The principle of ultrasound: Difference between revisions

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'''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.  

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