Ultrasound-based imaging methods of the kidney—recent developments
2016; Elsevier BV; Volume: 90; Issue: 6 Linguagem: Inglês
10.1016/j.kint.2016.06.042
ISSN1523-1755
AutoresJean‐Michel Correas, Dany Anglicheau, Dominique Joly, Jean‐Luc Gennisson, Mickaël Tanter, O. Hélénon,
Tópico(s)MRI in cancer diagnosis
ResumoIn recent years, several novel ultrasound (US)–based techniques have emerged for kidney diagnostic imaging, including tissue stiffness assessment with elastography, Ultrasensitive Doppler techniques, and contrast-enhanced ultrasonography to assess renal microvascularization. Renal elastography has become available with the development of noninvasive quantitative techniques, following the rapidly growing field of liver fibrosis diagnosis. With the increased incidence of chronic kidney disease, noninvasive diagnosis of renal fibrosis can be of critical value. However, it is difficult to simply extend the application of US elastography from one organ to the other due to anatomic and technical issues. Today, renal elastography appears to be a promising application that, however, still requires optimization and validation. New ultrasensitive Doppler techniques improve the detection of slow blood flow and can be used alone or after administration of US contrast agents. These microbubble-based agents are extremely well tolerated and can be administered even in cases of impaired renal function. Despite the lack of approval, they improve the characterization of atypical renal masses, complex cystic renal masses, and peripheral vascular disorders. Dynamic contrast-enhanced US is based on quantification of the signal intensity from region of interest and mathematical fits of the time-intensity curves. Perfusion-related parameters can be extracted for the monitoring of vascular changes in the renal parenchyma and in tumors in order to evaluate drug response. This estimation of renal perfusion depends on many parameters that should be kept constant for follow-up studies, and, when possible, an internal reference should be used to normalize the measurements. In recent years, several novel ultrasound (US)–based techniques have emerged for kidney diagnostic imaging, including tissue stiffness assessment with elastography, Ultrasensitive Doppler techniques, and contrast-enhanced ultrasonography to assess renal microvascularization. Renal elastography has become available with the development of noninvasive quantitative techniques, following the rapidly growing field of liver fibrosis diagnosis. With the increased incidence of chronic kidney disease, noninvasive diagnosis of renal fibrosis can be of critical value. However, it is difficult to simply extend the application of US elastography from one organ to the other due to anatomic and technical issues. Today, renal elastography appears to be a promising application that, however, still requires optimization and validation. New ultrasensitive Doppler techniques improve the detection of slow blood flow and can be used alone or after administration of US contrast agents. These microbubble-based agents are extremely well tolerated and can be administered even in cases of impaired renal function. Despite the lack of approval, they improve the characterization of atypical renal masses, complex cystic renal masses, and peripheral vascular disorders. Dynamic contrast-enhanced US is based on quantification of the signal intensity from region of interest and mathematical fits of the time-intensity curves. Perfusion-related parameters can be extracted for the monitoring of vascular changes in the renal parenchyma and in tumors in order to evaluate drug response. This estimation of renal perfusion depends on many parameters that should be kept constant for follow-up studies, and, when possible, an internal reference should be used to normalize the measurements. Conventional ultrasonography (US) has been available to study the kidney in routine practice for more than 40 years. It started with B-mode imaging and rapidly encompassed Color and Power Doppler US (CDUS and PDUS) and pulsed wave Doppler (PWD) to also detect renal blood flow disturbances. During the past 25 years, the performance