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Temporal bone radiology report classification using open source machine learning and natural langue processing libraries
© The Author(s). 2016
Received: 23 October 2015
Accepted: 1 June 2016
Published: 6 June 2016
Radiology reports are a rich resource for biomedical research. Prior to utilization, trained experts must manually review reports to identify discrete outcomes. The Audiological and Genetic Database (AudGenDB) is a public, de-identified research database that contains over 16,000 radiology reports. Because the reports are unlabeled, it is difficult to select those with specific abnormalities. We implemented a classification pipeline using a human-in-the-loop machine learning approach and open source libraries to label the reports with one or more of four abnormality region labels: inner, middle, outer, and mastoid, indicating the presence of an abnormality in the specified ear region.
Trained abstractors labeled radiology reports taken from AudGenDB to form a gold standard. These were split into training (80 %) and test (20 %) sets. We applied open source libraries to normalize and convert every report to an n-gram feature vector. We trained logistic regression, support vector machine (linear and Gaussian), decision tree, random forest, and naïve Bayes models for each ear region. The models were evaluated on the hold-out test set.
Our gold-standard data set contained 726 reports. The best classifiers were linear support vector machine for inner and outer ear, logistic regression for middle ear, and decision tree for mastoid. Classifier test set accuracy was 90 %, 90 %, 93 %, and 82 % for the inner, middle, outer and mastoid regions, respectively. The logistic regression method was very consistent, achieving accuracy scores within 2.75 % of the best classifier across regions and a receiver operator characteristic area under the curve of 0.92 or greater across all regions.
Our results indicate that the applied methods achieve accuracy scores sufficient to support our objective of extracting discrete features from radiology reports to enhance cohort identification in AudGenDB. The models described here are available in several free, open source libraries that make them more accessible and simplify their utilization as demonstrated in this work. We additionally implemented the models as a web service that accepts radiology report text in an HTTP request and provides the predicted region labels. This service has been used to label the reports in AudGenDB and is freely available.
KeywordsHuman-in-the-loop Machine learning Natural language processing Radiology Audiology
Electronic health records (EHRs) contain significant amounts of unstructured text that pose a challenge to their secondary use as a research data source [1, 2]. Prior to research utilization, EHR text data, such as physician notes and radiology reports typically must be converted to discrete values, e.g. outcome labels. In the absence of automated processing, this requires trained data abstractors to manually review the text sources and identify discrete values of interest. Such manual review may be time consuming and expensive, particularly for large data sets. Natural language processing (NLP) and machine learning (ML) methods present an alternative to manual text review. These methods have been applied to automate EHR text analysis in a variety of studies including phenotype extraction, adverse drug-event identification, and domain-specific radiology report classification [3–8].
In audiologic and otologic research, the ability to use anatomic information described in radiology is essential to understand the causes of hearing loss for research subjects and to develop new treatment modalities [9–12]. The Audiological and Genetic Database (AudGenDB), a public, de-identified observational research database derived from EHR data sources, contains over 16,000 de-identified, unlabeled radiologist reports . Because the reports are unlabeled, it is difficult for researchers to select reports that contain abnormalities in a specific region, e.g. the inner ear. Two straightforward methods to be considered are keyword searches and International Classification of Diseases (ICD9) based searches . As shown in the work presented here, these approaches lack sensitivity (recall) for this data set, and thus fail to identify most of the reports that contain an abnormality. Therefore, to facilitate the effective use of anatomic information contained in radiology reports for audiology research, we adopted a machine learning procedure.
Ideally, we would like to utilize a fully automated knowledge extraction system for which it would be necessary only to supply radiology reports. The system would generate labels that correspond to entities (e.g. vestibular aqueduct) and attributes (e.g. enlarged) identified in each report to be used for search indexing. Although significant progress has been made in the biomedical domain toward the development of such systems for text analysis [15, 16], fine-tuning is usually necessary to achieve acceptable performance for specific use cases. This requires training documents to be labeled with detailed concepts from the standardized ontology utilized by the system (e.g. UMLS ). The size and complexity of such ontologies places a potentially prohibitive burden on the labeler, typically a physician or study staff member, who is required to learn at least part of the ontology in order to perform the annotation task. Additionally, the granularity of the ontology may be inappropriate for the use case. For example, we sought to enable document search for abnormalities in broad ear regions for which highly granular labels are unnecessary. More generally, existing NLP systems [18, 19] can extract pre-specified features (e.g. word tokens, parts of speech tags) from natural language text. However, it remains necessary to determine which features extracted by these systems and which classification models are appropriate for our task. As these steps are likely to benefit from human input, we adopted an approach that utilizes aspects from domain-expert-centered knowledge discovery  and human-in-the-loop (HIL) machine learning . In this framework, the domain expert, a physician in our study, plays a central, rather than periphery, role in the knowledge extraction process that includes data selection criteria, document labeling requirements, analysis, and evaluation. The HIL approach enables humans to guide the machine learning process through data, feature and model selection based on expert knowledge and task specific requirements.
