Dynamic contrast-enhanced quantitative susceptibility mapping with ultrashort echo time MRI for evaluating renal function

Multiparametric Magnetic Resonance Investigations on Acute and Long-Term Kidney Injury

Acute kidney injury (AKI) is a frequent complication of critical illness and carries a significant risk of short- and long-term mortality. The prediction of the progression of AKI to long-term injury has been difficult for renal disease treatment. Radiologists are keen for the early detection of transition from AKI to long-term kidney injury, which would help in the preventive measures. The lack of established methods for early detection of long-term kidney injury underscores the pressing needs of advanced imaging technology that reveals microscopic tissue alterations during the progression of AKI. Fueled by recent advances in data acquisition and post-processing methods of magnetic resonance imaging (MRI), multiparametric MRI is showing great potential as a diagnostic tool for many kidney diseases. Multiparametric MRI studies offer a precious opportunity for real-time noninvasive monitoring of pathological development and progression of AKI to long-term injury. It provides insight into renal vasculature and function (arterial spin labeling, intravoxel incoherent motion), tissue oxygenation (blood oxygen level-dependent), tissue injury and fibrosis (diffusion tensor imaging, diffusion kurtosis imaging, T1 and T2 mapping, quantitative susceptibility mapping). The multiparametric MRI approach is highly promising but the longitudinal investigation on the transition of AKI to irreversible long-term impairment is largely ignored. Further optimization and implementation of renal MR methods in clinical practice will enhance our comprehension of not only AKI but chronic kidney diseases. Novel imaging biomarkers for microscopic renal tissue alterations could be discovered and benefit the preventative interventions. This review explores recent MRI applications on acute and long-term kidney injury while addressing lingering challenges, with emphasis on the potential value of the development of multiparametric MRI for renal imaging on clinical systems. EVIDENCE LEVEL: 1 TECHNICAL EFFICACY: Stage 2.

Rapid variable flip angle positive susceptibility contrast imaging for clinical metal seeds

Positive susceptibility contrast imaging (PSCI) based on susceptibility mapping exhibits excellent efficacy for visualizing magnetic resonance (MR)-compatible metallic devices because of their high magnetic susceptibility compared to that of human tissues. However, the long-acquisition time required by the two-dimensional fast spin echo (2D FSE)-based PSCI approach, impedes its practical applications in 3D imaging. In this study, a three-dimensional (3D) susceptibility-based variable flip angle (vFA) FSE sequence was proposed to accelerate data acquisition in the clinical radiotherapy applications of ex vivo and in vivo rapid 3D PSCI for the imaging of metal seeds. Here, the proposed scheme applied a 3D modulated vFA technique for refocused imaging with an extended echo-train sequence for sampling data. The scheme integrated the projection-onto-dipole fields (PDF) to remove the background field and accelerate PSCI by using a compressive sensing framework with a variable-densitysampling mask. The experiments involved some gelatin phantoms, porcine tissues and patients with scapular tumors and brachytherapy seeds. All of the experimental results showed that the proposed scheme could accelerate data acquisition of 3D PSCI at the reduction factors of 2 ∼ 5 while accurately localizing the actual positions of the brachytherapy seeds in the ex vivo and in vivo applications. The results were compared with those of the existing methods, including susceptibility gradient mapping using the original resolution (SUMO) and gradient echo acquisition for superparamagnetic particle (GRASP).

Nephron number and its determinants: a 2020 update

Studies of human nephron number have been conducted for well over a century and have uncovered a large variability in nephron number. However, the mechanisms influencing nephron endowment and loss, along with the etiology for the wide range among individuals are largely unknown. Advances in imaging technology have allowed investigators to revisit the principles of renal structure and physiology and their roles in the progression of kidney disease. Here, we will review the latest data on the influences impacting nephron number, innovations made over the last 6 years to understand and integrate renal structure and function, and new developments in the tools used to count nephrons in vivo.

