Nicols & O‘Rourke, McDonald‘s Blood Flow in Arteries
Blood Flow and Pressure Waves Abdominal Vasculature
PD Dr. Zöllner I Folie 157 I 9/9/2014
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 PD Dr. Zöllner I Folie 158 I 9/9/2014
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Blood velocity measurement using US
PD Dr. Zöllner I Folie 159 I 9/9/2014
Typical Values
speed of sound: velocity of blood: transmitted US-frequency:
Doppler-frequency " Audio Range PD Dr. Zöllner I Folie 160 I 9/9/2014
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Continuous Wave (CW) - US sender
fS
sender mixer IM
bandpass
base wave
receiver fE
flow signal fS-fE
display
echo
product
Doppler
PD Dr. Zöllner I Folie 161 I 9/9/2014
Pulse Wave (PW) - US
PD Dr. Zöllner I Folie 162 I 9/9/2014
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US Doppler Probes - Invasive
Nicols & O‘Rourke, McDonald‘s Blood Flow in Arteries
PD Dr. Zöllner I Folie 163 I 9/9/2014
MR Blood Flow Measurements (1)
# 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 PD Dr. Zöllner I Folie 164 I 9/9/2014
Lotz et al. , RadioGraphics 2002
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MR Blood Flow Measurements (2)
# flow encoding by means of difference in the bipolar gradients # acquisition of two images Lotz et al. , RadioGraphics 2002 # phase difference by substraction PD Dr. Zöllner I Folie 165 I 9/9/2014
MR Blood Flow Measurements (3) Phase difference expressed in formulars:
γ Δm ν
gyromagnetic ratio difference of the first moment of the gradient time curve velocity PD Dr. Zöllner I Folie 166 I 9/9/2014
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MR Blood Flow Measurements (4)
# ΔM relates to the velocity encoding (VENC) # gives sensitivity of measurable velocities # relates to the gradient strength PD Dr. Zöllner I Folie 167 I 9/9/2014
Diagram of MR PC sequence
PD Dr. Zöllner I Folie 168 I 9/9/2014
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PC-MR Images # receive 2 images # magnitude image with velocity # phase image with velocity and direction Flow encoding needs an operator defined value: VENC [cm/sec] -VENC < v < VENC PD Dr. Zöllner I Folie 169 I 9/9/2014
3D PC MRI # flow encoding is directional # depends on the gradient axis applied # therefore: 3D flow measurements possible in each voxel
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Examples
Courtesey of J Henning, Freiburg
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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 autoregulation 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 PD Dr. Zöllner I Folie 174 I 9/9/2014
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
PD Dr. Zöllner I Folie 175 I 9/9/2014
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Example – Slice Positioning
a. Abdominal aorta b. Images acquired from MR angiography c. Image slice through the vessel d. Imaging plane for PC-MRI (perpendicular to c)
Schönberg et al. , Radiology 1997
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Clinical Interpretation of Flow Profile
# 4 section within RR interval # Systolic peak (I) # Incision (II) followed by midsystolic peak (III) # Diastolic phase (IV)
PD Dr. Zöllner I Folie 177 I 9/9/2014
Schönberg et al. , Radiology 1997
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Flow Profile in Stenoic Patient
# First systolic peak and incision disappear with increasing degree of stenosis
Schönberg et al. , Radiology 1997 PD Dr. Zöllner I Folie 178 I 9/9/2014
Flow Measurement in Stenosis Model
measurement setup
stenosis model at different flow rates
Scheuer et al. Z Med Phys 2010 PD Dr. Zöllner I Folie 179 I 9/9/2014
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Flow Measurement in Stenosis Model
flow profiles from 3D PC-MRI at different locations along the stenosis model
Scheuer et al. Z Med Phys 2010 PD Dr. Zöllner I Folie 180 I 9/9/2014
Summary ! Basic laws to describe pressure and flow: Bernoulli & Poiseuille ! Blood flow in normal regimes laminar flow #
