Transducer characteristics, such as frequency and shape, determine ultrasound image quality. Ultrasound waves have frequencies that exceed the upper limit for audible human hearing, i.e., greater than 20 kHz. Medical ultrasound devices use sound waves in the range of 1–20 MHz. Proper selection of transducer frequency is an important concept for providing optimal image resolution in diagnostic and procedural Ultrasound.
High-frequency ultrasound waves (short wavelength) generate images of high axial resolution. Increasing the number of waves of compression and rarefaction for a given distance can more accurately discriminate between two separate structures along the axial plane of wave propagation. However, high-frequency waves are more attenuated than lower frequency waves for a given distance; thus, they are suitable for imaging mainly superficial structures (2-3cm from skin).
Conversely, low-frequency waves (long wavelength) offer images of lower resolution but can penetrate to deeper structures due to a lower degree of attenuation. For this reason, it is best to use high-frequency transducers (up to 10–15 MHz range) to image superficial structures (e.g. nerves in the interscalene, supraclavicular and axillary regions) and low-frequency transducers (typically 2–5 MHz) for infraclavicular block and imaging the lumbar neuraxial structures that are deep in most adults.
It is important to remember that:
- High frequency = high spatial resolution but limited depth of penetration
- Low frequency = greater depth of penetration but lower spatial resolution