2018 JRNL PP Iñaki Rabanillo - 1.pdf
Terbatas  Irwan Sofiyan
» Gedung UPT Perpustakaan
Terbatas  Irwan Sofiyan
» Gedung UPT Perpustakaan
Characterization of the noise distribution in
magnetic resonance images has multiple applications,
including quality assurance and protocol optimization.
Noise characterization is particularly important in the
presence of parallel imaging acceleration with multi-coil
acquisitions, where the noise distribution can contain
severe spatial heterogeneities. If the parallel imaging reconstruction
is a linear process, an accurate noise analysis
can be carried out by taking into account the correlations
between all the samples involved. However, for k-spacebased
techniques such as generalized autocalibrating partially
parallel acquisition (GRAPPA), the exact analysis has
been consideredcomputationallyprohibitive due to the very
large size of the noise covariance matrices required to
characterize the noise propagation from k-space to image
space. Previously proposed methods avoid this computational
burden by formulating the GRAPPA reconstruction as
a pixel-wise linear operation performed in the image space.
However, these methods are not exact in the presence of
non-uniform sampling of k-space (e.g., containing a calibration
region). For this reason, in this paper, we develop
an accurate characterization of the noise distribution for
self-calibrated parallel imaging in the presence of arbitrary
Cartesian sampling patterns. By exploiting the symmetries
and separability in the noise propagation process, the proposed
method is computationally efficient and does not
require large matrices. Under the assumption of a fixed
reconstruction kernel, this method provides the precise
distribution of the noise variance for each coil’s image.
These coil-by-coil noise maps are subsequently combined
according to the coil combination approach used in image
reconstruction, and therefore can be appliedwith both complex
coilcombination and root-sum-of-squaresapproaches.
In this paper, we present the proposed noise characterization
method and compare it to previous techniques using
Monte Carlo simulations as well as phantom acquisitions.