The principle of ultrasound: Difference between revisions

Continuous wave (CW) Doppler required 2 separate crystals, one that constantly transmits, and one that constantly receives data.  There is no damping using this mode of imaging.  One can measure very high velocities (i.e., velocities of aortic stenosis or mitral regurgitation).  The advantage of CW is high sensitivity and ease of detecting very small Doppler shifts.  The disadvantage of CW is the fact that echos arise from the entire length of the beam and they overlap between transmit and receive beams.  Thus one cannot determine where in the body the highest velocity is coming from – range ambiguity.

{{clr}}

'''Pulsed wave''' (PW) Doppler requires only one crystal.  It alternates between transmitting and receiving data.  The transducer “listens” for the data at a certain time only, since the sampling volume is coming from the location that is selected by the sonographer (i.e., the velocity at the LVOT or at the tips of the mitral valve).  This is called range resolution.  The major disadvantage of PW Doppler is aliasing.  In PW mode, the transducer has to sample a certain frequency at least twice to resolve it with certainty.  This put a limit on the max velocity that it can resolve with accuracy.  2 x Doppler frequency (Nyquist) = PRF.  If the velocity is greater than the sampling rate / 2, aliasing is produced.  The following maneuvers can be performed to eliminate aliasing: change the Nyquist limit (change the scale), select a lower frequency transducer, select a view with a shallower sample volume.  

{{clr}}