4d flow mri: principles and emmerging applications in...

4D Flow MRI: Principles and Emmerging Applications in Aortic disease

SFICV 2019 – JUNE 20TH, VICHY, FRANCE

Arshid AZARINECardio-Vascular ImagingHôpital Paris Saint-Joseph

Disclosure

Medical Adviser for Arterys ®

Introduced in late 80’s

2D-PC is a cine sequence ECG gated to cover a cardiac cycle

Principle: 2D-PC evaluates the loss of phase of mobile protons along a bipolar linear magnetic gradient

Thus we can assess velocity and direction ofMobile protons

Background: Fluxometry in MRI using 2DPhase Contrast

Stankovic & al. CD &T, 2014

Magnitude image:Signal intensity relates to the velocity but with no directional information.

2D Phase Contrast: One-directional through-plane (Z) velocity encoding sequence

Phase image: the blood flow is demonstrated with directional information

1 slice orthogonal to the vessel of interest axis Giving 2 images : • 1 Magnitude image• 1 Phase difference Image

Velocity Map:

Static TissueIntensity = 0

Moving blood:Positive / Negative value

Region of interestContouring XsectionOf the vessel

Magnitude image:Signal intensity relates to the velocity but with no directional information.

2D Phase Contrast: One-directional through-plane (Z) velocity encoding sequence

Phase image: the blood flow is demonstrated with directional information

In complex aortic diseaseMultiples slices needed orthogonal to each vessel aortic segment = Many breathholds + many fights and time to find the right plane… 6-10 minutes…

Results not always optimal

RA + Coarctation

4D Flow Imaging

Bogren & Buonocore JMRI 199915 slabs – 6mm Thickness/slabTAC: 60 minutes Temporal Resolution (TR) 112ms Hours/days of post processing

November 2018 TAC: 8:00 minutes – 1.5mm Thk – TR 30msOnline realtime post-processing

Antero-posterior Superior-Inferior

Magnitude Image

Y Z

X

Right - Left

4D Flow Raw Data

= 1Magnitude & 3 Phase Difference Volumes

4D Flow MR: how does it work?


Magnitude Image

Y Z

X

Right - Left

4D Flow Raw Data

•Eddy currents•Noise masking•Filtering of static tissue

= 1Magnitude & 3 Phase difference Volumes

Cloud processing (Protected Health Information)

Preprocessing:Phase Offsets &

Background correction

Helped by AI

After filtering

Accurate Filtering of eddi currents & offsets errors for reliable Flow Measurement, now helped by DL


Magnitude Image

Y Z

X

Right - Left

4D Flow Raw Data





Helped by AI

Post-Processing:

Visual Flow Analysis



Magnitude Image

Y Z

X

Right - Left

4D Flow Raw Data





Helped by AI

Post-Processing:

Visual Flow Analysis


Current trends•Forward Flow•Reverse Flow•Regurgitation Fraction•Peak Velocity

Advanced tools•Wall Shear Stress

Quantitative Flow Analysis

4D Flow - Checklist just before we start! Optimal ECG Gating (4D = 3D + Time!), retrospective

Explain the patient that the sequence will last 6–10 minutes

Free regular breathing +/- respiratory gating

To Inject or Not To Inject Gadolinium ?

4D Flow is possible without injection

Contrast media increases SNR: larger coverage & shorter acquisition time!

Temporal Resolution / number of Cardiac Frames

Acceleration factor (Compress sensing/ k-t undersampling)

Recommandation Reason Comment

ECG-gating Retrospective preferred Avoid sequence interruptionCover entire R-R cycle

Crucial for all anatomic areas

VENC Maximum velocity expected(10% higher when possible)

Avoid velocity aliasing Highest is the venc lower is the VNRUse Multiple Venc if available

Temporal Resolution Maximum, <40ms Accuracy Max 60ms (to save time)

Spatial Resolution Optimal, isotropic voxels2-2,5mm

Accuracy About 5 voxels in the vessel of interest

Field of View Better SNR & coverage

Flip Angle 7°(NC) - 15° (Post-Gd) Better CNR Higher if use of contrast agent

Contrast agent Macromolecular Larger coverageBetter SNR

Bolus followed by very slow perfusion

Offset errors correction Eddy current/phase unwrapping/static tissueNow helped by DL

AccuracyCorrection of offsets

Maxwell correction is done during preprocessing

Analysis step 1: Visual Analysis, 4D Flow offers new tools

Color coding:Red= high velocityBlue= low velocityAdjustable ScaleConsidering the vessel of interest

Velocity VectorsStreamlinesColor velocity 3D MIP

Streamlines: the path that a particle would take if released into the velocity field with the field held constant

3D Comprehensive visual analysis Of different blood flows:

