I’m currently competing in the Second Annual Data Science Bowl at Kaggle. This is by far the most difficult competition that I have entered to date. At the time of writing I am placed 62nd out of 755 entries, with only a day remaining to lock down my methodology. There’s a lot more I’d like to do to improve my model, but alas, I don’t have the time!
Here’s the problem that we are solving:
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We are given a set of medical images taken by MRI, across 30 time periods, and a variable number of location slices through the body.
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We are also given the volume of the left ventricle of the heart at times of diastole and systole.
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Our task is to design an automatic algorithm that inputs DICOM images and outputs a cumulative density function of the likelihood of different volumes at both diastole and systole.
The medical image files are in DICOM format, containing information about the patient (e.g. age and gender) and a set of monochrome images for each patient giving a 4 dimensional view of that patient’s chest. The key images are the “sax” (short axis) images, a set of slices perpendicular to the line that passes through the length of the heart (a heart isn’t circular, but more ovoid in shape), and there are typically 30 images for each sax, each being 1/30th the time period of a heartbeat, showing one complete cycle of the heart. There are a varying number of sax images for each patient, depending upon the length of the patient’s heart, and sometimes there are also repeated sax sets, where the scanning was repeated in an attempt to improve the image quality.
The image quality varies greatly between patients, with differing image resolutions, brightness, contrast and aspect ratio / rotation.
As you can see in the animated gifs above, some of the images are such poor quality that it is difficult for the human eye to discern the details. So my first challenge was to improve the brightness and contrast. One way to do this is to do a linear transformation on each image so that its pixels have a preset mean and standard deviation.
As you can see, while this approach helped, it did not work well enough for the problem images. I also tried non-linear transformations without much more success. No transformation function was flexible enough for the wide range of image qualities. Frequently part of the problem image gets washed out.
At the National Heart Centre Singapore, I spoke with Assistant Professor Calvin Chin about how doctors use imaging to assess heart volume. He explained that the very bright, washed out sections in some of the images are the result of fat deposits within the patient’s body. He also explained how to find the left ventricle chamber in an image (it is round with a thick lining surrounding it) and what to do about the dark patches inside the chamber (include them in the area of the chamber because they are blood vessels). This was really helpful. It pays to bring in some domain knowledge to a machine learning problem.
What I wanted was for all images to have similar brightness histograms. After much experimentation, I couldn’t find a transformation function that achieved this for me. But then I realised that I didn’t need to use a function – I could just use empirical histograms as my target function, mapped to my original image via the brightness ranking of each pixel. All I needed to do was select an exemplar image (or multiple exemplar images) and then order the pixel brightnesses, then map across. Here’s how I did it using R:
This gave me fairly consistent brightnesses and contrast, regardless of the quality of the original images, and also prevented washed out regions. You can see the results below:
Standardising the input data helps machine learning algorithms perform better because they don’t have to waste resources figuring out how to adjust for varying inputs where that variation is not a predictive feature.
In my next blog I will describe how I used the DICOM header information to further improve the model inputs, and to create extra features for my final model.
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