Jakob Meineke1, Fabian Wenzel1, Iain D. Wilkinson2, and Ulrich Katscher1
1Philips Research Europe, Hamburg, Germany, 2University of Sheffield, Sheffield, United Kingdom
Synopsis
Quantitative
Susceptibility Mapping (QSM) is used to study the deep gray-matter nuclei of
patients with Alzheimer’s Disease (AD) and healthy control subjects. QSM is
performed using “Joint background-field removal and segmentation-Enhanced
Dipole Inversion” (JEDI), which leverages the information from automated
model-based segmentation and allows the compact single-step formulation of the
ill-posed inversion problem of QSM. The tissue magnetic susceptibility shows a
trend for increase in the amygdala and the putamen of AD patients as compared
to healthy control subjects, in agreement with previous studies.Purpose
To assess the potential
of Quantitative Susceptibility Mapping (QSM) with Joint background-field
removal and segmentation-Enhanced Dipole Inversion (JEDI) for the fully
automated measurement of the magnetic susceptibility in subcortical gray-matter
nuclei of patients with Alzheimer’s Disease and healthy control subjects.
Introduction
Disturbed
iron regulation and increased iron levels in specific brain regions have long
been associated with Alzheimer’s Disease (AD) [1].
The specific role of iron in the framework of AD, its causes and its
consequences, are, however, still widely unclear, particularly due to the lack
of reliable in vivo measurements. In
recent years, MR-based Quantitative Susceptibility Mapping (QSM) has become
available to indicate changes in in vivo iron
concentration in deep gray matter quantitatively, assuming that magnetic
susceptibility of deep gray matter is predominantly determined by ferritin.
Methods
12 patients (age: 65$$$\pm$$$11) with AD and
14 healthy controls (HC) (age: 57$$$\pm$$$8) were scanned on a 3T scanner (Ingenia,
Philips Healthcare, Best, The Netherlands) using a 32-channel head-coil
(University of Sheffield) or 15-channel head-coil (Philips Research Hamburg) after
obtaining informed consent from the respective IRBs. The scans are part of an
ongoing prospective study within the framework of the EU VPH-DARE@IT project
and included a multi-echo gradient-echo sequence for QSM (FOV: (AP, FH, RL)
240×145×210 mm
3, true axial orientation, acquisition voxel: 0.6×0.6×2.0 mm
3,
FA=14°, TE=3.5 ms, ΔTE=4 ms, 7 echoes, TR=31 ms, bipolar readout, BW=275.9 Hz/vx,
SENSE (P/S) 1.8x1.2) and a T1-weighted magnetization-prepared TFE sequence which
is used for model-based segmentation (FOV: (AP, FH, RL) 240×240×170 mm
3, acq
voxel 0.94×0.94×1.0 mm
3, FA=8°, TR=8 ms, TFE factor=222, inversion delay 1000
ms, BW=191.5 Hz/vx, SENSE (P/S) 1.0/2.2).
To solve the ill-posed inverse problem of QSM,
the algorithm dubbed “Joint background-field removal and segmentation-Enhanced
Dipole Inversion” (JEDI) [2]
was chosen, which uses a priori
knowledge from automated anatomical segmentation within a single step
formulation of the inverse field-to-source problem. In previous studies, JEDI
demonstrated improved accuracy and less long-range modulations in numerical
simulations and in vivo volunteer
data. Reconstruction parameters were identical for all subjects. Within the
segmented regions, the median of the susceptibility was computed, referencing
to the median of the susceptibility in the corpus callosum.
Results
Figure 1 shows the median
susceptibility for segmented brain regions averaged within the AD and the HC
groups. Figure 2 shows the difference of the average median susceptibility
between the AD and the HC group. The largest increase in susceptibility between
the group of AD patients and the HC group are found in the amygdala and the
putamen. The differences between groups are about half the standard deviation
found within groups.
Small differences, below 0.005 ppm, both positive
and negative, are found in the caudate nucleus, the thalamus, the hippocampus
and the globus pallidus.
Discussion
The results
presented here are in agreement with previous work, which has used alternative
reconstruction methods for QSM, and showed elevated susceptibility values for
AD patients in the putamen [3]
along with indications for an increase in the amygdala. The variability of
median susceptibility values within the subject groups is unlikely to be caused
by the reconstruction method, and rather reflects subject-specific biological
variations. The reproducibility of the QSM reconstruction method used here has
been assessed by repeatedly scanning the same volunteer over several months,
finding a standard deviation of the median below 0.01 ppm for all brain regions.
The effect of age on the magnetic susceptibility depends on the brain region
considered [4, 5]
and might play a role for the putamen. In a volunteer study (N=19, ages between
32 and 60) performed in connection with the described study, no statistically
significant effect of age alone was observed.
Larger
cohort populations are needed to ascertain whether the observed trends reach
statistical significance.
The fully automated
reconstruction and region-specific evaluation is compatible with a clinical
setting.
Acknowledgements
This project has received funding from European
Union’s Seventh Framework Programme, grant no. 601055References
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Inversion,” ISMRM 2015, Toronto,
Canada, 3321.
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