advanced mri techniques (and applications) · advanced mri techniques (and applications) jeffry r....
TRANSCRIPT
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Advanced MRI Techniques (and Applications)
Jeffry R. Alger, PhD Department of Neurology
Ahmanson-Lovelace Brain Mapping Center Brain Research Institute
Jonsson Comprehensive Cancer Center University of California, Los Angeles
K30 course – January 2012
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(Selected) Advanced MRI Techniques • Gradient Echo MRI
– High resolution anatomic imaging – Magnetic Resonance Angiography
• Echo Planar MRI • Diffusion MRI
– Diffusion-weighted MRI – Diffusion Tensor MRI
• Perfusion MRI (contrast passage) • Diffusion and Perfusion MRI for acute
stroke assessment
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Gradient Echo MRI
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Gradient (Recalled) Echo Imaging
• More efficient than “conventional” forms of MRI – Makes 3-dimensional MRI practical – Time needed for a 3D acquisition is 6-15 min
• TR is much shorter than in conventional
MRI
– Inherently produces T1-weighted contrast
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3-Dimensional Gradient Echo MRI • 3-dimensional gradient echo imaging is
becoming more practical due to advances in MRI hardware design
Conventional Spin Echo Multiple Slice T1-weighted MRI (3 mm slice thickness)
3D Volumetric T1-weighted MRI (1 mm isotropic voxels)
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Rendering of surface anatomy
from MRI
Right Lateral
Left Lateral
Superior
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3-Dimensional Gradient Echo Imaging (Thompson PM et al)
• Quantification of brain tissue composition
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Image-guided Neurosurgery
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Magnetic Resonance Angiography (MRA)
• Imaging of the blood vessels using MRI – Usually arteries – Sometimes veins
• Most MRA techniques use gradient echo MRI
– Time-of-flight (TOF) MRA – Contrast enhanced MRA – Phase contrast MRA (not covered in this lecture)
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TOF-MRA
• Arteries appear as very hyperintense signal
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Time-of-Flight MRA
1 mm slice from volume acquisition
Volume maximum intensity projection
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Magnetic Resonance Angiography (MRA) with Contrast Agents
Magnevist
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Contrast Enhanced MRA • Fast Gradient echo MRI
– Usually 2D – 2 - 10 sec per time frame
• Venous injection of MRI contrast agent
• Image acquisition is timed such that the image is
collected when the contrast agent is in the arteries
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Contrast Enhanced MRA
• Complete angiographic coverage from arotic arch to Circle of Willis
• Other peripheral vascular applications
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Echo Planar Imaging (EPI)
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Echo Planar Imaging (EPI)
• Collects an entire slice in one TR (2 – 4 sec) – Greater degree of image
distortion • 10-20 slice images per
second – Facilitates studies that
require large numbers of images
• Typically provides T2-weighted images
• Typical resolution of 2 mm x 2 mm x 5 mm
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EPI-based Applications
• Diffusion imaging
• Diffusion tensor imaging
• Perfusion imaging
• BOLD Functional imaging
• Uncooperative patients
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Diffusion MRI
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“Flavors” of Diffusion MRI
• Diffusion Weighted Imaging (DWI)
• Apparent Diffusion Coefficient Imaging (ADCI)
• Diffusion Tensor MRI (DTI)
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www.wikipedia.com
Robert Brown (1773 – 1858)
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www.wikipedia.com
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Diffusion-Weighted MRI (DWI)
D. Le Bihan. Nature Reviews Neuroscience 2003;4:469-480
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Diffusion-Weighted MRI (DWI)
• The degree to which the pulse sequence is sensitive to diffusion is expressed through the “b-value” or “b-factor”
• The signal intensity of diffusion weighted images are – inversely related to the b-value – directly related to the degree of diffusion restriction
(at a particular b-value) – directly related to T2
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MRI of Acute Stroke
DWI (b = 1000)
T2w T1w PDw
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MRI of Acute Stroke
DWI (b = 1000)
T2w T1w PDw
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Swollen intracellular volume Increased volume average DWI Reduced volume average ADC
Cell Swelling Hypothesis
Intracellular diffusion is slower than extracellular diffusion
IC EC Normal
IC EC Ischemic
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DWI Hyperintensity in Stroke
• A marker of cytotoxic (cellular) edema seen in acute ischemic stroke
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Diffusion MRI
• Typical diffusion-weighted MRIs are also T2-weighted
– “T2 shine through”
• Apparent Diffusion Coefficient (ADC) calculation using
a series of diffusion-weighted images provides diffusion rate without the influence of T2
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Diffusion-Weighted MRI (DWI)
D. Le Bihan. Nature Reviews Neuroscience 2003;4:469-480
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ADC Calculation
ln(Sb)
L/R gradient sensitization S/I gradient sensitization A/P gradient sensitization
0 500 1000
b-value
Average Slope = - ADC Directionally averaged gradient sensitization
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Apparent Diffusion Coefficient (ADC) Imaging
DWI (b = 1000)
T2w ADC
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Apparent Diffusion Coefficient
• Quantitative measure of diffusion rate – Higher values mean less restricted diffusion – Lower values mean more restricted diffusion
• A diffusion “speedometer”
• Independent of T2
• “Numbers” that are comparable across subjects
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Diffusion Tensor Imaging (DTI)
• Formal evaluation of the “directionality” of the diffusion
• Relies on the ability to measure the diffusion speed in any arbitrary direction – Possible because of the directional
specificity of the gradient hardware
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White Matter Diffusion Restriction
• Water diffuses more readily parallel to the fiber bundles than perpendicular to the fiber bundles
• Typical values of water diffusion coefficient in brain
– Gray matter or white matter (directionally averaged): 750 - 1000 micron2/sec
– Parallel to white matter bundle: 1200 – 1500 micron2/sec
– Perpendicular to white matter bundle: 200 – 400 micron2/sec
– CSF: 3000 micron2/sec
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ADCL/R
ADCA/P
ADCS/I
ADCL/R ADCA/P ADCS/I >> =
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Diffusion Tensor Imaging • Measure the component of the ADC
– In at least 6 unique anatomic directions – In each voxel
• Determine in each voxel
– The direction in which the ADC is maximal – The value of the directionally averaged
ADC
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Diffusion Tensor Imaging
• Voxel is likely white matter fiber if there
is a great difference in the ADC between directions
• The direction showing the maximal ADC is parallel to the fiber
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ICBM Protocol
UCLA
1.5 T Sonata
5 acquisitions
March 4, 2005
A/P fiber
bundles
L/R fiber
bundles
S/I fiber
bundles
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b0 color fa Trace (ADC)
ICBM Protocol
UCLA
1.5 T Sonata
1 acquisition
March 4, 2005
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Parametric DTI Images • Trace (or ADC) conveys the “average”
diffusion speed – ADC is a quantitative measure that can be
compared across patients
• Fractional Anisotropy (FA) conveys the degree of directional coherence within the voxel – FA is an “index” such that 0 < FA < 1.0 – FA is a quantitative measure that can be
compared across patients
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DTI Fiber Tractography
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White Matter Fiber Tracking with DTI
Connect the principal direction between voxels with smooth stream lines
R. Bammer et al
Eur J Radiol 2003;45:223-234
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White Matter Fiber Tracking with DTI
D. Jones Institute of Psychiatry King’s College, London
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Catani Curr Opin Neurol 2006
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Contrast Bolus Passage “Perfusion” MRI
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Perfusion
• Perfusion is a physiologically-defined parameter
• Passage of blood through capillaries
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Field Distortion Around Capillaries Filled with Contrast Agent
• Gradient echo sequences are sensitive to magnetic field gradients
• Signal from water molecules outside of the capillaries is reduced due to magnetic field gradients
Buxton RB. Introduction to Functional Magnetic Resonance Imaging Principles & Techniques. Cambridge 2002
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MRI Bolus Passage Perfusion Imaging Design
• Perfusion imaging is performed using a timed bolus of contrast given during rapid image acquisition – Gradient echo echo planar imaging conveys
sensitivity to the passage of contrast – 2 sec time resolution for each 3D image
• Image intensity becomes hypointense as
contrast passes
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Perfusion Imaging using Contrast Passage
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Contrast Bolus Passage Perfusion Imaging
• Qualitative information – Tissue volumes that experience low contrast delivery – Tissue volumes experience delayed contrast delivery
• Quantitative information
– Fractional Cerebral Blood Volume – Cerebral Blood Flow (limited accuracy) – Mean Transit Time (limited accuracy)
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CBV
ml/100 g
CBF (R0)
ml/100 g/min
FHS
CBF (Rmax)
ml/100 g/min
Bolus max
FHD FHK
0.1
0.5
0.0
50.0
0.0
8.0
0
100 100
0 0
10
14
4 4
14
TMAX
(sec)
TTP
(sec)
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DSC-MRI-CBF Images 3T RRUCLAMC-1
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0
100
0
1000
MP-RAGE GRE FLAIR
PET-CBF MRI-CBF
cc per 100g per min cc per 100g
per min
Normal Same day Slice # 23
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Stroke Diffusion and Perfusion MRI
• The combined use of diffusion and perfusion MRI for acute stroke is based on the concept of the “ischemic penumbra”
• The “ischemic penumbra” is believed to be what can be saved with prompt treatment
• The combination of diffusion and perfusion MRI offers a means of visualizing the size of the ischemic penumbra
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Stroke Penumbra Imaging Diffusion and Perfusion MRI
• Perfusion MRI shows the entire affected territory
• Diffusion MRI shows the already damaged and unrepairable territory
• The penumbra is the difference (the mismatch)
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Diffusion-Perfusion Mismatch
Tmax
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Diffusion-Perfusion Mismatch
Tmax
At Risk
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Diffusion-Perfusion Mismatch
Tmax DWI
At Risk
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Diffusion-Perfusion Mismatch
Tmax DWI
Core
At Risk
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Diffusion-Perfusion Mismatch
Tmax DWI
At Risk
Core
At Risk
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Diffusion-Perfusion Mismatch
Tmax DWI
At Risk
Core
Salvageable At Risk
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Stroke Diffusion and Perfusion MRI
• The combined use of diffusion and perfusion MRI for acute stroke is based on the concept of the “ischemic penumbra”
• The “ischemic penumbra” is believed to be what can be saved with prompt treatment
• The combination of diffusion and perfusion MRI offers a means of visualizing the size of the ischemic penumbra
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Advanced MRI Techniques • Gradient Echo MRI
– 3D brain morphometry – Magnetic Resonance Angiography – Neurosurgical planning
• Diffusion MRI
– Diffusivity Imaging • Stroke assessment
– Diffusion Tensor Imaging • Visualization of white matter structure • Visualization of white matter connections
• Perfusion MRI (contrast passage)
• Stroke assessment
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