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Thứ Hai, 20 tháng 8, 2012

COMPARISON SWE to STRAIN ELASTOGRAPHY for THYROID NODULES


Abstract

Although elastography can enhance the dierential diagnosis of thyroid nodules, its diagnostic performance is not ideal at present. Further improvements in the technique and creation of robust diagnostic criteria are necessary. The purpose of this study was to compare the usefulness of strain elastography and a new generation of elasticity imaging called supersonic shear wave elastography (SSWE) in dierential evaluation of thyroid nodules. Six thyroid nodules in 4 patients were studied. SSWE yielded 1 true-positive and 5 true-negative results. Strain elastography yielded 5 false-positive results and 1 false-negative result. A novel finding appreciated with SSWE, were punctate foci of increased stiness corresponding to microcalcifications in 4 nodules, some not visible on B-mode ultrasound, as opposed to soft, colloid-inspissated areas visible on B-mode ultrasound in 2 nodules. This preliminary paper indicates that SSWE may outperform strain elastography in dierentiation of thyroid nodules with regard to their stiness. SSWE showed the possibility of dierentiation of high echogenic foci into microcalcifications and inspissated colloid, adding a new dimension to thyroid elastography. Further multicenter large-scale studies of thyroid nodules evaluating dierent elastographic methods are warranted.



Methods

During a few weeks trial time in 2010, four consecutive patients with single thyroid nodule (n = 1) and nodular goiter (n = 3) were evaluated. Approval for this study was obtained from the Ethics Committee of the Medical University of Warsaw, and all patients provided informed consent.

The Bmode and power Doppler ultrasound of whole thyroid and neck lymph nodes was performed. Six dominant thyroid nodules (in regard to B-mode and power Doppler ultrasound features) were evaluated with shear wave and strain elastography qualitatively and quantitatively as well as some with contrast-enhanced ultrasound (Sonovue (Bracco)). The examinations were performed with following scanner: AiXplorer (Supersonic Imagine Inc. France)—SSWE, Aplio XG (Toshiba, Japan)—strain elastography, Technos (Esaote, Italy)—contrast-enhanced ultrasound, with linear high-resolution transducers: 15–4MHz, 18–7MHz, and, 8–3MHz respectively. For strain elastography, we adopted qualitative scale of Rubaltelli et al. with threshold score of 2/3 and quantitative scale of Cantisani et al. with threshold thyroid tissue/nodule strain ratio of 2 measured with Elasto-Q (Toshiba). For shear wave elastography, we adopted quantitative scale of Sebag et al. with the threshold stiness (mean elastic modulus) of thyroid nodule of 65 kPa.

The final diagnosis was based on clinical evaluation, multiple FNB, 1 year followup, or surgery.


Discussion

Supersonic shear weave elastography consists of the generation of remote radiation force by focused ultrasonic beams, the so-called “pushing beams,” a patented technology called “Sonic Touch”. Using Sonic Touch, ultrasound beams are successively focused at dierent depth in tissues. The source is moved at a speed that is higher than the speed of the shear waves that are generated. In this supersonic regime, shear waves are coherently summed in a “Mach cone” shape, which increases their amplitude and improves their propagation distance. For a fixed acoustic power at a given location, Sonic Touch increases shear wave generation eciency by a factor of 4 to 8 compared to a nonsupersonic source. After generation of this shear wave, an ultrafast echographic imaging sequence is performed to acquire successive raw radiofrequency dots at a very high-frame rate (up to 20,000 frames per second). Based on Young’s modulus formula, the assessment of tissue elasticity can be derived from shear wave propagation speed. A color-coded image is displayed, which shows softer tissue in blue and stier tissue in red. Quantitative information is delivered; elasticity is expressed in kilo-Pascal (kPa).

This preliminary paper based on small number of cases indicates that SSWE indicated correctly thyroid nodules suspicious for cancer in contrast to strain elastography. False positives on strain elastography could be due to liquid or degenerative content of nodules.


However, imaging with SSWE, as a sensitive method of evaluation of stiness of human tissue, the operator should be aware of physiological processes influencing the elasticity and easily apply a few rules to avoid artifacts (Figures 4, 5 and 6). Among well-known artifacts on SSWE that should be mentioned is the one that can be encountered in any region when the SSWE can be applied: the increased stiness of the structures under externally applied pressure (Figures 4 and 5) that can be due to nonlinear elastic eects, well explained by theory.



Another artifact that can be encountered in thyroid SSWE is one of increased stiness in the isthmus of the thyroid due to trachea (Figure 6). It can be avoided with imaging in paracoronal plane of the nodule that does not incorporate the trachea. However, it is important to state that these artifacts when properly interpreted do not hinder the accurate diagnosis.



Supersonic shear wave elastography may add a new dimension to ultrasound evaluation of thyroid nodules in several ways, for example:

(a) improve general performance in elasticity dierentiation of thyroid nodules over strain elastography due to its high reproducibility, independence of examiners skill and numeral scale of elasticity measurement in kPa;

(b) overcome the limitations of strain elastography=

  (i) nodules with liquid components or with degenerative changes;

  (ii) small nodules (very good spatial resolution of the technique);

  (iii) large nodules (possibility of subsequent determination of sti regions even  of large nodules, without the need of visualizing the whole nodule on one image);

  (iv) multinodular goiter with no or scarce normal thyroid tissue as a reference;

(c) dierentiation between soft-inspissated colloid and sti microcalcifications;

(d) visualization of microcalcifications, even not visualized on B-mode imaging (may increase sensitivity and decrease specificity of thyroid cancer diagnosis);

(e) introduction of three-dimensional elastographic images to routine clinical practice and to national thyroid cancer databases, as this technique is
already available and enables rapid acquisition of 3D ultrasound and elastographic data. This would devoid diagnostic process and data archiving of image selection bias attributable to 2D ultrasound examination.


Further multicenter large scale studies of thyroid nodules evaluating dierent elastographic methods are warranted, including (a) investigation of developmental models of diseases that link biomechanical properties (elastography findings) with genetic, cellular, biochemical, and gross pathological changes; (b) comparison of accuracy of dierent elastographic methods; (c) establishment of optimal diagnostic elastographic criteria; (d) establishment of limitations of different elastographic methods in relation to evaluation of thyroid pathology.
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