medical image analysis
DESCRIPTION
Medical Image Analysis. Medical Imaging Modalities: Magnetic Resonance Imaging. Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011. Magnetic Resonance Imaging. Nuclear magnetic resonance The selected nuclei of the matter of the object - PowerPoint PPT PresentationTRANSCRIPT
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Medical Image AnalysisMedical Image AnalysisMedical Imaging Modalities: Magnetic Resonance Imaging
Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
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Magnetic Resonance Magnetic Resonance ImagingImagingNuclear magnetic resonance
◦The selected nuclei of the matter of the object
◦Blood flow and oxygenation◦Different parameters: weighted,
weighted, Spin-density◦Advance: MR Spectroscopy and
Functional MRI◦Fast signal acquisition of the order of
a fraction of a secondFigures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
1T 2T
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Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
Figure comes from the Wikipedia, www.wikipedia.org.
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Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
Figure 4.12. MR images of a selected cross-section that are obtained simultaneously using a specific imaging technique. The images show (from left to right), respectively, the T1-weighted, T-2 weighted and the Spin-Density property of the hydrogen protons present in the brain.
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Magnetic Resonance Magnetic Resonance ImagingImaging1H: high sensitivity and vast
occurrence in organic compounds13C: the key component of all
organic15N: a key component of proteins
and DNA19F: high relative sensitivity31P: frequent occurrence in organic
compounds and moderate relative sensitivity
Adapted from the Wikipedia, www.wikipedia.org.
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MR SpectroscopyMR Spectroscopy
Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
Figure comes from the Wikipedia, www.wikipedia.org.
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MR SpectroscopyMR Spectroscopy
Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
Figure comes from the Wikipedia, www.wikipedia.org.
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Functional MRIFunctional MRI
Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
Figure comes from the Wikipedia, www.wikipedia.org.
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MRI PrinciplesMRI Principles : spin-lattice relaxation time : spin-spin relaxation time : the spin density
Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
1T
2T
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MRI PrinciplesMRI Principles
1. Great web sites1. http://www.cis.rit.edu/htbooks/mri/
inside.htm
Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
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MRI PrinciplesMRI PrinciplesSpin
◦A fundamental property of nuclei with odd atomic weight and/or odd atomic numbers is the possession of angular moment
Magnetic moment◦The charged protons create a
magnetic field around them and thus act like tiny magnets
Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
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MRI PrinciplesMRI Principles : the spin angular moment : the magnetic moment : a gyromagnetic ratio, MHz/T
A hydrogen atom◦ :42.58 MHz/T
Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
J
J
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Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
NN
SS
JJ
JJ
Figure 4.13. Left: A tiny magnet representation of a charged proton with angular moment, J. Right: A symbolic representation of a charged proton with angular moment, J and a magnetic moment, μ.
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MRI PrinciplesMRI PrinciplesPrecession of a spinning proton
◦The interaction between the magnetic moment of nuclei with the external magnetic field
◦Spin quantum number of a spinning proton: ½
◦The energy level of nuclei aligning themselves along the external magnetic field is lower than the energy level of nuclei aligned against the external magnetic field
Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
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Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
Figure 4.14 (a) A symbolic representation of a proton with precession that is experienced by the spinning proton when it is subjected to an external magnetic field. (b) The random orientation of protons in matter with the net zero vector in both longitudinal and transverse directions.
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Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
MRI PrinciplesMRI PrinciplesEquation of motion for isolated
spin
Solution:
J
kHHdt
Jd
00
kHdt
d
0
00 H
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Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
Longitudinal Vector OX at the transverse position X
Net LongitudinalVector: Zero
Net TransverseVector: Zero
Net LongitudinalVector: Zero
Net TransverseVector: Zero
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Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
Lower Energy
Level
Higher Energy
Level
S
N
H
Lower Energy
Level
Higher Energy
Level
S
N
H0
Figure 4.15 (a). Nuclei aligned under thermal equilibrium in the presence of an external magnetic field. (b). A non-zero net longitudinal vector and a zero transverse vector provided by the nuclei precessing in the presence of an external magnetic field.
