1. G Practical MRI 1
Basic pulse sequences

2. Gradient Echo (GRE)
A class of pulse sequences that is primarily used for fast scanning
3D volume imaging
Cardiac imaging
Gradient reversal on the frequency-encoded axis forms the echo
A readout prephasing gradient lobe dephases the spins, then they are rephased with a readout gradient with opposite polarity
Can be fast because the flip angle is less than 90°
Why does that allows GRE to be fast?

3. Gradient Echo (GRE)
A class of pulse sequences that is primarily used for fast scanning
3D volume imaging
Cardiac imaging
Gradient reversal on the frequency-encoded axis forms the echo
A readout prephasing gradient lobe dephases the spins, then they are rephased with a readout gradient with opposite polarity
Can be fast because the flip angle is less than 90°
Fast T1 recovery  short TR can be used (e.g ms)

4. Small Flip-Angle RF Pulse
Bernstein et al. (2004) Handbook of MRI Pulse Sequences
What property of the small flip angle RF pulse is evident from this illustration?

5. Example of a GRE Pulse Sequence
The peak of the GRE occurs when the area under the two gradient lobes is equal.
Bernstein et al. (2004) Handbook of MRI Pulse Sequences

6. T2 and T2* Dephasing T2 dephasing: T2* dephasing
Inherent to tissue type
Molecular environment
Magnetic fields constantly changing in time
T2* dephasing
Imperfect static magnetic field
Air pockets (e.g. lungs) in body
Metal parts in body (e.g. stents, clips)
Magnetic fields that are constant in time
All of this PLUS T2 dephasing

7. Transverse Relaxation
Bernstein et al. (2004) Handbook of MRI Pulse Sequences
T2* is always shorter than T2

8. Response to a Series of RF Excitations
The excitation pulse is the only RF pulse in each TR (unless preparation pulses are used)
With a sufficient number of excitation pulses, Mz reaches a steady state
GRE sequences can be classified by the response of the transverse magnetization Mxy
Spoiled: if ~0 just before each excitation
Steady-state free precession (SSFP): if reaches a nonzero steady state

9. Spoiling Spoiling can be accomplished in different ways
The simplest method is to use TR ~ 5T2
Practical only with interleaved multi-slice acquisitions
End-of-sequence gradient spoiler
Not effective at spoiling the transverse steady state
Spatially non uniform because gradients produce spatially varying fields
RF spoiling
Phase-cycle the RF excitation pulses according to a predetermined schedule (i.e. flip the magnetization down in a different direction each time)

10. RF Spoiling
Stripe pattern artifact due to the spatially varying field produced by the gradients. (e.g. when the phase-encoding gradient is used as a spoiler, so no phase rewinding lobe is used)
(Bright stripes are unspoiled regions)
Bernstein et al. (2004) Handbook of MRI Pulse Sequences

11. RF Spoiling
Stripe pattern artifact due to the spatially varying field produced by the gradients. (e.g. when the phase-encoding gradient is used as a spoiler, so no phase rewinding lobe is used)
(Bright stripes are unspoiled regions)
RF spoiling: phase of the B1 field for the jth RF pulse in the rotating frame:
(equivalent to the phase twist imparted by the phase-encoding gradient)
Bernstein et al. (2004) Handbook of MRI Pulse Sequences
The recommended value for the starting phase increment is ϕ0 = 117°
During each TR the received MR signal must be shifted by the same phase, so
that k-space data are consistent

12. Steady State Mz for Spoiled GRE
Bernstein et al. (2004) Handbook of MRI Pulse Sequences
If the longitudinal magnetization at point A is MzA, after the excitation pulse MzB = MzAcosθ
In the TR between points B and C, T1 relaxation occurs, so:
When a steady state is reached MzA = MzC

13. Nobel Prize in Chemistry
Ernst Angle
Richard Ernst
August 14, 1933
1991
Nobel Prize in Chemistry
The signal Sspoil is caused by the gradient rephasing the FID at an echo time TE, so it is given by:
Which is equal to:
“Ernst angle”
The flip angle that maximize the signal is:

14. SSFP-FID (FISP) And SSFP-Echo
Standard GRE with greater signal than spoiled pulse sequences
Often at the cost of less contrast
SSFP-Echo less used
Conditions for SSFP:
phase coherent (RF pulses have the
same phase, or sign alternation, in
the rotating frame)
TR < T2
Accumulated phase is the same in
each TR ( same gradient area)
If met, than steady states for both Mz and Mxy will be established
(A FID-like signal just after the RF and a
time-reversed just before each pulse)
Bernstein et al. (2004) Handbook of MRI Pulse Sequences

15. SSFP-FID (FISP) And SSFP-Echo
Phase-coherent RF pulses with same flip-angle and constant TR < T2  steady state
Post-excitation signal (S+), FID arising from most recent RF pulse
Echo reformation signal (S-) when residual echo is refocused at the time of the subsequent RF pulse
Chavhan GB et al. (2008) Radiographics vol. 28(4)

