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<!DOCTYPE html>
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<div class="section" id="introduction">
<span id="id1"></span><h1>Introduction<a class="headerlink" href="#introduction" title="Permalink to this headline">¶</a></h1>
<div class="section" id="scientific-issues">
<h2>Scientific issues<a class="headerlink" href="#scientific-issues" title="Permalink to this headline">¶</a></h2>
<p>PyHRF aims to provide advanced tools for single-subject analysis of
functional Magnetic Resonance Imaging (fMRI) data acquired during an
experimental paradigm (i.e. not resting-state). The core idea of PyHRF is to
estimate the dynamics of brain activation by recovering the so-called
<strong>Hemodynamic Response Function (HRF)</strong> in a spatially varying manner. To this
end, PyHRF implements two different approaches:</p>
<ol class="arabic simple">
<li>a voxel-wise and condition-wise HRF estimation <a class="footnote-reference" href="#id5" id="id2">[1]</a>;</li>
<li>a parcel-wise spatially adaptive joint detection-estimation (JDE) algorithm <a class="footnote-reference" href="#id6" id="id3">[2]</a> <a class="footnote-reference" href="#id7" id="id4">[3]</a>.</li>
</ol>
<p>The second approach is more powerful since it jointly addresses (i) the
localization of evoked brain activity in response to external stimulation and
(ii) the estimation of parcelwise hemodynamic filters. To this end, a
parcellation (either functional or anatomical) has to be provided as input
parameter.</p>
<p>This tool hence provides interesting perspectives for understanding the HRF
differences between brain regions and also between individuals and
populations (infants, children, adults, patients, etc). Within the classical
workflow of fMRI data analysis (preprocessings, estimation, statistical
inference), PyHRF takes place at the estimation stage. However, we have also
implemented posterior probability maps (PPMs) to allow the user performing
statistical inference at the subject-level.</p>
<p>For the sake of computational efficiency, the variational
expectation-maximization (VEM) is used as default algorithm for computing
the solution. The results that are generated in the context are the following:</p>
<ul class="simple">
<li>the 3D effect size maps associated with each experimental condition;</li>
<li>the 3D contrast maps specified by the user;</li>
<li>the time-series describing the HRFs for the set of all brain regions
(4D volume). The analysis can also be performed on the cortical surface from
projected BOLD signals and then produces functional textures to be displayed
on the input cortical mesh.</li>
</ul>
<table class="docutils footnote" frame="void" id="id5" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id2">[1]</a></td><td>P. Ciuciu, J.-B. Poline, G. Marrelec, J. Idier, Ch. Pallier, and H. Benali,
“Unsupervised robust non-parametric estimation of the hemodynamic response
function for any fMRI experiment”, IEEE Trans. Medical Imaging;
22(10):1235-1251, Oct 2003</td></tr>
</tbody>
</table>
<table class="docutils footnote" frame="void" id="id6" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id3">[2]</a></td><td>T. Vincent, L. Risser and P. Ciuciu, “Spatially adaptive mixture modeling
for analysis of within-subject fMRI time series”, IEEE Trans. Medical
Imaging; 29(4):1059-1074, Apr 2010.</td></tr>
</tbody>
</table>
<table class="docutils footnote" frame="void" id="id7" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id4">[3]</a></td><td>L. Chari, T. Vincent, F. Forbes, M. Dojat and P. Ciuciu, “Fast joint
detection-estimation of evoked brain activity in event-related fMRI using a
variational approach”, IEEE Trans. Medical Imaging; 32(5):821-837, May 2013.</td></tr>
</tbody>
</table>
</div>
<div class="section" id="package-overview">
<h2>Package overview<a class="headerlink" href="#package-overview" title="Permalink to this headline">¶</a></h2>
<p>PyHRF is mainly written in <em>Python</em>, with some C-extensions that handle
computationally intensive parts of the algorithms. The package relies on
classical scientific libraries: <a class="reference external" href="https://github.com/numpy/numpy">numpy</a>,
<a class="reference external" href="https://github.com/scipy/scipy">scipy</a>,
<a class="reference external" href="https://github.com/matplotlib/matplotlib">matplotlib</a> as well as
<a class="reference external" href="https://github.com/nipy/nibabel">nibabel</a> to handle input/outputs and
<a class="reference external" href="https://github.com/nipy/nipy">nipy</a> which provides tools for functional data
analysis.</p>
<p>PyHRF can be used in a stand-alone fashion and provides a set of simple commands
in a modular fashion. The setup process is handled through XML files which can
be adapted by the user from a set of templates. This format was chosen for its
hierarchical organisation which suits the nested nature of the algorithm
parametrization. A dedicated XML editor is provided with a PyQt graphical
interface for a quicker edition and also a better review of the treatment
parameters. When such an XML setup file is generated <em>ab initio</em>, it defines a
default analysis which involves a small real data set shipped with the package.
This allows for a quick testing of the algorithm and is also used for
demonstration purpose.</p>
<p>An artificial fMRI data generator is provided where the user can test the
behaviour of the algorithms with different activation configurations, HRF shapes,
nuisance types (noise, low frequency drifts) and paradigms (slow/fast
event-related or bloc designs).</p>
<p>Concerning the analysis process, which can be computationally intensive, PyHRF
handles parallel computing through multiple core computers.</p>
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