Implementing this process, we developed a classification pipeline that uses freely available open-source ML and NLP libraries to label temporal bone radiology reports with one or more of four abnormality region labels: inner, middle, outer, and mastoid, indicating the presence of an abnormality in the specified ear region. We subsequently incorporated the pipeline into a web service that provides labels for reports submitted via HTTP requests to more broadly enable the audiology research community to effectively use the important anatomic information that is typically buried in radiology reports. Our successful application of these ML and NLP methods to a novel data source, AudGenDB, provides further evidence of their generalizability and that such methods can be effectively utilized in production biomedical software environments.
We conducted a retrospective analysis of de-identified radiologist reports obtained through the AudGenDB web query interface. Although AudGenDB now contains data from multiple institutions, radiology reports were only available from The Children’s Hospital of Philadelphia (CHOP) at the time of this study. The CHOP Internal Review Board approved the study under the overall AudGenDB project.
Each radiology report was reviewed by one of two study staff and annotated with one or more labels indicating ear regions in which structural abnormalities were noted. Following a human-in-the-loop approach, we derived annotation requirements from physician expert guidance . It was determined labels for inner, middle, and outer ear, and mastoid abnormalities were most appropriate for our task. The criteria for a positive label (indicating presence of an abnormality), developed through expert input, were: inner for abnormalities of the cochlea, vestibular aqueduct, vestibular nerves, vestibules, or semicircular canals; middle for abnormalities of the tympanic membrane, ossicles, stapes, incus, malleus, or scutum; outer for abnormalities of the external auditory canal; and mastoid for abnormalities of the mastoid regions. A report was labeled normal if no abnormalities were identified. Cohen’s Kappa statistic was calculated on a subset of 50 reports that were labeled by both reviewers to assess inter-rater agreement. The labeled dataset was stratified based on presence or absence of abnormalities in at least one region and randomly split into stratified training and test sets containing 80 and 20 % of the reports, respectively.
Document normalization and feature vector construction
Prior to the training and analysis, we extracted and normalized the findings and impression sections from each radiology report. The radiology report sections are consistently demarcated with section headers (e.g. FINDINGS). To extract these sections, we developed a custom Python program that reads each line of the file. It records all lines in the file after the FINDINGS header until the next section header. Similarly, it records all lines after the IMPRESSION header until the next section header. These extracted sections were then normalized. The normalization step uses the Python Natural Language Processing Toolkit (version 2.0.4) and custom regular expression patterns to: replace decimal numbers (e.g. 3.14) with number; replace units (e.g. years) with unit; remove common English words (except for “no”, “not”, and “under” which were deemed relevant to the specific classification task); and replace all words with their word-stems .
After normalization, every report was tokenized (separated into individual words). The tokens obtained from the training set reports were used to create an n-gram (sequence of n consecutive words or characters) collection representing all n-grams that occur in at least one training report. Note that test reports were also tokenized, but were not used to construct the n-gram collection. This implies that an n-gram that occurs in a test report but not in any training report will not appear in the n-gram collection. Because such an n-gram does not appear in the training set, it would provide no information during the training process and consequently not contribute to the classification model. After tokenization, we converted each report to a numerical feature vector (FV) using the Python scikit-learn library (version 0.16.1, with NumPy version 1.10.1) . FVs were constructed from the n-gram features in the report. Rather than adopt a pre-specified feature set or use unsupervised feature learning as would be done in a fully automated machine learning process, we employed a HIL technique and evaluated several potential candidate features identified from expert knowledge. The specific FV configuration was determined separately for each classifier as part of the model selection and evaluation process described below. We considered FVs composed of word only n-grams and character only n-grams with n in the range [1, 3]. For both word and character based FVs, we evaluated binary values, where element i (0 or 1) indicates the i th feature (from the complete n-gram collection) is absent or present in the report, and feature counts, where element i is the number of times the i th feature occurred in the report.
Model construction and evaluation
As with feature selection, we adopted a HIL procedure to select and tune the appropriate classification algorithms, in contrast to a fully automated process that would implement a pre-specified method. We evaluated logistic regression, support vector machine (SVM), decision tree, random forest, and naïve Bayes models. Model construction required hyperparameter selection for the associated learning algorithms. The hyperparameters include model specific parameters (e.g. regularization constant) and the specific FV configuration options (character vs. word N-grams, and binary features vs. counts). Only binary FVs were considered for the naïve Bayes classifier, as that is an algorithm assumption. We used the scikit-learn library, which includes implementations of the considered learning algorithms, to perform a grid search and select the optimal hyperparameters for each model. Hyperparameter combinations were evaluated by 5-fold cross validation . For each learning algorithm and ear region, the model hyperparameters and FV combination with the best cross-validation performance was selected and the model was re-trained using the entire training data set. These models were then run for the test set to evaluate generalization error and the best performing one in each ear region was selected for use in the report-labeling pipeline.
