• Advancing Unsupervised and Weakly Supervised Learning with Emphasis on Data-Driven Healthcare 

      Mikalsen, Karl Øyvind (Doctoral thesis; Doktorgradsavhandling, 2019-02-15)
      In healthcare, vast amounts of data are stored digitally in the electronic health records (EHRs). EHRs represent a largely untapped source of clinically relevant information, which combined with advances in machine learning, have the potential to transform healthcare into a more data-driven direction. However, due to the complexity and poor quality of the EHRs, data-driven healthcare is facing many ...
    • AI Risk Score on Screening Mammograms Preceding Breast Cancer Diagnosis 

      Larsen, Marthe; Olstad, Camilla Flåt; Koch, Henrik Wethe; Martiniussen, Marit Almenning; Hoff, Solveig Kristin Roth; Lund-Hanssen, Håkon; Solli, Helene; Mikalsen, Karl Øyvind; Auensen, Steinar; Nygård, Jan Franz; Lång, Kristina; Chen, Yan; Hofvind, Solveig Sand-Hanssen (Journal article; Tidsskriftartikkel; Peer reviewed, 2023-10-17)
      More than 38% of both screen-detected and interval cancers were assigned the highest artificial intelligence risk score on screening mammograms that preceded breast cancer diagnosis.<p> <p>Background - Few studies have evaluated the role of artificial intelligence (AI) in prior screening mammography.<p> <p>Purpose - To examine AI risk scores assigned to screening mammography in women who were ...
    • Analysis of free text in electronic health records for identification of cancer patient trajectories 

      Jensen, Kasper; Soguero-Ruiz, Cristina; Mikalsen, Karl Øyvind; Lindsetmo, Rolv-Ole; Kouskoumvekaki, Irene; Girolami, Mark; Skrovseth, Stein Olav; Augestad, Knut Magne (Journal article; Tidsskriftartikkel; Peer reviewed, 2017-04-07)
      With an aging patient population and increasing complexity in patient disease trajectories, physicians are often met with complex patient histories from which clinical decisions must be made. Due to the increasing rate of adverse events and hospitals facing financial penalties for readmission, there has never been a greater need to enforce evidence-led medical decision-making using available ...
    • A clinically motivated self-supervised approach for content-based image retrieval of CT liver images 

      Wickstrøm, Kristoffer; Østmo, Eirik Agnalt; Radiya, Keyur; Mikalsen, Karl Øyvind; Kampffmeyer, Michael; Jenssen, Robert (Journal article; Tidsskriftartikkel; Peer reviewed, 2023-05-09)
      Deep learning-based approaches for content-based image retrieval (CBIR) of computed tomography (CT) liver images is an active field of research, but suffer from some critical limitations. First, they are heavily reliant on labeled data, which can be challenging and costly to acquire. Second, they lack transparency and explainability, which limits the trustworthiness of deep CBIR systems. We address ...
    • Clinically relevant features for predicting the severity of surgical site infections 

      Boubekki, Ahcene; Myhre, Jonas Nordhaug; Luppino, Luigi Tommaso; Mikalsen, Karl Øyvind; Revhaug, Arthur; Jenssen, Robert (Journal article; Tidsskriftartikkel, 2021)
      Surgical site infections are hospital-acquired infections resulting in severe risk for patients and significantly increased costs for healthcare providers. In this work, we show how to leverage irregularly sampled preoperative blood tests to predict, on the day of surgery, a future surgical site infection and its severity. Our dataset is extracted from the electronic health records of patients who ...
    • Consensus Clustering Using kNN Mode Seeking 

      Myhre, Jonas Nordhaug; Mikalsen, Karl Øyvind; Løkse, Sigurd; Jenssen, Robert (Chapter; Bokkapittel, 2015-06-09)
      In this paper we present a novel clustering approach which combines two modern strategies, namely consensus clustering, and two stage clustering as represented by the mean shift spectral clustering algorithm. We introduce the recent kNN mode seeking algorithm in the consensus clustering framework, and the information theoretic kNN Cauchy Schwarz divergence as foundation for spectral clustering. In ...
    • Deforming the vacuum. On the physical origin and numerical calculation of the Casimir effect. 

