MRI Further Sequences 6.2.24

Question 1

What determines the filling of K-space?

Answer 1

Attitude of the phase encoding gradient determines which line of K space is filled.

Low amplitudes correspond to the central lines of K-space
High amplitudes correspond to the outer lines of K-space.

Question 2

What are the K-space differences between Conventional SE Vs FSE/TSE?

Answer 2

Conventional
- 1 line of K-space acquired each TR EG, 256×256 matrix needs, 256 TRs

Fast/turbo SE
- Multiple lines(n) of K-space acquired each TR
- Reduces total acquisition time by factor (n)
- each line has a different phase encoding gradient

Question 3

What is Echo train length (ETL)?

Answer 3

Number of echoes = number of lines per TR

Question 4

What is Effective echo time (TE eff)?

Answer 4

TE of echo acquired closest to the centre of K-space

Question 5

What is Echo spacing (ES) ?

Answer 5

Time between successive echoes

Question 6

What does Scan time equal?

Answer 6

TR x total number of lines / ETL

Question 7

What are the factors of TR for FSE /TSE in T1 and T2 imaging?

Answer 7

T2 weighting
- TE of last echo longer than conventional SE
- Reduces maximum of slices within TR
- increases TR above 3000ms

T1 weighting
- use short TR only with short ETL =3

Question 8

What are the characteristics of FSE/TSE?

Answer 8

• Bright Fat sat
• Blurring of short T2 structures
  - high spatial frequencies acquired later
  - esp proton density , T1 weighting
• Reduced signal from muscle, brain
  - magnetisation transfer effect

Question 9

What is Bright Fat signal?

What are the parameters for this sequence?

Answer 9

Fat sat signal brighter than conventional SE

Closely- spaced 180’ pulses reduce diffusion effects

T2 weighted abdo
- TR= 1800 ms
- TE = 101 ms
- TEL = 23
- 2.38 min

Question 10

What are the parameters for Short acquisition for T2 weighted brain?

Answer 10

TR = 5000 ms
TE = 120 ms
ETL = 15
= 1.05mins

Question 11

What is a DRIVE/RESTORE sequence?

What does it do to the magnetic field and how odes this affect the image appearances?

Answer 11

A (-) 90’ pulse applied at the end of the echo train

This transfers remaining Mxy magnetisation back along the positive Z axis

Allows faster T1 recovery of longT1/T2 tissues(fluid, CSF)

Allows T2 weighted imaging with shorter TRs

GE = fast recovery, Fast spin echo

Question 12

What is the ideal Magnetic field in homogeneity?
What happens in practice?

Answer 12

Ideal case - B0 perfectly uniform so the signal decays due to T2 relaxation only

In practice - inhomogenieties exist so there is a loss of phase coherence and signal decays more quickly -T2*

Question 13

What are some of the causes of magnetic field inhomogeniety?

Answer 13

+ intrinsic in homogeneities in B0 field
- Magnet design, installation (shimming)
+ magnetic, susceptibility variations
- Different materials introduced to the field i.e the patient
+ linear magnetic field gradients
- deliberately introduced for imaging purposes

Question 14

What happens to the image after decreasing the flip angle?

Answer 14

Increase in T2 weighing

Question 15

What are Transverse Coherences?
What does they cause?

Answer 15

Loss of Coherence = loss of transverse magnetisation

-Flip angle other than 180° creates transverse magnetisation

• If TR&raquo_space; T2 a train of RF pulses will generate only FID, otherwise, transverse, magnetisation, remain, and cause Hahn echoes and stimulated echoes

-If untreated, these can cause artefacts

Question 16

What does SSFP stand for?

Answer 16

Steady state free precession

Question 17

What are the gradient problems associated with Steady state free precession?

Answer 17

Overlapping signals caused interference patterns caused due to the presence of magnetic field inhomogeneities

Different signals, go out of phase and cancel

Question 18

What makes SSFP problems worse?

Answer 18

Poor homogeneity

Long TRs

Question 19

What improves SSFP problems?

Answer 19

Shimming
Short TRs

Question 20

What is the process of creating Steady state free precessions?

