Quantitative MRI of kidneys in renal disease

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.

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.

Assessing the Risk of Progression to Kidney Failure in Patients With Autosomal Dominant Polycystic Kidney Disease

While autosomal dominant polycystic kidney disease (ADPKD) is a dichotomous diagnosis, substantial variability in disease severity exists. Identification of inherited risk through family history, genetic testing, and environmental risk factors through clinical assessment are important components of risk assessment for optimal management of patients with ADPKD. Genetic testing is especially helpful in cases with diagnostic uncertainty, particularly in cases with no apparent family history, in young cases (age less than 25 years) where a definitive diagnosis is sought, or in atypical presentations with early, severe, or discordant findings. Currently, risk assessment in ADPKD may be performed with the use of age-adjusted estimated glomerular filtration rate thresholds, evidence of rapid estimated glomerular filtration rate decline on serial measurements, age- and height-adjusted total kidney volume by Mayo Clinic Imaging Classification, or evidence of early hypertension and urological complications combined with PKD1 or PKD2 mutation class; however, caveats exist with each of these approaches. Fine-tuning of risk stratification with advanced imaging features and biomarkers is the subject of research but is not yet ready for general clinical practice. While conservative treatment strategies will be advised for all patients, those with the greatest rate of disease progression will have the most benefit from aggressive disease-modifying therapy. In this narrative review, we will summarize the evidence behind the clinical assessment and risk stratification of patients with ADPKD.

Diffusion magnetic resonance imaging for kidney cyst volume quantification and non-cystic tissue characterisation in ADPKD

OBJECTIVES

Beyond total kidney and cyst volume (TCV), non-cystic tissue plays an important role in autosomal dominant polycystic kidney disease (ADPKD) progression. This study aims at presenting and preliminarily validating a diffusion MRI (DWI)-based TCV quantification method and providing evidence of DWI potential in characterising non-cystic tissue microstructure.

METHODS

T2-weighted MRI and DWI scans (b = 0, 15, 50, 100, 200, 350, 500, 700, 1000; 3 directions) were acquired from 35 ADPKD patients with CKD stage 1 to 3a and 15 healthy volunteers on a 1.5 T scanner. ADPKD classification was performed using the Mayo model. DWI scans were processed by mono- and segmented bi-exponential models. TCV was quantified on T2-weighted MRI by the reference semi-automatic method and automatically computed by thresholding the pure diffusivity (D) histogram. The agreement between reference and DWI-based TCV values and the differences in DWI-based parameters between healthy and ADPKD tissue components were assessed.

RESULTS

There was strong correlation between DWI-based and reference TCV (rho = 0.994, p < 0.001). Non-cystic ADPKD tissue had significantly higher D, and lower pseudo-diffusion and flowing fraction than healthy tissue (p < 0.001). Moreover, apparent diffusion coefficient and D values significantly differed by Mayo imaging class, both in the whole kidney (Wilcoxon p = 0.007 and p = 0.004) and non-cystic tissue (p = 0.024 and p = 0.007).

CONCLUSIONS

DWI shows potential in ADPKD to quantify TCV and characterise non-cystic kidney tissue microstructure, indicating the presence of microcysts and peritubular interstitial fibrosis. DWI could complement existing biomarkers for non-invasively staging, monitoring, and predicting ADPKD progression and evaluating the impact of novel therapies, possibly targeting damaged non-cystic tissue besides cyst expansion.

CLINICAL RELEVANCE STATEMENT

This study shows diffusion-weighted MRI (DWI) potential to quantify total cyst volume and characterise non-cystic kidney tissue microstructure in ADPKD. DWI could complement existing biomarkers for non-invasively staging, monitoring, and predicting ADPKD progression and evaluating the impact of novel therapies, possibly targeting damaged non-cystic tissue besides cyst expansion.

KEY POINTS

• Diffusion magnetic resonance imaging shows potential to quantify total cyst volume in ADPKD. • Diffusion magnetic resonance imaging might allow to non-invasively characterise non-cystic kidney tissue microstructure. • Diffusion magnetic resonance imaging-based biomarkers significantly differ by Mayo imaging class, suggesting their possible prognostic value.

