mr 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 • Phase-Contrast MR Angiography • Quantitative Flow Imaging 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 Vessel Signal Voids Early multi-slice spin echo images depicted vessels in the neck as signal voids Multi-slice Spin Echo Long TR 90o-180o Fast flow Spins not get refocused by 180o pulse MRI Slices Stationary Tissue Flowing Blood Slice #1 Slice #2 Slice #3 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 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 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 The MRI Signal: Amplitude & Phase Bo rf = B1 Net Magnetization Imaginary Real Real Imaginary Dephasing Due to Motion Gslice time +180o PHASE BLOOD: phase not zero TISSUE: phase equals zero time Phase Shift Due to Motion in a Gradient Field -180o t=0 Phase Contrast Imaging Velocity Encoded Image PHASE +180o -180o PHASE +180o -180o TISSUE: phase equals zero in BOTH images Phase Difference time Velocity Compensated Image Motion Compensation Gradient (Bipolar) Applied time Velocity Encoded Image BLOOD: phase is DIFFERENT in each image Magnetic Field Gradients in MRI (Two More Functions) • Slice Selection • Phase Encoding • Frequency Encoding • Sequence Timing (Dephase/Rephase) • Motion Compensation • Motion Encoding 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 images Velocity compensated / velocity encoded 2D Phase Contrast MRA Advantages •Short acquisition times •Variable velocity sensitivity •Good background suppression •Minimal saturation effects •Short T1 tissues not show up on images 2D Phase Contrast MRA Limitations •Single thick section projection •Vessel overlap artifact •Sensitive to flow in only one direction •Unstructured flow may cause problems 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 3D Phase-Contrast MRA Renal Circulation FP Coronal, Gd enhanced TR/TE = 7/1.4 ms 40o flip, false renal stenosis (FP) Coronal, 3D PC TR/TE = 33/6 ms 20o flip 3D Phase Contrast MRA Advantages •Thin slices •Quantitative flow velocity and direction •Excellent background suppression •Variable velocity sensitivity •Short T1 tissues not appear on images 3D Phase Contrast MRA Limitations •Long acquisition times •Long TE values Flow Measurement with PC-MRI • Typically uses 2DFT phase contrast method • Slice positioned perpindicular to axis of vessel • ROI drawn to delineate vessel lumen – Average value in ROI is mean velocity – Area of ROI is vessel cross-sectional area • Flow = mean velocity * Area • For pulsatile flow, multi-phase cine required Phase Contrast Velocity Images Magnitude Phase Contrast No Flow Stationary Flow Velocity 29 cm/s In Out In Out Velocity Encoding Range (Venc) +Venc Phase Difference (degrees) MRI Velocity (cm/s) 180o -180o -Venc True Flow Velocity (cm/s) 3D Cerebrovascular Flow Flow Encoding Right to Left Magnitude Flow Encoding Anterior to Posterior Saggital Sinus Ant Cerebral aa Basilar a Straight Sinus Flow Encoding Cranial to Caudal Summary Two different approaches to MRA are commonly used: Time-of-Flight (TOF-MRA) & Phase Contrast (PC-MRA) TOF-MRA is easy to implement and is robust but has difficulty with slow flow 3D TOF can be combined with fast imaging methods and Gd contrast agents to obtain improved depiction of vascular structures Summary PC-MRA requires more time to acquire more images but can result in high resolution, fewer flow related artifacts, and quantitative measurement of flow Phase-contrast MRI may provide the most accurate, noninvasive method for measuring blood flow in vivo