"""
=====================================================================
Reconstruction of the diffusion signal with the kurtosis tensor model
=====================================================================
The diffusion kurtosis model is an expansion of the diffusion tensor model
(see :ref:`example_reconst_dti`). In addition to the diffusion tensor (DT), the
diffusion kurtosis model quantifies the degree to which water diffusion in
biological tissues is non-Gaussian using the kurtosis tensor (KT)
[Jensen2005]_.
Measurements of non-Gaussian diffusion from the diffusion kurtosis model are of
interest because they can be used to charaterize tissue microstructural
heterogeneity [Jensen2010]_ and to derive concrete biophysical parameters, such
as the density of axonal fibres and diffusion tortuosity [Fieremans2011]_.
Moreover, DKI can be used to resolve crossing fibers in tractography and to
obtain invariant rotational measures not limited to well-aligned fiber
populations [NetoHe2015]_.
The diffusion kurtosis model expresses the diffusion-weighted signal as:
.. math::
S(n,b)=S_{0}e^{-bD(n)+\frac{1}{6}b^{2}D(n)^{2}K(n)}
where $\mathbf{b}$ is the applied diffusion weighting (which is dependent on
the measurement parameters), $S_0$ is the signal in the absence of diffusion
gradient sensitization, $\mathbf{D(n)}$ is the value of diffusion along
direction $\mathbf{n}$, and $\mathbf{K(n)}$ is the value of kurtosis along
direction $\mathbf{n}$. The directional diffusion $\mathbf{D(n)}$ and kurtosis
$\mathbf{K(n)}$ can be related to the diffusion tensor (DT) and kurtosis tensor
(KT) using the following equations:
.. math::
D(n)=\sum_{i=1}^{3}\sum_{j=1}^{3}n_{i}n_{j}D_{ij}
and
.. math::
K(n)=\frac{MD^{2}}{D(n)^{2}}\sum_{i=1}^{3}\sum_{j=1}^{3}\sum_{k=1}^{3}
\sum_{l=1}^{3}n_{i}n_{j}n_{k}n_{l}W_{ijkl}
where $D_{ij}$ are the elements of the second-order DT, and $W_{ijkl}$ the
elements of the fourth-order KT and $MD$ is the mean diffusivity. As the DT,
KT has antipodal symmetry and thus only 15 Wijkl elemments are needed to fully
characterize the KT:
.. math::
\begin{matrix} ( & W_{xxxx} & W_{yyyy} & W_{zzzz} & W_{xxxy} & W_{xxxz}
& ... \\
& W_{xyyy} & W_{yyyz} & W_{xzzz} & W_{yzzz} & W_{xxyy}
& ... \\
& W_{xxzz} & W_{yyzz} & W_{xxyz} & W_{xyyz} & W_{xyzz}
& & )\end{matrix}
In the following example we show how to fit the diffusion kurtosis model on
diffusion-weighted multi-shell datasets and how to estimate diffusion kurtosis
based statistics.
First, we import all relevant modules:
"""
import numpy as np
import matplotlib.pyplot as plt
import dipy.reconst.dki as dki
import dipy.reconst.dti as dti
import dipy.reconst.dki_micro as dki_micro
from dipy.data import fetch_cfin_multib
from dipy.data import read_cfin_dwi
from dipy.segment.mask import median_otsu
from scipy.ndimage.filters import gaussian_filter
"""
DKI requires multi-shell data, i.e. data acquired from more than one non-zero
b-value. Here, we use fetch to download a multi-shell dataset which was kindly
provided by Hansen and Jespersen (more details about the data are provided in
their paper [Hansen2016]_). The total size of the downloaded data is 192
MBytes, however you only need to fetch it once.
"""
fetch_cfin_multib()
img, gtab = read_cfin_dwi()
data = img.get_data()
affine = img.affine
"""
Function ``read_cenir_multib`` return img and gtab which contains respectively
a nibabel Nifti1Image object (where the data can be extracted) and a
GradientTable object with information about the b-values and b-vectors.
Before fitting the data, we preform some data pre-processing. We first compute
a brain mask to avoid unnecessary calculations on the background of the image.
"""
maskdata, mask = median_otsu(data, 4, 2, False, vol_idx=[0, 1], dilate=1)
"""
Since the diffusion kurtosis models involves the estimation of a large number
of parameters [TaxCMW2015]_ and since the non-Gaussian components of the
diffusion signal are more sensitive to artefacts [NetoHe2012]_, it might be
favorable to suppress the effects of noise and artefacts before diffusion
kurtosis fitting. In this example the effects of noise and artefacts are
suppress by using 3D Gaussian smoothing (with a Gaussian kernel with
fwhm=1.25) as suggested by pioneer DKI studies (e.g. [Jensen2005]_,
[NetoHe2012]_). Although here the Gaussian smoothing is used so that results
are comparable to these studies, it is important to note that more advanced
noise and artifact suppression algorithms are available in DIPY_ (e.g. the
non-local means filter :ref:`example-denoise-nlmeans`).
