Ultrasound and Doppler

Wavelength is the distance between two points of maximum compression or rarefaction. The frequency of a wave is the number of wavelengths per second expressed as hertz (Hz). High-frequency sounds have shorter wavelengths than low-frequency sounds. Interaction with an object is dependent on the wavelength being the same size or smaller than the object. Higher-frequency waves provide better resolution (higher quality) by interacting with smaller objects, but have lower penetration due to rapid attenuation. Conversely, lower frequencies penetrate further at the expense of resolution.

The amplitude of a wave is the signal intensity, or loudness of the sound, expressed in decibels.

Propagation velocity is the speed at which sound moves through a medium. It depends on the density and compressibility of the medium. Different tissues such as blood, bone, fat, lung and muscle have different propagation velocities. Taking into account these differences, the average speed at which ultrasound passes through human tissues is 1540 m·s⁻¹ at 37°C.

Ultrasound waves passing through tissues can undergo reflection, refraction, attenuation and scatter.

Reflection: ultrasound waves are partly reflected at the boundaries of tissue interfaces. These reflected sound waves form the basis of ultrasound imaging. The amount reflected depends on the difference in acoustic impedance between two interacting tissues. The law of reflection states that the angle of incidence equals the angle of reflection. Therefore, interfaces that are perpendicular to the beam will reflect waves back to the probe to the greatest extent.

Refraction: bending of the ultrasound beam occurs when it encounters tissues of differing acoustic impedance at oblique angles.

Attenuation: as the wave passes through tissues, it causes molecules to vibrate, leading to energy loss as heat. This results in diminution of the signal.

Scatter: small structures such as blood cells cause some of the ultrasound beam to scatter, contributing to signal attenuation.

Ultrasound transducers use piezoelectric crystals to generate and receive ultrasound. Applying an electric current to a piezoelectric crystal aligns the polarised particles with the crystal surface, resulting in a change in shape of the crystal. An alternating current causes rapid expansion and contraction of the crystal, which produces compressions and rarefactions (i.e. a sound wave). Using the same principle, the piezoelectric crystal can receive incoming sound waves to create an alternating electrical current.

Ultrasound images are created by the detection and subsequent display of reflected ultrasound waves. Piezoelectric crystals in the transducer generate a brief pulse of ultrasound waves, and then enter a receiving mode where they detect reflected ultrasound waves. Assuming the average speed of sound in human tissues is 1540 m·s⁻¹, the machine calculates the distance the sound wave travelled prior to reflection and plots this as a point on a screen. An ultrasound image is generated by plotting multiple points in this way.

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