1.6.1 Basics
Two-dimensional (2D) imaging still is the hallmark and the most crucial part of diagnostic ultrasound. Echocardiography enables the investigator to display and track structures throughout the cardiac cycle. The resolution is sometimes less than one millimeter. This is the result of tremendous technological advancements in transducer technology, beam formation and post-processing.
2D imaging is based on the concept that the distance between a border (reflector) and the transducer can be determined from the time it takes for a reflected ultrasound wave to return to the transducer. However, to do this we need to know the velocity at which the ultrasound travels in the specific medium. Velocity in human tissue is in the range of 1540 m/s and is very similar to the velocity at which ultrasound waves travel in blood (1570 m/s). Ultrasound diagnosis works because the speed of the ultrasound wave is very similar in all tissues of the body. If the velocity were not constant we would be unable to calculate distances.
The velocity of ultrasound is 1540 m/s in tissue and 1570 m/s in blood
To generate a cross-sectional image it is necessary to use multiple ultrasound rays. Therefore, transducers contain several piezoelectric elements which are able to send and receive ultrasound signals across a sector.
Transducer from the inside showing the embedded piezoelectric elements and the connecting wiring to each of the individual elements.
2D ultrasound is a "cut plane" technique. The orientation of the imaging plane determines which segment of the object is seen. This allows us to look inside the body and organs but has the disadvantage that we need to perform a mental reconstruction of multiple views in order to truly appreciate the three-dimensional extent of a structure.
Newer transducer technologies try to circumvent this limitation by providing matrix transducers which emit a pyramidal scan volume and thus allow three-dimensional reconstruction of structures (see Chapter 22, Other Technologies).
Generating a 2D image: Each ultrasound line is reflected at the boundary between water and the fish. By using multiple ultrasound lines one is able to obtain a 2D image.