Han Sang Lee1 and Junmo Kim1
1School of Electrical Engineering, KAIST, Daejeon, Korea, Republic of
Synopsis
Multi-atlas
segmentation has often suffered from the registration error. We propose a novel
method for multi-atlas registration for multi-atlas segmentation inspired by the template generation
and deep neural network. We first add an intra-atlas registration layer which
performs image-based registration between atlas images to duplicate the
atlases. We then add a label-wise registration layer which rectifies the
registered images by label-based registration. We present preliminary results
of our multi-layered atlas registration on brain MRI segmentation.Purpose
Multi-atlas segmentation
(MAS) is currently one of the major approaches for medical image segmentation. This
approach usually consists of two steps, multi-atlas registration (MAR) and label
fusion. In MAR, multiple annotated training images are
registered to target image to generate “transferred” labels. In label fusion,
the transferred labels are combines to compute target labels by means of
weighted voting. A popularity of multi-atlas approach comes from the generality
of this approach thanks to the recent presence of large scale annotated datasets
e.g. ADNI brain MRI or OAI knee joint MRI datasets. However, the MAS on these large
scale datasets has often suffered from the registration error. We propose a
novel method of MAR for MAS inspired by the template
generation [1] and deep neural network. We present preliminary results of our
multi-layered atlas registration (MLAR) on brain MRI MAS.
Methods
Five annotated brain MRI
were incorporated from the training data of MRBrainS challenge [2]. These 3D
T1-weighted scans were acquired on a 3.0T Philips Achieva system. All scans
have voxel size of 1.0mm x 1.0mm x 1.0mm, TR of 7.9ms, and TE of 4.5ms. All
scans were manually labeled by experts into 8 labels; cortical gray matter,
basal ganglia, white matter, white matter lesions, cerebrospinal fluid, ventricles, cerebellum, and brainstem. We construct the multi-atlas for
each image in leave-one-out manner; for one target image, other four images and
labels are used as atlas. In conventional multi-atlas registration (CMAR) for
MAS, the atlas labels are directly registered to the target image as shown in
Fig. 1 (a). It is straightforward however CMAR suffers from the registration
error. Recently, a novel method of duplicating
atlases [1] by generating additional atlas images and labels is proposed. In
[1], the new atlases are generated by registering the given atlases to other
unannotated images called “templates.” We extend this idea to propose the
single-layered atlas registration (SLAR) framework which generates the new
atlases by registering the atlases to other training atlases, as shown in Fig. 1 (b). In the figure, we first perform
atlas-by-atlas, so-called intra-atlas, image registration to generate NxN
atlases, where N is a number of atlases. These extended set of atlases is then
registered to the target image to transfer NxN atlas labels. Furthermore,
inspired by the deep neural network structure, we introduce MLAR framework by adding another type of registration layer to
the SLAR, as shown in Fig. 1 (c). In a new layer called the label-wise
registration layer, the registered atlases through SLAR are rectified according
to the ground-truth label for the registration target image SLAR. Since the input
atlas of label-wise registration layer is registered label-by-label to the
ground-truth label of the previously image-based-registered atlas, the number
of atlases does not change in this layer. In MLAR, these rectified set of
atlases is then registered to the target image to transfer NxN atlas labels. In
segmentation, the globally-weighed voting (GWV) scheme with the atlas
similarity weight is used as label fusion. In GWV, the atlas similarity weight
is computed by the patch-wise squared distance of registered and target images.
Results
Fig.
2 shows example images of segmentation results obtained from (a) CMAR, (b)
MLAR, and (c) manual segmentation. As shown in the figure, CMAR often results
in registration error which degenerates the segmentation accuracy as in yellow
box. MLAR reduces this registration error with the help of intra-atlas
registration and label-wise rectification. As a result, the results obtrained
from MLAR are the most visually similar to manual segmentation results. To
validate the performance, Dice similarity coefficients (DSC) for testing labels
are computed, as shown in Fig. 3. In evaluation, the 8 labels are combined and
rejected to generate 3 labels; (1) cerebrospinal fluid including ventricles,
(2) gray matter consisting of cortical gray matter and basal ganglia, and (3)
white matter including white matter lesions. As shown in the figure, MLAR
improved the overall DSCs compared to those of SLAR and CMAR. In cerebrospinal
fluid, average DSCs for CMAR, SLAR, and MLAR are 68.14%, 70.06%, and 70.19%,
respectively. In gray matter, average DSCs for CMAR, SLAR, and MLAR are 74.50%,
76.56%, and 77.04%, respectively. Finally, in white matter, average DSCs for
CMAR, SLAR, and MLAR are 80.03%, 82.25%, and 82.93%, respectively
Conclusions
A novel framework of MAR with multiple layers was produced. In experiments, we
showed promising results of our framework in brain MAS. Extensive validation
on brain MRI MAS is remained as future works. To
propose additional registration layers such as similar atlas selection layer,
will be further focused as future works.
Acknowledgements
No acknowledgement found.References
[1]
Pipitone J, et al. Multi-atlas segmentation of the whole hippocampus and
subfields using multiple automatically generated templates. NeuroImage. 2014;101;494-512.
[2] Mendrik A.M., et al. MRBrainS challenge: Online evaluation framework for
brain image segmentation in 3T MRI scans. Intell. Nerosci. 2015;813696.