Acoustic Artifacts Transducers
Lumify Ultrasound System 119
Philips Healthcare 4535 618 58571_A/795 * MAY 2016
Propagation speed errors occur when the assumed value for propagation speed by the
ultrasound system is incorrect. If the actual speed is greater than that assumed, the calculated
distance to a reflector is too small, and the reflector 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.
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.
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