Unit
- Ultrasound wave properties: Ultrasound waves are mechanical vibrations with frequencies above human hearing (typically 2-18 MHz for diagnostic imaging). These waves travel through tissues and reflect at interfaces between different acoustic impedances, producing echoes that form images. Understanding wave properties like frequency, wavelength, and amplitude is essential for optimizing image quality.
- Piezoelectric effect: The piezoelectric effect is the basis of ultrasound generation and reception. Certain crystals (e.g., quartz, lead zirconate titanate) deform when an electric field is applied, producing sound waves. Conversely, incoming sound waves deform the crystal, generating electrical signals that are processed into images.
- Acoustic impedance: Acoustic impedance (Z) is a measure of a medium's resistance to sound wave propagation. It depends on density and speed of sound in the medium. Differences in impedance between adjacent tissues determine the amount of reflection. Large impedance mismatches (e.g., soft tissue-bone) cause strong reflections, while small mismatches yield weaker echoes.
- Attenuation: Attenuation refers to the loss of ultrasound energy as it travels through tissue. This occurs through absorption, scattering, and reflection. Higher frequency waves attenuate more rapidly, limiting penetration depth. Operators must choose appropriate frequencies for the target anatomy: higher frequencies for superficial structures, lower frequencies for deeper structures.
- Speed of sound: The speed of sound in soft tissue is approximately 1540 m/s. This constant is used by ultrasound machines to calculate distances from echo time-of-flight. Variations in actual tissue speed (e.g., fat ~1450 m/s, bone ~3500 m/s) can lead to measurement errors if not accounted for.
- Resolution: Resolution in ultrasound includes axial (along the beam axis) and lateral (perpendicular to beam). Axial resolution depends on pulse length and is improved with higher frequencies. Lateral resolution depends on beam width and focusing. Spatial resolution directly impacts the ability to distinguish small structures.
- Doppler effect: The Doppler effect is used to measure blood flow velocity. Frequency shifts in echoes from moving red blood cells are proportional to velocity. Color Doppler, pulsed-wave Doppler, and continuous-wave Doppler are clinical applications that require careful adjustment of parameters like sample volume and angle correction.
- Safety indices: Ultrasound safety is governed by thermal and mechanical indices (TI and MI). TI indicates potential temperature rise, MI indicates potential cavitation. Operators must adhere to the ALARA (As Low As Reasonably Achievable) principle to minimize patient exposure while obtaining diagnostic quality images.
Regulatory Note
Regulation: The ALARA (As Low As Reasonably Achievable) principle is a core safety guideline for ultrasound. It requires operators to keep exposure time and intensity to the minimum necessary for diagnostic quality. The American Institute of Ultrasound in Medicine (AIUM) and the U.S. Food and Drug Administration (FDA) endorse this approach. Output display standards (TI, MI) are mandatory on modern equipment.
Source: AIUM Practice Parameter for the Performance of Diagnostic Ultrasound Examinations, 2023.
Quick Revision
- Ultrasound uses high-frequency sound waves and the piezoelectric effect for imaging.
- Acoustic impedance differences cause reflections; attenuation reduces wave energy with depth.
- Resolution is determined by frequency and beam characteristics; Doppler measures flow.
- Safety indices (TI, MI) and ALARA principle protect patients from unnecessary exposure.