Foundations of Medical Imaging and Signal Recording - Biomedical Signal Analysis: Contemporary Methods and Applications - page 24

organs and blood vessels, and supporting breast cancer detection.
The principle of the ultrasonic imaging is very simple: the acoustic
wave launched by a transducer into the body interacts with tissue and
blood, and some of the energy that is not absorbed returns to the
transducer and is detected by it. As a result, “ultrasonic signatures”
emerge from the interaction of ultrasound energy with different tissue
types that are subsequently used for diagnosis.
The speed of sound in tissue is a function of tissue type, temper-
ature, and pressure. Table 1.4 gives examples of acoustic properties of
some materials and biological tissues. Because of scattering, absorption
or reflection, an attenuation of the acoustic wave is observed. The atten-
uation is described by an exponential function of the distance, described
by A ( x )= A 0 exp ( αx ), where A is the amplitude, A 0 is a constant, α

is the attenuation factor, and x is the distance. The important charac-
teristics of the returning signal, such as amplitude and phase, provide
pertinent information about the interaction and the type of medium
that is crossed. The basic imaging equation is the pulse-echo equation,
which gives a relation among the excitation pulse, the transducer face,
the object reflectivity, and the received signal.
Ultrasound has the following imaging modes:

−

• A-mode (amplitude mode): the most simple method that displays the

envelope of pulse-echoes versus time. It is mostly used in ophthalmology
to determine the relative distances between different regions of the eye,
and also in localization of the brain midline or of a myocardial infarction.
Figure 1.13 visualizes this aspect.
•

B-mode (brightness mode): produced by scanning the transducer
beam in a plane, as shown in figure 1.14. It can be used for both
stationary and moving structures, such as cardiac valve motion.
• M-mode (motion mode): displays the A-mode signal corresponding

to repeated pulses in a separate column of a 2-D image. It is mostly
employed in conjunction with ECG for motion of the heart valves.

The two basic techniques used to achieve a better sensitivity of the
echoes along the dominant (steered) direction are the following:

• Beam forming: increases the transducer's directional sensitivity

• Dynamic focusing: increases the transducer's sensitivity to a particular

point in space at a particular time