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A-mode (Amplitude-mode) ultrasound is used to judge the depth of an organ, or otherwise assess an organ's dimensions. A-mode technology has been used in midline echoencephalography for rapid screening of intracranial mass lesions and ophthalmologic scanning. A-mode ultrasound imaging is now obsolete in medical imaging. The A-mode scan had also been used for early pregnancy assessment (detection of fetal heart beat), 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.
To make an echoencephalography scan, the first A-mode scan is obtained from the right side of the head and the image captured on film. Then the probe is placed at the corresponding point on the left side. The second exposure is made on the same film with 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 Ultrasound Biomicroscopy, A-scan, B-mode and the Infosheet about ultrasound modes.
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