Hardware Considerations for Preclinical Magnetic Resonance of the Kidney

Magnetic resonance imaging (MRI) is a noninvasive imaging technology that offers unparalleled anatomical and functional detail, along with diagnostic sensitivity. MRI is suitable for longitudinal studies due to the lack of exposure to ionizing radiation. Before undertaking preclinical MRI investigations of the kidney, the appropriate MRI hardware should be carefully chosen to balance the competing demands of image quality, spatial resolution, and imaging speed, tailored to the specific scientific objectives of the investigation. Here we describe the equipment needed to perform renal MRI in rodents, with the aim to guide the appropriate hardware selection to meet the needs of renal MRI applications.This publication 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 chapter on hardware considerations for renal MRI in small animals is complemented by two separate publications describing the experimental procedure and data analysis.

Feasibility of quantitative susceptibility mapping (QSM) of the human kidney

Objective

To evaluate the feasibility of in-vivo quantitative susceptibility mapping (QSM) of the human kidney.

Methods

An axial single-breath-hold 3D multi-echo sequence (acquisition time 33 s) was completed on a 3 T-MRI-scanner (Magnetom Prisma, Siemens Healthineers, Erlangen, Germany) in 19 healthy volunteers. Graph-cut-based unwrapping combined with the T₂*-IDEAL approach was performed to remove the chemical shift of fat and to quantify QSM of the upper abdomen. Mean susceptibility values of the entire, renal cortex and medulla in both kidneys and the liver were determined and compared. Five subjects were measured twice to examine the reproducibility. One patient with severe renal fibrosis was included in the study to evaluate the potential clinical relevance of QSM.

Results

QSM was successful in 17 volunteers and the patient with renal fibrosis. Anatomical structures in the abdomen were clearly distinguishable by QSM and the susceptibility values obtained in the liver were comparable to those found in the literature. The results showed a good reproducibility. Besides, the mean renal QSM values obtained in healthy volunteers (0.04 ± 0.07 ppm for the right and − 0.06 ± 0.19 ppm for the left kidney) were substantially higher than that measured in the investigated fibrotic kidney (− 0.43 ± − 0.02 ppm).

Conclusion

QSM of the human kidney could be a promising approach for the assessment of information about microscopic renal tissue structure. Therefore, it might further improve functional renal MR imaging.

Electronic supplementary material

The online version of this article (10.1007/s10334-020-00895-9) contains supplementary material, which is available to authorized users.

Feasibility of QSM in the human placenta

PURPOSE

Quantitative susceptibility mapping (QSM) is an emerging tool for the precise characterization of human tissue, including regional oxygenation. A critical function of the human placenta is oxygen transfer to the developing fetus, which remains difficult to study in utero. The purpose of this study is to investigate the feasibility of performing QSM in the human placenta in utero.

METHODS

In healthy pregnant women, 3D gradient echo data of the placenta were acquired with prospective respiratory gating at 1.5 Tesla and 3 Tesla. A brief period (6-7 min) of maternal hyperoxia was induced to increase placental oxygenation in a subset of women scanned at 3 Tesla, and data were acquired before and during oxygen administration. Susceptibility and T 2 ∗ / R 2 ∗ maps were reconstructed from gradient echo data, and mean and SD of these measures within the whole placenta were calculated.

RESULTS

A total of 54 women were studied at a mean gestational age of 30.7 ± 4.2 (range: 24 5/7-38 4/7) weeks. Susceptibility and T 2 ∗ maps demonstrated lobular contrast reflecting regional oxygenation difference at both field strengths. SD of susceptibilities, mean R 2 ∗ , and SD of R 2 ∗ of the placenta showed a linear relationship with gestational age (P < .01 for all). These measures were also responsive to maternal hyperoxia, and there was an increasing response with advancing gestational age (P < .01 for all).