Laminar, Helical, Eccentric…

Normal Laminar Systolic Flow

Eccentric Turbulent FlowDuring systole in an aortic regurgitation

(Helix: particles revolve around an axisand also have a net forward velocity parallel to the direction of the axis)

HelicalTurbulent Systolic FlowAfter a stenoticComplex Coarctation

Normal central flowWith high velocity

Analysis Step 2: Flow measurementMulti planar Reformat

• Retrospective 3D multi planar navigation and optimal positioning of any plane within the volume

• Cross sectional ROI placement orthogonal to the vessel axis

• Cloud computing: from anywhere with any device (laptop, tablette…)

Conventional parameters include as in 2D flow:• Forward Flow• Reverse Flow• Regurgitation Fraction• Peak VelocityQuality control apply:- Mass conservationPrinciple!- Comparaison toLV/RV Stroke Volumes

4D Flow allows Combined Visual and Quantitave Flow AnalysisThe more Complexe is the case the more 4D flow is usefull

2 - optimal measurement

1- risk of error in estimation of the pulmonary RF

Dilated Pulmonary Artery

Vortical turbulent flow

Pulmonary regurgitation

Site 1: 51% of Regugitation FractionSevere PR

Site 2: 42% of Regurgitation Fraction= Moderate pulmonary regurgitation

Combined visual and quantitative analysis is mandatory for a more accurate assessment

2D-PC“Blinded” to the FlowAssessment

4D Flow MR in Aortic Regurgitation

Vena Contracta = 8mm

PISA= 7mm

4D Flow

TTE

Classical doppler ultrasound parameters can be measured by 4D Flow. Here we find good correlation between TTE & 4D Flow

Reverse Vortical Flow in Aortic Regurgitation

Vortex: the flow rotates along an axis (just like in a whirlpool) During diastole, ReverseVortical flow is seen in blue « pushing » systolic flow to hit the anterior wall

Visual Analysis tools: Streamlines A Comprehensive visualisation of the blood flow

4D Flow MR in Valvular diseaseAortic Regurgitation

• Forward Flow• Reverse Flow

Volume• Regurgition

Fraction (FR)• Peak Velocity

Can be Retrospectively & optimaly

Assessed in any plane and location

Ascending Aorta

Vortical reverse flow in diastolein Aortic RegurgitationComprehensive visualization velocity vectors

Vortical reverse flowin Aortic Regurgitation: a whirlpool!Comprehensive visualization of different flowsparticles revolving around a point

SystoleAzarine A et Al. Radiographics 2019

Mesures après Visualisation des Flux Flux laminaires/Flux hélical / Flux vortical

Mesures optimales en évitant les zones de turbulences

Flux antérograde = Volume d’éjection Systolique calculé selon Simpson (en l’absence d’I. Mitrale)

Kaplan-Meier survival curve showing survival without surgery for conventional indicationsRF of 37% seems to be threshold for severity in CMR (35-40%)

To Further evaluate with 4D flow MRI

S G Myerson et al. Heart 2011;97:A93-A94

Aortic StenosisEmerging 2D Flow hemodynamic markers

Effective Orifice Area

Bernouilli continuity equation

Stroke Volume (SV) measured at LVOT

Velocity-Time Integral VTIAO

is measured at the aortic root 12mm above

CMR derived EOA=SV(LVOT)/VTIAo

Garcia et al. Journal of Cardiovascular Magnetic Resonance 2012

Assessment of Aortic StenosisCurrent flow-derived parameters

75 year-old-male with increasing dyspnea moderate aortic stenosis at TTE

75 yo M4D Flow demonstratedModerate Aortic stenosis &Mild aortic regurgitation (12% of FR)

4D FLOW also revealed Sinus venosus ASD with highly significant QP/QS=5!Protosystolic Inverted Right to Left Shunt (vector graph) - PHTRight Ventricle was dilated >150ml/m² with turbulent PA flow

& Partial Anomalous (right) pulmonary veinous return in SVC

PAPVR

4D Flow and pst procedure follow upPost TAVI Aortic Regurgitation• TAVI: frequency of percutaneous

replacement of the aortic valve is increasing worldwide

• Post TAVI Aortic Regurgitation (AR) is sometimes difficult to assess by TTE

• In this patient AR was diagnosed By TTE after TAVI Implantition, but it’s nature and severity where difficult to assess

• 4D Flow imaging demonstrated minim and mild para prosthetic leak at 2 points in this patient with very calcified annulus

Follow up can be pursued with 4D FLOW MR

Aortic valve stenosis +/- Aortic dilation4D Flow advanced tools

Garcia et al. J A C C : C A R D I O V A S C U L A R I M A G I N G , V O L . 1 2 , N O . 2 , 2 0 1 9

Viscous Energy Loss

Viscous Energy Loss and helical flowPronostic Factor to come?