Abdominal Imaging in ADPKD: Beyond Total Kidney Volume

In the context of autosomal dominant polycystic kidney disease (ADPKD), measurement of the total kidney volume (TKV) is crucial. It acts as a marker for tracking disease progression, and evaluating the effectiveness of treatment strategies. The TKV has also been recognized as an enrichment biomarker and a possible surrogate endpoint in clinical trials. Several imaging modalities and methods are available to calculate the TKV, and the choice depends on the purpose of use. Technological advancements have made it possible to accurately assess the cyst burden, which can be crucial to assessing the disease state and helping to identify rapid progressors. Moreover, the development of automated algorithms has increased the efficiency of total kidney and cyst volume measurements. Beyond these measurements, the quantification and characterization of non-cystic kidney tissue shows potential for stratifying ADPKD patients early on, monitoring disease progression, and possibly predicting renal function loss. A broad spectrum of radiological imaging techniques are available to characterize the kidney tissue, showing promise when it comes to non-invasively picking up the early signs of ADPKD progression. Radiomics have been used to extract textural features from ADPKD images, providing valuable information about the heterogeneity of the cystic and non-cystic components. This review provides an overview of ADPKD imaging biomarkers, focusing on the quantification methods, potential, and necessary steps toward a successful translation to clinical practice.

Utility of new image-derived biomarkers for autosomal dominant polycystic kidney disease prognosis using automated instance cyst segmentation

New image-derived biomarkers for patients affected by autosomal dominant polycystic kidney disease are needed to improve current clinical management. The measurement of total kidney volume (TKV) provides critical information for clinicians to drive care decisions. However, patients with similar TKV may present with very different phenotypes, often requiring subjective decisions based on other factors (e.g., appearance of healthy kidney parenchyma, a few cysts contributing significantly to overall TKV, etc.). In this study, we describe a new technique to individually segment cysts and quantify biometric parameters including cyst volume, cyst number, parenchyma volume, and cyst parenchyma surface area. Using data from the Consortium for Radiologic Imaging Studies of Polycystic Kidney Disease (CRISP) study the utility of these new parameters was explored, both quantitatively as well as visually. Total cyst number and cyst parenchyma surface area showed superior prediction of the slope of estimated glomerular filtration rate decline, kidney failure and chronic kidney disease stages 3A, 3B, and 4, compared to TKV. In addition, presentations such as a few large cysts contributing significantly to overall kidney volume were shown to be much better stratified in terms of outcome predictions. Thus, these new image biomarkers, which can be obtained automatically, will have great utility in future studies and clinical care for patients affected by autosomal dominant polycystic kidney disease.

Motion compensated renal diffusion weighted imaging

PURPOSE

To assess the effect of respiratory motion and cardiac driven pulsation in renal DWI and to examine asymmetrical velocity-compensated diffusion encoding waveforms for robust ADC mapping in the kidneys.

METHODS

The standard monopolar Stejskal-Tanner pulsed gradient spin echo (pgse) and the asymmetric bipolar velocity-compensated (asym-vc) diffusion encoding waveforms were used for coronal renal DWI at 3T. The robustness of the ADC quantification in the kidneys was tested with the aforementioned waveforms in respiratory-triggered and breath-held cardiac-triggered scans at different trigger delays in 10 healthy subjects.

RESULTS

The pgse waveform showed higher ADC values in the right kidney at short trigger delays in comparison to longer trigger delays in the respiratory triggered scans when the diffusion gradient was applied in the feet-head (FH) direction. The coefficient of variation over all respiratory trigger delays, averaged over all subjects was 0.15 for the pgse waveform in the right kidney when diffusion was measured in the FH direction; the corresponding coefficient of variation for the asym-vc waveform was 0.06. The effect of cardiac driven pulsation was found to be small in comparison to the effect of respiratory motion.

CONCLUSION

Short trigger delays in respiratory-triggered scans can cause higher ADC values in comparison to longer trigger delays in renal DWI, especially in the right kidney when diffusion is measured in the FH direction. The asym-vc waveform can reduce ADC variation due to respiratory motion in respiratory-triggered scans at the cost of reduced SNR compared to the pgse waveform.

Evaluation of renal fibrosis in various causes of glomerulonephritis by MR elastography: a clinicopathologic comparative analysis

BACKGROUND

Renal parenchymal fibrosis is the most important determinant of kidney disease progression and it is determined via biopsy. The aim of this study is to evaluate the renal stiffness noninvasively by magnetic resonance elastography (MRE) and to compare it with clinicopathologic parameters in glomerulonephritis and AA amyloidosis patients.

