Shear-Wave Elastography Variability Analysis and Relation with Kidney Allograft Dysfunction: A Single-Center Study

Kidney cortex shear wave motion simulations based on segmented biopsy histology

BACKGROUND AND OBJECTIVE

Biopsy stands as the gold standard for kidney transplant assessment, yet its invasive nature restricts frequent use. Shear wave elastography (SWE) is emerging as a promising alternative for kidney transplant monitoring. A parametric study involving 12 biopsy data sets categorized by standard biopsy scores (3 with normal histology, 3 with interstitial inflammation (i), 3 with interstitial fibrosis (ci), and 3 with tubular atrophy (ct)), was conducted to evaluate the interdependence between microstructural variations triggered by chronic allograft rejection and corresponding alterations in SWE measurements.

METHODS

Heterogeneous shear wave motion simulations from segmented kidney cortex sections were performed employing the staggered-grid finite difference (SGFD) method. The SGFD method allows the mechanical properties to be defined on a pixel-basis for shear wave motion simulation. Segmentation techniques enabled the isolation of four histological constituents: glomeruli, tubules, interstitium, and fluid. Baseline ex vivo Kelvin-Voigt mechanical properties for each constituent were drawn from established literature. The parametric evaluation was then performed by altering the baseline values individually. Shear wave velocity dispersion curves were measured with the generalized Stockwell transform in conjunction with slant frequency-wavenumber analysis (GST-SFK) algorithm. By fitting the curve within the 100-400 Hz range to the Kelvin-Voigt model, the rheological parameters, shear elasticity (µ1) and viscosity (µ2), were estimated. A time-to-peak algorithm was used to estimate the group velocity. The resultant in silico models emulated the heterogeneity of kidney cortex within the shear wave speed (SWS) reconstructions.

RESULTS

The presence of inflammation showed considerable spatial composition disparities compared to normal cases, featuring a 23 % increase in interstitial area and a 19 % increase in glomerular area. Concomitantly, there was a reduction of 12 % and 47 % in tubular and fluid areas, respectively. Consequently, mechanical changes induced by inflammation predominate in terms of rheological differentiation, evidenced by increased elasticity and viscosity. Mild tubular atrophy showed significant elevation in group velocity and µ1. Conversely, mild and moderate fibrosis exhibited negligible alterations across all parameters, compatible with relatively limited morphological impact.

CONCLUSIONS

This proposed model holds promise in enabling patient-specific simulations of the kidney cortex, thus facilitating exploration into how pathologies altering cortical morphology correlates to modifications in SWE-derived rheological measurements. We demonstrated that inflammation caused substantial changes in measured mechanical properties.

Shear wave elastography in chronic kidney disease - the physics and clinical application

Chronic kidney disease is a leading public health problem worldwide. The global prevalence of chronic kidney disease is nearly five hundred million people, with almost one million deaths worldwide. Estimated glomerular filtration rate, imaging such as conventional ultrasound, and histopathological findings are necessary as each technique provides specific information which, when taken together, may help to detect and arrest the development of chronic kidney disease, besides managing its adverse outcomes. However, estimated glomerular filtration rate measurements are hampered by substantial error margins while conventional ultrasound involves subjective assessment. Although histopathological assessment is the best tool for evaluating the severity of the renal pathology, it may lead to renal insufficiency and haemorrhage if complications occurred. Ultrasound shear wave elastography, an emerging imaging that quantifies tissue stiffness non-invasively has gained interest recently. This method applies acoustic force pulses to generate shear wave within the tissue that propagate perpendicular to the main ultrasound beam. By measuring the speed of shear wave propagation, the tissue stiffness is estimated. This paper reviews the literature and presents our combined experience and knowledge in renal shear wave elastography research. It discusses and highlights the confounding factors on shear wave elastography, current and future possibilities in ultrasound renal imaging and is not limited to new sophisticated techniques.

