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Susceptibility Weighted Imaging E.M. Haacke, Y. Xu, Y. N. Cheng, J. R.Reichenbach. Susceptibility Weighted Imaging (SWI) . MRM 52:612-618 (2004). MRSRL Journal Club 08/13/10. Background & Motivation for SWI. MRI often relies on magnitude images (clinic).

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MRSRL Journal Club 08/13/10

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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 Club

08/13/10


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.

E.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


Current Scanners

  • Packaged SWI technique available on Siemens Medical Systems.

  • SWAN on GE scanner: T2 Star Weighted Angiography. (multi-echo GRE) uses echoes.

c/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.


Susceptibility and Phase Relations

  • MRI equations

    • ω=γB0

    • ψ=ωt

    • Δψ=Δω*TE

  • Relating to susceptibility,

    • Since Δω=γΔBand ΔB=g*Δχ*B0

    • Δψ=-γΔB*TE [6]

    • And so, Δψ=-γgΔχB0*TE [7]


Effect of sample shape, orientation and susceptibility

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)


Outline of SWI Processing

  • Acquire high-res 3D fGRE.

  • Apply HPF to phase image to obtain “SWI filtered phase data.”

  • Create “phase” mask depending on sign.

  • Multiply phase mask by original magnitude image to obtain “merged SWI magnitude data.”


Step 1: Acquisition

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.

  • Method

    • Truncate original image ρ(r) to central n x n complex image ρn(r).

    • Zero-fill elements outside central n x n elements

    • Complex divide ρ(r) by ρn(r) to obtain a new image, ρ’(r) = ρ(r)/ρn(r)


Step 2: Phase Images

Raw phase image

HP-filtered (32x32)

HP-filtered (64x64)

Fig. 3 Haacke Review 1


Step 3: Phase Mask

f(x)

  • 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.

1

π

(x)

1

f(x)

(x)

π

E.M. Haacke et. al. Susceptibility Weighted Imaging. MRM 52:612-618 (2004).


Step 3: Phase Masking Process

Profile of mask created from A

Phase profile in filtered phase image

Fig. 6 Haacke Review 1


Step 4: Simulated images with phase mask multiplication

  • ρ(x)new= fm(x)ρ(x)

m=1m=4m=8m=16

Best CNR

Fig. 1 Haacke


Visibility as Function of Multiplication

M=4 gives peak CNR(m).

Visibility V = CNR(m)*p*sqrt(π); p = radius

Fig. 2


Overview: SWI Processing

Magnitude image

(phase mask)m

SWI image

SWI minIP

X

≥4 images

Phase image

c/o Samantha Holdsworth


Clinical Examples

  • Neuroimaging

  • Breast microcalcifications


Fat/Water Contrast

Phase;

Fat π out of phase with water

Magnitude

SWI m=2

SWI m=1

Fig. 4


GM/WM Contrast

  • T1w magn. image TE=5ms

  • SWI, m=4 using phase mask from TE=40ms data

Fig. 5


mIP of images

  • mIP of the original magnitude images.

  • b. Modified mIP of SWI using m=4.

  • c. mIP of filtered phase images

  • d. 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


Seeing Microbleeds with SWI

T1w

T2w

MRA

SWI

Multiple microbleeds in the brain are only evident on the SWI phase image (D), suspected cerebral amyloid angiopathy (CAA).

Haacke Review, Part 2


SWI in Multiple Sclerosis

MS Patient

Normal Volunteer

Iron build up in the pulvinar in MS indicated with SWI

Haacke Review, Part 2


Calcifications in the Brain

SWI Filtered Phase

CT

MRI Magnitude

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 2


Sturge Weber Syndrome in 5 y.o. girl

Post-contrast T1w

Leptomeninges (arrowhead)

Periventricular veins (arrow)

SWI – calcification of gyri (dotted/arrowhead)

Periventricular veins (arrow)

Sturge-Weber Syndrome found most often in children leads to vascular malformation

Haacke Review, Part 2


Questions?


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