2010 ISSUE 5 Newsletter

EFSUMB Newsletter

EFSUMB Newsletter European Federation of Societies for Ultrasound in Medicine and Biology

New Trends in Elasticity Imaging The assessment of tissue elasticity has gained significant interest in medicine due to the availability of this technology in the clinical environment. Elasticity is one of the most important physical para­ meters we experience from the very be­ ginning of our life. We explore each object by touching and squeezing it in order to estimate its stiffness. As we already know, the more force we need in order to gene­ rate a certain amount of deformation, the stiffer we say the material is. In medicine, elasticity or stiffness is an important cha­ racteristic of tissues that has been linked not only to malignancy but also to disease processes related to diffuse disorders.

Efforts to estimate tissue elasticity using ultrasound have been under development for a few decades. Methods have pro­ gressed from simple M–mode data acqui­ sition and simple motion tracking to so­ phisticated 3D/4D systems with quantita­ tive estimates of elastic moduli on an ab­

solute scale. Elasticity imaging methods combine some forms of tissue excitation with methods for detection of tissue res­ ponse. All ultrasound–based elasticity estimation methods have the same prin­ ciple consisting in acquiring a map of tis­ sue anatomy before and after some type of deformation. In conventional ultra­ sound elastography, biologic structures are compressed slightly. Normally, <1% deformation is recommended. Different methods of displacement and strain esti­ mation have been proposed using radio frequency ultrasound data pre– and post– compression, aiming to improve the smoothness of strain field, speed up the calculation, increase the strain image con­ trast and/or achieve robustness against de-correlation during compression. How­ ever, few methods have been implemen­ ted in commercialized ultrasound machi­ nes for real–time elasticity imaging.

Transient Elastography, is considered among the first clinical applications of elastography in medicine. Fibroscan (Echosens, Paris, France) has been suc­ cessfully used for liver fibrosis assess­ ment, being completely non–invasive. It uses an A–mode ultrasound with a very high frame rate to monitor shear wave propagation generated by a vibration source. A 5-MHz ultrasound transducer probe mounted on the axis of a vibrator is used. The vibrator generates a completely painless vibration (with a frequency of 50 Hz and amplitude of 2 mm) which pro­ duces an elastic share wave propagating through the skin and the subcutaneous tissue to the liver [1]. Liver stiffness is compared to the fibrosis stage obtained by liver biopsy. The wave propagation speed of the shear wave is used to calculate tis­ sue modulus [2]. While it can provide a quantitative measure for the elasticity, the device cannot provide a real–time B– mode image for localization and guiding.

Thus, the operator does not know the ex­ act portion of liver being tested.

Instead, Real-Time Elastography (RTE) is an ultrasound imaging method that over­ lays traditional B–mode imaging with a colored graphical representation of tissue elasticity. Using strain ratio assessment, RTE provides additional information about a lesion’s characteristics. RTE has been reported to be useful for the diagno­

EFSUMB

Fig. 1 Real-time elastography of (A) breast calcified fibroadenoma and malignant nodule of thyroid gland (B)

Lynne Rudd, General Secretary 36 Portland Place, London W1B 1LS, United Kingdom Tel.: +44 (0) 20 7099 7140 Fax: +44 (0) 20 7436 7934 Email: efsumb@efsumb.org

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EFSUMB Newsletter

Fig. 2 Real-time elastography of the liver: (A) fatty liver disease, (B) chronic viral C hepatitis and (C) cirrhosis.

sis and differentiation of many tumors, which are usually harder than normal sur­ rounding tissues, for e.g. in assessing breast and thyroid cancer diagnosis (qFig. 1), or in guiding minimally invasive treatment of prostate cancer [3,4]. Re­ cently, transabdominal RTE was proposed as a new method for non-invasive staging of liver fibrosis (qFig. 2) [5].

Furthermore, different solid tumors situ­ ated near the gastrointestinal tract might be also visualized by endoscopic ultra­ sound (EUS) RTE and potentially charac­ terized by this technique. EUS elastogra­ phy was already used in several studies for

the characterization and differentiation of benign and malignant lymph nodes, with variable sensitivity, specificity, and accu­ racy [6]. Furthermore, the value of EUS elastography was tested in focal pancrea­ tic lesions in large multicentric studies with good results (qFig. 3).