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Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
Non-zero Net Longitudinal Vector
x
y
z
x
y
zH0
Net Zero Transverse Vector
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MRI PrinciplesMRI PrinciplesThe precession frequency
◦Depends on the type of nuclei with a specific gyromagnetic ratio and the intensity of the external magnetic field
◦This is the frequency on which the nuclei can receive the Radio Frequency (RF) energy to change their states for exhibiting nuclear magnetic resonance
◦The excited nuclei return to the thermal equilibrium through a process of relaxation emitting energy at the same precession frequency
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MRI PrinciplesMRI Principles90-degree pulse
◦Upon receiving the energy at the Larmor frequency, the transverse vector also changes as nuclei start to precess in phase
◦Form a net non-zero transverse vector that rotates in the x-y plane perpendicular to the direction of the external magnetic field
Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
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Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
S
N
S
N x
y
z
Figure 4.16. The 90-degree pulse causing nuclei to precess in phase with the longitudinal vector shifted clockwise by 90-degrees as a result of the absorption of RF energy at the Larmor frequency.
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MRI PrinciplesMRI Principles180-degree pulse
◦If enough energy is supplied, the longitudinal vector can be completely flipped over with a 180-degree clockwise shidf in the direction against the external magnetic field
Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
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Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
S
N
S
N
x
y
z
Figure 4.17. The 180-degree pulse causing nuclei to precess in phase with the longitudinal vector shifted clockwise by 180-degrees as a result of the absorption of RF energy at the Larmor frequency.
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MRI PrinciplesMRI PrinciplesRelaxation
◦The energy emitted during the relaxation process induces an electrical signal in a RF coil tuned at the Larmor frequency
◦The free induction decay of the electromagnetic signal in the PF coil is the basic signal that is used to create MR images
◦The nuclear excitation forces the net longitudinal and transverse magnetization vectors to move
Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
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MRI PrinciplesMRI PrinciplesA stationary magnetization
vector
The total response of the spin system
Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
N
nnM
1
1
0
2
)(
T
kMM
T
jMiMHM
dt
Md zzyx
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Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
Dephasing
RF Pulse
Random Phase (Zero Transverse Vector)
In Phase Spin
Relaxation
Figure 4.18. The transverse relaxation process of spinning nuclei.
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MRI PrinciplesMRI PrinciplesThe longitudinal and transverse
magnetization vectors with respect to the relaxation times
where
Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
tiTtyxyx eeMtM 02/
,, )0()(
11 //0 )0()1()( Ttz
Ttzz eMeMtM
piyxyx eMM 0)0()0( ',',
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Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
t
t
Mx,y (t)
Mz (t)
Figure 4.19. (a) Transverse and (b) longitudinal magnetization relaxation after the RF pulse.
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MRI PrinciplesMRI PrinciplesThe RF pulse causes nuclear
excitation changing the longitudinal and transverse magnetization vectors
After the RF pulse is turned off, the excited nuclei go through the relaxation phase emitting the absorbed energy at the same Larmor frequency that can be detected as an electrical signal, called the Free Induction Decay (FID)
Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
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MRI PrinciplesMRI PrinciplesThe NMR spin-echo signal (FID
signal)
Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
dxdydzezyxMS zyxizyx
zyx )(0 ),,(),,(
zyxzyxi
zyx dddeSMzyx zyx )(0 ),,(),,(
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MR InstrumentationMR InstrumentationThe stationary external magnetic
field◦Provided by a large superconducting
magnet with a typical strength of 0.5 T to 1.5 T
◦Housing of gradient coils◦Good field homogeneity, typically on
the order of 10-50 parts per million◦A set of shim coils to compensate for
the field inhomogeneity Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
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Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
Gradient Coils
Magnet
Gradient Coils
RFCoils
PatientPlatform
MonitorMonitor Data-Acquisition
System
Figure 4.20. A general schematic diagram of a MR imaging system.
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Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
Figure comes from the Wikipedia, www.wikipedia.org.
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Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
Figure comes from the Wikipedia, www.wikipedia.org.
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MR InstrumentationMR InstrumentationAn RF coil
◦To transmit time-varying RF pulses◦To receive the radio frequency
emissions during the nuclear relaxation phase
◦Free Induction Decay (FID) in the RF coil
Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
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MR Pulse SequencesMR Pulse SequencesNMR signal
◦The frequency and the phaseSpatial encoding in MR imaging
◦Frequency encoding and phase encoding
Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
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Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
x
z
y
y
xx
y
z
z
Sagital
Coronal
Axial
Figure 4.21 (a). Three-dimensional object coordinate system with axial, sagittal and coronal image views. (b): From top left to bottom right: Axial, coronal and sagittal MR images of a human brain.