Shear wave elastography parameters adds prognostic value to adverse outcome in kidney transplantation recipients

INTRODUCTION

The tissue stiffness of donor kidneys in transplantation may increase due to pathological changes such as glomerulosclerosis and interstitial fibrosis, and those changes associate worse outcomes in kidney transplantation recipients. Ultrasound elastography is a noninvasive imaging examination with the ability to quantitatively reflect tissue stiffness. Aim of this study was to evaluate the prognostic value of ultrasound elastography for adverse kidney outcome in kidney transplantation recipients.

METHODS

Shear wave elastography (SWE) examinations were performed by two independent operators in kidney transplantation recipients. The primary outcome was a composite of kidney graft deterioration, all-cause re-hospitalization, and all-cause mortality. Survival analysis was calculated by Kaplan-Meier curves with the log-rank test and Cox regression analysis.

RESULTS

A total of 161 patients (mean age 46 years, 63.4% men) were followed for a median of 20.1 months. 27 patients (16.77%) reached the primary endpoint. The mean and median tissue stiffness at the medulla (hazard ratio: 1.265 and 1.229, respectively), estimated glomerular filtration rate (eGFR), and serum albumin level were associated with the primary outcome in univariate Cox regression. Adding mean or median medulla SWE to a baseline model containing eGFR and albumin significantly improved its discrimination (C-statistics: 0.736 for the baseline, 0.766 and 0.772 for the model added mean and median medulla SWE, respectively).

CONCLUSION

The medullary tissue stiffness of kidney allograft measured by shear wave elastography may provide incremental prognostic value to adverse outcomes in kidney transplantation recipients. Including SWE parameters in kidney transplantation recipients management could be considered to improve risk stratification.

Application of Real-Time Sound Touch Elastography for Evaluating Chronic Kidney Disease of Transplanted Kidneys

BACKGROUND

If chronic allograft nephropathy can be detected early and treated, the long-term survival rate of the transplanted kidney may be effectively improved.

PURPOSE

To compare the application value of real-time sound touch elastography (STE), strain elastography, and color Doppler flow imaging in evaluating chronic kidney disease of transplanted kidneys.

MATERIALS AND METHODS

A total of 101 patients with renal transplantation were divided into a normal group (serum creatinine <134 mol/L, 58 patients) and a chronic allograft nephropathy group after renal transplantation over 6 months (serum creatinine >134 mol/L, 43 patients). The maximum elastic modulus (Emax) was determined, and receiver operator characteristics were used to compare the diagnostic efficacy of STE ultrasound.

RESULTS

Emean, Emax, B/A (the strain rate of the internal oblique muscle tissue/ the strain rate of the central renal cortex) of cortical standard strain ratio in strain elastography, and resistance index (RI) between normal and chronic allograft nephropathy groups have statistical significance (P < .05). Emax is superior to B/A and arcuate artery RI in the chronic cortex in the diagnosis of renal dysfunction, and the area under the receiver operator characteristics curve is 0.88. The estimated glomerular filtration rate was negatively correlated with renal cortex Emax, B/A, and arcuate artery RI, among which Emax was the strongest (r = - 0.713, P < .001). The renal cortical Emax cut-off was 30.95 kPa, the sensitivity was 92%, the specificity was 88%, and the accuracy was 88%.

CONCLUSION

The STE technique to evaluate chronic renal dysfunction after renal transplantation is more sensitive than traditional strain-type elastography and hemodynamic parameters, with renal function decline, renal cortex Emax, renal cortical B/A, and arcuate artery RI gradually increased, and renal cortex Emax was particularly obvious.

Imaging fibrosis in pediatric kidney transplantation: A pilot study

BACKGROUND

Noninvasive alternatives to biopsy for assessment of interstitial fibrosis and tubular atrophy (IFTA), the major determinant of kidney transplant failure, remain profoundly limited. Elastography is a noninvasive technique that propagates shear waves across tissues to measure their stiffness. We aimed to test utility of elastography for early detection of IFTA in pediatric kidney allografts.

METHODS

We compared ultrasound (USE) and MR elastography (MRE) stiffness measurements, performed on pediatric transplant recipients referred for clinically indicated biopsies, and healthy controls.