of renal US has been continuously improved. The B-mode contrast and spatial resolution have been increased by changing the pulse sequence and the transducer capabilities. Nowadays, routine B-mode examinations are routinely performed using nonlinear harmonic imaging and spatial and frequency compounding. CDUS and power Doppler US have also benefited from increased sensitivity for the detection of deep and small vessels at much higher frame rate. However, conventional renal US still exhibits limitations for the evaluation of diffuse tissue disorders as well as for the detection of focal lesions (that will always depend on the accessibility to the ultrasound beam and on the contrast to the surrounding tissues) and for the characterization of renal masses. Recently, new renal ultrasound-based imaging methods have been leaving the research field to become available in routine practice. Of these new technologies, we focus on ultrasound elastography, micro-Doppler techniques, and contrast-enhanced US (CEUS). Noninvasive assessment of tissue stiffness should bring additional information to improve ultrasound diagnostic capabilities.1Sarvazyan A.P. Biophysical bases of elasticity imaging.in: Jones J.P. Acoustical Imaging. Springer US Plenum Press, New York, NY1995: 223-240Crossref Google Scholar Indeed, most parenchymal diseases are associated with tissue architecture changes that are affecting the tissue elasticity without necessarily changing the tissue ultrasound backscatter properties. Interstitial fibrosis is an example of such changes that has been widely studied in the liver for the detection and quantification of fibrosis in adults and children.2European Association for Study of Liver, Asociacion Latinoamericana para el Estudio del HigadoEASL-ALEH Clinical Practice Guidelines: non-invasive tests for evaluation of liver disease severity and prognosis.J Hepatol. 2015; 63: 237-264Abstract Full Text Full Text PDF PubMed Scopus (1214) Google Scholar, 3Barr R.G. Ferraioli G. Palmeri M.L. et al.Elastography assessment of liver fibrosis: Society of Radiologists in Ultrasound Consensus Conference Statement.Radiology. 2015; 276: 845-861Crossref PubMed Scopus (390) Google Scholar, 4Franchi-Abella S. Corno L. Gonzales E. et al.Feasibility and diagnostic accuracy of supersonic shear-wave elastography for the assessment of liver stiffness and liver fibrosis in children: a pilot study of 96 patients.Radiology. 2016; 278: 554-562Crossref PubMed Scopus (93) Google Scholar It seemed logical to extend this validated liver application to the noninvasive assessment of chronic kidney disease (CKD), particularly for the early stages when renal function is not yet significantly affected as well as for disease monitoring. The hypothesis that the development of the glomerular and interstitial fibrosis should lead to stiffness changes was supported by experimental findings in a rat model of CKD.5Derieppe M. Delmas Y. Gennisson J.-L. et al.Detection of intrarenal microstructural changes with supersonic shear wave elastography in rats.Eur Radiol. 2012; 22: 243-250Crossref PubMed Scopus (39) Google Scholar Renal elastography should be validated by comparison with renal pathology, glomerular filtration rate (GFR) and renal tissue stiffness changes in the course of CKD. Several ultrasound elastography technologies have been developed over the past 15 years.6Bamber J. Cosgrove D. Dietrich C.F. et al.EFSUMB guidelines and recommendations on the clinical use of ultrasound elastography. Part 1: basic principles and technology.Ultraschall Med. 2013; 34: 169-184Crossref PubMed Scopus (869) Google Scholar They all rely on the same principle with the following 3 steps: first, generation of an external (or, in rare cases, internal) constraint on the tissue; a second measurement of the very small displacement induced by the application of this constraint using US; and a third estimation of the elasticity modulus inverting the physical relationship between constraint and induced displacement. The constraint can be external (such as compression of the medium by the ultrasound transducer with obvious interoperator variability) or internal using an acoustic