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Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
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MR Pulse SequencesMR Pulse Sequences
Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
Z Gradient
90 RF Pulse(Slice Selection)
X Gradient
Phase-Encoding(x-scan selection)
Z Gradient
180 RF Pulse(Slice Echo Formation)
Y Gradient
Frequency Encoding(Read-Out Pulse)
Figure 4.22. (a): Three-dimensional spatial encoding for spin-echo MR pulse sequence. (b): A linear gradient field for frequency encoding. (c). A step function based gradient field for phase encoding.
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Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
Varying Spatially DependentLarmor Frequency
S
N
S
N
Linear Gradient
Precessing Nuclei
External Magnet
Positive PhaseChange
Negative PhaseChange
Phase -EncodingGradientStep
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MR Pulse SequencesMR Pulse SequencesFrequency encoding
◦A linear gradient is applied throughout the imaging space a long a selected direction
◦The effective Larmor frequency of spinning nuclei is also spatially encloded along the direction of the gradient
◦Slice selection for axial imaging
Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
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MR Pulse SequencesMR Pulse SequencesThe phase-encoding gradient
◦Applied in steps with repeated cycles◦If 256 steps are to be applied in the
phase-encoding gradient, the readout cycle is repeated 256 times, each time with a specific amount of phase-encoding gradient
Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
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Spin Echo ImagingSpin Echo Imaging :
◦Between the application of the 90 degree pulse and the formation of echo (rephasing of nuclei
:◦Between the 90 degree pulse and
180 degree pulse
Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
ET
2/ET
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Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
RF Energy: 90 Deg Pulse
Zero Net Vector:Random Phase
RelaxationDephasing
RF Energy: 180 Deg Pulse
Echo -Formation
RF Energy: 90 Deg Pulse
Zero Net Vector:Random Phase
In Phase
Rephasing
Echo -Formation
In Phase
Figure 4.23. The transverse relaxation and echo formation of the spin echo MR pulse sequence.
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Spin Echo ImagingSpin Echo ImagingK-space
◦The placement of raw frequency data collected through the pulse sequences in a multi-dimensional space
◦By taking the inverse Fourier transform of the k-space data, an image about the object can be reconstructed in the spatial domain
◦The NMR signals collected as frequency-encoded echoes can be placed as horizontal lines in the corresponding 2-D k-space
Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
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Spin Echo ImagingSpin Echo ImagingK-space
◦As multiple frequency encoded echoes are collected with different phase-encoding gradients, they are placed as horizontal lines in the corresponding k-space with the vertical direction representing the phase-encoding gradient values
Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
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Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
Figure comes from the Wikipedia, www.wikipedia.org.
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Spin Echo ImagingSpin Echo Imaging : the cycle repetition time weighted
◦A long and a long weighted
◦A short and a shortSpin-density
◦A long and a short
Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
RT
2T
RT ET
1T
RT ET
RT ET
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Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
90 deg RF pulse
180 deg RF pulse
Gz: Slice Selection Frequency EncodingGradient
Gx: Phase EncodingGradient
Gy: Readout Frequency EncodingGradient
TE /2
TE /2
TE
RF pulseTransmitter
NMRRF FIDSignal
Figure 4.24. A spin echo pulse sequence for MR imaging.
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Spin Echo ImagingSpin Echo Imaging
The effective transverse relaxation time from the field inhomogeneities
Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
12 1),,(),,( 0T
T
T
T RE
eezyxzyx
2
11
2*
2
H
TT
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Spin Echo ImagingSpin Echo ImagingThe effective transverse
relaxation time from a spatial encoding gradient
Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
2
11*
2**
2
Gd
TT
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Inversion Recovery Inversion Recovery ImagingImagingIR imaging
◦IR imaging pulse sequence allows relaxation of some or all of before spins are rephased through 90-degree pulse and therefore emphasizes the effect of longitudinal magnetization
◦ 180-degree pulse is first applied along with the slice selection frequency encoding gradient
Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
1T
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Echo Planar ImagingEcho Planar ImagingA single-shot fast-scanning
methodSpiral Echo Planar Imaging (SEPI)
◦where
)(1
)( tdt
dtG xx
)(1
)( tdt
dtG yy
tttx cos )(
ttty sin )(
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Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
90 deg RF pulse
90 deg RF pulse
Gz: Slice Selection Frequency EncodingGradient
Gx: OscillatingGradient
Gy: Readout Gradient
RF pulseTransmitter
NMRRF FIDSignal
Figure 4.25. A single shot EPI pulse sequence.
