Clinical overview
Ultrasound is the workhorse imaging modality of obstetrics and gynaecology, and in the South African public sector it is frequently the only imaging available at district and regional level. Unlike a radiograph that is captured once and read later, ultrasound is an operator-dependent, real-time examination: the diagnosis is only as good as the image the sonographer makes, and the image is only as good as the operator's command of the machine. "Knobology" — the deliberate, purposeful manipulation of the machine controls (transducer selection, frequency, gain, time-gain compensation, depth, focus, dynamic range, harmonics, and the Doppler controls) — is the practical skill that turns a noisy grey rectangle into a diagnostic study. Competence is not knowing that "gain controls brightness"; it is being able to optimise an image and operate Doppler safely in a pregnant patient.
The safety dimension is what distinguishes obstetric scanning from most other ultrasound. Diagnostic ultrasound is generally regarded as one of the safest imaging modalities and has no proven harmful effect at routine diagnostic intensities, but it is not energy-free. Ultrasound deposits energy as heat (thermal effect) and exerts mechanical forces (cavitation, radiation force). The embryo and fetus — particularly developing neural tissue and ossifying bone — are the tissues of greatest theoretical concern. Doppler modes, especially spectral pulsed-wave and colour Doppler, concentrate far more acoustic energy into tissue than B-mode (greyscale) imaging. The governing principle, demanded by Obstetric ultrasound practice and by every safety body, is ALARA — As Low As Reasonably Achievable: obtain the diagnostic information you need with the lowest output and shortest exposure that achieves it.
Core knowledge
Figure F11.1 — The greyscale knob drill: optimise before you diagnose — preset/probe, depth, frequency, focus, gain, TGC, dynamic range and harmonics — a good image first, diagnosis second.
How the image is formed
A transducer converts electrical energy to mechanical (sound) energy and back via the piezoelectric effect. Pulses of high-frequency sound (typically 2–18 MHz in O&G) travel into tissue, reflect at interfaces of differing acoustic impedance, and return as echoes. The machine times each echo's return to place it at the correct depth, and assigns its brightness from echo amplitude — this is B-mode (brightness mode), the standard greyscale image.
Two facts drive most knobology decisions:
- The frequency–penetration trade-off. Higher frequency = better axial resolution but poorer penetration (it attenuates faster); lower frequency penetrates deeper but resolves less detail. Hence a high-frequency transvaginal probe (classically ~5–9 MHz) gives exquisite detail of the pelvis at short range, while a lower-frequency curvilinear abdominal probe (classically ~2–5 MHz) reaches the posterior wall of a gravid uterus or an obese abdomen.
- Attenuation. Sound weakens with depth, so deeper echoes return weaker. The machine and operator must compensate for this to produce an evenly bright image.
The transducers you will use
- Curvilinear / convex abdominal probe (low frequency, wide field) — transabdominal obstetric and gynae scanning.
- Endocavitary / transvaginal probe (high frequency) — early pregnancy, ectopic assessment, the adnexa, cervical length, and detailed pelvic anatomy.
- Linear high-frequency probe — superficial structures (e.g. assessing a perineal/abdominal wall collection, vascular access).
Probe selection is itself a knobology decision: matching the right frequency and footprint to the target is the first optimisation step.
The core greyscale controls
- Gain amplifies all returning echoes uniformly — it brightens (or darkens) the whole image. Crucially, gain amplifies signal and noise; over-gaining fills anechoic structures (bladder, cyst, amniotic fluid) with false low-level echoes and can mimic debris, while under-gaining loses real soft-tissue detail.
- Time-gain compensation (TGC), a bank of sliders, selectively amplifies echoes from specific depths to offset attenuation, producing uniform brightness from near to far field. The classic error is a stepped or banded image from poorly set TGC sliders.
- Depth sets how far the image extends. Set it just beyond the structure of interest so the target fills the screen — excess depth wastes pixels and also slows the frame rate.
- Focus (focal zone) is the depth of narrowest beam width and therefore best lateral resolution. Place the focal marker at (or just deep to) the region of interest; multiple focal zones improve resolution but lower frame rate.
- Frequency / penetration can often be adjusted on a broadband probe (a "Res / Gen / Pen" or kHz toggle) — drop frequency for a difficult, deep, or obese patient; raise it for a thin patient or near-field detail.
- Dynamic range sets the number of grey shades displayed. Wide dynamic range = smoother, softer image (good for soft-tissue gradation); narrow = high-contrast, more black-and-white (useful for cystic/solid distinction).
- Tissue harmonic imaging (THI) uses harmonic frequencies generated within tissue to reduce near-field artefact and clutter — particularly useful in obese patients and to clarify cyst contents.
- Zoom (read/write zoom), cine loop (review the last few seconds frame-by-frame, invaluable for capturing a fleeting fetal view), and measurement calipers complete the everyday toolkit.