Comparison of Generalized Autocalibrating Partially Parallel Acquisitions and Modified Sensitivity Encoding for Diffusion Tensor Imaging
Y.A. Bhagata,
D.J. Emerya,b,
S. Naikb,
T. Yeob and
C. Beaulieua
a Department of Biomedical Engineering, University of Alberta, Edmonton, Alberta, Canada
b Radiology and Diagnostic Imaging, University of Alberta, Edmonton, Alberta, Canada
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Fig 1. Representative sets of b0 (T2-weighted), b1000 (isotropic diffusion weighted) images, and Trace/3 ADC and FA maps of one inferior section (level of pons) from one subject with conventional (R = 1), and mSENSE and GRAPPA R = 2 based DTI. Compared with R1-PPF and R1-no PPF, a reduction in distortions and off-resonance effects (arrowheads) are apparent with images obtained with mSENSE and GRAPPA R = 2 DTI. Although FA maps obtained with R1-PPF were deemed to be the best in 4 of 5 subjects based on their higher SNR and smoother profiles (qualitative analysis by neuroradiologists), closer inspection showed that peripheral white matter structures such as the middle temporal gyrus (magnified, below FA maps) affected by the distortions with conventional DTI were better resolved with the mSENSE and GRAPPA R = 2 based DTI methods.
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Fig 2. Image sets (b0, b1000) and Trace/3 ADC and FA maps of a middle section (level of corpus callosum) from one subject with conventional (R = 1), and mSENSE and GRAPPA R = 2 based DTI. Qualitative analysis of the b0 and b1000 images, and Trace/3 ADC maps indicated that mSENSE and GRAPPA R = 2 DTI generated the sharpest images and were more adept at handling spatial warping effects seen in images of the R1-PPF and R1-no PPF methods. Although FA maps derived from R1-PPF were considered to be smoother because of their intrinsically higher SNR in 4 of 5 cases, an in-depth comparison of these maps to those obtained by using mSENSE and GRAPPA R = 2 DTI showed that incorporating parallel imaging allowed better visualization of thinner white matter tracts such as the middle frontal gyrus (magnified, below FA maps).
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Fig 3. Poor quality Trace/3 ADC and FA maps of one middle section (level of corpus callosum) from one subject using mSENSE and GRAPPA R = 3 and 4-based DTI. Maps obtained from mSENSE R = 4, and GRAPPA R = 3, 4 suffered from reconstruction artifacts such as aliasing and were not considered for quantitative analysis. Trace/3 ADC maps from mSENSE R = 3 revealed regions with enhanced noise centrally and were also excluded from further analysis.
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Fig 4. Fractional anisotropy (FA) and Trace/3 ADC (B) values (mean ± SD) for 3 brain regions in 5 normal subjects using conventional DTI R1-PPF, R1-no PPF, and mSENSE and GRAPPA R = 2 DTI methods. The dashed lines in A indicate FA values obtained with conventional R1-PPF for different structures and help demonstrate the extent of variations with measurements made by using other techniques. Although subtle differences are evident, the values appear to be fairly consistent between parallel imaging and conventional DTI. Values for mSENSE and GRAPPA R = 3, 4 methods are not shown because of the poor quality of the maps. * indicates significant (P < .05) paired differences of R1-PPF versus R1-no PPF, mSENSE R = 2, and GRAPPA R = 2 methods.
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Fig 5. Relative (parallel acquisition technique [PAT] R = 2 / R1-PPF) signal-to-noise ratio (SNR) measurements (mean ± SD) of mSENSE or GRAPPA R = 2 for 3 brain regions in 5 normal subjects. SNR values were fairly close between mSENSE and GRAPPA R = 2 methods for the 3 structures evaluated. Relative SNR values were reduced by 25%–38% and 8%–10% in the corpus callosum and subcortical white matter in the gyri. Cortical gray matter demonstrated relative SNR values closer to unity with a higher percentage of error (35%–40%) in the measurements. Relative SNR values in the phantom decreased by 56% and 62% for the GRAPPA and mSENSE R = 2 methods, respectively, in accordance with theoretical considerations reflecting higher (>1.0) coil g-factors.9
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