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).

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

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

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

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

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

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)

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

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

Visibility as Function of Multiplication

16Visibility V = CNR(m)*p*sqrt(π); p = radius Fig. 2

M=4 gives peak CNR(m).

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

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

GM/WM Contrast

20a) T1w magn. image TE=5ms b) SWI, m=4 using phase mask from TE=40ms data

Fig. 5

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

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

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

Iron build up in the pulvinar in MS indicated with SWI

MS Patient Normal Volunteer

SWI in Multiple Sclerosis

24Haacke Review, Part 2

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

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)

Questions?

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