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Kidney Ultrasound for Nephrologists: A Review

2022; Elsevier BV; Volume: 4; Issue: 6 Linguagem: Inglês

10.1016/j.xkme.2022.100464

2590-0595

Rohit Singla, Matthew Kadatz, Robert Rohling, Christopher Nguan,

Pediatric Urology and Nephrology Studies

Ultrasound imaging is a key investigatory step in the evaluation of chronic kidney disease and kidney transplantation. It uses nonionizing radiation, is noninvasive, and generates real-time images, making it the ideal initial radiographic test for patients with abnormal kidney function. Ultrasound enables the assessment of both structural (form and size) and functional (perfusion and patency) aspects of kidneys, both of which are especially important as the disease progresses. Ultrasound and its derivatives have been studied for their diagnostic and prognostic significance in chronic kidney disease and kidney transplantation. Ultrasound is rapidly growing more widely accessible and is now available even in handheld formats that allow for bedside ultrasound examinations. Given the trend toward ubiquity, the current use of kidney ultrasound demands a full understanding of its breadth as it and its variants become available. We described the current applications and future directions of ultrasound imaging and its variants in the context of chronic kidney disease and transplantation in this review. Ultrasound imaging is a key investigatory step in the evaluation of chronic kidney disease and kidney transplantation. It uses nonionizing radiation, is noninvasive, and generates real-time images, making it the ideal initial radiographic test for patients with abnormal kidney function. Ultrasound enables the assessment of both structural (form and size) and functional (perfusion and patency) aspects of kidneys, both of which are especially important as the disease progresses. Ultrasound and its derivatives have been studied for their diagnostic and prognostic significance in chronic kidney disease and kidney transplantation. Ultrasound is rapidly growing more widely accessible and is now available even in handheld formats that allow for bedside ultrasound examinations. Given the trend toward ubiquity, the current use of kidney ultrasound demands a full understanding of its breadth as it and its variants become available. We described the current applications and future directions of ultrasound imaging and its variants in the context of chronic kidney disease and transplantation in this review. Ultrasound (US) imaging is a critical diagnostic tool for assessing human kidneys. A US transducer works by transmitting radiofrequency sound waves into the body. These waves interact with tissues and tissue interfaces, changing them and returning them to the transducer as echoes. Its piezoelectric crystals vibrate in response, converting the echoes into electrical signals, which are then processed using complex algorithms to provide cross-sectional images of the body's underlying tissue layers. US does not use ionizing radiation and is noninvasive, meaning that it does not require any penetration of the skin. In both acute care and ambulatory settings, US imaging can reveal information on kidney morphology, physical features, function, and probable anomalies. US is used as the first-line imaging modality for previously undiagnosed native and transplanted, abnormal kidney function.1Remer E.M. Papanicolaou N. Casalino D.D. et al.ACR appropriateness criteria® on renal failure.Am J Med. 2014; 127: 1041-1048Abstract Full Text Full Text PDF PubMed Scopus (25) Google Scholar,2Taffel M.T. Nikolaidis P. Beland M.D. et al.ACR appropriateness criteria® renal transplant dysfunction.J Am Coll Radiol. 2017; 14: S272-S281Abstract Full Text Full Text PDF PubMed Scopus (6) Google Scholar Several studies have argued for the formal incorporation of sonography into training and clinical practice, citing its increasing utility and accessibility in nephrology.3O'Neill W.C. Sonographic evaluation of renal failure.Am J Kidney Dis. 2000; 35: 1021-1038Abstract Full Text Full Text PDF PubMed Scopus (82) Google Scholar, 4Koratala A. Segal M.S. Kazory A. Integrating point-of-care ultrasonography into nephrology fellowship training: a model curriculum.Am J Kidney Dis. 2019; 74: 1-5Abstract Full Text Full Text PDF PubMed Scopus (18) Google Scholar, 5Niyyar V.D. O'Neill W.C. Point-of-care ultrasound in the practice of nephrology.Kidney Int. 2018; 93: 1052-1059Abstract Full Text Full Text PDF PubMed Scopus (24) Google Scholar, 6Koratala A. Olaoye O.A. Bhasin-Chhabra B. Kazory A. A blueprint for an integrated point-of-care ultrasound curriculum for nephrology trainees.Kidney360. 