7/28/2009. Outline. MR Angiography (MRA) Techniques and Applications. Vascular Abnormities. Human Vascular System. MR Angiography Techniques

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1 Outline MR Angiography (MRA) Techniques and Applications Chen Lin, PhD Indiana University School of Medicine & Clarian Health Partners The background The principle and techniques The challenges and solutions The applications Human Vascular System Intra cranial Carotid Aortic Coronary Pulmonary Abdominal Renal Peripheral Stenosis Aneurysm Vascular Abnormities Arterial Venous Malformation (AVM) Thrombus Plaque Internal bleeding MRA Related Properties of Blood Flow Velocity: cm/sec in abdominal aorta; cm/sec in peripheral arteries Steady versus Pulsatile: Peak arterial ms after ventricular contraction Laminar versus Turbulent T 1 ~ 1.5T; ~ 3T T 2 ~ 250ms for arterial blood; ~ 30ms for venous blood MR Angiography Techniques Contrast Enhanced MRA (CE-MRA) High contrast to noise ratio No flow induced de-phasing and signal lost Fast acquisition -> Time-resolved MRA Acquisition timing is important Gd related NSF is a concern Non-Enhanced MRA (NCE-MRA) Quantitative Prone to artifacts Different techniques specific to region 1

2 Injection Contrast Concentration Injection 7/28/2009 Contrast Enhanced MRA Became popular during Gd contrast agent reduces blood T 1 and increase contrast between blood and surrounding tissue in T1w sequence. High resolution and good coverage with fast 3D gradient acqusition. Short acquisition times allow breath-holding for visualization of abdominal vasculature. Basic CE-MRA Technique mm/kg (20-40ml) of Gd contrast injected at 2-3 ml/sec. Flush with 20-30ml of saline. 3D spoiled gradient echo based sequence. Min. TE and Min. TR. Partial k-space acquisition. 0.8 x 0.9 x 0.6 mm 3 CE-MRA Considerations 1. Amount of Gd Contrast 2. Proper acquisition window and timing Accurate bolus timing by test bolus or fluro-trigger Centric view ordering 3. Accelaration with partial k-space acquisition Partial Echo, Partial Fourier, Parallel imaging, Radial sampling 4. Time resolved MRA with view sharing Key-hole, TRICKS/TWIST 5. Multi-station bolus chasing and continuous moving table acquisition for peripheral MRA (pmra) 1. Amount of Contrast Pulmonary arteries 0.1 mmol/kg Aorta mmol/kg Renal arteries mmol/kg Portal vein 0.2 mmol/kg Peripheral arteries 0.3 mmol/kg Gd: 20ml Gd: 40ml Courtesy of M. Prince, Cornell, NY 2. Acquisition Window and Timing CE-MRA Acquisition Timing Time to k-space Center Recessed Elliptical Centric View Order Artery Patient Specific Delay Artery Vein 0 sec Vein 12 sec 18 sec 24 sec 30 sec Time Fluoro Triggering : Realtime 2D scan of ~1 fps) Test Bolus: 2D fast scan with small dose Time 2

3 Contrast Concentration Injection 7/28/ Acceleration by Parallel Imaging Hi-res CE-MRA of Carotid Artery Without SENSE CE-MRA w. SSD DSA With SENSE 4. Time-resolved CE-MRA (tmra) Acceleration with Parallel Imaging Artery Vein One volume per sec with ipat = 4 Time Combined P.Finn et al., UCLA, Los Angeles, USA Acceleration by Under-sampling VIPR Pulmonary tmra 7.5 s 11 s 14.5 s VIPR (Vastly under-sampled Isotropic PRojection) 18 s 21.5 s 25 s T. Gu American Journal of Neuroradiology 26: s 32 s 35.5 s 11 averaged time frames (7.5 s to 44 s, 37 s total) 9150 projections 3

4 Acceleration by Sharing of k-space Data Divide k-space into central and peripheral regions. Sample central k-space points more frequently than peripheral points No lose of SNR. Increase frame rate, by temporal base remains same (temporal interpolation). 4D (Spatial & Temporal) Information TWIST 5. Bolus Chase pmra (Run-off) Continuous Moving Table MRA 1st station: reverse centric k-space acquisition 2nd & 3rd stations: centric k-space acquisition 3D Gd MRA: 87 sec Vogt, F. M. et al. Radiology 2007 Whole (Body) MRA with Coil Arrays Non-Enhanced MRA (NCE-MRA) 1. Time of Flight (TOF) 3D TOF intracranial arterial 2D TOF Carotid and peripheral 2. Phase Contrast (PC) Intracranial, renal 3. Balanced SSFP (bffe/truefisp/fiesta) Coronary, Renal 4. ECG Triggered 3D FSE (Native SPACE) Abdominal, peripheral University Munich (LMU) 4

