Next: LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)

LINEAR ARRAY (ULTRASOUND) SCANNER

Typical Number in Hospital: 3

Cost Bands: 5,6

References: 1

The most common type of moving picture ultrasonic scanner uses a linear array transducer. This is a large transducer which has a contact area with the skin of approximately 1 cm by 10 cm. Inside the transducer the piezoelectric element is divided into many parallel elements along the 10 cm length and these are connected by electronic switching so that only a small area (e.g. 1 cm by 1 cm) of the transducer is active at any time. With the active area at one end of the transducer an ultrasonic pulse is transmitted, and echoes are detected from the tissue immediately beneath this area. On the B-scan display this produces a line of bright dots. Then the active area commutates along the transducer repeating the process so that many parallel lines of scan are produced on the screen. The process is repeated so that the complete scan is refreshed on to the screen at greater than the flicker frequency of the eye, thus producing a moving picture.

Moving picture scans have a number of advantages particularly where there is rapid movement of the structures in the scanning plane (e.g. heart). The scan is a 'simple' scan in that each point in the tissue is interrogated from one direction only, so theoretically some information may be lost. However, the diagnostic information is usually greater than with the static B-scanners except where a large field of view is required or where the skin surface cannot be flattened to accommodate the large contact area.

Most modern scanners include a frame freeze module in which the level of brightness (echo amplitude) at every point on the picture is stored in a computer memory so that the motion of the picture can be arrested for examination or recording. A larger field of view can be generated by freezing adjacent areas of the image on some scanners.

The main ultrasonic generation and processing sections of the scanner are the same as for the basic A-scanner. The only necessary extra is the control circuitry to activate the selected areas of the multi-element transducer and relate this to the correct lines on the picture.