Amplitude, Power & Intensity

Amplitude: The Size of the Variation

Amplitude describes the strength, or magnitude, of the variation in an acoustic variable — most commonly pressure — from its normal, undisturbed (baseline) value to its maximum value. A "loud" or "strong" pulse has a large pressure amplitude (a big swing between the peak of a compression and the trough of a rarefaction); a "quiet" or "weak" pulse has a small amplitude. Amplitude is controlled at the machine by output power (via the pulser/beam former, covered in Chapter 7) and is entirely independent of frequency — you can have a high-amplitude, low-frequency pulse or a low-amplitude, high-frequency pulse. Because pressure amplitude is the most clinically relevant of the four acoustic variables, it is commonly expressed in pascals (Pa) or megapascals (MPa) when describing peak rarefactional pressure for safety-index calculations (covered in Chapter 11), but for conceptual purposes it is simplest to think of amplitude as "how big the pressure swing is" above and below the undisturbed baseline value.

Power: Amplitude Squared

Power is the rate at which acoustic energy is transferred through the medium, measured in watts (W) or milliwatts (mW). Power is not simply proportional to amplitude — it follows a square-law relationship:

This square-law relationship has a large practical consequence: small changes in amplitude produce disproportionately large changes in power. Doubling amplitude does not double power — it quadruples it (2² = 4). Tripling amplitude produces nine times the power (3² = 9). This is the same square-law logic used throughout physics (kinetic energy proportional to velocity squared, for example) and it reappears on the SPI exam specifically in the amplitude/power relationship.

Intensity: Power Spread Over Area

Intensity takes power a step further by accounting for how tightly that power is focused. It is defined as:

For a fixed amount of power, concentrating it into a smaller cross-sectional area (a narrower beam) produces a higher intensity, and spreading the same power over a larger area produces a lower intensity. Because power itself is proportional to amplitude², intensity is also proportional to amplitude² for a fixed beam area — meaning a small increase in output amplitude at the machine can meaningfully raise the intensity delivered to the patient.

Naming the Many Faces of Intensity

Ultrasound intensity is not a single fixed number — it varies from moment to moment (as pulses turn on and off) and from location to location within the beam (highest at the beam's center, lower toward the edges). To describe intensity precisely, ultrasound physics uses a two-part naming convention:

• Spatial term — describes intensity across the beam's cross-section: peak (the single highest point in the beam) or average (averaged across the whole beam area)
• Temporal term — describes intensity over time: peak (during the pulse only), pulse average (averaged across just the pulse-on duration), or average (averaged across the entire pulse repetition period, including the "silent" listening time between pulses)

Because diagnostic ultrasound uses a very low duty factor (the pulse is "on" only a tiny fraction of the time — detailed in Chapter 3), the time-averaged values are dramatically smaller than the peak values: SATA < SPTA < SPPA < SPTP, from lowest to highest.

Why It Matters: The Link to Bioeffects

Intensity values are not just abstract labels — they are the basis for regulatory safety limits and bioeffect risk assessment (fully covered in Chapter 11). SPTA is the value most closely tied to thermal bioeffects, because it factors in the entire pulse repetition period, including the quiet listening time when tissue has a chance to cool — making it the best available proxy for cumulative heating. Higher amplitude leads to higher power, which leads to higher intensity, which leads to greater potential for tissue heating and, at very high pressures, cavitation. This chain of relationships (amplitude to power to intensity to bioeffect risk) is exactly why output power controls, TGC/gain, and machine safety indices are treated as physically distinct concepts on the SPI exam: gain amplifies received echoes electronically with no patient bioeffect cost, while output power changes the amplitude actually transmitted into the patient and does carry bioeffect implications.

SPI Exam Tip

Expect a direct amplitude/power squaring calculation ("if amplitude triples, power increases by what factor?") and at least one item asking you to identify which intensity abbreviation is used for a described averaging scenario, or which one is most relevant to thermal bioeffect limits (SPTA).