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Introduction to Magnetic Resonance Angiography

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Presentation on theme: "Introduction to Magnetic Resonance Angiography"— 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 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

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

5 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

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 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

9 The MRI Signal: Amplitude & Phase
Bo rf = B1 Net Magnetization Real Imaginary Real Imaginary

10 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

11 Pulsatile Motion Artifact
Aorta Artifact Artifact Artifact

12 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.

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

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

18 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

19 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

20 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

21 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

22 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

23 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

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

25 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

26 MIP #1 Maximum Intensity Projections 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
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

30 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

31 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

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

33 Circle of Willis 90o Time of Flight MRA

34 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

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 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

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

38 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 Phase Phase 1 Digital Subtraction X-ray Angiography Phase Phase 1 Schoenberg SO, et al. JMRI 1999; 10:

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

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

41 Phase-Contrast MRA Method
Bo Imaginary Real

42 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

43 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

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 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

46 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

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
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)

50 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

51 3D Phase Contrast MRA Limitations Long acquisition times
Long 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
Magnitude Phase Contrast No Flow Stationary In Out Flow Velocity 29 cm/s

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

55 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.

56 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

57 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

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