Ultrasound imaging is frequently utilized in modern anesthesiology practice in the context of central and peripheral venous access, placement of peripheral nerve blocks, and echocardiography. Medical ultrasound utilizes longitudinal, mechanically produced high-frequency sound waves to produce a real-time image of tissue.
PRODUCTION OF ULTRASONIC SOUND WAVES
Electrical energy is converted into mechanical waves in an ultrasound probe by a transducer. Most ultrasonic transducers contain artificial polycrystalline ferroelectric materials (crystals), such as lead zirconate titanate, to produce a piezoelectric effect. A voltage is applied to two electrodes attached to the surface of the crystal that creates an electric field resulting in a dimensional change. Serial dimensional changes produce the high-frequency sound waves emitted by the ultrasound probe. The thickness of the piezoelectric element in the probe determines the frequency of the sound waves emitted.
Frequencies of sound waves:
Infrasound: 0-20 Hz
Audible sound: 20-20 000 Hz
Ultrasound: greater than 20 000 Hz (>20 kHz)
Medical ultrasound: 2 500 000–15 000 000 Hz (2.5-15 MHz)
PROPAGATION AND REFLECTION OF ULTRASONIC WAVES IN TISSUE
Contrasting mechanical properties of tissues in different anatomic structures create interfaces that result in the reflection or echo of ultrasonic waves. A transducer in the ultrasound probe is able to use the same piezoelectric effect as discussed previously to convert the reflected mechanical wave back into electrical energy. The scanner’s computer represents the amplitude (strength) of the wave on the ultrasound image by a dot. The interface between tissues with differing mechanical properties, including density and compressibility, will reflect ultrasound waves with a variety of amplitudes, which are represented in the brightness of the dot on the image. The larger the mismatch is between the two tissues, a concept referred to as impedance mismatching, the larger the amplitude of the echo will be. The ability of a tissue interface to reflect an ultrasonic wave is called echogenicity. The term hyperechogenic or hyperechoic is used to describe tissue interfaces with many echoes. Tissue interfaces that do not produce echoes are said to be anechoic. Two tissues with the same echogenicity that are unable to be depicted separately are referred to as isoechogenic (Table 5-1).
TABLE 5-1General Echogenicity of Tissues ||Download (.pdf) TABLE 5-1 General Echogenicity of Tissues
|Hyperechoic (strong reflection) ||White dot on imaging ||Bone, tendons, ligaments, diaphragm, peripheral nerves, liver angiomas, tumor cells, blood vessels, fibrosis, and liver steatosis. |
|Hyperechoic (weaker reflections) ||Gray dot on imaging ||Most solid organs, thick fluid. |
|Anechoic (no reflection) ||No dot on imaging (black) ||Cysts, ascites, other fluid-filled regions. |
Although some waves are reflected at the interfaces between different tissues, other waves travel deeper into the body and are reflected from deeper structures. The amount of time between the production of the ultrasound wave and its return ...