Transducer Construction and Function Flashcards

Which component of an ultrasound transducer is specifically designed with an acoustic impedance intermediate between the piezoelectric crystal and the patient's skin to improve sound transmission?

Answer: The matching layer

The matching layer has an acoustic impedance that is between that of the piezoelectric element (crystal) and the soft tissue of the patient. This intermediate impedance reduces the large impedance mismatch at the transducer-skin interface, which in turn decreases reflection and allows more sound energy to be transmitted into the body.

A sonographer is using a transducer with a heavy backing material. Which of the following is a primary consequence of this design feature?

Answer: Improved axial resolution

The backing (or damping) material absorbs the backward-directed ultrasound energy and dampens the crystal's ringing. This shortens the spatial pulse length (SPL), which is crucial for improving axial resolution, the ability to distinguish two structures that are close to each other along the beam's path. A shorter SPL results in better axial resolution. Heavy damping leads to a wider bandwidth and decreased sensitivity.

The operating frequency of a single-crystal ultrasound transducer is most directly determined by which of the following physical characteristics?

Answer: The thickness of the piezoelectric crystal

The operating or resonant frequency of a transducer is determined by the thickness of the piezoelectric element. Specifically, the crystal is cut to a thickness that is equal to one-half of the desired wavelength of the ultrasound. Thinner crystals produce higher frequency sound waves, while thicker crystals produce lower frequency waves.

A phased array transducer achieves electronic steering and focusing of the ultrasound beam by:

Answer: Introducing precise time delays to the excitation of individual elements.

Phased array transducers consist of multiple small piezoelectric elements that are activated in complex patterns. By introducing minute time delays (phasing) in the electrical pulses sent to each element, the resulting ultrasound wavefront can be steered in different directions and focused electronically without physically moving the transducer.

Which of the following describes the piezoelectric effect as it is utilized in an ultrasound transducer during sound wave reception?

Answer: Conversion of a pressure wave into an electrical signal.

The piezoelectric effect is the ability of certain materials to generate an electric charge in response to applied mechanical stress. During reception, the returning sound echoes (which are pressure waves) cause the piezoelectric crystals to vibrate, and this mechanical deformation is converted back into an electrical signal for processing by the ultrasound system.

A transducer with a low Q-factor is characterized by:

Answer: A wide bandwidth and a short spatial pulse length.

The Quality factor (Q-factor) is the ratio of the center frequency to the bandwidth. A low Q-factor indicates a wide (or broad) bandwidth. Transducers with a wide bandwidth are heavily damped, which results in a short spatial pulse length (SPL). This combination is ideal for diagnostic imaging because a short SPL provides good axial resolution.