2021; 2: 1669-1676Crossref PubMed Google Scholar, 7Berns J.S. O'Neill W.C. Performance of procedures by nephrologists and nephrology fellows at US nephrology training programs.Clin J Am Soc Nephrol. 2008; 3: 941-947Crossref PubMed Scopus (59) Google Scholar Taken together, advances in beamforming, image acquisition, and image processing make portable US a reality while lowering costs by several orders of magnitude. One of these advances is point-of-care US. It is a US examination performed by nephrologists at the patient's bedside, often treated as an extension of physical examination.8Narula J. Chandrashekhar Y. Braunwald E. Time to add a fifth pillar to bedside physical examination: inspection, palpation, percussion, auscultation, and insonation.JAMA Cardiol. 2018; 3: 346-350Crossref PubMed Scopus (117) Google Scholar,9Rad A.H. Badeli H. Point-of-care ultrasonography: is it time nephrologists were equipped with the 21st century's stethoscope?.Iran J Kidney Dis. 2017; 11: 259-262PubMed Google Scholar Point-of-care US has become more accessible and widespread with increasingly portable, even handheld, US equipment.4Koratala A. Segal M.S. Kazory A. Integrating point-of-care ultrasonography into nephrology fellowship training: a model curriculum.Am J Kidney Dis. 2019; 74: 1-5Abstract Full Text Full Text PDF PubMed Scopus (18) Google Scholar To successfully use US as an adjunct to clinical decision making, a thorough understanding of the technology and its variants is required, particularly in the context of chronic kidney disease (CKD) and kidney transplantation. This review discusses US and its derivatives as well as their clinical applications. Brightness mode (B-mode), color Doppler, power Doppler, and spectral Doppler US are all types of US often used in disease evaluation. Additional types, including contrast-enhanced US and US-based elastography, have seen tremendous research efforts and may be worth considering for routine clinical use. This review focuses on the most common types for which published literature on CKD and kidney transplantation is accessible. This article summarizes the mechanism of action for each variation, reviews the type of data obtained, and synthesizes pertinent research demonstrating its therapeutic utility in CKD and transplantation. B-mode US is a 2-dimensional, cross-sectional depiction of anatomy in grayscale.10Szabo T.L. Chapter 9 - Scattering from tissue and tissue characterization.in: Szabo T.L. Diagnostic Ultrasound Imaging: Inside Out. 2nd ed. Elsevier Science & Technology, 2014: 295-363Crossref Google Scholar A pixel's depth in the image represents a tissue's distance from the transducer, and its intensity represents echogenicity, which is the ability of the tissue to reflect the US signal.10Szabo T.L. Chapter 9 - Scattering from tissue and tissue characterization.in: Szabo T.L. Diagnostic Ultrasound Imaging: Inside Out. 2nd ed. Elsevier Science & Technology, 2014: 295-363Crossref Google Scholar Tissue interfaces and microstructures influence echogenicity, capturing underlying tissue variations.10Szabo T.L. Chapter 9 - Scattering from tissue and tissue characterization.in: Szabo T.L. Diagnostic Ultrasound Imaging: Inside Out. 2nd ed. Elsevier Science & Technology, 2014: 295-363Crossref Google Scholar Lower imaging frequencies allow the imaging of deeper tissues, such as native kidneys in the abdominal cavity's back wall. The trade-off for penetration depth is that frequency is inversely linked to spatial resolution; lower frequencies have worse resolution (Fig 1). A transplanted kidney, located in the recipient's iliac fossa, is closer to the abdominal skin surface, allowing higher frequencies (Fig 1). The anthropomorphic measurements provided by B-mode US include the length and volume of kidneys. Kidney length is defined as the maximum distance from pole to pole, measured in a longitudinal view, with a normal range of 10-12 cm.11Koratala A. Bhattacharya D. Kazory A. Point of care renal ultrasonography for the busy nephrologist: a pictorial review.World J Nephrol. 