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

Spins do not get refocused by 180o pulse Multi-slice Spin Echo Long TR 90o-180o Fast flow MRI Slices Flowing Blood Stationary Tissue Spins do not get refocused by 180o pulse 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 in phase-encode 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 Real Imaginary Real Imaginary

Dephasing Due to Motion Gslice time +180o BLOOD: phase not zero TISSUE: phase equals zero PHASE time Phase Shift Due to Motion in a Gradient Field -180o t = 0

Pulsatile Motion Artifact Aorta Artifact Artifact Artifact

Motion Compensation Gradients Gslice time Phase Shift Due to Motion in a Gradient Field +180o PHASE time -180o BLOOD: phase equals zero t = 0 TISSUE: phase equals zero *Only applies for constant flow. More gradient lobes needed for acceleration.

Flow Artifact Correction Spatial pre-saturation pulses prior to entry of the vessel into the slices Surface coil localization Shortened pulse sequences Cardiac & respiratory gating Motion Compensation Gradients

Magnetic Resonance Angiography (MRA)

MRA Properties Utilizes artifactual signal changes caused by flowing blood to depict vessel lumen May include spin preparation to suppress signal from stationary tissues or discriminate venous from arterial flow Does not require exogenous contrast administration, but contrast agents may be used to enhance MRA for fast imaging

Methods of Magnetic Resonance Angiography Signal Amplitude Methods 2D Time-of-Flight 3D Time-of-Flight Signal Phase Methods 2D Phase Contrast (Velocity Imaging = Q-flow) 3D Phase Contrast

Time-of-Flight MRA Method Bo M Imaginary Real

Time of Flight Effect T1 of flowing water is effectively shorter than the T1 of stationary water Two contrast mechanisms are responsible: T1 saturation of the stationary tissue In-flow signal enhancement from moving spins

2D Time-of-Flight MRA Conditions Field Echo Imaging Short TE Partial Flip Angle generally large keeps stationary tissues saturated TR and flip angle adjusted to minimize stationary tissue adjusted to maximize blood

2D Time-of-Flight MRA Advantages Good stationary tissue to blood flow contrast Sensitive to flow Minimal saturation effects Short scan times Can be used with low flow rate

2D Time-of-Flight MRA Limitations Relatively poor SNR Poor in-plane flow sensitivity Relatively thick slices Long echo times (TE) Sensitive to short T1 species

Improving Contrast in Time-of-Flight MRA 1. Venous Pre-saturation (spatial suppression) 2. Magnetization Transfer Contrast (frequency selective irradiation) 3. Fat Saturation 4. Cardiac Gated MRA 5. Spatial variation of flip angle

Spatial Pre-saturation in Time-of-Flight MRA Saturates and dephases spins before they enter imaging slice Can be used to isolate arteries or veins Can be used to identify vessels feeding a given territory Can be used to establish the direction of flow in a particular vessel

Magnetization Transfer Contrast PROTON SPECTRUM Frequency (Hertz) “Free” Water Lipids “Bound” Water 217 Hz 1500 Hz Frequency (Hertz)

Gradient Echo with MTC Pulse Off-resonance rf pulse RF excitation TX Digitizer On Field Echo RX Slice Select Gsl Rephasing Spoilers Read Out Crushers or Spoilers Dephasing Gro Phase Encode Gpe

MIP #1 Maximum Intensity Projections MIP #2 OBJECT

2D TOF Application Abdominal Aneurysm

3D Time-of-Flight MRA Conditions Uses two phase encode gradients and volume excitation Maximum volume thickness limited by flow velocity Use minimum TR, adjust flip angle for best contrast

Three Dimensional Gradient Refocused Echo Imaging RF pulse (short time) TX Field Echo RX Slab Select Secondary Phase Encoding Digitizer On Gsl Rephasing Crusher Read Out Gro Dephasing Primary Phase Encoding Phase Rewinder Gpe

