22 Bone
The outer surface of cortical bone consists of dense fibrous connective tissue (the periosteum). There is a marked acoustic impedance mismatch between bone and soft tissue. This results in a bright line on ultrasound scans from reflection at the bone–soft tissue interface. Mature bone also is a great absorber of sound waves.1 Acoustic shadowing occurs deep to the interface because of extinction of the sound wave.
Clinical Pearls
• Neuraxial scans of pediatric patients are superior to those of adults, in part because skeletal maturity does not occur until about 2 years of age from an acoustic standpoint.
• The thermal index of bone (TIB) is greater than thermal index of soft tissue (TIS) because of the substantial absorption of ultrasound.
