SWI MRSRL

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 Haacke, Mark, et al Magnetic Resonance in Medicine 52:612–618 (2004) 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  Reichenbach, Haacke et al 1997   From 1997 – 2003   Lots of clinical papers Haacke et al 2004   “MR venography” or “BOLD venographic imaging” “Susceptibility Weighted Imaging” Caution: Sometimes the term susceptibility weighted imaging is used loosely! Major Clinical Applications for SWI      Stroke Brain Tumors Traumatic Brain Injury Vascular Malformations Neurodegenerative Diseases Stroke Nathaniel D Wycliffe, JMRI 20:372–377 (2004) Stroke minIP SWI vs CT Thomas, Bejoy, et al Neuroradiology (2008) v50 Brain Tumors CE T1 weighted vs SWI Sehgal, Vivek, et al Journal of Magnetic Resonance Imaging (2005) v22 Traumatic Brain Injury GRE Image vs SWI postprocessing Thomas, Bejoy, et al Neuroradiology (2008) v50 Vascular Malformations Routine GRE vs minIP SWI Thomas, Bejoy, et al Neuroradiology (2008) 50:108 Modeling the susceptibility effects in venous system   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! Difference fields for an infinitely extended circular cylinder Graph the result…  Since the local magnetic field in and around blood depends on venous blood volume fraction (λ), 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… Signal dependence on venous blood volume fraction (λ) and TE Phase image can be used to further enhance signal cancellation effects…  Referring back to the result for TE ~ 50ms φ = -π when θ = 0º (|| to B0)  -π < φ < for 0º < θ < 54º   A ‘negative’ phase mask filter can be created:    < φ < π : phase mask filter = -π < φ < 0: phase mask filter linearly scaled between and But! What about vessel orientations θ > 54º :   For 54º < θ < 90º, the phase φ > Therefore, negative phase mask will miss part of venous vascular information 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) Exposing SWI…… SWI at 1.5T reference Different phase mask orders SWI at 3T reference Different phase mask orders Final comparison at 3T: reference SWI image SWI minIP at different field strengths 1.5T 3T 7T Acquisition of SWI Current Method: 3D Gradient-Echo imaging (3D GRE)  Long scan time (32 partitions takes min.) Future Method? 3D multi-shot EPI 3D EPI   3D EPI has more k-space coverage per TR => faster scan time (~2 v min) and/or higher SNR Disadvantages: - geometric distortion (∝ 1/#shots) - signal dropout EPI GRE Multi-shot EPI Practical things:  Careful with your flip angle/TR, and no rfspoiling! 1.5T TE=40ms θ = 30º θ = 20º 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 Acknowledgements: Matus Straka, Karley Marty, Stefan Skare, Roland Bammer Thank You!! Questions? ... 14:453 (2001) Exposing SWI? ??… SWI at 1.5T reference Different phase mask orders SWI at 3T reference Different phase mask orders Final comparison at 3T: reference SWI image SWI minIP at different... Magnitude SWI Processed Image Enhanced magnitude comparison… Original image SWI image 4) Minimum Intensity Projection   A minIP, further enhances the contrast of susceptibilities in the final SWI. .. v50 Vascular Malformations Routine GRE vs minIP SWI Thomas, Bejoy, et al Neuroradiology (2008) 50:108 Neurodegenerative Diseases SWI minIP vs SWI phase image Thomas, Bejoy, et al Neuroradiology