CONCLUSION

This study demonstrates the feasibility of performing placental QSM in pregnant women and supports the potential for placental QSM to provide noninvasive in vivo assessment of placental oxygenation.

Exercise-induced calf muscle hyperemia: Rapid mapping of magnetic resonance imaging using deep learning approach

Exercise-induced hyperemia in calf muscles was recently shown to be quantifiable with high-resolution magnetic resonance imaging (MRI). However, processing of the MRI data to obtain muscle-perfusion maps is time-consuming. This study proposes to substantially accelerate the mapping of muscle perfusion using a deep-learning method called artificial neural network (NN). Forty-eight MRI scans were acquired from 21 healthy subjects and patients with peripheral artery disease (PAD). For optimal training of NN, different training-data sets were compared, investigating the effect of data diversity and reference perfusion accuracy. Reference perfusion was estimated by tracer kinetic model fitting initialized with multiple values (multigrid model fitting). Result: The NN method was much faster than tracer kinetic model fitting. To generate a perfusion map of matrix 128 × 128 on a same computer, multigrid model fitting took about 80 min, single-grid or regular model fitting about 3 min, while the NN method took about 1 s. Compared to the reference values, NN trained with a diverse group gave estimates with mean absolute error (MAE) of 15.9 ml/min/100g and correlation coefficient (R) of 0.949, significantly more accurate than regular model fitting (MAE 22.3 ml/min/100g, R 0.889, p < .001). Conclusion: the NN method enables rapid perfusion mapping, and if properly trained, estimates perfusion with accuracy comparable to multigrid model fitting.

High-resolution MRI of kidney microstructures at 7.05 T with an endo-colonic Wireless Amplified NMR detector

To map the hemodynamic responses of kidney microstructures at 7.05 T with improved sensitivity, a Wireless Amplified NMR Detector (WAND) with cylindrical symmetry was fabricated as an endoluminal detector that can convert externally provided wireless signal at 600.71 MHz into amplified MR signals at 300.33 MHz. When this detector was inserted inside colonic lumens to sensitively observe adjacent kidneys, it could clearly identify kidney microstructures in the renal cortex and renal medullary. Owing to the higher achievable spatial resolution, differential hemodynamic responses of kidney microstructures under different breathing conditions could be individually quantified to estimate the underlying correlation between oxygen bearing capability and local levels of oxygen unsaturation. The WAND's ability to map Blood Oxygen Level Dependent (BOLD) signal responses in heterogeneous microstructures will pave way for early-stage diagnosis of kidney diseases, without the use of contrast agents for reduced tissue retention and toxicity.

Dynamic contrast-enhanced MRI promotes early detection of toxin-induced acute kidney injury

Acute kidney injury (AKI) is a common cause of morbidity and mortality in hospitalized patients. Nevertheless, there is limited ability to diagnose AKI in its earliest stages through the collection of structural and functional information. Magnetic resonance imaging (MRI) is increasingly being used to provide structural and functional data that characterize the injured kidney. Dynamic contrast-enhanced (DCE) MRI is an imaging modality with robust spatial and temporal resolution; however, its ability to detect changes in kidney function following AKI has not been determined. We hypothesized that DCE MRI would detect a prolongation in contrast transit time following toxin-induced AKI earlier than commonly used serum and tissue biomarkers. To test our hypothesis, we injected mice with either vehicle or cisplatin (30 mg/kg) and performed DCE MRI at multiple time points. We found that commonly used kidney injury biomarkers, including creatinine, blood urea nitrogen, and neutrophil gelatinase-associated lipocalin, did not rise until day 2 following cisplatin. Tissue levels of the proinflammatory cytokines and chemokines, tumor necrosis factor-α, interleukin (IL)-1β, IL-1α, IL-6, C-C motif chemokine ligand 2, and C-X-C motif chemokine ligand 2 similarly did not upregulate until day 2 following cisplatin. However, the time to peak intensity of contrast in the renal collecting system was already prolonged at day 1 following cisplatin compared with vehicle-treated mice. This intensity change mirrored changes in kidney injury as measured by histological analysis and in transporter expression in the proximal tubule. Taken together, DCE MRI is a promising preclinical imaging modality that is useful for assessing functional capacity of the kidney in the earliest stages following AKI.

