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RAD512 > MRI Further Sequences 6.2.24 > Flashcards

MRI Further Sequences 6.2.24 Flashcards

1
Q

What determines the filling of K-space?

A

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.

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2
Q

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

A

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

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3
Q

What is Echo train length (ETL)?

A

Number of echoes = number of lines per TR

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4
Q

What is Effective echo time (TE eff)?

A

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

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5
Q

What is Echo spacing (ES) ?

A

Time between successive echoes

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6
Q

What does Scan time equal?

A

TR x total number of lines / ETL

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7
Q

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

A

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

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8
Q

What are the characteristics of FSE/TSE?

A
  • 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
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9
Q

What is Bright Fat signal?

What are the parameters for this sequence?

A

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

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10
Q

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

A

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

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11
Q

What is a DRIVE/RESTORE sequence?

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

A

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

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12
Q

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

A

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*

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13
Q

What are some of the causes of magnetic field inhomogeniety?

A

+ 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

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14
Q

What happens to the image after decreasing the flip angle?

A

Increase in T2 weighing

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15
Q

What are Transverse Coherences?
What does they cause?

A

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

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16
Q

What does SSFP stand for?

A

Steady state free precession

17
Q

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

A

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

Different signals, go out of phase and cancel

18
Q

What makes SSFP problems worse?

A

Poor homogeneity

Long TRs

19
Q

What improves SSFP problems?

A

Shimming
Short TRs

20
Q

What is the process of creating Steady state free precessions?

A
  1. After 3 pulses overlapping signals
  2. 3 signals co-exist in a steady state
  3. This leads to free state precession
21
Q

How do you spoil a SSFP signal?

A

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

Spoiled GE

22
Q

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

A

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

  • Interference pattern
  • Stronger signal
  • Flow compensated
23
Q

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

What are the effects of this?

A

Use gradients to refocus FID only for signal sampling

  • removes interference patterns
  • weaker signal
  • flow sensitive
24
Q

What does a High flip angle give for SSFP?

A

The best signal for SSFP methods

25
Q

What is FISP vs true FISP?

A

FISP - flow related signal loss due to pulsatile CSF motion

True FISP - interference patterns + higher CSF signal

26
Q

How has SSFP been improved?

A

Shorter TRs - faster gradient systems

Better magnetic field homogeneity
- improved magnets
- patient specific shimming - mandatory
- volume shimming

So interference patterns are negligible

27
Q

What are the Characteristics of SSFP?

What can/ can’t it be used for?

A

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.

28
Q

What is CINE?

How is CINE acquired?

A
  • 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
29
Q

What are the 3 Different types of diffusion?

A

Isotopic - random
Restricted
Bulk flow

30
Q

What is Diffusion in cellular environments?

A

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)

31
Q

What occurs when Spins are in static tissue?

A

= no diffusion

32
Q

What does ADC stand for?

A

Apparent diffusion coefficient

33
Q

What is T2 shine through artefact?

A

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

34
Q

What is a Susceptibility artefact?

A

DWI sequences are highly susceptible to magnetic field in-homogeneities

35
Q

What is a Patient motion artefact ?

A

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 .

36
Q

What are the New DWI sequences ?

A

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

37
Q

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

A

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

38
Q

What are some of the o ther applications of DWI

A
  • breast
  • prostate
  • female pelvis
  • renal
  • hepatic
  • spine
  • whole body

Diffusion Tensor imaging - white matter tracts between important brain areas identified through fMRI

39
Q

What ADC values do tumour have?

A

Low ADC values

RAD512 (26 decks)

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