SHAPE Society

1. Introduction to Nuclear
Magnetic Resonance
• Topics
– Nuclear spin and magnetism
– Resonance behavior and the Larmor
Frequency
• Larmor frequency
• flip angle
– Energy Absorption and Emission
• NMR spectroscopy
• Energy absorption in tissue (safety issues)
• Relaxometry
– T1,T2,T2* relaxation

2. Nuclear Magnetism
• Nucleons (protons, neutrons) have a quantum
property known as spin.
• Nucleons have been shown to obey Fermi
statistics, and thus have a maximum spin
magnitude of 1/2 Bohr magneton. (spin=1/2)
• In the absence of a magnetic field, nuclear spin
is not an observable
• In the presence of a homogeneous magnetic
field, the energy of the nucleus depends on the
relative orientation of the magnetic field and
the nuclear spin vector

3. single
voxel
single
voxel
net magnetizationnet magnetization
Individual nuclear spinsIndividual nuclear spins

4. Nuclear Magnetic Resonance:
Properties in Matter
• Relaxation
– After we have delivered energy to the nuclei
in our sample at the Larmor frequency,
there are two possible ways for the sample to
lose this energy (back to lowest energy state):
• spontaneous emission
• induced emission

5. • Spontaneous emission:
– negligible effect at RF frequencies (dominant
at visible frequencies)
• Induced emission
– Energy emission requires interaction of the
nucleus with its external environment
The nature of energy emission depends
strongly on the environment of the excited
nucleus (Relaxation)
3ω∝

6. • NMR Spectroscopy is the study of the
chemistry of matter using the NMR
absorption spectrum
• Relaxometry is the study of the chemistry of
matter using the NMR relaxation properties.
MRI generates tissue contrast based (mostly)
on NMR relaxation differences.

7. x
y
x
y
x
y
x
y
Spin Dephasing after excitation
Bo

8. Alignment of Spins in a
Magnetic Field
spin
magnetic moment
B0 field
M
M=0

9. Energy in a Magnetic Field
(Zeeman Splitting, Spin ½)
E+1/2= −γB0/2 E-1/2= +γB0/2
P+1/2= 0. 5000049 P-1/2= 0.4999951
1.5T, T=310K, P(E)∝exp(−E/kT)
mI = +½ mI = −½

10. Larmor Frequency
E+1/2= −γB0/2 E-1/2= +γB0/2
Allowed transitions ∆E = γB0
= ω0
mI = −½
mI = +½
ω0 = γB0

11. T1 Relaxation
Mz(t) = M0 + {Mz(0) − M0}exp(-t/T1)
Mz Mz
t t
saturation–recovery inversion–recovery
dMz(t) = − [Mz(t) − M0]
dt T1
M0 M0
Mz(0) = 0 Mz(0) = −M0

12. T2 Relaxation
Mxy(t) = Mxy(0) exp(−t/T2)
Mxy
t
dMxy(t) = − Mxy(t)
dt T2

13. • The relative populations of the spin states
can be altered in a well defined way by the
application of a resonant B1 field in the xy-
plane.
• Any fluctuating magnetic field that has a
component in the xy-plane that oscillates at
the resonant frequency can induce
transitions between the spin states.
The T1 Relaxation Process

14. Recovery Curve
time
Signal












−−=
1
0 exp1
T
t
SS

15. What effect does T1 have on
Images?
90° 90° 90° 90° 90°
Mz

16. What effect does T1 have on
Images?
t = 0 t = 3s t = 6s t = 9s t = 12s

17. • Assume the steady state has been reached.
• Use a flip angle of θdegrees.
• Find a condition where the transverse
magnetization following the flip is
maximized.
The Ernst Angle






−=
1
expcos
T
TR
θ

18. T1-Weighted Images

19. T1-Weighted Images

20. T1-Weighted Images

21. T1-Weighted Images

22. Transverse Relaxation
• Longitudinal relaxation is driven by field
oscillations in the transverse plane.
• Transverse relaxation is driven by field
oscillations in the longitudinal plane.
• Random fluctuations in B0 experienced by a
nucleus cause the resonant frequency of that
spin to change.

23. Transverse Relaxation
• If the field experienced by the molecule is
purely random then the effect would time
average to zero.
• Correlations in the motion cause a range of
frequencies.
• In solids where there is no molecular
tumbling the range of resonances is very
broad.