      Mikalsen, Karl Øyvind (Master thesis; Mastergradsoppgave, 2014-05-14)
      A new method for calculating the Casimir force between compact objects was introduced in May 2012 by Per Jakobsen and Isak Kilen. In this method a regularization procedure is used to reduce the pressure to the solution of an integral equation defined on the boundaries of the objects. In this thesis the method is further developed by extending from a 2D to a 3D massless scalar field, subject to ...
    • Improving Quality of ICD-10 (International Statistical Classification of Diseases, Tenth Revision) Coding Using AI: Protocol for a Crossover Randomized Controlled Trial 

      Chomutare, Taridzo Fred; Lamproudis, Anastasios; Budrionis, Andrius; Olsen Svenning, Therese; Hind, Lill Irene; Ngo, Phuong Dinh; Mikalsen, Karl Øyvind; Dalianis, Hercules (Journal article; Tidsskriftartikkel; Peer reviewed, 2024-03-12)
      Background: Computer-assisted clinical coding (CAC) tools are designed to help clinical coders assign standardized codes, such as the ICD-10 (International Statistical Classification of Diseases, Tenth Revision), to clinical texts, such as discharge summaries. Maintaining the integrity of these standardized codes is important both for the functioning of health systems and for ensuring data used ...
    • A Kernel to Exploit Informative Missingness in Multivariate Time Series from EHRs 

      Mikalsen, Karl Øyvind; Ruiz, Cristina Soguero; Jenssen, Robert (Chapter; Bokkapittel, 2020)
      A large fraction of the electronic health records (EHRs) consists of clinical measurements collected over time, such as lab tests and vital signs, which provide important information about a patient’s health status. These sequences of clinical measurements are naturally represented as time series, characterized by multiple variables and large amounts of missing data, which complicate the analysis. ...
    • The Kernelized Taylor Diagram 

      Wickstrøm, Kristoffer; Johnson, Juan Emmanuel; Løkse, Sigurd Eivindson; Camps-Valls, Gusatu; Mikalsen, Karl Øyvind; Kampffmeyer, Michael; Jenssen, Robert (Journal article; Tidsskriftartikkel; Peer reviewed, 2023-02-02)
      This paper presents the kernelized Taylor diagram, a graphical framework for visualizing similarities between data populations. The kernelized Taylor diagram builds on the widely used Taylor diagram, which is used to visualize similarities between populations. However, the Taylor diagram has several limitations such as not capturing non-linear relationships and sensitivity to outliers. To address ...
    • Learning representations of multivariate time series with missing data 

      Bianchi, Filippo Maria; Livi, Lorenzo; Mikalsen, Karl Øyvind; Kampffmeyer, Michael C.; Jenssen, Robert (Journal article; Tidsskriftartikkel; Peer reviewed, 2019-07-19)
      Learning compressed representations of multivariate time series (MTS) facilitates data analysis in the presence of noise and redundant information, and for a large number of variates and time steps. However, classical dimensionality reduction approaches are designed for vectorial data and cannot deal explicitly with missing values. In this work, we propose a novel autoencoder architecture based on ...
    • Learning similarities between irregularly sampled short multivariate time series from EHRs 

      Mikalsen, Karl Øyvind; Bianchi, Filippo Maria; Soguero-Ruiz, Cristina; Skrøvseth, Stein Olav; Lindsetmo, Rolv-Ole; Revhaug, Arthur; Jenssen, Robert (Conference object; Konferansebidrag, 2016-12-04)
      A large fraction of the Electronic Health Records consists of clinical multivariate time series. Building models for extracting information from these is important for improving the understanding of diseases, patient care and treatment. Such time series are oftentimes particularly challenging since they are characterized by multiple, possibly dependent variables, length variability and irregular ...
    • Maximizing Interpretability and Cost-Effectiveness of Surgical Site Infection (SSI) Predictive Models Using Feature-Specific Regularized Logistic Regression on Preoperative Temporal Data 

      Kocbek, Primoz; Fijacko, Nino; Soguero-Ruiz, Cristina; Mikalsen, Karl Øyvind; Maver, Uros; Brzan, Petra Povalej; Stozer, Andraz; Jenssen, Robert; Skrøvseth, Stein Olav; Stiglic, Gregor (Journal article; Tidsskriftartikkel; Peer reviewed, 2019-02-19)
      This study describes a novel approach to solve the surgical site infection (SSI) classification problem. Feature engineering has traditionally been one of the most important steps in solving complex classification problems, especially in cases with temporal data. The described novel approach is based on abstraction of temporal data recorded in three temporal windows. Maximum likelihood L1-norm ...
    • Mixing up contrastive learning: Self-supervised representation learning for time series 

