Magnetic resonance elastography of the kidneys: feasibility and reproducibility in young healthy adults

Development of a suitable vibration pad for renal MR elastography

The aim of this study was to develop a vibration pad suitable for renal MR elastography (MRE). Chronic kidney disease (CKD) is a progressive condition affecting >800 million people worldwide. Renal fibrosis is a common pathological feature of CKD that causes fibrotic regions to be much stiffer than those in normal renal tissues. Therefore, MRE can be used to diagnose CKD because it can image organ stiffness. In MRE, the shear modulus is obtained from the wavelength of the shear waves. Therefore, it is highly important to propagate shear waves with sufficient vibration strength in the tissue. By using a three-dimensional (3D) printer, we created a "Flexible Pad" suitable for renal MRE. The Flexible Pad was placed under the back of the participant in the supine position and deformed in response to the participant's weight, adhering closely to the body surface. Six healthy volunteers participated in this study. Our Flexible Pad allowed for coherent shear waves (clear waves with little scattering and interference) to be efficiently transmitted to the kidney deep-lying tissues in the abdomen. The shear moduli of the kidney (n = 6) were 8.95 ± 0.84 kPa in the right kidney and 9.70 ± 0.99 kPa in the left kidney. Our results indicate that using our Flexible Pad for renal MRE can provide a more reliable measurement of renal shear modulus.

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.

Magnetic resonance elastography resolving all gross anatomical segments of the kidney during controlled hydration

Introduction: Magnetic resonance elastography (MRE) is a non-invasive method to quantify biomechanical properties of human tissues. It has potential in diagnosis and monitoring of kidney disease, if established in clinical practice. The interplay of flow and volume changes in renal vessels, tubule, urinary collection system and interstitium is complex, but physiological ranges of in vivo viscoelastic properties during fasting and hydration have never been investigated in all gross anatomical segments simultaneously. Method: Ten healthy volunteers underwent two imaging sessions, one following a 12-hour fasting period and the second after a drinking challenge of >10 mL per kg body weight (60-75 min before the second examination). High-resolution renal MRE was performed using a novel driver with rotating eccentric mass placed at the posterior-lateral wall to couple waves (50 Hz) to the kidney. The biomechanical parameters, shear wave speed (cs in m/s), storage modulus (Gd in kPa), loss modulus (Gl in kPa), phase angle ( Υ = 2 π atan G l G d ) and attenuation (α in 1/mm) were derived. Accurate separation of gross anatomical segments was applied in post-processing (whole kidney, cortex, medulla, sinus, vessel). Results: High-quality shear waves coupled into all gross anatomical segments of the kidney (mean shear wave displacement: 163 ± 47 μm, mean contamination of second upper harmonics <23%, curl/divergence: 4.3 ± 0.8). Regardless of the hydration state, median Gd of the cortex and medulla (0.68 ± 0.11 kPa) was significantly higher than that of the sinus and vessels (0.48 ± 0.06 kPa), and consistently, significant differences were found in cs, Υ , and Gl (all p < 0.001). The viscoelastic parameters of cortex and medulla were not significantly different. After hydration sinus exhibited a small but significant reduction in median Gd by -0.02 ± 0.04 kPa (p = 0.01), and, consequently, the cortico-sinusoidal-difference in Gd increased by 0.04 ± 0.07 kPa (p = 0.05). Only upon hydration, the attenuation in vessels became lower (0.084 ± 0.013 1/mm) and differed significantly from the whole kidney (0.095 ± 0.007 1/mm, p = 0.01). Conclusion: High-resolution renal MRE with an innovative driver and well-defined 3D segmentation can resolve all renal segments, especially when including the sinus in the analysis. Even after a prolonged hydration period the approach is sensitive to small hydration-related changes in the sinus and in the cortico-sinusoidal-difference.

Magnetic Resonance Imaging in Clinical Trials of Diabetic Kidney Disease

Chronic kidney disease (CKD) associated with diabetes mellitus (DM) (known as diabetic kidney disease, DKD) is a serious and growing healthcare problem worldwide. In DM patients, DKD is generally diagnosed based on the presence of albuminuria and a reduced glomerular filtration rate. Diagnosis rarely includes an invasive kidney biopsy, although DKD has some characteristic histological features, and kidney fibrosis and nephron loss cause disease progression that eventually ends in kidney failure. Alternative sensitive and reliable non-invasive biomarkers are needed for DKD (and CKD in general) to improve timely diagnosis and aid disease monitoring without the need for a kidney biopsy. Such biomarkers may also serve as endpoints in clinical trials of new treatments. Non-invasive magnetic resonance imaging (MRI), particularly multiparametric MRI, may achieve these goals. In this article, we review emerging data on MRI techniques and their scientific, clinical, and economic value in DKD/CKD for diagnosis, assessment of disease pathogenesis and progression, and as potential biomarkers for clinical trial use that may also increase our understanding of the efficacy and mode(s) of action of potential DKD therapeutic interventions. We also consider how multi-site MRI studies are conducted and the challenges that should be addressed to increase wider application of MRI in DKD.

A diagnostic test of three-dimensional magnetic resonance elastography imaging for preoperative prediction of microvascular invasion in patients with T1 stage clear cell renal carcinoma

Background

Detection of microvascular invasion (MVI) of kidney tumors is important for selecting the optimal therapeutic strategy. Currently, the prediction of MVI lacks an accurate imaging biomarker. This study evaluated the performance of three-dimensional (3D) magnetic resonance elastography (MRE) imaging in predicting microvascular invasion (MVI) of T1 stage clear cell renal carcinoma (ccRCC).

