Chapter 2 — Physics of Ultrasound and Machine Operation

📚 Guideline basis

AIUM/ASE standards for equipment performance and safety; ASE 2019 recommendations for performing a comprehensive TTE examination (JASE 2019;32:1–64); AIUM statements on thermal and mechanical bioeffects.

Mechanisms

Wave propagation

Diagnostic ultrasound is a longitudinal mechanical pressure wave. Its behaviour is governed by:

c = fλ

where c = propagation speed (m/s), f = frequency (Hz), λ = wavelength (m). Soft tissue speed is assumed constant at 1540 m/s; every machine's depth calculation depends on this assumption, and it is the source of the speed-of-sound artefact when the beam traverses fat (≈1450 m/s) or bone (≈3500 m/s).

Cardiac transducers operate at 1.5–5 MHz (adult transthoracic typically 2–3.5 MHz; TEE 3.5–7 MHz). At 2 MHz, λ ≈ 0.77 mm.

The resolution–penetration trade-off

This is the single physical constraint that governs every scanning decision.

Axial resolution at 3 MHz with a two-cycle pulse is approximately 0.5 mm; lateral resolution is typically 3–5 times worse and is the reason that measurements should be made along the beam axis wherever possible.

Practical consequence: measure the LVOT diameter in the axial (vertical) dimension of a parasternal long-axis image, not the lateral one, and never measure a structure at the extreme edge of the sector.

Attenuation, impedance and reflection

Acoustic impedance Z = ρc (density × propagation speed). Reflection occurs at impedance mismatches; the fraction of intensity reflected at an interface is:

R = ((Z₂ - Z₁) / (Z₂ + Z₁))²

Air–tissue mismatch reflects essentially all incident energy, which is why lung, subcutaneous emphysema, and inadequate coupling gel destroy the image, and why a hyperinflated, high-PEEP ventilated chest is the defining technical problem of ICU echocardiography.

Machine controls: what each knob actually changes

Frame rate arithmetic

Frame rate is limited by the time required for each pulse to travel to maximum depth and return:

FR_max = c / (2 × D × N)

where D = imaging depth (m) and N = number of scan lines per frame. This is why reducing depth and narrowing the sector both increase frame rate, and why colour Doppler — which requires multiple pulses (the packet or ensemble) per scan line — collapses frame rate. In a patient at 140 beats/min, a 25 Hz colour frame rate samples roughly 10 frames per cardiac cycle, which is inadequate for timing a jet.

Harmonic imaging

Non-linear propagation generates harmonic frequencies within tissue. Receiving selectively at the second harmonic:

• Improves endocardial border delineation and reduces near-field and side-lobe clutter, which is the dominant reason it is the default cardiac preset.
• Degrades axial resolution and systematically thickens thin, bright structures.

Harmonics must be switched off when: measuring valve leaflet thickness; sizing a vegetation or mass; measuring the LVOT diameter (harmonics can inflate the measurement, and LVOT diameter is squared in the stroke volume equation — see below).

Safety and bioeffects

Two indices are displayed on every machine:

• Thermal index (TI) — estimated temperature rise. Cardiac imaging uses TIS (soft tissue) or TIB (bone).
• Mechanical index (MI) — cavitation risk, defined as peak rarefactional pressure divided by the square root of frequency.

MI = P_r / √(f)

Diagnostic scanners are capped at MI ≤ 1.9. The relevant ICU application is contrast (ultrasound enhancing agent) imaging, which requires a low MI (typically 0.1–0.3) to avoid microbubble destruction; high-MI imaging destroys the contrast bolus and produces a falsely opacified-then-empty ventricle.

ALARA (As Low As Reasonably Achievable) governs exposure. In practice, thermal risk in adult transthoracic cardiac imaging is negligible; the practical safety issues in the ICU are probe hygiene, line and drain displacement, and pressure injury from prolonged probe contact in the miniaturised TEE setting.

The error that propagates: LVOT diameter

Because stroke volume depends on the square of the LVOT diameter:

SV = π (D/2)² × VTI_LVOT

a 10% error in D produces a 21% error in stroke volume, and a 2 mm error on a 20 mm LVOT (10%) propagates identically. This single measurement is where machine settings translate directly into a wrong clinical decision. Requirements: zoomed parasternal long axis, harmonics off, mid-systolic frame, inner-edge to inner-edge, 0.5–1.0 cm proximal to the aortic annulus, parallel to the annular plane.

🛑 Critical pitfall: Over-gaining to "see better" in a poor window. Excess gain fills the cavity with speckle, blurs the endocardial border outward, and causes systematic underestimation of LV volumes and overestimation of ejection fraction — the opposite of the intended effect.

🛑 Critical pitfall: Leaving harmonics on while measuring the LVOT. The apparent boundary thickens, the measured diameter shrinks or expands depending on which edge is taken, and cardiac output is wrong by 20% or more.

• 💡 Clinical pearl: In a ventilated patient with poor windows, the fastest single manoeuvre is not more gel and more pressure — it is dropping transmit frequency and re-attempting from the subcostal window at end-expiration.
• 💡 Clinical pearl: Set sweep speed to 100 mm/s for all Doppler measurement and 50–100 mm/s for CW jets; the default 50 mm/s compresses the waveform and systematically degrades time-interval measurements (IVRT, deceleration time, pressure half-time).
• 💡 Clinical pearl: Frame rate is a measurable quantity displayed on screen. If it reads below 40 Hz while assessing a tachycardic patient, the problem is depth or sector width, not the patient.

References

1. Mitchell C, Rahko PS, Blauwet LA, et al. Guidelines for performing a comprehensive transthoracic echocardiographic examination in adults: recommendations from the American Society of Echocardiography. J Am Soc Echocardiogr 2019;32:1–64.
2. American Institute of Ultrasound in Medicine. Statement on mechanical bioeffects in the presence of gas bodies.
3. Porter TR, Mulvagh SL, Abdelmoneim SS, et al. Clinical applications of ultrasonic enhancing agents in echocardiography: 2018 ASE guidelines update. J Am Soc Echocardiogr 2018;31:241–74.