Jeff A Stainsby1, Chad T Harris1, Andrew T Curtis1, Philip J Beatty1, and Curtis N Wiens1
1Synaptive Medical, Toronto, ON, Canada
Synopsis
The feasibility
of generating diffusion tractography from data obtained on a head-only 0.5T
system is demonstrated and results are compared qualitatively to tractography generated
from clinical 1.5T data. DTI data from 0.5T compares favorably in quantitative
(FA, ADC) measures to literature values, and qualitative (segmented white
matter tracts) measures to 1.5T.
Introduction
Diffusion tensor
imaging (DTI) is emerging as an important tool for the pre-operative planning
of surgical treatment for many neurosurgical pathologies [1].
There is growing interest in increased accessibility to MR including locating
systems closer to the point-of-care [2]. Lower field scanners with
high-performance gradients have advantages in this context including less
restrictive siting requirements, improved spatial fidelity through reduced
geometric distortions, and diffusion imaging performance that can be comparable
to 1.5T [3]. In this work we compare DTI imaging on a head-only 0.5T system designed
for improved accessibility to standard clinical 1.5T imaging.Methods
Imaging was performed on two healthy volunteers with
informed consent in compliance with health and safety protocols.
Isotropic
3D T1-weighted images and DTI images were acquired on a head-only 0.5T system
and compared to previously acquired data from a 1.5T system (Philips Achieva,
Best, Netherlands). Images were then processed using a commercially available whole
brain tractography and surgical planning software (Modus PlanTM,
Synaptive Medical, Toronto). Acquisition parameters were as follows. 1.5T:
2.4 x 2.4 x 3mm resolution, 32 diffusion
directions, scan time 9:56; 0.5T: 2.4 x 2.4 x 3.0mm resolution, 60
diffusion directions, scan time 10:19.
Quantitative FA and ADC images were available at 0.5T and following the method
of Hakulinen et al [4] mean FA and ADC values were computed in ROIs placed in
the genu of the corpus callosum, the anterior corona radiata and the basal pons
and compared to literature values.
Whole brain DTI results were compared
qualitatively for the definition of major white matter tracts, applicability
for automatic white matter segmentation and appropriate definition of key white
matter bundles in automatically segmented results.Results
Mean FA and
ADC values at 0.5T were similar to values reported in the literature for regions
in various locations in the brain (Table 1). Qualitatively, FA, ADC and RGB
maps obtained at 0.5T compared favorably to those obtained at 1.5T in the clear
identification and directionality of major white matter tracts, and in the similarity
of ADC maps (Figure 1). Note that due to reduced chemical shift effects, fat
saturation was not applied at 0.5T.
Tractography
from both 0.5T and 1.5T were able to visualize major white matter tracts throughout
the brain including numerous branching tracts. DTI data from both volunteers at both field
strengths was of sufficient quality for successful automatic white matter
segmentation (Figure 2).
Qualitative comparisons between tractography results from 0.5T and
1.5T of the corticospinal tract, and the fornix demonstrated reasonable
identification of these key fiber bundles, and good agreement in the
characteristic of the bundles (Figure 3).Discussion
Quantitative
computed FA and ADC values are in close agreement with literature
values.
Processed RGB and FA maps
and whole brain tractography had similar appearance between 0.5T and 1.5T compared
to historical scans of the same volunteers from 2016. Due to the presence of
fat in the 0.5T images, FA, ADC and RGB images contain structure in the subcutaneous
fat of the scalp which can be addressed in the future with the use of fat suppression.
Despite the significantly lower SNR associated with 0.5T compared to 1.5T, DTI
results were similar. There is some apparent reduction in tract density at 0.5T.
Regardless, these results demonstrate that DTI on a head-only 0.5T system with high-performance gradients can produce a significant improvement in both image
quality and scan time over previously reported DTI performed at 0.5T [5].
Automatic white
matter segmentation was successful on all studies. Fiber tracts which are good
indicators of DTI fidelity include the fornix, and corticospinal tract due to
their complex paths and deep brain locations. There is good qualitative
agreement in the shape, location and extent of those structures.Conclusion
This work
has demonstrated that whole brain DTI at 0.5T (1) provides qualitatively similar segmented volumes and processed DTI images as 1.5T and (2) is of sufficient quality for automatic white matter segmentation.Acknowledgements
The authors would like to acknowledge Tim Hayes and Alicia McNeely for assistance with Modus Plan.References
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