Shear Wave Elasticity Imaging of Cervical Lymph Nodes

Abstract

A pilot study of real-time shear wave ultrasound elastography (SWE) for cervical lymphadenopathy in routine clinical practice was conducted on 55 nodes undergoing conventional ultrasound (US) with US-guided needle aspiration for cytology. Elastic moduli of stiffest regions in nodes were measured on colour-coded elastograms, which were correlated with cytology. Malignant nodes (n = 31, 56.4%) were stiffer (median 25.0 kPa, range 6.9–278.9 kPa) than benign nodes (median 21.4 kPa, range 8.9–30.2 kPa) (p = 0.008, Mann Whitney U test). A cut-off of 30.2 kPa attained highest accuracy of 61.8%, corresponding to 41.9% sensitivity, 100% specificity and 0.77 area under the receiver operating characteristic curve. Qualitatively, elastograms of benign nodes were homogeneously soft; malignant nodes were homogeneously soft or markedly heterogeneous with some including regions lacking elasticity signal. SWE is feasible for neck nodes. It appears unsuitable for cancer screening but may detect a subset of malignant nodes. The cause of spatial heterogeneity of malignant nodes on SWE is yet to be established.

Introduction

Ultrasound shear wave elastography (SWE) is a comparatively new development in elasticity imaging and has only recently been available on commercial clinical ultrasound (US) systems. Such systems generate acoustic radiation force impulses using specialized transducers, measure the speed of induced shear waves using ultrafast US tracking techniques, and from this data estimate tissue elastic modulus (stiffness) (Bercoff et al. 2004). At present, in systems using real-time SWE, this information can be displayed continually using a colour-coded overlay onto gray-scale images, while absolute measurements can be obtained within static elasticity image frames. As SWE permits absolute quantification of tissue stiffness, it represents a significant advance over conventional US (strain) elastography that at best produces semiquantitative estimates corresponding to relative tissue strain. Furthermore, by utilizing acoustic radiation to stimulate tissues, modern systems using SWE circumvent the requirement for manual tissue compression and, hence, may be less operator-dependent than conventional elastography. A handful of preliminary studies evaluating conventional strain elastography have documented moderate to high accuracies for differentiating benign and malignant cervical lymph nodes (73%–95%)(Alam et al., 2008, Aoyagi et al., 2009, Bhatia et al., 2010, Lyshchik et al., 2007, Saftoiu et al., 2006). The aim of this pilot study was to document our preliminary experience of performing SWE in the neck for cervical lymph nodes in routine clinical practice and, in particular, investigate its utility in identifying malignancy. It was not the purpose of this study to document the accuracy of conventional sonographic criteria for malignancy as this has been well documented in the medical literature.