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Ultrasound Physics & Imaging
⚛️ Physics Basics🩻
- 🔊 Ultrasound = sound waves above human hearing (>20 kHz). Medical imaging typically uses 1–20 MHz.
- 📡 When waves pass through tissues, they are transmitted or reflected depending on differences in acoustic impedance (density × speed of sound).
- ➡️ The greater the impedance difference at a tissue boundary, the more reflection occurs (bright echo).
- 🔬 Transducer: contains piezoelectric crystals. About 1% emit ultrasound; 99% detect echoes.
- 🧴 Coupling gel eliminates air between probe and skin, allowing transmission into the body.
- 📈 Frequency trade-off:
- High frequency → high resolution, shallow penetration (e.g., superficial structures, MSK).
- Low frequency → deeper penetration, lower resolution (e.g., abdomen, pelvis).
- 🖥️ Echoes displayed as 2D image with characteristic appearances:
- Bone & calculi: bright white with acoustic shadow beneath.
- Fluid (blood, urine, bile, water): black (anechoic).
- Solid organs: grey (variable echogenicity).
- Interfaces: may appear brighter (acoustic enhancement).
👶 Obstetric Imaging
Fetus seen in utero with ultrasound.
❤️ Cardiac Imaging
Heart seen on echocardiography (dynamic imaging of chambers, valves, flow).
🍃 Hepatobiliary Imaging
Liver, gallbladder, bile ducts and related structures on ultrasound.
📌 Exam Pearls
- Bone & gas = enemies of ultrasound (reflect/scatter waves, blocking deeper view).
- FAST scan in trauma = free intraperitoneal or pericardial fluid.
- Doppler ultrasound = assesses flow direction & velocity.
- Always optimise gain, depth, and frequency for best image.
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