Susceptibility Weighted Imaging (SWI)

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Susceptibility Weighted Imaging (SWI)
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Susceptibility Weighted Imaging?

• SWI is a magnetic resonance (MR) technique that
  utilizes the magnetic susceptibility differences
• Illuminate small vessels and veins in the brain
• Sensitive to iron calcification

Haacke, Mark, et. al. Magnetic Resonance in
Medicine 52612618 (2004)
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SWI Introduction

• Acquisition T2-weighted sequence to enhance
  the visibility of susceptibility differences.
• High-resolution 3D gradient-echo (with flow
  compensation)
• Long TE (40ms at 1.5T, 25ms at 3T) to get T2
  weighting
• Extra post processing using the phase image.

Magnitude
Phase
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History of SWI

• Reichenbach, Haacke et al. 1997
• MR venography or BOLD venographic imaging
• From 1997 2003
• Lots of clinical papers
• Haacke et al. 2004
• Susceptibility Weighted Imaging
• Caution Sometimes the term susceptibility
  weighted imaging is used loosely!

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Major Clinical Applications for SWI

• Stroke
• Brain Tumors
• Traumatic Brain Injury
• Vascular Malformations
• Neurodegenerative Diseases

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Stroke
Nathaniel D. Wycliffe, JMRI 20372377 (2004)
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Stroke
minIP SWI vs. CT
Thomas, Bejoy, et. al. Neuroradiology (2008) v50
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Brain Tumors
CE T1 weighted vs. SWI
Sehgal, Vivek, et. al. Journal of Magnetic
Resonance Imaging (2005) v22
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Traumatic Brain Injury
GRE Image vs. SWI postprocessing
Thomas, Bejoy, et. al. Neuroradiology (2008) v50
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Vascular Malformations
Routine GRE vs. minIP SWI
Thomas, Bejoy, et. al. Neuroradiology (2008)
50108
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Neurodegenerative Diseases

SWI minIP vs. SWI phase image
Thomas, Bejoy, et. al. Neuroradiology (2008) v50
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Post-processing Steps to a Susceptibility
Weighted Image
Background comes later!
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Brief Overview of Steps

1. Generate a high frequency phase image
2. Construct a normalized phase mask
3. Enhance the magnitude image with the phase mask
   to get the SWI
4. Optional produce a minimum intensity projection
   to produce a SWI minIP

3T
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1) Generate high frequency phase image

• Use a 2D Hanning filter (in k-space) to smooth
  the original image
• Divide the original image by the smoothed image
• A phase unwrapped image
• An image with high frequency phase information

Phasemap of original Image Phasemap of hanning
filtered image
High pass phase Image
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2) Phase Mask

• A phase mask is produced as follows
• - If phase gt0, then the resulting phase
  mask value 1
• - If phase lt 0, then the mask value is found
  by (ph(x) pi)/pi
• The resulting normalized phase mask (ranging from
  0 to 1).

High pass data
Normalized phase mask
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3) Enhance Magnitude Image

• Multiply the phase mask by the magnitude image
  (phase mask can be multiplied 3- 8 times)

SWI Processed Image
Phase mask.5
X Orig. Magnitude
X
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Enhanced magnitude comparison
Original image
SWI image
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4) Minimum Intensity Projection

• A minIP, further enhances the contrast of
  susceptibilities in the final SWI image
• A minIP usually done over 5 to 10 slices

SWI processed image
Final minIP
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More detail
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Modeling the susceptibility effects in venous
system
Difference fields for an infinitely extended
circular cylinder

• Venous imaging based on the magnetic
  susceptibility difference between oxygenated and
  deoxygenated hemoglobin
• Papers describing this
• Reichenbach Haacke, NMR Biomedicine 41453
  (2001)
• Springer, NMR in Physiology and Biomedicine 1994
  75
• Vessel to B0 intravascular frequency shift
• Vessel _ to B0 intravascular AND extravascular
  frequency shift
• SEE NOTES on WORD DOC!

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Graph the result
Signal dependence on venous blood volume fraction
(?) and TE

• Since the local magnetic field in and around
  blood depends on venous blood volume fraction
  (?), TE can be adjusted to reveal large signal
  cancellation
• Signal cancellation
• TE 40ms (1.5T), TE 25ms (3T) used to get
  maximum signal cancellation without phase
  aliasing
• But theres more we can use the phase
  information...

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Phase image can be used to further enhance signal
cancellation effects.

• Referring back to the result for TE 50ms
• ? -? when ? 0º ( to B0)
• -? lt ? lt 0 for 0º lt ? lt 54º
• A negative phase mask filter can be created
• 0 lt ? lt ? phase mask filter 1
• -? lt ? lt 0 phase mask filter linearly scaled
  between 0 and 1
• But! What about vessel orientations ? gt 54º
• For 54º lt ? lt 90º, the phase ? gt 0
• Therefore, negative phase mask will miss part of
  venous vascular information

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Negative Positive phase masks

• Complicated phase behaviour
• Can use triangular phase mask
• But result in fat vessels and blurring of veins
  gt negative phase mask used.

Reichenbach Haacke NMR in Biomedicine, 14453
(2001)
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Exposing SWI
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SWI at 1.5T
2
4
reference
Different phase mask orders
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8
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SWI at 3T
2
4
reference
Different phase mask orders
6
8
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Final comparison at 3T
reference
SWI image
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SWI minIP at different field strengths
1.5T
3T
7T
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Acquisition of SWI

• Current Method
• 3D Gradient-Echo imaging
• (3D GRE)
• Long scan time (32 partitions takes 7 min.)
• Future Method?
• 3D multi-shot EPI

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3D EPI

• 3D EPI has more k-space coverage per TR gt faster
  scan time (2 min v 7 min) and/or higher SNR.
• Disadvantages - geometric distortion (?
  1/shots)
• - signal dropout

GRE
EPI
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Multi-shot EPI
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Practical things

• Careful with your flip angle/TR, and no
  rfspoiling!

1.5T TE40ms
? 30º
? 20º
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• Conclusion
• SWI has promising applications in the clinics
  (probably just a good compliment to other
  techniques)
• Good delineation of venous network and some
  tissue pathologies
• Ability to image tumors without contrast agent
• Demonstrates vascular nature of a lesion
• Etc..
• Performs better with higher field strength
• A bit ambiguous?
• Disadvantage long scan times
• Advantage we can get abstracts by speeding it up

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Acknowledgements Matus Straka, Karley Marty,
Stefan Skare, Roland Bammer
Thank You!! Questions?