Methods

In this prospective study, we conducted pre-surgical imaging with a clinical 3.0 T magnetic resonance imaging (MRI) system. Firstly, 83 consecutive patients were enrolled in this study. A 3D MRE stiffness map was generated and transferred to a post-processing workstation. Contrast-enhanced computed tomography (CT) was conducted to calculate the tumor enhancement ratio. The presence of MVI was evaluated by histopathological analysis and graded according to the risk stratification based upon the number and distribution. The mean stiffness and CT tumor enhancement ratio was calculated for tumors with or without MVI. The diagnostic performance [sensitivity, specificity, positive predictive value, negative predictive value, area under the curve (AUC)] and independent predicting factors for MVI were investigated.

Results

Finally, A total of 80 patients (aged 46.7±13.2 years) were enrolled, including 22 cases of tumors with MVI. The mean MRE stiffness of kidney parenchyma and kidney tumors was 4.8±0.2 and 4.5±0.7 kPa, respectively. There was significant difference in the mean MRE stiffness between tumors with MVI (5.4±0.6 kPa) and tumors without MVI (4.1±0.3 kPa) (P<0.05). The sensitivity, specificity, positive predictive value, negative predictive value, and the AUC for mean stiffness in the prediction of MVI were 100%, 75%, 63%, 96%, and 0.87 [95% confidence interval (CI): 0.72, 0.94], respectively. The corresponding values for the CT tumor enhancement ratio were 90%, 80%, 63%, 96%, and 0.88 (95% CI: 0.71, 0.93), respectively. The odds ratio (OR) value for MRE tumor stiffness and CT kidney tumor enhancement ratio in the prediction of MVI was 2.9 (95% CI: 1.8, 3.7) and 1.2 (95% CI: 1.0, 1.7), respectively (P>0.05).

Conclusions

3D MRE imaging has promising diagnostic performance for predicting MVI in T1 stage ccRCC, which may improve the reliability of surgical strategy selection with T1 stage ccRCC.

Magnetic Resonance Elastography as Surrogate Marker of Interstitial Fibrosis in Kidney Transplantation: A Prospective Study

Background

Fibrosis progression is a major prognosis factor in kidney transplantation. Its assessment requires an allograft biopsy, which remains an invasive procedure at risk of complications.

Methods

We assessed renal stiffness by magnetic resonance elastography (MRE) as a surrogate marker of fibrosis in a prospective cohort of kidney transplant recipients compared with the histologic gold standard. Interstitial fibrosis was evaluated by three methods: the semi-quantitative Banff ci score, a visual quantitative evaluation by a pathologist, and a computer-assisted quantitative evaluation. MRE-derived stiffness was assessed at the superior, median, and inferior poles of the allograft.

Results

We initially enrolled 73 patients, but only 55 had measurements of their allograft stiffness by MRE before an allograft biopsy. There was no significant correlation between MRE-derived stiffness at the biopsy site and the ci score (ρ=-0.25, P=0.06) or with the two quantitative assessments (pathologist: ρ=-0.25, P=0.07; computer assisted: ρ=-0.21, P=0.12). We observed negative correlations between the stiffness of both the biopsy site and the whole allograft, with either the glomerulosclerosis percentage (ρ=-0.32, P=0.02 and ρ=-0.31, P=0.02, respectively) and the overall nephron fibrosis percentage, defined as the mean of the percentages of glomerulosclerosis and interstitial fibrosis (ρ=-0.30, P=0.02 and ρ=-0.28, P=0.04, respectively). At patient level, mean MRE-derived stiffness was similar across the three poles of the allograft (±0.25 kPa). However, a high variability of mean stiffness was found between patients, suggesting a strong influence of confounding factors. Finally, no significant correlation was found between mean MRE-derived stiffness and the slope of eGFR (P=0.08).

Conclusions

MRE-derived stiffness does not directly reflect the extent of fibrosis in kidney transplantation.

Advanced non-invasive diagnostic techniques for visualization and estimation of kidney fibrosis

Kidney fibrosis is marked by excessive extracellular matrix deposition during disease progression. Unfortunately, existing kidney function parameters do not predict the extent of kidney fibrosis. Moreover, the traditional histology methods for the assessment of kidney fibrosis require liquid and imaging biomarkers as well as needle-based biopsies, which are invasive and often associated with kidney injury. The repetitive analyses required to monitor the disease progression are therefore difficult. Hence, there is an unmet medical need for non-invasive and informative diagnostic approaches to monitor kidney fibrosis during the progression of chronic kidney disease. Here, we summarize the modern advances in diagnostic imaging techniques that have shown promise for non-invasive estimation of kidney fibrosis in pre-clinical and clinical studies.

Evaluation of Renal Fibrosis by Mapping Histology and Magnetic Resonance Imaging

Background

Renal fibrosis is a key driver of progression in chronic kidney disease (CKD). Recent advances in diagnostic imaging techniques have shown promising results for the noninvasive assessment of renal fibrosis. However, the specificity and accuracy of these techniques are controversial because they indirectly assess renal fibrosis. This limits fibrosis assessment by imaging in CKD for clinical practice. To validate magnetic resonance imaging (MRI) assessment for fibrosis, we derived representative models by mapping histology-proven renal fibrosis and imaging in CKD.