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Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
gy
gx
x
y
Figure 4.26. The k-space representation of the EPI scan trajectory.
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Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
x
y
SEPI Trajectory
Data SamplingPoints
Figure 4.27. The spiral scan trajectory of SEPI pulse sequence in the k-space.
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Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
90 deg RF pulse
180 deg RF pulse
Gz: Slice Selection Frequency EncodingGradient
Gx Gradient
Gy Gradient
TE /2
TE /2
TE
RF pulseTransmitter
NMRRF FIDSignal
TD
Figure 4.28. The SEPI pulse sequence
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Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
Figure 4.29. MR images of a human brain acquired through SEPI pulse sequence.
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Gradient Echo ImagingGradient Echo ImagingFast low angle shot (FLASH) imaging
◦Utilize low-flip angle RF pulses to create multiple echoes in repeated cycles to collect the data required for image reconstruction
◦A low-flip angle (as low as 20 degrees)◦The readout gradient is inverted to re-
phase nuclei leading to the gradient echo during the data acquisition
◦The entire pulse sequence time is much shorter than the spin echo pulse sequence
Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
![Page 61: Medical Image Analysis](https://reader037.vdocuments.site/reader037/viewer/2022110101/5681322e550346895d98947a/html5/thumbnails/61.jpg)
Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
Low Flip Angle RF pulse
Gz: Slice Selection Frequency EncodingGradient
RF pulseTransmitter
Gx: Phase EncodingGradient
Gy: Readout Frequency EncodingGradient TE
NMRRF FIDSignal
Figure 4.30. The FLASH pulse sequence for fast MR imaging.
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Flow ImagingFlow ImagingTracking flow
◦Diffusion (incoherent flow) and perfusion (partially coherent flow)
◦The FID signal generated in the RF receiver coil by the moving nuclei and velocity-dependent factors
MR angiography
Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
![Page 63: Medical Image Analysis](https://reader037.vdocuments.site/reader037/viewer/2022110101/5681322e550346895d98947a/html5/thumbnails/63.jpg)
Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
90 deg RF pulse
180 deg (selective)RF pulse
Gz: Slice Selection Frequency EncodingGradient
Gx: Phase EncodingGradient
Gy: Readout Frequency EncodingGradient
TE /2
TE
RF pulseTransmitter
NMRRF FIDSignal
Figure 4.31. A flow imaging pulse sequence with spin echo.
![Page 64: Medical Image Analysis](https://reader037.vdocuments.site/reader037/viewer/2022110101/5681322e550346895d98947a/html5/thumbnails/64.jpg)
Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
Figure 4.32: Left: A proton density image of a human brain. Right: The corresponding perfusion image.
![Page 65: Medical Image Analysis](https://reader037.vdocuments.site/reader037/viewer/2022110101/5681322e550346895d98947a/html5/thumbnails/65.jpg)
Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
90 degree RF pulse
Gz: Slice Selection Frequency EncodingGradient
RF pulseTransmitter
Gx: Phase EncodingGradient
Gy: Readout Frequency EncodingGradient
TE NMRRF FIDSignal
Next 90 degree RF pulse
TR
Figure 4.33. Gradient echo based MR pulse sequence for 3-D MR volume angiography.
![Page 66: Medical Image Analysis](https://reader037.vdocuments.site/reader037/viewer/2022110101/5681322e550346895d98947a/html5/thumbnails/66.jpg)
Figures come from the textbook: Medical Image Analysis, Second Edition, by Atam P. Dhawan, IEEE Press, 2011.
Figure 4.34. An MR angiography image.
