Image formation

The first attempts to use ultrasound for medical diagnosis were based on the expectation that it would be possible to demonstrate tissue masses within the body and, particularly, within the brain, because of differences in attenuation. Dussik et al (1947) constructed a scanner in which a beam of ultrasound was directed through the patient’s head and detected by a receiver placed in line with the transmitter. The images which were formed by scanning the beam in a raster pattern seemed to represent the intracerebral structures, including the ventricles. Using a similar scanner operating at a frequency of 2.5 MHz and an intensity of about 1 W cm- Hueter and Bolt (1951) concluded that ‘a preliminary evaluation indicates that the echo-reflection method is considerably less promising (than the transmission method) for general ventriculography, mainly because of the small amount of reflection at the interface between the tissue and the ventricular fluid’. The subsequent demonstration (Ballantine et al 1954) that an empty skull gave rise to similar pictures, because of the coincidental transmission properties of the bone, halted work on this approach and arguably held back progress in ultrasonic imaging research for several years.

Pulse-echo ultrasound was shown to have practical value for the detection of flaws in metals during World War II; the publications of Firestone (1946) in the USA and Desch et al (1946) in the UK appeared as soon as the constraints of military secrecy were relaxed. Using this same technique, research into medical applications soon began in Denver, where Howry and Bliss (1952) constructed a water-immersion two-dimensional scanner, and in Minneapolis, where Wild and Reid (1952) started to develop high-frequency real-time two-dimensional imaging. From this early work, researchers world-wide and in increasing numbers began to explore the potential of the new technique, although perhaps initially more slowly in the USA than elsewhere, because of the set-back with transmission imaging.

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