Because of the inherent bias induced by selection of images from a dynamic se­ quence of elastography, different authors reported on the utility of using computeraided diagnosis by averaging images from a dynamic sequence and calculating hue histograms, as a better way to describe, semiquantitatively, elastography movies

Fig. 3 Endoscopic ultrasound real-time elastography of pancreatic cancer (A) and mediastinal ­malignant lymph node (B).

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[7]. The new advances incorporated in ne­ wer ultrasound systems allow this analy­ sis in real-time, using the software of the device (Strain Histogram Measurement, Hitachi, Japan) shortening in this way, the diagnosis timing and offering a quantita­ tive assessment of the structures.

Acoustic Radiation Force Impulse (ARFI) is a suitable technology for the evaluation of deep tissues, not accessible by superficial compression elastography. Virtual Touch Imaging software (Siemens, Europe) pro­ vides a qualitative gray scale map of rela­ tive stiffness for a user defined region of interest. Using this method, stiff tissue may be differentiated from soft tissue even if they appear isoechoic in conventi­ onal ultrasound imaging. ARFI imaging technology involves the mechanical exci­ tation of tissue using short-duration acou­ stic pulses (push pulses) in a region of in­ terest chosen by the examiner, producing shear waves that spread away from the re­ gion of interest, perpendicularly to the acoustic push pulse, generating localized, micron-scale displacements in the tissue.

It provides accurate numerical measure­ ments related to tissue stiffness at userdefined anatomical locations [8]. ARFI technology quantifies stiffness without manual compression, the tissue being compressed by acoustic energy. Further­ more, effective tumor localization and in­ tra–procedural monitoring are critical to treatment success during thermal ablati­ on. ARFI imaging showed great potential in determining the size and shape of the ablated area (protein denaturation and

EFSUMB Newsletter water vaporization increase the tissue elastic modulus).

Magnetic Resonance Elastography (MRE), a non–invasive MR–based approach, is very well–suited to obtain patient–speci­ fic biomechanical properties of tumoral tissue. It can directly visualize and quan­ titatively measure propagating mechani­ cal shear waves in biological tissues. An important advantage of MRE is the possi­ bility of measuring displacements accura­ tely in all three directions. The technique spatially maps and measures the shear wave displacement patterns [9]. The wave images are processed to generate local quantitative values of shear modulus of tissues in maps. It can provide relevant pre–operative information on the consis­ tency of the tumor and surrounding healthy tissue. MRE has recently been shown to be useful for non-invasive as­ sessment of liver fibrosis. Studies have de­ monstrated that MRE can be used to dif­ ferentiate normal liver from fibrotic liver with a very high degree of accuracy. In other applications, MRE has been found to have promising results for differentiating benign breast and brain lesions from ma­ lignant tumors.

The Supersonic Shear Imaging (SSI) tech­ nique is based on the radiation force indu­ ced by a conventional ultrasonic probe to generate a planar shear wave deep into tissue. The shear wave propagation throughout the medium is caught in real– time due to an ultrafast ultrasound scan­ ner (up to 5000 frames/s). Using modified sequences and post–processing, this tech­ nique is implemented with curved arrays in order to get a larger field of view of liver tissue. This real–time elasticity mapping using an ultrasonic curved probe offers better signal–to–noise ratio than linear arrays and a larger area in the patient‘s liver [10]. This gives more confidence about the accuracy of the diagnosis of the fibrosis stage. Furthermore, the elasticity parameters obtained with SSI give access to the shear wave group velocity and the phase velocity. As a consequence, the SSI assessment of liver stiffness could poten­ tially give more information on the visco­ elasticity properties of the liver.

In conclusion, elastography has become an efficient and easy-to-perform compo­ nent of the ultrasound examination with a rapidly increasing number of clinical ap­ plications. New techniques, including 3D and 4D elastography, as well as fusion imaging, are currently tested in research laboratories in order to discover the real potential of elasticity imaging.

Dan Ionuţ Gheonea, Adrian Săftoiu Research Center of Gastroenterology and Hepatology University of Medicine and Pharmacy, Craiova, Romania

1 Lupsor M, Badea R, Stefanescu H, et al. Per­ formance of unidimensional transient elas­ tography in staging non-alcoholic steatohe­ patitis. J Gastrointestin Liver Dis. 2010; 19: 53–60 2 Zheng YP, Huang ZM, Zhou YJ, He JF, Cheung J. Simultaneous ultrasound B–mode imaging and elasticity measurement using vibration based on a conventional ultrasound scanner.

