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 ...