Blood Flow Measurement Techniques Typical values for blood flow ...

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PD Dr. Zöllner I Folie 155 I 9/9/2014

Blood Flow Measurement Techniques

!  not only blood pressure but blood flow important information

!  describes the supply of blood in organs and tissue

!  abnormal changes in the blood flow or blood velocity gives rise to malformatuon of vessels

!  blood flow Q: volume of blood per time [ml/min]

!  perfusion: blood supply in capillary bed [ml/min/100g]


Typical values for blood flow [cm/s]:

Aorta 100 – 250 Abdominal 100 Carotis commuis 80 – 150 Vena Cava 5 – 40

Blood Flow Values Body

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Blood Flow and Pressure Waves Abdominal Vasculature

Nic

ols

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rke,

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onal

d‘s

Blo

od F

low

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rterie

s


Electromagnetic flow meter

!  based on Faraday‘s law of induction

!  blood is a conductor of electricity

!  can be ‚aligned‘ by external magnet field

Nicols & O‘Rourke, McDonald‘s Blood Flow in Arteries

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Blood velocity measurement using US


Typical Values

speed of sound:

velocity of blood:

transmitted US-frequency:

Doppler-frequency

" Audio Range

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Continuous Wave (CW) - US

base wave

echo

product

Doppler

sender

mixer

bandpass display

sender

receiver

flow signal

fS

fE

IM

fS-fE


Pulse Wave (PW) - US

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US Doppler Probes - Invasive

Nicols & O‘Rourke, McDonald‘s Blood Flow in Arteries


MR Blood Flow Measurements (1)

Lotz et al. , RadioGraphics 2002

#  spins moving along a magnetic field gradient acquire a difference in the phase (right)

#  amount of phase difference proportional to the velocity of the moving spins

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Lotz et al. , RadioGraphics 2002

#  flow encoding by means of difference in the bipolar gradients

#  acquisition of two images #  phase difference by substraction



Phase difference expressed in formulars:

γ gyromagnetic ratio Δm difference of the first moment of the

gradient time curve ν velocity

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#  ΔM relates to the velocity encoding (VENC) #  gives sensitivity of measurable velocities #  relates to the gradient strength


Diagram of MR PC sequence

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PC-MR Images

Flow encoding needs an operator defined value:

VENC [cm/sec]

-VENC < v < VENC

#  receive 2 images

# magnitude image with velocity

#  phase image with velocity and direction


#  flow encoding is directional

#  depends on the gradient axis applied

#  therefore: 3D flow measurements possible in each voxel

3D PC MRI

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Courtesey of J Henning, Freiburg

Examples


MIP Projection – Renal Artery

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RAS, Secondary Hypertension, etc.

Parameters to detect status: !  Flow reduction !  Flow velocity ! Kidney size ! Overall and split renal

function

!  RAS leading cause for secondary hypertension " CKD

!  Reduced flow triggers auto-regulation of systemic circulation

!  Loss of renal parenchyma in the stenosed kidney

!  Microangiopathy in the contralateral kidney

Allow for stenosis grading and better selection of patients for PTA


Example - Acquisition Protocol

ECG gated PC cine MR sequence with:

!  TE=4ms, TR=37ms, FA=30°

! VENC=100 cm/s

!  ca. 20 images / cardiac cycle

! Resolution 0.5 x0.5 mm

! Slice thickness 6mm

1.5T Siemens Symphony Vision

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Example – Slice Positioning

Schönberg et al. , Radiology 1997

a.  Abdominal aorta

b.  Images acquired from MR angiography

c.  Image slice through the vessel

d.  Imaging plane for PC-MRI (perpendicular to c)


Clinical Interpretation of Flow Profile


#  4 section within RR interval

#  Systolic peak (I)

#  Incision (II) followed by midsystolic peak (III)

#  Diastolic phase (IV)

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Flow Profile in Stenoic Patient


#  First systolic peak and incision disappear with increasing degree of stenosis


Flow Measurement in Stenosis Model

Scheuer et al. Z Med Phys 2010

measurement setup stenosis model at different flow rates

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Flow Measurement in Stenosis Model

Scheuer et al. Z Med Phys 2010

flow profiles from 3D PC-MRI at different locations along the stenosis model


Summary

!  Basic laws to describe pressure and flow: Bernoulli & Poiseuille

!  Blood flow in normal regimes laminar flow

#  Reynolds number

#  Parabolic shape of flow in tubes

!  Blood pressure measurement techniques

#  Korotkof sounds

#  Cuff & stetoscope

!  Blood flow measurement techniques

#  EM flow meters

#  Doppler US

#  PC-MRI

Blood Flow Measurement Techniques Typical values for blood flow ...

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