Final answer:
To reduce ringing in a piezoelectric crystal, the transducer requires efficient backing material, which quickly absorbs energy and minimizes vibrations for clearer imaging.
Step-by-step explanation:
The part of the transducer that is necessary to reduce the amount of ringing of the piezoelectric (PZT) crystal is the backing material. This material is designed to quickly absorb the energy of the vibrating PZT crystal, thus dampening the vibrations and reducing the duration of the 'ringing' effect after an electrical pulse is applied. The quicker the vibrations cease, the clearer the resulting ultrasound image will be, as this allows for better resolution of structures in the body by minimizing the overlap of subsequent ultrasound pulses.