METHODS

Thirty-four patients with glomerular filtration rate (GFR) over 20 ml/min/1.73m² had non-contrast MRE prospectively. Kidney stiffness values were obtained from whole kidney, cortex, and medulla. Values were correlated with GFR, albuminuria, proteinuria, and degree of fibrosis that are assessed via renal biopsy. Patients were grouped clinicopathologically to assess the relation between stiffness and chronicity.

RESULTS

Mean whole kidney, cortex, and medulla stiffnesses were 3.78 (± 1.26), 3.63 (± 1.25), and 4.77 (± 2.03) kPa, respectively. Mean global glomerulosclerosis was 22% (± 18%) and median segmental glomerulosclerosis was 4% (min-max: 0%-100%). Extent of tubulointerstitial fibrosis was less than 25% in 26 of the patients (76.5%), 25%-50% in 6 of the patients (17.6%), and higher than 50% in 2 of the patients (5.9%). Fourteen patients were defined to have chronic renal parenchymal injury. MRE-derived stiffness values correlated negatively with parameters of fibrosis. Lower stiffness values were observed in patients with chronic renal injury compared to those without (P < 0.05 for whole kidney and medulla MRE-derived stiffness).

CONCLUSION

MRE-derived stiffness values were lower in patients with chronic injury. Stiffness decreases as glomerulosclerosis and tubulointerstitial fibrosis progresses in patients with primary glomerulonephritis and AA amyloidosis. With future studies, there may be a role for MRE to assess renal function in concert with conventional markers.

Application of MR Imaging Features in Differentiation of Renal Changes in Patients With Stage III Type 2 Diabetic Nephropathy and Normal Subjects

OBJECTIVE

The objective of the study was to explore the value of MRI texture features based on T1WI, T2-FS and diffusion-weighted imaging (DWI) in differentiation of renal changes in patients with stage III type 2 diabetic nephropathy (DN) and normal subjects.

MATERIALS AND METHODS

A retrospective analysis was performed to analyze 44 healthy volunteers (group A) and 40 patients with stage III type 2 diabetic nephropathy (group B) with microalbuminuria. Urinary albumin to creatinine ratio (ACR) <30 mg/g, estimated glomerular filtration rate (eGFR) in the range of 60-120 ml/(min 1.73 m²), and randomly divided into primary cohort and test cohort. Conventional MRI and DWI of kidney were performed using 1.5 T magnetic resonance imaging (MRI). The outline of the renal parenchyma was manually labeled in fat-suppressed T2-weighted imaging (FS-T2WI), and PyRadiomics was used to extract radiomics features. The radiomics features were then selected by the least absolute shrinkage and selection operator (LASSO) method.

RESULTS

There was a significant difference in sex and body mass index (BMI) (P <0.05) in the primary cohort, with no significant difference in age. In the final results, the wavelet and Laplacian-Gaussian filtering are used to extract 1,892 image features from the original T1WI image, and the LASSO algorithm is used for selection. One first-order feature and six texture features are selected through 10 cross-validations. In the mass, 1,638 imaging extracts features from the original T2WI image.1 first-order feature and 5 texture features were selected. A total of 1,241 imaging features were extracted from the original ADC images, and 5 texture features were selected. Using LASSO-Logistic regression analysis, 10 features were selected for modeling, and a combined diagnosis model of diabetic nephropathy based on texture features was established. The average unit cost in the logistic regression model was 0.98, the 95% confidence interval for the predictive efficacy was 0.9486-1.0, specificity 0.97 and precision 0.93, particularly. ROC curves also revealed that the model could distinguish with high sensitivity of at least 92%.

CONCLUSION

In consequence, the texture features based on MR have broad application prospects in the early detection of DN as a relatively simple and noninvasive tool without contrast media administration.

Automatic semantic segmentation of kidney cysts in MR images of patients affected by autosomal-dominant polycystic kidney disease

PURPOSE

For patients affected by autosomal-dominant polycystic kidney disease (ADPKD), successful differentiation of cysts is useful for automatic classification of patient phenotypes, clinical decision-making, and disease progression. The objective was to develop and evaluate a fully automated semantic segmentation method to differentiate and analyze renal cysts in patients with ADPKD.

