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Susceptibility Weighted Imaging (SWI). 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.

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susceptibility weighted imaging
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 52:612–618 (2004)

swi introduction
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

history of swi
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!
major clinical applications for swi
Major Clinical Applications for SWI
  • Stroke
  • Brain Tumors
  • Traumatic Brain Injury
  • Vascular Malformations
  • Neurodegenerative Diseases
stroke
Stroke

Nathaniel D. Wycliffe, JMRI 20:372–377 (2004)

stroke7
Stroke

minIP SWI vs. CT

Thomas, Bejoy, et. al. Neuroradiology (2008) v50

brain tumors
Brain Tumors

CE T1 weighted vs. SWI

Sehgal, Vivek, et. al. Journal of Magnetic Resonance Imaging (2005) v22

traumatic brain injury
Traumatic Brain Injury

GRE Image vs. SWI postprocessing

Thomas, Bejoy, et. al. Neuroradiology (2008) v50

vascular malformations
Vascular Malformations

Routine GRE vs. minIP SWI

Thomas, Bejoy, et. al. Neuroradiology (2008) 50:108

neurodegenerative diseases
Neurodegenerative Diseases

SWI minIP vs. SWI phase image

Thomas, Bejoy, et. al. Neuroradiology (2008) v50

brief overview of steps
Brief Overview of Steps:
  • Generate a high frequency phase image
  • Construct a normalized phase mask
  • Enhance the magnitude image with the phase mask to get the SWI
  • Optional: produce a minimum intensity projection to produce a SWI minIP

3T

1 generate high frequency phase image
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

2 phase mask
2) Phase Mask
  • A phase mask is produced as follows:

- If phase >=0, then the resulting phase mask value = 1

- If phase < 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

3 enhance magnitude image
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

enhanced magnitude comparison
Enhanced magnitude comparison…

Original image

SWI image

4 minimum intensity projection
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

modeling the susceptibility effects in venous system
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 41:453 (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!
graph the result
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 there’s more – we can use the phase information…...
phase image can be used to further enhance signal cancellation effects
Phase image can be used to further enhance signal cancellation effects….
  • Referring back to the result for TE ~ 50ms
    •  = - when θ = 0º (|| to B0)
    • - <  < 0 for 0º < θ < 54º
  • A ‘negative’ phase mask filter can be created:
      • 0 <  <  : phase mask filter = 1
      • - <  < 0: phase mask filter linearly scaled between 0 and 1
  • But! What about vessel orientations θ > 54º :
    • For 54º < θ < 90º, the phase  > 0
    • Therefore, negative phase mask will miss part of venous vascular information
negative positive phase masks
Negative & Positive phase masks
  • Complicated phase behaviour
  • Can use triangular phase mask
  • But result in fat vessels and blurring of veins => negative phase mask used.

Reichenbach & Haacke NMR in Biomedicine, 14:453 (2001)

swi at 1 5t
SWI at 1.5T

2

4

reference

Different phase mask orders

6

8

swi at 3t
SWI at 3T

2

4

reference

Different phase mask orders

6

8

final comparison at 3t
Final comparison at 3T:

reference

SWI image

acquisition of swi
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

3d epi
3D EPI
  • 3D EPI has more k-space coverage per TR => faster scan time (~2 min v 7 min) and/or higher SNR.
  • Disadvantages: - geometric distortion ( 1/#shots)

- signal dropout

GRE

EPI

slide33
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