Eighth International Conference on the Ult­ rasonic Measurement and Imaging of Tissue Elasticity. Vlissingen, Zeeland, The Nether­ lands, September, 2009 3 Cochlin DL, Ganatra RH, Griffiths DFR. Elas­ tography in the detection of prostatic cancer.

Clinical Radiology 2002; 57: 1014–1020 4 Garra BS, Céspedes EI, Ophir J, et al. Elasto­ graphy of breast lesions: initial clinical re­ sults. Radiology 1997; 202: 79–86 5 Gheonea DI, Saftoiu A, Ciurea T, Gorunescu F, Iordache S, Popescu GL, Belciug S, Gorune­ scu M, Sandulescu L. Real-time sono-elasto­ graphy in the diagnosis of diffuse liver disea­ ses. World Journal of Gastroenterology 2010; 16: 1720–1726 6 Săftoiu A, Vilmann P, Ciurea T, et al. Dynamic analysis of EUS used for the differentiation of benign and malignant lymph nodes. Gas­ trointest Endosc 2007; 66: 291–300.

7 Săftoiu A, Vilmann P, Gorunescu F, et al. Neu­ ral network analysis of dynamic sequences of EUS elastography used for the differential diagnosis of chronic pancreatitis and pan­ creatic cancer. Gastrointest Endosc. 2008; 68: 1086–1094.

8 Fierbinteanu-Braticevici C, Andronescu D, Usvat R, Cretoiu D, Baicus C, Marinoschi G. Acoustic radiation force imaging sonoelasto­ graphy for noninvasive staging of liver fibro­ sis. World J Gastroenterol 2009; 15: 5525– 32.

9 Venkatesh SK, Yin M, Glockner JF, Takahashi N, Araoz PA, Talwalkar JA, Ehman RL. MR elastography of liver tumors: preliminary results. AJR Am J Roentgenol 2008; 190: 1534–1540.

10 Bavu E, Gennisson JL, Osmanski BF, et al.

Non–invasive liver fibrosis staging using su­ personic shear imaging: a clinical study on 150 patients. Eighth International Confe­ rence on the Ultrasonic Measurement and Imaging of Tissue Elasticity. Vlissingen, Zee­ land, The Netherlands, September, 2009

Literature about safety issues The Safety Committee/ECMUS regularly publishes reviews of articles on ultra­ sound safety. The latest update listed below can be found at www.efsumb.org in the Safety Committee’s microsite.

▶▶Review 159: Mulvagh S et al. 2008 American Society of Echocardiography consensus statement on the clinical applications of ultrasound contrast agents in echocardiography. Journal of American Society of Echocardiogra­ phy: 21: 1179–1201 ▶▶Review 160: Wei K et al. 2008 The sa­ fety of Definity and Optison for ultra­ sound image enhancement: A retros­ pective analysis of 78,383 administe­ red contrast doses. Journal of Ameri­ can Society of Echocardiography; 21: 1202–1206 ▶▶Review 162: Gabriel R et al. 2008 Safe­ ty of ultrasound contrast agents in stress echocardiology. Am J Cardiol; 102: 1269–1272 ▶▶Review 172: Piscaglia F et al. Know­ ledge of the bio-effects of ultrasound among physicians performing clinical ultrasonography: Results of a survey conducted by the Italian Society for Ultrasound in Medicine and Biology (SIUMB) ▶▶Review 174: Dolan M et al. 2009 Safety and efficacy of commercially available ultrasound contrast agents for rest and stress echocardiography. J Am Coll Car­ diol; 53: 32–38 ▶▶Review 175: Duck FA 2008 Hazards, risks and safety of diagnostic ultra­ sound. Medical Engineering & Physics; 30: 1338–1348 ▶▶Review 178: 2009 Grether Jk et al. An­ tenatal ultrasound and risk of autism spectrum disorders. J Autism Dev Dis­ ord 2009. DOI 10.1007/s10803-0090859-4. ▶▶Review 181: Schabrun S et al. 2008 How accurate are therapeutic ultra­ sound machines? Hong Kong Physio­ therapy Journal 26: 39–44

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