The principle of ultrasound: Difference between revisions

Range equation – since ultrasound systems measure the time of flight and the average speed of ultrasound in soft tissue is known (1540 m/s), then we can calculate the distance of the object location.  Distance to boundary (mm) = go-return time (microsecond) x speed (mm/microsecond) / 2.

So far we have defined the ultrasound variables and parameters.  In the next section will talk more about pulsed ultrasound.  Pulse Duration is defined as the time that the pulse is on.  It is determined by the number of cycles and the period of each cycle.  In clinical imaging, a pulse is comprised of 2-4 cycles and the pulse duration is usually between 0.5 to 3 microseconds.  Pulse duration does not change with depth, thus it cannot be changed by the sonographer.  '''Pulse Duration''' (msec) = # of cycles x period (msec).  Since Wavelength (mm) = Propagation speed in tissue (mm/microsecond) / frequency (MHz), this can be rewritten as 1/frequency = wavelength / propagation speed.  And since period = 1/frequency, then the Pulse Duration = (# of cycles x wavelength) / Propagation speed.

'''Pulse Repetition Period''' or PRP is the time between the onset of one pulse till the onset of the next pulse.  Aagain, it is measured in units of time.  This parameter includes the time the pulse is “on” and the listening time when the ultrasound machine is “off”.  It can be changed by the sonographer by varying the depth to which the signal is send.  Since the Pulse Duration time is not changed, what is changed is the listening or the “dead time”.  PRP = 13 microseconds x the depth of view (cm).  It follows from this equation that the deeper is the target, the longer is the PRP.  The typical values of PRP in clinical echo are form 100 microseconds to 1 millisecond.