radiation force impulse (ARFI). The first technique consists of using external compression-decompression cycles applied by the transducer. It is called quasi-static elastography (or strain elastography). It is a qualitative technique that supposes a uniform deformation of the tissue of interest. The stiffness estimation depends on the stiffness of the tissues located inside and outside the elastographic box. The estimated parameter is the local strain that is related to local stiffness through Hooke's law involving the local stress field usually unknown in clinical conditions. The elasticity is color coded, and the color range is distributed between the softest and hardest tissues (Figure 1a), and thus the position of the elastographic box can induce additional variability. Strain elastography is widely available from many US manufacturers and is currently used mainly for breast, thyroid, and prostate stiffness evaluation.7Barr R.G. Nakashima K. Amy D. et al.WFUMB guidelines and recommendations for clinical use of ultrasound elastography: part 2: breast.Ultrasound Med Biol. 2015; 41: 1148-1160Abstract Full Text Full Text PDF PubMed Scopus (324) Google Scholar, 8Lyshchik A. Higashi T. Asato R. et al.Thyroid gland tumor diagnosis at US elastography.Radiology. 2005; 237: 202-211Crossref PubMed Scopus (539) Google Scholar, 9Brock M. von Bodman C. Palisaar R.J. et al.The impact of real-time elastography guiding a systematic prostate biopsy to improve cancer detection rate: a prospective study of 353 patients.J Urol. 2012; 187: 2039-2043Abstract Full Text Full Text PDF PubMed Scopus (97) Google Scholar Its value for kidney elastography is very limited due to the depth of the organ, the difficulty of applying a reproducible homogeneous external deformation, and the previously mentioned technical limitations (including the inability to achieve absolute stiffness measurements10Franchi-Abella S. Elie C. Correas J.-M. Ultrasound elastography: advantages, limitations and artefacts of the different techniques from a study on a phantom.Diagn Interv Imaging. 2013; 94: 497-501Crossref PubMed Scopus (51) Google Scholar). In contrast, transient elastography (Fibroscan, Echosens, Paris, France) allows quantitative evaluation of the tissue stiffness based on the measurement of the shear wave velocities (SWVs) propagating perpendicularly to the ultrasound beam direction. It uses a piston to generate a tiny shock in between the intercostal space, and a single ultrasound crystal monitors the propagation of the shear waves inside the tissue. It provides a single point measurement without imaging capabilities and has been validated for the diagnosis of liver fibrosis.11Ferraioli G. Tinelli C. Malfitano A. et al.Performance of real-time strain elastography, transient elastography, and aspartate-to-platelet ratio index in the assessment of fibrosis in chronic hepatitis C.AJR Am J Roentgenol. 2012; 199: 19-25Crossref PubMed Scopus (54) Google Scholar, 12Cosgrove D. Piscaglia F. Bamber J. et al.EFSUMB guidelines and recommendations on the clinical use of ultrasound elastography. Part 2: clinical applications.Ultraschall Med. 2013; 34: 238-253Crossref PubMed Scopus (742) Google Scholar The volume of tissue involved in the measurement is at fixed depth and has a length of 30 to 40 mm, which could help to guide the site of measurement. For liver fibrosis quantification, the intercostal space allows standardization for depth and pressure applied by the device, but this placement cannot be used for renal stiffness measurements. Due to these limitations, this technique is not suited for kidney elastography in practice, with only a single report in the literature.13Arndt R. Schmidt S. Loddenkemper C. et al.Noninvasive evaluation of renal allograft fibrosis by transient elastography–a pilot study.Transpl Int. 2010; 23: 871-877PubMed Google Scholar Another approach for the generation of shear waves was introduced in 2002 using an ultrasound pulse produced by the imaging US transducer to apply an acoustic radiation force the tissue (ARFI).14Nightingale K. Soo M.S. Nightingale R. Trahey G. Acoustic radiation force impulse imaging: in vivo demonstration