Medical of Physics in chart of the r.ppt

physics

1. Part III Physics: Medical
Physics Magnetic Resonance
Imaging
1999
Part III Physics:
Medical Physics Option
Magnetic Resonance
Imaging
Dr T A Carpenter
http://www.wbic.cam.ac.uk/~tac12

2. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Lecture Content
Lecture I
–Overview of Nuclear Magnetic
Resonance
–Excitation and Signal detection
–One pulse and Two pulse experiments
–Hardware

3. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Lecture Content
Lecture II
–How does NMR become MRI
–Effects of Magnetic Field Gradients
–Imaging pulse sequences
–contrast
–examples

4. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Lecture Content
Lecture III
–functional MRI
–Diffusion MRI
–interventional MRI
–examples

5. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Useful Web Sites
Rochester Institute:
http://www.cis.rit.edu/htbooks/mri/mri-
main.htm
UCLA Brain Mapping Centre:
http://brainmapping.loni.ucla.edu/
BMD_HTML/SharedCode/Shared.htm

6. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging

7. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging

8. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging

9. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging

10. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging

11. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
NMR History
1921: Compton: electron spin
1924: Pauli: Proposes nuclear spin
1946: Stanford/Harvard group
detect first NMR signal
mid -50 to mid 70’s NMR become
powerful tool for structural analysis
mid-70 first superconducting
magnets

12. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
NMR History
1976: Lauterbur: First NMR image of
sample tubes in a chemical spectrometer
1981: First commercial scanners <0.2T
1985: 1.5T scanner
1986: Rapid developments in SNR,
resolution etc
1998: Whole body 8T at OSU

13. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Nuclear Zeeman Effect
Application of strong magnetic field B
0
lifts degeneracy
of nuclear spin levels
For spin 1/2:
E =  h B
0
 Gyromagnetic ratio (constant of nucleus)
For hydrogen  = 42.5 Mhz/T
E

14. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Population Difference
Given by Boltzman Statistics:
n

exp(
-hBo
/
kT )
n
population difference is small <1 in 10
6
NMR is very insensitive

15. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Semi-Classical Model
Gyroscopic motion of magnetic moment about B
0
B
0
Use classical
mechanics(Larmor)
w
0 = -  B
0


16. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Ensemble Average
M

17. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Rotating Frame
Consider precessing moment in a frame of
reference rotating at the larmor frequency
around B
0
x
y
 = B
o
X’ Y’

18. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Rotating Frame
Classical treatment of M
Effect of RF in
laboratory
Frame:
Y
X
Equivalent to
sinusoidal Brf

19. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Rotating Frame
Classical treatment of M
Effect of RF in
rotating Frame:
Y
X
X’ Y’
B
0
B
rf

20. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Rotating Frame
Classical treatment of M
Effect of RF in
rotating Frame:
Y
X
X’ Y’
B
0
B
rf

21. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Rotating Frame
Classical treatment of M
Effect of RF in
rotating Frame:
Y
X
X’ Y’
B
0
B
rf

22. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Rotating Frame
Classical treatment of M
Effect of RF in
rotating Frame:
Y
X
X’ Y’
B
0
B
rf

23. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Rotating Frame
Classical treatment of M
Effect of RF in
rotating Frame:
Y
X
X’ Y’
B
0
B
rf

24. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Rotating Frame
Classical treatment of M
Effect of RF in
rotating Frame:
Y
X
X’ Y’
B
0
B
rf

25. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Rotating Frame
Classical treatment of M
Effect of RF in
rotating Frame:
Y
X
X’ Y’
B
0
B
rf

26. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Signal Detection
rotating Frame:
Y’X’
B
0
YX

27. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Fourier Transformation
FT
Sampling frequency = 2 expected frequency spread
(Nyquist)

28. Basic Spin Echo Imaging 28Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Effect of RF pulses:
B
1 (rf)
y’ y’
x’ x’
z zB
0
90
o

degree
pulse

29. Basic Spin Echo Imaging 29Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Effect of RF pulses:
B
1 (rf)
y’ y’
x’ x’
z zB
0
90
o

degre
e
pulse

30. Basic Spin Echo Imaging 30Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Effect of RF pulses:
B
1 (rf)
y’ y’
x’ x’
z zB
0
180
o
pulse
(invertin
g pulse)

