On the determination of optimal tuning parameters for a space-variant LASSO problem using geometric and convex analysis techniques

Compressed Sensing (CS) comprises a wide range of theoretical and applied techniques to recover signals given a partial knowledge of their coefficients. It finds its applications in several fields, such as mathematics, physics, engineering, and many medical sciences, to name a few. Driven by our interest in the mathematics behind Magnetic Resonance Imaging (MRI) and Compressed Sensing (CS), we use convex analysis techniques to determine analytically the optimal tuning parameters of the space-variant LASSO with voxel-wise weighting, under assumptions on the fidelity term, either on the sign of its gradient or orthogonality-like conditions on its matrix. Finally, we conclude conjecturing what the explicit form of optimal parameters should be in the most general setting (hypotheses-free) of the space-variant LASSO

Consensus-based technical recommendations for clinical translation of renal BOLD MRI.

Harmonization of acquisition and analysis protocols is an important step in the validation of BOLD MRI as a renal biomarker. This harmonization initiative provides technical recommendations based on a consensus report with the aim to move towards standardized protocols that facilitate clinical translation and comparison of data across sites. We used a recently published systematic review paper, which included a detailed summary of renal BOLD MRI technical parameters and areas of investigation in its supplementary material, as the starting point in developing the survey questionnaires for seeking consensus. Survey data were collected via the Delphi consensus process from 24 researchers on renal BOLD MRI exam preparation, data acquisition, data analysis, and interpretation. Consensus was defined as ≥ 75% unanimity in response. Among 31 survey questions, 14 achieved consensus resolution, 12 showed clear respondent preference (65-74% agreement), and 5 showed equal (50/50%) split in opinion among respondents. Recommendations for subject preparation, data acquisition, processing and reporting are given based on the survey results and review of the literature. These technical recommendations are aimed towards increased inter-site harmonization, a first step towards standardization of renal BOLD MRI protocols across sites. We expect this to be an iterative process updated dynamically based on progress in the field

FUNCTIONAL MAGNETIC RESONANCE IMAGING OF THE KIDNEYS AS A TOOL TO ASSESS KIDNEY FUNCTION AND PREDICT KIDNEY FUNCTION DECLINE

Le projet de doctorat présenté dans cette thèse est constitué de trois sous-projets, tous impliquant l'Imagerie par Résonance Magnétique nucléaire (IRM) chez l'humain et tous en relation avec la fonction rénal. Les reins extraient (filtrent] des métabolites et des produits toxiques de la circulation. Chez l'adulte sains, ~100-120 ml/min de plasma sont filtrés par les glomérules. Lorsque que le dit Débit de Filtration Glomérulaire estimé [DFGe) est en dessous de 60 ml/min/1.73 m2, ou lorsque de l'albumine est perdue dans l'urine (>30mg/jour) à cause de glomérules endommagée pendant au moins 3 mois, la personne concernée souffre par définition d'Insuffisance Rénale Chronique (IRC). Malgré le fait que l'IRC est devenu un problème de santé publique majeur avec une prévalence global d'environ 10%, sa pathophysiologie reste incomplètement comprise. De plus, la prédiction de manière précise du déclin rénal ainsi que d'événements adverses reste impossible. Une des causes de cette lacune est l'absence de méthode de mesure précise in vivo pour l'humain. Le but principal de ce projet de doctorat était d'évaluer si l'IRM fonctionnel permettait d'améliorer la compréhension de la physiologie rénale et de gagner de l'information sur l'apparition et la progression de l'IRC afin de mieux prédire sont évolution a long terme. Les trois sous-projets ont impliqué trois techniques différentes utilisées dans l'imagerie par résonance magnétique, ces techniques étant susceptibles de gagner de l'information sur l'oxygénation rénale, la perfusion rénale et la régulation du sodium respectivement. Le but du premier sous-projet fut le développement d'une nouvelle technique semi- automatique pour l'analyse d'image de reins obtenue par l'IRM BOLD (de l'anglais Blood Oxygénation Level Dépendent) et de l'application de cette technique aux images BOLD de 225 patients et volontaires dans le cadre d'une étude clinique. La variabilité inter¬observateur de cette technique s'est avérée basse et son application dans l'étude clinique à révélée qu'une haute valeur R2* (correspondant à une basse oxygénation des tissus) dans le cortex rénal prédisait un déclin de la fonction rénale. Le deuxième sous-projet consista en l'implémentation et l'amélioration du protocole IRM pour la mesure de la Fraction de Perfusion (FP) dans les reins à l'aide de l'IRM de diffusion et du model IVIM (de l'anglais Intravoxel Incohérent Motion). Il s'est avéré que les résultats fourni par cette technique étaient plus vraisemblables si l'acquisition des images pondérées en diffusion était déclenchée au moment ou le sang avait une vitesse maximale dans les reins. Le troisième sous-projet consista en l'élaboration d'un dispositif expérimental et du post processing permettant la quantification de la concentration de sodium dans les muscles de la cuisse à l'aide de l'IRM au sodium. Deux dispositifs différents, comportant deux séquences IRM différentes, ont été testés. -- The PhD project presented in this thesis consists of three subprojects, ail involved in nuclear Magnetic Resonance Imaging (MRI) of humans and in relation with the kidney function. The kidneys remove ("filter") metabolites and toxic products from the circulation. In healthy adults, ~100-120 ml/min of plasma is filtered by the glomeruli. When the so called estimated Glomerular Filtration Rate (eGFR) is less than 60 ml/min/1.73 m2, or when albumin is lost in the urine (>30mg/day) due to damaged glomeruli for at least three months, a person suffers by définition from Chronic Kidney Disease (CKD). Despite the fact that CKD has become a major public health problem with a global prevalence around 10%, its pathophysiology is incompletely understood. Moreover, accurately predicting rénal function décliné and adverse outcome is still not possible. One of the reasons is the absence of précisé in vivo measurement methods for humans. The main aim of this PhD project was to assess whether functional MRI allows to increase the understanding of kidney physiology and to gain information on the development and progression of CKD in order to better predict its outcome. The three subprojects focus on three différent MRI related techniques susceptible to gain information on respectively kidney oxygénation, perfusion and sodium handling. The first subproject consisted in the development of a new semi-automatic post processing technique for the analysis of so called "Blood Oxygénation Level Dépendent MRI" (BOLD MRI) of kidneys and in the application of this technique in a clinical study involving 225 patients and volunteers. The inter observer variability of this new analysis method was low, and its applications in the clinical study revealed that the presence of high cortical R2* values [corresponding to low tissue oxygénation) predicts rénal function décliné. The second subproject consisted in integrating and improving existing MRI protocols for the measurement of the Perfusion Fraction (PF) in kidneys by the mean of diffusion MRI and the Intravoxel Incohérent Motion (IVIM) model. It was found that the Diffusion Weighted Images (DWI's) should be acquired at the instant of maximal blood velocity in kidneys for the measurement of the perfusion fraction. The third subproject consisted in setting up an experimental environment and the post processing for the quantification of sodium in the leg muscle with the help of sodium MRI. Two différent set ups, including two différent MRI sequences, were tested

