Post on 05-Jan-2016
description
Contrast Mechanisms in MRI
Introduction to Cardiovascular Engineering
Michael Jay Schillaci, PhDManaging Director, Physicist
Tuesday, September 16th, 2008
Overview
Image Acquisition Basic Pulse Sequences Unwrapping K-Space Image Optimization
Contrast Mechanisms Static and Motion Contrasts
T1 & T2 Weighting, Field Strength, T2*, Dephasing Endogenous Contrasts
BOLD Imaging Motion Contrasts
Time of Flight, Diffusion, Perfusion
Basic Pulse Sequences
Image Formation Integrate magnetization to get MRI signal
Select a z “slice” and form image of XY plane variations
Contrast from difference in magnetization in different tissues— Image at several times to get average
dxdyetyxMtSArea
dtyGxGi
XYz
t
YX
0,,)(
Horizontal density
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tica
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nsit
y
Basic MRI Scan Terminology Orientation:
Coronal Sagittal Axial
Matrix Size: # of Voxels in dimension
Field of view (FOV): Spatial extent of dimension
Resolution: FOV/Matrix size.
Axial Orientation64x64 Matrix192x192mm FOV3x3mm Resolution
Sagittal Orientation256x256 Matrix256x256mm FOV1x1mm Resolution
CoronalCoronal SagittalSagittal AxialAxial
Image Creation
The scanning process1. Protocol sets Gradients and Encodes K-Space Weights2. Signal is Determined with Fourier Transform
3. Image Created with Inverse Transform
dtGkt
xx 0
2
dtGkt
yy 0
2
1
0
1
0
,,x
ln
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ln
y N
nyx
xtikytik
yx
N
lnl dkdkeekksyxm
dxdyetyxMtSArea
dtyGxGi
XYz
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Step 1
Step 2
Step 3
Image Acquisition
sxpeyy TGTGy
FOV
221
TNGTGxFOV
sxsxx
221
Gy varies in each cycle
Data Acquisition (DAQ)
Gradient Field Ensures Field Greater on “Top”
Larmor Frequency Depends on z Position
RF pulse Energizes “Matched” Slice
Slice Selection Gradient: Gsl
Fie
ld S
tren
gth
Z Position
Frequency Encoding Gradient: Gro
Apply transverse gradient when we wish to acquire image.
Slice emits signal at Larmor frequency, e.g. lines at higher fields will have higher frequency signals.
X P
ositi
on
Field Strength
Phase Encoding Gradient: Gpe
Apply Orthogonal RF pulse Apply before readout Adjusts the phase along the dimension (usually Y)
Y P
ositi
on
Field Strength
Unwrapping K-Space
TNGTGxFOV
sxsxx
221
sxpeyy TGTGy
FOV
221
x
x
N
FOVx
Image Adapted from Prof. Yao Wang’s Medical Imaging course notes at: http://eeweb.poly.edu/~yao/EL5823
y
y
N
FOVy
Pixel Size:
Field of View:
Choose phase encoding
time so that yx
Image Optimization
Adjustment of Flip Angle Parameter Maximum SNR typically between 30 and 60 degrees Long TR sequences (2D)
Increase SNR by increasing flip angle Short TR sequences (TOF & 3D)
Decrease SNR by increasing flip angle
1cos TTR
E e
*2
1
1
cos1
1sin0
TTE
TTR
TTR
Spoil ee
eMS
Maximizing the signal
gives the: Ernst Angle:
1. Assume perfect “spoiling” -transverse magnetization is zero before each excitation:
2. Spin-Lattice (T1) Relaxation occurs between excitations:
1. Assume steady state is reached during repeat time (TR):
2. Spoiled gradient rephases the FID signal at echo time (TE):
Gradient Echo Imaging
coszAzB MM
11 10
TTR
TTR
zBzC eMeMM
*2
1
1
cos1
1sin0
TTE
TTR
TTR
Spoil ee
eMS
*2sin T
TE
zASpoil eMS
zAzC MM
Spin Echo Imaging
Spin echo sequence applies a 180º “refocusing pulse” Half way between 90º pulse and DAQ Allows measurement of true T2 time
T2
T2*
The “Refocusing Pulse”
Sig
na
l
0
1
T2
T2*
0.5 TE 0.5 TE
Actual Signal
Spins Rotate at Different Rates Refocusing Pulse Re-Aligns Spins
1mm Gap
2mm Thick
Volume Reconstruction 3D volumes
composed of 2D slices
Slice thickness. Thicker slices have more hydrogen so
more signal (shorter scan time) Thinner slices provide higher resolution
(longer scan time)
Optional: gap between slices. Reduces RF interference (SNR)
Fewer slices cover brain3mm
NVSNR
Static Contrast Mechanisms
T1 and T2 Weighting T1 Contrast
Echo at T2 min Repeat at T1 max
T2 Contrast Echo at T2 max Repeat at T1 min
Net Magnetization is
T1 Contrast Weighting
T2 Contrast Weighting
TE
TR TE
TR
Min T2 Contrast Max T1 Contrast
Max T2 Contrast Min T1 Contrast
decay
T
TE
eryre
T
TR
