Ultrasound Physics

Basic acoustic principles

Sound is a series of pressure waves traveling through a medium. One full cycle of the wave is a complete positive and negative pressure change; the wavelength is the distance covered during one cycle, and frequency — measured in Hertz, or cycles per second — describes how many of those cycles occur each second.

Audible sound for most people falls between 20 Hz and 20,000 Hz (20 kHz). Ultrasound refers to any sound above that range, and diagnostic ultrasound specifically uses frequencies between about 2 and 20 million Hz (MHz).

How fast a wave travels through a medium depends on that medium's stiffness — the stiffer the material, the faster the wave moves, which is why sound travels faster through solids than through liquids or gases. In human soft tissue, sound travels at roughly 1,540 meters per second, or about one mile per second.

Attenuation

Attenuation is the loss of intensity and amplitude as sound travels through tissue. The main cause in soft tissue is absorption, where acoustic energy converts into heat. Reflection, refraction, and scatter also contribute:

• Reflection — occurs at the boundary between two different tissues; some of the wave bounces back toward the source as an echo, with the angle of the reflected wave matching the angle it arrived at, while the rest continues into the second tissue.
• Refraction — when the two tissues have different stiffness, the change in propagation speed bends the wave off its original path; the amount of bending depends on how different the two tissues' stiffness is.
• Scatter — occurs when a wave meets a surface with an uneven, heterogeneous structure. Most of the wave keeps traveling along its original path, but a small portion scatters off in random directions.

The pulse-echo principle

Ultrasound image generation relies on the pulse-echo principle. The source of the wave is a piezoelectric crystal housed in the transducer, which converts electrical current into mechanical pressure waves and can also do the reverse — converting returning pressure waves back into an electrical signal. After sending a pulse, the crystal switches from "sending" to "listening" mode to wait for the echo. In practice, transducers spend over 99% of their time listening rather than transmitting, and this send-and-listen cycle repeats several million times per second. The direction, timing, and amplitude of the returning waves are what the machine translates into an image.

Frequency and resolution trade off against each other: lower frequencies penetrate deeper into tissue but produce lower resolution, while higher frequencies produce finer detail at the cost of penetration depth.

Ultrasound modes

B-mode (brightness mode)

B-mode is the standard two-dimensional, structural view of tissue, with brightness determined by the amplitude of the returning echoes.

• Anechoic / echolucent — little to no returning signal; appears black.
• Hypoechoic — few returning echoes; appears darker than surrounding tissue.
• Hyperechoic / echogenic — strong returning echoes; appears brighter than surrounding tissue.

M-mode (motion mode)

M-mode captures the returning echoes along a single line of the B-mode image and displays how they change over time, making it possible to visualize movement of structures along that line. M-mode is often displayed alongside a real-time B-mode image on the same screen.

Doppler modes

Doppler modes measure the direction and speed of tissue motion or blood flow using the Doppler shift — the change in frequency between the sent and returning wave, which is generated when sound reflects off moving particles. The amount of shift correlates with the velocity and direction of that motion.

Artifacts

Artifacts are features the machine displays that don't actually exist in the tissue. They can either aid interpretation or confuse it.

Attenuation artifacts

• Shadowing — caused by partial or total reflection or absorption of sound energy; a much weaker signal returns from behind a strong reflector (like air) or a strongly absorbing structure (like a gallstone, kidney stone, or bone).
• Edge shadowing — a thin acoustic shadow behind the lateral edges of cystic structures, caused when sound hits a curved surface at a tangential angle and refracts with few echoes returning to the transducer.
• Posterior enhancement — the area behind an echo-weak or echo-free structure appears brighter than its surroundings, because neighboring signals pass through more attenuating tissue and return weaker by comparison. A common example is the area behind the anechoic bladder.

Propagation artifacts

• Reverberation — occurs when sound bounces back and forth between two highly reflective layers before returning to the transducer; the longer travel time is read as greater distance, displaying extra "reverberated" images deeper in the tissue than they really are.
• Comet tail artifact — similar to reverberation, produced by the front and back of a very strong reflector (such as an air bubble); the reverberations are so closely spaced they blend into a small band.
• Mirroring — a duplicate image appears on the opposite side of a strong reflective surface, caused when waves reflect off that surface, bounce off another structure, and return with a longer travel time that gets displayed as an extra structure deeper than the reflective surface.

Miscellaneous artifacts

• Ring down — caused by resonance from a collection of gas bubbles, producing a continuous, uninterrupted echo that looks similar to a comet tail artifact.
• Side lobe — low-energy "side lobes" of the main ultrasound beam hit a highly reflective structure (such as bowel gas); if the returning echo is strong enough, it gets misattributed to the main beam and displayed at the wrong location, usually appearing as bright, rounded lines within hypoechoic or echo-free structures.

Probes

A transducer is built from an active element (the piezoelectric crystal), damping material, and a matching layer. Different arrangements of the active element produce different probe types, each suited to different uses.

Image acquisition and probe positioning

Imaging planes

• Transverse (axial / cross-sectional) — runs perpendicular to the ground in a supine patient, separating superior from inferior (head from feet).
• Sagittal — runs perpendicular to the ground, separating left from right.
• Coronal (frontal) — runs parallel to the ground, separating anterior from posterior (front from back).
• Oblique — angled between the coronal, sagittal, and transverse planes rather than aligned with any of them.

Probe manipulation

• Slide — moving the probe along its long axis across the body's surface while staying perpendicular to the target.
• Sweep — moving the probe along its short axis across the body's surface while staying perpendicular to the target.
• Rock — tilting the probe along its long axis without moving its point of contact with the body.
• Fan — tilting the probe along its short axis without moving its point of contact with the body.
• Pressure / compression — pressing the probe into the body's surface while keeping full footprint contact and staying perpendicular to the target.
• Rotate — turning the probe clockwise or counterclockwise while keeping full footprint contact and staying perpendicular to the target.

Ultrasound machine functions

These core functions are common to most ultrasound machines, though exact controls vary by manufacturer: