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A Hybrid Approach for Biomedical Relation Extraction Using Finite State Automata and Random Forest-Weighted Fusion

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The automatic extraction of relations between medical entities found in related texts is considered to be a very important task, due to the multitude of applications that it can support, from question answering systems to the development of medical ontologies. Many different methodologies have been presented and applied to this task over the years. Of particular interest are hybrid approaches, in which different techniques are combined in order to improve the individual performance of either one of them. In this study, we extend a previously established hybrid framework for medical relation extraction, which we modify by enhancing the pattern-based part of the framework and by applying a more sophisticated weighting method. Most notably, we replace the use of regular expressions with finite state automata for the pattern-building part, while the fusion part is replaced by a weighting strategy that is based on the operational capabilities of the Random Forests algorithm. The experimental results indicate the superiority of the proposed approach against the aforementioned well-established hybrid methodology and other state-of-the-art approaches.
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A Hybrid Approach for Biomedical Relation Extraction
Using Finite State Automata and Random Forest-
Weighted Fusion
Thanassis Mavropoulos1, Dimitris Liparas1, Spyridon Symeonidis1, Stefanos
Vrochidis1 and Ioannis Kompatsiaris1
1 Information Technologies Institute, Centre for Research and Technology Hellas, Thermi-
Thessaloniki, Greece
{mavrathan, dliparas, spyridons, stefanos, ikom}@iti.gr
Abstract. The automatic extraction of relations between medical entities found
in related texts is considered to be a very important task, due to the multitude of
applications that it can support, from question answering systems to the devel-
opment of medical ontologies. Many different methodologies have been pre-
sented and applied to this task over the years. Of particular interest are hybrid
approaches, in which different techniques are combined in order to improve the
individual performance of either one of them. In this study, we extend a previ-
ously established hybrid framework for medical relation extraction, which we
modify by enhancing the pattern-based part of the framework and by applying a
more sophisticated weighting method. Most notably, we replace the use of regu-
lar expressions with finite state automata for the pattern-building part, while the
fusion part is replaced by a weighting strategy that is based on the operational
capabilities of the Random Forests algorithm. The experimental results indicate
the superiority of the proposed approach against the aforementioned well-
established hybrid methodology and other state-of-the-art approaches.
Keywords: Natural Language Processing, Relation Extraction, Supervised
Learning, Support Vector Machines, Random Forests, Weighted Fusion
1 Introduction
The onset of the digital era and notably the advent of the internet have not only
changed the way people communicate and entertain themselves but have also altered
fundamentally their working practices and needs. The medical domain has been on
the forefront of these changes, as medical professionals have been exploiting the latest
advancements of research and technology in order to improve their services since the
very beginning. But this wealth of information is sometimes overwhelming and diffi-
cult to tackle manually. A certain level of automation in information extraction is
imperative, especially when non-medical practitioners, like patients or their families,
are involved. In most cases these people do not possess the ability to fully understand
the language used by the professionals since there is a great knowledge gap between
the two groups. The rich in terminology patient history reports is one such area, espe-
cially when these are riddled with acronyms tailored to the medical domain. The same
holds for online resources, like dedicated medical sites and forums, which users often
consider when soliciting for information on drugs, diseases or treatments.
Medical concept relation extraction deals with the automatic extraction of relations
that exist between entity types relevant to this domain, such as treatment, test or dis-
ease, among others. This task has been the focal point for a lot of researchers, due to
many applications that it can support, such as the creation of medical ontologies and
content representation that could serve as basis for medical content retrieval and ques-
tion answering systems, as well as decision support services for doctors. According to
[1], "identifying relations between medical entities in clinical data can help in strati-
fying patients by disease susceptibility and response to therapy, reducing the size,
duration, and cost of clinical trials, leading to the development of new treatments,
diagnostics, and prevention therapies".
Traditionally, studies on medical relation extraction have relied on rule/pattern-
based linguistic approaches, machine learning ones and also on hybrid systems that
combine linguistic templates and machine learning in order to improve their results.