Garcia et al. J A C C : C A R D I O V A S C U L A R I M A G I N G , V O L . 1 2 , N O . 2 , 2 0 1 9

4D Flow advanced features:Flow derived Wall Shear Stress (WSS)

Shear Stress

BloodVelovity Profile

Vessel Wall

WSS= v/x v

x

= Blood Viscosity

WSS is expressed in Pa or N/m²

v/x denotes radial velocity gradient at the vessel wall

Indicator for impact of Flow changes onEndothelial cell and extra cellular matrix function

WSS is defined as the tangential viscous shear forces per unit area that is exerted by blood flow to the surface of the vessel

• Assuming blood flow along the vessel, the magnitude of WSS is proportional to the radial velocity gradient at the vessel wall

Elevated WSS correlated to Elastin degeneration in aortic wall

Flow Eccentricity BAV patients

• Flow eccentricity was the most sensitive to differences in BAV phenotype• WSS or jet impingement angle and jet flow eccentricity may help better risk-

stratification in patients with ascending aorta aneurysms

Mahadevia R et al. Circulation. 2014 Feb 11; 129(6): 673–682

4D Flow MR in resistant hypertension in a young adult

Imaging of renal arteries demonstrating large collaterals suggesting coarctation in a Young 38 year old adult with resistant hypertension:Complexe severe coarctation was demonstrated by CMR with 4D Flow

Charpentier, Azarine Vasc Med 2017

4D Flow MR in Aortic DiseaseAortic Coarctation without bicuspid valve

Systole Diastole

Viscous Energy Loss in Descending Aorta

Systolo-diastolicTurbulent flox through the coarctation

4D Flow MR in Aortic DiseaseFollow up aortic dissection

True lumen

No regression of the size of the false lumen was noted after endograft. TYPE I or II?4D Flow MRI demonstrates systolic type 1 proximal endoleak (arrow).

Endovascular Abdominal Aortic Repair (EVAR) Follow up

Inflow leak from lombar arteries to the aneurysmal sac

L= AORTIC LUMEN, LA= LOMBAR ARTERIES, E= ENDOLEAK, IMA= INFERIOR MESENTERIC ARTERY

L

IMA

E

LALA

EVAR: Type II b Endoleak

Right LAInflowLA together 0,04L/min

Left LA

Inflow leak from lombar arteries to the aneurysmal sac

IMA: outflow0,04L/min

4D Flow MR in Aortic DiseaseTakayasu disease

18 yo FemalewithTakayasu disease under TNF Arterial HypertensionDespite multi drug T.No Renal A. stenosis

Higher velocity in areas w/Aortic wall thickenningElevated aortic Stiffness??

95

72

4D Flow MR in Aortic DiseaseTakayasu disease

Velocity derived Wall shear stress measurement:Increased Abdominal Aorta WSS in a 18 yo F w/ Takayashu diseaseHypertension without RA Stenosis

WSS MAPduring Diastole

Increased WSSduring Systole

Flow acceleration in the areas of aortic wall thickening

Persistent Inflamatory parietal thickening

4D Flow MR in Takayasu disease: exploring different vascular beds

With high & low velocity blood flows

Aliasing!!Flow = AUC

CorrectionAlgorythmsNot accurate

If VENC not high enoughPhase wrap artifacts (Aliasing) : measurement errors

Set VENC 10% higher than the Maximum velocity expected

But higher is the VENC, higher is noise!

SCOUT Venc (some constructors)2D-PC Test sequence

Assessing multiple vascular bedsDual or Multiple VENC sequences!

Brand new! We can avoid phase wrap artifact withDual Venc sequences (x1,5 longer but 2 Vencs)

High Venc (aortic and pulmonaryHemodynamics)

Low Venc (Vena cava connections IVC & SVC flows)

Low-Venc dataset: Assessment of low flowIVC and SVC connections patency after Senning surgery

High-Venc dataset: assessment of pulmonary and aortic hemodynamics (Aliased on the low-venc dataset)

Exemple of Dual Venc sequence in a patient followed-upAfter Senning repair for Transposition of Great Arteries

Simultaneous Assessment ofLow velocities(IVC/SVC connections)With goodVNR

AndHigh velocitiesWithout aliasing

4D Flow is a revolutionary new tool in CMR enabling

Optimal flow measurements

A new comprehensive approach by visualizing blood flows

4D Flow enters the field of clinical applications now!

Congenital heart disease, aortic and Valvular heart disease are next

Widespread of new applications are appearing

The more complex the disease the more useful is 4D Flow

Reliability: compare to stroke volume, mass conservation law

Aortic Regurgitation: next Gold Standard??

Dual Venc sequences, when needed, enables good assessment of low and high blood velocities Within the same sequence

4D Flow is ready to go!

merciThank you for your attention…

You can findThese Pictures Movies, tables & moreIn RadiographicsMay-June 2019ByAzarine A et al.

4d flow mri: principles and emmerging applications in...

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