"""
fwhm = 1.25
gauss_std = fwhm / np.sqrt(8 * np.log(2)) # converting fwhm to Gaussian std
data_smooth = np.zeros(data.shape)
for v in range(data.shape[-1]):
data_smooth[..., v] = gaussian_filter(data[..., v], sigma=gauss_std)
"""
Now that we have loaded and pre-processed the data we can go forward
with DKI fitting. For this, the DKI model is first defined for the data's
GradientTable object by instantiating the DiffusionKurtosisModel object in the
following way:
"""
dkimodel = dki.DiffusionKurtosisModel(gtab)
"""
To fit the data using the defined model object, we call the ``fit`` function of
this object:
"""
dkifit = dkimodel.fit(data_smooth, mask=mask)
"""
The fit method creates a DiffusionKurtosisFit object, which contains all the
diffusion and kurtosis fitting parameters and other DKI attributes. For
instance, since the diffusion kurtosis model estimates the diffusion tensor,
all diffusion standard tensor statistics can be computed from the
DiffusionKurtosisFit instance. For example, we show below how to extract the
fractional anisotropy (FA), the mean diffusivity (MD), the axial diffusivity
(AD) and the radial diffusivity (RD) from the DiffusionKurtosisiFit instance.
"""
FA = dkifit.fa
MD = dkifit.md
AD = dkifit.ad
RD = dkifit.rd
"""
Note that these four standard measures could also be computed from DIPY's DTI
module. Theoretically, computing these measures from both models should be
analogous. However, according to recent studies, the diffusion statistics from
the kurtosis model are expected to have better accuracy [Veraar2011]_,
[NetoHe2012]_. For comparison purposes, we calculate below the FA, MD, AD, and
RD using DIPY's ``TensorModel``.
"""
tenmodel = dti.TensorModel(gtab)
tenfit = tenmodel.fit(data_smooth, mask=mask)
dti_FA = tenfit.fa
dti_MD = tenfit.md
dti_AD = tenfit.ad
dti_RD = tenfit.rd
"""
The DT based measures can be easily visualized using matplotlib. For example,
the FA, MD, AD, and RD obtained from the diffusion kurtosis model (upper
panels) and the tensor model (lower panels) are plotted for a selected axial
slice.
"""
axial_slice = 9
fig1, ax = plt.subplots(2, 4, figsize=(12, 6),
subplot_kw={'xticks': [], 'yticks': []})
fig1.subplots_adjust(hspace=0.3, wspace=0.05)
ax.flat[0].imshow(FA[:, :, axial_slice].T, cmap='gray', vmin=0, vmax=0.7,
origin='lower')
ax.flat[0].set_title('FA (DKI)')
ax.flat[1].imshow(MD[:, :, axial_slice].T, cmap='gray', vmin=0, vmax=2.0e-3,
origin='lower')
ax.flat[1].set_title('MD (DKI)')
ax.flat[2].imshow(AD[:, :, axial_slice].T, cmap='gray', vmin=0, vmax=2.0e-3,
origin='lower')
ax.flat[2].set_title('AD (DKI)')
ax.flat[3].imshow(RD[:, :, axial_slice].T, cmap='gray', vmin=0, vmax=2.0e-3,
origin='lower')
ax.flat[3].set_title('RD (DKI)')
ax.flat[4].imshow(dti_FA[:, :, axial_slice].T, cmap='gray', vmin=0, vmax=0.7,
origin='lower')
ax.flat[4].set_title('FA (DTI)')
ax.flat[5].imshow(dti_MD[:, :, axial_slice].T, cmap='gray', vmin=0, vmax=2.0e-3,
origin='lower')
ax.flat[5].set_title('MD (DTI)')
ax.flat[6].imshow(dti_AD[:, :, axial_slice].T, cmap='gray', vmin=0, vmax=2.0e-3,
origin='lower')
ax.flat[6].set_title('AD (DTI)')
ax.flat[7].imshow(dti_RD[:, :, axial_slice].T, cmap='gray', vmin=0, vmax=2.0e-3,
origin='lower')
ax.flat[7].set_title('RD (DTI)')
plt.show()
fig1.savefig('Diffusion_tensor_measures_from_DTI_and_DKI.png')
"""
.. figure:: Diffusion_tensor_measures_from_DTI_and_DKI.png
:align: center
Diffusion tensor measures obtained from the diffusion tensor estimated
from DKI (upper panels) and DTI (lower panels).