Methods

Ninety-seven adult Chinese CKD participants with histology were studied. The kidney cortex interstitial extracellular matrix volume was calculated by the Aperio ScanScope system using Masson's trichrome slices. The kidney cortex microcirculation was quantitatively assessed by peritubular capillary density using CD34 staining. The imaging techniques included intravoxel incoherent motion diffusion-weighted imaging and magnetic resonance elastography (MRE) imaging. Relevant analyses were performed to evaluate the correlations between MRI parameters and histology variables. Multiple linear regression models were used to describe the relationships between a response variable and other variables. The best-fit lines, which minimize the sum of squared residuals of the multiple linear regression models, were generated.

Results

MRE values were negatively associated with the interstitial extracellular matrix volume (Rho = -0.397, p < 0.001). The best mapping model of extracellular matrix volume with the MRE value and estimated glomerular filtration rate (eGFR) we obtained was as follows: Interstitial extracellular matrix volume = 218.504 - 14.651 × In(MRE) - 18.499 × In(eGFR). DWI-fraction values were positively associated with peritubular capillary density (Rho = 0.472, p < 0.001). The best mapping model of peritubular capillary density with DWI-fraction value and eGFR was as follows: Peritubular capillaries density = 17.914 + 9.403 × (DWI - fraction) + 0.112 × (eGFR).

Conclusions

The study provides histological evidence to support that MRI can effectively evaluate fibrosis in the kidney. These findings picture the graphs of the mapping model from imaging and eGFR into fibrosis, which has significant value for clinical implementation.

Uterine leiomyomas: correlation between histologic composition and stiffness via magnetic resonance elastography - a Pilot Study

OBJECTIVES

To evaluate magnetic resonance elastography as a tool for characterizing uterine leimyomas.

MATERIAL AND METHODS

At total of 12 women with symptomatic leiomyomas diagnosed in physical and ultrasound examinations were enrolled in this pilot study. Before surgery, all patients underwent magnetic resonance elastography of the uterus using a 1.5 T MR whole-body scanner (Optima, GE Healthcare, Milwaukee, WI, USA). Surgical specimens were forwarded for histological examination. The findings were allocated into 3 categories depending on the percentage content of connective tissue: below 15%, from 15 to 30% and more than 30%. The median stiffness of leiomyomas for each of the group was calculated. The U-Mann Whitney test was used for statistical analysis.

RESULTS

The stiffness of the leiomyomas ranged between 3.7-6.9 kPa (median value 4.9 kPa). The concentration of extracellular components in the leiomyomas did not exceed 40%. An increasing trend of the stiffness with the growing percentage of extracellular component was observed. Stiffness of the leiomyomas obtained by MRE varies depending on microscopic composition.

CONCLUSIONS

The value of stiffness shows a trend of increasing with the percentage of extracellular component of the leiomyoma. Further studies are required to assess the usefulness of MRE in diagnostics of uterine leiomyomas.

Nonlinear Inversion MR Elastography With Low-Frequency Actuation

Magnetic resonance elastography (MRE) has been developed to noninvasively reconstruct mechanical properties for tissue and tissue-like materials over a frequency range of 10 ~200 Hz. In this work, low frequency (1~1.5 Hz) MRE activations were employed to estimate mechanical property distributions of simulated data and experimental phantoms. Nonlinear inversion (NLI) MRE algorithms based on viscoelastic and poroelastic material models were used to solve the inverse problems and recover images of the shear modulus and hydraulic conductivity. Data from a simulated phantom containing an inclusion with property contrast was carried out to study the feasibility of our low frequency actuated approach. To verify the stability of NLI algorithms for low frequency actuation, different levels of synthetic noise were added to the displacement data. Spatial distributions and property values were recovered well for noise level less than 5%. For the presented experimental phantom reconstructions with regularizations, the computed storage moduli from viscoelastic and poroelastic MRE gave similar results. Contrast was detected between inclusions and background in recovered hydraulic conductivity images. Results and findings confirm the feasibility of future in vivo neuroimaging examinations using natural cerebrovascular pulsations at cardiac frequencies, which can eliminate specialized equipment for high frequency actuation.

Magnetic resonance elastography-derived stiffness of the kidneys and its correlation with water perfusion

Stiffness plays an important role in diagnosing renal fibrosis. However, kidney stiffness is altered by perfusion changes in many kidney diseases. Therefore, the aim of the current study is to determine the correlation of kidney stiffness with water intake. We hypothesize that kidney stiffness will increase with 1 L of water intake due to increased water perfusion to the kidneys. Additionally, stiffness of the kidneys will correlate with apparent diffusion coefficient (ADC) and fractional anisotropy (FA) values before and after water intake. A 3 T MRI scanner was used to perform magnetic resonance elastography and diffusion tensor imaging of the kidneys on 24 healthy subjects (age range: 22-66 years) before and after water intake of 1 L. A 3D T1-weighted bladder scan was also performed to measure bladder volume before and after water intake. A paired t-test was performed to evaluate the effect of water intake on the stiffness of kidneys, in addition to bladder volume. A Spearman correlation test was performed to determine the association between stiffness, bladder volume, ADC and FA values of both kidneys before and after water intake. The results show a significant increase in stiffness in different regions of the kidney (ie, percentage increase ranged from 3.6% to 7.5%) and bladder volume after water intake (all P < 0.05). A moderate significant negative correlation was observed between change in kidney stiffness and bladder volume (concordance correlation coefficient = -0.468, P < 0.05). No significant correlation was observed between stiffness and ADC or FA values before and after water intake in both kidneys (P > 0.05). Water intake caused a significant increase in the stiffness of the kidneys. The negative correlation between the change in kidney stiffness and bladder volume, before and after water intake, indicates higher perfusion pressure in the kidneys, leading to increased stiffness.