31. Basic Spin Echo Imaging 31Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Effect of RF pulses:
B
1 (rf)
y’ y’
x’ x’
z zB
0
180
o
pulse
(invertin
g pulse)

32. Basic Spin Echo Imaging 32Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Effect of 180
o
RF pulses:
B
1 (rf)
y’ y’
x’ x’
z zB
0
180
o

degre
e
pulse

33. Basic Spin Echo Imaging 33Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Effect of 180
o
RF pulses:
B
1 (rf)
y’ y’
x’ x’
z zB
0
180
o

degre
e
pulse

34. Basic Spin Echo Imaging 34Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Effect of 180
o
RF pulses:
B
1 (rf)
y’ y’
x’ x’
z zB
0
180
o

degre
e
pulse

35. Basic Spin Echo Imaging 35Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Effect of 180
o
RF pulses:
B
1 (rf)
y’ y’
x’ x’
z zB
0
180
o

degre
e
pulse

36. Basic Spin Echo Imaging 36Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Effect of 180
o
RF pulses:
B
1 (rf)
y’ y’
x’ x’

37. Basic Spin Echo Imaging 37Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Effect of 180
o
RF pulses:
B
1 (rf)
y’ y’
x’ x’

38. Basic Spin Echo Imaging 38Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Effect of 180
o
RF pulses:
B
1 (rf)
y’ y’
x’ x’

39. Basic Spin Echo Imaging 39Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Effect of 180
o
RF pulses:
B
1 (rf)
y’ y’
x’ x’

40. Basic Spin Echo Imaging 40Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Effect of 180
o
RF pulses:
B
1
(rf)
y’
x’
y’
x’
y’
x’
y’
x’

41. Basic Spin Echo Imaging 41Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Effect of 180
o
RF pulses:
B
1
(rf)
y’
x’
y’
x’
y’
x’
y’
x’

42. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Two Pulse sequences (I)
90—— ——90
Saturation recovery
Two Pulse sequences (I)
180—— ——90
Inversion recovery
123456
T
1
123456
T
1

43. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
T
1 Spin Lattice Relaxation
Time
Describes the return to equilibrium
for spins from the excited state
Spins loose heat to the rest of the
world
Requires fluctuating magnetic field
near the Larmor frequency for an
effective transfer of energy from a
spin to surrounding lattice

44. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Two Pulse sequences (II)
90—— ——180 —— ——
Spin Echo sequence
y’
x’

45. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Two Pulse sequences (II)
90—— ——180 —— ——
Spin Echo sequence
y’
x’

46. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Two Pulse sequences (II)
90—— ——180 —— ——
Spin Echo sequence
y’
x’

47. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Two Pulse sequences (II)
90—— ——180 —— ——
Spin Echo sequence
y’
x’

48. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Two Pulse sequences (II)
90—— ——180 —— ——
Spin Echo sequence
y’
x’
e
-t
/
T2
*
e
-t
/
T2

49. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Two Pulse sequences (II)
90—— ——180 —— ——
Spin Echo sequence
y’
x’
e
-t
/
T2
*
e
-t
/
T2

50. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Two Pulse sequences (II)
90—— ——180 —— ——
Spin Echo sequence
y’
x’

51. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Two Pulse sequences (II)
90—— ——180 —— ——
Spin Echo sequence
y’
x’

52. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Two Pulse sequences (II)
90—— ——180 —— ——
Spin Echo sequence
y’
x’

53. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Two Pulse sequences (II)
90—— ——180 —— ——
Spin Echo sequence
y’
x’

54. Basic Spin Echo Imaging 54Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
T
2 and T
2
*
e
-t
/
T2
*
e
-t
/
T2
H
O
H H
O
H

55. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Spin-Spin Relaxation Time
Static inhomogeneities refocussed
by 180 pulse
Time varying imhomogeneity are
not
T
2 changes in disease give rise to
diagnostic value of MRI

56. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Superconducting Magnet
Helium vessel
containing super-
con coil
Vacuu
m

57. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Superconducting Magnet
Bore B0
100cm 0  4T
80cm 0  8T

58. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Shimming

59. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Other Magnet Types
Permanent magnet, e.g.
light weight rare earth
magnets, <0.3T

60. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Other Magnet Types

61. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Other Magnet Types
Electromagnet <0.3T

62. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Special Superconducting
Magnets
Active Shielding
–Extra coils reduce
stray field
–Improves siting
10
12
4
2
5mT contour
0.5T wholebody 3T AS wholebody

63. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
RF Coils
Remember B
rf
must be  B
0
Field is  subject, can use solenoid.

64. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
RF Coils
Remember B
rf
must be  B
0
Field is  subject, cannot use solenoid.
Saddle coil, B
rf
is  coil
access. Efficiency is
low, and homogeneity
is poor

65. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging

66. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
How to Make Images
Impose (separately):
B
z
x
B
z
y
B
z
z
X gradient
Gx
Y gradient
Gy
Z gradient
Gz
Typical values are 10-100 mT/m

67. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
How to make images
For a Z gradient
-hz +hz
z = -(B
0
+ G
z
.z)

68. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
How to make images

69. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Imaging Gradients
Special coils (together with power
supplies) provide linear variation in
B
0 in X, Y and Z directions
Z
B
0Z

70. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Imaging Gradients
Special coils (together with power
supplies) provide linear variation in
B
0 in X, Y and Z directions
X,Y

71. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Selection of Slice
Use Fourier relationship:
RF Amplitude (volts)

72. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Selection of slice
Slice thickness
adjusted by
changeimg
gradient strength
or slice bandwith
(longer pulse has
narrower
frequency spread)
Slice position
adjusted by
changing the
centre frequency
of the pulse

73. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
k-space
k-space is the raw data space before
fourier transformation into the
image
2D image will be represented by a
2D array of data points spread
throughout k-space
Differing the k-space trajectory will
alter image contrast

74. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Image vs k-space
(r) S(k)
k(t)= /2G(t)dt

75. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Image vs k-space
(r) S(k)
k(t)= /2G(t)dt

76. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Image vs k-space
(r) S(k)
k(t)= /2G(t)dt

77. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Image vs k-space
(r) S(k)
k(t)= /2G(t)dt

78. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Image vs k-space
(r) S(k)
k(t)= /2G(t)dt
FT

79. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
GE k-space trajectory
(r) S(k)
k(t)= /2G(t)dt
RF
GS
GR
GP
S(t)

80. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
(r) S(k)
k(t)= /2G(t)dt
RF
GS
GR
GP
S(t)
-k
r
+k
r
GE k-space trajectory

81. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
(r) S(k)
k(t)= /2G(t)dt
RF
GS
GR
GP
S(t)
-k
r
+k
r
GE k-space trajectory

82. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
(r) S(k)
k(t)= /2G(t)dt
RF
GS
GR
GP
S(t)
-k
r
+k
r
-k
p
+k
p
GE k-space trajectory

83. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
(r) S(k)
k(t)= /2G(t)dt
RF
GS
GR
GP
S(t)
-k
r
+k
r
-k
p
+k
p
GE k-space trajectory

84. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
(r) S(k)
k(t)= /2G(t)dt
RF
GS
GR
GP
S(t)
-k
r
+k
r
-k
p
+k
p
GE k-space trajectory

85. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
(r) S(k)
k(t)= /2G(t)dt
RF
GS
GR
GP
S(t)
-k
r
+k
r
-k
p
+k
p
GE k-space trajectory

86. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
(r) S(k)
k(t)= /2G(t)dt
RF
GS
GR
GP
S(t)
-k
r
+k
r
-k
p
+k
p
GE k-space trajectory

87. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
(r) S(k)
k(t)= /2G(t)dt
RF
GS
GR
GP
S(t)
-k
r
+k
r
-k
p
+k
p
GE k-space trajectory

88. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
(r) S(k)
k(t)= /2G(t)dt
RF
GS
GR
GP
S(t)
-k
r
+k
r
-k
p
+k
p
GE k-space trajectory

89. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
(r) S(k)
k(t)= /2G(t)dt
RF
GS
GR
GP
S(t)
-k
r
+k
r
-k
p
+k
p
GE k-space trajectory

90. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
(r) S(k)
k(t)= /2G(t)dt
RF
GS
GR
GP
S(t)
-k
r
+k
r
-k
p
+k
p
GE k-space trajectory

91. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
(r) S(k)
k(t)= /2G(t)dt
RF
GS
GR
GP
S(t)
-k
r
+k
r
-k
p
+k
p
GE k-space trajectory

92. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
(r) S(k)
k(t)= /2G(t)dt
RF
GS
GR
GP
S(t)
-k
r
+k
r
-k
p
+k
p
GE k-space trajectory

93. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
(r) S(k)
k(t)= /2G(t)dt
RF
GS
GR
GP
S(t)
-k
r
+k
r
-k
p
+k
p
GE k-space trajectory

94. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
(r) S(k)
k(t)= /2G(t)dt
RF
GS
GR
GP
S(t)
-k
r
+k
r
-k
p
+k
p
GE k-space trajectory

95. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
(r) S(k)
k(t)= /2G(t)dt
RF
GS
GR
GP
S(t)
-k
r
+k
r
-k
p
+k
p
GE k-space trajectory

96. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
(r) S(k)
k(t)= /2G(t)dt
RF
GS
GR
GP
S(t)
-k
r
+k
r
-k
p
+k
p
GE k-space trajectory

97. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
(r) S(k)
k(t)= /2G(t)dt
RF
GS
GR
GP
S(t)
-k
r
+k
r
-k
p
+k
p
GE k-space trajectory

98. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
(r) S(k)
k(t)= /2G(t)dt
-k
r
+k
r
-k
p
+k
p
SE k-space trajectory
RF
GS
GR
GP
S(t)

99. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
(r) S(k)
k(t)= /2G(t)dt
-k
r
+k
r
-k
p
+k
p
SE k-space trajectory
RF
GS
GR
GP
S(t)

100. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
(r) S(k)
k(t)= /2G(t)dt
-k
r
+k
r
-k
p
+k
p
SE k-space trajectory
RF
GS
GR
GP
S(t)

101. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
(r) S(k)
k(t)= /2G(t)dt
-k
r
+k
r
-k
p
+k
p
SE k-space trajectory
RF
GS
GR
GP
S(t)

102. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
(r) S(k)
k(t)= /2G(t)dt
-k
r
+k
r
-k
p
+k
p
SE k-space trajectory
RF
GS
GR
GP
S(t)

103. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
(r) S(k)
k(t)= /2G(t)dt
-k
r
+k
r
-k
p
+k
p
SE k-space trajectory
RF
GS
GR
GP
S(t)

104. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Definitions
RF
GS
GR
GP
S(t)
TR

105. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Definitions
RF
GS
GR
GP
S(t)
TE

106. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
123456
T
1
123456
T
2
Controlling contrast

107. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
123456
T
1
123456
T
2
Proton Density
TR TE

108. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
123456
T
1
123456
T
2
T
2 Contrast
TR TE

109. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
0.5T
Multislice
Multiech
o
TR2000/3
0..90
30ms 90ms

110. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
123456
T
1
123456
T
2
T
1 Contrast
TR TE

111. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging

112. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Effect of Flip angle 
X’ Y’
B
0
B
rf

113. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Effect of Flip angle 
X’ Y’
B
0
B
rf
90
o
pulse
Maximum
signal but
have to wait
5T
1
for
recovery

114. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Effect of Flip angle 
X’ Y’
B
0
B
rf

115. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Effect of Flip angle 
X’ Y’
B
0
B
rf
Flip angle 30
o:
detect M
0sin  = 0.5 M
0
remaining M
0
cos = 0.87 M
0

116. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
TR/
TE/
41/9/15
500/9/15
41/9/9041/9/60
500/9/90
Contras
t versus

Contras
t versus
TR

117. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Why ?
freeze involuntary patient motion
visualization of dynamic process
–fast imaging: minutes
–turbo imaging: seconds
More complex MRI experiments
–obtain multiple images vary some
parameter e.g. TI
reduce patient examination time

118. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Why does MRI take so long
Answer
–Only one phase encode line acquired per
excitation
–Spin Echo: 256*3s for T2, 256*0.6s for T1
–Gradient Echo: 256*35ms (but have to do
3D
Solution
–get more phase encode lines per
excitation

119. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Echo Planar Imaging
Fastest imaging method
Typical AQ time is 30-100ms
Low RF deposition
Very fast gradient switching
Highly demanding on MRI hardware
–B
0 homogeneity
–gradient switching

120. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
(r) S(k)
k(t)= /2G(t)dt
RF
GS
GR
GP
S(t)
-k
r
+k
r
-k
p
+k
p
GE-PEI k-space trajectory

121. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
(r) S(k)
k(t)= /2G(t)dt
RF
GS
GR
GP
S(t)
-k
r
+k
r
-k
p
+k
p
GE EPI k-space trajectory

122. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
(r) S(k)
k(t)= /2G(t)dt
RF
GS
GR
GP
S(t)
-k
r
+k
r
-k
p
+k
p
GE EPI k-space trajectory

123. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
(r) S(k)
k(t)= /2G(t)dt
RF
GS
GR
GP
S(t)
-k
r
+k
r
-k
p
+k
p
GE EPI k-space trajectory

124. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
(r) S(k)
k(t)= /2G(t)dt
RF
GS
GR
GP
S(t)
-k
r
+k
r
-k
p
+k
p
GE EPI k-space trajectory

125. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
(r) S(k)
k(t)= /2G(t)dt
RF
GS
GR
GP
S(t)
-k
r
+k
r
-k
p
+k
p
GE EPI k-space trajectory

126. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
(r) S(k)
k(t)= /2G(t)dt
RF
GS
GR
GP
S(t)
-k
r
+k
r
-k
p
+k
p
GE EPI k-space trajectory

127. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
(r) S(k)
k(t)= /2G(t)dt
RF
GS
GR
GP
S(t)
-k
r
+k
r
-k
p
+k
p
GE EPI k-space trajectory

128. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
(r) S(k)
k(t)= /2G(t)dt
RF
GS
GR
GP
S(t)
-k
r
+k
r
-k
p
+k
p
GE EPI k-space trajectory

129. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
(r) S(k)
k(t)= /2G(t)dt
RF
GS
GR
GP
S(t)
-k
r
+k
r
-k
p
+k
p
GE EPI k-space trajectory

130. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
(r) S(k)
k(t)= /2G(t)dt
RF
GS
GR
GP
S(t)
-k
r
+k
r
-k
p
+k
p
GE EPI k-space trajectory

131. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
GE vs EP Imaging
TE
ms
TR
ms
AQ
ms
BW
khz
Gread
mT/m
Switch

s
GE103510252.5500
EPI50

0.525025100
Assume FOV 25cm AQ = 10ms
Matrix 256 time/sample = 10
-2
/256
Bandwidth = 25kHz Gread = 25 x 10
3
/0.25
= 100 000Hz/m
= ~ 2.5 mT/m

132. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
GE vs EP Imaging
TE
ms
TR
ms
AQ
ms
BW
khz
Gread
mT/m
Switch

s
GE103510252.5500
EPI50

0.525025100
Assume FOV 25cm AQ = 0.5ms
Matrix 128 time/sample = 5x10
-4
/128
Bandwidth = 250kHz Gread = 250 x
10
3
/0.25
= 1 000 000Hz/m
= ~ 25 mT/m

133. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
MRI at 3T
128x128 single shot,
GE echo planar.
X,Y,Z shim only
(~30s)
No template or
navigator correction
Straight FFT after row
reversal

134. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
fMRI (functional MRI)
Monitor T2 or T2* contrast during cognitive
task
eg acquire 20-30 slices every 4 seconds
Design experiment to have alternating blocks
of task and control condition
Look for statistically significant signal intenisty
changes correlated with task blocks

135. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Echo-Planar fMRI
responsestimulus
GE-images with EPI
fMRI correlation maps
Signal response
averaged over region



136. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
oxyhaemoglobin
deoxyhaemoglobin
Resting
O
2 & glucose

137. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
O
2 & glucose
Blood flow
‘over-compensation’
%O
2
Activated
ATP
ADP
BOLD signal

138. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Effect of Intravascular
Oxygenation level
Blood vessel
Paramagne
tic
T
2
(and T
2
*
)
reduced
because of
diffusion
through field
gradients
Diamagnet
ic
deoxy oxy

139. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
T2* curves activated and
rest
time (ms)
signal
activated
rest
TE Signal difference ~ 1-5 %
oxyhaemoglobin
deoxyhaemoglobin
resting activated

140. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Unilateral Finger
Opposition (high res)

141. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Definitions
Diffusion relates to the microscopic
Brownian thermal motion of molecules
Perfusion, classically is defined as that
process that results in the delivery of
nutrients to cells, normally expressed
as ml/min/100g wet weight of tissue

142. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Effect of Diffusion on NMR
Rms. of an ensemble is zero
For a single molecule diffusion results in
a gaussian distribution of displacements
r

143. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Diffusion and Spin echoes
 


144. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Diffusion and Spin echoes
I/I
0
= e
-bD
b = 
2
g
2

2
(-/3)

145. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
D and ADC
0
2
4
6
8
10
0 500 1000 1500
water
DMSO
I/I
0
= e
-bD
b = 
2
g
2

2
(-/3)
H
2O = 2.1 x 10
-3
mm
2
s
-1
DMSO = 0.55 x 10
-3
mm
2
s
-1
normal = 0.71 x 10
-3
mm
2
s
-1
ischaemic = 0.55 x 10
-3
mm
2
s
-1
b
L
o
g

(
I
/
I
0
)

146. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Diffusion Weighted
Imaging
RF
G
s
G
r
G
p

147. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Diffusion Weighted
Imaging
RF
G
s
G
r
G
p
G
diffusion

148. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
 

0
2
4
6
8
10
0 500 1000 1500
water
DMSO
Typical Values:
 = 20,  = 50
Gmax b
0.5 31
1 124
5 3104
10 12418
b
L
o
g

(
I
/
I
0
)

149. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Practical Problems in Human
DWI
Gross Motion
–Head motion
–breathing
Pulsitility
–CSF/brain pulsation
Anisotropy
–D is direction dependant, especially
white matter

150. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Practical Problems in Human
DWI
Gross Motion
–Echo Planar Imaging
–navigator echoes
Pulsitility
–gating plus navigator echoes
Anisotropy
–Measure trace, Dxx + Dyy + Dzz
–Measure full tensor (all matrix elements)

151. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Diffusion Weighted EPI (b=1570 s/mm
2
)
READ
PHASE SLICE
FOV 25cm, TE 118ms TY
DW-EPI 128x128 interpolated to
256x256
Partial k-acquisition (62.5%)
4 interleaves,  = 28ms ;  = 66 ms

152. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Cambridge NIH van Zijl
A
D
C

t
r
a
c
e
Diffusion Weighted EPI (b=1570 s/mm
2
)

153. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Diffusion Weighted EPI (b=1570 s/mm
2
)
A
n
i
s
t
r
o
p
y

I
n
d
e
x

154. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
MRI and O
15
water PET

155. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Gadolinium blous experiment in rat brain
Image number (relative to blous injection)
-20-100 102030405060
R
e
l
a
x
a
t
i
o
n

r
a
t
e

c
h
a
n
g
e

(
s
-
1
)
-1
0
1
2
3
4
5
6

156. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Effect of Intravascular Gd
Blood vessel
Tissue
Tissue

157. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Effect of Intravascular Gd
Blood vessel
Tissue
Tissue
T
2
(and T
2
*
)
reduced because
of difussion
through field
gradients

158. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Gadolinium blous experiment in rat brain
Image number (relative to blous injection)
-20-100 102030405060
R
e
l
a
x
a
t
i
o
n

r
a
t
e

c
h
a
n
g
e

(
s
-
1
)
-1
0
1
2
3
4
5
6

159. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Data Analysis
Fit first pass of the bolus (avoid
recirculation)
Gamma variate, or (better) Monte
Carlo
Estimate arterial input function
from large vessel signal
rrCBV, rrCBF but absolute MTT

160. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Perfusion weighted MRI of a patient with a high
grade stenosis (>90%) of the right internal carotid
artery leading to a terminal supply zone
infarction in the region of the middle cerebral
artery, from
http://www.picker.com/mr/acr/perfusn/perfusn.htm
T2 weighted FSE images
(3555/80/4)
rrCBV-map map of the bolus
delay (MTT image)

161. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
Caution
Numbers obtained are not for true
perfusion (as measured by PET)
Similar to dynamic CT, DSC
measures micro-capillary flow
However good correlation between
PET and DSC (in pigs), in humans??

162. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
True Perfusion by MRI
Arterial spin labeling
–EPISTAR, ASL, QUIPS
–label arterial blood on the way into
brain
–subtract images with and without
labelling
–difference is due to arterial water that
has entered tissue, i.e. perfusion

163. Part III Physics: Medical Physics
Option Magnetic Resonance
Imaging
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