Web service implementation
To make the report classifiers available to applications, specifically AudGenDB, we implemented a web service that provides the predicted labels for each of the four ear regions. The web service is a REST implementation created in Python using the open-source Flask library. The REST service accepts HTTP POST requests that include one or more reports to be labeled and responds with the predicted labels.
We considered two alternative approaches for comparison to our ML models. First, we created a keyword list for each ear region [see Additional file 1]. We then performed a keyword search where we considered a report abnormal for a specific ear region if it contains any region specific keywords. In the second alternative, we identified a list of ICD9 diagnosis codes that pertain to abnormalities of the ear. These were assigned to one or more ear regions based on the code description [see Additional file 1]. For each radiology report, we obtained the ICD9 codes annotated to the associated patient from AudGenDB. A radiology report was considered abnormal for a given ear region if the patient’s diagnosis codes included any of the ICD9 codes assigned to that ear region.
Our dataset consisted of 726 radiology reports composed primarily of computed tomography (CT) scans of the temporal bones and a small number of Magnetic Resonance Imaging (MRI) scans of the brain. Inter-rater agreement was near perfect (kappa 0.94, 0.90, 0.88, and 0.82 for the inner, middle, outer, and mastoid regions, respectively) . Based on these results, we determined that single report assessment by reviewers working in parallel to generate a larger training set was the most efficient resource utilization. A double review with consensus reconciliation of discordant assessment may have yielded marginally better labels, but would have been feasible only for a smaller report set.
Abnormal annotation distribution
At least one
62.41 % (362)
62.33 % (91)
26.72 % (155)
26.03 % (38)
37.59 % (218)
40.41 % (59)
13.79 % (80)
14.38 % (21)
30.86 % (179)
36.99 % (54)
Best classifier hyperpameters by ear region
Feature Vector Hyperparameters
Cost parameter, C = 0.1
Regularization cost, l = 0.1
Cost parameter, C = 0.333
Max depth = 2
Best classifier test set performance metrics
90 % (+16.0)
90 % (+30.4)
93 % (+7.38)
82 % (+19.0)
Best classifier confusion matrices by ear region
Inner Ear: SVM Linear Kernel
Middle Ear: Logistic Regression
Outer Ear: SVM Linear Kernel
Mastoid: Decision Tree
Keyword and ICD9 search method performance
As described in the methods, we extracted the findings and impressions sections of the radiology report and performed text normalization prior to training and testing. As these steps were not part of the cross validation grid search, we examined their impact by evaluating the performance of the best logistic regression classifier for each region when separately removing each one of the preprocess steps. The best logistic regression classifier refers to the hyperparameter configuration that yielded the best cross validation results for the logistic regression model for a given region. In the case where extraction of the findings and impression sections was not performed, i.e. the entire report was used in training and test set evaluation, the test set accuracy difference was less than 2.1 % across all regions. The McNemar test statistic p-value was greater than 0.6 across all regions when comparing the output labels with and without separate extraction of the findings and impression sections. Similarly, we evaluated the best logistic regression classifier when text normalization was not performed. The test set accuracy difference was less than 4.1 % across all regions and the McNemar test statistic p-value was greater than 0.05 across all regions. These observations suggest that these preprocess steps may provide little benefit for this particular data set. We ultimately chose to keep them in our pipeline because they did not hinder performance and they are common practice in similar studies.
Robust methods that reduce the need for manual chart review to identify pertinent radiology reports are critical to support observational clinical studies for hearing loss research. We have demonstrated that standard ML and NLP methods may address this challenge when supported by human expert guidance. These methods significantly outperformed the keyword and ICD9 based search alternatives. Although other baseline methods may be superior, the intent of this work was to demonstrate the effectiveness of ML methods on an audiologic data set as compared to approaches likely to be used by most researchers performing observational clinical studies. Our fundamental criteria for baseline method selection was therefore to rely only on filtering methods and information readily available in the AudGenDB database. We also chose to rely on well-reported ML and NLP methods that are available in free libraries for many programming languages including Python (as we used for our experiment) and R [23, 27]. It is possible that superior results could be obtained through more advanced methods. For instance, the word count vectors used to represent documents could be replaced with word embedding methods such as word2vec and GloVe that capture additional semantic information [28, 29]. More advanced models such as recurrent neural networks that readily account for word order and relations between words would also likely be beneficial . However, we were motivated in part to demonstrate that standard ML and NLP methods could achieve reasonable success on a novel clinical data set.
