susceptibility weighted imaging e.m. haacke, y. xu, y. n. cheng, j. r.reichenbach. susceptibility...
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Susceptibility Weighted ImagingE.M. Haacke, Y. Xu, Y. N. Cheng, J. R.Reichenbach. Susceptibility Weighted
Imaging (SWI). MRM 52:612-618 (2004).
MRSRL Journal Club08/13/10
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Background & Motivation for SWI
• MRI often relies on magnitude images (clinic).• Phase images contain information about local
susceptibility changes between tissues.• Susceptibility differences can be used as a
new type of contrast ~ T1, T2, T2*, PD.
2E.M. Haacke, Y. Xu, Y. N. Cheng, J. R.Reichenbach. Susceptibility Weighted Imaging (SWI). MRM 52:612-618 (2004).
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Applications of SWI
• SWI offers information about tissues with different susceptibilities from surrounding tissues.– deoxygenated blood, hemosiderin, ferritin, calcium
• Numerous Clinical applications– Hemorrhages– Cerebrovascular and ischemic brain diseases– Traumatic brain injuries– Arteriovenous malformations– Neurodegenerative diseases– Breast microcalcifications
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Current Scanners
• Packaged SWI technique available on Siemens Medical Systems.
• SWAN on GE scanner: T2 Star Weighted Angiography. (multi-echo GRE) uses echoes.
4c/o Catherine Moran
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Magnetic Susceptibility• When an object is placed in an external magnetic
field H, magnetization is induced in the object.• Magnetic susceptibility is the magnetic response
of a material when it is placed in a magnetic field.– M = χH– χ = susceptibility (ppm)– M = induced magnetization– H = applied field
• If diamagnetic, like Ca3(PO)4, χ < 0• If paramagnetic, like deoxygenated blood, χ > 0
E.M. Haacke et. Al. Susceptibility-Weighted Imaging: Technical Aspects and Clinical Applications, Part 1. AJNR Am J Neuroradiol 30:19-30. Jan 2009. 5
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Susceptibility and Phase Relations
• MRI equations– ω=γB0
– ψ=ωt– Δψ=Δω*TE
• Relating to susceptibility,– Since Δω=γΔB and ΔB=g*Δχ*B0
– Δψ=-γΔB*TE [6] – And so, Δψ=-γgΔχB0*TE [7]
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Effect of sample shape, orientation and susceptibility
7 Schenck, JF. The role of magnetic susceptibility in magnetic resonance imaging: MRI magnetic compatibility of the first and second kinds. Med Phys. 1996 Jun;23(6):815-50
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Geometric Effects
• Distortion manifests as change in phase• For example, a blood vessel can be modeled
as a cylinder at an angle θ to B0,– ΔBin=ginΔχB0, where gin=(3cos2θ-1)/6
– ΔBout=ΔχB0sin2θcos(2ϕ)a2/(2r2)
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Outline of SWI Processing
1. Acquire high-res 3D fGRE.2. Apply HPF to phase image to obtain “SWI
filtered phase data.”3. Create “phase” mask depending on sign.4. Multiply phase mask by original magnitude
image to obtain “merged SWI magnitude data.”
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Step 1: Acquisition
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Magnitude
c/o Samantha Holdsworth
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Step 2: Creating HPF
• Uses 64x64 low-pass filter divided into the original phase image to create a HP-filter effect.
• Method1. Truncate original image ρ(r) to central n x n
complex image ρn(r).2. Zero-fill elements outside central n x n elements3. Complex divide ρ(r) by ρn(r) to obtain a new
image, ρ’(r) = ρ(r)/ρn(r)
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Step 2: Phase Images
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Raw phase image HP-filtered (32x32) HP-filtered (64x64)
Fig. 3 Haacke Review 1
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Step 3: Phase Mask
• Negative Phase Mask• 0 < < : phase mask filter = 1• - < < 0: phase mask filter
linearly scaled between 0 and 1
• Positive Phase Mask• > > 0 : phase mask filter = 1• 0 > > - : phase mask filter
linearly scaled between 0 and 1.
E.M. Haacke et. al. Susceptibility Weighted Imaging. MRM 52:612-618 (2004). 13
-π π
1
(x)
f(x)
π-π
1
(x)
f(x)
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Step 3: Phase Masking Process
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Phase profile in filtered phase image Profile of mask created from A
Fig. 6 Haacke Review 1
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Step 4: Simulated images with phase mask multiplication
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m=1 m=4 m=8 m=16
ρ(x)new= fm(x)ρ(x)
Fig. 1 Haacke
Best CNR
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Visibility as Function of Multiplication
16Visibility V = CNR(m)*p*sqrt(π); p = radius Fig. 2
M=4 gives peak CNR(m).
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Overview: SWI Processing
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(phase mask)m
Phase image
Magnitude image
X
SWI image
c/o Samantha Holdsworth
SWI minIP
≥4 images
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Clinical Examples
• Neuroimaging• Breast microcalcifications
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Fat/Water Contrast
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Magnitude Phase;Fat π out of phase with water
SWI m=1SWI m=2
Fig. 4
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GM/WM Contrast
20a) T1w magn. image TE=5ms b) SWI, m=4 using phase mask from TE=40ms data
Fig. 5
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mIP of images
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a. mIP of the original magnitude images.
b. Modified mIP of SWI using m=4.
c. mIP of filtered phase imagesd. Slice from a cadaver brain
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Breast Microcalcifications
A. Fatemi-Ardekani, C. Boylan, M. D. Noseworthy. Identification of breast calcification using magnetic resonance imaging. Medical Physics, Vol. 36, No. 12, December 2009 22
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Seeing Microbleeds with SWI
Multiple microbleeds in the brain are only evident on the SWI phase image (D), suspected cerebral amyloid angiopathy (CAA).
T1w T2w
MRA SWI
Haacke Review, Part 223
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Iron build up in the pulvinar in MS indicated with SWI
MS Patient Normal Volunteer
SWI in Multiple Sclerosis
24Haacke Review, Part 2
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Calcifications in the Brain
MRI Magnitude CT SWI Filtered Phase
Both calcification and hemorrhage are hypointense on magnitude images but calcifications are distinct on CT and SWI phase filtered images (patient w/ history of neurocysticercosis).
Haacke Review, Part 225
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Sturge-Weber Syndrome found most often in children leads to vascular malformation
Sturge Weber Syndrome in 5 y.o. girl
Haacke Review, Part 226
Post-contrast T1wLeptomeninges (arrowhead)Periventricular veins (arrow)
SWI – calcification of gyri (dotted/arrowhead)Periventricular veins (arrow)
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Questions?
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