Diagnostic ultrasound ( PDFDrive )

CHAPTER 1 Physics of Ultrasound 9
FIG. 1.11 Dynamic Range. The ultrasound receiver must compress the wide range of amplitudes returning to the transducer into a range that
can be displayed to the user. Here, compression and remapping of the data to display dynamic ranges of 35, 40, 50, and 60 dB are shown. The
widest dynamic range shown (60 dB) permits the best differentiation of subtle differences in echo intensity and is preferred for most imaging
applications. The narrower ranges increase conspicuity of larger echo differences.
for some means of automatic TGC, the manual adjustment of
this control is one of the most important user controls and may
have a profound efect on the quality of the ultrasound image
provided for interpretation.
Another important function of the receiver is the compression
of the wide range of amplitudes returning to the transducer into
a range that can be displayed to the user. he ratio of the highest
to the lowest amplitudes that can be displayed may be expressed
in decibels and is referred to as the dynamic range. In a typical
clinical application, the range of relected signals may vary by a
factor of as much as 1 : 10 12 , resulting in a dynamic range of up
to 120 dB. Although the ampliiers used in ultrasound machines
are capable of handling this range of voltages, gray-scale displays
are limited to display a signal intensity range of only 35 to 40 dB.
Compression and remapping of the data are required to adapt
the dynamic range of the backscattered signal intensity to the
dynamic range of the display (Fig. 1.11). Compression is performed
in the receiver by selective ampliication of weaker signals.
Additional manual postprocessing controls permit the user to
map selectively the returning signal to the display. hese controls
afect the brightness of diferent echo levels in the image and
therefore determine the image contrast.
Image Display
Ultrasound signals may be displayed in several ways. Over the
years, imaging has evolved from simple A-mode (amplitudemode)
and bistable display to high-resolution, real-time, grayscale
imaging. he earliest A-mode devices displayed the voltage
produced across the transducer by the backscattered echo as a
vertical delection on the face of an oscilloscope. he horizontal
time sweep of the oscilloscope was calibrated to indicate the
distance from the transducer to the relecting surface. In this
form of display, the strength or amplitude of the relected sound
is indicated by the height of the vertical delection displayed on
the oscilloscope. With A-mode ultrasound, only the position
and strength of a relecting structure are recorded.
Another simple form of imaging, M-mode (motion-mode)
ultrasound, displays echo amplitude and shows the position of
A
B
A
B
FIG. 1.12 M-Mode Display. M-mode ultrasound displays changes
of echo amplitude and position with time. Display of changes in echo
position is useful in the evaluation of rapidly moving structures such as
cardiac valves and chamber walls. Here, the three major moving structures
in the upper gray-scale image of the fetus are recorded in the corresponding
M-mode image and include the near ventricular wall (A), the interventricular
septum (B), and the far ventricular wall (C). The baseline is
a time scale that permits the calculation of heart rate from the M-mode
data.
moving relectors (Fig. 1.12). M-mode imaging uses the brightness
of the display to indicate the intensity of the relected signal.
he time base of the display can be adjusted to allow for varying
degrees of temporal resolution, as dictated by clinical application.
M-mode ultrasound is interpreted by assessing motion patterns
of speciic relectors and determining anatomic relationships
from characteristic patterns of motion. Currently, the major
application of M-mode display is evaluation of embryonic and
fetal heart rates, as well as in echocardiography, the rapid motion
of cardiac valves and of cardiac chamber and vessel walls. M-mode
imaging may play a future role in measurement of subtle changes
in vessel wall elasticity accompanying atherogenesis.
he mainstay of imaging with ultrasound is provided by
real-time, gray-scale, B-mode display, in which variations in
display intensity or brightness are used to indicate relected signals
C
C