24. Transverse Relaxation
Long T2
Short T2
frequency

25. Decay Curve
time
Signal






−=
2
0 exp
T
t
SS

26. PD
T1
T2

28. What is T2
*?
• Spin-spin relaxation represents a loss of
coherence in the transverse magnetization
due to local effects on spin.
• Loss of the coherence of the transverse
magnetization also occurs as a result of bulk
magnetic effects

29. What is T2
*?

30. What effect does T2
* have on
Images?
• T2 and T2
* have the same effect on images.
• T2
* effects dominate when there is no spin
echo.
• From now on, we will assume that T2* is
more important, since in imaging it often is.

31. What effect does T2
* have on
Images?
• Effect of echo time

32. What effect does T2
* have on
Images?
• Effect of echo time

33. What effect does T2
* have on
Images?
• Effect of echo time

34. What effect does T2
* have on
Images?
• Effect of echo time

35. What effect does T2
* have on
Images?
× =
⊗ =
⇓ ⇓ ⇓FT FTFT
Perfect FID T2
*
Decay Actual FID
Perfect Image Point Spread Function Actual Image

36. What effect does T2
* have on
Images?
× =
⊗ =
⇓ ⇓ ⇓FT FTFT
Perfect FID T2
*
Decay Actual FID
Perfect Image Point Spread Function Actual Image

38. Contrast in MRI
• Contrast based on Relaxation Time
• Contrast Agents
• BOLD Contrast
• Perfusion and Diffusion Contrast

39. To start with….
• No Spins, no contrast.
• Spin density is the most fundamental of
contrasts in MRI.
• In our case spin density is related to the
water content of the tissue.

40. Water Content of Tissue
Tissue % Water Content
Grey Matter 70.6
White Matter 84.3
Heart 80.0
Blood 93.0
Bone 12.2

41. Enhancing the Signal
from Spin Density
• Gastrointestinal imaging
– Drinking water
• Lung imaging
– Inhaling hyperpolarized inert gas. A laser
polarises the helium gas increasing its
magnetisation.

42. T1 Contrast
• The shorter the repetition time (TR) the
greater the T1 contrast.
• Prepare the magnetisation with a saturation
or inversion pulse followed by a suitable
delay.

43. T2 or T2
* Contrast
• Sequences without a spin echo will be T2
*-
weighted rather than T2-weighted.
• The longer the echo time (TE) the greater
the T2 contrast.

44. Magnetism
• units of magnetism
– 1T = 10,000 Gauss
– Earth’s field = 0.5 Gauss
• types of magnetism
– ferromagnetism
– paramagnetism
– superparamagnetism
– diamagnetism

45. Magnetic Susceptibility
• the degree to which a material
becomes magnetized when placed
in a magnetic field
• magnetizability

46. Ferromagnetism
• Property of certain metals
– iron
– nickel
– cobalt
• Positive susceptibility
• consist of magnet domains
• able to remain magnetized without
the influence of an external field

47. Paramagnetism
• Oxygen
• metal ions with unpaired electrons
resulting in positive susceptibility
– Fe
– Gd
– Mg
• less than 1/1000 th effect of
ferromagnetism
• shortens T1 and T2 times

48. Superparamagnetism
• halfway between ferro and
paramagnetic
• property of individual domains of
elements that have ferromagnetic
properties in bulk
• iron containing contrast agents
for bowel and lymph nodes are
superparamagnetic

49. Diamagnetism
• no intrinsic atomic magnetic moment
• In a magnetic field weakly repel the field
– negative magnetic susceptibility
• water, copper, nitrogen, barium sulfate,
and most tissues are diamagnetic.
• contributes to the loss of signal seen in
bowel on MRI after administration of
barium sulfate suspensions.

50. Contrast Agents
• Sometimes it is desirable to enhance the
natural contrast in the images.
– Highlighting a tumour.
– Measuring perfusion rates.
• MR contrast agents act to change the
relaxation times of the substrate they are in.

51. Contrast Agents
• Contain an ion with a large number of
unpaired electrons.
• This gives it a larger magnetic moment than
the proton.
• This enhances the local magnetic fields that
fluctuate, due to thermal motion, in the
vicinity of a proton.

52. Contrast Agents
• These are usually paramagnetic ions.
e.g. Gadolinium
• By themselves these ions are highly toxic so
they are bound up in large molecules.
e.g. DTPA
• They tend not to permeate the blood brain
barrier and so enhance tumours that lack
this barrier.