of clinical feasibility.Ultrasound Med Biol. 2002; 28: 227-235Abstract Full Text Full Text PDF PubMed Scopus (943) Google Scholar The axial displacement of a few micrometers induced by the US pulse produces shear waves that propagate in the transverse plane. The speed of the shear waves increases with tissue stiffness, and the elasticity measurement can be expressed in meters per second or converted into kilopascals, under a few hypotheses about the acoustic properties of the tissue (homogeneity, isotropy, and density) (ARFI-Shear Wave Velocity ARFI-SWV).1Sarvazyan A.P. Biophysical bases of elasticity imaging.in: Jones J.P. Acoustical Imaging. Springer US Plenum Press, New York, NY1995: 223-240Crossref Google Scholar, 14Nightingale K. Soo M.S. Nightingale R. Trahey G. Acoustic radiation force impulse imaging: in vivo demonstration of clinical feasibility.Ultrasound Med Biol. 2002; 28: 227-235Abstract Full Text Full Text PDF PubMed Scopus (943) Google Scholar The measurement is performed in a single small region of interest (ROI), 10,000 Hz thanks to ultrafast imaging processing.6Bamber J. Cosgrove D. Dietrich C.F. et al.EFSUMB guidelines and recommendations on the clinical use of ultrasound elastography. Part 1: basic principles and technology.Ultraschall Med. 2013; 34: 169-184Crossref PubMed Scopus (869) Google Scholar, 15Bercoff J. Tanter M. Fink M. Supersonic shear imaging: a new technique for soft tissue elasticity mapping.IEEE Trans Ultrason Ferroelectr Freq Control. 2004; 51: 396-409Crossref PubMed Scopus (1900) Google Scholar It solves many of the limitations of conventional shear wave elasticity imaging techniques by avoiding the repetition of successive shear wave tracking sequences and tracking in real time the propagation of the shear waves in a single ultrafast acquisition. Multiple ROIs with variable shapes can be positioned upon an area of interest on the color-coded elastographic box, or on the displayed reference morphological B-mode image, upon the cortex, the medulla, or any abnormal tissue, avoiding the peri-renal fat or the sinus (Figure 1b). The workflow does not need to be changed as its use is very similar to that of CDUS, by pressing the appropriate button with the same ultrasound transducer. This real-time capability is helpful for kidney elastography because the most stable acquisition can be selected for measurements, and pressure artifacts can easily be recognized. It is available on linear transducers for superficial renal transplants and convex transducers for native kidneys and deep renal transplants. Many confounding factors affect renal elastography. Any tissue compression with the transducer should be avoided, not only because it increases local stiffness (Figure 2) but also because it changes the acoustic properties of the tissues, including nonlinear propagation and speed of shear waves.16Gennisson J.-L. Rénier M. Catheline S. et al.Acoustoelasticity in soft solids: assessment of the nonlinear shear modulus with the acoustic radiation force.J Acoust Soc Am. 2007; 122: 3211-3219Crossref PubMed Scopus (147) Google Scholar Anatomic factors include renal anisotropy, blood perfusion, and hydronephrosis. The effect of anisotropy has been demonstrated in muscle and kidney elastography due to their spatial organization.17Gennisson J.-L. Deffieux T. Macé E. et al.Viscoelastic and anisotropic mechanical properties of in vivo muscle tissue assessed by supersonic shear imaging.Ultrasound Med Biol. 2010; 36: 789-801Abstract Full Text Full Text PDF PubMed Scopus (496) Google Scholar, 18Gennisson J.-L. Grenier N. Combe C. Tanter M. Supersonic shear wave elastography of in vivo pig kidney: influence of blood pressure, urinary pressure and tissue anisotropy.Ultrasound Med Biol. 2012; 38: 1559-1567Abstract Full Text Full Text PDF PubMed Scopus (177) Google Scholar When shear wave propagation is parallel to the renal tubules and interlobular arteries (and the ultrasound beam is perpendicular to these structures), the velocity of the shear waves is increased. Elasticity measurements performed in perpendicular to the long axis of the pyramids are greater for all renal compartments. Renal perfusion strongly affects renal elastography (Figure 3), with a drop in the medulla ranging from 44% to 72.7% in cases of renal artery occlusion, and an increase >500% in the case of renal vein thrombosis.18Gennisson J.