Magnetic Resonance Imaging to Diagnose and Predict the Outcome of Diabetic Kidney Disease—Where Do We Stand?

Diabetic kidney disease (DKD) is a major public health problem and its incidence is rising. The disease course is unpredictable with classic biomarkers, and the search for new tools to predict adverse renal outcomes is ongoing. Renal magnetic resonance imaging (MRI) now enables the quantification of metabolic and microscopic properties of the kidneys such as single-kidney, cortical and medullary blood flow, and renal tissue oxygenation and fibrosis, without the use of contrast media. A rapidly increasing number of studies show that these techniques can identify early kidney damage in patients with DKD, and possibly predict renal outcome. This review provides an overview of the currently most frequently used techniques, a summary of the results of some recent studies, and our view on their potential applications, as well as the hurdles to be overcome for the integration of these techniques into the clinical care of patients with DKD

Magnetic Resonance Imaging to Diagnose and Predict the Outcome of Diabetic Kidney Disease—Where Do We Stand?

Diabetic kidney disease (DKD) is a major public health problem and its incidence is rising. The disease course is unpredictable with classic biomarkers, and the search for new tools to predict adverse renal outcomes is ongoing. Renal magnetic resonance imaging (MRI) now enables the quantification of metabolic and microscopic properties of the kidneys such as single-kidney, cortical and medullary blood flow, and renal tissue oxygenation and fibrosis, without the use of contrast media. A rapidly increasing number of studies show that these techniques can identify early kidney damage in patients with DKD, and possibly predict renal outcome. This review provides an overview of the currently most frequently used techniques, a summary of the results of some recent studies, and our view on their potential applications, as well as the hurdles to be overcome for the integration of these techniques into the clinical care of patients with DKD

Consensus-based technical recommendations for clinical translation of renal BOLD MRI.

Harmonization of acquisition and analysis protocols is an important step in the validation of BOLD MRI as a renal biomarker. This harmonization initiative provides technical recommendations based on a consensus report with the aim to move towards standardized protocols that facilitate clinical translation and comparison of data across sites. We used a recently published systematic review paper, which included a detailed summary of renal BOLD MRI technical parameters and areas of investigation in its supplementary material, as the starting point in developing the survey questionnaires for seeking consensus. Survey data were collected via the Delphi consensus process from 24 researchers on renal BOLD MRI exam preparation, data acquisition, data analysis, and interpretation. Consensus was defined as ≥ 75% unanimity in response. Among 31 survey questions, 14 achieved consensus resolution, 12 showed clear respondent preference (65-74% agreement), and 5 showed equal (50/50%) split in opinion among respondents. Recommendations for subject preparation, data acquisition, processing and reporting are given based on the survey results and review of the literature. These technical recommendations are aimed towards increased inter-site harmonization, a first step towards standardization of renal BOLD MRI protocols across sites. We expect this to be an iterative process updated dynamically based on progress in the field