METHODS

An automated deep learning approach using a convolutional neural network was trained, validated, and tested on a set of 60 MR T2-weighted images. A three-fold cross-validation approach was used to train three models on distinct training and validation sets (n = 40). An ensemble model was then built and tested on the hold out cases (n = 20), with each of the cases compared to manual segmentations performed by two readers. Segmentation agreement between readers and the automated method was assessed.

RESULTS

The automated approach was found to perform at the level of interobserver variability. The automated approach had a Dice coefficient (mean ± standard deviation) of 0.86 ± 0.10 vs Reader-1 and 0.84 ± 0.11 vs. Reader-2. Interobserver Dice was 0.86 ± 0.08. In terms of total cyst volume (TCV), the automated approach had a percent difference of 3.9 ± 19.1% vs Reader-1 and 8.0 ± 24.1% vs Reader-2, whereas interobserver variability was - 2.0 ± 16.4%.

CONCLUSION

This study developed and validated a fully automated approach for performing semantic segmentation of kidney cysts in MR images of patients affected by ADPKD. This approach will be useful for exploring additional imaging biomarkers of ADPKD and automatically classifying phenotypes.

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.

Renal pH Imaging Using Chemical Exchange Saturation Transfer (CEST) MRI: Basic Concept

Magnetic Resonance Imaging (MRI) has been actively explored in the last several decades for assessing renal function by providing several physiological information, including glomerular filtration rate, renal plasma flow, tissue oxygenation and water diffusion. Within MRI, the developing field of chemical exchange saturation transfer (CEST) has potential to provide further functional information for diagnosing kidney diseases. Both endogenous produced molecules as well as exogenously administered CEST agents have been exploited for providing functional information related to kidney diseases in preclinical studies. In particular, CEST MRI has been exploited for assessing the acid-base homeostasis in the kidney and for monitoring pH changes in several disease models. This review summarizes several CEST MRI procedures for assessing kidney functionality and pH, for monitoring renal pH changes in different kidney injury models and for evaluating renal allograft rejection.This chapter is based upon work from the COST Action PARENCHIMA, a community-driven network funded by the European Cooperation in Science and Technology (COST) program of the European Union, which aims to improve the reproducibility and standardization of renal MRI biomarkers. This introduction chapter is complemented by two separate chapters describing the experimental procedure and data analysis.

Application of multiparametric MR imaging to predict the diversification of renal function in miR29a-mediated diabetic nephropathy

Diabetic nephropathy (DN) is one of the major leading cause of kidney failure. To identify the progression of chronic kidney disease (CKD), renal function/fibrosis is playing a crucial role. Unfortunately, lack of sensitivities/specificities of available clinical biomarkers are key major issues for practical healthcare applications to identify the renal functions/fibrosis in the early stage of DN. Thus, there is an emerging approach such as therapeutic or diagnostic are highly desired to conquer the CKD at earlier stages. Herein, we applied and examined the application of dynamic contrast enhanced magnetic resonance imaging (DCE-MRI) and diffusion weighted imaging (DWI) to identify the progression of fibrosis between wild type (WT) and miR29a transgenic (Tg) mice during streptozotocin (STZ)-induced diabetes. Further, we also validate the potential renoprotective role of miR29a to maintain the renal perfusion, volume, and function. In addition, Ktrans values of DCE-MRI and apparent diffusion coefficient (ADC) of DWI could significantly reflect the level of fibrosis between WT and Tg mice at identical conditions. As a result, we strongly believed that the present non-invasive MR imaging platforms have potential to serveas an important tool in research and clinical imaging for renal fibrosis in diabetes, and that microenvironmental changes could be identified by MR imaging acquisition prior to histological biopsy and diabetic podocyte dysfunction.

Assessing Polycystic Kidney Disease in Rodents: Comparison of Robotic 3D Ultrasound and Magnetic Resonance Imaging

