Advancing Artificial Intelligence in Sensors, Signals, and Imaging Informatics

Summary

Objective: To identify research works that exemplify recent developments in the field of sensors, signals, and imaging informatics.

Method: A broad literature search was conducted using PubMed and Web of Science, supplemented with individual papers that were nominated by section editors. A predefined query made from a combination of Medical Subject Heading (MeSH) terms and keywords were used to search both sources. Section editors then filtered the entire set of retrieved papers with each paper having been reviewed by two section editors. Papers were assessed on a three-point Likert scale by two section editors, rated from 0 (do not include) to 2 (should be included). Only papers with a combined score of 2 or above were considered.

Results: A search for papers was executed at the start of January 2019, resulting in a combined set of 1,459 records published in 2018 in 119 unique journals. Section editors jointly filtered the list of candidates down to 14 nominations. The 14 candidate best papers were then ranked by a group of eight external reviewers. Four papers, representing different international groups and journals, were selected as the best papers by consensus of the International Medical Informatics Association (IMIA) Yearbook editorial board.

Conclusions: The fields of sensors, signals, and imaging informatics have rapidly evolved with the application of novel artificial intelligence/machine learning techniques. Studies have been able to discover hidden patterns and integrate different types of data towards improving diagnostic accuracy and patient outcomes. However, the quality of papers varied widely without clear reporting standards for these types of models. Nevertheless, a number of papers have demonstrated useful techniques to improve the generalizability, interpretability, and reproducibility of increasingly sophisticated models.

Keywords

Sensors, signals, imaging informatics - medical informatics - artificial intelligence - machine learning

Introduction

No other field has experienced the magnitude of impact of recent developments in artificial intelligence (AI) and machine learning (ML) like sensors, signals, and imaging informatics (SSII). Keeping pace with the rapid evolution in this field is challenging. Readers are encouraged to refer to a number of high quality review articles that have been published in 2018 in the areas of sensors and signals [1], [2], imaging informatics [3], [4], and both [5]. The selected candidate and best papers highlighted in this synopsis provide a representative sampling of noteworthy work being conducted internationally that have advanced aspects of AI/ ML in healthcare involving sensors, signals, and imaging informatics data.

Paper Selection Process

The process of searching the literature for candidate best papers by the SSII section remained largely unchanged from prior years. Two large bibliographic indices, PubMed and Web of Science, were searched using a combination of Medical Subject Headings (MeSH) and terms provided by the section editors.

The objective of the search process was to generate a manageable list of papers for section editors to review while encompassing the breadth of work that has been conducted this past year and the diversity of journals. For sensors and signals, terms encompassed computer-assisted processing, physiologic monitoring, and biosensing techniques (e.g., electroretinogram, photoplethysmography). For imaging informatics, terms included imaging modalities (e.g., computed tomography, electrocardiogram), image processing (e.g., segmentation, registration), and machine and deep learning techniques (e.g., convolutional neural networks, generative adversarial networks). The query was further restricted to journal articles written in English, describing original research, including an abstract, and published in 2018. Scripts used to execute the full queries are available upon request from the corresponding author.

The original search was performed during the first week of January 2019. The search result was made of 1,459 papers published in 119 unique journals. Results were imported into BibReview [6], a software tool primarily used to standardize the formatting of citation and abstract formats and to perform an initial review. Citations were then exported into Google Sheets, which allowed real-time concurrent review of the citations by all section editors.

The identification of candidate best papers proceeded in three phases. In the first phase, each section editor was assigned a set of papers to identify relevant papers based on their titles and/or abstracts. The assignments were made such that each paper was reviewed by two section editors. Papers were rated on a Likert scale between 0 to 2, where 0 was assigned to papers that were irrelevant and should be removed from consideration, and 2 was given to papers that were highly relevant and should be considered. Papers with a cumulative score of 2 or above (n=24) were kept. In the second phase, every section editor independently reviewed these remaining papers and again assigned a score. The section editors discussed the papers based on the results of the scoring and nominated 14 candidate best papers for the SSII section. In the third phase, the candidate best papers were sent to a group of eight external reviewers, comprised of other Yearbook editorial members and representative researchers from the SSII community who were not involved in the production of the Yearbook. Each paper was considered by at least five external reviewers and assigned an overall score based on criteria such as scientific impact, quality of content, originality, and clarity. Section editors nominated five best papers taking the external scores into account while selecting papers that are representative of the diverse research, institutions, and journals in the field. Finally, the Yearbook editorial board discussed the nominations and identified any potential overlap with other sections, and settled on four best papers for the SSII section (see [Table 1]).

Table 1 Best paper selection of articles for the IMIA Yearbook of Medical Informatics 2019 in the section 'Sensors, Signals, and Imaging Informatics’. The articles are listed in alphabetical order of the first author’s surname.

Section Sensors, Signals, and Imaging Informatics
▪ Bandeira Diniz JO, Bandeira Diniz PH, Azevedo Valente TL, Correa Silva A, de Paiva AC, Gattass M. Detection of mass regions in mammograms by bilateral analysis adapted to breast density using similarity indexes and convolutional neural networks. Comput Methods Programs Biomed 2018 Mar;1 56:1 91-207.
▪ Lee H, Yune S, Mansouri M, Ki M, Tajmir SH, Guerrier CE, Ebert SA, Pomerantz SR, Romero JM, Kamalian S, Gonzalez RG, Lev MH, Do S. An explainable deep-learning algorithm for the detection of acute intracranial hemorrhage from small datasets. Nat Biomed Eng 2019 Mar;3(3):1 73-82.
▪ Samad MD, Ulloa A, Wehner GJ, Jing L, Hartzel D, Good CW, Williams BA, Haggerty CM, Fornwalt BK. Predicting survival from large echocardiography and electronic health record datasets: optimization with machine learning. JACC Cardiovasc Imaging 2019 Apr;12(4):681-9.
▪ Vasilakakis MD, lakovidis DK, Spyrou E, Koulaouzidis A. DINOSARC: color features based on selective aggregation of chromatic image components for wireless capsule endoscopy. Comput Math Methods Med 2018 Sep 3;201 8:2026962.

