3D Flow Vector Generation From Decorrelation
TW Ketai, TA Tuthill, JB Fowlkes
Background/Objective
The purpose of this research is to develop a new angle independent technique for measuring blood flow speed and direction
using ultrasound. It combines the established technique of Doppler imaging to determine in-plane velocities
and decorrelation to determine out-of-plane velocities. These three components can then be combined to
determine total 3D velocities. Decorrelation, D, represents the change in pixel intensity over time as the image
changes, and can be related to velocity, V, by the formula D^2=Vx^2/Bx^2+Vy^2/By^2+Vz^2/Bz^2 where Bx, By and Bz
are properties of the transducer's beamwidth in the axial, lateral and elevational directions.
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Methods
For this research, a set of experiments involving the simulation of blood vessels in the body was conducted.
A saline solution with contrast agent was chosen as a suitable blood-mimicking fluid. The experimental setup
consisted of a tank filled with water with 6.4 mm dialysis tubing running through it. A 10 MHz transducer was positioned
above the tube first perpendicular, then rotated to 20 and 40 degrees from perpendicular. As the saline and
contrast agent was pumped through the tube at 15 ml/min, cine loops were collected with the beam steered 20
degrees to the left and right. These data were then processed using MATLAB programs to determine the
direction and velocity of the flow. |
Data/Results
The processing of the data we obtained produced the three vectors plots shown below. They may be rotated to
visualize the slant of the vectors by playing the movies or by sliding the marker on the play bar.
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The vectors above were generated from an experiment where the fluid was flowing
normal (at 90 degrees) to the plane of the beam.
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In the second trial, the fluid was flowing at 70 degrees laterally to the beam.
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Here, the flow was at 50 degrees to the beam. The displayed vectors are not at 50 degrees from
horizontal because of aliasing that occurred due to the limited frame rate.
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Discussion
In current clinical settings, to measure blood velocities using Doppler, the sonographer must estimate the angle
between the transducer and vessel. Our technique automates this process for determining the vessel orientation.
The data above show that the method for finding and displaying the velocity vectors works fairly well.
The error in the angle estimate for trial 3 is due to the aliasing in the Doppler measurements. The large
velocity component in the lateral (x) direction results in a large frequency shift that can not be measured with the
limited frame rate. With a higher frame rate or use of a higher firing rate along a single line, this error
would be eliminated. For the smaller angles, however the data show that our method can, using grayscale
images and doppler data, determine the angle at which a fluid in the field is flowing.
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Conclusions
In conclusion, this method for flow analysis looks very promising. We need to do more trials to determine the
accuracy to which the techinique works, but if this comes out to a reasonable range, this could be very
helpful in medical procedures where the details of blood flow in the patient are needed. By scanning a vessel
twice from the same location, with the beam of teh transducer steered oppositely in each scan, it should be
possible to determine how fast and in what direction the blood in the vessel is flowing.
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