2019; 8: 44-58Crossref PubMed Google Scholar The left kidney is longer than the right, with median lengths of 11.2 and 10.9 cm, respectively.12Emamian S.A. Nielsen M.B. Pedersen J.F. Ytte L. Kidney dimensions at sonography: correlation with age, sex, and habitus in 665 adult volunteers.AJR Am J Roentgenol. 1993; 160: 83-86Crossref PubMed Scopus (310) Google Scholar Widjaja et al13Widjaja E. Oxtoby J.W. Hale T.L. Jones P.W. Harden P.N. McCall I.W. Ultrasound measured renal length versus low dose CT volume in predicting single kidney glomerular filtration rate.Br J Radiol. 2004; 77: 759-764Crossref PubMed Scopus (75) Google Scholar used US imaging to quantify the native kidney lengths of 69 adults and compared them with single kidney function using radionuclide imaging. They found the length to have a moderate positive correlation (r2 = 0.48) with the estimated glomerular filtration rate (eGFR) of the native single kidneys, although Zanoli et al14Zanoli L. Romano G. Romano M. et al.Renal function and ultrasound imaging in elderly subjects.Sci World J. 2014; 2014 (830649): 7Crossref Scopus (4) Google Scholar discovered that the correlation between length and eGFR varied based on the eGFR equation. Measured in a longitudinal view from the kidney's midpole boundary to the base of the medullary pyramids, cortical thickness is a common metric. The normal cortical thickness is 7-10 mm; reduced cortical thickness may indicate progressive kidney disease or decreased eGFR.11Koratala A. Bhattacharya D. Kazory A. Point of care renal ultrasonography for the busy nephrologist: a pictorial review.World J Nephrol. 2019; 8: 44-58Crossref PubMed Google Scholar,15Korkmaz M. Aras B. Güneyli S. Yılmaz M. Clinical significance of renal cortical thickness in patients with chronic kidney disease.Ultrasonography. 2018; 37: 50-54Crossref PubMed Scopus (14) Google Scholar,16Beland M.D. Walle N.L. Machan J.T. Cronan J.J. Renal cortical thickness measured at ultrasound: is it better than renal length as an indicator of renal function in chronic kidney disease?.AJR Am J Roentgenol. 2010; 195: W146-W149Crossref PubMed Scopus (103) Google Scholar With respect to volume, the normal range in men is 110-190 mL and in women is 90-150 mL.17Cheong B. Muthupillai R. Rubin M.F. Flamm S.D. Normal values for renal length and volume as measured by magnetic resonance imaging.Clin J Am Soc Nephrol. 2007; 2: 38-45Crossref PubMed Scopus (177) Google Scholar It is possible to measure volume by simplifying the kidney's shape as spherical or ellipsoid and measuring several distances. Better still is to contour the kidney in each US frame and sum the contoured area. Manual contouring is the gold standard for US-based volume estimation; however, it is very time consuming and labor intensive. Studies by Zanoli et al14Zanoli L. Romano G. Romano M. et al.Renal function and ultrasound imaging in elderly subjects.Sci World J. 2014; 2014 (830649): 7Crossref Scopus (4) Google Scholar and Sanusi et al18Sanusi A.A. Arogundade F.A. Famurewa O.C. et al.Relationship of ultrasonographically determined kidney volume with measured GFR, calculated creatinine clearance and other parameters in chronic kidney disease (CKD).Nephrol Dial Transplant. 2009; 24: 1690-1694Crossref PubMed Scopus (58) Google Scholar reported favorable relationships between kidney volume and 24-hour creatinine clearance as well as eGFR estimated using various equations. Volume interpretation requires caution as Kim et al19Kim H.C. Yang D.M. Lee S.H. Cho Y.D. Usefulness of renal volume measurements obtained by a 3-dimensional sonographic transducer with matrix electronic arrays.J Ultrasound Med. 2008; 27: 1673-1681Crossref PubMed Scopus (23) Google Scholar found that 2-dimensional, US-based volumes underestimated computed tomography volumes by 15%. Nephrologists can use these parameters to understand the state of kidney disease. A kidney length of ≤8 cm is related to kidney failure.20Fiorini F. Barozzi L. The role of ultrasonography in the study of medical nephropathy.J Ultrasound. 