An example of a hybrid framework for medical relation extraction is the approach
introduced in [2] and further evaluated in [3], which relied on two different method-
ologies: a) relation patterns defined by human experts via regular expressions and b)
Support Vector Machine (SVM)-based classification based on three types of extracted
features, namely lexical, morphosyntactic and semantic features. Fusion of the results
from these two methodologies was achieved by means of a strategy, which relied on
the training examples of a given dataset, giving more influence to the relation patterns
when few training examples were available for a certain relation type and more influ-
ence to the machine learning approach when enough examples were provided.
In this paper, the focus is shifted towards the relation extraction task of the 2010
i2b2/VA challenge, which required the extraction of eight types of semantic relation-
ships found between the medical concepts of the given dataset. The other parts of the
contest involved the extraction of the medical concepts themselves and also the anno-
tation of the assertions made about these concepts. We are inspired by the hybrid
approach described above and we extend it with an innovative pattern-construction
method, based on finite state automata, and a novel weighted fusion strategy. More
specifically, we approach the creation of linguistic patterns not via the use of regular
expressions, as in the case of [2], but by using node-based finite state automata, which
can include information like the part of speech (POS) and the inflection of a lexical
unit or even contain whole gazetteers of words inside a node.
As an additional novelty, we introduce the use of a Random Forests (RF) classifi-
cation model, which provides the weighted fusion values for the pattern-based and
machine learning modules of the relation extraction framework based on its opera-
tional performance on the training set, with the use of the out-of-bag (OOB) error
estimate [4]. It should be noted that we keep the use of the SVM classifier for the
machine learning module of our framework, due to its demonstrated superiority in
many natural language processing (NLP)-related classification tasks. Our hybrid
framework is applied to the currently available partial version of the 2010 i2b2/VA
challenge dataset [5] and the experimental results demonstrate its superior perfor-
mance, compared to a number of considered approaches.
The rest of this paper is organised as follows: In Section 2, the theoretical back-
ground and an outline of the relevant literature are provided. In Section 3 the pro-
posed hybrid relation extraction approach is described, while Section 4 provides the
experimental framework of our study. In Section 5, the results of the experiments are
presented and discussed. Finally, Section 6 concludes the paper.
2 Related work and theoretical background
In this section, since the biomedical domain constitutes the point of interest of the
current study, we report previous work on relation extraction in this field. In addition,
we provide information on the theoretical background, as well as the related work for
the Random Forests (RF) and Support Vector Machines (SVMs) machine learning
methods.
As already mentioned in Section 1, three main types of methodologies have been
proposed over the years for concept relation extraction: the rule/pattern-based linguis-
tic approaches, the statistical/machine learning approaches and the hybrid ones, which
combine both approaches.
Pattern-based systems have been used in the biomedical domain since the early
2000s and have mainly approached the problem as a text classification one. [6] tried
to extract and structure information related to molecular pathways with their Ge-
neWays system. A year later, [7] attempted to extract similar relationships between
genes, proteins, drugs and diseases.
However, the term “relation extraction” is only part of the problem called “relation
classification”, which was first introduced in [8] and entails the extraction of the se-
mantic roles and the recognition of the relationship that holds between them. It was a
very influential study that explored five generative graphical models and a neural
network to identify seven different relationships that can be found between “treat-
ment” and “disease” entities. The corpus that was used in their work originates from
“The BioText Project”, is known as the “MEDLINE 2001” corpus and has since been
widely used in relation extraction tasks. In [9], a Conditional Random Fields (CRF)
classifier was used because of the need to detect the medical entities and at the same
time, the relations between them. The semantic relations between diseases and treat-
ments, as well as between genes and treatments were targeted, which were classified
into seven and five predefined types respectively. All experiments were conducted on
the MEDLINE 2001 corpus. Relation extraction between entities in literature text
(Medline abstracts) was conducted by [10], via the use of kernel-based learning meth-
ods. The method involved a customization of the standard tree kernel function “by
incorporating a trace kernel to capture richer contextual information” and resulted in
outperforming word and sequence kernels.
The framework that currently claims the best results between treatments and dis-
eases on the MEDLINE 2001 corpus is the one presented in [11], which uses a hybrid
feature set for the classification of relations. The major differentiation is in the seman-
tic feature set, where verb phrases are ranked using the Unified Medical Language
System (UMLS), while the relations are classified by SVM and Naïve Bayes models.