3D Time-of-Flight MRA Advantages Higher resolution (thinner slices) available allowing for delineation of smoother edges Higher signal-to-noise than 2D methods Lower slice select gradient amplitudes results in fewer phase effect artifacts than 2D method Short duration RF pulses can be used to excite slab – TE can be reduced

3D Time-of-Flight MRA Limitations Blood signal is easily saturated with slow flow Relatively poor background suppression Short T1 tissues may be mistaken for vessels

3D-TOF Application: Cerebral Arteries – Circle of WIllis TR /TE = 40 / 4.7 ms 64 partitions, 48 mm slab, 0.75 mm per partition Flip angle = 25o 256 x 256, 18 cm FOV, 0.78 x 1.56 mm pixel MTC contrast Venous Presaturation

Circle of Willis 90o Time of Flight MRA

Cerebral Venous Angiogram TOP Use of arterial presaturation allows visualization of cerebral venous vessels Saggital Sinus FRONT Straight Sinus Transverse Sinus Confluence Of Sinuses Cerebven.mpeg

Multi-Slab 3D TOF MRA Hybrid of 2D and 3D methods: Thin 3D slabs used Good inflow enhancement Multiples slabs to cover volume of interest High resolution Short TE Relatively time inefficient

Gd Contrast Enhanced MRA Gd contrast agents decrease T1 and increase CNR of blood and soft tissue Along with ultra-fast 3D sequences, allow coverage of larger VOI’s Shorter acquisition times allow breath-holding for visualization of central and pulmonary vasculature

MRI Compatible Power Injectors Programmable Automatic Injection MRI Compatible Allows rapid arterial injection of Gd-DTPA www.medrad.com

3D CE-MRA of Aortic Aneurysm 44 slices 32 sec scan TR/TE = 2.3/1.1 ms 1.5 x 1.8 x 1.8 mm pixel Phase 3 Phase 2 Phase 1 Digital Subtraction X-ray Angiography Phase 2 Phase 1 Schoenberg SO, et al. JMRI 1999; 10:347-356

Bolus Chase 3D MRA Station 1 Station 2 Station 3 Earlier venous enhancement noted with fast injection Ho VB et al. JMRI 1999; 10: 376-388

Image of tissue surrounding vessel can be manually striped off Normal Runoff MRA Image of tissue surrounding vessel can be manually striped off http://www.uth.tmc.edu/radiology/publish/mra/gallery.html

Phase-Contrast MRA Method Bo  Imaginary Real

Dephasing Due to Motion Gslice time +180o BLOOD: phase not zero TISSUE: phase equals zero PHASE time Phase Shift Due to Motion in a Gradient Field -180o t = 0

Phase Contrast Imaging Velocity Encoded Image +180o Phase Difference PHASE time Velocity Compensated Image -180o Motion Compensation Gradient (Bipolar) Applied +180o PHASE time Velocity Encoded Image -180o TISSUE: phase equals zero in BOTH images 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 Good for slow flow doesn’t rely on T1 effects Good for slow flow Motion is imaged in only one direction usually slice select Requires 2 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 do 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, 3D PC TR/TE = 33/6 ms 20o flip Coronal, Gd enhanced TR/TE = 7/1.4 ms 40o flip, false renal stenosis (FP)

3D Phase Contrast MRA Advantages Thin slices Quantitative flow velocity and direction Excellent background suppression Variable velocity sensitivity Short T1 tissues do 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 In Out Flow Velocity 29 cm/s

Velocity Encoding Range (Venc) MRI Velocity (cm/s) Phase Difference (degrees) -180o -Venc True Flow Velocity (cm/s)

3D Cerebrovascular Flow Flow Encoding Right to Left Magnitude Saggital Sinus Flow Encoding Anterior to Posterior Flow Encoding Cranial to Caudal Straight Sinus Ant. Cerebral aa. Basilar a.

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