2007; 10: 161-167Crossref PubMed Scopus (23) Google Scholar If the kidney loses its length or volume, this is attributed to atrophy or fibrosis.8Narula J. Chandrashekhar Y. Braunwald E. Time to add a fifth pillar to bedside physical examination: inspection, palpation, percussion, auscultation, and insonation.JAMA Cardiol. 2018; 3: 346-350Crossref PubMed Scopus (117) Google Scholar A unilateral decrease may specifically indicate hypoperfusion, such as in renal artery stenosis,8Narula J. Chandrashekhar Y. Braunwald E. Time to add a fifth pillar to bedside physical examination: inspection, palpation, percussion, auscultation, and insonation.JAMA Cardiol. 2018; 3: 346-350Crossref PubMed Scopus (117) Google Scholar whereas bilateral hypotrophic kidneys with decreased volumes may indicate advanced disease, such as in kidney failure. Fluid retention, inflammation, protein deposition, and acute tubular necrosis or neoplasms are linked to increased kidney volume.8Narula J. Chandrashekhar Y. Braunwald E. Time to add a fifth pillar to bedside physical examination: inspection, palpation, percussion, auscultation, and insonation.JAMA Cardiol. 2018; 3: 346-350Crossref PubMed Scopus (117) Google Scholar Renal echogenicity is frequently used as a kidney health biomarker. The echogenicity of renal parenchyma is assessed by comparing it with a reference tissue, such as the liver, which should be less echogenic.11Koratala A. Bhattacharya D. Kazory A. Point of care renal ultrasonography for the busy nephrologist: a pictorial review.World J Nephrol. 2019; 8: 44-58Crossref PubMed Google Scholar,21Manley J.A. O'Neill W.C. How echogenic is echogenic? Quantitative acoustics of the renal cortex.Am J Kidney Dis. 2001; 37: 706-711Abstract Full Text PDF PubMed Scopus (49) Google Scholar Moghazi et al22Moghazi S. Jones E. Schroepple J. et al.Correlation of renal histopathology with sonographic findings.Kidney Int. 2005; 67: 1515-1520Abstract Full Text Full Text PDF PubMed Scopus (142) Google Scholar retrospectively evaluated the correlation between cortical echogenicity and histopathologic parameters, finding a strong correlation with severe disease and increased relative echogenicity. Similarly, Page et al23Page J.E. Morgan S.H. Eastwood J.B. et al.Ultrasound findings in renal parenchymal disease: comparison with histological appearances.Clin Radiol. 1994; 49: 867-870Abstract Full Text PDF PubMed Scopus (39) Google Scholar found a link between glomerular sclerosis detected using biopsy and cortical echogenicity. Echogenicity must be carefully considered because it depends on the US machine setup, image acquisition factors, or hydration status. Additionally, occult liver disease may increase the liver's echogenicity, causing a misleading negative comparison with the renal cortex. The echogenicity of the cortex and medulla are easily compared. The inability to distinguish between them is known as the loss of corticomedullary differentiation.24Faubel S. Patel N.U. Lockhart M.E. Cadnapaphornchai M.A. Renal relevant radiology: use of ultrasonography in patients with AKI.Clin J Am Soc Nephrol. 2014; 9: 382-394Crossref PubMed Scopus (64) Google Scholar This observation is nonspecific in both native and transplanted kidneys.24Faubel S. Patel N.U. Lockhart M.E. Cadnapaphornchai M.A. Renal relevant radiology: use of ultrasonography in patients with AKI.Clin J Am Soc Nephrol. 2014; 9: 382-394Crossref PubMed Scopus (64) Google Scholar Infection, autosomal recessive kidney disease, renal vein thrombosis, and rejection are all conditions that may present with the loss of corticomedullary differentiation.25Browne R.F.J. Tuite D.J. Imaging of the renal transplant: comparison of MRI with duplex sonography.Abdom Imaging. 2006; 31: 461-482Crossref PubMed Scopus (44) Google Scholar,26Gulati M. Cheng J. Loo J.T. Skalski M. Malhi H. Duddalwar V. Pictorial review: renal ultrasound.Clin Imaging. 