Multicenter reproducibility of quantitative susceptibility mapping in a gadolinium phantom using MEDI+0 automatic zero referencing

PURPOSE

To determine the reproducibility of quantitative susceptibility mapping at multiple sites on clinical and preclinical scanners (1.5 T, 3 T, 7 T, and 9.4 T) from different vendors (Siemens, GE, Philips, and Bruker) for standardization of multicenter studies.

METHODS

Seven phantoms distributed from the core site, each containing 5 compartments with gadolinium solutions with fixed concentrations between 0.625 mM and 10 mM. Multi-echo gradient echo scans were performed at 1.5 T, 3 T, 7 T, and 9.4 T on 12 clinical and 3 preclinical scanners. DICOM images from the scans were processed into quantitative susceptibility maps using the Laplacian boundary value (LBV) and MEDI+0 automatic uniform reference algorithm. Region of interest (ROI) analyses were performed by a physicist to determine agreement between results from all sites. Measurement reproducibility was assessed using regression, Bland-Altman plots, and the intra-class correlation coefficient (ICC).

RESULTS

Quantitative susceptibility mapping (QSM) from all scanners had similar, artifact-free visual appearance. Regression analysis showed a linear relationship between gadolinium concentrations and average QSM measurements for all phantoms (y = 350x - 0.0346, r² >0.99). The SD of measurements increased almost linearly from 32 ppb to 230 ppb as the measured susceptibility increased from 0.26 ppm to 3.56 ppm. A Bland-Altman plot showed the bias, upper, and lower limits of agreement for all comparisons were -10, -210, and 200 ppb, respectively. The ICC was 0.991 with a 95% CI (0.973, 0.99).

CONCLUSIONS

QSM shows excellent multicenter reproducibility for a large range of susceptibility values encountered in cranial and extra-cranial applications on a diverse set of scanner platforms.

Renal Functional MRI and Its Application

Renal function varies according to the nature and stage of diseases. Renal functional magnetic resonance imaging (fMRI), a technique considered superior to the most common method used to estimate the glomerular filtration rate, allows for noninvasive, accurate measurements of renal structures and functions in both animals and humans. It has become increasingly prevalent in research and clinical applications. In recent years, renal fMRI has developed rapidly with progress in MRI hardware and emerging postprocessing algorithms. Function-related imaging markers can be acquired via renal fMRI, encompassing water molecular diffusion, perfusion, and oxygenation. This review focuses on the progression and challenges of the main renal fMRI methods, including dynamic contrast-enhanced MRI, blood oxygen level-dependent MRI, diffusion-weighted imaging, diffusion tensor imaging, arterial spin labeling, fat fraction imaging, and their recent clinical applications.

LEVEL OF EVIDENCE

5 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2018;48:863-881.

MRI-Based Quantification of Magnetic Susceptibility in Gel Phantoms: Assessment of Measurement and Calculation Accuracy

The local magnetic field inside and around an object in a magnetic resonance imaging unit depends on the magnetic susceptibility of the object being magnetized, in combination with its geometry/orientation. Magnetic susceptibility can thus be exploited as a source of tissue contrast, and susceptibility imaging may also become a useful tool in contrast agent quantification and for assessment of venous oxygen saturation levels. In this study, the accuracy of an established procedure for quantitative susceptibility mapping (QSM) was investigated. Three gel phantoms were constructed with cylinders of varying susceptibility and geometry. Experimental results were compared with simulated and analytically calculated data. An expected linear relationship between estimated susceptibility and concentration of contrast agent was observed. Less accurate QSM-based susceptibility values were observed for cylindrical objects at angles, relative to the main magnetic field, that were close to or larger than the magic angle. Results generally improved for large objects/high spatial resolution and large volume coverage. For simulated phase maps, accurate susceptibility quantification by QSM was achieved also for more challenging geometries. The investigated QSM algorithm was generally robust to changes in measurement and calculation parameters, but experimental phase data of sufficient quality may be difficult to obtain in certain geometries.

