SonoSite M-Turbo, Service Manual

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6 Chapter 2: System Overview
Color
Doppler
(Color) In color Doppler, a real-time, two-dimensional cross-section of blood flow is displayed. The
2D cross-section may be presented as a rectangle, parallelogram, trapezoid, sector, or a full
circle, depending on the particular transducer used.
The 2D cross-section is presented as a full color display, with various colors being used to
represent the velocity, both positive and negative, of the blood flow echoes. Often, to
provide spatial orientation, the full color blood flow cross-section is overlaid on top of the
gray scale cross-section of soft tissue structure (2D echo). For each pixel in the overlay, the
decision of whether to display VCD, gray scale (echo) information or a blended combination
is based on the relative strength of echoes from the soft-tissue structures and from the red
blood cells.
A high pass filter (wall filter) is used to remove the signals from stationary or slowly moving
structures. Tissue motion is discriminated from blood flow by assuming that blood is moving
faster than the surrounding tissue, although additional parameters may also be used to
enhance the discrimination. The remaining signal after wall filtering may be averaged over
time (persistence) to present a steady state image of blood flow distribution. Variance
information may also be displayed to provide information when large variance is observed in
the velocity information.
Color Power
Doppler
(CPD) In CPD, a real-time two-dimensional cross-section of blood flow is displayed. The 2D
cross-section may be presented as a rectangle, parallelogram, trapezoid, sector, or a full
circle, depending on the particular transducer used.
The 2D cross-section is presented as a full color display, with various colors being used to
represent the power in blood flow echoes. Often, to provide spatial orientation, the full color
blood flow cross-section is overlaid on top of the gray scale cross-section of soft tissue
structure (2D echo). For each pixel in the overlay, the decision of whether to display CPD, gray
scale (echo) information or a blended combination is based on the relative strength of
echoes from the soft-tissue structures and from the red blood cells.
A high pass filter (wall filter) is used to remove the signals from stationary or slowly moving
structures. Tissue motion is discriminated from blood flow by assuming that blood is moving
faster than the surrounding tissue, although additional parameters may also be used to
enhance the discrimination. The power in the remaining signal after wall filtering may be
averaged over time (persistence) to present a steady state image of blood flow distribution.
Continuous
Wave (CW)
Doppler CW provides a real-time representation of blood flow and is displayed as a
velocity-versus-time sweeping output. Velocity (or frequency) is presented as the vertical
axis with time along the horizontal axis. The magnitude of the detected signal is represented
as different gray scale values.
CW Doppler mode provides the clinician with the ability to obtain blood flow velocities
focused about a user specified focal region. A continuous transmit waveform of ultrasound
energy with a known frequency is transmitted and focused by the system; on the receive
side, the transducer receive echoes are continuously amplified, focused about the focal
region and converted to a base band quadrature signal. The signal is analyzed by a
quadrature phase detector that establishes two receive channels to allow detection of flow
direction. These two channels are then analyzed by a fast complex Fourier transform (FFT)
circuit to establish the spectrum of frequencies present in the echoes. The data are displayed
as spectrum frequencies with respect to time.
CW can be used alone but is normally used in conjunction with a 2D image for spatial
reference. The 2D image has a graphical line (D-line) superimposed on the 2D image
indicating where the M-mode beam is located.