The source level is the acoustic power (sound pressure) in decibels (dB) referenced to 1.0 microPascal measured at 1 meter from the sound source transmitted through a unit area in a specified direction.
The unit of intensity is watt per square meter, but source level is commonly given as a relative intensity in dB.

Microwave thermotherapy is a type of ultrasound treatment in which body tissue is exposed to high temperatures. Focused ultrasound waves with high intensity damage and kill cancer cells or to make cancer cells more sensitive to the effects of radiation and certain anticancer drugs.
Examples for such treatments are the transurethral microwave thermotherapy (TUMT) and transurethral needle ablation (TUNA) that are used for benign prostatic hyperplasia.

See also High Intensity Focused Ultrasound and Ultrasound Therapy.

A voxel is a volume element (volumetric and pixel) representing a value in the three dimensional space, corresponding to a pixel for a given slice thickness. Voxels are frequently used in the visualization and analysis of medical data. The ultrasound pixel intensity is proportional to the signal intensity of the appropriate voxel.

See also Slice Thickness Artifact.

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).

A-mode (Amplitude-mode) ultrasound is a technique used to assess organ dimensions and determine the depth of an organ. While A-mode technology was previously employed in midline echoencephalography for rapid screening of intracranial mass lesions and ophthalmologic scanning, it is now considered obsolete in medical imaging. Nonetheless, the A-mode scan has found applications in early pregnancy assessment (specifically the detection of fetal heartbeats), cephalometry, and placental localization.
When the ultrasound beam encounters an anatomic boundary, the received sound impulse is processed to appear as a vertical reflection of a point. On the display, it looks like spikes of different heights (the amplitude). The intensity of the returning impulse determined the height of the vertical reflection and the time it took for the impulse to make the round trip would determine the space between verticals. The distance between these spikes can be measured accurately by dividing the speed of sound in tissue (1540 m/sec) by half the sound travel time.
During an echoencephalography scan, the first A-mode scan is acquired from the right side of the head and captured on film. Subsequently, the probe is positioned at the corresponding point on the left side, and a second exposure is captured on the same film, displaying inverted spikes. The A-mode ultrasound could be used to identify structures normally located in the midline of the brain such as the third ventricle and falx cerebri. The midline structures would be aligned in normal patients but show displacement in patients with mass lesion such as a subdural, epidural, or intracranial hemorrhage.

See also 2D Ultrasound, 3D Ultrasound, 4D Ultrasound, Ultrasound Biomicroscopy, A-scan, B-mode and the Infosheet about ultrasound modes.