will be displayed too far from the transducer. Speed error can cause a structure
to be displayed with incorrect size and shape.
Range ambiguity can occur when reflections are received after the next pulse
is transmitted. In ultrasound imaging, it is assumed that for each pulse produced,
all reflections are received before the next pulse is sent out. The ultrasound
system calculates the distance to a reflector from the echo arrival time assuming
that all echoes were generated by the last emitted pulse. The maximum depth
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 screen.
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 screen. 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.
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iU22 User Manual
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8
Transducers
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