Katharina Paul1, Till Huelnhagen1, Oliver Stachs2, and Thoralf Niendorf1,3
1Berlin Ultrahigh Field Facility (B.U.F.F), Max Delbrück Center for Molecular Medicine in the Helmholtz Association (MDC), Berlin, Germany, 2Department of Ophthalmology, University Medicine Rostock, Rostock, Germany, 3Experimental and Clinical Research Center, a joint cooperation between the Charité Medical Faculty and the Max-Delbrueck-Center for Molecular Medicine in the Helmholtz Association, Berlin, Germany
Synopsis
Diffusion-weighted
imaging of the eye and orbit is an emerging MRI application to provide guidance
during diagnostic assessment and treatment of ophthalmological diseases. It has
been shown that RARE based diffusion-sensitized imaging (ms-RARE) provides
images free of geometric distortions. Though, artifacts induced by involuntary
eye motion remain a concern. Applying inner volume imaging (IVI) offers the possibility
to shorten acquisition times by reducing the number of acquired phase encoding
lines. This study examines the applicability of IVI in conjunction with ms-RARE
with the goal to reduce the propensity to bulk eye motion in
diffusion-sensitized ophthalmic imaging.Purpose
MRI of the spatial
arrangements of the eye segments and their masses is an emerging application
increasingly used in (pre)-clinical imaging and diagnostic radiology [1-5].
Diffusion‑weighted MRI (DWI) probes self-diffusion of water in tissue on a
microscopic level and holds the promise to enhance the diagnostic accuracy over
anatomic ophthalmic imaging [6]. Diffusion-sensitized segmented split-echo
rapid acquisition with relaxation enhancement (ms-RARE) imaging provides high-spatial
resolution images of the eye, orbit and Nervus opticus at 3.0 T and 7.0 T [7].
Geometric distortions that are observed for EPI-DWI approaches even at lower
field strengths are offset by DWI using ms-RARE, but artifacts induced by
involuntary eye motion remain a concern. Inner volume imaging (IVI) [8] is
particularly suited for ophthalmic imaging since the field of view (FOV) can be
conveniently adjusted to the target region. This study examines the
applicability of IVI in conjunction with ms-RARE with the goal to shorten
acquisition time and to reduce the propensity to bulk eye motion in diffusion-sensitized
ophthalmic imaging.
Methods
In vivo studies in healthy volunteers (n=3) were
performed on a 3.0 T whole body MR system (Siemens Healthcare, Erlangen,
Germany). Informed written consent was obtained from each volunteer prior to
the study in compliance with the local institutional review board guidelines. IVI
was realized by applying the excitation radiofrequency pulse selectively in
phase encoding direction. Diffusion-sensitized ms-RARE (Figure 1) was conducted
using: TR=3300ms, TE=46ms, receiver bandwidth=263kHz, ESP=6.6ms, spatial
resolution=(0.5x0.5x5)mm
3, six b-values ranging from 0 to 500s/mm
2. Full FOV
imaging ((140x140)mm
2, acquisition matrix 256x256, acquisition time 58s per
b-value) was performed in comparison to a 50% reduced FOV acquisition (140x70)mm
2,
acquisition matrix 256x128, acquisition time 31 s per b-value). To further tailor
the FOV to the target region, the FOV was reduced to (120x48)mm
2 with an
acquisition matrix of 192x78 resulting in an in-plane spatial resolution of
(0.6x0.6)mm
2 and an acquisition time of 21s per b-value. Apparent diffusion
coefficient maps were generated by fitting the data points obtained for a
series of b-values to a linear decay after taking the logarithm of the signal
intensity.
Results
The anatomical images (b=0s/mm
2) (Figure2a-c) demonstrate that
IVI allows adjustment of the FOV to the eye and the orbit without folding
artifacts. For the full FOV configuration phase encoding was set L-R to
circumvent folding artifacts; for the reduced FOV set-up it could be changed to
A-P which better fits the target anatomy. Simultaneously, the acquisition time
was reduced by a factor of two for the 50% FOV configuration. The reduction in
signal-to-noise ratio (SNR) for ms-RARE with IVI did not impede diagnostic
image quality of the morphological images. ADC mapping free of geometric
distortion was feasible for all FOV configurations (Figure2d-f). For strong
diffusion weighting the SNR was further lowered generating a higher noise level
in the ADC maps obtained for reduced FOV acquisition. Despite of this
confinement no diagnostic information was lost since the complete bulbus was
well displayed.
Discussion
Reducing
the FOV along the phase encoding direction is beneficial for ocular MRI but
also for optic nerve imaging where only a narrow region is in the focus of
interest. Imaging the eye and the orbit involves strategies to reduce
involuntary motion including application of a triggering scheme dictating
fixing and relaxing periods or asking the volunteer to avoid blinking by fixing
the view on a fixation cross. Both strategies benefit from shorter acquisition
times ultimately resulting in better image quality and enhanced volunteer
comfort. The proposed IVI technique is restricted to single-slice acquisitions.
Incorporating ZOOM imaging [9] offers means for multi-slice imaging. Replacing
the current Cartesian sampling pattern by radial views – preferably PROPELLER [10]
– promises to further reduce the susceptibility of diffusion-weighted ms-RARE to
motion and hence would relax the demands of volunteer cooperation.
Conclusions
This
study demonstrates that diffusion-sensitized ms-RARE in conjunction with inner
volume imaging provides distortion-free images of the eye and orbit. At the
same time acquisition time is drastically shortened compared to the traditional
full FOV ms-RARE approach without compromising diagnostic value. The relaxed
time constraints of IVI ms-RARE reduce the propensity of conventional ms-RARE to
eye motion. This benefit is very instrumental en route to fast and robust high-spatial resolution DWI of the eye
and the orbit with high anatomic fidelity for the diagnosis, therapy planning and
treatment response monitoring of ocular and orbital diseases.
Acknowledgements
No acknowledgement found.References
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