During a Doppler exam, the transmitted signal frequency is determined by which of the following?

• A. Backing material
• B. Attenuation factor of the tissue
• C. Bandwidth
• D. The frequency of the electrical signal applied to the transducer

Answer: (D)

The transmitted frequency is set by the drive signal applied to the transducer. The piezoelectric crystal vibrates in response to whatever electrical frequency you drive it with, so the ultrasound it emits travels at that same frequency (within the transducer’s design limits). Tissue attenuation affects how much of the signal returns and is not about the emitted frequency, while bandwidth describes how wide the emitted frequencies are around the center frequency. The backing material influences damping and thus bandwidth and pulse shape, not the drive frequency itself. So the drive frequency determines the transmitted frequency.