Selected Best Papers and Their Implications

The paper by Lee et al., [7] investigates variants of deep convolutional neural networks to classify the presence of intracranial hemorrhage (ICH) and if present, its subtype. The work is noteworthy for its attempt to visually explain how the model is processing input images. As models become increasingly complex, the uninterpretable “black box” nature of ML algorithms needs to be addressed [8]. Class activation maps and activation maximization are techniques that help spatially visualize features or regions where the networks are focusing to make the subtype prediction [5]. Lee et al., [7] provided an effective demonstration of how attention maps are able to help radiologist readers understand regions that contributed to the model’s prediction of ICH subtype. They found a 78.1% overlap rate when interpreting regions where the model focused its attention versus segmentations of ICH regions done by a radiologist. Moreover, the authors investigated variants of the model architecture (i.e., VGG16, ResNet-50, Inception-v3, Inception-ResNet-v2) and the use of various data augmentation and preprocessing techniques to generate the best performing ensemble model.

The paper by Samad et al., [9] predicts five-year mortality using a combination of clinical, ejection function, and echocardio- graphic measurements. Authors showed that a random forest model was able to achieve high performance (AUC = 0.89) as compared to existing clinical risk scores with value demonstrated from the inclusion of echocardiography-derived features. The study’s dataset is impressive in scope. The cohort consisted of 171,510 patients who underwent 331,317 echocardiograms and were extracted from the authors’ institutional medical record system. However, despite the sheer size of the dataset, missing values remained a practical challenge. The authors discussed techniques such as multivariate imputation by chained equations to address this issue. They also utilized the “mean decrease impurity” approach [10] to characterize the relative importance of features in the random forest model. In summary, the authors described a large-scale study leveraging ML framework to generate accurate outcome predictions using real-world clinical cardiology data.

The paper by Bandeira Diniz et al., [11] describes a pipeline to automate the detection of masses in mammograms. Using a set of models based on convolutional neural networks, the authors first classified whether the mammogram showed dense or non-dense breast tissue. The images were then registered, breast parenchyma was segmented, and candidate masses were extracted. Two separate models tailored to dense versus non-dense breasts were used to perform the final determination of whether a candidate was a mass. The manuscript was notably detailed in the description of the method developed, outlining the entire pipeline. The work was also noteworthy in its use of domain knowledge to facilitate classification tasks. While the field has largely relied on the inherent ability of deep learning techniques to discover salient features directly from the data, incorporating domain knowledge as part of the process of modeling training can improve overall performance. The authors demonstrated that the use of bilateral analysis (comparing mammograms of each breast to each other with the assumption that they should be symmetrical) and knowledge of breast density (dense versus non-dense) would produce better models.

The paper by Vasilakakis et al., [12] presents an approach to extract salient features from a large set of color images taken during wireless capsule endoscopy (WCE). During this procedure, a pill containing a color camera passes through a patient’s bowel, collecting images with the goal of detecting abnormalities such as bleeding, polyps, and ulcers. The reliable extraction of salient features from a high dimensional dataset is a critical aspect of ML. The paper describes an automated approach to generate color-based salient points that are then used to generate local and global image descriptors that characterized potential abnormalities. The authors showed that their approach, distances on selective aggregation of chromatic image components (DINOSARC), generated more relevant points in a more efficient manner as compared to other detectors such as scale-invariant feature transform (SIFT).

Discussion and Conclusion

Identifying the best papers for this year’s Yearbook afforded an opportunity to gain a high-level characterization of the current trends in the broad field of SSII. The use of deep learning methods remains a trending area of research. However, as researchers attempt to bridge the gap from experimentation to clinical translation of these methods, papers are addressing issues such as model interpretability [7].

Evaluations that involve comparing the performance of human readers against AI/ ML are increasingly being reported [13], [14]. Studies have also moved towards training and testing on larger datasets [9], [11], [14], [15]. While a majority of papers utilized deep learning techniques, several of the represented papers apply traditional machine learning techniques (e.g., logistic regression, random forest, fuzzy support vector machines) [9], [16], traditional signal processing [17], and structural co-occurrence matrix [18]. Moreover, papers touched upon the topics that are agnostic to the modeling approach, including the detection of salient features from a high dimensional feature space [12], and the integration of data from multiple heterogeneous data sources [9].

The diversity of institutions and countries represented in the 14 candidate best papers has increased with the greatest number of papers coming from institutions in the United States, the United Kingdom, and China. Nevertheless, the field continues to see a wide range of heterogeneity in how models and their results are reported in the literature. The availability of datasets used to train models or the models themselves is scarce, making fair comparisons of model performance difficult. One candidate paper proposed a standardized pipeline to facilitate model comparisons, which is a step towards this direction [19].

In conclusion, SSII are rapidly evolving fields with a growing number of successful examples of how AI/ML can improve sensor, signal, and imaging data. The next horizon for AI in SSII is the continued development, refinement, and translation of these algorithms in an ethical, interpretable, and reproducible manner.

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