Reflection

Ultrasonography measures the amplitude of the return echo as a function of time.¹ Sound waves are reflected at the interface of tissues with different acoustic impedances. The acoustic impedance (kg/[m²-sec]) is the product of the density (kg/m³) and velocity (m/sec). The extent of reflection is governed by the reflection coefficient: R = (Z1 − Z2)/(Z1 + Z2). If Z1 = Z2, there is no reflected wave.² Ultrasound characteristics of biologic tissue and interventional materials are summarized in Table 4-1.

Reflections off a smooth surface are called specular. If two specular reflectors are close to each other, reverberation within the sound field can result, displayed as parallel, equally spaced lines deep to the reflectors. Comet-tail artifact, which is a form of reverberation artifact, is caused by multiple internal reflections from a small, highly reflective interface.^3,4

FIGURE 4-1 Reverberation artifact from a block needle placed nearly parallel to the active face of the transducer.

FIGURE 4-2 Comet-tail artifact from the peritoneum during rectus sheath block. The peritoneum and pleura have similar appearances on ultrasound scans.

FIGURE 4-3 A strong echo and acoustic shadowing are observed when air is inadvertently injected during musculocutaneous nerve block in the axilla. Sonograms before injection (A) and after injection (B) are shown.

FIGURE 4-4 Acoustic properties of a steroid suspension. Although the local anesthetic injected for most regional blocks is anechoic, the particles of this steroid suspension are sufficiently large to produce a strong echo.