16. SSFP-FID And SSFP-Echo Signals
If TR >> T2 

17. SSFP-FID And SSFP-Echo Signals
If TR >> T2 
If θ << 1 
(PD-weighting at low flip angles)

18. Balanced SSFP (True FISP)
For SSFP the gradient area on any axis must not vary among TR intervals
For Balanced SSFP the gradient area on any axis is zero during each TR
Peaks of SSFP-FID and SSFP-Echo combine at TE (coherent sum of two signals
The magnitude of the signal changes for sign alternated pulses
Used in practice because of greater signal
If the balanced SSFP signal is rephased in the center of the TR interval (i.e. TE = TR/2), the decay is governed by T2 rather than T2*
decreasing TE can increase susceptibility weighting in balanced SSFP
(the contrary happens for spoiled GRE and SSFP-FID)

19. Scheffler K and Lehnhardt S (2003) Eur Radiol vol. 13
Balanced SSFP
Scheffler K and Lehnhardt S (2003) Eur Radiol vol. 13

20. Artifacts of Balanced SSFP
In regions where a phase shift removes the sign alternation there is a signal loss
Banding artifact
Unwanted phase shifts are always present
Short TR (e.g. less than 7 ms) are needed
Question: are balanced SSFP easier or more difficult to implement at higher field strength?

21. Banding Artifacts in Balanced SSFP
Scheffler K and Lehnhardt S (2003) Eur Radiol vol. 13

22. Examples of Banding Artifacts

23. Artifacts of Balanced SSFP
In regions where a phase shift removes the sign alternation there is a signal loss
Banding artifact
Question: for example what could cause a phase shift?
Unwanted phase shifts are always present
Short TR (e.g. less than 7 ms) are needed
Question: are balanced SSFP easier or more difficult to implement at higher field strength?

24. Artifacts of Balanced SSFP
In regions where a phase shift removes the sign alternation there is a signal loss
Banding artifact
Unwanted phase shifts are always present
Short TR (e.g. less than 7 ms) are needed
Question: are balanced SSFP easier or more difficult to implement at higher field strength?

25. Artifacts of Balanced SSFP
In regions where a phase shift removes the sign alternation there is a signal loss
Banding artifact
Unwanted phase shifts are always present
Short TR (e.g. less than 7 ms) are needed
More difficult to implement at high field
Increased susceptibility variations
SAR associated with very short TR

26. Particular Cases of Balanced SSFP
For short TR (TR << T2 < T1) the signal formula becomes:
Question: what does the formula tells you about the signal from fluids in balanced SSFP images?

27. Particular Cases of Balanced SSFP
For short TR (TR << T2 < T1) the signal formula becomes:
The signal is maximized for:
At flip angles ~ 90° becomes more highly T2 / T1 weighted:
Max of nearly M0/2 when T2 = T1
 extremely strong signal for a short TR pulse sequence

28. Example SSFP-FID and Spoiled GRE: TR = 14 ms TE = 6 ms Balanced SSFP:

29. Inversion Recovery (IR)
Pulse sequences with an inversion pulse followed by a time delay prior to an RF excitation
Produce images with T1-weighted contrast. Why?

30. Inversion Recovery (IR)
Pulse sequences with an inversion pulse followed by a time delay prior to an RF excitation
Produce images with T1-weighted contrast.
Time delay is know as the inversion time (TI)
Consists of two parts:
Inversion pulse, spoiler gradient (optional), slice selection gradient (if selective inversion pulse)
A self-contained pulse sequence (e.g. GRE) after TI
Require long TR (2-11 s) to preserve the contrast
2D IR sequences more frequently used
Benefits from real rather than magnitude reconstruction
Why?

31. Inversion Recovery (IR)
Pulse sequences with an inversion pulse followed by a time delay prior to an RF excitation
Produce images with T1-weighted contrast. Why?
Time delay is know as the inversion time (TI)
Consists of two parts:
Inversion pulse, spoiler gradient (optional), slice selection gradient (if selective inversion pulse)
A self-contained pulse sequence (e.g. GRE) after TI
Require long TR (2-11 s) to preserve the contrast
2D IR sequences more frequently used
Benefits from real rather than magnitude reconstruction
Mz ranges from –M0 and +M0  increased tissue contrast

32. Diagram of IR Pulse Sequence
Bernstein et al. (2004) Handbook of MRI Pulse Sequences
Besides T1-weighted images, what is another application of IR pulse sequences that we mentioned during a previous lecture?

