Piezoelectric pulse transmitter, A-mode vs B-mode

Transmitter circuit

The piezoelectric crystal needs a short, high-amplitude electrical pulse to produce a strong ultrasound pulse.

 HV supply --[R]--+--[C]--+-------------+--> T/R switch --> receiver amp
                  |       |             |
           [SCR / switch] (crystal)  [R damp]
                  |       |             |
                 gnd     gnd           gnd
                  ^
           trigger pulse (clock)

Working: the capacitor C charges slowly through the resistor R from a high-voltage supply (hundreds of volts). A trigger from the clock turns on the fast switch (SCR, thyratron or MOSFET), so C discharges almost instantly through the crystal (and a damping resistor). The sharp high-voltage spike shocks the crystal into oscillation at its resonant frequency (1 to 10 MHz), producing a short damped burst of ultrasound (about 1 µs). The cycle repeats at the pulse repetition frequency (1 to 5 kHz). Between pulses, the same crystal acts as the receiver for the echoes.

Use of pulses to view internal structures (pulse-echo)

1. The transducer, placed on the skin with coupling gel, sends a pulse into the body.
2. At each boundary between tissues of different acoustic impedance, part of the pulse is reflected as an echo.
3. The echo returns to the crystal, which converts it to an electrical signal; this is amplified (with time-gain compensation) and detected.
4. The depth of the reflector is d = c·t/2 (c = 1540 m/s, t = time between pulse and echo); the echo strength shows the reflectivity.
5. The signals are displayed in A, B or M mode to show the organs (liver, kidney, foetus, heart).

A-mode versus B-mode