Gijs van Tulder

PhD student in machine learning   |   computer scientist   |   the Netherlands

I work on transfer learning and feature learning, and their applications in model-based medical image analysis. I am interested in the theoretical and practical aspects of machine learning and computer vision.

After a reasonably successful but unfulfilling start as a student of economics and business, I had great pleasure as a computer science student at the Delft University of Technology. At the end of 2012 I completed my Master of Science in Media and Knowledge Engineering with a thesis on a theoretical aspect of active learning. In early 2013 I started as a PhD student at the Biomedical Imaging Group Rotterdam.

I like programming and web development. During my studies I worked as a freelance web developer, mostly for De Digitale School and Dalton Voorburg. You’ll find some examples of my work below.

Publications

Combining Generative and Discriminative Representation Learning for Lung CT Analysis with Convolutional Restricted Boltzmann Machines

Gijs van Tulder and Marleen de Bruijne

The choice of features greatly influences the performance of a tissue classification system. Despite this, many systems are built with standard, predefined filter banks that are not optimized for that particular application. Representation learning methods such as restricted Boltzmann machines may outperform these standard filter banks because they learn a feature description directly from the training data. Like many other representation learning methods, restricted Boltzmann machines are unsupervised and are trained with a generative learning objective; this allows them to learn representations from unlabeled data, but does not necessarily produce features that are optimal for classification. In this paper we propose the convolutional classification restricted Boltzmann machine, which combines a generative and a discriminative learning objective. This allows it to learn filters that are good both for describing the training data and for classification. We present experiments with feature learning for lung texture classification and airway detection in CT images. In both applications, a combination of learning objectives outperformed purely discriminative or generative learning, increasing, for instance, the lung tissue classification accuracy by 1 to 8 percentage points. This shows that discriminative learning can help an otherwise unsupervised feature learner to learn filters that are optimized for classification.

Why Does Synthesized Data Improve Multi-sequence Classification?

Gijs van Tulder and Marleen de Bruijne

RBM

Restricted Boltzmann machines can be used to
predict a missing sequence given the others

The classification and registration of incomplete multi-modal medical images, such as multi-sequence MRI with missing sequences, can sometimes be improved by replacing the missing modalities with synthetic data. This may seem counter-intuitive: synthetic data is derived from data that is already available, so it does not add new information. Why can it still improve performance? In this paper we discuss possible explanations. If the synthesis model is more flexible than the classifier, the synthesis model can provide features that the classifier could not have extracted from the original data. In addition, using synthetic information to complete incomplete samples increases the size of the training set.

We present experiments with two classifiers, linear support vector machines (SVMs) and random forests, together with two synthesis methods that can replace missing data in an image classification problem: neural networks and restricted Boltzmann machines (RBMs). We used data from the BRATS 2013 brain tumor segmentation challenge, which includes multi-modal MRI scans with T1, T1 post-contrast, T2 and FLAIR sequences. The linear SVMs appear to benefit from the complex transformations offered by the synthesis models, whereas the random forests mostly benefit from having more training data. Training on the hidden representation from the RBM brought the accuracy of the linear SVMs close to that of random forests.

Learning Features for Tissue Classification with the Classification Restricted Boltzmann Machine

Gijs van Tulder and Marleen de Bruijne

Diagram

Filters learned from lung tissue data

Performance of automated tissue classification in medical imaging depends on the choice of descriptive features. In this paper, we show how restricted Boltzmann machines (RBMs) can be used to learn features that are especially suited for texture-based tissue classification. We introduce the convolutional classification RBM, a combination of the existing convolutional RBM and classification RBM, and use it for discriminative feature learning. We evaluate the classification accuracy of convolutional and non-convolutional classification RBMs on two lung CT problems. We find that RBM-learned features outperform conventional RBM-based feature learning, which is unsupervised and uses only a generative learning objective, as well as often-used filter banks. We show that a mixture of generative and discriminative learning can produce filters that give a higher classification accuracy.

