Ultrasound Physics: Core Principles & Artifacts

Overview

Medical ultrasound utilizes high-frequency sound waves (typically 2-15 MHz) to generate real-time structural images. Unlike X-rays and CT, ultrasound does not use ionizing radiation, making it uniquely safe for obstetrics, pediatrics, and repeated dynamic imaging.

The Piezoelectric Effect

The foundational principle of modern ultrasound relies on piezoelectric crystals housed within the transducer probe:

• Transmission: An alternating electrical current is applied to the crystals, causing them to rapidly rapidly expand and contract. This mechanical vibration generates the high-frequency sound wave.
• Reception: When returning sound echoes strike the crystals, the mechanical pressure physically deforms them, generating a tiny electrical voltage that is processed into an image.

Acoustic Impedance & Reflection

Sound waves are reflected back to the probe when they encounter a boundary between two tissues with different "acoustic impedances" (tissue density × speed of sound). The larger the difference, the stronger the echo (reflection).

• Bone / Soft Tissue: Massive impedance difference. Almost 100% of sound is reflected, causing bone to appear bright white with a complete "acoustic shadow" behind it.
• Air / Soft Tissue: Massive impedance difference. Sound is completely scattered and reflected by air. This is why coupling gel is absolutely mandatory to remove air from the skin-probe interface.
• Liver / Kidney: Very small impedance difference. Only a tiny fraction of sound is reflected, allowing the remaining wave to transmit deeper into the body.

Frequency vs. Penetration Trade-off

Selecting the correct transducer is governed by the immutable trade-off between spatial resolution and tissue penetration:

• High-Frequency (10-15 MHz): e.g., Linear Arrays. Provides magnificent, microscopic spatial resolution but cannot penetrate deep tissues. Ideal for thyroid, breast, testes, and superficial MSK.
• Low-Frequency (2-5 MHz): e.g., Curvilinear Arrays. Provides vastly deeper tissue penetration at the cost of grainier spatial resolution. Mandatory for abdominal and obstetric imaging in adults.

Core Ultrasound Artifacts

Artifacts are not just errors; they are vital diagnostic clues.

• Posterior Acoustic Enhancement: Tissues deep to a purely fluid-filled structure (like a simple cyst or full bladder) appear artificially brighter than adjacent tissues because sound travels through fluid without significant attenuation.
• Posterior Acoustic Shadowing: Tissues deep to a highly attenuating or reflecting structure (like a gallstone or rib) appear completely dark because no sound reaches them.
• Reverberation: Repeated bouncing of sound between two highly reflective parallel surfaces, creating equidistant horizontal false echoes (e.g., A-lines in lung ultrasound).
• Comet-Tail (Ring-Down): A specific type of continuous reverberation artifact classically caused by tiny pockets of cholesterol crystals (adenomyomatosis) or small air bubbles.

High Yield Facts