fund bioimag 2013 13-1 13: advanced mri contrast mechanisms 1.how does moving blood affect the image...
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Fund BioImag 201313-1
13: Advanced MRI Contrast Mechanisms
1. How does moving blood affect the image phase ? 2. What is the effect of self-diffusion on the MR signal ?3. Why is diffusion in vivo not isotropic ?
• Fiber tracking4. How do the different imaging modalities compare ?
• Capabilities• Limitations• Choice• Comparison by examples
After this week you1. Understand the influence of motion on the phase of magnetization2. Understand how random motion leads to echo amplitude reduction 3. Are able to calculate the attenuation of the MR signal due to diffusion 4. Understand how diffusion-weighted MRI signal reflects cellular structure and
how this can be exploited to track nerve fibers, among others5. Have a firm grasp on the premises and limitations of the imaging modalities
covered in this course
Fund BioImag 201313-2
Blood moving with velocity v
timeT 2T0
13-1. How does Bulk Motion affect the Rephased Signal ?(Blood Flow)
Freq. Encode (Gx)
TE T T
T xx dtvtxGdtvtxGT0
2)()()2(
T
T x
T
x dttxGdttxGt2
0)()(
2)2( GvTT
x(t)=x0+vt
T
x
tvxtG
0
2
)2
( T
T
x
tvxtG
22
)2
(
f(t)
For transverse magnetization at point (x,y):
xtikxdttGixx eeyxm )()(
),(
Phase f of the magnetization:
)()0()( tieMtM
(Gradient along x)
f does not depend on x
Entire echo has phase f at TE
2
2),(
TE
Gvii eeyxm
f(TE)
t
x dttxtGt0
')'()'(
Fund BioImag 201313-3
13-2. How does self-Diffusion influence the MR signal ?
<r> = 20 mmD = 0.1 s
<r> = 45 mmD = 0.5 s
<r> = 63 mmD = 1 s
Dr 6
Einstein random walk:
D: self diffusion coefficient
<r>: root mean square displacement after D seconds
Fund BioImag 201313-4
What is the effect of random motion on magnetization phase ?when applying pulsed gradient
RF
G
90°
a
b
c
d
a b c d
Static magnetization:
Magnetization in motion:
stationary
spins
Displacedspins
net transverse magnetization
c: Particle displaces by r
Fund BioImag 201313-5
Ex. Effect of Diffusion on MagnetizationPhase f of Mxy
Inco
here
nt m
otio
n
Absence of incoherent motion: Echo formation
timeT
TG(t)
f(t) All in-phase: max. echo formation
f(t)
Not all in-phase: reduced echo amplitude
No diffusion
With diffusion
f
Fund BioImag 201313-6
gradient echo, i.e. sensitive to T2*
How is the effect of diffusion on the MR signal described ?
Mathematical description
Degree of echo signal reduction1. Strength of the diffusion process (D)
2. Delay between dephasing and rephasing gradient (D)
3. Area of the dephasing gradient (strength G, duration d) D
d
G
bDoeSbS )( 3/2 Gb
Attenuation of the signal (echo amplitude) due to diffusion in the direction of G
D: apparent diffusion coefficient (ADC)
D
d
GG
1800
RF
Equivalent sequence (spin echo, i.e. sensitive to T2)
Fund BioImag 201313-7
13-3. How is Anisotropic Water Diffusion described ?
Consider structure along (myelinated) axon (or myofibril)
Anisotropic mean displacement Anisotropic diffusion coefficient
Diffusion coefficient depends on gradient orientation
→ Diffusion tensor Dij
zzzyzx
yzyyyx
xzxyxx
DDD
DDD
DDD
D
Motion (diffusion) of water molecules:
Restricted by cell membranes
Corpus callosum
CSF (isotropic)
GxGz Gy
Fund BioImag 201313-8
Diffusion tensor imaging (DTI)imaging anisotropic diffusion
λ3
λ1λ2
3
2
1
00
00
00
DT
Diffusion tensor symmetric: Dij = Dji
3 orthogonal Eigenvectors
→ Eigenvalues li
For each voxel determine direction of principal eigenvector (largest l):
Pseudocolor directionality
Mean diffusivity (trace Dij)
Fractional Anisotropy
(l1-l3)
Fund BioImag 201313-9
Application: Fiber Tracking using Diffusion MRI
from diffusion anisotropy to connectivity1. Image of diffusion anisotropy
2. Directionality of water diffusion connects adjacent voxels (spaghettis)
3. Establish fiber tracks
Fund BioImag 201313-10
13-4. Bio-imaging modalities comparison
I. contrast and limitations
Major limitationsstrong e- density differences (bone)
Ionizing radiation
g emitters available
non-uniform spatial resolution & sensitivity
sensitivity
time-consuming & motion-sensitive
complex methodology
does not penetrate hard objects (e.g. bone)
Major limitationsstrong e- density differences (bone)
Ionizing radiation
g emitters available
non-uniform spatial resolution & sensitivity
sensitivity
time-consuming & motion-sensitive
complex methodology
does not penetrate hard objects (e.g. bone)
Contrast mechanismsContrast mechanisms
CT e- density, Z
MR (Spin concentration)
Relaxation of magnetization
Fat/Water (chemical shift)
Diffusion
(etc …)
SPECT
PETTracer distribution in tissue
USBoundaries of tissues with different mechanical properties
Fund BioImag 201313-11
Comparison IISNR, reconstruction, contrast agents
Image reconstructionImage reconstruction
CT
SPECT
PET
Directionality of photon
→Radon transform
Projection reconstruction
MRprecession of M (gradient G)→ Frequency analysis Fourier transform
Contrast agents(contrast modifiers)
Contrast agents(contrast modifiers)
CT, x-ray Compounds with high Z
MR Compounds shortening relaxation times (T1, T2, or T2*)
Maximize SNRMaximize SNRCT Increase radiation dose
MR Increase magnetic field
SPECT
PETIncrease tracer dose
Limited by
Effective radiation dose
Equilibrium magnetization(Boltzmann distribution)
Scatter noiseRadiation dose
Fund BioImag 201313-12
Which bioimaging modality is right for you ?
Rapid and least invasive assessment of tissue close to surface
Contrast between air-tissue or bone-tissue
Rapid scan with high spatial resolution
Image receptors, glucose metabolism, transport, perfusion
Biochemical information of tissue
Exquisite soft tissue contrast with mm spatial resolution (rodent 100µm)
Functional information
US
X-ray, CT
SPECT, PET
NMR spectroscopy
MRI
MRI
Immobile spins
BoneAir
Air-Tissue
interface
multiple exposures
(ionizing radiation)
Metallic implants
& devices
Moving blood (angiography)MRI (multiple means)CT (contrast agents)
Doppler US
Fund BioImag 201313-13
13-5. Comparison of imaging modalities: Brain
Skull gets in the way of X-ray imaging:– Bone scatters X-rays much more than soft tissue– MRI radio waves pass unimpeded through bone
Images have been “skull stripped”
Same patient
FBPA-PET
PETMRI (T1)
Contrast agents
CT
MRI
pre post
MRI (T1)
MRI (T2)
TE=30 ms TE=80 ms
FDG-PET MRI (T1)
MRI (T2)
post
High grade astrocytoma
Fund BioImag 201313-14
http://www.eradimaging.com/site/article.cfm?ID=327
PET MRI
SPECT
MRI
Heart
Lung LiverWhole body
CT
MRI
US
3D CT of mouse
Comparison of modalities: Body