2018; 51: 133-154Abstract Full Text Full Text PDF PubMed Scopus (10) Google Scholar. B-mode imaging lies in point-of-care adoption. The feasibility and usability of these morphologic and echogenic properties are considerably boosted by accessible US imaging. Further research on understanding how easily nephrologists can acquire clinically meaningful images is required, as is research on existing technical challenges to further adoption. Doppler imaging is used to assess renal vascular flow and patency. When a pulsed US wave encounters a moving medium, the resultant echo undergoes a frequency shift.10Szabo T.L. Chapter 9 - Scattering from tissue and tissue characterization.in: Szabo T.L. Diagnostic Ultrasound Imaging: Inside Out. 2nd ed. Elsevier Science & Technology, 2014: 295-363Crossref Google Scholar In the human body, this moving medium is commonly blood inside vessels. The frequency decreases as the medium moves away from the transducer and increases as it moves toward it.10Szabo T.L. Chapter 9 - Scattering from tissue and tissue characterization.in: Szabo T.L. Diagnostic Ultrasound Imaging: Inside Out. 2nd ed. Elsevier Science & Technology, 2014: 295-363Crossref Google Scholar Derived from these shifts is flow velocity.10Szabo T.L. Chapter 9 - Scattering from tissue and tissue characterization.in: Szabo T.L. Diagnostic Ultrasound Imaging: Inside Out. 2nd ed. Elsevier Science & Technology, 2014: 295-363Crossref Google Scholar The types of Doppler US can be color Doppler, power Doppler, or spectral Doppler imaging. In Fig 2, color Doppler imaging overlays the color map of flow velocity onto a B-mode image also showing directionality. Power Doppler imaging employs a color map that sacrifices directionality for increased sensitivity to detect blood flow. Spectral Doppler imaging produces a "waveform" of velocities across time and can be employed in continuous or pulsed wave patterns. Continuous waves measure high velocities but cannot localize them because they employ different crystals to send and receive radiofrequency waves at the same time. Pulsed waves can localize velocities but are subject to aliasing. Doppler US is widely used to assess transplanted kidneys, although it is difficult to be performed in native kidneys because of the depth of the organ. It can be used to detect renal vascular disorders such as stenosis or thrombosis. For example, increased peak systolic velocities surpassing 200 cm/sec, a 3.5:1 peak systolic velocity ratio between the renal artery and the vascular tributary proximal to it, and aliasing are all signs of the renin angiotensin system.27Al-Katib S. Shetty M. Jafri S.M. Jafri S.Z. Radiologic assessment of native renal vasculature: a multimodality review.Radiographics. 2017; 37: 136-156Crossref PubMed Scopus (25) Google Scholar The use of pulsed-wave spectral Doppler imaging is required. A systematic review found that peak systolic velocity had the highest sensitivity (85%) and specificity (92%) compared with other US parameters in the diagnosis of this devastating complication.28Williams G.J. Macaskill P. Chan S.F. et al.Comparative accuracy of renal duplex sonographic parameters in the diagnosis of renal artery stenosis: paired and unpaired analysis.AJR Am J Roentgenol. 2007; 188: 798-811Crossref PubMed Scopus (108) Google Scholar With increasing kidney size and reversed parenchymal vascular flow, the absence of venous flow from renal vein thromboses is instructive. Doppler US can also be used after biopsy to evaluate postprocedure complications, such as intrarenal arteriovenous fistula, which occurs up to 18% of the time.29Whittier W.L. Complications of the percutaneous kidney biopsy.Adv Chronic Kidney Dis. 2012; 19: 179-187Abstract Full Text Full Text PDF PubMed Scopus (56) Google Scholar Power Doppler imaging is useful in the diagnosis of kidney ischemia or infarction because its greater sensitivity helps in the detection of minor vascular alterations such as capillary bed changes.30Chen P. Maklad N. Redwine M. Color and power Doppler imaging of the kidneys.World J Urol. 1998; 16: 41-45Crossref PubMed Scopus (23) Google Scholar The renal resistive index (RRI) can be derived from Doppler US. This is a unitless, surrogate measure of intrarenal parenchymal pathology, defined as the ratio of the difference between peak systolic velocity and end-diastolic velocity to the peak systolic velocity. It is measured in arcuate or interlobar arteries. When RRI is ≤0.7, it reassures the nephrologist of the absence of severe kidney damage.31Platt J.F. Ellis J.H. Rubin J.M. DiPietro M.A. Sedman A.B. Intrarenal arterial Doppler sonography in patients with nonobstructive renal disease: correlation of resistive index with biopsy findings.AJR Am J Roentgenol. 