MRI Further Sequences 6.2.24 Flashcards
What determines the filling of K-space?
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.
What are the K-space differences between Conventional SE Vs FSE/TSE?
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
What is Echo train length (ETL)?
Number of echoes = number of lines per TR
What is Effective echo time (TE eff)?
TE of echo acquired closest to the centre of K-space
What is Echo spacing (ES) ?
Time between successive echoes
What does Scan time equal?
TR x total number of lines / ETL
What are the factors of TR for FSE /TSE in T1 and T2 imaging?
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
What are the characteristics of FSE/TSE?
- 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
What is Bright Fat signal?
What are the parameters for this sequence?
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
What are the parameters for Short acquisition for T2 weighted brain?
TR = 5000 ms
TE = 120 ms
ETL = 15
= 1.05mins
What is a DRIVE/RESTORE sequence?
What does it do to the magnetic field and how odes this affect the image appearances?
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
What is the ideal Magnetic field in homogeneity?
What happens in practice?
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*
What are some of the causes of magnetic field inhomogeniety?
+ 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
What happens to the image after decreasing the flip angle?
Increase in T2 weighing
What are Transverse Coherences?
What does they cause?
Loss of Coherence = loss of transverse magnetisation
-Flip angle other than 180° creates transverse magnetisation
- If TR»_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
What does SSFP stand for?
Steady state free precession
What are the gradient problems associated with Steady state free precession?
Overlapping signals caused interference patterns caused due to the presence of magnetic field inhomogeneities
Different signals, go out of phase and cancel
What makes SSFP problems worse?
Poor homogeneity
Long TRs
What improves SSFP problems?
Shimming
Short TRs
What is the process of creating Steady state free precessions?
- After 3 pulses overlapping signals
- 3 signals co-exist in a steady state
- This leads to free state precession
How do you spoil a SSFP signal?
Completely destroy or spoil the steady state signal
+ Only measure the FID
+ Use spoiler gradient pulses
+ Use RF spoiling
Spoiled GE
What is involved in No spoiling of SSFP signal (all signals)?
What are the effects of this?
Gradient to refocus all signals for signal sampling ‘ balanced gradients’
- Interference pattern
- Stronger signal
- Flow compensated
What is involved in No spoiling of SSFP signal ( FID only) ?
What are the effects of this?
Use gradients to refocus FID only for signal sampling
- removes interference patterns
- weaker signal
- flow sensitive
What does a High flip angle give for SSFP?
The best signal for SSFP methods
What is FISP vs true FISP?
FISP - flow related signal loss due to pulsatile CSF motion
True FISP - interference patterns + higher CSF signal
How has SSFP been improved?
Shorter TRs - faster gradient systems
Better magnetic field homogeneity
- improved magnets
- patient specific shimming - mandatory
- volume shimming
So interference patterns are negligible
What are the Characteristics of SSFP?
What can/ can’t it be used for?
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.
What is CINE?
How is CINE acquired?
- 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
What are the 3 Different types of diffusion?
Isotopic - random
Restricted
Bulk flow
What is Diffusion in cellular environments?
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)
What occurs when Spins are in static tissue?
= no diffusion
What does ADC stand for?
Apparent diffusion coefficient
What is T2 shine through artefact?
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
What is a Susceptibility artefact?
DWI sequences are highly susceptible to magnetic field in-homogeneities
What is a Patient motion artefact ?
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 .
What are the New DWI sequences ?
Segmented EPI (resolve) - effectively removes susceptibility artefact
Not single shot - so takes longer
What are some of the Applications of DWI in the CNS ?
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
What are some of the o ther applications of DWI
- breast
- prostate
- female pelvis
- renal
- hepatic
- spine
- whole body
Diffusion Tensor imaging - white matter tracts between important brain areas identified through fMRI
What ADC values do tumour have?
Low ADC values