53. Gadolinium
• shortens T1 relaxation of water
– high SI on T1 weighted images in lower
concentrations
• shortens T2 relaxation of water
– lowers SI on T1 weighted images in high
concentrations
• seven unpaired electrons
– high magnetic moment
– unpaired electrons react with protons in
adjacent water molecules shortening their
relaxation time

54. Gd-DTPA
COO-
CH2
N - CH2 - CH2 - N - CH2 - CH2 - N
Gd3
+
CH2 - COO-
CH2 - COOH
COO- - CH2
COOH - CH2

55. Haemoglobin

56. Haemoglobin
Oxy-haemoglobin
Deoxy-haemoglobin
Paramagnetic
Diamagnetic

57. Perfusion Imaging
• Contrast agent methods
• Spin tagged methods
– Continuously labelled
– IVIM
– Pulse labelled

58. Perfusion Imaging
• Perfusion is the rate constant determining
the delivery of metabolic substrates to the
local tissue and clearance of products of
metabolism.
• If a volume of tissue V is supplied with
arterial blood at a rate of F then perfusion
f = F / V (ml/100g/min)

59. Contrast Agent Methods
• Inject contrast agent.
• Causes a bolus of blood to have a different
signal intensity to the blood in the brain.
• This tagged bolus perfuses the brain.
• Level of signal change gives an indication
of rate of perfusion.

60. Contrast Agent Methods
Artery
Capillary Bed
Vein

61. Contrast Agent Methods
Artery
Capillary Bed
Vein

62. Contrast Agent Methods
Artery
Capillary Bed
Vein

63. What is k-space?
• a mathematical concept
• not a real “space” in the patient nor in
the MR scanner
• key to understanding spatial encoding
of MR images

64. k-space and the MR Image
x
y
f(x,y)
kx
ky
KKK---spacespacespace
F(kx,ky)
ImageImageImage---spacespacespace
ℑ

65. k-space and the MR Image
• each individual point in the MR image
is reconstructed from every point in
the k-space representation of the
image
– like a card shuffling trick: you must have all of
the cards (k-space) to pick the single correct
card from the deck
• all points of k-space must be collected
for a faithful reconstruction of the
image

66. Discrete Fourier Transform
F(kx,ky) is the 2D discrete Fourier transform of the
image f(x,y)
x
y
f(x,y)
kx
ky
ℑ
K-space
F(kx,ky)
f x y
N
F k k e
xk yk
kk
x y
j
N
x j
N
yNN
yx
( , ) ( , )=
+






=
−
=
−
∑∑
1
2
2 2
0
1
0
1 π π
image-space

67. k-space and the MR Image
• If the image is a 256 x 256 matrix size,
then k-space is also 256 x 256 points.
• The individual points in k-space
represent spatial frequencies in the
image.
• Image contrast is represented by low
spatial frequencies; detail is
represented by high spatial frequencies.

68. low spatial
frequencies
low spatial
frequencies
high spatial
frequencies
high spatial
frequencies
all
frequencies
all
frequencies

69. Spatial Frequencies
• low frequency = contrast
• high frequency = detail
• The most abrupt change occurs at an
edge. Images of edges contain the
highest spatial frequencies.

70. Properties of k-space
• k-space is symmetrical
• all of the points in k-space must be known
to reconstruct the signal faithfully
• truncation of k-space results in loss of
detail, particularly at edges
• most important information centered
around the middle of k-space
• k-space is the Fourier representation of the
waveform

71. MRI and k-space
• The nuclei in an MR experiment
produce a radio signal (wave) that
depends on the strength of the main
magnet and the specific nucleus being
studied (usually H+).
• To reconstruct an MR image we need
to determine the k-space values from
the MR signal.

72. RF signal
A/D
conversion
image space
FT
k-space

73. MRI
• Spatial encoding is accomplished by
superimposing gradient fields.
• There are three gradient fields in the
x, y, and z directions.
• Gradients alter the magnetic field
resulting in a change in resonance
frequency or a change in phase.

74. MRI
• For most clinical MR imagers using
superconducting main magnets, the main
magnetic field is oriented in the z direction.
• Gradient fields are located in the x, y, and
z directions.

75. MRI
• The three magnetic gradients work together
to encode the NMR signal with spatial
information.
• Remember: the resonance frequency
depends on the magnetic field strength.
Small alterations in the magnetic field by the
gradient coils will change the resonance
frequency.

76. Gradients
• Consider the example of MR imaging in the
transverse (axial) plane.
Z gradient: slice select
X gradient: frequency encode (readout)
Y gradient: phase encode

77. Slice Selection
• For axial imaging, slice selection occurs
along the long axis of the magnet.
• Superposition of the slice selection gradient
causes non-resonance of tissues that are
located above and below the plane of
interest.

