PICANO Eugenio

Eugenio PICANO

CNR, Institute of Clinical Physiology, Italy

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The underlying hypothesis in tissue characterization studies is that a different biochemical structure, internal architectural arrangement or physiologic state of normal versus diseased tissue can affect the physical properties of the tissue and can therefore be detected by ultrasound. Tissue characterization can be performed using two main methods: videodensitometry and radiofrequency analysis. Videodensitometry is technologically simpler and widely applicable: for instance, a normal myocardium appears brighter in end-diastole and darker in end-systole, with a 20％-40％ cyclic variation in gray-level amplitude （1,2）. Radiofrequency analysis is more technologically demanding and theoretically more accurate, since the native ultrasonic signal is sampled, which is not distorted by the postprocessing function of the imaging chain （3,4）. Both approaches have been applied with promising results to identification of fibrotic, ischemic and viable myocardium （1-3）.
According to recent recommendations, “the long history of the ultrasound tissue characterization technique compared with its rare clinical use tells its own story in relation to its difficulty” （5）. This procedure is technically demanding, subject to artifacts related to the presence of other myocardial/atrial reflectors, image settings, and the exact location of the sample volume. In the era of 2D-strain measurement, there is little to recommend the ongoing measurement of cyclic variation as a marker of contractility. Calibrated backscatter still has a value as a marker of fibrosis, but again it is hard to think of it as competitive with DE-CMR. Therefore - the guidelines conclude - this methodology remains more of a research instrument than a clinical tool.
The second, major possible clinical application of tissue characterization is the acoustic identification of vulnerable or high-risk plaques, which are thin-cap fibroatheromas （without calcification） which can be identified in vitro （6-9） and in vivo by radiofrequency invasive intravascular ultrasound imaging or （with less favourable signal-to-noise ratio） with transcutaneous or transesophageal B-mode ultrasound. The potential of acoustically heterogeneous, soft, irregular profile plaques to predict future events has been at present only proven for Radiofrequency Intravascular Ultrasound （10）, but remains a challenging but achievable target - as recently outlined by NHBLBI Working Group （11） - for future research in the field.

References
1.Picano E, Faletra F, Marini C, et al. Increased echodensity of transiently asynergic myocardium in humans: a novel echocardiographic sign of myocardial ischemia. J Am Coll Cardiol 1993; 21:199-207
2.Marini C, Ghelardini G, Picano E, et al. Effects of coronary blood flow on myocardial gray level amplitude in two-dimensional echocardiography: an experimental study. Cardiovasc Res 1993; 27:279-283
3.Picano E, Pelosi G, Marzilli M, et al. In vivo quantitative ultrasonic evaluation of myocardial fibrosis in man. Circulation 1990; 81:58-64
4.Lattanzi F, Di Bello V, Picano E, et al. A normal ultrasonic myocardial reflectivity in atlete with increased left ventricular mass: a tissue characterization study. Circulation 1992; 85:1828-1834
5.Mor-Avi V, et al. Current and evolving echocardiographic techniques for the quantitative evaluation of cardiac mechanics: ASE/EAE consensus statement on methodology and indications endorsed by the Japanese Society of Echocardiography. Eur J Echocardiogr. 2011;12:167-205
6.Picano E, Landini L, Distante A, et al. Fibrosis, lipids, and calcium in human atherosclerotic claque. In vitro differentiation from normal aortic walls by ultrasonic attenuation. Circ Res 1985; 56:556-562
7.Picano E, Landini L, Distante A, et al. Angle dependence of ultrasonic backscatter in arterial tissues: a study in vitro. Circulation 1985; 72:572-576
8.Picano E, Landini L, Lattanzi F, et al. The use of frequency histograms of ultrasonic backscatter amplitudes for the detection of atherosclerosis in vitro. Circulation 1986; 74:1093-1098
9.Picano E, Landini L, Lattanzi F, et al. Time domain echo pattern evaluation from normal and atherosclerotic arterial walls: a study in vitro. Circulation 1988; 77:654-659
10.Stone GW, et al. A prospective natural-history study of coronary atherosclerosis. N Engl J Med 2011; 364: 226-235
11.Fleg JL, et al. Detection of high-risk atherosclerotic plaque: report of the NHLBI Working Group on current status and future directions. JACC Cardiovasc Imaging. 2012;5:941-55