Polycystic kidney disease (PKD) is an inherited disorder characterized by renal cyst formation and enlargement of the kidney. PKD severity can be staged noninvasively by measuring total kidney volume (TKV), a promising biomarker that has recently received regulatory qualification. In preclinical mouse models, where the disease is studied and potential therapeutics are evaluated, the most popular noninvasive method of measuring TKV is magnetic resonance imaging (MRI). Although MRI provides excellent 3D resolution and contrast, these systems are expensive to operate, have long acquisition times, and, consequently, are not heavily used in preclinical PKD research. In this study, a new imaging instrument, based on robotic ultrasound (US), was evaluated as a complementary approach for assessing PKD in rodent models. The objective was to determine the extent to which TKV measurements on the robotic US scanner correlated with both in vivo and ex vivo reference standards (MRI and Vernier calipers, respectively). A cross-sectional study design was implemented that included both PKD-affected mice and healthy wild types, spanning sex and age for a wide range of kidney volumes. It was found that US-derived TKV measurements and kidney lengths were strongly associated with both in vivo MRI and ex vivo Vernier caliper measurements (R ²=0.94 and 0.90, respectively). In addition to measuring TKV, renal vascular density was assessed using acoustic angiography (AA), a novel contrast-enhanced US methodology. AA image intensity, indicative of volumetric vascularity, was seen to have a strong negative correlation with TKV (R ²=0.82), suggesting impaired renal vascular function in mice with larger kidneys. These studies demonstrate that robotic US can provide a rapid and accurate approach for noninvasively evaluating PKD in rodent models.

Multiparametric Renal MRI: An Intrasubject Test-Retest Repeatability Study

Background

Renal multiparametric magnetic resonance imaging (MRI) is a promising tool for diagnosis, prognosis, and treatment monitoring in kidney disease.

Purpose

To determine intrasubject test–retest repeatability of renal MRI measurements.

Study Type

Prospective.

Population

Nineteen healthy subjects aged over 40 years.

Field Strength/Sequences

T₁ and T₂ mapping, R₂* mapping or blood oxygenation level‐dependent (BOLD) MRI, diffusion tensor imaging (DTI), and intravoxel incoherent motion (IVIM) diffusion‐weighted imaging (DWI), 2D phase contrast, arterial spin labelling (ASL), dynamic contrast enhanced (DCE) MRI, and quantitative Dixon for fat quantification at 3T.

Assessment

Subjects were scanned twice with ~1 week between visits. Total scan time was ~1 hour. Postprocessing included motion correction, semiautomated segmentation of cortex and medulla, and fitting of the appropriate signal model.

Statistical Test

To assess the repeatability, a Bland–Altman analysis was performed and coefficients of variation (CoVs), repeatability coefficients, and intraclass correlation coefficients were calculated.

Results

CoVs for relaxometry (T₁, T₂, R₂*/BOLD) were below 6.1%, with the lowest CoVs for T₂ maps and highest for R₂*/BOLD. CoVs for all diffusion analyses were below 7.2%, except for perfusion fraction (F_P), with CoVs ranging from 18–24%. The CoV for renal sinus fat volume and percentage were both around 9%. Perfusion measurements were most repeatable with ASL (cortical perfusion only) and 2D phase contrast with CoVs of 10% and 13%, respectively. DCE perfusion had a CoV of 16%, while single kidney glomerular filtration rate (GFR) had a CoV of 13%. Repeatability coefficients (RCs) ranged from 7.7–87% (lowest/highest values for medullary mean diffusivity and cortical F_P, respectively) and intraclass correlation coefficients (ICCs) ranged from −0.01 to 0.98 (lowest/highest values for cortical F_P and renal sinus fat volume, respectively).

Data Conclusion

CoVs of most MRI measures of renal function and structure (with the exception of F_P and perfusion as measured by DCE) were below 13%, which is comparable to standard clinical tests in nephrology.

Level of Evidence

2

Technical Efficacy

Stage 1

Role of oxygen and the HIF-pathway in polycystic kidney disease

Kidney cyst growth in ADPKD is associated with regional hypoxia, presumably due to a mismatch between enlarged cysts and the peritubular capillary blood supply and compression of peritubular capillaries in cyst walls. Regional hypoxia leads to activation of hypoxia-inducible transcription factors, with the two main HIF isoforms, HIF-1 and HIF-2 expressed in cyst epithelia and pericystic interstitial cells, respectively. While HIF-2 activation is linked to EPO production, mitigating the anemia that normally accompanies chronic kidney disease, HIF-1 promotes cyst growth. HIF-dependent cyst growth is primarily due to an increase in chloride-dependent fluid secretion into the cyst lumen. However, given the broad spectrum of HIF-target genes, additional HIF-mediated pathways may also contribute to cyst progression. Furthermore, hypoxia can influence cyst growth through the generation of reactive oxygen species. Since cyst expansion aggravates regional hypoxia, a feedforward loop is established that accelerates cyst expansion and disease progression. Inhibiting the HIF pathway and/or HIF target genes that are of particular relevance for HIF-dependent cyst fluid secretion may therefore represent novel therapeutic approaches to retard the progression of APDKD.