2010 was a year that marked a great insurgence of research in the medical concept
extraction domain and this was due in no small part to the respective i2b2 Shared-
Task and Workshop. The contest gave the research community the incentive by sup-
plying a pre-annotated corpus with concepts, relations and assertions. Since then, the
contest’s best ranking systems are considered as the reference, against which all new
ones are benchmarked.
The research, which is underway in the extraction of biomedical relationships, has
also been receiving growing attention, “with numerous biological and clinical applica-
tions including those in pharmacogenomics, clinical trial screening and adverse drug
reaction detection”, as [12] are outlining in great detail. In addition, there have been
some recent approaches based solely on Convolutional Neural Network (CNN) mod-
els. For instance, in [13], a CNN-based model is implemented in order to extract the
semantic relations found between medical concepts and with the goal “to learn fea-
tures automatically and thus reduce the dependency on manual feature engineering”.
The method is applied to the currently available partial version of the 2010 i2b2/VA
challenge dataset with promising results.
Random Forests (RF) is a well-known machine learning method [4], used with
great success in many applications. Its basic idea is the construction of a multitude of
decision trees, which can be used for classification and regression purposes. There is
randomness in the operational procedures of RF in two different ways: 1) Each deci-
sion tree is constructed on a different group of data, sampled randomly with replace-
ment (bootstrap) from the training set, and 2) During the construction of each decision
tree, the best split at each node is determined based on a randomly selected subset of
the variable set. An estimation of the generalisation error of RF can be provided by
means of an inherent method called out-of-bag (OOB) error. In a nutshell, only ap-
proximately 2/3 of the original data examples are used in a specific bootstrap sample
during the construction of a decision tree. The rest of the original data examples (ap-
proximately 1/3), called OOB data, are used for testing the performance of the con-
structed decision tree. The OOB error is the averaged prediction error for each train-
ing case, using only the decision trees that do not have that training case in their boot-
strap sample. As already mentioned, RF has been successfully applied to many disci-
plines. Specifically in the biomedical domain, there have been applications of RF for
automated diagnosis of diseases [14], electromyography (EMG) signal classification
[15], or in the context of brain-computer interfaces (BCI) [16], among others. Finally,
the use of late fusion strategies based on RF’s operational capabilities in the context
of multimodal news articles classification has been investigated in [17].
Support Vector Machines (SVMs) [18] are supervised learning methods used for
solving pattern recognition problems. Their basic notion lies in hyperplanes, which
are used to separate sets of data points with different class memberships in multidi-
mensional spaces. The effectiveness of SVMs in NLP classification tasks and more
specifically, for relation extraction, can be highlighted by the fact that the highest
performance for the relation extraction task in the 2010 i2b2/VA challenge was
achieved by [19] with their supervised approach. This approach employed an SVM
classifier to identify relations, which was informed by several resources such as Wik-
ipedia, WordNet, General Inquirer and a relation similarity metric. Furthermore, the
only hybrid system participating in the challenge, employing an SVM classifier to-
gether with manually constructed linguistic patterns was developed by [20]. Finally,
[1] used an SVM classifier with a combination of lexical, syntactic and semantic fea-
tures, terms extracted from a vector-space model created using a random projection
algorithm, as well as additional contextual information extracted at sentence-level to
detect relations.
3 Hybrid relation extraction approach
In this section we present the proposed framework for the medical relation extraction
problem, which is illustrated in Figure 1. It consists of two main modules for relation
extraction (a pattern-based and a machine learning one) and a weighting module for
the fusion of the results provided by each module.
Fig. 1. Proposed relation extraction framework
Pattern-based module. While developing a pattern based method one has to con-
sider the many forms that are often utilised in natural language to express the same
thing. These variations need to be taken into consideration when devising the manual-
ly constructed rules and patterns, in order for the system to deliver the optimal results.
This exact fact is also what makes pattern based methods complex and time consum-
ing to develop. The method of choice revolves around finite state automata, which,
while being the simplest level of grammar and well understood by users who write
rules, is also a technique versatile enough to enable detailed description of complex
linguistic phenomena as well as permit the generation of output files rich in linguistic
information.