Functional MRI as a Tool for Evaluating Interstitial Fibrosis and Prognosis in Kidney Disease

Background

Kidney fibrosis is a key driver of progression of kidney diseases. Renal biopsies remain the gold-standard approach to specifically diagnose and stage renal fibrosis at present. However, there is a lack of multi-dimensional pictures showing renal function, histology, and imaging of the fibrotic kidney.

Summary

Magnetic resonance imaging (MRI) strategies have been rapidly evolving during the past couple of decades, especially driven by advances in technology. Recently, several pioneer and remarkable studies demonstrated that advanced functional MRI (fMRI) tools could be useful for the evaluation of kidney fibrosis and progression, which provides more opportunities to benefit from the significant value of fMRI tools for clinical implementation.

Key Messages

fMRI will be a novel approach to evaluate interstitial fibrosis and prognosis in kidney disease.

Magnetic Resonance Elastography of kidneys: SE-EPI MRE reproducibility and its comparison to GRE MRE

The purpose of this study is 1) to demonstrate reproducibility of spin echo-echo planar imaging (SE-EPI) magnetic resonance elastography (MRE) to estimate kidney stiffness; and 2) to compare SE-EPI MRE and gradient recalled echo (GRE) MRE-derived stiffness estimations in various anatomical regions of the kidney. Kidney MRE was performed on 33 healthy subjects (8 for SE-EPI MRE reproducibility and 25 for comparison with GRE MRE; age range: 22-66 years) in a 3 T MRI scanner. To demonstrate SE-EPI MRE reproducibility, subjects were scanned for the first scan and then asked to leave the scan room and repositioned again for the second (repeat) scan. Similar set-up was used for GRE MRE as well. The displacement data was then processed to obtain overall stiffness estimates of the kidney. Concordance correlation analyses were performed to determine SE-EPI MRE reproducibility and agreement between GRE MRE and SE-EPI MRE derived stiffness. A high concordance correlation (ρ_c  = 0.95; p-value<0.0001) was obtained for SE-EPI MRE reproducibility. Good concordance correlation was observed (ρ_c  = 0.84; p < 0.0001 for both kidneys, ρ_c  = 0.91; p < 0.0001 for right kidney and ρ_c  = 0.78; p < 0.0001 for left kidney) between GRE MRE and SE-EPI MRE derived stiffness measurements. Paired t-test results showed that stiffness value of medulla was significantly (p < 0.0001) greater than cortex using SE-EPI MRE as well as GRE MRE. SE-EPI MRE was reproducible and good agreement was observed in MRE-derived stiffness measurements obtained using SE-EPI and GRE sequences. Therefore, SE-EPI can be used for kidney MRE applications.

Technical recommendations for clinical translation of renal MRI: a consensus project of the Cooperation in Science and Technology Action PARENCHIMA

Purpose

The potential of renal MRI biomarkers has been increasingly recognised, but clinical translation requires more standardisation. The PARENCHIMA consensus project aims to develop and apply a process for generating technical recommendations on renal MRI.

Methods

A task force was formed in July 2018 focused on five methods. A draft process for attaining consensus was distributed publicly for consultation and finalised at an open meeting (Prague, October 2018). Four expert panels completed surveys between October 2018 and March 2019, discussed results and refined the surveys at a face-to-face meeting (Aarhus, March 2019) and completed a second round (May 2019).

Results

A seven-stage process was defined: (1) formation of expert panels; (2) definition of the context of use; (3) literature review; (4) collection and comparison of MRI protocols; (5) consensus generation by an approximate Delphi method; (6) reporting of results in vendor-neutral and vendor-specific terms; (7) ongoing review and updating. Application of the process resulted in 166 consensus statements.

Conclusion

The process generated meaningful technical recommendations across very different MRI methods, while allowing for improvement and refinement as open issues are resolved. The results are likely to be widely supported by the renal MRI community and thereby promote more harmonisation.

Noninvasive assessment of renal fibrosis by magnetic resonance imaging and ultrasound techniques

Renal fibrosis is a useful biomarker for diagnosis and guidance of therapeutic interventions of chronic kidney disease (CKD), a worldwide disease that affects more than 10% of the population and is one of the major causes of death. Currently, tissue biopsy is the gold standard for assessment of renal fibrosis. However, it is invasive, and prone to sampling error and observer variability, and may also result in complications. Recent advances in diagnostic imaging techniques, including magnetic resonance imaging (MRI) and ultrasonography, have shown promise for noninvasive assessment of renal fibrosis. These imaging techniques measure renal fibrosis by evaluating its impacts on the functional, mechanical, and molecular properties of the kidney, such as water mobility by diffusion MRI, tissue hypoxia by blood oxygenation level dependent MRI, renal stiffness by MR and ultrasound elastography, and macromolecule content by magnetization transfer imaging. Other MR techniques, such as T1/T2 mapping and susceptibility-weighted imaging have also been explored for measuring renal fibrosis. Promising findings have been reported in both preclinical and clinical studies using these techniques. Nevertheless, limited specificity, sensitivity, and practicality in these techniques may hinder their immediate application in clinical routine. In this review, we will introduce methodologies of these techniques, outline their applications in fibrosis imaging, and discuss their limitations and pitfalls.