-L. Grenier N. Combe C. Tanter M. Supersonic shear wave elastography of in vivo pig kidney: influence of blood pressure, urinary pressure and tissue anisotropy.Ultrasound Med Biol. 2012; 38: 1559-1567Abstract Full Text Full Text PDF PubMed Scopus (177) Google Scholar Hydronephrosis also results in renal elasticity increase, with a correlation between urinary tract pressure and cortical stiffness varying from 119% to 137% between 5 and 40 mm Hg.18Gennisson J.-L. Grenier N. Combe C. Tanter M. Supersonic shear wave elastography of in vivo pig kidney: influence of blood pressure, urinary pressure and tissue anisotropy.Ultrasound Med Biol. 2012; 38: 1559-1567Abstract Full Text Full Text PDF PubMed Scopus (177) Google Scholar Finally, other confounding factors are due to the physics of US elastography; stiffness values slightly vary with the technology and the transmit frequency, as viscoelastic properties of the tissues are frequency dependent. In a study of the liver, we found an average 1-kPa difference between the linear high-frequency transducer and the convex abdominal low-frequency transducer.4Franchi-Abella S. Corno L. Gonzales E. et al.Feasibility and diagnostic accuracy of supersonic shear-wave elastography for the assessment of liver stiffness and liver fibrosis in children: a pilot study of 96 patients.Radiology. 2016; 278: 554-562Crossref PubMed Scopus (93) Google Scholar The transmit pulse attenuation also affects elastography because when the signal-to-noise ratio is decreasing, small tissue displacements due to beating vessels and breathing motion will be confused with shear waves. This effect explains why stiffness values in the deep field are typically lower compared with superficial field measurements; using ARFI-SWV, the SWV was reduced by 27% when the depth increased from 2 to 3 cm to 6 to 7 cm (2.95 ± 0.41 m/s and 2.16 ± 0.61 m/s, respectively).19Bota S. Bob F. Sporea I. et al.Factors that influence kidney shear wave speed assessed by acoustic radiation force impulse elastography in patients without kidney pathology.Ultrasound Med Biol. 2015; 41: 1-6Abstract Full Text Full Text PDF PubMed Scopus (40) Google Scholar However, this phenomenon is complex and not constant.20Guo L.-H. Xu H.-X. Fu H.-J. et al.Acoustic radiation force impulse imaging for noninvasive evaluation of renal parenchyma elasticity: preliminary findings.PloS One. 2013; 8: e68925Crossref PubMed Scopus (88) Google Scholar At present, no cross-correlation tables are available between transducers and manufacturers or for depth compensation.Figure 3Living donor renal transplantation with immediate acute bleeding after anastomosis of the 2 renal arteries. After declamping, the graft recovered a homogeneous color pattern, but initial diuresis was poor. Multiparametric ultrasonography (US) was performed 6 hours after surgery for detection of early complications. (a) With color Doppler US (CDUS), no vessel was detected in the cortex. A single artery was recorded in the sinus and pulse wave Doppler showed a demodulated blood flow with reduced systolic velocities; initial diagnosis was subocclusion of the renal artery and massive infarct of the graft. (b) Shear wave elastography (SWE) revealed a patchy cortical pattern with areas of normal stiffness (arrow) and softer areas (arrowhead). (c) Micro-Doppler US performed with a linear transducer at the lower pole confirmed the presence of areas of persisting vascularity (arrow) and an infarcted area at the lower pole (arrowhead). (d) Contrast-enhanced US was performed after the i.v. administration of 1.2 ml of SonoVue (Bracco SA, Milan, Italy). It confirmed the presence of a lower pole infarct. The remaining parenchyma was still perfused, and the information was concordant with SWE and micro-Doppler ultrasonography. Conventional CDUS sensitivity to reduced blood flow was not sufficient to display the true vascular information.View Large Image Figure ViewerDownload (PPT) Only a few studies have evaluated kidney stiffness in normal and pathologic conditions compared with the extensive literature devoted to liver elasticity. Using ARFI-SWV in native kidneys, normal cortical stiffness values range from 2.15 to 2.54 m/s, with similar values between the 2 kidneys (that can be converted to ∼13.9–19.3 kPa).19Bota S. Bob F. Sporea I. et al.Factors that influence kidney shear wave speed assessed by acoustic radiation force impulse elastography in patients without kidney pathology.Ultrasound Med Biol. 2015; 41: 1-6Abstract Full Text Full Text PDF PubMed Scopus (40) Google Scholar, 20Guo L.