5 1. Time of Flight (TOF) MRA The Principle of TOF No Flow Max. Saturation Slow Flow Fast Flow Partial Saturation No Saturation Imaging Slice Miyazaki, M. et al. Radiology 2008;248:20-43 The effective T 1 is reduced due to in-flow Elimination of Venous Signal 2D versus 3D for TOF MRA Arterial Flow Venous Flow 2D Less saturation, more sensitive to slow flow Better contrast between blood and stationary tissue 3D Better resolution in slab direction More efficient acquisition, greater SNR Variable FA Excitation Slice (TONE) Tracking Saturation Band Can be used to identify vessels feeding a given territory Multiple Overlapping Thin Slab Acquisition (MOTSA) TOF MRA Optimization Background Suppression with MT Orienting the slice/slab perpendicular to the direction of flow. Variation of flip angle across slice profile. Background tissue suppress with MT. Fat saturation. Cardiac gating to reduce pulsatile flow artifact. Acceleration. 3DTOF without MT 3DTOF with ECR MT 5

6 Accelerated 3D TOF MRA at 3.0T The Advantage of High Field 3DTOF (12:06) 3DTOFEC (7:11) 3DTOFEC + SENSE (3:41) Aneurysm (2.8 mm) of the Middle Cerebral Artery 1.5 T?? DSA 3.0T Higher SNR + Longer T1 + MT still possible : MT Region 2. Phase Contrast (PC) MRA Motion Dependent Phase Difference G t A Bipolar Gradient A t Z Df Moving spins Df = gat V Y Stationary spins or VENC = 0 t X M XY Moving spins Phase Contrast Images Need to acquire two images: 1) w/o flow encoding and 2) flow encoded f2-f1 phase difference forward flow bright background mid-gray reverse flow black f2-f1 magnitude of phase difference ALL flow bright background suppressed PC MRA Technique Requires two acquisitions Long scan time. Good background suppression from subtraction. Independent selection of VENC in each direction. Sensitive for slow flow provided there is adequate SNR and long T2*. No saturation as in TOF. Lost of signal due to turbulent flow at bifurcation and stenosis. Can be quantitative: Flow velocity ~ f2 - f1 6

7 3D PC MRA of Renal Circulation 3. Balanced SSFP (bssfp) MRA 3D CE-MRA DSA Finn, J. P. et al. Radiology 2006;241: D PC-MRA bssfp MRA Technique Flow compensated (Does not rely on blood flow) T2/T1 Contrast Pre-saturation of venous blood and stationary fluid signal with IR. (so that only fresh arterial blood produce signal.) RESP-triggered for abdominal. ECG-triggered for coronary. o Susceptible to off-resonance artifacts o High SAR 3D bssfp with NAV and FS Finn, J. P. et al. Radiology 2006;241: bssfp versus CE MRA for Renal Artery 4. ECG Triggered FSE MRA A B C D Navigator Triggered SSFP CE-MRA Maki, J. H. et al. Am. J. Roentgenol. 2007;188:W540-W546 Dr. Vivian Lee et al., NYU, USA, Jian Xu, Siemens, Alto Stemmer, Siemens 7

8 Inter-leaved Dual-Phase FSE Velocity Contrast ECG Diastolic triggering Systolic triggering Diastolic triggering Systolic triggering Delay 1 Delay 2 Delay 1 Delay 2 slice 1 slice 1 RF IR pulse 90 o 180 o 180 o 180 o 180 o 180 o slice 2 slice 2 - = Echo Diastolic (V+A) Systolic (V) MRA (A) FSE MRA Technique Signal lost due to fast flow during systole. Requires ECG triggering and correct setting of acquisition delays. Independent of flow direction. NATIVE versus CE for Peripheral MRA Courtesy of LMU, Munich, Germany Native versus CE MRA for Aortic Artery Renal NCE-MRA Source Image MIP bssfp PC TOF 8

9 Thank You! 9