Ultrasound for medical imaging is produced and detected using the piezoelectric effect.Some crystals, such as natural quartz and the ceramic lead zirconate titanate (PZT), are made of ions arranged so that squeezing the crystal shifts the positive and negative charges slightly relative to each other. The piezoelectric effect is the production of a potential difference across a crystal when it is compressed or stretched. Compressing it gives a p.d. one way; stretching it gives a p.d. the opposite way.The effect also works in reverse. Applying a p.d. across the crystal makes it change shape: one polarity makes it expand, and reversing the p.d. makes it contract.This gives two uses:• Transmitting: an alternating p.d. makes the crystal expand and contract repeatedly, so it vibrates at the same frequency as the p.d. The vibrating faces push on the medium and send out ultrasound. The vibrations are largest when the frequency of the p.d. matches the resonant frequency of the crystal.
• Receiving: when ultrasound echoes hit the crystal, the pressure changes squeeze and stretch it, producing an alternating p.d. at the frequency of the echo. This p.d. can be amplified and processed.
Example: an alternating p.d. of frequency 4.0 MHz is applied to a PZT crystal. The crystal vibrates at 4.0 MHz and emits 4.0 MHz ultrasound. A returning 4.0 MHz echo produces an alternating p.d. of 4.0 MHz across the same crystal.So one crystal can act as both the source and the detector of ultrasound.
• Receiving: when ultrasound echoes hit the crystal, the pressure changes squeeze and stretch it, producing an alternating p.d. at the frequency of the echo. This p.d. can be amplified and processed.
Example: an alternating p.d. of frequency 4.0 MHz is applied to a PZT crystal. The crystal vibrates at 4.0 MHz and emits 4.0 MHz ultrasound. A returning 4.0 MHz echo produces an alternating p.d. of 4.0 MHz across the same crystal.So one crystal can act as both the source and the detector of ultrasound.