S	Raw scans
I	Preprocessed images set
N	Number of images
$I_j$	Random image from the set I
$I^{\prime }$	Aligned I
$I_{wc}$	Weighted center image
$I_c$	Unweighted center image
T	Template
t	Iteration counter
$T^t$	Template at iteration t
$\Delta$	Difference between $T^t$ and $T^{t-1}$
$T_P$	Template from patch-based estimation
$T_V$	Template from voxel-based averaging
F	Features
$PC$	Principal components
$\Sigma$	Covariance matrix
${\Sigma }^{-1}$	Inverse of $\Sigma$
W	Image similarity weights
$NW$	Normalized image similarity weights
$d_{ji}$	Displacement from $I_j$ to each $I_i\in I:i\ne j$
$d_{wj}$	Weighted displacement for $I_j$ to reach $I_{wc}$
$d_{ic}$	Displacement from $I_i\in I$ to center image
$P_T$	Set of template patches
$P_I$	Set of $I^{\prime }$ patches
D	Euclidean distances
h	Gaussian bandwidths
w	$I^{\prime }$ nonzero voxel weights
$nw$	$I^{\prime }$ nonzero voxel normalized weights
V	Set of nonzero voxel indices
K	Number of nonzero voxel indices
v	Voxel index
*	Element-wise multiplication
$WD$	Weighted Displacement
$AGM$	Average Gradient Magnitude
$NMC$	Normalized Michelson Contrast
$D_{KL}$	Kullback-Leibler Divergence
$W{M}_{\max }$	Maximum intensity in WM
$G{M}_{\min }$	Minimum intensity in GM
${\parallel .\parallel }_2$	The L2-norm
$\mid ·\mid$	The absolute value
x	Intensity or voxel value
M	Number of template voxels
$P\left(x\right)$	Probability of x in template distribution
$Q\left(x\right)$	Probability of x in $I^{\prime }$ distribution
$J_v$	Jacobian matrix at v
$\det \left(J_v\right)$	Determinant of $J_v$
∇	Gradient operator
X	Total computational time
C	Processing cores