Principles of Ultrasound

Introduction

• Ultrasound is integral in regional anesthesia for guidance of peripheral nerve blocks and catheters, in cardiac anesthesia for transesophageal echocardiography, and in multiple aspects of perioperative anesthesia for multiorgan diagnostic point-of-care ultrasound and for procedural guidance of vascular access (e.g., arterial, central, and peripheral catheters).
• Electricity excites piezoelectric crystals within the ultrasound probe which emit high-frequency sound waves.^1,2
• These sound (or pressure) waves propagate through tissue, which scatter ultrasound waves in multiple directions, reflecting some of the energy back to the ultrasound transducer.
• The energy that reflects back to the transducer mechanically stresses the transducer’s piezoelectric crystals causing them to generate electrical signals.
• The electrical signals are converted by software in the ultrasound machine into an image that represents the varying densities of the underlying structures.^1,2,4

Resolution

• Two types of resolution: spatial and temporal
• Spatial resolution is akin to image quality, specifically the ability to discern the difference between two structures.
  • Spatial resolution has two aspects: axial (or longitudinal) and lateral (Figure 1).
  • Increased frequency bandwidth improves axial resolution (i.e., whether two objects are resolvable in the direction parallel to the sound beam).
  • Both beam width and frequency bandwidth impart lateral resolution (i.e., whether two objects are resolvable in the direction perpendicular to the sound beam).

• • Spatial resolution is determined by the spatial pulse length (wavelength x number of cycles in a pulse of ultrasound) (Figure 2 and 3).

Temporal resolution

• The ability to distinguish between events in time, e.g., in rapidly moving heart structures during a cardiac cycle.
• An ultrasound clip is generated as a series of frames over time, and each frame is created from repeated pulses that form scan lines.
• Temporal resolution is equal to the time from the beginning of one frame to the next.

Frequency

• The frequency of an ultrasound beam (f) is inversely proportional to its wavelength (λ) and varies with the specific velocity of sound in a given tissue, by the formula λ = c / f
• Frequency, resolution and depth (Figure 4)
  • Shorter wavelengths → higher frequencies → higher resolution images, but shallower penetration
  • Longer wavelengths → lower frequencies → lower resolution images, but penetrate deeper
• Ultrasonography uses frequencies between 2-18 MHz.
  • High-frequency (8-15 MHz) linear transducers penetrate shallow structures (target 3-4cm) and offer higher resolution.³
  • Low-frequency (3-5 MHz) curvilinear transducers penetrate deeper structures (target >5cm) and yield lower resolution.³
  • Once a transducer is selected, further adjustment within the frequency range can be made to optimize images.

Depth

• Depth of penetration depends on frequency as discussed above.
• This can be adjusted with each transducer selected.
• Similar to aperture on a camera, the ultrasound beam can be focused, which increases resolution at a certain depth (Figure 4).

Resonance

• Acoustic resonance: system amplifies sound waves whose frequencies are the same as its own natural frequencies of vibration.
• May result in a reverberation artifact (i.e., repeating hyperechoic structures such as a needle) which can occur with hyperresonance² (Figure 5, 6, and 7).