Physics of Ultrasound and Doppler
Clinical Overview
Ultrasound is a physics instrument before it is a clinical picture. The probe sends short pulses of high-frequency sound, receives echoes, and the machine converts echo timing and strength into a greyscale image. Doppler then adds motion information by measuring frequency shifts from moving blood. In O&G this underpins early pregnancy assessment, adnexal masses, fetal biometry, placental localisation, fetal growth restriction, uterine artery screening, middle cerebral artery assessment and theatre-adjacent procedures.
Useful scanning judgement depends on knowing why one image is better than another, why an artefact can mislead or help, why Doppler angle matters, and why ultrasound safety is active decision-making rather than passive trust in the machine.
High-yield chains:
Higher frequency -> shorter wavelength -> better resolution -> poorer penetration -> transvaginal probes see fine pelvic detail but cannot replace deep transabdominal obstetric imaging.
Large acoustic impedance mismatch -> strong reflection -> bright line and shadow -> bone, gas and calcification hide deeper structures.
Fluid has low attenuation -> few internal echoes and posterior enhancement -> a simple cyst looks dark with increased brightness deep to it.
Moving red cells -> reflected frequency shift -> Doppler waveform -> downstream resistance inferred from systolic and diastolic flow.
Higher output and longer dwell time -> more thermal/mechanical bioeffect potential -> ALARA, TI/MI awareness and restraint with first-trimester Doppler.
Core Knowledge
Start from one sentence and build everything from it: ultrasound makes a picture by sending sound into the body, timing the echoes, and turning echo timing into depth and echo strength into brightness. Every control, artefact, Doppler index and safety rule in this chapter is a consequence of that single loop. The sections below ascend deliberately — sound in tissue, then how echoes form an image, then how we sharpen it, then how movement is measured, then how we keep it safe — so that each idea rests only on the one before it.
Sound Waves and Tissue
Ultrasound is sound above human hearing. It is a longitudinal mechanical wave, so particles in tissue oscillate back and forth in the same direction as wave travel. It needs a medium and, unlike X-rays, it does not ionise tissue — it carries far too little energy per cycle to strip electrons from atoms. Diagnostic O&G ultrasound commonly uses megahertz frequencies, typically about 2 to 15 MHz, which is roughly two hundred times higher than the highest pitch an adult ear can hear.
| Term | Meaning | O&G consequence |
|---|---|---|
| Frequency | Cycles per second | Higher frequency improves resolution but reduces penetration |
| Wavelength | Distance occupied by one cycle | Shorter wavelength separates small structures better |
| Velocity | Speed of sound through tissue | Machine assumes about 1540 m/s in soft tissue |
| Amplitude | Pressure variation of the wave | Relates to echo strength and displayed brightness |
| Intensity | Power per unit area | Relevant to exposure and bioeffects |
| Attenuation | Loss of beam energy with depth | Worse with higher frequency, bone, calcification and gas |
The important trade-off is unavoidable: resolution competes with penetration. A 7-9 MHz transvaginal probe can show early pregnancy, endometrium and adnexal detail because targets are near the probe. A lower-frequency transabdominal probe is needed when the target is deeper, as in later pregnancy or a high pelvic mass, but the image loses fine detail.
Frequency, Attenuation and Probe Choice
Attenuation increases with frequency and distance. The beam loses energy by absorption, scatter and reflection, so deeper echoes are weaker even when the target is normal. Time-gain compensation exists because the same tissue would otherwise look darker simply because it is deeper.
| Probe choice | Physics advantage | Physics cost | O&G use |
|---|---|---|---|
| High-frequency transvaginal | short wavelength and near target | poor deep penetration and narrow field | early pregnancy, cervix, adnexa |
| Lower-frequency abdominal | better penetration and wider field | lower spatial resolution | later obstetrics, large pelvic masses |
| Curvilinear probe | broad sector from small footprint | variable resolution across depth | routine obstetric scanning |
| Linear probe | high near-field resolution | shallow penetration and limited field | superficial vulval, abdominal wall or vascular questions |
Probe choice is therefore a clinical argument. If an early pregnancy scan is uncertain on a low-frequency abdominal view, the next step is not a confident diagnosis of pregnancy of unknown location; it is often transvaginal imaging with the right depth, focus and gain if acceptable and available. If a late-pregnancy structure is deep, the price of penetration is reduced detail, so limitations must be documented.
Pulse-Echo Imaging
The transducer contains piezoelectric material. An electrical pulse deforms the crystal and emits sound. Returning echoes deform the crystal and generate electrical signals. The machine calculates depth from time-of-flight and brightness from echo amplitude.
| Step | Physics | Image consequence |
|---|---|---|
| Pulse emission | A short sound burst enters tissue | Shorter pulse improves axial resolution |
| Propagation | Sound travels through tissues at assumed velocity | Wrong velocity assumptions create distance errors |
| Reflection/scatter | Interfaces and small structures return echoes | Tissue texture and boundaries appear |
| Reception | Echoes deform the crystal | Weak echoes need appropriate gain |
| Processing | Time and amplitude are mapped to pixels | Image quality depends on physics and settings |
Acoustic impedance is tissue density multiplied by sound velocity. Echoes arise when a beam meets an interface between tissues with different impedance. Small impedance differences produce weak reflections; large differences produce strong reflection and shadowing. Soft tissue to fluid gives useful transmission. Soft tissue to bone or gas gives strong reflection and poor visualisation beyond the interface.
Tissue Interactions
| Interaction | Mechanism | Clinical example |
|---|---|---|
| Reflection | Beam returns from a boundary | Gestational sac wall, fetal skull, fibroid capsule |
| Refraction | Beam changes direction at an angled boundary | Misregistration of deep structures |
| Scattering | Small irregular reflectors send echoes in many directions | Parenchymal texture, speckle |
| Absorption | Energy converted to heat | Contributor to attenuation and safety concern |
| Diffraction | Beam spreads around apertures or obstacles | Limits a perfectly narrow beam |
| Transmission | Beam passes through | Full bladder acoustic window, cyst assessment |
The machine assumes echoes travel straight out and back. When refraction, side-lobes or multipath reflection break that assumption, the displayed structure may be misplaced or duplicated. This is why scanning is not just image collection; it is hypothesis testing from several planes and settings.