The use of three-dimensional ultrasound for thyroid volumetry.

Conventional two-dimensional (2-D) ultrasound is the standard method for the investigation of thyroid morphology. Volume calculations need model assumptions and are observer dependent. The present study performed with a commercially available three-dimensional (3-D) system Freescan added to a conventional ultrasound scanner compares the accuracy of conventional thyroid volumetry to several methods of 3-D volume determination. In vitro measurements were performed on thyroid phantoms with known volumes. The standard deviation of the normalized differences was 8.0% (3-D segmentation) and 10.5% (conventional). For the accuracy of volume determination in human thyroids we performed a postmortem study. The thyroid volume was calculated conventionally by the ellipsoid model and by two 3-D methods (segmentation and the newly developed multiplanar volume approximation). The reference volume was determined after resection by submersion. The standard deviation of the normalized differences was 26.9% for the conventional method, 9.7% for 3-D segmentation and 11.5% for the multiplanar volume approximation, showing significant better results for both 3-D methods and no significant difference between the 3-D methods. The 3-D system, therefore, achieves a better accuracy for thyroid volumetry than the conventional volumetry using planar images. In addition, the 3-D images are stored electronically and can be used for follow-up studies.