Chapter 50

Ultrasound imaging utilizes high-frequency sound waves (3–17 MHz). Because the speed of sound in soft tissue is fairly constant (1540 m/sec), the position of objects can be inferred from the time of flight of their received echoes. The product of wavelength and frequency is the speed of sound, so high-frequency sound waves have shorter wavelengths, and therefore provide better axial resolution. Attenuation of sound waves is frequency-dependent (approximately 0.75 dB/cm/MHz), so penetration of high-frequency sound waves into deep tissue is limited. For interventional guidance, one of the biggest advantages of ultrasound over other imaging modalities is the real-time acquisition of images. Frame rates of 30 Hz or higher are common in clinical practice.

Theoretically, ultrasound imaging can cause warming of tissue through absorption of sound waves (quantified by the thermal index). Transmission of sound waves also can cause cavitation (gas body formation, quantified by the mechanical index). However, no adverse biological effects have been confirmed for diagnostic ultrasound. Nevertheless, it is prudent to limit scanning to that necessary for clinical care and related education.

The most common artifact associated with ultrasound imaging is contact artifact. Contact artifact is defined as loss of acoustic coupling between transducer and skin. Scanning gel is normally applied to exclude air from the transducer–skin interface. This interface can be disrupted simply because the transducer does not touch the skin. Another common cause is trapping of air bubbles under the sterile cover of the transducer. If the block needle is inserted too close to the transducer, the skin contact will be disturbed. Firm, even pressure with the transducer (like holding a mask to ventilate an anesthetized patient) is required to produce optimal scans. Manual compression exerted with the transducer is usually optimal for regional block when sufficient to just cause coaptation of the walls of superficial veins within the field of imaging.

Throughout this chapter I will use the American Institute of Ultrasound in Medicine (AIUM) standard nomenclature for transducer manipulation: tilting, rocking, sliding, compression, and rotation.1

In 1842 Christian Johann Doppler described the frequency shift that occurs when a wave source or receiver moves. Doppler's stellar observation has been applied to estimate the velocity of moving reflectors in the body (typically red blood cells) by measuring the frequency shift of sound waves. Modern color Doppler velocity imaging maps the mean velocity to a color scale. Specifically, color flow mapping systems overlay a pseudocolor velocity map on a gray-scale, two-dimensional image.2

Recently a new color Doppler technology has been developed.3 Rather than estimate the mean Doppler frequency shift, these new technologies are based on estimating the integrated Doppler power spectrum. The advantage of power Doppler is that it is more sensitive at detecting blood flow than velocity imaging (by a factor of 3 to 5 in some cases). In addition, the integrated power Doppler signal is almost independent of the angle between the vessel and ...

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