Bioengineering 278" Magnetic Resonance Imaging" " Winter 2011" Lecture 9! Time of Flight MRA!

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1 Bioengineering 278" Magnetic Resonance Imaging" " Winter 2011" Lecture 9 Motion Encoding using Longitudinal Magnetization: Magnetic Resonance Angiography Time of Flight Contrast Enhanced Arterial Spin Labeling ASL Basics ASL for fmri Velocity Selective ASL Vessel Encoded ASL Time of Flight MRA Spoiled gradient echo with high flip angle and short TR Static magnetization becomes highly saturated Relaxed inflowing blood has much higher signal Signal (M 0 ) Flip=5:5:40 TR=20ms T 1 =1600ms Time (s) M z (tr) = M 0 (1" ((1" M z (tr "1)cos(#))e "TR /T 1 )) Signal(tr) = M z (tr)sin(#)

2 Contrast Enhanced MRA MRA acquired during the passage of a bolus of Gd based contrast agent T 1 reduced as low as 50ms T 1 is so short, no need to rely on TOF effect for contrast Allows for very short TR and high flip angle Dramatically improves speed and/or SNR After first pass, Gd leaks into tissues 4s per frame Contrast Enhanced MRA GEMS web site

3 Arterial Spin Labeling Inflow and T 1 Decay Tag by Magnetic Inversion Acquire image of tissue + tagged blood Control Acquire image of tissue + relaxed blood ASL Signal = Control - Tag CBF Tissue Signal subtracted out. Blood Water Dynamics Tissue Water E~95% in brain Perfusion: ml blood per 100 gram tissue per min Brain: 0.01s -1 Heart: 0.01s -1 Kidney: 0.01s -1 Lung: 0.01s -1 Skeletal Muscle: s s 0.5-1s s? Arteries Capillaries Veins

4 Tissue Dealing with Tissue Signal Pre-saturation (pre-sat): Saturation on the imaging slab Back-Ground Suppression (BGS): Globally applied inversion pulses to null tissue signal Mz Control pre-sat Mz Control BGS ASL ASL signal signal Tissue t t Tag Imaging Tag Inversion Imaging Types of ASL Imaging Plane CASL Flow Dependent Inversion Plane PASL Pulsed Inversion Slab Global Velocity Selective Tag VSASL

5 CASL: Flow Driven Adiabatic Inversion Effective field in frame that rotates at " L : B e = B ˆ 1 i + z (t )G ˆ Z k z z z z(t)g Z # B e y y y z(t)g Z B e x x x z(t)g Z RF B e G Z PASL: Pulsed Adiabatic Inversion Effective field in frame that rotates with pulse: B e = B ˆ 1 i + kˆ " z # B e " # z y # y y " B e x z x # " B e x #" $ Gz

6 Transit Delay - PASL PASL Inflow Curves 1500 ms #M ANATOMICAL CALCULATED PERFUSION DELAY MAP 0 ms QUIPSS II %t$ ($ TI < %t : #M(&')=0 %t < TI < %t+( : #M(&')=2) 0B (TI-%t) CBF e -TI/T1B %t+( < TI : #M(&')=2) 0B (() CBF e -TI/T1B TI 1 #&'$ INVERSION TAG AFTER SATURATION ACQUIRE IMAGES If TI 1 <( and #TI>%t then #M=2) 0B TI 1 CBF e -TI2/T1B Continuous vs Pulsed ASL Imaging Plane Imaging Plane T 1 Decay CASL Flow Dependent Inversion Plane Bolus Width Delay + Image PASL CASL PASL CASL vs PASL Larger ASL signal Higher SAR Pulsed Inversion Slab

7 Water Exchange Good Tissue Water Extraction ~95% Capillary Transit ~1s Arteries Capillaries Veins On the T 1 time scale, tagged blood behaves like short lived microspheres Not so good TI Pic: B=22 B=0 T 1 shift changes relaxation rate of tag during TI T 2 /T 2 * shift changes ASL signal during image acquisition CBF and BOLD Time Series RAW CBF BOLD For Pulsed ASL with Presat: Control - Tag = ASL Signal Control + Tag = Tissue Signal M z t

8 Simultaneous Flow and BOLD fmri Anatomy CBF Change BOLD Change Transit Delays in Diagnostic ASL Problem: Long transit delays (delay >>T 1 ) limit the diagnostic potential of ASL in some disease states Solution: Apply tag pulses that are not spatially selective but velocity selective

9 Velocity Selective ASL 1 Tagging based purely on velocity Tag has no spatial selectivity Transit delay eliminated Control Mz Image Tag Tag 0 90x 180y -90x READOUT Velocity (cm/s) T1=1.6s Vessel Encoded Pseudo-Continuous ASL " y1 % "(1 1 (1 1%"R% $ ' $ '$ ' $y2 ' = $ 1 (1 (1 1'$L' $y3 ' $(1 (1 1 1'$B' $ ' $ '$ ' #y4 & # &# S & 10

10 Vascular Mixing 90 Mixing Angle Akash Kansagra Linear Model Assign Colors By Histogram Vessel Encoded ASL Explicit identification of territories with probabilities Nonlinear encoding Vascular Source Imaging sin/cos modulation Multi-dimensional clustering 0