New and Emerging Applications of Magnetic Resonance Elastography of Other Abdominal Organs

Increasing clinical experience and ongoing research in the field of magnetic resonance elastography (MRE) is leading to exploration of its applications in other abdominal organs. In this review, the current research progress of MRE in prostate, uterus, pancreas, spleen, and kidney will be discussed. The article will describe patient preparation, modified technical approach including development of passive drivers, modification of sequences, and inversion. The potential clinical application of MRE in the evaluation of several disease processes affecting these organs will be discussed. In an era of increasing adoption of multiparametric magnetic resonance imaging approaches for solving complex abdominal problems, abdominal MRE as a biomarker may be seamlessly incorporated into a standard magnetic resonance imaging examination to provide a rapid, reliable, and comprehensive imaging evaluation at a single patient appointment in the future.

Correlation of Point Shear Wave Velocity and Kidney Function in Chronic Kidney Disease

OBJECTIVES

Point shear wave elastography is a quantitative ultrasound-based imaging method used in the assessment of renal disease. Among point shear wave elastographic options, 2 techniques have been studied considerably: Virtual Touch quantification (VTQ; Siemens AG, Erlangen, Germany) and ElastPQ (EPQ; Philips Healthcare, Bothell, WA). Both rely on the tissue response to an acoustic beam generated by the ultrasound transducer. The data on renal VTQ are more extensive, whereas EPQ has been used less thus far in the assessment of the kidneys. This study aimed to evaluate the performance of EPQ in the kidney and compare it with VTQ.

METHODS

We studied 124 participants using EPQ: 22 with no renal disease and 102 with chronic kidney disease (CKD). Ninety-one were studied with both the EPQ and VTQ methods. We obtained 5 valid measurements in each kidney, expressed in meters per second.

RESULTS

The mean kidney stiffness measurements ± SD obtained with EPQ in the healthy control group were as follows: right kidney, 1.23 ± 0.33 m/s; and left kidney, 1.26 ± 0.32 m/s (P = .6). In the patients with CKD (all stages), the mean kidney stiffness measurements obtained were significantly lower: right kidney, 1.09 ± 0.39 m/s; and left kidney, 1.04 ± 0.38 m/s (P = .4). We observed that, similar to VTQ, EPQ values decreased with CKD progression, based on analysis of variance results using different CKD stages. From a receiver operating characteristic curve analysis, the cutoff value for an estimated glomerular filtration rate of less than 45 mL/min was 1.24 m/s, and the value for an estimated glomerular filtration rate of less than 30 mL/min was 1.07 m/s.

CONCLUSIONS

When using EPQ, the kidney shear wave velocity is decreased in patients with CKD, an observation similar to that obtained by using the VTQ method.

Functional MRI in transplanted kidneys

Renal transplantation is the therapy of choice for patients with end-stage renal diseases. Improvement of immunosuppressive therapy has significantly increased the half-life of renal allografts over the past decade. Nevertheless, complications can still arise. An early detection of allograft dysfunction is mandatory for a good outcome. New advances in magnetic resonance imaging (MRI) have enabled the noninvasive assessment of different functional renal parameters in addition to anatomic imaging. Most of these techniques were widely tested on renal allografts in past decades and a lot of clinical data are available. The following review summarizes the comprehensive, functional MRI techniques for the noninvasive assessment of renal allograft function and highlights their potential for the investigations of different etiologies of graft dysfunction.

Characterization of Small Renal Tumors With Magnetic Resonance Elastography: A Feasibility Study

OBJECTIVES

The aim of this study was to explore the feasibility of magnetic resonance elastography (MRE) for characterizing indeterminate small renal tumors (SRTs) as part of a multiparametric magnetic resonance (MR) imaging protocol.

MATERIALS AND METHODS

After institutional review board approval and informed consent were obtained, 21 prospective adults (15 men; median age, 55 years; age range, 25-72 years) with SRT were enrolled. Tumors (2-5 cm Ø) were imaged using 3-directional, gradient echo MRE. Viscoelastic parametric maps (shear wave velocity [c] and attenuation [α]) were analyzed by 2 independent radiologists. Interobserver agreement (Bland-Altman statistics and intraclass correlation coefficients) was assessed. Anatomical T2-weighted, dynamic contrast-enhanced (DCE) and diffusion sequences completed the acquisition protocol. Imaging parameters were compared between groups (Mann-Whitney U test).

RESULTS

Quality of MRE was good in 18 cases (mean nonlinearity <50%), including 1 papillary renal cell carcinoma and 1 metanephric adenoma. A cohort of 5 oncocytomas and 11 clear-cell renal cell carcinomas (ccRCCs) was analyzed for statistical differences. The MRE viscoelastic parameters were the strongest imaging discriminators: oncocytomas displayed significantly lower shear velocity c (median, 0.77 m/s; interquartile range [IQR], 0.76-0.79) (P = 0.007) and higher shear attenuation α (median, 0.087 mm; IQR, 0.082-0.087) (P = 0.008) than ccRCC (medians, 0.92 m/s and 0.066 mm; IQR, 0.84-0.97 and 0.054-0.074, respectively). T2 signal intensity ratio (tumor/renal cortex) was lower in oncocytomas (P = 0.02). The DCE and diffusion MR parameters overlapped substantially (P ≥ 0.1). Oncocytomas displayed a consistent MRE viscoelastic profile, corresponding to data point clustering in a bidimensional scatter plot. Values for MRE intraclass correlation coefficient were 0.982 for c and 0.984 for α, indicating excellent interobserver agreement.

CONCLUSIONS

Magnetic resonance elastography is feasible for SRT characterization; MRE viscoelastic parameters were stronger discriminators between oncocytoma and ccRCC than anatomical, DCE and diffusion MR imaging parameters.