-H. Xu H.-X. Fu H.-J. et al.Acoustic radiation force impulse imaging for noninvasive evaluation of renal parenchyma elasticity: preliminary findings.PloS One. 2013; 8: e68925Crossref PubMed Scopus (88) Google Scholar, 21Asano K. Ogata A. Tanaka K. et al.Acoustic radiation force impulse elastography of the kidneys: is shear wave velocity affected by tissue fibrosis or renal blood flow?.J Ultrasound Med. 2014; 33: 793-801Crossref PubMed Scopus (89) Google Scholar, 22Bob F. Bota S. Sporea I. et al.Kidney shear wave speed values in subjects with and without renal pathology and inter-operator reproducibility of acoustic radiation force impulse elastography (ARFI)–preliminary results.PloS One. 2014; 9: e113761Crossref PubMed Scopus (44) Google Scholar, 23Bob F. Bota S. Sporea I. et al.Relationship between the estimated glomerular filtration rate and kidney shear wave speed values assessed by acoustic radiation force impulse elastography: a pilot study.J Ultrasound Med. 2015; 34: 649-654Crossref PubMed Scopus (41) Google Scholar, 24Gallotti A. D'Onofrio M. Pozzi Mucelli R. Acoustic Radiation Force Impulse (ARFI) technique in ultrasound with Virtual Touch tissue quantification of the upper abdomen.Radiol Med (Torino). 2010; 115: 889-897Crossref PubMed Scopus (151) Google Scholar In 9- to 16-year-old children, higher ARFI-SWV stiffness values were found, ranging from 3.00 to 3.33 m/s (mean, 3.13 ± 0.09 m/s); however, the acquisition protocol mentioned that a certain degree of compression was applied with the ultrasound transducer. Surprisingly, normal kidney stiffness was found to exhibit an inverse, statistically significant relationship with patient age (P = 0.0003).19Bota S. Bob F. Sporea I. et al.Factors that influence kidney shear wave speed assessed by acoustic radiation force impulse elastography in patients without kidney pathology.Ultrasound Med Biol. 2015; 41: 1-6Abstract Full Text Full Text PDF PubMed Scopus (40) Google Scholar In a study performed in healthy people 18 to 30, 51 to 65, and older than 65 years of age, ARFI-SWV was 2.94 ± 0.60 m/s, 2.48 ± 0.8 m/s, and 1.82 ± 0.63 m/s, respectively. In the same study, a statistically significant difference was found between women and men.19Bota S. Bob F. Sporea I. et al.Factors that influence kidney shear wave speed assessed by acoustic radiation force impulse elastography in patients without kidney pathology.Ultrasound Med Biol. 2015; 41: 1-6Abstract Full Text Full Text PDF PubMed Scopus (40) Google Scholar Using SWE, similar values were found in a small series of normal volunteers with a superficial kidney, with an average cortical stiffness of 15.4 ± 2.5 kPa.25Grenier N. Gennisson J.-L. Cornelis F. et al.Renal ultrasound elastography.Diagn Interv Imaging. 2013; 94: 545-550Crossref PubMed Scopus (88) Google Scholar The stiffness of the renal medulla has been evaluated only in a few studies. It is lower than cortical stiffness in our experience and that of another group,26Grenier N. Poulain S. Lepreux S. et al.Quantitative elastography of renal transplants using supersonic shear imaging: a pilot study.Eur Radiol. 