to be imaged unambiguously by the system determines its maximum pulse
repetition frequency.
Reverberation is the continuing reception of a particular signal because of
reverberation rather than reflection from a particular acoustic interface. This
phenomenon is analogous to the effect created by mirrors positioned on opposite
walls when an object, a head for instance, is placed between the mirrors. The
image of the head is reflected back and forth infinitely between the two mirrors,
creating the optical illusion of multiple heads. Reverberations are easily identifiable,
because they are equally spaced on the display.
Scattering is the diffuse, low-amplitude sound waves that occur when acoustic
energy reflects off tissue interfaces smaller than a wavelength. In diagnostic
ultrasound, Doppler signals come primarily from acoustic energy back-scattered
from red blood cells.
Shadowing is the reduction in echo amplitude from reflectors that lie behind
a strongly reflecting or attenuating structure. This phenomenon occurs when
scanning a lesion or structure with an attenuation rate higher than that of the
surrounding tissue. The lesion causes a decrease in beam intensity, which results
in decreased echo signals from the structures beyond the lesion. Consequently,
a dark cloud behind the lesion image forms on the display. This cloud, or shadow,
is useful as a diagnostic clue.
Side lobes (from single-element transducers) and grating lobes (from array
transducers) cause objects that are not directly in front of the transducer to be
displayed incorrectly in lateral position.
Speckle appears as tissue texture close to the transducer but does not
correspond to scatterers in tissue. It is produced by ultrasound wave interference
and results in general image degradation.
Spectral broadening is a display phenomenon that occurs when the number
of energy-bearing Fourier frequency components increases at any given point in
time. As a consequence, the spectral display is broadened. Spectral broadening
can indicate the disturbed flow caused by a lesion, and therefore it is important
diagnostically. However, broadening can also result from interaction between
flow and sample volume size, in which case it is an artifact.
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Transducers
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