|Year : 2019 | Volume
| Issue : 2 | Page : 91-93
Elastography in mediastinal ultrasound, where do we stand?
Malay Sharma1, Piyush Somani2
1 Department of Gastroenterology, Jaswant Rai Speciality Hospital, Meerut, Uttar Pradesh, India
2 Department of Gastroenterology, Thumbay Hospital, Dubai, UAE
|Date of Web Publication||28-Feb-2019|
Dr. Malay Sharma
Department of Gastroenterology, Jaswant Rai Speciality Hospital, Meerut, Uttar Pradesh
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Sharma M, Somani P. Elastography in mediastinal ultrasound, where do we stand?. Lung India 2019;36:91-3
The advent of computer-aided tomography and magnetic resonance imaging has led to an increased detection of enlarged mediastinal lymph nodes (LNs) at different stations. The evaluation of mediastinal LN at these stations is possible by endoscopic ultrasound (EUS) and endobronchial ultrasound (EBUS). EBUS and EUS are also useful for sampling by fine-needle aspiration (FNA) if evaluation suggests LNs of pathological origin. Sampling of LNs from stations 2L, 4, 5, 6, 7 of International Association for the Study of Lung Cancer is possible both by EBUS and EUS. Sampling of LNs from stations 8 and 9 is possible by EUS alone, and sampling of LNs from stations 2R, 4R, 10, 11, and 12 is possible by EBUS alone [Figure 1]. Currently, pulmonologists prefer the route of bronchus for obtaining tissue by EBUS-FNA, and gastroenterologists prefer the route of esophagus by EUS-FNA. EUS-FNA and EBUS-FNA are nowadays considered complementary for a comprehensive evaluation of mediastinal lymphadenopathy.
|Figure 1: The lymph node stations can be evaluated by endobronchial ultrasound (shown in green), endoscopic ultrasound (shown in blue), and both by endoscopic ultrasound and endobronchial ultrasound (shown in pink)|
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The selection of a pathological LN for sampling is difficult as mediastinum is filled with a significant number of normal LNs, which may be as large as 1–2 cm at stations 7, 4R, and 4L. Unnecessary sampling may be avoided by proper characterization of LNs with gray-scale (B-mode) and color Doppler imaging. The gray-scale imaging can describe the site, size, shape, echotexture, and border of the nodes. The color mode imaging provides information about the vascularity of the node. The features of malignant LNs are hypoechoic echotexture, distinct margin, roundness, and a diameter ≥10 mm. Malignancy can be predicted with 100% accuracy when all the four features are present. Absence of hyperechoic LN hilum, lack of a central nodal vessel, occurrence of hyperechoic coagulation necrosis, and heterogeneous echo pattern are other features of malignant LN. However, some features predictive of malignancy, in particular coagulation necrosis and heterogeneous echotexture, are also observed in tubercular LNs.
The two most common benign etiologies for mediastinal lymphadenopathy are tuberculosis (TB) and sarcoidosis. Both are granulomatous disorders with lots of diagnostic dilemmas. The characteristic features of tuberculous LNs are fusion of LN (conglomeration), presence of hypoechoic areas and hyperechoeic foci due to necrotic debris, indistinct margins, heterogeneous echotexture, calcifications, caseating granuloma, and abscess formation. The characteristic features of sarcoidosis LNs are distinct margins, noncaseating granuloma, granular (sandpaper) appearance, and homogeneous echotexture. The EBUS/EUS-guided FNA helps in making the final diagnosis in such cases, but EBUS/EUS-FNA is not 100% percent accurate.
Additional improvement in routine imaging techniques of B-mode and color mode imaging of LN is possible by contrast-enhanced EUS and EUS/EBUS elastography. In the latest issue of Lung India, Saurabh Mittal et al. presented their early experience of EBUS elastography in mediastinal lymphadenopathy in two cases. Multiple studies have been published regarding the role of EUS elastography for mediastinal lymphadenopathy since 2006. Till now, around 16 studies of EBUS elastography have been published from all over the world since the year 2013, and this is the second study published from India.