      Wickstrøm, Kristoffer; Kampffmeyer, Michael; Mikalsen, Karl Øyvind; Jenssen, Robert (Journal article; Tidsskriftartikkel; Peer reviewed, 2022-02-14)
      The lack of labeled data is a key challenge for learning useful representation from time series data. However, an unsupervised representation framework that is capable of producing high quality representations could be of great value. It is key to enabling transfer learning, which is especially beneficial for medical applications, where there is an abundance of data but labeling is costly and time ...
    • Noisy multi-label semi-supervised dimensionality reduction 

      Mikalsen, Karl Øyvind; Soguero-Ruiz, Cristina; Bianchi, Filippo Maria; Jenssen, Robert (Journal article; Tidsskriftartikkel; Peer reviewed, 2019-01-29)
      Noisy labeled data represent a rich source of information that often are easily accessible and cheap to obtain, but label noise might also have many negative consequences if not accounted for. How to fully utilize noisy labels has been studied extensively within the framework of standard supervised machine learning over a period of several decades. However, very little research has been conducted ...
    • On the Use of Time Series Kernel and Dimensionality Reduction to Identify the Acquisition of Antimicrobial Multidrug Resistance in the Intensive Care Unit 

      Escudero-Arnanz, Oscar; Rodríguez-Álvarez, Joaquín; Mikalsen, Karl Øyvind; Jenssen, Robert; Soguero-Ruiz, Cristina (Conference object; Konferansebidrag, 2021)
      The acquisition of Antimicrobial Multidrug Resistance (AMR) in patients admitted to the Intensive Care Units (ICU) is a major global concern. This study analyses data in the form of multivariate time series (MTS) from 3476 patients recorded at the ICU of University Hospital of Fuenlabrada (Madrid) from 2004 to 2020. 18% of the patients acquired AMR during their stay in the ICU. The goal of ...
    • Performance and clinical applicability of machine learning in liver computed tomography imaging: a systematic review 

      Radiya, Keyur; Joakimsen, Henrik Lykke; Mikalsen, Karl Øyvind; Aahlin, Eirik Kjus; Lindsetmo, Rolf Ole; Mortensen, Kim Erlend (Journal article; Tidsskriftartikkel; Peer reviewed, 2023-05-12)
      Objectives Machine learning (ML) for medical imaging is emerging for several organs and image modalities. Our objectives were to provide clinicians with an overview of this field by answering the following questions: (1) How is ML applied in liver computed tomography (CT) imaging? (2) How well do ML systems perform in liver CT imaging? (3) What are the clinical applications of ML in liver CT ...
    • RELAX: Representation Learning Explainability 

      Wickstrøm, Kristoffer; Trosten, Daniel Johansen; Løkse, Sigurd Eivindson; Boubekki, Ahcene; Mikalsen, Karl Øyvind; Kampffmeyer, Michael; Jenssen, Robert (Journal article; Tidsskriftartikkel; Peer reviewed, 2023-03-11)
      Despite the significant improvements that self-supervised representation learning has led to when learning from unlabeled data, no methods have been developed that explain what influences the learned representation. We address this need through our proposed approach, RELAX, which is the first approach for attribution-based explanations of representations. Our approach can also model the uncertainty ...
    • Robust clustering using a kNN mode seeking ensemble 

      Myhre, Jonas Nordhaug; Mikalsen, Karl Øyvind; Løkse, Sigurd; Jenssen, Robert (Journal article; Tidsskriftartikkel; Peer reviewed, 2017-12-02)
      In this paper we present a new algorithm for parameter-free clustering by mode seeking. Mode seeking, especially in the form of the mean shift algorithm, is a widely used strategy for clustering data, but at the same time prone to poor performance if the parameters are not chosen correctly. We propose to form a <i>clustering ensemble</i> consisting of repeated and bootstrapped runs of the recent kNN ...
    • Selective Imputation for Multivariate Time Series Datasets with Missing Values 

      Blazquez-Garcia, Ane; Wickstrøm, Kristoffer Knutsen; Yu, Shujian; Mikalsen, Karl Øyvind; Boubekki, Ahcene; Conde, Angel; Mori, Usue; Jenssen, Robert; Lozano, Jose A. (Journal article; Tidsskriftartikkel, 2023-01-31)
      Multivariate time series often contain missing values for reasons such as failures in data collection mechanisms. Since these missing values can complicate the analysis of time series data, imputation techniques are typically used to deal with this issue. However, the quality of the imputation directly affects the performance of downstream tasks. In this paper, we propose a selective imputation ...