2012; 22: 2138-2146Crossref PubMed Scopus (92) Google Scholar except for 1 study using ARFI-SWV (Figure 4).21Asano K. Ogata A. Tanaka K. et al.Acoustic radiation force impulse elastography of the kidneys: is shear wave velocity affected by tissue fibrosis or renal blood flow?.J Ultrasound Med. 2014; 33: 793-801Crossref PubMed Scopus (89) Google Scholar In patients with CKD, the most consistent predictor of outcome, even in diseases of glomerular origin (i.e., IgA nephropathy), is the extent of tubulointerstitial fibrosis. This may be not surprising because the most important proportion of any renal biopsy originates from the tubulointerstitial compartment. However, histologic assessment of fibrosis carries specific risks related to the procedure and a risk of sampling error because of the small amount of parenchyma studied. A noninvasive alternative to renal biopsy is required to monitor the progression of CKD and to study the effect of therapeutic interventions. In kidney transplantation, progressive scarring of the allograft characterized by interstitial fibrosis and tubular atrophy (IFTA), vascular changes, and glomerulosclerosis (formerly called chronic allograft nephropathy) is responsible for chronic allograft dysfunction; these lesions are quantified histologically using the Banff classification.27Solez K. Colvin R.B. Racusen L.C. et al.Banff 07 classification of renal allograft pathology: updates and future directions.Am J Transplant. 2008; 8: 753-760Crossref PubMed Scopus (1615) Google Scholar Although renal allograft fibrosis is characterized by a progressive deterioration of GFR, serum creatinine levels and GFR are poor predictors of the severity of histologic lesions (IFTA) and functional parameters other than GFR (i.e., early after transplantation). IFTA correlates inversely with renal graft survival and long-term graft function. Early detection of IFTA could therefore be useful to predict the risk of subsequent graft function deterioration and potentially to determine the efficacy of specific therapeutic interventions. The current gold-standard test for the diagnosis of IFTA remains histologic examination of allograft biopsy specimens. An early diagnosis of IFTA, before the onset of clinically evident graft dysfunction, would require multiple biopsies. A noninvasive test that could provide diagnosis and/or prognosis early on is therefore urgently needed to detect early IFTA to avoid repeated biopsies and to allow early targeted therapeutic intervention and ultimately improvement in patient management. Renal stiffness is supposed to increase due to changes in the tissue architecture and the accumulation of extracellular matrix in the glomerulus, vessel walls, and renal interstitium. The early noninvasive identification and noninvasive quantification of renal fibrosis would have highly significant prognostic implications and represent a crucial preliminary step for the development and follow-up of antifibrotic treatment by alleviating the need for repeat biopsies. To date, several studies have reported a correlation between renal stiffness and fibrosis or renal function. In experimental models of glomerulosclerosis, the cortical stiffness correlated with the degree of renal dysfunction.5Derieppe M. Delmas Y. Gennisson J.-L. et al.Detection of intrarenal microstructural changes with supersonic shear wave elastography in rats.Eur Radiol. 2012; 22: 243-250Crossref PubMed Scopus (39) Google Scholar In humans, this correlation has remained highly variable in both native and transplanted kidneys due to technical and anatomic limitations. Some groups have reported a correlation between renal stiffness and fibrosis or renal function with several different techniques.13Arndt R. Schmidt S. Loddenkemper C. et al.Noninvasive evaluation of renal allograft fibrosis by transient elastography–a pilot study.Transpl Int. 2010; 23: 871-877PubMed Google Scholar, 20Guo L.-H. Xu H.-X. Fu H.-J. et al.Acoustic radiation force impulse imaging for noninvasive evaluation of renal parenchyma elasticity: preliminary findings.PloS One. 2013; 8: e68925Crossref PubMed Scopus (88) Google Scholar, 21Asano K. Ogata A. Tanaka K. et al.Acoustic radiation force impulse elastography of the kidneys: is shear wave velocity affected by tissue fibrosis or renal blood flow?.J Ultrasound Med. 2014; 33: 793-801Crossref PubMed Scopus (89) Google Scholar, 28Stock K.F. Klein B.S. Cong M.T.V. et al.ARFI-based tissue elasticity quantification and kidney graft dysfunction: first clinical experiences.Clin Hemorheol Microcirc. 2011; 49: 527-535Crossref PubMed Scopus (54)
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