RESULTS

Ten transplant recipients (median age 16 years) and eight controls (median age 16.5 years) were enrolled. Three transplant recipients had "stable" allografts and seven had Banff Grade 1 IFTA. Median time from transplantation to biopsy was 12 months. Mean estimated glomerular filtration rate was 61.5 mL/min/1.73m2 by creatinine-cystatin-C CKiD equation at time of biopsy. Mean stiffness, calculated through one-way ANOVA, was higher for IFTA allografts (23.4 kPa USE/5.6 kPa MRE) than stable allografts (13.7 kPa USE/4.4 kPa MRE) and controls (9.1 kPa USE/3.6 kPa MRE). Pearson's coefficient between USE and MRE stiffness values was strong (r = .97). AUC for fibrosis prediction in transplanted kidneys was high for both modalities (0.91 USE and 0.89 MRE), although statistically nonsignificant (p > .05). Stiffness cut-off values for USE and MRE were 13.8 kPa and 4.6 kPa, respectively. Both values yielded a sensitivity of 100% but USE specificity (72%) was slightly higher than MRE (67%).

CONCLUSION

Elastography shows potential for detection of low-grade IFTA in allografts although a larger sample is imperative for clinical validation.

3D hypoxia-mimicking and anti-synechia hydrogel enabling promoted neovascularization for renal injury repair and regeneration

In-situ renal tissue engineering is promising yet challenging for renal injury repair and regeneration due to the highly vascularized structure of renal tissue and complex high-oxidative stress and ischemic microenvironment. Herein, a novel biocompatible 3D porous hydrogel (DFO-gel) with sustained release capacity of hypoxia mimicking micromolecule drug deferoxamine (DFO) was developed for in-situ renal injury repair. In vitro and in vivo experimental results demonstrated that the developed DFO-gels can exert the synchronous benefit of scavenging excess reactive oxygen species (ROS) regulating inflammatory microenvironment and promoting angiogenesis for effective renal injury repair by up-regulating hypoxia-inducible factor-1 alpha (HIF-1α) and vascular endothelial growth factor (VEGF). The in-situ neogenesis of neonatal glomerular- and tubular-like structures in the implanted areas in the partially nephrectomized rats also suggested the potential for promoting renal injury repair and regeneration. This multifunctional hydrogel can not only exhibit the sustained release and promoted bio-uptake capacity for DFO, but also improve the renal injured microenvironment by alleviating oxidative and inflammatory stress, accelerating neovascularization, and promoting efficient anti-synechia. We believe this work offers a promising strategy for renal injury repair and regeneration.

Ultrasound elastography in chronic kidney disease: a systematic review and meta-analysis

Ultrasound elastography (USE) is a noninvasive technique for assessing tissue elasticity, and its application in nephrology has aroused growing interest in recent years. The purpose of this article is to systematically review the clinical application of USE in patients with chronic kidney disease (CKD), including native and transplanted kidneys, and quantitatively investigate differences in elasticity values between healthy individuals and CKD patients. Furthermore, we provide a qualitative analysis of the studies included, discussing the potential interplay between renal stiffness, estimated glomerular filtration rate, and fibrosis. In January 2022, a systematic search was carried out on the MEDLINE (PubMed) database, concerning studies on the application of USE in patients with CKD, including patients with transplanted kidneys. The results of the included studies were extracted by two independent researchers and presented mainly through a formal narrative summary. A meta-analysis of nine study parts from six studies was performed. A total of 647 studies were screened for eligibility and, after applying the exclusion and inclusion criteria, 69 studies were included, for a total of 6728 patients. The studies proved very heterogeneous in terms of design and results. The shear wave velocity difference of - 0.82 m/s (95% CI: - 1.72-0.07) between CKD patients and controls was not significant. This result agrees with the qualitative evaluation of included studies that found controversial results for the relationship between renal stiffness and glomerular filtration rate. On the contrary, a clear relationship seems to emerge between USE values and the degree of fibrosis. At present, due to the heterogeneity of results and technical challenges, large-scale application in the monitoring of CKD patients remains controversial.

Shear wave elastography using high-frequency linear probe for transplant kidney monitoring: A methodological study

OBJECTIVES

To investigate the influencing factors of the image quality of shear wave elastography (SWE) performed using a high-frequency probe and its reproducibility for renal allografts.

