MR Angiography: Techniques

Transcription

1 MR Angiography: Techniques Dharshan Vummidi MRCP, FRCR Department of Radiology University of Michigan

2 Disclosures Author for Expert Ddx: Chest, Amirsys Inc.

3 MRA: Principles & Physics Objectives: Essential MRA Principles Three distinct mechanisms Acquisition Techniques Application

4 MRA Mechanism #1 Time-of-Flight TOF TOF relates to time of inflow TR short relative to tissue T1 TR long enough for flow to replenish slice Static tissue is saturated weak signal TR > (Slice Thickness / Flow Speed) Gradient-recalled echo (GRE) only Flow in GRE slice-select pulse signal Flow in Spin echo may miss 90o or 180o slice-select pulses no signal

5 TOF Effect Critical Speed minimum speed for full replenishment Relative TOF Signal rf pulses TR Slice thickness d e.g. TR = 50msec; d = 5mm Flow direction Vcritical = 5mm/50ms = 10cm/sec Flow Speed

6 2D TOF MRA Neck 2D TOF GRE TR/TE/Flip=50/min/50; FOV ; 256 x 128; slc 1.5mm; FlowComp Sup Sat; 6-10min; gating optional

7 2D TOF Pelvis / Legs 2D TOF GRE TR/TE/Flip 46/6/60 FOV 380 slc 2.9mm; 100 loc 256 x 192 FlowComp Inf Sat 9 each slice 15min 100 slices 2D TOF slow!

8 Advantages 2D TOF MRA easy sensitive to slow thru-plane flow non-invasive & good for neck-up and pelvis down Disadvantages modest resolution (slice ~ 5mm) saturates in-plane flow misregistration artifact (false stenosis?) signal loss in large voxels (dephasing) incomplete background suppression long scantime 5-15min

9 Enhanced by: Increased SNR Increased tissue T1 vessel/background contrast 3D TOF 3T

10 Advantages 3D TOF MRA easy & reasonably fast (5-8min) high spatial resolution good for intermediate and fast flow can use multi-slab for more coverage short TE and small voxels reduce flow dephasing non-invasive & great for head MRA Disadvantages poor for slow flow (ok for arteries, less so for veins) incomplete background suppression venetian-blind effect

11 Abdominal TOF MRA Greatly Compromised by Respiratory Motion!

12 MRA Mechanism #2: Phase Contrast (PC) Effect Moving spins through a magnetic field gradient accumulate phase relative to stationary spins

13 Phase Flow Velocity Magnitude Signal Strength Recall MRI Signal Has Magnitude and Phase Magnitude Strength of magnetization Dependent on TR, T1, TOF z Phase ML M Orientation of transverse magnetization Bo y Dependent on gradient waveform x MT Uniformity of flow speeds within voxel

14 Gradient Flow Phase Shift Velocity-encoding gradients Additional bi-polar gradients to induce additional phase shift in proportion to flow speed along given gradient direction! Flow Through Field Gradient V = 40cm/sec 30cm/sec 20cm/sec 10cm/sec 0cm/sec Flow at +Venc shifted +180 o Flow at -Venc shifted -180 o Time (msec) Phase Shift TE 0 G( t) x( t) dt VG 2 flow speed Venc

15 Choose Venc ~ Max Flow Speed If Venc too high reduced sensitivity to flow If Venc too low aliased flow (false reversal) VENC = 100 VENC = 40 VENC = 25

16 Phase Contrast MRA of Neck PC MIP PC Speed PC Velocity 2D TOF

17 Phase Contrast MRA Vessel Localizer Magnitude of Signal Magnitude of Phase of Signal

18 Axial 3D Phase Contrast in Abdomen TR: 25 msec TE: 8 msec FOV: cm Matrix: 256 x 192 x 28 (2.5 mm thick slices) Velocity Encoding Strength young: cm/sec old: 30 cm/sec sick: cm/sec Axial 3D phase contrast

19 CINE PC Flow Quantitation Encode flow along slice-select axis only Pulsatile flow cardiac synchronization Quantitative within range Venc

20 CINE PC Flow Quantitation Venc = 50cm/sec Venc = 100cm/sec

21 Flow Velocity (cm/sec) CINE PC Flow Quantitation Venc = 100cm/sec 50 PC Quantitative Flow: Venc=100cm/sec J Vein Carotid 25 0 Time (msec)

22 Turbulence-related Signal Loss Flow thru imaging gradient additional phase Wide range of flow speeds in a voxel wide range of phase shift in voxel Flow turbulence / acceleration wide range of phase shift in voxel Wide range of phase shift in voxel flow dephasing signal loss

23 MRA via steady-state free precession / balanced FFE / true FISP / FIESTA Blood bright in SSFP due to favorable T2/T1 ratio Non contrast-enhanced abdominal 3D MRA via: Slab-selective inversion of tissue with TI-delay chosen to null tissue signal Inflowing blood not nulled Respiratory navigator and cardiac triggered for SSFP bright blood Saturate unwanted venous entering slab See Wyttenbach, R. et.al. Radiology (2007): v245;