Elastography is the imaging equivalent of the ancient palpation and works as a real-time method for the evaluation of tissue stiffness. The principle of elastography is that softer parts of tissues deform easier under compression than harder parts do, allowing an objective determination of tissue consistency and showing differences in hardness between normal and diseased tissues. There are two types of elastography: a qualitative one based on tissue's response to an external or internal generated force, called strain elastography, and a quantitative one, based on measurements of the shear waves generated by a “push-pulse” of low frequency, called shear wave elastography. In qualitative elastography, elasticity (on a scale of 1–255) is depicted using a color map (red, green, and blue), wherein hard tissue is shown in dark blue, tissue with intermediate hardness in green, medium soft tissue in yellow, and soft tissue in red. At present, there are two methods for qualitative elastography: the strain ratio and the strain histogram. Strain ratio is based on standard qualitative EUS elastography data. Two different areas (A and B) are selected. Area A which is selected includes as much of the target lesion as possible without including the surrounding tissues. Area B is selected within a soft (red) reference area outside the target lesion. The strain ratio is calculated as the quotient of B/A [Figure 2] and [Figure 3].
|Figure 2: A malignant-looking lymph node is seen on endobronchial ultrasound. Qualitative elastography shows a strain ratio of 273.7 between the hardest and softest tissues. Higher strain ratio suggests higher possibility of malignancy|
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|Figure 3: A comparison of elastography in malignant- and benign-looking lymph nodes in mediastinum. (a) A rounded lymph node. (b) Qualitative elastography shows a high strain ratio of 77. (c) Conglomerated lymph nodes. (d) The conglomerated node shows heterogeneous appearance with blue areas mixed with green areas, and even the strain ratio is high. The heterogeneity suggests a nonmalignant lesion, and fine-needle aspiration cytology reveals a tubercular lymph node|
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For strain histogram, the largest measurement box that touches the inside boundary of lesion within the region of interest (ROI) is selected. The X-axis in the strain histogram represents the elasticity from 0 (hardest) to 255 (softest) of the tissue. This method analyzes the range and distribution of strains within a large ROI. It shows several key parameters such as mean strain, standard deviation, percent of area (blue), and complexity (relation of circumference to the area of blue patches) [Figure 4].,
|Figure 4: The large-sized transducer of endoscopic ultrasound scope may give better qualitative assessment during elastography. (a) Endobronchial ultrasound image and (b) (endoscopic ultrasound image) show a comparison of strain histogram with endobronchial ultrasound and endoscopic ultrasound scope|
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EUS elastography is useful in differentiating benign from malignant LNs, with a sensitivity of 88% and a specificity of 85% according to the meta-analysis of Xu et al. Benign (physiological and reactive) LNs are characterized by a homogeneous or scattered soft pattern (predominantly green or mixed red-yellow-green) [Figure 3]. Malignant LNs often display a homogeneous hard pattern on elastography [Figure 3]. Elastography may be useful for the identification of the harder, most suspicious LN, or of the hardest regions within LNs to be targeted for EUS/EBUS-FNA. Thus, EUS elastography may be helpful by reducing the number of false-negative results and repeated endoscopies. However, criticisms of EUS elastography include variability of the elastographic images, the difficulty of interpretation, and the endosonographers' inability to control tissue compression. Although potentially capable in defining the characteristics of benign and malignant lesions, EUS/EBUS elastography needs improvement of the specificity and clear definitions of the criteria required for an accurate elastographic evaluation. Elastographic strain ratio has been used for a long time for grading, but currently, the strain histogram is used with the latest version of updated software in ultrasound scanners in latest EBUS and EUS transducers.
In summary, elastography complements conventional EUS/EBUS mediastinal LN imaging with minimal prolongation of the examination time, minimum cost, and no added complication or death. Although the differential diagnosis of malignant and benign LNs cannot be solved for all cases with elastography, it seems to be an excellent method for targeting different areas of the LN to avoid unnecessary needle passes during the sampling of node.,
Both EBUS-FNA and EUS-FNA are used for sampling mediastinal nodes. Studies have shown that the yield of EUS-FNA is better than that of EBUS-FNA. The technique of elastography is better standardized in EUS in view of multiple studies which were started many years before the use of elastography in EBUS. However, the use of elastography in mediastinal LN is still not the regular feature of EUS evaluation. In this situation, the question arises whether there is a need of EBUS elastography when it can be assessed by EUS. The answer to this question will be answered in future studies. The main reasons for limited advantage of elastography in EBUS are: (a) recent introduction of elastography in EBUS, (b) possibility of better transducer in EUS, (c) wider field of vision with EUS scope, and (d) lack of proper comparative studies with EUS. At present, the role of EBUS elastography, especially for mediastinal nodes, appears to be limited.
We would like to thank Mr. Pran Prakash Sharma for making the line arts and technical help.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4]