradient echo

Question 35

Where does the gradient echo occur at?

Answer 35

the point in time where the net phase is zero, i.e. the amount of dephasing is equal to the amount of rephasing – Signal reaches a maximum at the gradient echo

Question 36

What is gradient echo signal?

Answer 36

T2* dependent | There is no 180 degree pulse to refocus the dephasing of isochromat due to B0 field inhomogeneities

Question 37

What is the first step in the formation of a gradient echo?

Answer 37

90° pulse is applied and isochromats are tipped into the transverse plane

Question 38

What is the second step in the formation of a gradient echo?

Answer 38

Dephasing gradient is applied and the isochromat dephase as a result of this grafient

Question 39

What is the third step in the formation of a gradient echo?

Answer 39

Equal and opposite gradient is applied (a rephasing gradient) -isochromat phases are reversed and a gradient echo is formed at the point in time where the amounts of rephasing and dephasing by gradients are exactly equal

Question 40

What does T2* describe?

Answer 40

Total transverse relaxation due to both spin-spin interaction and B0 field inhomogeneities

Question 41

What does magnet homogeneity directly relate to?

Answer 41

Image quality and various artefacts

Question 42

What are the types of problem associated with poor magnet homogeneity?

Answer 42

- Shading - Spatial distortion - Blurring - Intensity loss - Curved slice profiles - Zebra banding artefacts

Question 43

What is inversion recovery pulse sequence?

Answer 43

spin echo pulse sequence preceded by a 180° RF pulse

Question 44

What does the Inversion recovery pulse sequence do?

Answer 44

inverts longitudinal magnetisation (Mz), namely, it flips Mz to its negative value, -Mz.Tissues regain Mz at different longitudinal (T1) relaxation rates determined by their T1 relaxation times

Question 45

What are the Inversion recovery contrast?

Answer 45

1. Short T1 - GM is white - WM is darker - contrast: T1 and PD 2. Longer T1 - WM is white - GM is darker 3. Longer still T1 - WM and GM are iso-intense 4. Very long T1 - GM is white - WM darker - contrast: mainly due to PD