Acoustic Radiation Force Impulse-Imaging for the evaluation of the thyroid gland: a limited patient feasibility study.

PURPOSE Real-time tissue elastography, a qualitative elastography method, has shown promising results in the diagnostic work up of thyroid nodules. However, to our knowledge no study has evaluated a quantitative elastography method in the thyroid gland. The present study is a feasibility study evaluating Acoustic Radiation Force Impulse-Imaging, a novel quantitative elastography method in the thyroid gland. METHODS ARFI-imaging involves the mechanical excitation of tissue using short-duration acoustic pulses to generate localized displacements in tissue. The displacements induce a lateral shear-wave propagation which is tracked using multiple laterally positioned ultrasound "tracking" beams. Inclusion criteria were: thyroid nodules ≥1cm, non-functioning or hypo-functioning on radionuclide scanning, and cytological/histological assessment of thyroid nodule as reference method. All patients received conventional ultrasound, and examination of the thyroid gland including Power Doppler Ultrasound using a 9MHz linear transducer, in addition real-time elastography (RTE) was performed at 9MHz frequency and ARFI-imaging was performed at 4MHz using Siemens (ACUSON S2000) B-mode-ARFI combination transducer. RESULTS Sixty nodules in 55 patients were analyzed. Three nodules were papillary carcinoma. The stiffer the tissue the faster the shear wave propagates. The results obtained indicated that the shear wave velocity in thyroid lobes ranged between 0.5 and 4.9m/s. The median velocity of ARFI-imaging in the healthy nodule-free thyroid gland, as well as in benign and malignant thyroid nodules was 1.98m/s (range: 1.20-3.63m/s), 2.02m/s (range: 0.92-3.97m/s), and 4.30m/s (range: 2.40-4.50m/s), respectively. While no significant difference in median velocity was found between healthy thyroid tissue and benign thyroid nodules, a significant difference was found between malignant thyroid nodules on the one hand and healthy thyroid tissue (p=0.018) or benign thyroid nodules (p=0.014) on the other hand. Specificity of ARFI-imaging for the differentiation of benign and malignant thyroid nodules was comparable with RTE (91-95%). CONCLUSIONS ARFI can be performed in the thyroid tissue with reliable results.