Magnetic resonance elastography can monitor changes in medullary stiffness in response to treatment in the swine ischemic kidney

OBJECTIVE

Low-energy shockwave (SW) therapy attenuates damage in the stenotic kidney (STK) caused by atherosclerotic renal artery stenosis (ARAS). We hypothesized that magnetic resonance elastography (MRE) would detect attenuation of fibrosis following SW in unilateral ARAS kidneys.

MATERIALS AND METHODS

Domestic pigs were randomized to control, unilateral ARAS, and ARAS treated with 6 sessions of SW over 3 consecutive weeks (n = 7 each) starting after 3 weeks of ARAS or sham. Four weeks after SW treatment, renal fibrosis was evaluated with MRE in vivo or trichrome staining ex vivo. Blood pressure, single-kidney renal-blood-flow (RBF) and glomerular-filtration-rate (GFR) were assessed.

RESULTS

MRE detected increased stiffness in the STK medulla (15.3 ± 2.1 vs. 10.1 ± 0.8 kPa, p < 0.05) that moderately correlated with severity of fibrosis (R2 = 0.501, p < 0.01), but did not identify mild STK cortical or contralateral kidney fibrosis. Trichrome staining showed that medullary fibrosis was increased in ARAS and alleviated by SW (10.4 ± 1.8% vs. 2.9 ± 0.2%, p < 0.01). SW slightly decreased blood pressure and normalized STK RBF and GFR in ARAS. In the contralateral kidney, SW reversed the increase in RBF and GFR.

CONCLUSION

MRE might be a tool for noninvasive monitoring of medullary fibrosis in response to treatment in kidney disease.

Magnetic Resonance Elastography to Assess Fibrosis in Kidney Allografts

BACKGROUND AND OBJECTIVES

Fibrosis is a major cause of kidney allograft injury. Currently, the only means of assessing allograft fibrosis is by biopsy, an invasive procedure that samples <1% of the kidney. We examined whether magnetic resonance elastography, an imaging-based measure of organ stiffness, could noninvasively estimate allograft fibrosis and predict progression of allograft dysfunction.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Kidney allograft recipients >1 year post-transplant undergoing an allograft biopsy first underwent free-breathing, flow-compensated magnetic resonance elastography on a 3.0-T magnetic resonance imaging scanner. Each patient had serial eGFR measurements after the elastography scan for a follow-up period of up to 1 year. The mean stiffness value of the kidney allograft was compared with both the histopathologic Banff fibrosis score and the rate of eGFR change during the follow-up period.

RESULTS

Sixteen patients who underwent magnetic resonance elastography and biopsy were studied (mean age: 54±9 years old). Whole-kidney mean stiffness ranged between 3.5 and 7.3 kPa. Whole-kidney stiffness correlated with biopsy-derived Banff fibrosis score (Spearman rho =0.67; P<0.01). Stiffness was heterogeneously distributed within each kidney, providing a possible explanation for the lack of a stronger stiffness-fibrosis correlation. We also found negative correlations between whole-kidney stiffness and both baseline eGFR (Spearman rho =-0.65; P<0.01) and eGFR change over time (Spearman rho =-0.70; P<0.01). Irrespective of the baseline eGFR, increased kidney stiffness was associated with a greater eGFR decline (regression r2=0.48; P=0.03).

CONCLUSIONS

Given the limitations of allograft biopsy, our pilot study suggests the potential for magnetic resonance elastography as a novel noninvasive measure of whole-allograft fibrosis burden that may predict future changes in kidney function. Future studies exploring the utility and accuracy of magnetic resonance elastography are needed.

Magnetic Resonance Imaging of the Fibrotic Kidney

Magnetic resonance imaging (MRI) has been used for many years for anatomic evaluation of the kidney. Recently developed methods attempt to go beyond anatomy to give information about the health and function of the kidneys. Several methods, including diffusion-weighted MRI, renal blood oxygen level-dependent MRI, renal MR elastography, and renal susceptibility imaging, show promise for providing unique insight into kidney function and severity of fibrosis. However, substantial limitations in accuracy and practicality limit the immediate clinical application of each method. Further development and improvement are necessary to achieve the ideal of a noninvasive image-based measure of renal fibrosis. Our brief review provides a short explanation of these emerging MRI methods and outlines the promising initial results obtained with each as well as current limitations and barriers to clinical implementation.

Innovative Perspective: Gadolinium-Free Magnetic Resonance Imaging in Long-Term Follow-Up after Kidney Transplantation

Since the mid-1980s magnetic resonance imaging (MRI) has been investigated as a non- or minimally invasive tool to probe kidney allograft function. Despite this long-standing interest, MRI still plays a subordinate role in daily practice of transplantation nephrology. With the introduction of new functional MRI techniques, administration of exogenous gadolinium-based contrast agents has often become unnecessary and true non-invasive assessment of allograft function has become possible. This raises the question why application of MRI in the follow-up of kidney transplantation remains restricted, despite promising results. Current literature on kidney allograft MRI is mainly focused on assessment of (sub) acute kidney injury after transplantation. The aim of this review is to survey whether MRI can provide valuable diagnostic information beyond 1 year after kidney transplantation from a mechanistic point of view. The driving force behind chronic allograft nephropathy is believed to be chronic hypoxia. Based on this, techniques that visualize kidney perfusion and oxygenation, scarring, and parenchymal inflammation deserve special interest. We propose that functional MRI mechanistically provides tools for diagnostic work-up in long-term follow-up of kidney allografts.

