Ultrasound Elastography

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Information about Ultrasound Elastography

Published on September 30, 2015

Author: Apollo_Hospitals

Source: slideshare.net

1. Ultrasound Elastography

2. Technology Update PRINCIPLES AND TECHNICAL ASPECTS Tissue elasticity is characterized by the amount of tissue displacement or distortion in response to the application of an external load. A region of tissue is subjected to a compression force (called stress) and the degree to which it distorts (known as strain) is assessed.While in principle any imaging technique could be used Ultrasound has the advantage of good resolution in both space and time, is safe, repeatable, so has emerged as the dominant technique while MR also has a role. Basically, tissue elasticity imaging methods based on ultrasound fall into three main groups [1]. (i) Methods where a quasi-static compression is applied to tissue and the resulting components of displacement or of the strain is estimated. These methods include elastography based on a global compression of the medium and Acoustic Radiation Force Impulse (ARFI) that uses localized compression. (ii) Methods based on a monochromatic low frequency, vibration such as sonoelasticity which uses Doppler signals to estimate tissue displacement and vibro acoustography which uses ultrasound stimulated acoustic emission. (iii) Transient elastography which relies on the observation of the propogation of a transient shear wave through tissues. These propogations are followed by pulse echo acquisitions and the measured velocity of propo- gations is directly related to tissue stiffness. (Results are expressed in kilopascals). Qualitative and quantitative assessment methods of tissue elasticity are available. In the qualitative assessment using Real Time Ultrasound Imaging a baseline B mode sonographic reference image is obtained. After this an external mechanical compression force (stress) is exerted to produce a strain. Depending on its specific stiffness properties the given tissue will be displaced and the motion displacement of the tissue is estimated. In practical terms, RF ultrasonic data before and after the applied compression is acquired and speckle tracking technique (e.g.) Cross correlation methods are employed to calculate the resulting strain. The resulting strain image is called elastogram [2]. There are two types of elastograms - gray scale and color. The hard and soft areas appear in the grey scale elastogram as dark and bright respectively (Fig.1). In the color increasing tissue hardness appears in ascending order as red, yellow, green and blue. These colors represent the relative hardness of the tissues in the elastography (red - soft tissue, blue-hard tissue, green and yellow (intermediate). The color coding might vary in different equipments. The more recent and quantitative technology shear wave elastography has emerged usingAcoustic Radiation Force Impulse Imaging (Figs. 2,3) [3,4].An important benefit of generating the displacement acoustically in that it removes the user dependence of strain elastography, because, no manual compression is applied to the tissues. It is easy to perform, reproducible and provides numerical values (wave velocity measurements (m/s) of tissue stiffness at the precise image based Anatomical Location Region of Interest (ROI). Various applications of ultrasound elastography breast Various studies indicate that breast elastography has good sensitivity for differentiation between malignant ULTRASOUND ELASTOGRAPHY Meera Krishnakumar Consultant, Department of Radiology, Apollo Hospitals, Greams Road (off Greams Lane),Chennai 600 006, India. E-mail: mikimeera@yahoo.co.in Ultrasound Elastography is a new imaging technique that allows a noninvasive estimation and imaging of tissue elasticity distribution within biological tissues using conventional, Real Time Ultrasound equipment with modified software. It can be viewed as an electronic palpation of tissues. Introduced by Ophir et al in 1991, it subsequently evolved into a Real Time Imaging tool. Key words : Elastography, Tissue-elasticity, Acoustic Radiation Force Impulse(ARFI ), Qualitative and quantitative assessment, Region of Interest (ROI). Apollo Medicine, Vol. 7, No. 3, September 2010 224

3. Technology Update 225 Apollo Medicine, Vol. 7, No. 3, September 2010 Benign tumours reduce in size on mechanical compression while malignant tumours being stiff stay the same size or appear bit larger. A qualitative assessment of the tumoral pattern and an Elasticity score 1-5 has been developed for breast lesions; the higher score (4 or 5), the stiffer the tissue and higher probability of malignant lesion. Thyroid It is a potentially adjunctive tool in identifying thyroid cancer and useful in diagnosis of metastatic cervical nodes. In the case of multiple nodules it can help to target biopsies. [6]. Prostate Can accurately locate prostate tumour foci, determine extent and take targeted biopsies. Skin It can detect dermatological malignancies and identifies at times tissue indurations and abscesses not seen on B mode imaging. EUS It is an useful add-on to endoscopic ultrasound for differentiation between benign and malignant pancreatic lesions and lymph nodes can be useful in differentiating the benign or malignant GIST located in the wall of the gastro intestinal tract. Vascular imaging Cardiological applications are also under review especially regarding functioning of cardiac muscles. Studies on quantifying the elasticity of carotid plaques which might predict the risk of stroke are also under progress. Evaluation of blood vessel wall and venous thrombi also shows great potential. Liver [1,2] TheARFI imaging (Virtual Touch Tissue Qualification mode provided by Siemens Acuson S2000) is a highly promising method for assessing liver fibrosis. Comparative studies with transient elastography and liver biopsy are underway. The shear wave velocity measurements strongly correlate with the fibrosis stage of the liver and predict cirrhosis. It can be considered as a virtual biopsy of the liver and found useful to: (a) Differentiate focal lesions. (b) Identify malignant lesions in a cirrhotic liver. Fig.1. Colour and grey scale elastography of a malignant lesion. Fig.2. Schematic diagram of ARFI imaging. Fig.3. ARFI imaging of a malignant liver neoplasm. and benign tissue, thus reducing the number of unnecessary biopsies performed. It can be used as next line imaging technique when mammography detects a suspicious lesion [5].

4. Apollo Medicine, Vol. 7, No. 3, September 2010 226 Technology Update (c) Identify metastasis. (d) Diagnose fatty liver. (e) Grade fibrosis and help in early referral for liver transplant. (f) Help in target biopsies. (g) Post liver transplant assessment. (h) Follow up in alcoholic liver and hepatitis under treatment. (i) Follow up of post radiofrequency ablation cases. Pancreas ARFI is helpful in detecting adenocarcinomas and in cystic lesions of pancreas (serous vs mucinous cystadenomas). Others Various studied are underway for numerous other applications of elastography viz.in acute appendicitis, musculoskeletal pathology (shoulder, ankle, plantar fascitis), assessing the severity of lymphedema or evaluating the kidneys and spleen. In conclusion, with the fast paced advancement in technology and research and availability of accurate, practical quantitative and /qualitative or (focal/regional) elastographic assessments, ELASTOGRAPHY promises to make an important contribution to ultrasound practice by improving specificity. It is definitely a new revolutionary method and shows potential as a reliable and non invasive way to assess lesions. REFERENCES 1. Liana Gheorghe, et al. Real Time Sonoelastography – A New Application. Clinical Imaging 2008; 17. 2. Ultrasound Elastography Seminar LUT 2 University of Kuopio, Finland (Joseph Jaros). 3. Roee S. Lazabnik. Tissue Strain Analytics, Virtual Touch Tissue Imaging and Quantification. Siemens Medical Solutions, USA. 4. Farzana Alam, Kumiho Naito, et al. 5. Barbara Dowell. Applications Ultrasound 2008; 16:123- 127. 6. Accuracy of Sonographic Elastography in the different diagnosis of enlarged cervical lymphnodes.

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