SHAPE Society

1. Physics of MRI
• Course syllabus
– Lecture 1: Intro to NMR Dr. Lowe
– Lecture 2: Imaging Sequences I Dr. Buckwalter
– Lecture 3: Imaging Sequences II Dr. Buckwalter
– Lecture 4: Spatial encoding I Dr. Buckwalter
– Lecture 5: Spatial encoding II Dr. Buckwalter
– Lecture 6: Spin prepped imaging Dr. Lowe
– Lecture 7: Ultrafast imagingDr. Lowe
http://www.indyrad.iupui.edu/public/lectures/mri/iu_lectures/mri_homepage.htm

2. Loose Ends

3. Energy Absorption
β0
M0=x M0=z
900
tip
900
RF pulse
ω=ω0

4. Relaxation
β0
t=t0
RF
t=t1
ML=0
t=t2
ML=a
t=t3
ML=b
t=∞
ML=1
….
t
ML
t0 t1 t2 t3

5. Relaxation and Imaging
• FID (free induction decay) is
the relaxation behavior
following a single RF pulse
• most imaging done with
repetitive RF energy deposition
• the interval between the RF
energy pulses is called the TR
interval (time to repetition)

6. Relaxation
β0
t=t0
900
RF
t=t3
ML=b
t=t4
ML<b
900
RF
t=t3+
ML=0
900
RF
t=t4+
ML=0
t=t5
ML<<b
TR TR

7. Equilibrium
• after 5 or so
repetitions, the
system reaches
equilibrium
• similar to water
flowing into a
leaky bucket
relaxation
RF in
equilibrium

8. Differential Relaxation
• short TR
• lower absolute ML
• marked difference
in relative signal
• long TR
• higher absolute ML
• minimal difference
in relative signal
fat protons
water protons

9. T1 Relaxation
0
0.2
0.4
0.6
0.8
1
1.2
0 1000 2000 3000 4000 5000 6000
msec
ML
long T1
short T1

10. Image Contrast and T1
Relaxation
• shorter TRs maximize
differences in T1 relaxation,
generating tissue contrast
• longer TRs minimize
differences in T1 relaxation,
reducing T1 tissue contrast

11. Imaging Sequences
part I
• Gradient Echo
• Spin Echo
• Fast Spin Echo
• Inversion Recovery

12. Goals of Imaging
Sequences
• generate an RF signal
perpendicular to β0
• generate tissue contrast
• minimize artifacts

13. Measuring the MR Signal
z
y x
RF signal from
precessing protons
RF antenna
β0

14. Gradient Echo
• simplest sequence
–alpha flip-gradient recalled echo
• 3 parameters
–TR
–TE
–flip angle
• reduced SAR
• artifact prone

15. Gradient Echo
FID gradient recalled
echo
α
RF pulse
rephase
dephase
signal
gradient

16. z
y x
z
y x
α0
RF
t=t0 t=t0+
Partial Flip
α0
ML
MXY
M
MXY = M sin(α)
ML = M cos(α)

17. Dephasing in the xy-plane
view from the top
y
x
z Mxy
y
x
z
Mxy≈0
dephase
phase coherency phase dispersion

18. y
x
z Mxy
phase coherency
minus t2* decay
Rephasing in the xy-plane
view from the top
rephase
y
x
z
Mxy≈0
phase dispersion

19. MR Signal During
Rephasingz
y x
RF signal
“echo”
RF antennaβ0

20. T2* decay
• occurs between the dephasing and
the rephasing gradients
• rephasing incompletely recovers
the signal
• signal loss is greater with longer
TEs
• decay generates image contrast

21. T2* decay
• T2* decay is always faster than
T2 decay
• gradient echo imaging cannot
recover signal losses from
–magnetic field inhomogeneity
–magnetic susceptibility
–water-fat incoherence

22. T2 and T2* Relaxation
• T2* relaxation influences
contrast in gradient echo
imaging
• T2 relaxation influences
contrast in spin echo imaging

23. Gradient Echo
pulse timing
echo
RF
signal
readout
α0
phase
slice
TE

24. Gradient Echo
advantages
• faster imaging
–can use shorter TR and shorter
TEs than SE
• low flip angle deposits less
energy
–more slices per TR than SE
–decreases SAR
• compatible with 3D acquisitions