78. Slice Selection
ββ
z
y
x
0
imaging plane
ββ++
ββ−−
ββ00
z gradientz gradient

79. Slice Selection
slice thickness is determined by gradient strength
β
β0
ω
ω0
RF bandwidth
1
3
2
ΖΖ
tt11
tt22
tt33

80. Slice Selection
Selection of an axial
slice is accomplished
by the z gradient.
zz gradient directiongradient direction
ω = ω0
graph of the z magnetic gradient
z-axis
β β = β0
β > β0
β < β0

81. Slice Selection
slice location is determined by the null point of the z gradient
β
β0
ω
ω0
RF bandwidth
slice 1
ΖΖ
slice 2 slice 3
Ζ1 Ζ2 Ζ3

82. Frequency Encoding
• Within the imaging plane, a small gradient is
applied left to right to allow for spatial
encoding in the x direction.
• Tissues on the left will have a slightly higher
resonance frequency than tissues on the right.
• The superposition of an x gradient on the
patient is called frequency encoding.
• Frequency encoding enables spatial localization
in the L-R direction only.

83. Frequency Encoding
z
y
x
x gradientx gradient
higher frequency
lower frequency
LR

84. Frequency Encoding
RF signalRF signal
fromfrom entireentire sliceslice
A/D conversion, 256 pointsA/D conversion, 256 points 1 line of
k-space

85. Phase Encoding
• An additional gradient is applied in the
y direction to encode the image in the
remaining direction.
• Because the x gradient alters the
frequencies in the received signal according
to spatial location, the y gradient must alter
the phase of the signal.
• Thus, the points of k-space are revealed by
recording the digitized RF signal after a
phase encoding gradient application.

86. Phase Encoding
• The technique of phase encoding the second
dimension in the imaging plane is
sometimes referred to as spin warping.
• The phase encoding gradient is “stepped”
during the acquisition of image data for a
single slice. Each step provides a unique
phase encoding.
• For a 256 x 256 square image matrix, 256
unique phase encodings must be performed
for each image slice. The second 256 points
in the x direction are obtained by A to D
conversion of the received signal.

87. Phase Encoding
z
y
x
yy gradient,gradient,
phase step #192phase step #192
yy gradient,gradient,
phase step #64phase step #64

88. Phase Encoding
2D k-space matrix
gradient strength +128
RF in RF outRF out A/D conversion
gradient strength N
RF in RF outRF out A/D conversion
gradient strength -128
RF in RF outRF out A/D conversion
                 






END
BEGIN
line 128
line N
line -128
                 
                 

89. Spin Echo Imaging
RF
z gradient
echo
90°180°
echo
90°180°
echo
90°180°
y gradient
x gradient
slice select
phase
readout

90. Spin Echo Imaging
view -128
view -55
view 40
                 
                 
                 
k-space
256 x 256 points
row 40
row -55
row -128
A/D, 256 points
kx = frequency
ky = phase

91. • Acquisition of spatially encoded data as
described allows for reconstruction of the
MR image.
• The frequency and phase data are acquired
and form points in a 2D array .
• Reconstruction of the image is provided by
2D inverse Fourier transform of the
2D array.
• This method of spatially encoding the MR
image is called 2D FT imaging.
MR Image Reconstruction

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

93. 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

94. 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)

95. 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

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

97. 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

98. 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

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

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

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

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

103. 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)

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

105. 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

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

107. 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

108. 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

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

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

111. 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

112. 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

113. Gradient Echo
changing TE
TE 9
FA 30
TE 9
FA 30
TE 30
FA 30
TE 30
FA 30
susceptibility effectsusceptibility effect T2* weightingT2* weighting

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

115. 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

116. Conventional GR
TE 20, FA 15

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

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

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

120. MTC GR
TE 13, FA 50

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

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

123. 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

124. 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

125. 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

126. 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

127. 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

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

129. WNMR Race
t=0
900 RF

130. WNMR Race

131. WNMR Race
t=TE/2
1800 RF

132. t=TE
WNMR Race

133. 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

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

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

136. 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

137. 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

138. T1 weightedT1 weighted T2 weightedT2 weighted
Spin Echo Contrast

139. Spin Echo Contrast
PD weightedPD weighted T2 weightedT2 weighted

140. Contrast
• the ability to discriminate different
tissues based on their relative
brightness

147. Caveat
• windowing affects the relative
contrast of tissues
– intensity values of pixels are relative
to one another, unlike CT
• windowing can make a solid tumor
look like a “cyst”

148. T1 SET1 SET2 FSET2 FSE
“CYST”“CYST”

149. T1 SET1 SET2 FSET2 FSE
CYST?CYST?

150. T2 FSET2 FSE T2 FSET2 FSE
CYST?CYST?

151. Summary
• need visible differences in intensity
to discriminate tissues
• surrounding tissues can make an
intermediate signal tissue appear
dark or bright
• windowing affects image and tissue
contrast