Thus, for the semantic relation extraction task, a set of patterns is constructed for
each target relation after examining the structure of certain natural language expres-
sions and detecting common forms in them. This is usually possible with the use of
regular expressions and by exploiting keywords usually found in clinical texts, like
cure, treat, drug and side effect. It is the most commonly used method and the one
employed by [3] in their MEANS system. However, the current paper adopts an ap-
proach which is based on the exploitation of finite state automata (or graphs) via the
use of the corpus processing suite Unitex [21], in order to overcome any limitations
that are encountered when utilising regular expressions. The pattern-building proce-
dure is done through a powerful interface that enables the manipulation of intercon-
necting nodes, in order for the user to achieve the most descriptive pattern possible.
These nodes may contain a POS, a regular expression, a multitude of linguistic filters
(e.g. the feminine plural forms of an adjective) or even whole graphs. A major differ-
entiation compared to simple regular expressions, which ultimately plays a pivotal
role in the effectiveness of a Unitex-made graph, is the ability to exploit the rich in
linguistic information incorporated dictionaries. These have been manually created
and contain the grammatical attributes, such as POS or inflection, for the whole of the
English vocabulary. In addition to the default integrated dictionaries, Unitex also sup-
ports the creation of custom ones which can be populated with specialised entries
such as disease or treatment terminology.
Each relation targeted by the pattern-based module is being represented by a num-
ber of dedicated, manually constructed patterns that locate medical entities/concepts,
which appear in pairs in a sentence. A weighted label of specificity is allocated to
each pattern in order to solve ambiguous matches, since different relations can be
expressed in similar manners (for each pattern, the more detailed the representation of
the lexical context, the more specific the weight that gets allocated). The pattern
weights that correspond to the assigned labels take the values of 1 for the most specif-
ic relation type pattern, 0.75 for a fairly specific one and 0.50 for low specificity pat-
terns (i.e. R1=1, R2=0.75, R3=0.50, with R1 being the most specific relation (R)).
When the entity pair meets the criteria laid out by one of these patterns, the respective
label is assigned. To be more precise via an example, the phrase “He had been noting
night sweats, increasing fatigue, anorexia, and dyspnea, which were not particularly
improved by increased transfusions or alterations of hydroxy urea.” can be represent-
ed with the automaton of Figure 2, while one of the possible output sentences is rep-
resented as (E1=entity1 and E2=entity2): He had been noting night sweats, increasing
fatigue, anorexia, and <E2>dyspnea</E2>which were not particularly
<TrWP2>improved by</TrWP2><E1>increased transfusions </E1>or alterations
of hydroxy urea.
All grey boxes invoke secondary graphs with similar formalism to this one, which
contain relevant information to their title. The nodes “disease/signORsymptom” and
“treat/cadec_drug/gene_unknown” enclose the relevant dictionaries, while the nodes
“negation”, “possession”, “conjunction” describe the respective syntactic functions.
Lastly, the white node, which is the only one not evoking another graph, is determin-
ing the output of the box, which in this case is the relation type <TrWP2> (Treatment
Worsens Problem with level 2 specificity). In total, around 350 patterns were created,
a number that also includes assistive graphs, like the ones used to handle lexical units
of trivial importance found between or around the target entities
(test_{10}/test_{20}).
Fig. 2. Finite state automaton representing the “TrWP” relation type.
Machine learning module. In the training phase, a linear SVM classifier is trained
on features extracted from a given dataset in order to describe each example. The
features fall into three types: lexical, morphosyntactic and semantic features.
The lexical features include the entities position in the phrase, the words that form
each entity and their immediate context; the words before, after and between them.
Also of importance are their lemmas. The morphosyntactic features include the POS
(extracted by the Stanford CoreNLP suite [22]) of the lexical units in question, the
number of words that form each entity, the verbs before, after and between the entity
pairs. Finally, the semantic features refer to the concepts associated to the target enti-
ties, as well as those found in their close vicinity; before, after and between them.
They are all derived from the online resource UMLS [23], which is a software suite
that encompasses various health related vocabularies and standards to allow for inter-
actions between computer systems. Another type of feature, which carries semantic
information and is provided in the dataset, is the concept type of each entity. Howev-
er, it was decided that, while such a feature is positively helpful and already available
in the given dataset, it wouldn’t be included in the feature set of the used classifier.
