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Introduction to Magnetic Resonance Angiography

Introduction to Magnetic Resonance Angiography. Geoffrey D. Clarke, Ph.D. Division of Radiological Sciences University of Texas Health Science Center at San Antonio. Overview. Flow-Related Artifacts in MRI Time-of-Flight MR Angiography Contrast-Enhanced MR Angiography

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Introduction to Magnetic Resonance Angiography

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  1. Introduction to Magnetic Resonance Angiography Geoffrey D. Clarke, Ph.D. Division of Radiological Sciences University of Texas Health Science Center at San Antonio

  2. Overview • Flow-Related Artifacts in MRI • Time-of-Flight MR Angiography • Contrast-Enhanced MR Angiography • Phase-Contrast MR Angiography • Quantitative Flow Imaging

  3. Flow Voids & Enhancements • In spin echo imaging vessels appear as signal voids • same volume of blood does not experience both 90o and 180o pulses • In flow effect • may cause unsaturated blood to appear bright in slice that is most proximal to heart • Saturation effects • cause diminished signals in blood flowing parallel to image plane

  4. Vessel Signal Voids Early multi-slice spin echo images depicted vessels in the neck as signal voids

  5. Multi-slice Spin Echo MRI Slices Long TR 90o-180o Fast flow Flowing Blood Stationary Tissue Spins do not get refocused by 180o pulse Slice #1 Slice #2 Slice #3

  6. Field Echoes & Bright Blood • Partial Flip Angle/Field Echo Images • Short TR, Short TE • Only one TX RF pulse (o) • Blood has Greater Proton Density than Stationary Tissues

  7. Bright Blood Images Using gradient (field) echo images with partial flip angles allowed blood which flowed through the 2D image plane to be depicted as being brighter than stationary tissue.

  8. Motion Artifacts • in read-out direction • data acquired in time short compared to motion • blurring of edges • in phase-encode direction • ghosting presenting as lines & smudges • in slice-select direction • variable partial volume, difficult to detect

  9. The MRI Signal: Amplitude & Phase Bo rf = B1 Net Magnetization Real Imaginary Real Imaginary

  10. Dephasing Due to Motion Gslice time +180o BLOOD: phase not zero TISSUE: phase equals zero PHASE time Phase Shift Due to Motion in a Gradient Field -180o t = 0

  11. Pulsatile Motion Artifact Aorta Artifact Artifact Artifact

  12. Motion Compensation Gradients Gslice time Phase Shift Due to Motion in a Gradient Field +180o PHASE time -180o BLOOD: phase equals zero t = 0 TISSUE: phase equals zero *Only applies for constant flow. More gradient lobes needed for acceleration.

  13. Flow Artifact Correction • Spatial pre-saturation pulses prior to entry of the vessel into the slices • Surface coil localization • Shortened pulse sequences • Cardiac & respiratory gating • Motion Compensation Gradients

  14. Magnetic Resonance Angiography (MRA)

  15. MRA Properties • Utilizes artifactual signal changes caused by flowing blood to depict vessel lumen • May include spin preparation to suppress signal from stationary tissues or discriminate venous from arterial flow • Does not require exogenous contrast administration, but contrast agents may be used to enhance MRA for fast imaging

  16. Methods of Magnetic Resonance Angiography • Signal Amplitude Methods • 2D Time-of-Flight • 3D Time-of-Flight • Signal Phase Methods • 2D Phase Contrast • (Velocity Imaging = Q-flow) • 3D Phase Contrast • (Velocity Imaging = Q-flow)

  17. Time-of-Flight MRA Method Bo M Imaginary Real

  18. Time of Flight Effect • T1 of flowing water is effectively shorter than the T1 of stationary water • Two contrast mechanisms are responsible: • T1 saturation of the stationary tissue • In-flow signal enhancement from moving spins

  19. 2D Time-of-Flight MRA Conditions • Field Echo Imaging • Short TE • Partial Flip Angle • generally large • keeps stationary tissues saturated • TR and flip angle • adjusted to minimize stationary tissue • adjusted to maximize blood

  20. 2D Time-of-Flight MRA Advantages • Good stationary tissue to blood • flow contrast • Sensitive to flow • Minimal saturation effects • Short scan times • Can be used with low flow rate

  21. 2D Time-of-Flight MRA Limitations • Relatively poor SNR • Poor in-plane flow sensitivity • Relatively thick slices • Long echo times (TE) • Sensitive to short T1 species

  22. Improving Contrast in Time-of-Flight MRA • 1. Venous Pre-saturation • (spatial suppression) • 2. Magnetization Transfer Contrast • (frequency selective irradiation) • 3. Fat Saturation • (frequency selective irradiation) • 4. Cardiac Gated MRA • 5. Spatial variation of flip angle

  23. Spatial Pre-saturation in Time-of-Flight MRA • Saturates and dephases spins before they enter imaging slice • Can be used to isolate arteries or veins • Can be used to identify vessels feeding • a given territory • Can be used to establish the direction of flow in a particular vessel

  24. Magnetization Transfer Contrast PROTON SPECTRUM 0 Frequency (Hertz) “Free” Water Lipids “Bound” Water 0 217 Hz 1500 Hz Frequency (Hertz)

  25. Gradient Echo with MTC Pulse Off-resonance rf pulse RF excitation TX Digitizer On Field Echo RX Slice Select Gsl Rephasing Spoilers Read Out Crushers or Spoilers Dephasing Gro Phase Encode Gpe