Automated Segmentation of Tissues Using CT and MRI: A Systematic Review

RATIONALE AND OBJECTIVES

The automated segmentation of organs and tissues throughout the body using computed tomography and magnetic resonance imaging has been rapidly increasing. Research into many medical conditions has benefited greatly from these approaches by allowing the development of more rapid and reproducible quantitative imaging markers. These markers have been used to help diagnose disease, determine prognosis, select patients for therapy, and follow responses to therapy. Because some of these tools are now transitioning from research environments to clinical practice, it is important for radiologists to become familiar with various methods used for automated segmentation.

MATERIALS AND METHODS

The Radiology Research Alliance of the Association of University Radiologists convened an Automated Segmentation Task Force to conduct a systematic review of the peer-reviewed literature on this topic.

RESULTS

The systematic review presented here includes 408 studies and discusses various approaches to automated segmentation using computed tomography and magnetic resonance imaging for neurologic, thoracic, abdominal, musculoskeletal, and breast imaging applications.

CONCLUSION

These insights should help prepare radiologists to better evaluate automated segmentation tools and apply them not only to research, but eventually to clinical practice.

The feasibility of b-value maps based on threshold DWI for detection of breast cancer: A case-control STROBE compliant study

Diffusion-weighted imaging (DWI) plays an important role in the diagnosis of breast cancer as well as the evaluation of treatment effects. A novel technique named b-value map based on thresholded DWI images has been proposed and can achieve good contrast for demonstrating prostate lesions only by manipulating the window width and center of the images. Its application on the breast has not yet explored, so the aim of the study was to investigate the feasibility of b-value maps based on threshold DWI for detection of breast cancer. A total of 25 patients with pathologically proven invasive ductal breast carcinoma were included and underwent preoperative magnetic resonance imaging (MRI) examinations including DWI at 3T. The capabilities to display lesions of DWIb=800, b-value maps and optimal computed DWI (cDWI) images were evaluated by using a 4-point method of scoring. Apparent diffusion coefficient (ADC) values of lesions were measured for the breast carcinoma. Mean scores indicating the display capability were compared among DWIb=800, optimal cDWI and b-value maps by using Kruskal-Wallis test followed by Nemenyi test. The scores of both b-value maps (3.92 ± 0.28) and optimal cDWI images (3.80 ± 0.41) were higher than that of DWIb=800 (3.48 ± 0.51), with statistical differences (P = .001 and P = .033, respectively). The optimal b values for manifesting breast carcinoma based on cDWI were 1000 to 1200 s/mm. The b-value map enables fast identification for breast lesions and shows similar performance to the optimal cDWI images.

Phase-contrast magnetic resonance imaging to assess renal perfusion: a systematic review and statement paper

Objective

Phase-contrast magnetic resonance imaging (PC-MRI) is a non-invasive method used to compute blood flow velocity and volume. This systematic review aims to discuss the current status of renal PC-MRI and provide practical recommendations which could inform future clinical studies and its adoption in clinical practice.

Methodology

A comprehensive search of all the PC-MRI studies in human healthy subjects or patients related to the kidneys was performed.

Results

A total of 39 studies were included in which PC-MRI was used to measure renal blood flow (RBF) alongside other derivative hemodynamic parameters. PC-MRI generally showed good correlation with gold standard methods of RBF measurement, both in vitro and in vivo, and good reproducibility. Despite PC-MRI not being routinely used in clinical practice, there are several clinical studies showing its potential to support diagnosis and monitoring of renal diseases, in particular renovascular disease, chronic kidney disease and autosomal dominant polycystic kidney disease.

Discussion

Renal PC-MRI shows promise as a non-invasive technique to reliably measure RBF, both in healthy volunteers and in patients with renal disease. Future multicentric studies are needed to provide definitive normative ranges and to demonstrate the clinical potential of PC-MRI, likely as part of a multi-parametric renal MRI protocol.

Electronic supplementary material

The online version of this article (10.1007/s10334-019-00772-0) contains supplementary material, which is available to authorized users.

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.

Change in kidney volume after kidney transplantation in patients with autosomal polycystic kidney disease

Background

The indication to bilateral nephrectomy in patients with autosomal dominant polycystic kidney scheduled for kidney transplantation is controversial. Indeed, the progressive enlargement of cysts may increase the risk of complications and the need for nephrectomy. However, very few studies investigated the change in kidney volume after kidney transplantation.

