Continuous vs Pulsed Wave; PRF & PRP

Continuous Wave vs. Pulsed Wave

Chapter 2 introduced sound as a mechanical, longitudinal wave and defined the basic wave parameters — period, frequency, wavelength, and propagation speed. Those parameters describe a single, uninterrupted wave train. But diagnostic B-mode, M-mode, and pulsed-wave (PW) Doppler imaging cannot use an uninterrupted wave train. Instead, nearly every clinical imaging application depends on pulsed ultrasound: short bursts of sound separated by comparatively long periods of silence.

Continuous wave (CW) ultrasound, by contrast, transmits sound non-stop. A CW transducer actually contains two separate piezoelectric elements — one dedicated entirely to transmitting and one dedicated entirely to receiving — because a single element cannot simultaneously send and listen. CW ultrasound is used clinically in one specific place on the SPI blueprint: continuous-wave Doppler (covered fully in Chapter 9), which is prized for measuring very high blood-flow velocities without any upper limit.

The reason CW cannot be used for grayscale imaging is fundamental: because the transmitter never stops, the machine has no way to know when any particular returning echo was originally sent. Depth is calculated from the round-trip travel time of an echo (the range equation, covered in Chapter 4), and that calculation is only possible if the system knows the exact instant of transmission. Pulsed ultrasound solves this by transmitting a brief burst, going silent, and listening for the echo before transmitting again — the "listen after you shout" pattern that makes depth measurement, and therefore two-dimensional image formation, possible at all.

Pulse Repetition Frequency and Pulse Repetition Period

Two closely related parameters describe how often the system pulses:

• Pulse repetition frequency (PRF) — the number of pulses transmitted per second, expressed in Hz or kHz.
• Pulse repetition period (PRP) — the time from the beginning of one pulse to the beginning of the very next pulse (one complete transmit-and-listen cycle).

PRF and PRP are reciprocals of one another, exactly the same reciprocal relationship period and frequency have in Chapter 2:

⚠ PRP = 1/PRF

Typical imaging PRF runs about 1–10 kHz (1,000 to 10,000 pulses per second). PRF is not something a sonographer dials in directly as a number — it is set automatically by the scanner, primarily driven by the selected imaging depth. A shallow superficial scan (small parts, thyroid) can use a comparatively high PRF near the top of that range; a deep abdominal or obstetric scan uses a comparatively low PRF near the bottom of it.

Why Deeper Imaging Requires a Lower PRF

The system cannot safely transmit the next pulse until it has finished listening for echoes from the deepest structure of interest in the current pulse. If a new pulse were sent too soon, an echo returning late from a deep target could arrive during the "listening window" assigned to a later pulse and be misplaced on the image at the wrong, shallower-looking depth — a range-ambiguity artifact (covered in Chapter 10). To avoid this, the deeper the imaging depth, the longer the system must wait for the echo to return, which lengthens PRP and therefore lowers PRF. Shallow, superficial imaging can tolerate a higher PRF because echoes return quickly.

The 13-Microsecond-per-Centimeter Rule

Because the machine must wait for a full round trip — the pulse travels down to the reflector and the echo travels all the way back to the transducer — the required listening time depends directly on the speed of sound in soft tissue (1,540 m/s, from Chapter 2) and on the factor-of-two round-trip distance. Working this relationship out yields a simple, memorizable rule that recurs throughout the SPI exam:

⚠ Round-trip travel time ≈ 13 microseconds (µs) for every 1 cm of depth.

This rule is the practical link between imaging depth and PRF: the scanner must make PRP at least as long as the round-trip time to the deepest structure being imaged, plus a small safety margin, which in turn caps how high PRF can be set. A sonographer who increases the depth control on the machine is, without necessarily realizing it, forcing PRF downward — a relationship that resurfaces later when frame rate (Chapter 6) and the maximum measurable Doppler velocity, or Nyquist limit (Chapter 9), are discussed, since both of those also fall as PRF falls.

Summary of the PRF/PRP/Depth Relationship

• Imaging must use pulsed, not continuous, wave ultrasound so depth can be timed.
• PRF = pulses per second; PRP = 1/PRF = time per full pulse cycle.
• Typical imaging PRF ≈ 1–10 kHz, set automatically from the selected depth.
• Round-trip time ≈ 13 µs per cm of depth; deeper targets require longer listening time.
• Therefore, PRF and imaging depth are inversely related — increasing depth always lowers the maximum usable PRF.

Understanding this chain — pulsed instead of continuous wave, PRF and PRP as reciprocals, and the 13 µs/cm rule tying depth to wait time — is essential groundwork before Section 3.2 examines what happens within a single pulse (pulse duration and spatial pulse length) and how much of the total PRP is actually spent transmitting (duty factor).