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Flow ImagingFlow Imaging
Figure comes from the Wikipedia, www.wikipedia.org.
angiography image
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FMRIFMRIfMRI imaging
◦Measure blood oxygen level during sensory stimulation or any task that causes a specific neural activity
◦Visual or auditory stimulation, finger movement, or a cognitive task
◦Blood oxygenated level dependence (BOLD)
◦Oxygenated hemoglobin ( ) is diamagnetic, while deoxygenated hemoglobin ( ) is paramagnetic
2HbO
Hb
![Page 69: Medical Image Analysis](https://reader037.vdocuments.site/reader037/viewer/2022110101/5681322e550346895d98947a/html5/thumbnails/69.jpg)
Figure comes from the Wikipedia, www.wikipedia.org.
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FMRIFMRIfMRI imaging
◦A reduction of the relative deoxy-hemoglobin concentration due to an increase of blood flow and hence increased supply of fresh oxy-hemoglobin during neural activity is measured as an increase in or weighted MR signals
2T2T
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Diffusion ImagingDiffusion ImagingDiffusion process
◦Water molecules spread out over time that is represented by Brownian motion
◦An anisotropic Gaussian distribution along a given spatial axis such that the spread of the position of molecules after a time along a spatial axis can be represented with a variance of
◦where is diffusion coefficient in the tissue
Tx
D
DTx 22
![Page 72: Medical Image Analysis](https://reader037.vdocuments.site/reader037/viewer/2022110101/5681322e550346895d98947a/html5/thumbnails/72.jpg)
Figure comes from the Wikipedia, www.wikipedia.org.
DTI color image
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Diffusion ImagingDiffusion ImagingDiffusion process
◦Anisotropic diffusion in the white matter
◦Isotropic diffusion in the gray matter◦Motion probing gradients (MPG) to
examine the motion of water molecules in the diffusion process in a specific direction
◦The MR FID signal is decreased for healthy tissue, and increased with trapped-in water molecules
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Diffusion ImagingDiffusion ImagingDiffusion process
◦where is the gyromagnetic ratio, is diffusion coefficient, and is the strength of two MPG gradients each with duration separated by applied in spatial directions
DNGeSS )/(0
222
DG
N
![Page 75: Medical Image Analysis](https://reader037.vdocuments.site/reader037/viewer/2022110101/5681322e550346895d98947a/html5/thumbnails/75.jpg)
Diffusion ImagingDiffusion ImagingDiffusion process
z
y
x
zzzyzx
yzyyyx
xzxyxx
zyx
u
u
u
DDD
DDD
DDD
uuuD ),,(
321][trace zzyyxx DDDD
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Diffusion ImagingDiffusion ImagingDiffusion process
◦Fractional anisotropy (FA)
◦Multiple sclerosis, strokes, tumors, Parkinson’s and Alzheimer’s disease
◦Attention deficit hyperactivity disorder (ADHS)
23
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2
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FA
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Contrast, Spatial Resolution, Contrast, Spatial Resolution, and SNRand SNRSpin-echo imaging pulse
sequence
Inversion recovery (180-90-180) imaging pulse sequence
211 T
T
T
T ER
eekS
21121 T
T
T
T
T
T ERI
eeekS
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Contrast, Spatial Resolution, Contrast, Spatial Resolution, and SNRand SNRGradient echo imaging pulse
sequence
1
*21
cos1
sin1
T
T
T
T
T
T
R
ER
e
ee
kS
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Contrast, Spatial Resolution, Contrast, Spatial Resolution, and SNRand SNRParamagnetic contrast agent
◦gadolinium (Gd) to change the susceptibility of the net magnetization vector
◦Reduces relaxation time and increases the signal intensity of -weighted images
Noise and field inhomogeneities◦RF noise, field inhomogeneities,
motion, chemical shift
1T1T
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Contrast, Spatial Resolution, Contrast, Spatial Resolution, and SNRand SNRChemical shift
◦The deviation of its effective resonance frequency in the presence of other nuclei from a standard reference without any other nuclei with their local magnetic fields present
◦ppmref
ref
610)(
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Contrast, Spatial Resolution, Contrast, Spatial Resolution, and SNRand SNR
◦Induced magnetic field in alkenes
◦Induced magnetic field in alkynes
Figure comes from the Wikipedia, www.wikipedia.org.