A transducer is a device that converts a signal in one form of energy to a signal in another form of energy. An annular array transducer is a transducer consisting of several circular elements, arranged concentrically, and is typically used to convert electrical pulses into ultrasound pulses for medical applications.
An annular array transducer consists of an array of piezoelectric elements -- elements in which an applied potential difference, or voltage, produces vibration -- rather than a single element. The elements are created by cutting grooves into the transducer base, or substrate. Lower frequency arrays are typically machined with a diamond cutter or laser beam. Higher frequency arrays require more sophisticated techniques, such as photolithography -- in which a circuit pattern is photographed on a light sensitive background and the background is etched away, chemically -- because of their small dimensions.
Ultrasound refers to sound with a frequency above 20,000 cycles per second, or 20 kilohertz, although ultrasound imaging equipment for medical diagnosis operates in the range between 2 million cycles per second, or 2 megahertz, and 20 megahertz. An annular array transducer is placed directly against the human body and sends pulses of ultrasound -- created by the vibration of the array elements -- into the body tissues. The body tissues reflect the ultrasound pulses back to the transducer, where they are converted back into electrical pulses, which are processed to create an image.
An annular array transducer is focused by applying an electrical pulse to each element in turn, starting with the outermost element and working inward toward the center of the array. The individual pulses combine, constructively, to create a composite pulse, which is focused on a point at a specific distance from the transducer. The distance to the focal point depends on the time delay between the electrical pulses applied to the transducer elements. By delaying the pulses to and from each element, the ultrasound beam can be focused to different depths within the human body.
One of the principal advantages of an annular array transducer over a linear transducer array is that it allows ultrasound pulses to be focused in two dimensions rather than one. Pulses can be focused, electronically, into small, well-defined beams, which provide highly detailed images of tissue structures at various depths within the body. The ultrasound beam produced by annular array transducers is symmetrical, which means that it produces a thinner scan slice than other types of array.
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