METHODS

A total of 211 patients with transplanted kidneys who underwent SWE examination performed using high-frequency or low-frequency probes were recruited for the study. The reproducibility of inter- and intraobserver agreements were analysed by using the intraclass correlation coefficient (ICC). According to the colour filling of the area of interest and imaging noise when conducting SWE, the image quality was classified as three grades: "good", "common", and "poor". A logistic regression was used to analyse the independent factors for SWE quality.

RESULTS

In the comparative analysis, high frequency, transection measurement and middle pole were selected as the appropriate measurement methods. Regarding reproducibility, the ICCs) of the intra- and interobserver agreements were 0.85 and 0.77, respectively. Multivariate analysis indicated that only the skin allograft distance and kidney width were independent variables for SWE quality. In the subgroup analysis of the skin-allograft distance, the "good" and "common" rates of images decreased as the distance increased, but the CV (coefficients of variation) showed the opposite trend. The SWE quality of kidney width <5.4 cm was significantly better than that of kidney width ≥5.4 cm.

CONCLUSIONS

High-frequency SWE can be used in the evaluation of transplanted kidneys due to its good repeatability and high successful measurement rate, but we should pay attention to the influence of the skin-allograft distance and kidney width on SWE quality.

Measurement of renal congestion and compliance following intravenous fluid administration using shear wave elastography

Objective

Ultrasound shear wave elastography (SWE) is a novel technique that may provide non-invasive measurements of renal compliance. We aimed to investigate the relationship between intravenous (IV) fluid administration and change in SWE measurements. We hypothesised that following IV fluid administration in healthy volunteers, global kidney stiffness would increase and that this increase in stiffness could be quantified using SWE. Our second hypothesis was that graduated doses of IV fluids would result in a dose-dependent increase in global kidney stiffness measured by SWE.

Design

Randomised prospective study.

Setting

Intensive Care Unit.

Participants

Healthy volunteers aged 18-40 years.

Interventions

Participants were randomised to receive 20 ml/kg, 30 ml/kg, or 40 ml/kg of normal saline. The volume of fluid infused was based on the actual body weight recorded.

Main outcome measures

We recorded average SWE stiffness (kPa with standard deviation of the mean), median SWE stiffness (kPa), and the interquartile range.

Results

Ninety-eight percent of participants (44/45) demonstrated an increase in global kidney stiffness following administration of IV fluids. The average SWE pre fluid administration was 7.572 kPa ± 2.38 versus 14.9 kPa ± 4.81 post fluid administration (p < 0.001). In subgroup analysis, there were significant changes in global kidney stiffness pre and post fluid administration with each volume (ml/kg) of fluid administered. Average percentage change in global kidney stiffness from baseline was compared between the three groups. There was no significant difference when comparing groups 1 and 2 (197.1% increase ± 49.5 vs 216.1% ± 72.0, p ¼ 0.398), groups 2 and 3 (216.1% increase ± 72.0 vs 197.8% ± 59.9, p ¼ 0.455), or groups 1 and 3 (197.1% increase ± 49.5 vs 197.8% ± 59.9, p ¼ 0.972).

Conclusions

Fluid administration results in immediately visible and quantifiable changes in global kidney stiffness across all infused volumes of fluid.

Nomogram based on high-frequency shear wave elastography (SWE) to evaluate chronic changes after kidney transplantation

OBJECTIVES

To construct a nomogram with high-frequency shear wave elastography (SWE) as a noninvasive method to accurately assess chronic changes in renal allografts.

METHODS

A total of 191 renal transplantation patients (127 cases in the training group and 64 cases in the verification group) were included in this study. All patients received conventional ultrasound and high-frequency SWE examination, followed directly by biopsy the next day. The chronic changes were divided into mild, moderate, and severe. Multivariate logistic analyses were used to select significant variables, which were used to develop the nomogram. Nomogram models were assessed by receiver operating characteristic (ROC) curves, calibration plots, and decision curve analysis (DCA).