A MRI-Compatible Combined Mechanical Loading and MR Elastography Setup to Study Deformation-Induced Skeletal Muscle Damage in Rats

Deformation of skeletal muscle in the proximity of bony structures may lead to deep tissue injury category of pressure ulcers. Changes in mechanical properties have been proposed as a risk factor in the development of deep tissue injury and may be useful as a diagnostic tool for early detection. MRE allows for the estimation of mechanical properties of soft tissue through analysis of shear wave data. The shear waves originate from vibrations induced by an external actuator placed on the tissue surface. In this study a combined Magnetic Resonance (MR) compatible indentation and MR Elastography (MRE) setup is presented to study mechanical properties associated with deep tissue injury in rats. The proposed setup allows for MRE investigations combined with damage-inducing large strain indentation of the Tibialis Anterior muscle in the rat hind leg inside a small animal MR scanner. An alginate cast allowed proper fixation of the animal leg with anatomical perfect fit, provided boundary condition information for FEA and provided good susceptibility matching. MR Elastography data could be recorded for the Tibialis Anterior muscle prior to, during, and after indentation. A decaying shear wave with an average amplitude of approximately 2 μm propagated in the whole muscle. MRE elastograms representing local tissue shear storage modulus G_d showed significant increased mean values due to damage-inducing indentation (from 4.2 ± 0.1 kPa before to 5.1 ± 0.6 kPa after, p<0.05). The proposed setup enables controlled deformation under MRI-guidance, monitoring of the wound development by MRI, and quantification of tissue mechanical properties by MRE. We expect that improved knowledge of changes in soft tissue mechanical properties due to deep tissue injury, will provide new insights in the etiology of deep tissue injuries, skeletal muscle damage and other related muscle pathologies.

General review of magnetic resonance elastography

Magnetic resonance elastography (MRE) is an innovative imaging technique for the non-invasive quantification of the biomechanical properties of soft tissues via the direct visualization of propagating shear waves in vivo using a modified phase-contrast magnetic resonance imaging (MRI) sequence. Fundamentally, MRE employs the same physical property that physicians utilize when performing manual palpation - that healthy and diseased tissues can be differentiated on the basis of widely differing mechanical stiffness. By performing “virtual palpation”, MRE is able to provide information that is beyond the capabilities of conventional morphologic imaging modalities. In an era of increasing adoption of multi-parametric imaging approaches for solving complex problems, MRE can be seamlessly incorporated into a standard MRI examination to provide a rapid, reliable and comprehensive imaging evaluation at a single patient appointment. Originally described by the Mayo Clinic in 1995, the technique represents the most accurate non-invasive method for the detection and staging of liver fibrosis and is currently performed in more than 100 centers worldwide. In this general review, the mechanical properties of soft tissues, principles of MRE, clinical applications of MRE in the liver and beyond, and limitations and future directions of this discipline -are discussed. Selected diagrams and images are provided for illustration.

Magnetic resonance elastography of abdomen

Many diseases cause substantial changes in the mechanical properties of tissue, and this provides motivation for developing methods to noninvasively assess the stiffness of tissue using imaging technology. Magnetic resonance elastography (MRE) has emerged as a versatile MRI-based technique, based on direct visualization of propagating shear waves in the tissues. The most established clinical application of MRE in the abdomen is in chronic liver disease. MRE is currently regarded as the most accurate noninvasive technique for detection and staging of liver fibrosis. Increasing experience and ongoing research is leading to exploration of applications in other abdominal organs. In this review article, the current use of MRE in liver disease and the potential future applications of this technology in other parts of the abdomen are surveyed.

Magnetic resonance elastography (MRE) in cancer: Technique, analysis, and applications

Tissue mechanical properties are significantly altered with the development of cancer. Magnetic resonance elastography (MRE) is a noninvasive technique capable of quantifying tissue mechanical properties in vivo. This review describes the basic principles of MRE and introduces some of the many promising MRE methods that have been developed for the detection and characterization of cancer, evaluation of response to therapy, and investigation of the underlying mechanical mechanisms associated with malignancy.

Reliability of magnetic resonance elastography using multislice two-dimensional spin-echo echo-planar imaging (SE-EPI) and three-dimensional inversion reconstruction for assessing renal stiffness

BACKGROUND

To evaluate the reliability of MRE using a spin-echo echo-planar imaging (SE-EPI) renal MRE technique in healthy volunteers.

METHODS

Institutional review board approved prospective study in which all participants provided written informed consent. Sixteen healthy volunteers comprising seven males and nine females with a median age of 35 years (age range: 23 to 59 years) were included. Coronal 90 Hz and 60 Hz MRE acquisitions were performed twice within a 30-min interval between examinations. Renal MRE reliability was assessed by (i) test-retest repeatability, and (ii) inter-rater agreement between two independent readers. The MRE-measured averaged renal stiffness values were evaluated using: intraclass correlation coefficient (ICC), Bland-Altman and the within-subject coefficient of variation (COV).

RESULTS

For test-retest repeatability, Bland-Altman showed a mean stiffness difference between examinations of 0.07 kPa (95% limits of agreement: -1.41, 1.54) at 90 Hz and 0.01 kPa (95% limits of agreement: -0.51, 0.53) at 60 Hz. Coefficient of repeatability was 1.47 kPa and 0.52 kPa at 90 Hz and 60 Hz, respectively. The within-subject COV was 13.6% and 7.7% at 90 Hz and 60 Hz, respectively. ICC values were 0.922 and 0.907 for test-retest repeatability and 0.998 and 0.989 for inter-rater agreement, respectively (P < 0.001).