In addition to the standard diffusion statistics, the DiffusionKurtosisFit
instance can be used to estimate the non-Gaussian measures of mean kurtosis
(MK), the axial kurtosis (AK) and the radial kurtosis (RK).
Kurtosis measures are susceptible to high amplitude outliers. The impact of
high amplitude kurtosis outliers can be removed by introducing as an optional
input the extremes of the typical values of kurtosis. Here these are assumed to
be on the range between 0 and 3):
"""
MK = dkifit.mk(0, 3)
AK = dkifit.ak(0, 3)
RK = dkifit.rk(0, 3)
"""
Now we are ready to plot the kurtosis standard measures using matplotlib:
"""
fig2, ax = plt.subplots(1, 3, figsize=(12, 6),
subplot_kw={'xticks': [], 'yticks': []})
fig2.subplots_adjust(hspace=0.3, wspace=0.05)
ax.flat[0].imshow(MK[:, :, axial_slice].T, cmap='gray', vmin=0, vmax=1.5,
origin='lower')
ax.flat[0].set_title('MK')
ax.flat[1].imshow(AK[:, :, axial_slice].T, cmap='gray', vmin=0, vmax=1.5,
origin='lower')
ax.flat[1].set_title('AK')
ax.flat[2].imshow(RK[:, :, axial_slice].T, cmap='gray', vmin=0, vmax=1.5,
origin='lower')
ax.flat[2].set_title('RK')
plt.show()
fig2.savefig('Kurtosis_tensor_standard_measures.png')
"""
.. figure:: Kurtosis_tensor_standard_measures.png
:align: center
Kurtosis tensor standard measures obtained from the kurtosis tensor.
The non-Gaussian behaviour of the diffusion signal is larger when water
diffusion is restricted by compartments and barriers (e.g., myelin sheath).
Therefore, as the figure above shows, white matter kurtosis values are smaller
along the axial direction of fibers (smaller amplitudes shown in the AK map)
than for the radial directions (larger amplitudes shown in the RK map).
As mentioned above, DKI can also be used to derive concrete biophysical
parameters by applying microstructural models to DT and KT estimated from DKI.
For instance, Fieremans et al. [Fieremans2011]_ showed that DKI can be used to
estimate the contribution of hindered and restricted diffusion for well-aligned
fibers. These tensors can be also interpreted as the influences of intra- and
extra-cellular compartments and can be used to estimate the axonal volume
fraction and diffusion extra-cellular tortuosity. According to recent studies,
these latter measures can be used to distinguish processes of axonal loss from
processes of myelin degeneration [Fieremans2012]_.
The model proposed by Fieremans and colleagues can be defined in DIPY by
instantiating the 'KurtosisMicrostructureModel' object in the following way:
"""
dki_micro_model = dki_micro.KurtosisMicrostructureModel(gtab)
"""
Before fitting this microstructural model, it is useful to indicate the
regions in which this model provides meaningful information (i.e. voxels of
well-aligned fibers). Following Fieremans et al. [Fieremans2011]_, a simple way
to select this region is to generate a well-aligned fiber mask based on the
values of diffusion sphericity, planarity and linearity. Here we will follow
these selection criteria for a better comparision of our figures with the
original article published by Fieremans et al. [Fieremans2011]_. Nevertheless,
it is important to note that voxels with well-aligned fibers can be selected
based on other approaches such as using predefined regions of interest.
"""
well_aligned_mask = np.ones(data.shape[:-1], dtype='bool')
# Diffusion coefficient of linearity (cl) has to be larger than 0.4, thus
# we exclude voxels with cl < 0.4.
cl = dkifit.linearity.copy()
well_aligned_mask[cl < 0.4] = False
# Diffusion coefficient of planarity (cp) has to be lower than 0.2, thus
# we exclude voxels with cp > 0.2.
cp = dkifit.planarity.copy()
