Figure 2. Ultrasound scan of a large consolidation of ventral lung in a Thoroughbred racehorse. The branching airway is filled with homogeneous fluid (see arrow).

caused by discrete abscesses (Figure 3), tumors and edema due to cardiac failure. Trauma leading to lung contusion is not commonly diagnosed in the adult, but can be found in the neonate after dystocia, especially secondary to rib fractures.

Horses with a history of exercise-induced pulmonary hemorrhage will usually have roughened areas in the caudodorsal lung surfaces bilaterally. Recent hemorrhage will produce consolidations resembling those in Figure 1. With time they usually decrease in size and remain as irregu- larities on the lung surfaces. Horses that continue to bleed continue to have consolidations of varying size in these areas.

References

1. Rantanen, N.W.: Ultrasound appearance of normal lung

Figure 3. Ultrasound scan of a surface abscess in a Stan- dardbred racehorse (see arrow). The inner margins are rounded and composite fluid is present.

borders and surrounding viscera in the horse. Vet Radio/., 22:217,1981.

2. Rantanen, N.W.: Disease ofthethorax. VetClin. NorthAm., 2:49, 1986.

3. Powis, R.L. and Powis, W. J.: A thinker's guide to ultrasonic imaging. Baltimore, Urban and Schwarzenberg, 1984.

THE SAGA OF THE LINGERING TRANSDUCER

Raymond L. Powis, Ph.D.

Question: My ultrasound machine uses both mechanical and phased array transducers. What's the real difference

A. Curved Transducer. B. Curved Lens. C. Phased Array

Volume 13, Number 6, 1993 323

between these two? Why are mechanical scanheads still around?

Answer: To start with, mechanical transducers are still around for three rea- sons: they are relatively safe to use, low cost to produce, and they work. The quality of your machine points out two additional facts: no single transducer can handle all the application needs of the current clinical environment, and trans- ducer design conforms to application needs.

Transducers are changers of energy: electrical to mechanical and vice versa. But they do more than that, they form both transmitting and receiving beams, and therefore set system performance. If you are looking for the heart of your system, it is the transducer.

All transducer focusing depends upon the predictable behavior of waves. Focusing requires making a large part of the energy in a moving wavefront pass through a single region in space. This requires curving a starting wavefront so that the individual waves converge to- ward a focal spot. The same rules apply to receiving, only now the transducer fa- vors listening to a single region (called directivity). The multiplication of trans-

mit focus and receive directivity produce the final beam characteristics.

The single, wafer-type transducer that is part of a mechanical scanhead focuses by curving the wavefront com- ing off the transducer face. This wave- shaping comes from either a lens on the body-side of the transducer, or by curv- ing the transducer itself. The transducer size and frequency determines which tech- nique happens to be the best for a particu- lar design.

The phased array also produces a curved wavefront to focus the beam. The array comprises a linear arrangement of small, individually unfocused transducer elements, each only a fraction of a wave- length wide. In this case, the system electronically makes the transducer act as if they were part of a much larger curved transducer. The system does this by controlling the timing (phase) among the array elements (i.e., phased array). This electronic control not only permits many different transmit and receive focal points within the same image, but also beam steering.

To make a real-time image, every ultrasound system must move or scan the ultrasound beam in space. A single ele-

ment transducer requires not only me- chanical focusing, but also mechanical movement to scan the beam. The phased array uses the same time-delay electron- ics that focuses the beam to steer the beam.

And that brings us back to why me- chanical scanners are still valuable. As it turns out, despite the f'Lxed, mechanical focusing and the belts, levers and gears required to move the beam, mechanical systems are still a low cost way to see smaller signals deeper than phased arrays with a beam organization that stays the same regardless of position. In contrast, phased arrays are electronic, silent, small and light (their main attractions), but very sensitive to physical design, and costly to produce. They have direction-dependent beam formation, providing better resolu- tion over portions of the image and poorer resolution over the rest.

Phased array designs continue to improve, and they may eventually re- place all mechanical systems. Until then, these very different transducer designs will let you tradeoff performance and price to get the imaging you need.

(Reprinted with permission from Ultrasound Review.)

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324 JOURNAL OF EQUINE VETERINARY SCIENCE