Sophia Kronthaler1, Christof Böhm1, Dominik Weidlich1, Maximilian N. Diefenbach1,2, and Dimitrios C. Karampinos1
1Department of Diagnostic and Interventional Radiology, Technical University of Munich, Munich, Germany, 2Division of Infectious Diseases and Tropical Medicine, Munich, Germany
Synopsis
Osteoporosis is characterized by a loss in bone mass and structural decrease of bone tissue which is leading to morbidity and an increased fracture risk. Fractures occur predominantly in areas with trabecular bone and the assessment of the bone volume and microstructural changes in the trabecular bone are therefore highly relevant in fracture prediction. The present study characterizes the decay of trabecularized bone marrow signal in multi-echo imaging sampling both UTEs and conventional TEs. A novel methodology is proposed for simultaneous cortical and trabecular bone imaging in the presence of bone marrow.
Purpose
Trabecular bone imaging is highly relevant in osteoporotic
fracture prediction1. Two main MR approaches have been proposed for
measuring trabecular bone properties2. The first approach encompasses
techniques focusing on bone marrow either by high-resolution imaging of the
bone matrix voids within the marrow3,4 or by quantifying magnetic
susceptibility effects using the marrow exponential signal decay at later echo
times (TEs) (T2*/T2’
mapping)5 or its gaussian decay at short TEs6,7. The
second approach encompasses techniques aiming on direct imaging of the bone
matrix, using ultra-short TE (UTE) techniques in combination with fat/long T2-species
suppression8-9. Techniques measuring bone marrow properties use
conventional TEs and cannot image the cortical bone. Techniques measuring
directly the bone matrix employ UTEs and image the cortical bone10,
but need to suppress the marrow signal for trabecular bone imaging and suffer
from lower sensitivity due to the low bone proton density11. This
work characterizes the decay of trabecularized bone marrow signal in multi-echo
imaging sampling both UTEs and conventional TEs and proposes a novel methodology
for simultaneous cortical and trabecular bone imaging.Theory
We assume that a tissue consists of bone mineral matrix and fatty bone
marrow. Given the different magnetic susceptibilities χ between bone matrix and marrow and the low-fat diffusion coefficient, the
static dephasing regime in magnetically inhomogeneous tissues applies6:
the fatty marrow signal shows gaussian decay at short TEs and exponential decay
at later TEs. If the MR signal from the bone mineral matrix is neglected across
TEs (low proton density and short T2*), the total tissue signal
equals the bone marrow signal. The bone marrow signal at later TEs extrapolated
at t=0 would be higher compared to the measured bone marrow signal at t=012
and the signal difference would be sensitive to the bone matrix volume fraction
(ν).
$$ ζ=e^{ν}=\frac{S_{extrapolated}(t=0) - S_{measured}(t=0)}{S_{extrapolated}(t=0) + S_{measured}(t=0)}$$Methods
Bone cubes forward simulation
Bone masks were generated by thresholding human
trabecular bone micro CT scans (dx=45.6µm, Figure 2A)13. Microscopic
field-maps were forward simulated (Figure 2B) at different BV/TVs, different
χ-difference of bone matrix to marrow and different orientations of
the main magnetic field B0 (Figure 3A-B). The signal was then
downsampled to the ‘macroscopic’ MR-like resolution (dx=1.5mm) and the
simulations were repeated for different signal models: a) without bone matrix
signal and T2,fat=60ms, b) with bone matrix signal, T2,fat=60ms,
T2*bone=0.5ms,
ρbone=0.3, c) with/without bone matrix
signal and the 9-peak marrow fat spectrum model14 (Figure 3C).
In vivo measurements
3D-UTE relaxometry measurements were performed with a stack-of-stars
center-out radial acquisition and phase-encoding in the third cartesian
dimension (Figure 1A, SENSE R=2) on a 3T system (Ingenia Elition, Philips
Healthcare, Best, The Netherlands). All TEs of one spoke, with randomized order of the TEs15, were acquired before
the spoke was rotated (Figure
1B).
A scan with extensive echo sampling was performed with 26 TEs, range=0.19ms-10.14ms,
resolution 1.5x1.5x4mm3, scan time 38min, flip-angle 5°,
TR 12ms, in fat phantoms (PDFF 0%, 5%, 15%, 100%) and in the knee of two
subjects. To simultaneously image cortical and trabecular bone regions using
the ζ, 7 TEs were acquired in the knee of two subjects
[0.19,1.29,2.39,3.49,4.59,5.69,6.79 ms] with a resolution of 1.5x1.5x2mm3
and a scan time of 15.4min.
Reconstruction and
postprocessing
The UTE k-space trajectory was corrected by means of a gradient impulse
response function16. A common-R2* water-fat signal model was used17,
including echoes with a TE<1.54ms. The difference ζ between the measured signal and the
extrapolated fitted curve was estimated at TE=0.19ms.
Results
For TEs smaller than a characteristic time tc, the simulated
signal decays exponentially with an argument that depends quadratically on TE
(Figure 2). An exponential fit for points larger tc yields a T2*
decay including a T2’ introduced due to the intra-voxel field inhomogeneities.
For different orientations of B0, the T2* varied. S(t=0) of the
extrapolated fitted curve was independent of microstructure changes.
Figure 2C
includes the fat chemical shift effect, showing an overestimation of the signal
at t=0. Even after considering a fast decaying signal coming from the bone
matrix, a time interval, where the extrapolated signal exceeded the measured
signal, was observed.
Figure 4 shows the signal decay of the 26-echoes in vivo data-set. The
fit with a common-T2* water-fat signal model TEs>1.5ms showed a small
residual in all ROIs. The extrapolated signal at TEs<1.5ms matched the
measured data in bone and muscle. In cortical bone, the signal was
underestimated and in ROIs containing trabecular bone the signal was
overestimated using the extrapolation.
Figure 5 illustrates the
difference between the measured and extrapolated signal. For ζ > 0 trabecular bone
was visible and areas with a higher BV/TV lighted up (ζtrabbone =0.05 compared to ζmuscle =0.001). For ζ < 0 areas with short
T2* components showed higher |ζ| values such as cortical bone and fascia.Discussion & Conclusion
Numerical simulations showed T2* is sensitive to intra-voxel dephasing
whereas ζ, parametrizing the difference of the measured and
extrapolated data at t=0, is only dependent on BV/TV. Preliminary in
vivo results indicated that ζ scales with BV/TV in trabecularized bone regions. Based on a 7-echo measurement
sampling one UTE and 6 conventional TEs, the ζ map allowed the visualization of both trabecularized
bone and cortical bone regions.Acknowledgements
Acknowledgement: The present
work was supported by the European Research Council (grant agreement No 677661,
ProFatMRI). This work reflects only the authors view and the EU is not
responsible for any use that may be made of the information it contains. The
authors also acknowledge research support from Philips Healthcare.References
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