XY eeMM
2
cov
10 1
Static Contrast Images
T2 Weighted Image (T2WI)(Gray Matter – CSF Contrast)
T1 Weighted Image (T1WI)(Gray Matter – White Matter)
Examples from the Siemens 3T
“Anatomical Image” “Diagnostic Image”
Flip Angle Variation
RF Pulse Magnitude Determines Flip Angle Duration and magnitude are important
cosMM Z
Adapted from: http://www.mri.tju.edu/phys-web/1-T1_05_files/frame.htm
B0
M
BC
MZ
+z
+x
+yMXY
sinMM XY
Field Strength Effects
Increased field strength Net magnetization in material is greater Increased contrast means signal is increased Image1 resolution is better
1MRI adapted from: http://www.mri.tju.edu/phys-web/1-T1_05_files/frame.htm
Muscle
Tissue
Tissue Contrast and Dephasing
Dephasing of H2O and Fat MRI signal is a composite of Fat and H2O signals
H2O and Fat resonate at different frequencies
T1F = 210 ms, T1W = 2000 ms ( T1F > T1W → fat is brighter) Relative phase gives TE dependence
Anti-Parallel (ΦFW = 180o )
@ TE = 15.66 ms
Parallel ( ΦFW = 0o )
@ TE = 13.42 ms
ΦFW
MF
MW
Endogenous Contrast
BOLD Imaging
Blood Oxyenation Level Dependent Contrast dHb is paramagnetic, Hb is less Susceptibility of blood increases linearly with oxygenation BOLD subject to T2* criteria
Oxygen is extracted from capillaries Arteries are fully oxygenated Venous blood has increased proportion of dHb Difference between Hb and dHb is greater for veins Therefore BOLD is result of venous blood changes
Sources of the BOLD Signal
Neuronal activity Metabolism
Blood flow
Blood volume
[dHb]BOLDsignal
BOLD is a very indirect measure of activity…
Neuronal Origins of BOLD
Adapted from Logothetis et al. (2002)
BOLD response predicted by dendritic activity (LFPs)
Increased neuronal activity results in increased MR (T2*) signal
LFP=Local Field Potential; MUA=Multi-Unit Activity; SDF=Spike-Density Function
BASELINE ACTIVE
The BOLD Signal
BASELINE ACTIVE
BOLD Imaging
Blood Oxyenation Level Dependent Contrast Susceptibility of blood changes with oxygenation Blood flow correlated with task performance Differential activations can be mapped
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0.50
1.00
1.50
2.00
-5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
0
2
Static Contrast - T2* Relaxation
T2* accounts for magnetic defects and effects
T2 is relaxation due to spin-spin interaction of nuclei
T2M is relaxation induced by inhomogeneities of main magnet
T2MS is relaxation induced by magnetic susceptibility of material
0BM
MSM TTTT 2
1
2
1
2
1
2
1*
mM
BOLD artifacts
fMRI is a T2* image – we will have all the artifacts that a spin-echo sequence attempts to remove.
Dephasing near air-tissue boundaries (e.g., sinuses) results in signal dropout.
BOLDNon-BOLD
Motion Contrast
Flow Weighting
• Time-of-Flight Contrast
No Flow
Medium Flow
High Flow
No Signal
Medium Signal
High Signal
Vessel
AcquisitionSaturation Excitation
Vessel Vessel
Diffusion Coefficients Magnitude (ADC) Maps “Proton pools” Direction (Anisotropy) Maps “Velocity” Reconstruct Fiber Tracks with “Clustering”
Diffusion Tensor ImagingADCADC AnisotropyAnisotropy
Dtl 2322
3
2TGD
oeSS
FA VectorMD
Indices of Diffusion Anisotropy
2 2 2
1 2 3 3RA
Relative anisotropy:
Fractional anisotropy: 2 2 2 2 2 2
1 2 3 1 2 33 2FA
Healthy
DTI in Stroke Research Examine integrity of fiber tracts
Tractography - trace white matter paths in gray matter Assess neglect as a disconnection syndrome
Stroke
Arterial Spin Labeling
Perfusion Flow of fluid into vessels to supply nutrients/oxygen
The amount and direction of flow matters
AlternatingAlternatingInversionInversion
Pulsed Labeling
AlternatingInversion
Imaging PlaneImaging Plane
FAIRFAIRFlow-sensitive Alternating IRFlow-sensitive Alternating IR
EPISTAREPISTAREPI Signal Targeting with Alternating RadiofrequencyEPI Signal Targeting with Alternating Radiofrequency
ASL Pulse Sequences
RF
Gx
Gy
Gz
Image
90o 180o
Alternating oppositeDistal Inversion
OddScan
EvenScan
180o
RF
Gx
Gy
Gz
Image
90o180o180o
AlternatingProximal Inversion Odd Scan
Even Scan
FA
IRF
AIR
EP
IST
AR
EP
IST
AR
Flow-sensitive Alternating IRFlow-sensitive Alternating IR
EPI Signal Targeting with Alternating RadiofrequencyEPI Signal Targeting with Alternating Radiofrequency