How a Two Dimensional Ultrasound Image is Made

Two-Dimensional Ultrasound

©Walter Rasmussen, R.D.C.S.

The ultrasound probe consists of a linear array of tiny crystals which by a complex activation sequence creates a focused ultrasound beam of a few millimeters thickness that is projected into the body.

probe

An array of piezoelectric crystals on the face of the probe emit ultrasound waves in a focused beam.  

Above: Pulsed ultrasound beam  reflects off of tissue strata.    

By sweeping the ultrasound beam through a sector, lines of returning echoes from the ultrasound pulses are placed adjacent to each other at a rapid rate (30-60 Hz), and a two-dimensional moving image is assembled. 

Sector Scan.

Pulses of ultrasound are emitted  as the ultrasound beam is steered through a 90-degree sector in order to to reveal a slice of tissue at rates sufficient to capture fast motion.  Each pulse must be allowed to travel to and from the targeted area, which is then termed a “line” of information.

As the pulse of ultrasound encounters tissue of varying density and elasticity, some of its energy is reflected back to the transducer in succession, thus creating  a record of the strata that is encountered.

The returning ultrasound stimulates the  crystals inside of the transducer, producing electrical impulses of varying strengths dependent upon the intensity of the reflected sound waves.  The ultrasound machine hardware and software then digitize the electrical signals and create corresponding image pixels of varying brightness, corresponding to the intensity of the reflections, producing an image that conveys form and texture (B Mode).

The primary determinants of image resolution are the  frequency of the ultrasound beam, the diameter of the ultrasound beam, the size of the image pixels and numerous processing operations such as smoothing and contrast adjustments that are selected.

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