Master’s thesis

Sample reusability in importance-weighted active learning

A classification problem

Which example should we label next?

Active learning sounds like a wonderful idea: select the most interesting examples to learn the best classifier with the least effort. Not every example is equally helpful for training your classifier. If labelling examples is expensive, it makes sense to label only those examples that you expect will give a large improvement to your classifier.

Unfortunately, using a non-random and unrepresentative selection of samples violates the basic rules of machine learning. It is therefore not surprising that active learning can sometimes lead to poor results because its unnatural sample selection produces the wrong classifier.

Modern active learning algorithms try to avoid these problems and can be reasonably successful at it. One of the remaining problems is that of sample reusability: if you used active learning to select a dataset tailored for one type of classifier, can you also use that same dataset to train another type of classifier?

In my thesis I investigate the reusability of samples selected by the importance-weighted active learning algorithm, one of the current state-of-the-art active learning algorithms. I conclude that importance-weighted active learning does not solve the sample reusability problem completely. There are datasets and classifiers where it does not work.

In fact, as I argue in the second part of my thesis, I think it is impossible to have an active learning algorithm that can always guarantee sample reusability between every pair of classifiers. To specialise your dataset for one classifier, you must necessarily exclude samples that could be useful for others. If you want to do active learning, decide what classifier you want to use before you start selecting your samples.

Web development work (mostly in Dutch)

Jam! A game of maths

Screenshot Jam!

A simple educational game.

A fellow student and I built this game as part of a course on educational software. Intended for high-school students, it lets you explore mathematical formulas and their corresponding graphs. Balls enter the screen and tumble down the graphs that you have placed there. By choosing the right formulas to build your graphs, you guide the balls from the entrance to the exit.

Try it yourself!

Wrts

Screenshot Wrts

Online vocabulary training tool.

Wrts helps you to learn a foreign language: enter the words you want to learn and let the computer test you. Although not a very original idea – there are many tools like this – the easy-to-use (and free) Wrts quickly became quite successful among Dutch high school students. At the start of 2010 the site had over 1 million accounts.

The text-to-speech function helps students to learn not just the spelling but also the pronunciation of the words.

Wrts is available in Dutch and English.

Leermiddelendatabase

Screenshot leermiddelendatabase

Learning materials exchange site for Dutch primary and secondary education.

De Digitale School, a Dutch foundation, operates a number of free communities for Dutch and Belgian teachers. The leermiddelendatabase allows these teachers to share and exchange their lesson ideas and materials.

When uploading their learning objects, teachers provide metadata that describes the contents of the lesson. The learning objects and metadata are also exchanged via EduStandaard, the Dutch education metadata initiative.

The leermiddelendatabse is in Dutch.

Dalton Voorburg

Screenshot www.daltonvoorburg.nl

School website with cms and other applications.

Starting in 2002, I built and maintained a content management system for the website of my former high school. With a few modifications the system is still in use (as of 2013). The content management system makes it easy for teachers to upload and maintain their own course pages.

Besides the main website I built several smaller web-based tools.

The website is in Dutch.

Student projects

Question Classification by Weighted Combination of Lexical, Syntactic and Semantic Features

Babak Loni, Gijs van Tulder, Pascal Wiggers, David M.J. Tax, Marco Loog

For a course project on ‘Real-Time AI and Automated Speech Recognition’ at the Delft University of Technology, Babak Loni and I did some work on question classification. Babak further developed and published a question classification library that started from this project.

Bachelor’s thesis: Greetinq voicemail

For the final project of my Bachelor of Science in Computer Science I did an internship at Greetinq in Delft. During three months, Roland Heinrichs and I developed a web-based prototype for an iPhone interface to their personalised voicemail service.

Old web development articles

My (still) surprisingly popular article explaining an SQL data structure:

I wrote several articles for evolt.org, a web development community:

I published one Dutch article on PHPen.nl (now offline):

And a few notes I have only shown on my own site: