MR Angiography

1. MR Angiography Hattie Dong MRSRL Study Group Friday, July 18, 2008

2. What and Why • MR angio: study of vessels and blood flow by generating contrast b/w blood and background • Applications: • Detect aneurysms, atherosclerosis, and other vessel diseases. • Visualize blood flow (e.g. during kidney transplant). • Guide surgeons when performing implantation or using a stent.

3. Overview • Black blood: dark blood signal • RF spin echo • Inversion recovery • Bright blood: bright blood signal • Time-of-flight (TOF) • Projection angiography • Contrast-enhanced MRA (CEMRA)

4. Spatial Saturation • Removes signals from a specific location • Can be easily added to angio sequences

5. Black Blood Methods

6. RF Spin Echo

7. RF Spin Echo • Complete outflow from imaging slice z when through-plane velocity exceeds: • Single echo: • Double echo: • Adv: obtained for “free” • Drawback: slow/recirculating flow not suppressed

8. Inversion Recovery • T1blood 1200ms • Choose TI such that blood signal recovers to approximately zero. • Single inversion drawback: static blood and tissues with T1 similar to blood also suppressed

9. Double IR • Non-selective 180  slice-selective 180  TI  imaging sequence • Complete suppression condition: • Common to add DIR module to FSE

10. Multislice DIR-FSE • Yarnykh, JMRI 2003: Invert a slab and readout at different slice locations.

11. Multislice DIR-FSE

12. Triple Inversion Recovery • Simonetti, Radiology 1996: Add one more selective 180 inversion to suppress fat (T1=300ms)

14. Bright Blood Methods

15. Time-of-Flight • Imaging volume excited with short TR, reducing steady state Mz of static tissue • Inflowing blood experiences fewer excitations.

16. Flow-Related Enhancement • Incomplete T1 relaxation in between RF excitations, unless T1<<TR. • GRE steady state FID:

17. FRE • If spins experience j excitations: • j   FRE  j   Ernst  (Ernst is angle that maximizes FRE)

18. 2D & 3D TOF • 2D: Fresh spins enter slice after each excitation • Spatial saturation to suppress venous flow • 3D: blood traverses entire slab, Mz lowers over time • Higher FRE at entrance than exit • MIP to recon

19. MOTSA • Tradeoff: image volume vs. blood “freshness”  MOTSA • Parker, MRM 1991: Overlapping short 3D slabs acquired • Slice in overlap region: acquire as entry slice from one slab and exit slice from another, then take maximum

21. Projection Angiography • TOF recon uses MIP: contrast reduced at high resolution • If background signal sufficiently suppressed, can directly perform 2D projection of the 3D volume

22. Multiple Inversion Recovery Mani, MRM 1997

23.   sm m-1 m Mz(m-1) Mz(m) MIR • Propagate through n tip pulses and image at a time Q after spatial saturation, =, Mz(0)=0:

24. MIR • 4 inversions, 3 species to null  nonlinear underdetermined system • Quasi-Newton optimization (BFGS), least norm of final Mz’s.

25. MIR Implementation • Algorithm sensitive to B0 & B1 inhomogeneity • Saturation: CHESS (Moonen, MRM 1990) • Inversion: Adiabatic sech pulses

27. Contrast-Enhanced MRA • Contrast agent gadolinium chelate (molecular cage) • T1 reduction of blood: • Causes skin thickening in kidney failure patients

28. Discussions • Black blood methods: • RF spin echo and IR-related • Intravoxel dephasing due to complex flow further reduces blood signal • Pulsatile flow does not produce ghosting as blood nulled • Bright blood methods: • 2D & 3D TOF, MIR PA, CEMRA • Clear depiction of vessels without static signal distraction

29. Q & A