Sound Meets Tissue
Ultrasound imaging sends mechanical pressure waves into the body at frequencies far above human hearing. In soft tissue, sound travels at approximately 1540 m/s — a value remarkably consistent across muscle, liver, and fat. When a pulse crosses a boundary between tissues of different acoustic impedance (density times sound speed), part of the energy reflects. The time delay of each returning echo encodes the depth of the reflecting interface.
Resolution vs Penetration
The fundamental trade-off in ultrasound is frequency versus depth. A 15 MHz linear probe resolves structures as small as 0.05 mm but penetrates only 3-4 cm — ideal for superficial tendons and thyroid nodules. A 2 MHz curvilinear probe reaches 20+ cm into the abdomen but with resolution limited to ~0.4 mm. Attenuation in tissue scales linearly with both frequency and depth, following the rule of thumb: 0.5 dB/cm/MHz.
Acoustic Impedance & Reflection
The reflection coefficient at an interface depends on the impedance mismatch. Soft tissue boundaries reflect 1-5% of incident energy — enough for imaging while allowing the beam to continue deeper. Tissue-bone interfaces reflect over 40%, and tissue-air interfaces reflect nearly 100%. This is why gel is essential between probe and skin — eliminating the air gap — and why ultrasound struggles with lungs and bones.
B-Mode Display
In B-mode (brightness mode), each echo is plotted as a bright dot at the corresponding depth along the scan line. Sweeping or steering the beam across a region builds a 2D cross-sectional image in real time — typically 30-100 frames per second. This simulator shows how changing frequency and depth alters the resolution limit and penetration, helping you understand the physical parameters behind every clinical ultrasound image.