The 2D-mode (2-Dimensional-mode) is a spatially oriented B-mode (brightness) ultrasound. The imaged structures are displayed 2 dimensional as a function of depth and width. The brightness level is based on the echo signal amplitude.
Most of the ultrasound devices in medical imaging are 2D real-time scanner. The image is created by a rapidly back and forth swept sound beam over the region of interest.

See also Gray Scale.

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.

As low as reasonably achievable (ALARA) is in ultrasound imaging (ultrasonography) as well as in other medical imaging modalities (MRI, X-RAY, etc.) the guiding principle to keep patient exposure as low as possible for the diagnostic result.

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Amersham Health, now GE Healthcare was the imaging diagnostics and therapy business of Amersham plc. The imaging business produced contrast agents that are injected into the body to improve pictures obtained through medical imaging scans.

The wider the ultrasound beam, the more severe the problem with volume averaging and the beam-width artifact, to avoid this, the ultrasound beam can be shaped with lenses.
Different possibilities to focus the beam:

Conventional multi-element transducers are electronically focused in order to minimize beam width. This transducer type can be focused electronically only along the long axis of the probe where there are multiple elements, along the short axis (elevation axis) are conventional transducers only one element wide. Electronic focusing in any axis requires multiple transducer elements arrayed along that axis. Short axis focusing of conventional multi-element transducers requires an acoustic lens which has a fixed focal length.
For operation at frequencies at or even above 10 MHz, quantization noise reduces contrast resolution. Digital beamforming gives better control over time delay quantization errors. In digital beamformers the delay accuracy is improved, thus allowing higher frequency operation. In analog beamformers, delay accuracy is in the order of 20 ns.
Phased beamformers are suitable to handle linear phased arrays and are used for sector formats such as required in cardiography to improve image quality. Beamforming in ultrasound instruments for medical imaging uses analog delay lines. The signal from each individual element is delayed in order to steer the beam in the desired direction and focuses the beam.
The receive beamformer tracks the depth and focuses the receive beam as the depth increases for each transmitted pulse. The receive aperture increase with depth. The lateral resolution is constant with depth, and decreases the sensitivity to aberrations in the imaged tissue. A requirement for dynamic control of the used elements is given. Since often a weighting function (apodization) is used for side lobe reduction, the element weights also have to be dynamically updated with depth.

See also Huygens Principle.