well_aligned_mask[cp > 0.2] = False
# Diffusion coefficient of sphericity (cs) has to be lower than 0.35, thus
# we exclude voxels with cs > 0.35.
cs = dkifit.sphericity.copy()
well_aligned_mask[cs > 0.35] = False
# Removing nan associated with background voxels
well_aligned_mask[np.isnan(cl)] = False
well_aligned_mask[np.isnan(cp)] = False
well_aligned_mask[np.isnan(cs)] = False
"""
Analogous to DKI, the data fit can be done by calling the ``fit`` function of
the model's object as follows:
"""
dki_micro_fit = dki_micro_model.fit(data_smooth, mask=well_aligned_mask)
"""
The KurtosisMicrostructureFit object created by this ``fit`` function can then
be used to extract model parameters such as the axonal water fraction and
diffusion hindered tortuosity:
"""
AWF = dki_micro_fit.awf
TORT = dki_micro_fit.tortuosity
"""
These parameters are plotted below on top of the mean kurtosis maps:
"""
fig3, ax = plt.subplots(1, 2, figsize=(9, 4),
subplot_kw={'xticks': [], 'yticks': []})
AWF[AWF == 0] = np.nan
TORT[TORT == 0] = np.nan
ax[0].imshow(MK[:, :, axial_slice].T, cmap=plt.cm.gray, interpolation='nearest',
origin='lower')
im0 = ax[0].imshow(AWF[:, :, axial_slice].T, cmap=plt.cm.Reds, alpha=0.9,
vmin=0.3, vmax=0.7, interpolation='nearest', origin='lower')
fig3.colorbar(im0, ax=ax.flat[0])
ax[1].imshow(MK[:, :, axial_slice].T, cmap=plt.cm.gray, interpolation='nearest',
origin='lower')
im1 = ax[1].imshow(TORT[:, :, axial_slice].T, cmap=plt.cm.Blues, alpha=0.9,
vmin=2, vmax=6, interpolation='nearest', origin='lower')
fig3.colorbar(im1, ax=ax.flat[1])
fig3.savefig('Kurtosis_Microstructural_measures.png')
"""
.. figure:: Kurtosis_Microstructural_measures.png
:align: center
Axonal water fraction (left panel) and tortuosity (right panel) values
of well-aligned fiber regions overlaid on a top of a mean kurtosis all-brain
image.
References
----------
.. [TaxCMW2015] Tax CMW, Otte WM, Viergever MA, Dijkhuizen RM, Leemans A
(2014). REKINDLE: Robust extraction of kurtosis INDices with
linear estimation. Magnetic Resonance in Medicine 73(2):
794-808.
.. [Jensen2005] Jensen JH, Helpern JA, Ramani A, Lu H, Kaczynski K (2005).
Diffusional Kurtosis Imaging: The Quantification of
Non_Gaussian Water Diffusion by Means of Magnetic Resonance
Imaging. Magnetic Resonance in Medicine 53: 1432-1440
.. [Jensen2010] Jensen JH, Helpern JA (2010). MRI quantification of
non-Gaussian water diffusion by kurtosis analysis. NMR in
Biomedicine 23(7): 698-710
.. [Fieremans2011] Fieremans E, Jensen JH, Helpern JA (2011). White matter
characterization with diffusion kurtosis imaging. NeuroImage
58: 177-188
.. [Fieremans2012] Fieremans E, Jensen JH, Helpern JA, Kim S, Grossman RI,
Inglese M, Novikov DS. (2012). Diffusion distinguishes between
axonal loss and demyelination in brain white matter.
Proceedings of the 20th Annual Meeting of the International
Society for Magnetic Resonance Medicine; Melbourne, Australia.
May 5-11.
.. [Hansen2016] Hansen, B, Jespersen, SN (2016). Data for evaluation of fast
kurtosis strategies, b-value optimization and exploration of
diffusion MRI contrast. Scientific Data 3: 160072
doi:10.1038/sdata.2016.72
.. [NetoHe2012] Neto Henriques R, Ferreira H, Correia M, (2012). Diffusion
kurtosis imaging of the healthy human brain. Master
Dissertation Bachelor and Master Programin Biomedical
Engineering and Biophysics, Faculty of Sciences.
http://repositorio.ul.pt/bitstream/10451/8511/1/ulfc104137_tm_Rafael_Henriques.pdf
.. [NetoHe2015] Neto Henriques R, Correia MM, Nunes RG, Ferreira HA (2015).
Exploring the 3D geometry of the diffusion kurtosis tensor -
Impact on the development of robust tractography procedures and
novel biomarkers, NeuroImage 111: 85-99
.. [Veraar2011] Veraart J, Poot DH, Van Hecke W, Blockx I, Van der Linden A,
Verhoye M, Sijbers J (2011). More Accurate Estimation of
Diffusion Tensor Parameters Using Diffusion Kurtosis Imaging.
Magnetic Resonance in Medicine 65(1): 138-145
.. include:: ../links_names.inc
"""