The reason behind this decision lies in the non-existent availability of a reliable re-
source/procedure that can provide equivalent values in a reallife, non-laboratory
scenario.
In the testing phase, for any instance where its relation type is considered to be un-
known, the trained SVM model outputs a prediction of the relation type in the form of
probability scores.
Weighting module. The probability scores from the pattern-based and machine
learning modules are combined using weighted fusion. Different weights are assigned
to each module and for each class (relation type). In order to output the final probabil-
ity that a case is relevant to a class R, the predicted scores Ppb (from the pattern-based
module) and Pml (from the machine learning module) are first multiplied by their cor-
responding weights Wpb and Wml and are then summed, as in equation (1). The relation
type with the highest fused probability score is assigned to each test set instance.
Pfused(R) = (Wpb(R) * Ppb(R)) + (Wml(R) * Pml(R))
(1)
In this study, we propose a weighting method, which relies on a different classifier
than the one used in the machine learning module. Specifically, a RF model is trained
on the training examples in order to leverage an operational capability exclusive to
this algorithm. This capability is the out-of-bag (OOB) error, which provides an esti-
mation of the generalisation error of RF. During the training of the RF model, a por-
tion of the original data examples, called OOB data, are used for testing the perfor-
mance of each constructed decision tree. The accuracy of the trained RF model on the
OOB data is calculated for each class separately and the corresponding scores are
assigned as weight values to the machine learning module. The sum of the weights for
the two modules must be strictly equal to 1. This means that the pattern-based weight
for a relation R is the complement of the corresponding machine learning weight,
Wpb(R) + Wml(R) = 1.
4 Experimental framework
Dataset. The proposed approach was evaluated on the relation extraction task of the
2010 i2b2/VA challenge, which has been the reference for nearly every competing
system working on medical relation extraction. The task’s focus was on eight relation
categories, as it can be seen in Table 1. The eight relationships can be further classi-
fied into three sub-groups of the treatment-problem (TrIP, TrWP, TrCP, TrAP,
TrNAP), test-problem (TeRP, TeCP) and problem-problem (PIP) variety. The vast
majority of training examples that can be found in the dataset belongs to the “TrAP”,
“PIP” and “TeRP” relations, with 885, 755 and 992 examples respectively. This num-
ber amounts to 84.39% of the dataset examples, which is a problem in itself as the
remaining 15.61% that represents the five less populated classes is not enough to
effectively feed the training procedure of the classifier in order to produce acceptable
results. This fact alone renders the presence of a pattern-based module imperative,
which not only rectifies the problem of the sub-populated classes, but also aids in the
amelioration of the final results in their entirety.
The original dataset consisted of 394 training reports, 477 test reports, and 877 un-
annotated reports, while currently, the dataset is only partially available for research,
due to IRB limitations, with 170 training and 256 test reports, respectively.
Experimental setup. The LibSVM [24] wrapper class contained in the Weka ma-
chine learning software was used to train the linear SVM models of the machine
learning module. The main SVM parameters C and gamma, received values of 1 and
0, respectively. In the training procedure one binary classifier (mono-class) was
trained for each relation type. For weight assignment, two different strategies were
tested. In the first strategy (proposed in [2]), the weight values are directly analogous
to the frequency of each relation type in the training set examples. The second strate-
gy is the one we propose for our hybrid approach, based on the RF OOB error esti-
mate. The RF classification model was trained using the scikit-learn python library.
Finally, for the evaluation of the performance of all configurations the micro-
averaged values for the precision, recall and F-score measures were computed.
Table 1. Details of the dataset.
Relation Type
Examples
TrIP
Treatment improves medical problem relations.
51
TrWP
Treatment worsens medical problem relations.
24
TrCP
Treatment causes medical problem relations.
184
TrAP
Treatment is administered for medical problem rela-
tions.
885
TrNAP
Treatment is not administered because of medical
problem relations.
62
PIP
Medical problem indicates medical problem rela-
tions.
755
TeRP
Test reveals medical problem relations.
992
TeCP
Test conducted to investigate medical problem rela-
tions.