  26. MIP #1 Maximum Intensity Projections MIP #2 OBJECT

  27. 2D TOF Application Abdominal Aneurysm

  28. 3D Time-of-Flight MRA Conditions • Uses two phase encode gradients and volume excitation • Maximum volume thickness limited by flow velocity • Use minimum TR, adjust flip angle for best contrast

  29. Three Dimensional Gradient Refocused Echo Imaging RF pulse (short time) TX Field Echo RX Slab Select Secondary Phase Encoding Digitizer On Gsl Crusher Rephasing Read Out Gro Dephasing Primary Phase Encoding Phase Rewinder Gpe

  30. 3D Time-of-Flight MRA Advantages • Higher resolution (thinner slices) available allowing for delineation of smoother edges • Higher signal-to-noise than 2D methods • Lower slice select gradient amplitudes results in fewer phase effect artifacts than 2D method • Short duration RF pulses can be used to excite slab – TE can be reduced

  31. 3D Time-of-Flight MRA Limitations • Blood signal is easily saturated with slow flow • Relatively poor background suppression • Short T1 tissues may be mistaken for vessels

  32. 3D-TOF Application:Cerebral Arteries – Circle of WIllis • TR /TE = 40 / 4.7 ms • 64 partitions, 48 mm slab, 0.75 mm per partition • Flip angle = 25o • 256 x 256, 18 cm FOV, 0.78 x 1.56 mm pixel • MTC contrast • Venous Presaturation

  33. Circle of Willis Time of Flight MRA 90o

  34. Cerebral Venous Angiogram TOP Saggital Sinus Use of arterial presaturation allows visualization of cerebral venous vessels FRONT Straight Sinus Transverse Sinus Confluence Of Sinuses Cerebven.mpeg

  35. Multi-Slab 3D TOF MRA Hybrid of 2D and 3D methods: • Thin 3D slabs used • Good inflow enhancement • Multiples slabs to cover volume of interest • High resolution • Short TE • Relatively time inefficient

  36. Gd Contrast Enhanced MRA • Gd contrast agents decrease T1 and increase CNR of blood and soft tissue • Along with ultra-fast 3D sequences, allow coverage of larger VOI’s • Shorter acquisition times allow breath-holding for visualization of central and pulmonary vasculature

  37. MRI Compatible Power Injectors Programmable Automatic Injection MRI Compatible Allows rapid arterial injection of Gd-DTPA www.medrad.com

  38. 3D CE-MRA of Aortic Aneurysm • 44 slices • 32 sec scan • TR/TE • = 2.3/1.1 ms • 1.5 x 1.8 x 1.8 mm pixel Phase 3 Phase 2 Phase 1 Digital Subtraction X-ray Angiography Phase 2 Phase 1 Schoenberg SO, et al. JMRI 1999; 10:347-356

  39. Bolus Chase 3D MRA Station 1 Station 2 Station 3 Earlier venous enhancement noted with fast injection Ho VB et al. JMRI 1999; 10: 376-388

  40. Normal Runoff MRA Image of tissue surrounding vessel can be manually striped off http://www.uth.tmc.edu/radiology/publish/mra/gallery.html

  41. Phase-Contrast MRA Method Bo  Imaginary Real

  42. Dephasing Due to Motion Gslice time +180o BLOOD: phase not zero TISSUE: phase equals zero PHASE time Phase Shift Due to Motion in a Gradient Field -180o t = 0

  43. Phase Contrast Imaging Velocity Encoded Image +180o Phase Difference PHASE time Velocity Compensated Image -180o Motion Compensation Gradient (Bipolar) Applied +180o PHASE time Velocity Encoded Image -180o TISSUE: phase equals zero in BOTH images BLOOD: phase is DIFFERENT in each image

  44. Magnetic Field Gradients in MRI(Two More Functions) • Slice Selection • Phase Encoding • Frequency Encoding • Sequence Timing (Dephase/Rephase) • Motion Compensation • Motion Encoding

  45. 2D Phase Contrast MRA Features • Can use minimum TR • doesn’t rely on T1 effects • Good for slow flow • Motion is imaged in only one direction • usually slice select • Requires 2 images • Velocity compensated / velocity encoded

  46. 2D Phase Contrast MRA Advantages • Short acquisition times • Variable velocity sensitivity • Good background suppression • Minimal saturation effects • Short T1 tissues do not show up on images

  47. 2D Phase Contrast MRA Limitations • Single thick section projection • Vessel overlap artifact • Sensitive to flow in only one direction • Unstructured flow may cause problems

  48. 3D Phase Contrast MRA Features • Images obtained at higher spatial • resolution than 2D PC • 3D PC requires at least four images: • flow compensated • x-encoded • y-encoded • z-encoded • Low velocity imaging in tortuous vessels • Takes the most time

  49. 3D Phase-Contrast MRA Renal Circulation FP Coronal, 3D PC TR/TE = 33/6 ms 20o flip Coronal, Gd enhanced TR/TE = 7/1.4 ms 40o flip, false renal stenosis (FP)

  50. 3D Phase Contrast MRA Advantages • Thin slices • Quantitative flow velocity and direction • Excellent background suppression • Variable velocity sensitivity • Short T1 tissues do not appear on images

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