The term Doppler should be
capitalized because it refers to
Christian Johann Doppler, an Austrian
physicist (1803–1853). He described a
phenomenon whereby the frequency
of sound changes when it is reflected
off a moving object. If the object is
moving towards the observer, reflected
sound frequency is increased (blue
shift), while if the reflecting object
is moving away from the observer,
the sound frequency decreases (red
shift). This is analogous to a moving
ambulance with its siren on; the
siren pitch increases as ambulance
approaches and decreases as it
receeds.
The Doppler Effect may be used
also in ultrasound. With normal grey
scale ultrasound we rely on amplitude
of reflected sound waves and the reflectivity of anatomic structures
is proportional to the intensity (or
amplitude) of the reflected sound
and hence brightness on the scanner
display. If frequency (rather than
amplitude) is analysed, one can
detect motion on the basis of Doppler
shifts. These frequency shifts can
be quantified reasonably accurately
according to direction, velocity and
also acceleration, all of which provide
specific signatures that help identify
both normal and diseased blood
vessels.peer-reviewe
