EQUIPMENT DESIGN AND OPERATION

TEE probes are a marvel of engineering: a miniaturized echocardiographic transducer (about 40 mm long, 13 mm wide, and 11 mm thick) mounted on the tip of a gastroscope. Typically, the transducer is a phased-array configuration with 64 piezoelectric elements operating at 3.7 to 7.5 MHz. By means of sequential firing of the elements and an acoustic lens in the transducer housing, the ultrasound waves are formed into a 90-degree beam approximately 1 mm in thickness that emanates at right angles to the transducer. Like standard gastroscopes, two rotary knobs ("wheels") control movement of the tip of the scope. One of the wheels anteflexes and retroflexes the transducer (i.e., moves the transducer toward and away from the heart). The other wheel flexes the transducer rightward and leftward ( Fig. 33-6 ).

Multiplane transducers use the same transducer technology, but mount the transducer on a rotating device that allows it to spin on its axis from 0 to 180 degrees within the tip of the gastroscope (transducer housing) ( Fig. 33-6 ). Because cardiac structures and blood flow are not precisely aligned relative to the transducer, this design has significantly refined imaging capability. By reducing the number of crystals and further miniaturizing transducers, manufacturers have produced transducers small enough for use in infants and neonates ( Fig. 33-7 ).

Ultrasonographs contain high-powered computers capable of initiating the ultrasound beam and processing the returning data. A series of electronic transforms (some guarded commercial secrets) produce the real-time images displayed on the video screen. All ultrasonographs share common technical aspects, including gain, depth, and Doppler controls. However, the differences in technical aspects between manufacturers and even between models from the same manufacturer are sufficiently great that they prevent the formulation of any universal operating instructions. Fortunately, detailed instructions for each model are available in the operator's manual supplied with each ultrasonograph. Alternatively, cardiac sonographers are often excellent sources of instruction in the operation of these machines.