The duty factor is the fraction of time that the ultrasound system is actively transmitting a signal. Continuous wave (CW) Doppler has the highest duty factor because it continuously transmits and receives ultrasound waves. Unlike pulsed wave Doppler, which alternates between sending and receiving signals, CW Doppler does not have a listening period, resulting in a duty factor of nearly 100%. Therefore, CW Doppler has the highest duty factor among the modes listed.
Reference:
ARDMS Sonography Principles & Instrumentation Guidelines
Hedrick WR, Hykes DL, Starchman DE. Ultrasound Physics and Instrumentation. 4th ed. Philadelphia, PA: Elsevier Saunders; 2005.

Comprehensive and Detailed Explanation From Exact Extract:
Attenuation increases with both frequency and depth. The highest attenuation occurs at the highest frequency and greatest depth. 7.5 MHz attenuates faster than 5 MHz, and 5 cm represents deeper tissue penetration.
Principles and Instrumentation state:
"Attenuation is directly proportional to frequency and path length: Attenuation = Frequency × Depth × Attenuation Coefficient." Therefore, the correct answer is D: 7.5 MHz at 5 cm.
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Comprehensive and Detailed Explanation From Exact Extract:
Harmonic imaging operates by transmitting at the fundamental frequency and receiving echoes at the second harmonic frequency (which is twice the fundamental frequency). Therefore, the transducer must have a broad enough bandwidth to include both frequencies.
According to sonography instrumentation reference:
"For harmonic imaging, the transducer bandwidth must accommodate both the transmitted fundamental frequency and the received second harmonic frequency." Therefore, the correct answer is A: Fundamental and second harmonic frequencies.
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Continuous wave Doppler uses two crystals - one for transmitting and one for receiving ultrasound waves continuously. This allows for the measurement of high velocities without aliasing, a common limitation in pulsed wave Doppler. However, continuous wave Doppler does not have range specificity, meaning it cannot precisely determine the depth from which the Doppler signal is returning.
Reference: ARDMS Sonography Principles and Instrumentation, Chapter on Doppler Ultrasound.

Comprehensive and Detailed Explanation From Exact Extract:
Cross talk (spectral mirror artifact) appears as a duplication of the spectral waveform above and below the baseline. One primary cause is excessive Doppler gain, which amplifies weak signals and noise symmetrically. Reducing gain minimizes this artifact.
Principles and Instrumentation state:
"Cross talk in spectral Doppler may result from high gain settings. Lowering the gain can reduce or eliminate this artifact." PRF adjustments affect aliasing, not cross talk.
Baseline shifts affect waveform positioning.
Wall filter affects low-frequency signals but not cross talk.
Therefore, the correct answer is A: By decreasing the Doppler gain.