166
5 Experimental results
The test set results from the experiments conducted in this study are compared in
Table 2 with state-of-the-art systems. Rows 2 and 3 of Table 2 contain the results
from our system and from the one we use as a baseline approach. It should be noted
that all experiments for these two hybrid systems were conducted with the use of our
own patterns, as it is not possible to recreate the exact patterns used in [2]. We ob-
serve a 2.6% relative improvement (in terms of micro-averaged F-score) in the per-
formance of our system, when compared to the baseline system. This improvement is
satisfactory, considering that only the weighting strategy changes are taken into ac-
count. No reliable comparison can be made on a pattern level, until the two systems
are compared on the same dataset. In row 4, [13] trained a convolutional neural net-
work on the exact same limited I2b2 dataset that we also used in our experiments.
Rows 5-8 of Table 2 present the performance and type of the relation extraction sys-
tems that scored the highest in the I2b2/VA challenge (they used the full dataset, so
the machine learning part was trained with more data). We notice that our proposed
system outperforms all considered state-of-the-art approaches, to a lesser or greater
extent. Most notably, there is an approximate 7% relative improvement in our sys-
tem’s performance, compared to the best I2b2 hybrid system [20].
Furthermore, Table 3 presents the added value that the integration of the pattern-
based module brings to our hybrid system, compared to the use of the machine learn-
ing module only. We notice an overall improvement in the F-score values for the
different relation types of the dataset. The biggest gains are observed in the TrNAP
and TeCP relation types, with a relative improvement of 320.6% and 133.6%, respec-
tively. It becomes evident that the performance improvements warrant the manual
effort needed for the construction of our hybrid system’s pattern-based module.
Table 2. Performance evaluation of the proposed hybrid system vs. the baseline sys-
tem and state-of-the-art approaches.
System
Approach
F-score
Our method
Hybrid
0.758
Abacha & Zweigenbaum
Hybrid
0.739
Sahu et al. [13]
Semi-supervised
0.712
Roberts et al. [25]
Supervised
0.737
DeBruijn et al. [26]
Semi-supervised
0.731
Grouin et al. [20]
Hybrid
0.709
Patrick et al. [27]
Supervised
0.702
Table 3. Performance difference (in terms of F-score) from the integration of the
pattern-based module into the proposed system.
Relation type
Supervised
Hybrid
Relative difference
TrIP
0.240
0.279
+16.2%
TrWP
0.0
0.275
N/A
TrCP
0.456
0.516
+13.2%
TrAP
0.749
0.782
+4.4%
TrNAP
0.068
0.286
+320.6%
PIP
0.792
0.823
+3.9%
TeRP
0.817
0.829
+1.5%
TeCP
0.125
0.292
+133.6%
6 Conclusions and future work
In this study, we have proposed a novel medical concept relation extraction frame-
work by extending [2] with the use of a more sophisticated pattern-constructing
method and a weighting strategy, which leverages an inherent operational feature of
the RF algorithm. Based on experiments conducted on a well-known dataset for rela-
tion extraction, we have demonstrated that our methodology outperforms a number of
state-of-the-art approaches. It should be noted that in [2] the evaluation is conducted
on the MEDLINE 2001 corpus and the patterns of the corresponding module are con-
structed in a different way. In the future, we plan to fully compare our approach with
the latter on the MEDLINE 2001 corpus, as well as investigate the use of alternative
weighting strategies for our framework.
Acknowledgments. This work was supported by the project KRISTINA (H2020-
645012), funded by the European Commission. Deidentified clinical records used in
this research were provided by the i2b2 National Center for Biomedical Computing
funded by U54LM008748 and were originally prepared for the Shared Tasks for
Challenges in NLP for Clinical Data organized by Dr. Ozlem Uzuner, i2b2 and
SUNY.
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... Text processing is handled by Stanford's CoreNLP suite [38], which performs linguistic analysis utilising tools like part-of-speech (POS) parsers, tokenisers, and chunkers to extract dependencies between sentence words, concepts, the underlying relations, named entities, etc. The produced output receives supplementary processing in order to retrieve probable disease/treatment-related relations in user queries by applying a hybrid relation extraction tool [39]. ...
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