Answer 20

1. After 3 pulses overlapping signals
2. 3 signals co-exist in a steady state
3. This leads to free state precession

Question 21

How do you spoil a SSFP signal?

Answer 21

Completely destroy or spoil the steady state signal
+ Only measure the FID
+ Use spoiler gradient pulses
+ Use RF spoiling

Spoiled GE

Question 22

What is involved in No spoiling of SSFP signal (all signals)?
What are the effects of this?

Answer 22

Gradient to refocus all signals for signal sampling ‘ balanced gradients’

• Interference pattern
• Stronger signal
• Flow compensated

Question 23

What is involved in No spoiling of SSFP signal ( FID only) ?

What are the effects of this?

Answer 23

Use gradients to refocus FID only for signal sampling

• removes interference patterns
• weaker signal
• flow sensitive

Question 24

What does a High flip angle give for SSFP?

Answer 24

The best signal for SSFP methods

Question 25

What is FISP vs true FISP?

Answer 25

FISP - flow related signal loss due to pulsatile CSF motion True FISP - interference patterns + higher CSF signal

Question 26

How has SSFP been improved?

Answer 26

Shorter TRs - faster gradient systems Better magnetic field homogeneity - improved magnets - patient specific shimming - mandatory - volume shimming So interference patterns are negligible

Question 27

What are the Characteristics of SSFP? What can/ can’t it be used for?

Answer 27

Signal related to T2/T1 ration - bright fluid, bright fat Beware - Not T2 weighted - cannot use for pathology in the same way - Some tumours can be isointense with normal tissue - Good for vessels and cardiac applications - Localisers.

Question 28

What is CINE? How is CINE acquired?

Answer 28

- CSF study to diagnose cardiac function - CINE is a movie and utilises SSFP - Images are required continuously using ECG gating retrospectively filled into K-space so all of K-space is acquired within a breath hold (6-10s) In reality, all the cardiac sequences are required in a gated mode during diastole of the heartbeat, using ECG gating and when the chest is relatively still using breath gating

Question 29

What are the 3 Different types of diffusion?

Answer 29

Isotopic - random Restricted Bulk flow

Question 30

What is Diffusion in cellular environments?

Answer 30

Molecules cellular environment impeded by cellular membranes Restriction of diffusion of water Molecules is directly proportional to tissue cellularity. Directionally restrictive diffusion observed in: - white matter tracts and nerves (myelin sheath) - muscle fibres (heart)

Question 31

What occurs when Spins are in static tissue?

Answer 31

= no diffusion

Question 32

What does ADC stand for?

Answer 32

Apparent diffusion coefficient

Question 33

What is T2 shine through artefact?

Answer 33

DWI Imaging is T2 weighted B-value images have both diffusion and T2 weighted Tissues with long T2 values may appear bright but do not restrict diffusion

Question 34

What is a Susceptibility artefact?

Answer 34

DWI sequences are highly susceptible to magnetic field in-homogeneities

Question 35

What is a Patient motion artefact ?

Answer 35

Gross patient motion and physiological motion More pronounced in phase encoding direction ADC values will be inaccurate where ghosting has occurred Motion, artefact from breathing can be mitigated by using a breath hold or gated techniques .

Question 36

What are the New DWI sequences ?

Answer 36

Segmented EPI (resolve) - effectively removes susceptibility artefact Not single shot - so takes longer

Question 37

What are some of the Applications of DWI in the CNS ?

Answer 37

Widely used in majority of brain protocols B-values from 0-1000 Applications include : - stroke and tIA - DWI/FLAIR mismatch for accurate identification of patients to benefit from thrombolysis - dementia, Alzheimer’s and MS - Tumours (lower ADC values = higher grade) - Pseudo-response to antiangioegnic therapy - Distinguish between inflammation and tumour - abscess

Question 38

What are some of the o ther applications of DWI

Answer 38

- breast - prostate - female pelvis - renal - hepatic - spine - whole body Diffusion Tensor imaging - white matter tracts between important brain areas identified through fMRI

Question 39

What ADC values do tumour have?

Answer 39

Low ADC values