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Introduction to Magnetic Resonance Angiography Geoffrey D. Clarke, Ph.D. Division of Radiological Sciences University of Texas Health Science Center at.

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Presentation on theme: "Introduction to Magnetic Resonance Angiography Geoffrey D. Clarke, Ph.D. Division of Radiological Sciences University of Texas Health Science Center at."— Presentation transcript:

1 Introduction to Magnetic Resonance Angiography Geoffrey D. Clarke, Ph.D. Division of Radiological Sciences University of Texas Health Science Center at San Antonio

2 Overview Flow-Related Artifacts in MRI Time-of-Flight MR Angiography Contrast-Enhanced MR Angiography Phase-Contrast MR Angiography Quantitative Flow Imaging

3 Flow Voids & Enhancements In spin echo imaging vessels appear as signal voids –same volume of blood does not experience both 90 o and 180 o 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

4 Vessel Signal Voids Early multi-slice spin echo images depicted vessels in the neck as signal voids

5 FlowingBlood Multi-slice Spin Echo MRISlices Stationary Tissue Long TR 90 o -180 o Fast flow Slice #1 Slice #2 Slice #3 Spins do not get refocused by 180 o pulse

6 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

7 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.

8 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

9 The MRI Signal: Amplitude & Phase Net Magnetization BoBoBoBo rf = B 1 Real Imaginary Real Imaginary

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

11 Pulsatile Motion Artifact Aorta Artifact Artifact Artifact

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

13 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

14 Magnetic Resonance Angiography (MRA)

15 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

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

17 Time-of-Flight MRA Method Real Imaginary BoBoBoBo M

18 Time of Flight Effect T 1 of flowing water is effectively shorter than the T 1 of stationary water Two contrast mechanisms are responsible: –T 1 saturation of the stationary tissue –In-flow signal enhancement from moving spins

19 2D Time-of-Flight MRA Conditions Field Echo ImagingField Echo Imaging Short TEShort TE Partial Flip AnglePartial Flip Angle generally largegenerally large keeps stationary tissues saturatedkeeps stationary tissues saturated TR and flip angleTR and flip angle adjusted to minimize stationary tissueadjusted to minimize stationary tissue adjusted to maximize bloodadjusted to maximize blood

20 2D Time-of-Flight MRA Advantages Good stationary tissue to bloodGood stationary tissue to blood flow contrast flow contrast Sensitive to flowSensitive to flow Minimal saturation effectsMinimal saturation effects Short scan timesShort scan times Can be used with low flow rateCan be used with low flow rate

21 2D Time-of-Flight MRA Limitations Relatively poor SNRRelatively poor SNR Poor in-plane flow sensitivityPoor in-plane flow sensitivity Relatively thick slicesRelatively thick slices Long echo times (TE)Long echo times (TE) Sensitive to short T 1 speciesSensitive to short T 1 species

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

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

24 Magnetization Transfer Contrast “Free” Water Lipids “Bound” Water PROTON SPECTRUM Frequency (Hertz) 217 Hz 1500 Hz 0 0

25 Gradient Echo with MTC Pulse TX RX G sl G ro G pe RF excitation Field Echo Crushers or Spoilers Phase Encode Dephasing Rephasing Slice Select Read Out Digitizer On Off-resonance rf pulse Spoilers

26 Maximum Intensity Projections MIP #1 MIP #2 OBJECT

27 2D TOF Application Abdominal Aneurysm

28 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

29 Three Dimensional Gradient Refocused Echo Imaging TX RF pulse (short time) Field Echo RX G sl G ro G pe Crusher Primary Phase Encoding Phase Rewinder Dephasing Rephasing Slab Select Read Out Digitizer On Secondary Phase Encoding

30 3D Time-of-Flight MRAAdvantages Higher resolution (thinner slices) available allowing for delineation of smoother edgesHigher resolution (thinner slices) available allowing for delineation of smoother edges Higher signal-to-noise than 2D methodsHigher signal-to-noise than 2D methods Lower slice select gradient amplitudes results in fewer phase effect artifacts than 2D methodLower 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

