Basic Principles MRI related to Neuroimaging

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Basic Principles MRI related to Neuroimaging

• Xiaoping Hu
• Department of Biomedical Engineering
• Emory University/Georgia Tech
• xhu_at_bme.emory.edu

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Outline

• Basic NMR/MRI Physics
• Imaging sequences
• Contrast Mechanisms
• Pitfalls and Limitations

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In the absence of magnetic field
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In the presence of magnetic field
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Bulk Nuclear Magnetization in the Presence of a
Static Magnetic Field
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Precession
nuclear spin inside a magnetic field
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Larmor Frequency
? is frequency of precession and
resonance usually in the radiofrequency (RF) range
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Resonance
Resonance occurs when the external influence
exerted to a system matches the systems natural
frequency. E.g., pushing a swing In MRI, the
natural frequency, called the Larmor frequency,
is proportional to the applied magnetic field. At
1.5 T, it is 64 Mhz (1Mhz1000,000 hz FM radio
uses 88-106 Mhz).
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Generation of NMR signal

• Excitation
• an RF pulse is applied to tip the magnetization
  such that it has a transverse component
• Reception
• precessing transverse component of M induces an
  emf in a receiving RF coil
• Relaxation
• The processes with which the magnetization
  returns to equilibrium. They determine the
  intensity/contrast of the image

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Spatial discriminationachieved with magnetic
field gradients
B0
x
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Selective Excitation Application of a
band-limited RF pulse in the presence of a
gradient along the direction perpendicular to the
desired slice
B0
w
RF power
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Lauterbur, 242, 190, Nature, 1973.
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w
B0
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phase
frequency
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FT
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RF
Gss
Gpe
Gro
Signal
timing diagram of a spin-echo sequence
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frequency encoding

phase encoding
k-space traversal of a spin-echo sequence
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Temporally interleaved multislice imaging
slice 1 acquisition
slice 2 acquisition

slice n acquisition
TR
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Effects of Slice Spacing and Order
nominal thickness
with gap or skip
no interleave
1
2
7
8
9
10
11
12
3
4
5
6
interleave
1
7
4
10
5
11
6
12
2
8
3
9
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timing diagram of a blipped EPI sequence
RF
Gss
Gpe
Gro
Signal
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frequency encoding

phase encoding
k-space traversal of an EPI sequence
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Spiral Pulse Sequence
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Spiral k-space trajectory
i?(t)
k k(t) e k(t) C t ?(t) C k(t)
(Archimedian)
1
2
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CONTRAST MECHANISMS in MRI T1 (Spin-lattice
Relaxation time) relaxation along Bo T2
(Spin-spin relaxation time) relaxation
perpendicular to Bo T2 (Signal decay
perpendicular to Bo ) due to dephasing plus T2
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Relaxation and Contrast
z
T1-relaxation
y
T2-relaxation
x
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T1 relaxation
TR


90 pulse
90 pulse
M0
M
TR
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Signal decay due to transverse relaxation
Irreversible processes (T2) Dephasing due to
different frequency of precession in the
presence of magnetic field inhomogeneities
(reversible) (T2).
1/T21/ T2 1/T2 Characterizes decay due to
both processes.
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TE
180 pulse
90 pulse
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90 pulse
time
-TE/T2
S(TE) So e
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Relaxation and Contrast
T1-relaxation Growth of magnetization for next
nutation
T2-relaxation decay of magnetization being
detected
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T1w Imaging at 3 Tesla
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Brain Tumor Imaging
T2W Pre-contrast
T1W Pre-contrast
T1W Post-contrast
MRI for brain tumor
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Spatial resolution

• Signal-to-noise ratio
• Imaging time
• Gradient performance parameters
• Physics
• Diffusion
• Signal decay

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State of the Art

• Structural imaging of human subjects
• 1mm 1mm 1mm
• Anatomic imaging of rodents
• 50?m 50 ?m 50 ?m
• NMR microscopy (of samples)
• 10?m 10 ?m 10 ?m
• Functional studies
• Humans 3mm 3mm 5mm
• Animals 100?m 100 ?m 500 ?m
• In vivo proton spectroscopy
• Human 7mm 7mm 7mm
• Animal 1mm 1mm 1mm

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Temporal resolution

• Signal-to-noise ratio
• Image resolution
• Gradient performance parameters
• Physics
• Relaxation

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State of the Art

• High resolution 3-D structural imaging
• 10-20 min
• Multislice imaging
• minutes
• Anatomic imaging of animals
• hours
• NMR microscopy (of samples)
• hours to days
• Functional studies
• Sec/image, minutes/study
• In vivo proton spectroscopy
• Human 10s of minutes
• Animal hours

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High-resolution imaging with reduced FOV Zoomed
imaging by outer volume saturation
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Limitations of ultrafast sequences

• EPI
• Nyquist ghost
• Spatial distortion
• Spiral
• Blurring
• EPI and Spiral
• Signal dropout
• Resolution degradation due to T2 decay

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Nyquist ghost
k-space data
image
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image
k-space data
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B0 inhomogeneity induced distortion

• Several possible causes
• Static field inhomogeneity
• Subject-dependent susceptibility
• Field inhomogeneity disturbs the conditions of
  Fourier imaging
• Image distortion and artifacts are encountered
  with severe inhomogeneity

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EPI image distortion due to field inhomogeneity
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Phase map
original
corrected
flash
Single-Shot EPI
Segmented EPI
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Spiral (before correction)
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Spiral (after correction)
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Problems in both EPI and Spiral

• signal loss due to T2 decay
• resolution degraded and limited by T2

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7 Tesla T2-weighted images (TE 15 msec)

5-mm
? 1-mm
z-shim

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Pulse Sequence for a Single-Shot EPI with
Susceptibility Compensation
Song, MRM 46, 407, 2001.
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Combined images from the single-shot
acquisitioncompared with conventional
single-shot acquisition at 4T
Song, MRM 46, 407, 2001.
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Spiral-In/Out Experiments
TE
Acquisitions - spiral-out (A)
- spiral-in (B) - combined
spiral-in/out (C)
Glover Law, MRM, 45, 515, 2001
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Spiral-In/Out Combination
spiral-out
S
spiral-in
wtd ave
Glover Law, MRM, 45, 515, 2001