CONCLUSION

SE-EPI renal MRE is a reliable technique.

Imaging of cardiovascular complications in patients with systemic lupus erythematosus

In the long-term survival of patients with systemic lupus erythematosus (SLE), cardiovascular disease (CVD) is a leading cause of death. Recently, multimodality cardiovascular imaging methods have been adopted for the evaluation of cardiovascular risk, which has shown to be associated with both traditional cardiovascular risk factors and SLE-specific conditions. Quantitative imaging biomarkers, which can describe both morphological and functional abnormalities in the heart, are expected to provide new insights to stratify cardiovascular risks and to guide SLE management by assessing individual responses to therapies either protecting the cardiovascular system or suppressing the autoimmune reactions. In this review, we will discuss cutting-edge cardiovascular imaging techniques and potential clinical applications and limitations of those techniques for the evaluation of major SLE-related heart disorders.

Ultrasound Elastography and MR Elastography for Assessing Liver Fibrosis: Part 2, Diagnostic Performance, Confounders, and Future Directions

OBJECTIVE

The purpose of the article is to review the diagnostic performance of ultra-sound and MR elastography techniques for detection and staging of liver fibrosis, the main current clinical applications of elastography in the abdomen.

CONCLUSION

Technical and instrument-related factors and biologic and patient-related factors may constitute potential confounders of stiffness measurements for assessment of liver fibrosis. Future developments may expand the scope of elastography for monitoring liver fibrosis and predict complications of chronic liver disease.

Ultrasound-guided therapeutic focused ultrasound: current status and future directions

This paper reviews ultrasound imaging methods for the guidance of therapeutic focused ultrasound (USgFUS), with emphasis on real-time preclinical methods. Guidance is interpreted in the broadest sense to include pretreatment planning, siting of the FUS focus, real-time monitoring of FUS-tissue interactions, and real-time control of exposure and damage assessment. The paper begins with an overview and brief historical background of the early methods used for monitoring FUS-tissue interactions. Current imaging methods are described, and discussed in terms of sensitivity and specificity of the localisation of the FUS effects in both therapeutic and sub-therapeutic modes. Thermal and non-thermal effects are considered. These include cavitation-enhanced heating, tissue water boiling and cavitation. Where appropriate, USgFUS methods are compared with similar methods implemented using other guidance modalities, e.g. magnetic resonance imaging. Conclusions are drawn regarding the clinical potential of the various guidance methods, and the feasibility and current status of real-time implementation.

Comparison of magnetic resonance imaging with radionuclide methods of evaluating the kidney

Nuclear medicine and MRI provide information about renal perfusion, function (glomerular filtration rate), and drainage. Some tracers that are used in nuclear medicine (technetium-diethylene triamine pentaacetic acid ([(99m)Tc-DTPA] and (51)chromium-EDTA) and some contrast media (CM) that are used for MRI (gadolinium-DTPA for instance) share the same pharmacokinetic properties, though, detection techniques are different (low-spatial resolution 2-dimensional projection with a good concentration-to-signal linearity for nuclear medicine and high-resolution 3-dimensional localization with nonlinear behavior for MRI). Thus, though based on the same principles, the methods are not the same and they provide somewhat different information. Many MRI perfusion studies have been conducted; some of them were compared with nuclear medicine with no good agreement. Phase contrast can reliably assess global renal blood flow but not perfusion at a tissular level. Arterial spin labeling has not proven to be a reliable tool to measure renal perfusion. Techniques using CM theoretically can assess perfusion at the tissular level, but they have not proven to be precise. To assess renal function, many models have been proposed. Some MRI techniques using CM, both semiquantitative (Patlak) and quantitative, have shown ability to roughly assess relative function. Some quantitative methods (Annet's and Lee's methods) have even showed that they could roughly estimate absolute renal function, with better results than estimated glomerular filtration rate. Quantification of drainage has not been much studied using MRI.

High-resolution mechanical imaging of the kidney

The objective of this study was to test the feasibility and reproducibility of in vivo high-resolution mechanical imaging of the asymptomatic human kidney. Hereby nine volunteers were examined at three different physiological states of urinary bladder filling (a normal state, urinary urgency, and immediately after urinary relief). Mechanical imaging was performed of the in vivo kidney using three-dimensional multifrequency magnetic resonance elastography combined with multifrequency dual elastovisco inversion. Other than in classical elastography, where the storage and loss shear moduli are evaluated, we analyzed the magnitude |G(⁎)| and the phase angle φ of the complex shear modulus reconstructed by simultaneous inversion of full wave field data corresponding to 7 harmonic drive frequencies from 30 to 60Hz and a resolution of 2.5mm cubic voxel size. Mechanical parameter maps were derived with a spatial resolution superior to that in previous work. The group-averaged values of |G(⁎)| were 2.67±0.52kPa in the renal medulla, 1.64±0.17kPa in the cortex, and 1.17±0.21kPa in the hilus. The phase angle φ (in radians) was 0.89±0.12 in the medulla, 0.83±0.09 in the cortex, and 0.72±0.06 in the hilus. All regional differences were significant (P<0.001), while no significant variation was found in relation to different stages of bladder filling. In summary our study provides first high-resolution maps of viscoelastic parameters of the three anatomical regions of the kidney. |G(⁎)| and φ provide novel information on the viscoelastic properties of the kidney, which is potentially useful for the detection of renal lesions or fibrosis.