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Introduction to Acoustics

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868 Part F Biological and Medical <strong>Acoustics</strong><br />

Part F 21.4<br />

Organ<br />

Patient<br />

Ultrasound<br />

transducer<br />

Posterior Anterior<br />

Right Left<br />

every 20 µs, advancing the focus 15 mm each time along<br />

with the corresponding aperture. This focal adjustment<br />

is called dynamic focusing and is similar in concept <strong>to</strong><br />

time-gain compensation. It is also possible <strong>to</strong> acquire<br />

data along the ultrasound line using several pulse-echo<br />

cycles, each with a transmit burst that is focused at a dif-<br />

a)<br />

b)<br />

c)<br />

Fig. 21.41a–f Scan head details. This curved linear-array scan head consists of an array of 128 transducer elements cut<br />

from a single block of PZT piezoelectric ceramic. The array block is 64 mm long and 5 mm wide, cut in<strong>to</strong> elements 0.4mm<br />

long and 5 mm wide with a 0.1 mm kerf between. The block is curved <strong>to</strong> form a curved linear array. (a) Transducer in<br />

housing with cable; (b) Transducer housing removed showing the copper foil shielding over the internal electronics;<br />

(c) Shielding removed <strong>to</strong> show the 64 electronic matching circuits on this side of the circuit board; (d) 128 individual<br />

coaxial cables housed in the ultrasound scan head cable; (e) Scan head plug showing the connec<strong>to</strong>r electrodes for the 128<br />

coaxial cables; (f) Front panel of the ultrasound scanner showing the sockets for the scan head connections. The front<br />

panel also shows connec<strong>to</strong>rs for other instruments including: electrocardiograph, plethysmograph, phonocardiograph<br />

microphone and computer disk drive for data exchange<br />

d)<br />

f)<br />

Fig. 21.40 Apodized aperture. Apodization is used <strong>to</strong> limit<br />

the side-lobes of the ultrasound beam pattern. Transducers<br />

near the edge of the aperture are excited with a low voltage<br />

during transmit compared <strong>to</strong> the central transducers and<br />

are amplified with a lower gain compared <strong>to</strong> the central<br />

transducers during receive. The drawing on the right shows<br />

the image line corresponding <strong>to</strong> the beam pattern with the<br />

superficial and deep echoes plus the mark on the right,<br />

indicating the focal depth<br />

ferent depth. Although dynamic receive focusing takes<br />

no more time than static focusing, using double transmit<br />

focusing <strong>to</strong> further improve the lateral resolution of<br />

the image at a greater range of depths does reduce the<br />

frame rate by half. The location of the transmit focus is<br />

indicated by a carrot on the right of the image.<br />

e)

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