Biomedical Engineering Reference
In-Depth Information
Fig. 2.1.
The block diagram of the components of an ultrasonic echograph
For example, more sound is reflected from an interface between gaseous and
solid media than from a liquid-solid interface. Whether the ultrasound is re-
flected by an interface also depends on the relative sizes of the mismatched
media and the wavelength. The total thickness presented to the ultrasonic
beam must be at least one fourth the wavelength of the sound. Thus, ultraso-
und with a higher frequency or a shorter wavelength can reflect sound from
smaller objects. The loss of ultrasound as it traverses a medium is known as
attenuation, which is a combination of absorption and scattering.
The instrument used to create an image using ultrasound is known as an
echograph. Figure 2.1 shows a block diagram of an echograph. The trans-
ducer both sends and receives the ultrasound, and the transmitter regulates
the sending of the ultrasound by way of a timer that controls the duration
and frequency of the ultrasonic pulse. The transducer converts the returning
echoes to electrical impulses, which in turn go to the receiver and the signal
amplifier. The returning echoes or impulses are processed so that they can
be displayed. The impulse is converted from a spike to a dot and, within
limits, the taller the echo, the brighter the dot. This presentation is known
as B-mode, the B standing for brightness. This type of display was the back-
bone of ultrasonography for many years. If the interface from which the echo
is derived is constantly moving, then the echo position changes constantly
with reference to the transducer. The echo signal moves back and forth on
the face of the display. If the intensity-modulated dots sweep from left to
right, M-mode tracing, where M stands for motion, is obtainable. If the spa-
tial orientation of the transducer is tracked electronically, one could obtain
a spatially oriented M-mode examination (Fig. 2.2). In this figure, when the
ultrasonic transducer is close to the top of the beaker, it traverses the circular
object at the point at which the two echoes are relatively close together (
A
).
When the transducer is moved downward, the two B-mode echoes are farther
apart and in the center of the display (
B
), and then the two echoes are closer
