Biophysics and Imaging for MRCOG Part 1
Ultrasound Physics
Diagnostic ultrasound uses frequencies of roughly 2–10 MHz, generated by a piezoelectric crystal that both transmits and receives.
The governing trade-off, and the one every question turns on:
Higher frequency = better axial resolution, less tissue penetration.
So a 2–5 MHz curvilinear probe images a third-trimester fetus; a 5–10 MHz transvaginal probe images an early gestational sac.
| Term | Meaning | |---|---| | Axial resolution | Along the beam. Determined by pulse length, hence frequency. Better than lateral. | | Lateral resolution | Across the beam. Determined by beam width, best at the focal zone. | | Acoustic impedance | Density × propagation velocity. Reflection occurs at interfaces where it changes. | | Attenuation | Loss of intensity with depth, from absorption, scatter and reflection. Rises with frequency. |
Large impedance mismatches explain the classic artefacts: air and bone reflect almost everything, so bowel gas obscures and bone shadows. Coupling gel exists to remove the skin–air interface.
Artefacts worth naming: acoustic shadowing (behind calcification or bone), posterior acoustic enhancement (behind a fluid-filled structure such as a cyst or full bladder), reverberation, and mirror-image artefact.
Doppler
Doppler measures frequency shift from moving reflectors, principally red cells. The shift depends on the cosine of the insonation angle, so:
- At 0° (beam parallel to flow) the measured velocity is maximal and accurate.
- At 90° the cosine is zero and there is no measurable shift — the vessel looks static.
- Keep the angle below about 60°, above which small angle errors produce large velocity errors.
Indices used in obstetrics are angle-independent because they are ratios:
- Pulsatility index (PI) = (peak systolic − end diastolic) / mean
- Resistance index (RI) = (peak systolic − end diastolic) / peak systolic
- S/D ratio = peak systolic / end diastolic
Absent or reversed end-diastolic flow in the umbilical artery indicates high placental resistance. Aliasing occurs when the Doppler shift exceeds half the pulse repetition frequency (the Nyquist limit) and is corrected by raising the scale, shifting the baseline, or using a lower frequency.
Safety. Thermal index and mechanical index describe potential bioeffects. The ALARA principle applies. Pulsed Doppler deposits more energy than B-mode, which is why routine pulsed Doppler over the embryo in the first trimester is discouraged.
Ionising Radiation
| Quantity | Unit | Meaning | |---|---|---| | Absorbed dose | gray (Gy) | Energy deposited per kg | | Equivalent dose | sievert (Sv) | Absorbed dose weighted for radiation type | | Effective dose | sievert (Sv) | Weighted for tissue sensitivity — used for risk |
Effects divide into:
- Deterministic — a threshold exists, and severity rises with dose. Cataract, skin erythema, and fetal effects such as growth restriction.
- Stochastic — no threshold; probability rises with dose. Carcinogenesis and heritable effects.
In pregnancy: the fetal dose from a chest radiograph or CT pulmonary angiogram is very small; a CTPA delivers a higher maternal breast dose, while a V/Q scan delivers a higher fetal dose — the trade-off named in GTG 37b. Fetal risk is highest during organogenesis. No diagnostic radiograph delivers a dose near the deterministic threshold, and a necessary investigation should not be withheld.
MRI
MRI uses a strong static magnetic field and radiofrequency pulses; it is non-ionising. It is generally regarded as safe in pregnancy at 1.5 T, and is preferred over CT where it answers the question. Gadolinium crosses the placenta and is avoided unless the benefit clearly justifies it. The principal hazards are projectile injury from ferromagnetic objects, and implanted devices.
Electrosurgery
- Monopolar: current flows from the active electrode through the patient to a return plate. Burns occur when the plate contacts poorly.
- Bipolar: current passes only between the two limbs of the instrument. Safer near bowel and in patients with pacemakers.
- Cutting uses a continuous low-voltage waveform; coagulation uses an interrupted high-voltage waveform producing more lateral thermal spread.
- Capacitive coupling and insulation failure are the mechanisms of laparoscopic bowel injury remote from the operative site — often recognised days later, when the patient re-presents unwell.
High-Yield Exam Points
- Higher frequency buys resolution and costs penetration. This single sentence answers most ultrasound questions.
- A Doppler beam at 90° to flow measures nothing.
- PI, RI and S/D are ratios and therefore angle-independent.
- V/Q gives a higher fetal dose; CTPA gives a higher maternal breast dose.
- Gadolinium crosses the placenta; the magnetic field itself does not harm the fetus at 1.5 T.
- Delayed presentation after laparoscopy suggests capacitive coupling or insulation failure, not a sharp injury.
MRCOG AI is an independent educational tool. It is not affiliated with, endorsed by, or connected to the Royal College of Obstetricians and Gynaecologists (RCOG).