1990; 154: 1223-1227Crossref PubMed Scopus (260) Google Scholar In the native kidney setting, a controversy arises when elevated RRI and histopathologic markers are associated. Some studies have linked increased RRI with glomerulosclerosis progression, glomerular disease, arteriolosclerosis, and interstitial fibrosis, whereas others have found no correlation.32Gigante A. Barbano B. Di Mario F. et al.Renal parenchymal resistance in patients with biopsy proven glomerulonephritis: correlation with histological findings.Int J Immunopathol Pharmacol. 2016; 29: 469-474Crossref PubMed Scopus (11) Google Scholar, 33Sugiura T. Nakamori A. Wada A. Fukuhara Y. Evaluation of tubulointerstitial injury by Doppler ultrasonography in glomerular diseases.Clin Nephrol. 2004; 61: 119-126Crossref PubMed Scopus (52) Google Scholar, 34Ikee R. Kobayashi S. Hemmi N. et al.Correlation between the resistive index by Doppler ultrasound and kidney function and histology.Am J Kidney Dis. 2005; 46: 603-609Abstract Full Text Full Text PDF PubMed Scopus (182) Google Scholar, 35Kirkpantur A. Yilmaz R. Baydar D.E. et al.Utility of the Doppler ultrasound parameter, resistive index, in renal transplant histopathology.Transplant Proc. 2008; 40: 104-106Crossref PubMed Scopus (18) Google Scholar, 36Garcia-Covarrubias L. Martinez A. Morales-Buenrostro L.E. et al.Parameters of Doppler ultrasound at five days posttransplantation as predictors of histology and renal function at one year.Transplant Proc. 2010; 42: 262-265Crossref PubMed Scopus (6) Google Scholar Elevated RRI may have prognostic value because it has been associated with worsening kidney function,37Heine G.H. Reichart B. Ulrich C. Köhler H. Girndt M. Do ultrasound renal resistance indices reflect systemic rather than renal vascular damage in chronic kidney disease?.Nephrol Dial Transplant. 2007; 22: 163-170Crossref PubMed Scopus (67) Google Scholar,38Bigé N. Lévy P.P. Callard P. et al.Renal arterial resistive index is associated with severe histological changes and poor renal outcome during chronic kidney disease.BMC Nephrol. 2012; 13: 1-9Crossref PubMed Scopus (69) Google Scholar cardiovascular risk scores.,37Heine G.H. Reichart B. Ulrich C. Köhler H. Girndt M. Do ultrasound renal resistance indices reflect systemic rather than renal vascular damage in chronic kidney disease?.Nephrol Dial Transplant. 2007; 22: 163-170Crossref PubMed Scopus (67) Google Scholar and progressive diabetic nephropathy.39Milovanceva-Popovska M. Dzikova S. Progression of diabetic nephropathy: value of intrarenal resistive index (RI).Prilozi. 2007; 28: 69-79PubMed Google Scholar In kidney transplantation, renal vein and renal artery thromboses have been associated with low RRI values in the immediate postoperative period.40Nezami N. Tarzamni M.K. Argani H. Nourifar M. Doppler ultrasonographic indices after renal transplantation as renal function predictors.Transplant Proc. 2008; 40: 94-99Crossref PubMed Scopus (21) Google Scholar Wang et al reported that donor kidneys from deceased individuals have higher RRIs (0.73 ± 0.10) than control kidneys from living donors (0.66 ± 0.11). RRI is yet to be proven as a predictive factor for kidney transplant outcomes, including long-term allograft survival.41McArthur C. Geddes C.C. Baxter G.M. Early measurement of pulsatility and resistive indexes: correlation with long-term renal transplant function.Radiology. 2011; 259: 278-285Crossref PubMed Scopus (55) Google Scholar Some researchers have contended that variation in RRI is related to extrarenal variables rather than inherent structural changes in the kidney, both in native and transplanted contexts. RRI is affected by age, heart rate, blood pressure, systemic vascular resistance, and hydration.42Platt J.F. Imaging: refining noninvasive ultrasound evaluation of the kidneys.Nat Rev Nephrol. 2012; 8: 557-558Crossref PubMed Scopus (1) Google Scholar O'Neill43O'Neill W.C. Renal resistive index: a case of mistaken identity.Hypertension. 2014; 64: 915-917Crossref PubMed Scopus (54) Google Scholar presented a mathematical approach to the interpretation of RRI, suggesting that it is more easily impacted by pulse pressure, renal capillary wedge pressure, and vascular compliance. Rather than intrarenal changes, the author argued that it reflects systemic hemodynamics.43O'Neill W.C. Renal resistive index: a case of mistaken identity.Hypertension. 