RESULTS

The cutoff value of SWE in mild, moderate, and severe chronic changes was 18.9, 22.5, and 27.6 kPa, respectively. The areas under the curve (AUCs) of SWE in the differential diagnosis of mild and moderate to severe chronic changes and mild to moderate and severe chronic changes were 0.817 and 0.870, respectively. Multivariate analysis showed that time since transplantation, proteinuria, glomerular filtration rate, echogenicity, and SWE were independent diagnostic factors for moderate to severe chronic changes (all p < 0.05); thus, a nomogram was successfully developed. The AUCs of the nomogram in the training and validation groups were 0.905 and 0.938, respectively. The high agreement between the model predictions and the actual observations was confirmed by calibration plot and DCA.

CONCLUSIONS

Based on SWE, the nomogram provided an insightful and applicable tool to evaluate chronic changes in renal allografts.

KEY POINTS

• In kidney transplantation, compared with acute changes, chronic changes are significantly correlated with cortical stiffness. • SWE shows good performance in identifying mild to moderate and severe chronic changes, with an AUC of 0.870. • Time since transplantation, proteinuria, glomerular filtration rate, echogenicity, and SWE are independent diagnostic factors for moderate to severe chronic changes in renal allografts.

Effects of body parameters on renal cortical stiffness measured by shear-wave elastography in patients with kidney transplantation

OBJECTIVES

The results of elastic imaging in evaluating the function and histopathological changes of allogeneic renal transplantation are contradictory, one of the important reasons may be that there are differences in human parameters related to kidney transplantation among individuals. The purpose of this study is to explore the related human body parameters on shear-wave elastography (SWE) effects on quantitative stiffness of graft cortex.

METHODS

From March 2021 to November 2021, a total of 63 patients with allogeneic kidney transplantation in the Department of Ultrasonography, Third Xiangya Hospital, Central South University, were selected to collect the parameters of two-dimensional, color Doppler and SWE. The subjects were divided into a <20% group and a 20%-30% group according to the variation of cortical hardness measurement. Mann Whitney U test was used to compare the differences in relevant human parameters, and Spearman rank correlation was used to analyze the correlation between relevant human parameters and cortical hardness of transplanted kidney.

RESULTS

There was no significant difference between the 2 groups in age, sex, postoperative time, resistance index of interlobar artery, SCr, blood uric acid, ratio of fat layer to muscle layer, and BMI (all P>0.05). Compared with the <20% group, the patients in the 20%-30% group had smaller cortical hardness of the transplanted kidney, greater total distance between the transplanted kidney and the skin surface, and thicker fat layer or muscle layer in front of the transplanted kidney (all P<0.05). The age of patients, the total distance from the transplanted kidney to the skin surface, the thickness of fat layer and muscle layer, the ratio of fat layer to muscle layer, BMI, and the variation of cortical hardness were significantly negatively correlated with the cortical hardness of the transplanted kidney (all P<0.05).

CONCLUSIONS

Human parameters relevant to kidney transplantation affect the accuracy of SWE in measuring the cortical hardness of the transplanted kidney. It is very important to obtain the highly stabile elastic measurement value and interpret the elastic measurement results according to different levels of human body related parameters in combination with individual conditions.

Precision Imaging Guidance in the Era of Precision Oncology: An Update of Imaging Tools for Interventional Procedures

Interventional oncology (IO) procedures have become extremely popular in interventional radiology (IR) and play an essential role in the diagnosis, treatment, and supportive care of oncologic patients through new and safe procedures. IR procedures can be divided into two main groups: vascular and non-vascular. Vascular approaches are mainly based on embolization and concomitant injection of chemotherapeutics directly into the tumor-feeding vessels. Percutaneous approaches are a type of non-vascular procedures and include percutaneous image-guided biopsies and different ablation techniques with radiofrequency, microwaves, cryoablation, and focused ultrasound. The use of these techniques requires precise imaging pretreatment planning and guidance that can be provided through different imaging techniques: ultrasound, computed tomography, cone-beam computed tomography, and magnetic resonance. These imaging modalities can be used alone or in combination, thanks to fusion imaging, to further improve the confidence of the operators and the efficacy and safety of the procedures. This article aims is to provide an overview of the available IO procedures based on clinical imaging guidance to develop a targeted and optimal approach to cancer patients.