25. Gradient Echo
disadvantages
• difficult to generate good T2
weighting
• magnetic field inhomogeneities
cause signal loss
–worse with increasing TE times
–susceptibility effects
–dephasing of water and fat
protons

26. Gradient Echo
changing TE
TE 9
FA 30
TE 30
FA 30
susceptibility effect T2* weighting

27. Gradient Echo
magnetic susceptibility
post-surgical change
“blooming” artifact

28. Gradient Echo
• image contrast depends on
sequence
• conventional GR scan
–aka GRASS, FAST
–decreased FA causes less T1
weighting
–increased TE causes more T2*
weighting

29. Conventional GR
TE 20, FA 15

30. Gradient Echo
• Spoiled GR
–aka SPGR, RF-FAST
–spoiling destroys accumulated
transverse coherence
–maximizes T1 contrast

31. Gradient Echo
• Contrast enhanced GR
–aka SSFP, CE-FAST
–infrequently used because of
poor S/N
–generates heavily T2* weighted
images

32. Gradient Echo
• other varieties
–MTC
• T2 - like weighting
–IR prepped
• 180 preparatory pulse
–DE (driven equilibrium) prepped
• 90-180-90 preparatory pulses
• T2 contrast

33. MTC GR
TE 13, FA 50

34. Spin Echo
• widely used sequence
–90-180-echo
• 2 parameters
–TR
–TE
• generates T1, PD, and T2
weighted images
• minimizes artifacts

35. Spin Echo
FID spin
echo
900
RF pulse
readoutfrequency encode
signal
gradient
1800
RF pulse

36. Gradient versus Spin Echo
Spin Echo
FID spin
echo
900RF pulse
readoutfrequency encode
signal
gradient
1800RF pulse
Gradient Echo
FID gradient recalled
echo
α
RF pulse
rephase
dephase
signal
gradient

37. 900
Flip
z
y x
z
y x
900
RF
t=t0 t=t0+
900
After
ML=0
MXY=M
Before
ML=M
MXY=0

38. Dephasing in the xy-plane
view from the top
y
x
z Mxy
y
x
z
Mxy≈0
phase coherency phase dispersion
Dephasing begins
immediately after
the 900
RF pulse.
t=0 t=TE/2
900
RF

39. y
x
z Mxy
phase coherency
minus t2 decay
Rephasing in the xy-plane
view from the top
y
x
z
Mxy≈0
phase dispersion
t=TE/2 t=TE
1800
RF

40. z
y x
z
y x
z
y x
z
y x
t=TE/2 t=TE
1800
RF
t=0
900
RF
dephased
rephased
1800
Flip

41. Spin Echo
pulse timing
echo
RF
signal
readout
900
phase
slice
TE
1800

42. WNMR Race
t=0
900
RF

43. WNMR Race

44. WNMR Race
t=TE/2
1800
RF

45. t=TE
WNMR Race

46. Effects of the 1800
Pulse
• eliminates signal loss due to
field inhomogeneities
• eliminates signal loss due to
susceptibility effects
• eliminates signal loss due to
water/fat dephasing
• all signal decay is caused by
T2 relaxation only

47. Spin Echo
advantages
• high signal to noise
• least artifact prone sequence
• contrast mechanisms easier to
understand

48. Spin Echo
disadvantages
• high SAR than gradient echo
because of 900
and 1800
RF
pulses
• long TR times are incompatible
with 3D acquisitions

49. Spin Echo Contrast
• T1 weighted
–short TR (450-850)
–short TE (10-30)
• T2 weighted
–long TR (2000 +)
–long TE (> 60)
• PD weighted
–long TR, short TE

50. Spin Echo Contrast
T2 Relaxation
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 100 200 300 400 500
msec
Mxy
long T2
short T2
T1 Relaxation
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 1000 2000 3000 4000 5000
msec
ML
long T1
short T1
T1 weighted - T1 relaxation predominates
•Short TE minimizes differences in T2 relaxation
•Short TR maximizes differences in T1 relaxation
T2 weighted - T2 relaxation predominates
•Long TE maximizes differences in T2 relaxation
•Long TR minimizes differences in T1 relaxation

51. T1 weighted T2 weighted
Spin Echo Contrast

52. Spin Echo Contrast
PD weighted T2 weighted