2014; 64: 915-917Crossref PubMed Scopus (54) Google Scholar Others have suggested that increases in posttransplantation RRI are linked to the organ's new vascular environment in the recipient.44de Freminville J.B. Vernier L.M. Roumy J. et al.Impact on renal resistive index of diabetes in renal transplant donors and recipients: a retrospective analysis of 1827 kidney transplant recipients.J Clin Hypertens (Greenwich). 2019; 21: 382-389Crossref PubMed Scopus (3) Google Scholar Seiler et al45Seiler S. Colbus S.M. Lucisano G. et al.Ultrasound renal resistive index is not an organ-specific predictor of allograft outcome.Nephrol Dial Transplant. 2012; 27: 3315-3320Crossref PubMed Scopus (27) Google Scholar showed that RRI may not be specific to kidneys. They reported a correlation between RRI and increased common carotid thickness but not kidney-specific markers.45Seiler S. Colbus S.M. Lucisano G. et al.Ultrasound renal resistive index is not an organ-specific predictor of allograft outcome.Nephrol Dial Transplant. 2012; 27: 3315-3320Crossref PubMed Scopus (27) Google Scholar However, as Grün et al46Grün O.S. Herath E. Weihrauch A. et al.Does the measurement of the difference of resistive indexes in spleen and kidney allow a selective assessment of chronic kidney injury?.Radiology. 2012; 264: 894-902Crossref PubMed Scopus (18) Google Scholar reported in their investigation of variations in resistive indices between the spleen and the kidney, the use of 2 indices may reduce extrarenal influence and enhance kidney specificity. Overall, these studies suggest caution while interpreting RRI as a kidney-specific pathology indication. Doppler US-acquired values are subject to external factors, such as the imaging plane, operator technique, and transducer location, making valid, accurate, and repeatable measurements difficult. Future research should consider extrarenal hemodynamic variables that may affect RRI values. Given the difficulty in conducting high-quality Doppler imaging studies without experienced personnel, the use of Doppler US at the bedside may be best suited for determining vascular patency or its absence. A technique known as contrast-enhanced US is used to investigate blood flow and perfusion in greater detail.47Chang E.H. An introduction to contrast-enhanced ultrasound for nephrologists.Nephron. 2018; 138: 176-185Crossref PubMed Scopus (25) Google Scholar A contrast medium—a nontoxic, biologically inert microbubble with a diameter <10 μm—is injected into the systemic circulation.47Chang E.H. An introduction to contrast-enhanced ultrasound for nephrologists.Nephron. 2018; 138: 176-185Crossref PubMed Scopus (25) Google Scholar The contrast medium dissolves on its own within minutes of being injected and has no effect on kidney function or the systemic circulation in any other manner. Although contrast-enhanced US is less expensive than color Doppler imaging, it provides a more accurate assessment of the microvasculature and perfusion in narrow blood arteries, where color Doppler imaging may require significant operator expertise or blood flow may be too slow.48Zeisbrich M. Kihm L.P. Drüschler F. Zeier M. Schwenger V. When is contrast-enhanced sonography preferable over conventional ultrasound combined with Doppler imaging in renal transplantation?.Clin Kidney J. 2015; 8: 606-614Crossref PubMed Scopus (20) Google Scholar Contrast-enhanced US enables improved visualization of perfusion and the provision of absolute, quantitative values through the observation of destruction and replenishment of the contrast medium, which is characterized using time-intensity curves.48Zeisbrich M. Kihm L.P. Drüschler F. Zeier M. Schwenger V. When is contrast-enhanced sonography preferable over conventional ultrasound combined with Doppler imaging in renal transplantation?.Clin Kidney J. 2015; 8: 606-614Crossref PubMed Scopus (20) Google Scholar These curves can be used to compute perfusion characteristics, such as time to peak, rising time, area under the curve, peak intensity, and mean transit time. The contrast agents are typically injected intravenously as either a bolus or an infusion.47Chang E.H. An introduction to contrast-enhanced ultrasound for nephrologists.Nephron. 2018; 138: 176-185Crossref PubMed Scopus (25) Google Scholar Wit