GE Voluson 730, Руководство по обслуживанию

Страница: 127 / 270

GE M
EDICAL
S
YSTEMS
- K
RETZTECHNIK
U
LTRASOUND
D
IRECTION
105844, R
EVISION
1 V
OLUSON
® 730 S
ERVICE
M
ANUAL
Chapter 5 Components and Functions (Theory) 5-7
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 is then averaged over time (persistence) to present
a steady state image of blood flow distribution. Power Doppler can be used in combination with 2D and
Spectral Doppler modes as well as with 3D mode.
5-2-1-4 Pulsed (PW) Doppler:
PW Doppler processing is one of two spectral Doppler modalities, the other being CW Doppler. In
spectral Doppler, blood flow is presented as a scrolling display, with flow velocity on the Y-axis and time
on the X-axis. The presence of spectral broadening indicates turbulent flow, while the absence of
spectral broadening indicates laminar flow. PW Doppler provides real time spectral analysis of pulsed
Doppler signals. This information describes the Doppler shifted signal from the moving reflectors in the
sample volume. PW Doppler can be used alone but is normally used in conjunction with a 2D image
with an M-line and sample volume marker superimposed on the 2-D image indicating the position of the
Doppler sample volume. The sample volume size and location are specified by the operator. Sample
volume can be overlaid by a flow direction cursor which is aligned, by the operator, with the direction of
flow in the vessel, thus determining the Doppler angle. This allows the spectral display to be calibrated
in flow velocity (m/sec) as well as frequency (Hz). PW Doppler also provides the capability of performing
spectral analysis at a selectable depth and sample volume size. PW Doppler can be used in
combination with 2D and Color Flow modes.
5-2-2 3D Imaging
The Voluson® 730 Ultrasound System will be used to acquire multiple, sequential 2D images which can
be combined to reconstruct a three dimensional image. These 3D images are useful in visualizing three-
dimensional structures, and in understanding the spatial or temporal relationships between the images
in the 2D sequence. The 3D image is presented using standard visualization techniques, such as
surface or volume rendering.
5-2-2-1 3D Data Collection and Reconstruction:
2D gray scale including Power Doppler images may be reconstructed. The acquisition of volume data
sets is performed by sweeping 2D-scans with special transducers (called 3D-transducers) designed for
the 2D-scans and the 3D-sweep.
Images are spatially registered, using internal probe position sensing and a position control to ensure
geometric accuracy of the 3D data.
2D ultrasound imaging modes are used to view a two dimensional cross-sections of parts of the body.
For example in 2D gray scale imaging, a 2 dimensional cross-section of a 3-dimensional soft-tissue
structure such as the heart is displayed in real time. Typically, the user of an ultrasound machine
manipulates the position and orientation of this 2D cross-section in real time during an ultrasound exam.
By changing the position of the cross-section, a variety of views of the underlying structure are obtained,
and these views can be used to understand a 3-dimensional structure in the body.
To complete survey a 3-dimensional structure in the body, it is necessary to collect 2D images which
span a volume containing the structure. One way is to sweep the imaging cross-section by translating
it in a direction perpendicular to the cross-section. Another example method is to rotate the cross
section about a line contained in the cross section. The Voluson® 730 Ultrasound System uses the
automated so called C-Scan for the motion perpendicular to automated B-scan.
Once a representative set of 2D cross-sections are obtained, standard reconstruction techniques can
be used to construct other 2D cross-sections, or to view the collection of the cross-sections as a 3D
images.