Sound and ultrasound waves consist of a mechanical disturbance of a medium such as air. The disturbance passes through the medium at a fixed speed causing vibration. The rate at which the particles vibrate is the frequency, measured in cycles per second or Hertz (Hz).
The pressure of sound is reported on a logarithmic scale called sound-pressure level, expressed in decibel (dB) referenced to the weakest audible 1 000 Hz sound pressure of 2*10^-5 Pascal (20 mP). Sound level meters contain filters that simulate the ear's frequency response. The most commonly used filter provides what is called 'A' weighting, with the letter 'A' appended to the dB units, i.e. dBA.
Sound becomes inaudible to the human ear above about 20 kHz and is then known as ultrasound. Diagnostic imaging uses much higher frequencies, in the order of MHz.
See also Spatial Peak Intensity.

Sound frequencies:

(short for ultrasound beam) The sound beam is the confined, directional beam of ultrasound traveling as a longitudinal wave from the transducer face into the propagation medium. The near field and the far field are two separate regions along the beam. Sound beams are either steered mechanically or electrically. Both rapidly sweep sound waves through tissues.

See also Sheer Wave, Beam Vessel Angle, Beam Steering, and Huygens Principle.

(US) Ultrasound is very high frequency sound above about 20,000 Hertz. Any frequency above the capabilities of the human ear is referred to as ultrasound.
Diagnostic ultrasound imaging uses much higher frequencies, in the order of megahertz. The frequencies present in usual sonograms can be anywhere between 2 and 13 MHz. The sound beam produce a single focused arc-shaped sound wave from the sum of all the individual pulses emitted by the transducer.

See also Medical Imaging.

2D ultrasound imaging is a widely used technique in medical imaging that provides two-dimensional visual representations of internal structures. A handheld device known as a probe or transducer contains piezoelectric crystals that emit and receive ultrasound waves which penetrate tissues and bounce back as echoes. The echoes are detected and converted into electrical signals. These signals are processed and displayed on a monitor, creating a real-time 2D grayscale image, with different shades of gray representing various tissue densities. The brighter areas on the image correspond to structures that reflect more ultrasound waves, while darker areas represent structures that reflect fewer waves or are attenuated by intervening tissues. The 2D-mode (or B-mode) provides cross-sectional views of the scanned area, showing a single plane or slice of the scanned area at a time.

Key Features and Uses of 2D Ultrasound:

Comparison with 3D and 4D Ultrasound:
See also Ultrasound Technology, Sonographer, Ultrasound Elastography, Obstetric and Gynecologic Ultrasound.

In 3D ultrasound (US) several 2D images are acquired by moving the probe across the body surface or rotating inserted probes. 3D-mode uses the same basic concept of a 2D ultrasound but rather than take the image from a single angle, the sonographer takes a volume image. The volume image that is displayed on the screen is a software rendering of all of the detected soft-tissue combined by specialized computer software to form three-dimensional images.
The 3D volume rendering technique (VR) does not rely on segmentation (segmentation techniques are difficult to apply to ultrasound pictures) and makes it possible to obtain clear 3D ultrasound images for clinical diagnosis. A 3D ultrasound produces a still image. Diagnostic US systems with 3D display functions and linear array probes are mainly used for obstetric and abdominal applications. The combination of contrast agents, harmonic imaging and power Doppler greatly improves 3D US reconstructions.

3D imaging shows a better look at the organ being examined and is used for:

Fusion 3D imaging methods for generating compound images from two sets of ultrasound images (B-mode and Doppler images) enable the observation of the structural relationships between lesions and their associated blood vessels in three dimensions (maximum intensity projection).