imaging (MRI) and may be difficult to localize on grayscale ultrasound being isoechoic to surrounding hepatic parenchyma. Realtime monitoring of ablation with ultrasonography usually shows an irregular hyperechoic enlarging cloud of gas and bubbles around the electrode tip, which can make needle repositioning challenging because the hyperechoic gas cloud obscures the lesion.26
Previous demonstrations of mechanical contrast within abdominal tumors and thermal lesions in RFA have shown that elastographic imaging could help both in guiding and monitoring ablation procedures.28,29
In one study,
the feasibility of ARFI imaging in assisting probe placement in ‘difficult to see lesions’ was demonstrated and, thus, Virtual Touch tissue imaging can increase operator confidence in probe placement.30
ARFI imaging could be used to assess RFA as it is performed, based on the principle that coagulated tissue is stiffer than normal healthy liver tissue. For successful ablation to occur, coagulative necrosis must be induced throughout the entire tumor and should include a 0.5–1 cm rim of surrounding normal hepatic parenchyma.31,32
imaging protocols allow for monitoring of the radial growth of the ablation zone in real time, before significant gas cloud formation.31 Although there has been no in vivo testing of this as yet, ex vivo testing has suggested clinical feasibility.31
The high temperatures encountered during ablation cause protein denaturation, which
1. Nightingale K, Palmeri M, Trahey G, Analysis of contrast in images generated with transient acoustic radiation force, Ultrasound Med Biol, 2006;3(1):61–72.
2. Nightingale K, Stutz D, Bentley R, Trahey G, Acoustic radiation force impulse imaging: ex vivo and in vivo demonstration of transient shear wave propagation. Presented at: Proceedings of the 2002 IEEE International Symposium on Biomedical Imaging: Macro to Nano, Washington, DC, US, 7–10, July, 2002:556–9.
3. Nightingale KR, Palmeri ML, Acoustic radiation force impulse (ARFI) imaging: fundamental concepts and image formation, In: Fatim M, Al-Jumaily A (eds), Biomedical Applications of Vibration and Acoustics in Imaging and Characterizations, New York, NY: ASME Digital Library.
4. Nightingale K, Soo MS, Nightingale R, Trahey G, Acoustic radiation force impulse imaging: in vivo demonstration of clinical feasibility, Ultrasound Med Biol, 2002;28(2):227–35.
5. Melodelima D, Bamber JC, Duck FA, Shipley JA, Transient elastography using impulsive ultrasound radiation force: a preliminary comparison with surface palpation elastography, Ultrasound Med Biol, 2007;33(6):959–69.
6. Lazebnik RS, Tissue strain analytics: Virtual Touch Tissue Imaging and Quantification. In: Siemens Medical Solutions, Mountview, CA, US.
7. Nightingale K, McAleavey S, Trahey G, Shear-wave generation using acoustic radiation force: in vivo and ex vivo results, Ultrasound Med Biol, 2003;29(12):1715–23.
8. Garra BS, Imaging and estimation of tissue elasticity by ultrasound, Ultrasound Q, 2007;23(4):255–68.
9. Rifai K, Cornberg J, Mederacke I, et al., Clinical feasibility of liver elastography by acoustic radiation force impulse imaging (ARFI), Dig Liver Dis, 2011;43(6):491–7.
10. Cohen EB, Afdhal NH, Ultrasound-based hepatic elastography: origins, limitations, and applications, Clin Gastroenterol, 2010;44(9):637–45.
11. Palmeri ML, Wang MH, Dahl JJ, et al., Quantifying hepatic shear modulus in vivo using acoustic radiation force,
elevates the stiffness of the treated region and in turn produces high-contrast elastograms of the treated region.29
To date, no studies are available to show the utility of ARFI imaging in detecting recurrence at or around the site of previously ablated lesions and further research is needed to assess the potential of ARFI for this purpose.
ARFI or Virtual Touch tissue applications have opened a new era in diagnostic and therapeutic imaging of the liver and its applications extend beyond the liver to other solid organs and tissues. Virtual Touch tissue imaging and Virtual Touch tissue quantification are the first commercially available applications of ARFI technology that allow for conventional ultrasound imaging in addition to ARFI evaluation. ARFI measurements and imaging are quick to perform and can be incorporated into the routine workflow of a busy imaging department. The ability to measure liver stiffness while screening for focal liver lesions and performing a Doppler of the liver concurrently enables increased cost-effectiveness and greater clinical convenience for both patient and healthcare provider. To date, the clinical utility of Virtual Touch quantification is best established in evaluating liver fibrosis. Both Virtual Touch tissue imaging and quantification show promise in providing complementary information in the detection and characterization of focal liver lesions. The potential to assist with radiofrequency ablation of liver lesions shows promise and preliminary results are encouraging. n
Ultrasound Med Biol, 2008;34(4):546–58.
