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One of the datasets available for the OHBM Hackathon is the Q1 public data release from the Human Connectome Project. In addition to the imaging data, which are mirrored on S3 for easy access from AWS, a great deal of imaging metadata and associated non-imaging data is accessible through ConnectomeDB, a web application built on the XNAT imaging informatics platform.
pyxnat is a library that provides a Python language API to XNAT's RESTful web services. In this tutorial, we'll use pyxnat to access behavioral measures stored in ConnectomeDB. Even if you're not a Pythonista, read on, as the underlying XNAT REST API can be accessed from just about any language. I have small examples of code using the REST API in bash, Java and Clojure, and I'd probably find it amusing to cook up an example in your favorite language; send me mail if you'd like details.
You'll need Python (version 2.7.x recommended) and pyxnat to follow along. Someday soon we'll have a hackathon-customized version of pyxnat to provide easier access to the S3-hosted data, but there's nothing AWS-specific about this introduction, so plain old pyxnat will be fine. I'm writing this using Python 2.7.1 on Mac OS X 10.7.5, but I regularly use pyxnat on Gentoo Linux; other people use pyxnat on other Linuxes and even Windows, and in principle this all should work just about anywhere you can run Python. Send me mail if you run into trouble.
Aside for Python experts: because I'm working on pyxnat and not just with it, I usually don't install pyxnat to the system Python; instead I set up a virtualenv and install to that. We'll probably have to do this in a later tutorial, as we start using not-yet-published pyxnat extensions for working with the S3-hosted data.
We'll look at some behavioral measures in ConnectomeDB: the Non-Toolbox Data Measures, a variety of tests that aren't part of the NIH Toolbox. (NIH Toolbox scores are forthcoming but not available in the Q1 data release.) The non-Toolbox measures are documented in detail here. nontoolbox.xsd is an XML Schema document that specifies the non-Toolbox data type in ConnectomeDB; it's not particularly readable, but it does provide the exact naming conventions used in ConnectomeDB.
Let's start by firing up a Python session, loading pyxnat, and setting up a connection to ConnectomeDB.
This Interface object creates a session on ConnectomeDB. Be warned: if the session is idle for a while – say, for example, you're too busy reading documentation to keep typing -- ConnectomeDB may close the session. You can tell that the session has gone stale if, when you try to do a query:
you get a plateful of nonsense that looks like:
If this happens, just create a new Interface:
Any query result objects that you created from the stale Interface will also need to be refreshed. There's an example later in this tutorial.
Exploring the ConnectomeDB data hierarchy
ConnectomeDB's data is organized into projects, which are the main access control structure in XNAT. If you have access to a project, you can see that project's data. Let's see what projects we have access to:
cdb.select.projects() asks ConnectomeDB for project details and turns the result into a collection of project objects. The
get() method returns the identifiers for each object in the collection. We could get the same result using a list comprehension; let's try that now, because that will be a more convenient form in general:
Since we're interested in the HCP Q1 data, let's get a handle on just that project:
Note that if the session goes stale, so will this object
q1. So in addition to refreshing cdb, you'll probably need to refresh
q1, too, by reissuing this command:
Querying for Subjects in the Q1 Project
What's inside of this project object? Each project contains subjects and experiments. Let's look at the list of subjects:
subject.label() instead of
subject.id(), which inside the list comprehension would have given the same result as
label() instead of
id()? The label is the human-readable name for the subject within a specified project (HCP_Q1 in our case); the first label in the list is 100307, which is the HCP-assigned name for that subject. The subject id is the XNAT site-wide unique identifer for that subject, a not-intended-for-human-consumption identifier; the id for subject 100307 is 'ConnectomeDB_S00230'. In principle, different projects might assign different labels to the same subject, or different subjects might share the same label in different projects. We aren't engaging in those sorts of shenanigans on ConnectomeDB, but we do inherit a little complexity from XNAT's flexibility.
Querying for Experiments for each Subject
What data are available for subject 100307? Let's ask:
There are three "experiments" here: 100307_3T contains the imaging data and associated metadata acquired on the HCP 3T Skyra; 100307_SubjMeta holds some bookkeeping about what data have been collected for this subject; and 100307_NonToolbox has the non-Toolbox scores. Again we use
label() instead of
get() on the experiments collection), because each project has a human-readable label for the experiment, whereas the id is the site-wide, XNAT-generated identifier.
Exploring Experiment Data
The experiments are represented by XML documents; we can view the XML for 100307_NonToolbox to see what's inside:
That's a lot of stuff. Let's take it line-by-line.
The first line,
<?xml version="1.0" ... , just tells us that this is an XML document.
The second line,
<nt:NTScores ID="ConnectomeDB_E00299" ..., is the start of the actual content. It tells us that this is a N(on)T(oolbox)Scores document, gives us the experiment ID (the XNAT site-wide identifier), the project ID, the experiment labels (the human-readable, in-project-context name), and ends with a bunch of namespace information in case we want to validate this document against the schema we were looking at earlier. (I don't. You're welcome to if you like.)
The next few lines,
</xnat:sharing>, tell us what projects know about this experiment. We can skip over this. (Yes, there's an HCP_Q2 project. No, it's not ready for you to look at yet.)
Next comes the subject ID; again, this is the XNAT site-wide ID, not the human-readable name (label). We can use pyxnat to ask ConnectomeDB for the label in a specified project:
After that come the scores (and lots of them), organized into a few groups. The schema document nontoolbox.xsd may be useful in helping to decipher this. We can ask for individual scores by walking the XML DOM:
That's a slow way of retreiving scores, since we need a full HTTP request and response for each field. (Actually, pyxnat does some caching so the requests aren't repeated. Probably. Usually. I'd still recommend doing something else.) If we want multiple scores -- either more than one score from a single experiment, or one or more scores from each of multiple experiments, there are more efficient methods.
Let's start with selecting multiple scores for a single experiment. A reasonable approach is to grab and parse the entire experiment XML document, using the Python standard library module ElementTree:
Getting scores from multiple experiments can be done either by iterating over experiment IDs with the methods described above (single-attribute or XML document requests), or by using the pyxnat search interface, which will be covered in an update coming soon.
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