24 Phase Contrast MRA Advantages reasonably fast (PCMRA 2-5min; Venc Scout ~10sec) directional flow and in-plane slow flow (~1cm/sec) through fast (~100cm/sec) 2D, 3D, and cardiac synchronize options complete background suppression quantitative for thru-plane flow Disadvantages signal loss flow-dephasing in large voxels / turbulence flow aliasing - choose appropriate Venc

25 MRA Mechanism #3: Contrast - Enhanced (CE) 3D MRA IV injection of contrast agent gadolinium-based Rapid 3D (breathhold) MRI TR < 6msec; TE < 2msec; high flip angle (~30 o 45 o ) High resolution 3D (256; TRx128x32 20 ) In-plane flow imaging (coronal abdominal MRA!)

26 Abdominal Arterial-phase Timing Synchronize Three Events: Contrast agent in vessels of interest how much & how fast agent injected patient circulation properties Data acquisition Repetition time TR minimum (< 4ms) Resolution: phase-enc Mx s (>40)x(>200) ~25 scan) k-space acquisition order Breathhold

27 Maximum Arterial Gadolinium During Center of K-space

28 Signal Enhancement Center of k-space Arterial Venous (renal) ~20 seconds

29 Ringing artifact from scanning too early [Gd] still increasing during traversal through center of k-space

30 Approaches to Timing 1. Empiric / Guess 2. Test bolus 3. Automatic Gd arrival detection 4. 2D Fluoro - Manual switch to 3D 5. Multi-Phase fast scanning

31 Test Dose Contrast timing method Inject small amount of Gd (1-2cc) while rapid 2D imaging (1 image/sec) & have in-flow TOF removed by saturation bands Measure test dose time-to-peak in tissue of interest Plan for same delay to center of k-space for full Gad injection: Scan Delay Time To Peak Duration of FullGad 2 Injection Time To Center of k space

32 Intensity ROI plotted Example Pulmonary MRA Test Dose Time in Seconds

33 Pulmonary MRA

34 Contrast arrival detection Gad-Monitor Volume Ortho- 90 o -180 o Spin-Echo Imaging Volume 3D SPGR

35 Integral of Echo Speed Automatic Bolus Timing with MR Smartprep 2000 Start scanning Start Injecting Gd Detect Gd Time (sec) 40 ml Gd

36 Fluoro-Trigger / Bolus Track Gad Arrival Method

37 Bolus Chase MRA

38 Imaging flow kinetics: Multiphase 3D MRA 4D MRA

39 Enhancement Pulmonary Aortic Arch T- Aorta A- Aorta Multiphase 3D-Gd-MRA Venous (renal) seconds

40 Original Dynamic 3D Acquisition Technique 11 seconds 22 seconds 33 seconds 44 seconds Total Scan time = 0:44 with 4 phases

41 Methods to Increase Temporal & Spatial Resolution Minimize TR / TE Asymmetric k-space coverage Parallel imaging SENSE / ASSET / SMASH / GRAPPA Will increase noise and maybe artifact Serial central k-space sampling and reuse peripheral k-space

42 k-space

43 Multi-phase: k-space Acquisition Order Schemes and many variations! Pre Gd >>>> Inject Gd & Dynamic ksp Center Acq >>>> Post Gd kspace kspace Synthesize high-res dynamic scan: ksp Center + Pre Gad Synthesize high-res dynamic scan: ksp Center + Post Gad

44 Dynamic 3D MRA 4D MRA Rapid serial acquisition of the center ( keyhole ) of k-space (every 1-3 secs) Re-use peripheral k-space acquired earlier or later Be aware of spatial / temporal resolution tradeoffs!!! eg. Brain AVM at 2 resolution

45 Hand 4D MRA (2sec resolution) MRA DSA

46 Limitation of Shared k-space Methods Each dynamic timepoint comprised of Current coarse contrast features (ie central kspace) Plus finer contrast features borrowed from the past And/or finer contrast features borrowed from the future Vessels may appear enhanced before contrast arrives!!!

47 4D MRA of Legs Pre-Gd Mask Dyn Phase #1 (before Gd arrival) Dyn Phase #30 (after Gd Arrival)

48 Gad Contrast-Enhanced MRA Advantages High-resolution 3D & 4D MRA of all anatomic regions Abdominal MRA in a breath-hold Principles parallel conventional angiography including kinetics / function Simpler and more robust than TOF and PC in most applications Disadvantages Use of contrast material NSF / NFD risk FDA recommends against Gad for GFR < 30 4D MRA: confusion over spatial vs temporal tradeoff

49 MRA Summary Applications by mechanism Time - of - Flight 2D and 3D MRA: neck-up and pelvis-down Phase Contrast 2D and 3D MRA head and neck; renal MRA Quantitative of flow Contrast Enhanced 3D MRA in chest/body very successful Runoffs 4D Kinetic / Function information