31 3D Time-of-Flight MRALimitations Blood signal is easily saturated with slow flowBlood signal is easily saturated with slow flow Relatively poor background suppressionRelatively poor background suppression Short T 1 tissues may be mistaken for vesselsShort T 1 tissues may be mistaken for vessels

32 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 = 25 o 256 x 256, 18 cm FOV, 0.78 x 1.56 mm pixel MTC contrast Venous Presaturation

33 Circle of Willis Time of Flight MRA 90 o

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

35 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

36 Gd Contrast Enhanced MRA Gd contrast agents decrease T 1 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

37 MRI Compatible Power Injectors Programmable Automatic Injection MRI Compatible Allows rapid arterial injection of Gd-DTPA

38 3D CE-MRA of Aortic Aneurysm Digital Subtraction X-ray Angiography Phase 3 Phase 2 Phase 1 Phase 2 Phase 1 44 slices 32 sec scan TR/TE = 2.3/1.1 ms 1.5 x 1.8 x 1.8 mm pixel Schoenberg SO, et al. JMRI 1999; 10:

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

40 Normal Runoff MRA Image of tissue surrounding vessel can be manually striped off

41 Phase-Contrast MRA Method Real Imaginary BoBoBoBo 

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

43 Phase Contrast Imaging time PHASE -180 o +180 o time BLOOD: phase is DIFFERENT in each image TISSUE: phase equals zero in BOTH images PHASE -180 o +180 o Velocity Encoded Image Velocity Compensated Image Phase Difference Velocity Encoded Image Motion Compensation Gradient (Bipolar) Applied

44 Magnetic Field Gradients in MRI (Two More Functions) Slice Selection Phase Encoding Frequency Encoding Sequence Timing (Dephase/Rephase) Motion Compensation Motion Encoding

45 2D Phase Contrast MRA Features Can use minimum TRCan use minimum TR  doesn’t rely on T 1 effects Good for slow flowGood for slow flow Motion is imaged in only one direction Motion is imaged in only one direction  usually slice select Requires 2 images Requires 2 images  Velocity compensated / velocity encoded

46 2D Phase Contrast MRA Advantages Short acquisition times Variable velocity sensitivity Good background suppression Minimal saturation effects Short T 1 tissues do not show up on images

47 2D Phase Contrast MRA Limitations Single thick section projection Vessel overlap artifact Sensitive to flow in only one direction Unstructured flow may cause problems

48 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

49 3D Phase-Contrast MRA Renal Circulation Coronal, Gd enhanced TR/TE = 7/1.4 ms 40 o flip, false renal stenosis (FP) Coronal, 3D PC TR/TE = 33/6 ms 20 o flip FP

50 3D Phase Contrast MRA Advantages Thin slices Quantitative flow velocity and direction Excellent background suppression Variable velocity sensitivity Short T 1 tissues do not appear on images

51 3D Phase Contrast MRA Limitations Long acquisition timesLong acquisition times Long TE valuesLong TE values

52 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

53 Phase Contrast Velocity Images No Flow Flow Velocity 29 cm/s Magnitude Phase Contrast Stationary In Out In Out

54 Velocity Encoding Range (V enc ) -V enc +V enc Phase Difference (degrees) MRI Velocity (cm/s) 180 o -180 o True Flow Velocity (cm/s)

55 3D Cerebrovascular Flow Magnitude Flow Encoding Cranial to Caudal Flow Encoding Right to Left Flow Encoding Anterior to Posterior Saggital Sinus Straight Sinus Ant. Cerebral aa. Basilar a.

56 Summary 1.Two different approaches to MRA are commonly used: Time-of-Flight (TOF-MRA) & Phase Contrast (PC-MRA) 2.TOF-MRA is easy to implement and is robust but has difficulty with slow flow 3.3D TOF can be combined with fast imaging methods and Gd contrast agents to obtain improved depiction of vascular structures

57 Summary 4.PC-MRA requires more time to acquire more images but can result in high resolution, fewer flow related artifacts, and quantitative measurement of flow 5.Phase-contrast MRI may provide the most accurate, noninvasive method for measuring blood flow in vivo


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