Material and methods

In this prospective cohort study, the change in native kidney volume in polycystic patients was evaluated with magnetic resonance imaging. Forty patients were included in the study. Kidney diameters and total kidney volume were evaluated with magnetic resonance imaging in patients who underwent simultaneous nephrectomy and kidney transplantation and in patients with kidney transplant alone, before transplantation and 1 year after transplantation.

Results

There was a significant reduction of kidney volume after transplantation, with a mean degree of kidney diameters reduction varying from 12.24% to 14.43%. Mean total kidney volume of the 55 kidney considered in the analysis significantly reduced from 1617.94 ± 833.42 ml to 1381.42 ± 1005.73 ml (P<0.05), with a mean rate of 16.44% of volume decrease. More than 80% of patients had a volume reduction in both groups.

Conclusions

Polycystic kidneys volume significantly reduces after kidney transplantation, and this would reduce the need for prophylactic bilateral nephrectomy in asymptomatic patients.

Diffusion-weighted magnetic resonance imaging to assess diffuse renal pathology: a systematic review and statement paper

Diffusion-weighted magnetic resonance imaging (DWI) is a non-invasive method sensitive to local water motion in the tissue. As a tool to probe the microstructure, including the presence and potentially the degree of renal fibrosis, DWI has the potential to become an effective imaging biomarker. The aim of this review is to discuss the current status of renal DWI in diffuse renal diseases. DWI biomarkers can be classified in the following three main categories: (i) the apparent diffusion coefficient-an overall measure of water diffusion and microcirculation in the tissue; (ii) true diffusion, pseudodiffusion and flowing fraction-providing separate information on diffusion and perfusion or tubular flow; and (iii) fractional anisotropy-measuring the microstructural orientation. An overview of human studies applying renal DWI in diffuse pathologies is given, demonstrating not only the feasibility and intra-study reproducibility of DWI but also highlighting the need for standardization of methods, additional validation and qualification. The current and future role of renal DWI in clinical practice is reviewed, emphasizing its potential as a surrogate and monitoring biomarker for interstitial fibrosis in chronic kidney disease, as well as a surrogate biomarker for the inflammation in acute kidney diseases that may impact patient selection for renal biopsy in acute graft rejection. As part of the international COST (European Cooperation in Science and Technology) action PARENCHIMA (Magnetic Resonance Imaging Biomarkers for Chronic Kidney Disease), aimed at eliminating the barriers to the clinical use of functional renal magnetic resonance imaging, this article provides practical recommendations for future design of clinical studies and the use of renal DWI in clinical practice.

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.

Alterations in apparent diffusion coefficient values of the kidney during the cardiac cycle: Evaluation with ECG-triggered diffusion-weighted MR imaging

PURPOSE

To evaluate dynamic changes in apparent diffusion coefficient (ADC) values of the kidney at different time points during the cardiac cycle using electrocardiographic (ECG)-triggered diffusion-weighted MR imaging in normal subjects, and to elucidate the differences in ADC values between the right and left kidneys during a cardiac cycle.

MATERIALS AND METHODS

The study was approved by our institutional review board and informed consent was obtained from subjects. Twenty healthy volunteers who underwent ECG-triggered diffusion-weighted MR imaging of the kidney were included. The differences in ADC values of each kidney during different cardiac phases were compared. Additionally, the differences in maximum and minimum ADC values between the right and left kidney were also evaluated.

RESULTS

ADC values in the right and left kidney changed significantly during the cardiac cycle (P < 0.00001). Maximum and minimum ADC values during the cardiac cycle of the left kidney were significantly higher (P = 0.026 and 0.017, respectively) than those of the right kidney. Maximum ADC value in the left kidney had a significantly strong positive correlation with the left renal vein ratio (r = 0.83, P < 0.00001). In the right kidney, maximum ADC showed a weakly positive correlation with the diameter of the right renal vein (r = 0.45, P = 0.048).

CONCLUSION

ADC values of the kidney obtained using ECG-triggered diffusion-weighted MR imaging change significantly during the cardiac cycle. Maximum (systolic) ADC during the cardiac cycle of the left kidney was significantly higher than that of the right kidney, probably due to the anatomical difference in the renal vein.