Shear wave elastography and Doppler ultrasound in kidney transplant recipients

Objective

To evaluate the association between shear wave elastography parameters and arterial resistance in kidney transplant recipients.

Materials and Methods

This was a prospective cross-sectional study involving consecutive adult kidney transplant recipients. All patients underwent color Doppler to evaluate the resistive index (RI) and ultrasound shear wave elastography for the quantification of renal allograft stiffness.

Results

We evaluated 55 patients, of whom 9 (16.4%) had an RI defined as abnormal (≥ 0.79) and 46 (83.6%) had an RI defined as normal (< 0.79). The mean age was higher in the abnormal RI group than in the normal RI group (68.0 ± 8.6 years vs. 42.6 ± 14.1 years; p < 0.001), as was the mean shear wave velocity (2.6 ± 0.4 m/s vs. 2.2 ± 0.4 m/s; p = 0.013). Multivariate analysis identified two independent predictors of arterial resistance: age (OR = 1.169; 95% CI: 1.056 to 1.294; p = 0.003) and shear wave velocity (OR = 17.1; 95% CI: 1.137 to 257.83; p = 0.040).

Conclusion

We observed an association between rigidity in the cortex of the transplanted kidney, as evaluated by shear wave elastography, and arterial resistance, as evaluated by color Doppler, in kidney transplant recipients.

Color Doppler Ultrasound and Hemodynamics for Evaluating Graft Dysfunction in Recurrent Immunoglobulin A Nephropathy

BACKGROUND The aim of this study was to investigate the diagnostic and prognostic utility of color Doppler ultrasound for graft dysfunction in recurrent immunoglobulin A nephropathy (IgAN). MATERIAL AND METHODS We selected a series of 78 biopsies diagnostic of recurrent IgAN following living-donor transplantation from July 2004 to January 2019. Based on Lee's classification, Doppler parameters in different degrees of histopathological injury were retrospectively analyzed. RESULTS The 4-year cumulative graft survival rate after biopsy was 66.3%, and the difference among the Kaplan-Meier curves of Lee's classification (P<0.01) was significant. Doppler parameters showed that echo enhancement, decreasing blood flow distribution, decreasing end-diastolic velocity (EDV) of the main renal artery (MRA), segmental renal atery (SRA) and interlobar renal artery (IRA), and an elevated resistance index (RI) of the arcuate renal artery (ARA) were significantly different among grades I-V of Lee's classification (P<0.05). Logistic multivariate analysis indicated that echo enhancement (HR 13.6, 95% CI 2.7-68.4) and decreasing EDV of the SRA (HR 1.1 for a 1-cm/s, 95% CI 1.0-1.2) were independent predictors of severe injury (IV-V). The ROC curve fitted by echo enhancement and decreasing EDV of the SRA had an area under the curve of 0.87. The cutoff was 17.5 cm/s (decreasing EDV of the SRA) without echo enhancement. The sensitivity and specificity were 72.2% and 91.7%, respectively. CONCLUSIONS Color Doppler ultrasound successfully evaluated the graft dysfunction in recurrent IgAN; a decreasing EDV of the SRA indicated severe histopathological injury and poor prognosis.

Novel Uses of Ultrasound to Assess Kidney Mechanical Properties

Ultrasound is a key imaging tool for evaluating the kidney. Over the last two decades, methods to measure the mechanical properties of soft tissues have been developed and used in clinical practice, although use in the kidney has not been as widespread as for other applications. The mechanical properties of the kidney are determined by the structure and composition of the renal parenchyma and perfusion characteristics. Because pathologic processes change these factors, the mechanical properties change and can be used for diagnostic purposes and for monitoring treatment or disease progression. Ultrasound-based elastography methods for evaluating the mechanical properties of the kidney use focused ultrasound beams to perturb the kidney and then high frame-rate ultrasound methods are used to measure the resulting motion. The motion is analyzed to estimate the mechanical properties. This review will describe the principles of these methods and discuss several seminal studies related to characterizing the kidney. Additionally, an overview of the clinical use of elastography methods in native and kidney allografts will be provided. Perspectives on future developments and uses of elastography technology along with other complementary ultrasound imaging modalities will be provided.