12. D'Onofrio M, Gallotti A, Mucelli RP, Tissue quantification with acoustic radiation force impulse imaging: measurement repeatability and normal values in the healthy liver, AJR Am J Roentgenol, 2010;195(1):132–6.
13. Toshima T, Shirabe K, Takeishi K, et al., New method for assessing liver fibrosis based on acoustic radiation force impulse: a special reference to the difference between right and left liver, J Gastroenterol, 2011;46(5):705–11.
14. Fierbinteanu-Braticevici C, Andronescu D, Usvat R, et al., Acoustic radiation force imaging sonoelastography for non-invasive staging of liver fibrosi, World J Gastroenterol, 2009;15(44):5525–32.
15. Haque M, Robinson C, Owen D, et al., Comparison of acoustic radiation force impulse imaging (ARFI) to liver biopsy histologic scores in the evaluation of chronic liver disease: a pilot study, Ann Hepatol, 2010;9(3):289–93.
16. De Alwis NM, Day CP, Non-alcoholic fatty liver disease: the mist gradually clears, J Hepatol, 2008;48(Suppl. 1):S104–12.
17. Petta S, Muratore C, Craxì A, Non-alcoholic fatty liver disease pathogenesis: the present and the future, Dig Liver Dis, 2009;41(9):615–25.
18. Palmeri ML, Wang MH, Rouze NC, et al., Noninvasive evaluation of hepatic fibrosis using acoustic radiation force-based shear stiffness in patients with nonalcoholic fatty liver disease, J Hepatol, 2011;55:666–72.
19. Yoneda M, Suzuki K, Kato S, et al., Nonalcoholic fatty liver disease: US-based acoustic radiation force impulse elastography, Radiology, 2010;256(2):640–7.
20. Wilson SR, Burns PN, Microbubble-enhanced US in body imaging: what role?, Radiology, 2010;257(1):24–39.
21. Jang HJ, Yu H, Kim TK, Contrast-enhanced ultrasound in the detection and characterization of liver tumors, Cancer Imaging, 2009;9:96–103.
22. Shuang-Ming T, Ping Z, Ying Q, et al., Usefulness of acoustic radiation force impulse imaging in the differential diagnosis of
benign and malignant liver lesions, Acad Radiol, 2011;18:810–5.
23. Davies G, Koenen M, Acoustic radiation force impulse elastography in distinguishing hepatic haemangiomata from metastases: preliminary observations, Br J Radiol, 2011;84:939–43.
24. Cho SH, Lee JY, Han JK, Choi BI, Acoustic radiation force impulse elastography for the evaluation of focal solid hepatic lesions: preliminary findings, Ultrasound Med Biol, 2010;36(2):202–8.
25. Gallotti A, D'Onofrio M, Romanini L, et al., Acoustic Radiation Force Impulse (ARFI) ultrasound imaging of solid focal liver lesions, Eur J Radiol, 2011; [Epub ahead of print].
26. Shankar S, van Sonnenberg E, Silverman SG, Tuncali K, Interventional radiology procedures in the liver. Biopsy, drainage, and ablation, Clin Liver Dis, 2002;6(1):91–118.
27. Meloni MF, Livraghi T, Filice C, et al., Radiofrequency ablation of liver tumors: the role of microbubble ultrasound contrast agents, Ultrasound Q, 2006;22(1):41–7.
28. Yeh WC, Li PC, Jeng YM, et al., Elastic modulus measurements of human liver and correlation with pathology, Ultrasound Med Biol, 2002;28(4):467–74.
29. Bharat S, Techavipoo U, Kiss MZ, et al., Monitoring stiffness changes in lesions after radiofrequency ablation at different temperatures and durations of ablation, Ultrasound Med Biol, 2005;31(3):415–22.
30. Fahey BJ, Hsu SJ, Wolf PD, et al., Liver ablation guidance with acoustic radiation force impulse imaging: challenges and opportunities, Phys Med Biol, 2006;51(15):3785–808.
31. Fahey BJ, Nightingale KR, Stutz DL, Trahey GE, Acoustic radiation force impulse imaging of thermally- and chemically- induced lesions in soft tissues: preliminary ex vivo results, Ultrasound Med Biol, 2004;30(3):321–8.
32. Fahey BJ, Nelson RC, Hsu SJ, et al., In vivo guidance and assessment of liver radio-frequency ablation with acoustic radiation force elastography, Ultrasound Med Biol, 2008;34(10):1590–603.
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