Errors as a result of metal in the near environment when using an electromagnetic locator with freehand three-dimensional echocardiography.

BACKGROUND

Quantitative ventriculography by freehand 3-dimensional (3D) echocardiography with an acoustic spatial locator has been proven to provide highly accurate reproducible measurements of left ventricular volume, mass, and function. It has been shown to be 2 to 3 times better than conventional 2-dimensional echocardiographic techniques. Although accurate, the acoustic spatial locator uses a spark gap to generate hypersound for locating and is somewhat bulky. The Bird direct current electromagnetic locator (Ascension Technology Corp, Burlington, Vt) is a notable alternative locator for the freehand 3D system because it is small and easily portable. However, conductive metals in the near environment may adversely affect electromagnetic locator accuracy. To determine the feasibility of using the electromagnetic locator in a freehand 3D echocardiographic system in the conventional hospital environment, a series of experiments was carried out assessing the accuracy of such a system under various conditions of exposure to conductive metal.

METHODS

Using tissue equivalent ellipsoid phantoms of known volumes, we compared volume measurement accuracy of the freehand 3D echocardiographic system equipped with the standard Bird or miniBird electromagnetic locator systems with our freehand acoustic spatial locator 3D echocardiographic system in 3 experiments: (experiment 1) no metal within 30 in (76.2 cm) of the phantoms and electromagnetic locator; (experiment 2) phantoms placed on a standard metal hospital stretcher with conductive metal less than 10 in (25.4 cm) from the phantoms and electromagnetic locator and with the echocardiographic machine greater than 30 in (76.2 cm) from the electromagnetic locator; and (experiment 3) phantoms placed on the same stretcher with conductive metal less than 10 in (25.4 cm) from the phantoms and electromagnetic locator and with the echocardiographic machine in its usual position approximately 10 in (25.4 cm) from the electromagnetic locator.

RESULTS

For experiment 1 there was no significant volume error (<1%) by any system; no significant difference among the 3 locator systems (acoustic, Bird, or miniBird). For experiment 2 there was significant volume underestimation error by both electromagnetic locator systems (-10.9%, P <.05). For experiment 3 there was significant and greater volume underestimation error by both electromagnetic locator systems (-14.7%, P <.05) in close proximity to the echocardiographic machine. Interobserver variability was 5.1%.

CONCLUSION

For quantitative ventriculography by a freehand 3D echocardiographic system, electromagnetic locator systems should not be used if conductive metal is in the near environment (<30 in [76.2 cm] from the locator). Accurate quantitative ventriculography may be performed with an electromagnetic locator system if the near environment is free of conductive metals.