PHASED ARRAY (ULTRASONIC) SCANNER

Typical Number in Hospital: 1

Cost Bands: 6

References: 1

Moving picture ultrasonic B-scans can be produced by a variety of methods including rotating or rocking transducers, electronic switching of transducer elements (linear array scanners), or by electronic steering of the ultrasound beam to produce a sector scan.

Phased array scanners use a transducer with a small contact area with the skin (e.g. 1 cm by 3 cm) which can project and receive the ultrasound in a number of directions to produce a triangular image diverging from the point of contact. Inside the transducer the piezoelectric element (crystal) is divided into several parallel strips (typically 16) which are connected to the transmit pulse generator and to the receiving amplifier via a set of delay lines. If all the elements are subject to the same time delays on transmit and receive then the ultrasound beam is projected straight ahead and echoes are received from the same direction. If the left-hand end of the transducer is subject to less delay then the beam will be deflected towards the right. By progressively changing the time delay the beam may be switched or steered from left to right and back again. The electronic control over the delay lines can be improved to effect focusing of the beam on transmit, and dynamic focusing (focal length changes with time) on receive.

Such scanners are useful where the ultrasound beam must be transmitted through a small 'window' to the tissue being studied. This is particularly useful in cardiology where the heart may be viewed through a space between the ribs, or with neonates, in which case the probe is placed on top of the head before the bones have closed over. This type of scanner is relatively expensive because of its complexity.