Blood Flow Dynamics and MRI Flow Phenomena in Neuroimaging

Blood flow motion

Spiral Flow, Laminar Flow, Turbulent flow, and vortex flow

Laminar Flow

Normal, Constant, orderly, and parallel

1st order motion

Laminar Flow

Parabolic in context of Laminar flow

Velo Profile, fast @ center, slow @ walls

Gradient Nulling

Compensates Laminar Flow

Blood flow in Large/Curved arteries

Spiral

Vortex Flow

Swirling/rotational motion

Turbulent flow

Random fluctuations and chaotic

Flow phenomenon

Time of Flight, Intravoxel dephasing, Entry slice phenomenon

Time of flight

Flow Phenomenon that describes stationary spins return more signal than flowing b/c spins can exit image before receiving both excite and rephase pulse

TOF effects

As TOF increases, signal intensity decreases

High velo signal loss

Signal loss due to TOF effects

TOF phenomenon more prominent during SE than GE

True

Why TOF phenomenon more prominently in SE

Alpha and Rephasing pulses used are slice selective, and TE values are long

As Flow velo increases

TOF effects increase

TE increase

TOF effects increase

Entry slice

Flow Phenom when flowing spins enter stack of slices unsat'd, increased sign intensity

Inflow effect

Term Synonymous with 'entry slice phenom'.

Counter and Co current

Perpendicular flow relative to stack of slices

Prolonged RF exposure

What happens to spins flowing parallel in ESP (entry slice Phenom)

Intravoxel dephase

More prevalent in GE than SE.

DWI

An MRA imaging technique.

GE sequences

Type of pulse sequence used in contrast enhanced MRA

VENC parameter, Bipolar gradient application

Two things involved in phase contrast imaging

VENC gradient

Allows velo encoding to go in any direction.

2D TOF imaging

Used for slower blood flow

3D TOF imaging

Used for imaging anatomicals that need a smaller FOV.

3D TOF

Provides better spatial res than 2D TOF.

Parameters for 2D TOF

50 deg F/A, 30ms, min tr

Gradient pulses

Applied before and after 180 deg rf rephase pulse during DWI to create sensitivity to water.

Anisotropic

Terms synonymous with 'Directional Diffusion'.

Isotropic

A term commonly synonymous with 'non-directional diffusion'.

White Matter

Best demonstrated by ANISOTROPIC diffusion during DWI sequences.

B-Value

Net movement of water molecules within biological tissues driven by random thermal motion.

B - Factor

Synonym for B-value.

Units for b-value

s/cm^2

Typical B factor values

500-1500s/mm^2

B-Value increase = DWi increase

True.

SNR decreases

How does increasing B value in DWI affect SNR?

DWI trace images

Initial Images of DWI.

T2

Artifact that occurs with long t2 decay time.

DTI

Used to visualize white matter tracts.

DTI sequences

Used before neuro surgery.

GE sequence

Pulse sequence used in SWI.

Echo time

Susceptibility variations primarily affect which MR Signal in SWI.

SWI Uses

Blood, Calcifications, Venous structures.

PWI

MR Technique used to assess blood supply to tissue.

Perfusion

Physiological process of delivering oxygen and nutrients to tissues via blood flow.

CBV Measuring unit

Ml per 100g of tissue.

Reduced CBF may indicate ischemic areas

During PWI, it causes.

Dynamic susceptibility contrast (DSC)

Uses parameters like Cerebral Blood volume (CBV), Cerebral Blood Flow (CBF), Mean transit time (MTT), Ktrans (Volume transfer constant).

Perfusion parameter

Rate of the contrast agent when it moves from vascular space into extracellular, extravascular space.

Dynamic susceptibility contrast (DSC) perfusion imaging

Imaging method that uses Wash-in and wash-out phases.