As described recently , the free availability of NLP tools in “out of the box” software packages makes them more accessible to various researchers in numerous settings. However, as the work presented here illustrates, in most cases some form of human guidance is required to develop a successful machine learning application. Accordingly, we adopted concepts from human-in-the-loop machine learning [20, 21] for document annotation, feature selection, and model evaluation. We hypothesize that additional HIL ML concepts could be beneficial to this and similar studies. Specifically, as shown in , human knowledge can be integrated into the ML model training process to reduce the necessary training data by starting with a small training set and having the human expert select and label additional examples iteratively based on model performance. This is supported by our results as seen in the learning curves in Fig. 2 that indicate three region classifier models required only a small fraction of the overall training set to achieve maximum performance. This suggests that an iterative HIL approach could have significantly reduced our training data annotation requirements. This is important because a limitation of this study is that training and test reports originated from a single institution. It is likely that the classifiers must be re-trained for other institutions due to report feature differences between institutions. Thus, although the pipeline training and implementation software are directly transferable between institutions, each institution would need to provide a labeled data set. Utilizing a HIL iterative approach could reduce the number of required training samples and therefore increase portability to other institutions. Interactive HIL ML concepts could potentially address another limitation of this study, namely our pipeline produces abnormality labels for anatomical regions (e.g. inner ear), rather than specific feature abnormalities (e.g. enlarged vestibular aqueduct). It is much more difficult to automate such classification primarily because it places significant burden on the domain experts to label a larger training set and to use detailed ontology concepts for annotation. An interactive HIL ML procedure could potentially be used to simplify the detailed annotation. Similar to reducing the overall training set size, in this approach the domain expert initially provides high-level concept labels that are used by the learning algorithm to suggest more detailed labels that the expert can accept or reject . In this manner, granular labels are achieved without requiring the expert annotator to learn a complex ontology.
The presented work may be readily extended to clinical settings. As shown by the receiver operator characteristic results, the logistic regression model can be applied across all regions and is easily tuned to improve sensitivity (specificity) without severely impacting specificity (sensitivity). This may enable further applications of the described approach such as the use of NLP as a screening tool. For example, tuning to achieve high sensitivity may be used to reduce the need for physician review of predicted normal reports.
Our application of open source machine libraries and human-in-the-loop machine learning approaches on a novel pediatric audiologic data source further validates their potential and serves to demonstrate the generalizability of these methods. Additionally, while many biomedical research studies have considered ML and NLP approaches, their adoption in production biomedical software settings has been limited. This work provides evidence that such methods can be effectively utilized in production biomedical software environments.
Our study was designed with the objective of enabling researchers to search the AudGenDB database for radiology reports that contain abnormalities in specific ear regions. The results indicate that the ML and NLP methods can achieve accuracy scores across the four regions (range 82–93 %) that approach physician expert performance as observed in previous studies [32–35]. We have implemented the classifier models in a web service that is utilized to generate labels for the radiology reports in AudGenDB. The labels are provided as a data field in the AudGenDB interface and can be used to filter reports for abnormalities in the four ear regions. To date, the web service has been used to label and enhance the search capability of over 16,000 radiologist reports in the AudGenDB database.
In summary, we developed an automated pipeline that labels radiologist text reports as normal or abnormal relative to four ear regions. Our results indicate that standard ML and NLP methods implemented in freely available software libraries in concert with HIL ML techniques can accurately identify abnormal regions noted in text reports for the otologic domain. This is encouraging in that it aligns with previous studies that have indicated the ability of such methods to accurately provide radiology report outcome labels and suggests general applicability of the approach.
AUC, area under the curve; AudGenDB, Audiological and Genetic Database; CHOP, The Children’s Hospital of Philadelphia; CT, computed tomography; EHR, electronic health record; FV, feature vector; G-SVM, Gaussian support vector machine; HIL, human in the loop; ML, machine learning; NLP, natural language processing; REST, representational state transfer; ROC, receiver operating characteristic; SVM, support vector machine.
This work is funded by the National Institutes of Deafness and Other Communication Disorders of the National Institutes of Health, project number R24 DC012207.
Availability of data and materials
The data used for this study is freely available via the AudGenDB application website, http://audgendb.chop.edu/, by navigating to the Workspace tab and selecting the public query, AARC Manuscript Query. Our application source code is freely available at https://github.com/chop-dbhi/arrc.
AJM conceived of the study, implemented the models, performed the analysis and drafted the manuscript. RWG participated in the analysis and helped draft the manuscript. JWP and EBC participated in the study design and helped draft the manuscript. JAG provided clinical expertise necessary to create the gold standard report set and helped draft the manuscript. All authors have read and approved this manuscript.
The authors declare that they have no competing interests.
Consent for publication
Ethics approval and consent to participate
This study used de-identified radiology reports obtained retrospectively from The Children’s Hospital of Philadelphia (CHOP). The CHOP Internal Review Board approved the study under the overall AudGenDB project.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.
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