Ultrasound Elastography of the Thyroid

It has long been recognized that palpably hard thyroid nodules are suspicious for cancer. Conventional ultrasound provides information regarding characteristics shown to be correlated with risk of cancer, such as shape, echogenicity, edge definition, calcification, and vascular flow. However, it does not provide direct information corresponding to the hardness of a nodule. Elastography is a newly developed technique that utilizes ultrasound to analyze the stiffness of a nodule by measuring the amount of distortion that occurs when the nodule is subjected to external pressure. The technique was first described ten years ago, but has only recently been tested on thyroid nodules. Preliminary results have shown an excellent correlation between the hardness of a nodule determined by elastography and subsequent pathology determined by biopsy or excision.

Two techniques have been employed to provide external pressure and strain to a nodule. The most common technique is to apply external pressure using the transducer. After placing a linear transducer over the region of interest, the ultrasonographer manually applies light pressure with the transducer. Multiple sites within and around the nodule are analyzed, and the ultrasound software compares the deformation of the nodule to the surrounding tissue (strain index). The relative stiffness is shown on a color display, superimposed on a B-mode image. An alternative technique uses pulsation from the carotid artery as the compression source. This may be useful, particularly when the nodule is in the lateral aspect of the gland, near the carotid. One preliminary study has indicated that carotid pulsation can be used as the pressure source for elastography. On the other hand, artifacts introduced by carotid pulsation have been reported to adversely affect image quality on real-time elastograms.