33. Principles of IR Immediately after the inversion pulse:
During the time interval TI
(for long TR)
If θinv = 180°:
If θinv = 90°:
Saturation Recovery
(SR)

34. IR and SR Curves SR IR
nulling time
Bernstein et al. (2004) Handbook of MRI Pulse Sequences
The TI value that nulls the longitudinal magnetization is called the “nulling time” or “zero-crossing point”

35. Examples of IR Applications
T1 mapping
A series of IR images are acquired from the same location with different TI (everything else the same)
Long TR used to avoid signal saturation
Non-linear fitting (for magnitude IR, first need to obtain the zero-crossing and negate signals before it)
Lipid suppression (STIR)
Improves contrast for lesions embedded in fat (e.g. edema in bone marrow), as lipids appear bright like many lesions in post-contrast
Water signal loss (any tissue with T1 similar to fat)
Long acquisition time

36. Radiofrequency Spin Echo (SE)
Formed by an excitation pulse and one or more refocusing pulse
Usually a 90° pulse followed by 180° pulse
Typically 2D mode using interleaved multislice
Allows to obtain a specific contrast weighting
Greater immunity to off-resonance artifacts
Why?

37. Radiofrequency Spin Echo (SE)
Formed by an excitation pulse and one (or more in multi-echo SE) refocusing pulse
Usually a 90° pulse followed by 180° pulse
Typically 2D mode using interleaved multislice
Allows to obtain a specific contrast weighting
Greater immunity to off-resonance artifacts because of the 180° refocusing pulse
As T2 > T2*  heavily T2-weighted images possible with long TE without much signal loss (dephasing)
Only a single phase-encoding step in any TR interval

38. Single-Echo SE
Bernstein et al. (2004) Handbook of MRI Pulse Sequences

39. Determination of TE
The gradient area on the frequency-encoding axis determines the temporal location of the peak of the echo (when the area under readout gradient balances the area of the prephasing gradient lobe)
Sometimes Δ is nonzero due to systems imperfections (e.g. eddy currents that shift gradient lobes)
What is the effect?
Bernstein et al. (2004) Handbook of MRI Pulse Sequences

40. Determination of TE
The gradient area on the frequency-encoding axis determines the temporal location of the peak of the echo (when the area under readout gradient balances the area of the prephasing gradient lobe)
Sometimes Δ is nonzero due to systems imperfections (e.g. eddy currents that shift gradient lobes)
 The signal will have some T2* weighting
Note: some specialized sequences use
nonzero Δ intentionally
Bernstein et al. (2004) Handbook of MRI Pulse Sequences

41. Partial-Echo SE What differences do you notice?
Bernstein et al. (2004) Handbook of MRI Pulse Sequences
What differences do you notice?

42. Partial-Echo SE
Bernstein et al. (2004) Handbook of MRI Pulse Sequences
The peak of the echo (not the center of the readout) occurs when the RF spin would have refocused in the absence of imaging gradients
Used to avoid T2* weighting of the signal and reduce minimum TE
Achieved by reducing the area of the prephasing lobe
Image reconstruction with partial Fourier methods

43. Signal Formula for SE = 90° = 180° short pulse (no T1 relaxation
Bernstein et al. (2004) Handbook of MRI Pulse Sequences
= 90°
= 180°
Mxy negligible
(TR >> T2, or
spoiler gradient)
MzA
short pulse (no T1 relaxation
between A and B, or C and D)

44. Multi-Echo SE
The transverse magnetization can be repeatedly refocused into subsequent SEs by playing additional RF refocusing pulse
The series of echoes is called an echo train
Each echo number fits its own independent k-space
The length of the echo train is limited by T2 decay
In most cases we are interested in 2 echoes (an early and a late one). Question: if TR is long, what contrast will have the 2 resulting images?

45. Multi-Echo SE
The transverse magnetization can be repeatedly refocused into subsequent SEs by playing additional RF refocusing pulse
The series of echoes is called an echo train
Each echo number fits its own independent k-space
The length of the echo train is limited by T2 decay
In most cases we are interested in 2 echoes (an early and a late one).  if TR is long, the two images will be PD- and T2-weighted, respectively

46. Example of Dual-Echo SE Acquisition
Proton density-weighted
TE/TR = 17/2200 ms
T2-weighted
TE/TR = 80/2200 ms

47. Dual-Echo SE
Bernstein et al. (2004) Handbook of MRI Pulse Sequences

48. T2-Mapping
It is a common application of acquiring longer echo trains (otherwise more than two echoes per TR are rarely acquired in MRI)
In theory we can acquire long echo train of SEs and fit the signal intensity at each pixel to calculate T2
In practice there are systematic errors that make it difficult to fit a monoexponential decay curve
Variable flip angle across slice profile
Stimulated echoes can introduce unwanted T1-weighting variations into the echo-train signals
If magnitude reconstruction is used, the noise floor has nonzero mean leading to incorrectly larger T2 values

49. Any questions?

50. See you on Thursday!