Measurement of Murine Single-Kidney Glomerular Filtration Rate Using Dynamic Contrast-Enhanced MRI

PURPOSE

To develop and validate a method for measuring murine single-kidney glomerular filtration rate (GFR) using dynamic contrast-enhanced MRI (DCE-MRI).

METHODS

This prospective study was approved by the Institutional Animal Care and Use Committee. A fast longitudinal relaxation time (T1 ) measurement method was implemented to capture gadolinium dynamics (1 s/scan), and a modified two-compartment model was developed to quantify GFR as well as renal perfusion using 16.4T MRI in mice 2 weeks after unilateral renal artery stenosis (RAS, n = 6) or sham (n = 8) surgeries. This approach was validated by comparing model-derived GFR and perfusion to those obtained by fluorescein isothiocyanante (FITC)-inulin clearance and arterial spin labeling (ASL), respectively, using the Pearson's and Spearman's rank correlations and Bland-Altman analysis.

RESULTS

The compartmental model provided a good fitting to measured gadolinium dynamics in both normal and RAS kidneys. The proposed DCE-MRI method offered assessment of single-kidney GFR and perfusion, comparable to the FITC-inulin clearance (Pearson's correlation coefficient r = 0.95 and Spearman's correlation coefficient ρ = 0.94, P < 0.0001, and mean difference -7.0 ± 11.0 μL/min) and ASL (r = 0.92 and ρ = 0.84, P < 0.0001, and mean difference 4.4 ± 66.1 mL/100 g/min) methods.

CONCLUSION

The proposed DCE-MRI method may be useful for reliable noninvasive measurements of single-kidney GFR and perfusion in mice. Magn Reson Med 79:2935-2943, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

MRI gradient-echo phase contrast of the brain at ultra-short TE with off-resonance saturation

Larmor-frequency shift or image phase measured by gradient-echo sequences has provided a new source of MRI contrast. This contrast is being used to study both the structure and function of the brain. So far, phase images of the brain have been largely obtained at long echo times as maximum phase signal-to-noise ratio (SNR) is achieved at TE = T2* (∼40 ms at 3T). The structures of the brain, however, are compartmentalized and complex with a wide range of signal relaxation times. At such long TE, the short-T2 components are largely attenuated and contribute minimally to phase contrast. The purpose of this study was to determine whether proton gradient-echo images of the brain exhibit phase contrast at ultra-short TE (UTE). Our data showed that UTE images acquired at 7 T without off-resonance saturation do not contain significant phase contrast between gray and white matter. However, UTE images of the brain can attain strong phase contrast even at a nominal TE of 106 μs by using off-resonance RF saturation pulses, which provide direct saturation of ultra-short-T2 components and indirect saturation of longer-T2 components via magnetization transfer. In addition, phase contrast between gray and white matter acquired at UTE with off-resonance saturation is reversed compared to that of the long-T2 signals acquired at long TEs. This finding opens up a potential new way to manipulate image phase contrast of the brain. By accessing short and ultra-short-T2 species, MRI phase images may further improve the characterization of tissue microstructure in the brain.

Novel contrast mixture achieves contrast resolution of human bladder wall suitable for T1 mapping: applications in interstitial cystitis and beyond

PURPOSE

Instillation of novel contrast mixture (NCM) was recently shown to improve the contrast resolution of rat bladder wall with high contrast-to-noise ratio (CNR). Here, the clinical safety and the feasibility of NCM-enhanced MRI to achieve artifact-free visualization of human bladder wall suitable for quantitative measurement of the magnetic resonance (MR) longitudinal relaxation time (T1) was assessed.

