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A transducer is a device that converts one form of energy into another.

They have two functions:

During transmission, electrical energy from the ultrasound machine is converted into sound.
During reception, the reflected sound pulse is converted into electricity.

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The PZT crystal is physically moved to create the sector-shaped image. 

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Interesting Note: Phased array always means "adjustable" or "multi-focus."

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The disc-like active elements in an annular phased array are displayed in multiple ring shapes that look similar to a bull's eye target. 

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Inner rings of the array obtain images from shallow depts, and the outer rings obtain images from deeper depths.

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Sound pulses originate from different locations along the face of the probe when different sets of crystals are excited. 

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In older models of linear sequential arrays, the beams were usually focused internally with a curved element or by putting a lens in front of the active element. With these settings, the sonographer could not make any changes. In more modern linear sequential arrays focusing is attained electronically through electronic phase delays.

Electronic transmit focusing is done by a curved pattern of the active element being fired. The curved pattern is achieved by the outer active elements being fired earlier than the inner elements.

Receive focusing is done by electrical delays in the returning signals.

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Think of external focusing as a pair of eyeglasses and internal focusing as Lasik.

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Convex probes have a wider field of view at a lower frequency and are mostly used for abdominal imaging because of the deep depths it can penetrate.

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Vector transducers are good for scanning large structures deep in tissue, such as the kidneys. Also, structures like the bladder, gastrointestinal tract, liver and renal transplants.