152. Noise
• constant at a given machine setup
• reduces the ability to visualize low
contrast structures
• adds to or subtracts from the
average signal intensity of a given
pixel

153. Noise
• increasing the available signal will
reduce the relative effects of noise
• machine parameters must be
chosen to maximize signal without
significantly extending exam times
• S/N is a relative measure allowing
for comparison in a variety of
circumstances

154. frequency
SI
frequency
SI
Signal versus Noise
• high signal
• high SNR
• low signal
• low SNR

155. Noiseless Conditions
0
10
20
30
40
50
60
70
80
90
100
Tissue A Tissue B
Tissue Type
RelativeSignalIntensity

156. High Signal/Low Noise
0
10
20
30
40
50
60
70
80
90
100
Tissue A Tissue B
Tissue Type
RelativeSignalIntensity

157. Low Signal/High Noise
0
10
20
30
40
50
60
70
80
90
100
Tissue A Tissue B
Tissue Type
RelativeSignalIntensity

158. Noiseless Conditions
0
10
20
30
40
50
60
70
80
90
100
1
3
5
7
9
11
13
15
17
19
21
23
25
27
29
Relative Pixel Location
RelativeSignalIntensity

159. High Signal/Low Noise
0
10
20
30
40
50
60
70
80
90
100
1
3
5
7
9
11
13
15
17
19
21
23
25
27
29
Relative Pixel Location
RelativeSignalIntensity

160. Low Signal/High Noise
0
10
20
30
40
50
60
70
80
90
100
1
3
5
7
9
11
13
15
17
19
21
23
25
27
29
Relative Pixel Location
RelativeSignalIntensity

161. Image Contrast
100% noise

162. Image Contrast
80% noise

163. Image Contrast
60% noise

164. Image Contrast
40% noise

165. Image Contrast
20% noise

166. Image Contrast
0% noise

167. Factors Affecting SNR
• strength of main magnet
• coil selection
• voxel size
• phase encoding
• number of averages
• receiver bandwidth
• pulse sequence parameters

168. • stronger main magnet
• proper imaging coil
• larger voxel size
• decreased phase encoding
• increased number of averages
• decreased receiver bandwidth
• (pulse sequence parameters)
Factors INCREASING SNR

169. Stronger Main Magnet
S/N effect Downside
• linear increase • less T1 weighting
at high fields
• increased
chemical shift
effects in RO
direction

170. Coil Selection
S/N effect Downside
• increase in signal
with surface coils
• quadrature
provides 40%
increase S/N over
linear
• phased array
increased over
quadrature
• limited coverage
with surface coils
• more complex
coils are more
expensive

171. Larger Voxel Size
S/N effect Downside
• linear increase in
either RO or PE
direction
• linear increase
with increased
slice thickness
• decreased
resolution

172. Decreased Phase Encodings
S/N effect Downside
• square root
increase in signal
to noise
• linear decrease in
scan time
• decreased
resolution in PE
direction
• Gibb’s
phenomenon in
PE direction

173. Increased Signal Averages
S/N effect Downside
• square root
increase in signal
to noise
• linear increase in
scan time

174. Decreased Receiver BW
S/N effect Downside
• square root
increase in signal
to noise
• increase in
chemical shift
artifact in RO
direction

175. Pulse Sequence Parameters
• SE imaging
– increased TR provides nonlinear
increase in SNR with linear increase
in scan time
– decreased TE provides nonlinear
increase in SNR with no effect on
scan time and less T2 weighting

176. Pulse Sequence Parameters
• GE imaging
– complex effects
– maximum SNR typically between 30
and 60 degrees
– long TR sequences (2D)
• increase SNR with increased flip angle
– short TR sequences (TOF & 3D)
• decreased SNR with increased flip angle

177. SNR Application
• pituitary imaging
– baseline:
• 16 cm FOV, 3 mm slice thickness, 192
phase encodes, 4 NEX
– new goal:
• reduced scan time, same SNR

178. FOV RO PE
Slice
Thickness
(mm) NEX
Imaging Time
(TR=500
msec)
Relative
SNR
160 256 192 3 4 6.40 43.30
160 256 170 4 2 2.83 43.39
190 256 192 3 2 3.20 43.18
160 256 144 3 3 3.60 43.30
SNR Example

179. Fat Suppression and SNR
• non fat-suppressed image
– each image pixel comprised of signal
from water and fat in the imaging
voxel
• fat-suppression
– reduces total signal by suppression of
fat from the voxel
– reduces SNR

180. frequency
SI
frequency
SI
Fat Suppression
• without fat
suppresion
• high SNR
• with fat
suppression
• lower SNR
water
plus
fat water only