METHODS

Six female subjects [two controls and two with Hunner-type interstitial cystitis IC and two with non-Hunner-type IC] consented for MRI at 3 T before and after instillation of NCM [4 mM gadobutrol and 5 mM ferumoxytol in 50 mL of sterile water for injection]. Single breath-hold fast MR acquisition in large readout bandwidth for 5-mm-thick single slice with variable flip angle was applied to minimize the motion and chemical shift artifacts in measurements of bladder wall thickness (BWT), CNR and T1 from 20 pixels.

RESULTS

NCM instillation in subjects did not evoke pain or discomfort. Fourfold increase in bladder wall CNR (*p < 0.02) and pixel size of 0.35 mm with minimal influence of artifacts allowed accurate determination of bladder wall thinning ~ 0.46 mm from 50 mL NCM (*p < 0.05). Pre-contrast bladder wall T1 of 1544 ± 34.2 ms was shortened to 860.09 ± 13.95 ms in Hunner-type IC (*p < 0.0001) relative to only 1257.42 ± 20.59 and 1258.16 ± 6.16 ms in non-Hunner-type IC and controls, respectively.

CONCLUSION

Findings demonstrate the safety and feasibility of NCM-enhanced MRI to achieve artifact-free differential contrast and spatial resolution of human bladder wall, which is suitable for measuring BWT and pixel-wise measurement of T1 in post-contrast setting.

The clinical utility of QSM: disease diagnosis, medical management, and surgical planning

Quantitative susceptibility mapping (QSM) is an MR technique that depicts and quantifies magnetic susceptibility sources. Mapping iron, the dominant susceptibility source in the brain, has many important clinical applications. Herein, we review QSM applications in the diagnosis, medical management, and surgical treatment of disease. To assist in early disease diagnosis, QSM can identify elevated iron levels in the motor cortex of amyotrophic lateral sclerosis patients, in the substantia nigra of Parkinson's disease (PD) patients, in the globus pallidus, putamen, and caudate of Huntington's disease patients, and in the basal ganglia of Wilson's disease patients. Additionally, QSM can distinguish between hemorrhage and calcification, which could prove useful in tumor subclassification, and can measure microbleeds in traumatic brain injury patients. In guiding medical management, QSM can be used to monitor iron chelation therapy in PD patients, to monitor smoldering inflammation of multiple sclerosis (MS) lesions after the blood-brain barrier (BBB) seals, to monitor active inflammation of MS lesions before the BBB seals without using gadolinium, and to monitor hematoma volume in intracerebral hemorrhage. QSM can also guide neurosurgical treatment. Neurosurgeons require accurate depiction of the subthalamic nucleus, a tiny deep gray matter nucleus, prior to inserting deep brain stimulation electrodes into the brains of PD patients. QSM is arguably the best imaging tool for depiction of the subthalamic nucleus. Finally, we discuss future directions, including bone QSM, cardiac QSM, and using QSM to map cerebral metabolic rate of oxygen. Copyright © 2016 John Wiley & Sons, Ltd.

MRI tools for assessment of microstructure and nephron function of the kidney

MRI can provide excellent detail of renal structure and function. Recently, novel MR contrast mechanisms and imaging tools have been developed to evaluate microscopic kidney structures including the tubules and glomeruli. Quantitative MRI can assess local tubular function and is able to determine the concentrating mechanism of the kidney noninvasively in real time. Measuring single nephron function is now a near possibility. In parallel to advancing imaging techniques for kidney microstructure is a need to carefully understand the relationship between the local source of MRI contrast and the underlying physiological change. The development of these imaging markers can impact the accurate diagnosis and treatment of kidney disease. This study reviews the novel tools to examine kidney microstructure and local function and demonstrates the application of these methods in renal pathophysiology.