Page 25 - Libro vascular I

Chap-02.qxd 29~8~04 13:20 Page 16
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PERIPHERAL VASCULAR ULTRASOUND
Field of view
Wavelets
Wavefront
A
B
A
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1–5 first beam
2–6 second beam
3–7 third beam
B
Figure 2.14 A: A group of elements within an array can be excited simultaneously, and the resulting wavelets will interfere to produce a wavefront perpendicular to the transducer face. B: The group of elements excited within an array can be varied to produce beams following parallel adjacent paths.
same at depth as it is close to the transducer (Fig. 12.13A).
A sector image can be produced by arranging the elements in a curvilinear array (Fig. 2.13B). As the beam paths diverge, the image fans out and therefore the scan lines run more closely in the portion of the image near to the transducer and become more spread out at depth. This leads to some loss of image quality at depth but allows a larger field of view compared with that produced by a linear array transducer. Curvilinear arrays are mainly used for abdominal imaging.
Using several elements to form the ultrasound beam enables the beam shape to be manipulated. If the elements used to form the beam are excited at slightly different times, the wavefronts produced by the elements will interfere differently than they would if they were all excited at the same time. For example, if the element on the far right in the array (Fig. 2.15A) is excited first, with the next element excited after a very short delay, and so forth, the
C
A: Linear array transducer. This is typically made up of 128 elements in a row and produces a
rectangular field of view. B: Curvilinear array transducer. This produces a sector image, with a field of view that diverges with depth. C: Phased array transducer. This uses a smaller array of elements and electronically steers the beam to produce a sector image.
Figure 2.13
produce ultrasound beams that follow parallel adjacent paths (Fig. 2.14B)—e.g., elements 1–5 produce the first beam, 2–6 the second, 3–7 the third and so on. A linear array transducer produces a rectangular image in which the field of view is the