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Effect of echo time and T₂-weighting on GRASE-based T₁w/T₂w ratio measurements at 3T

Md Nasir Uddin¹, Teresa D Figley¹, Chase R Figley²

¹Department of Radiology, University of Manitoba, Winnipeg, MB, Canada; Division of Diagnostic Imaging, Health Sciences Centre, Winnipeg, MB, Canada; Neuroscience Research Program, Kleysen Institute for Advanced Medicine, Winnipeg, MB, Canada.

Magnetic Resonance Imaging

|April 23, 2018

Abstract:

Tissue contrast can be enhanced by dividing T₁-weighted (T₁w) images by T₂-weighted (T₂w) images to map the so-called T₁w/T₂w ratio, which has become an increasingly popular technique for quantifying brain tissue changes associated with neurodevelopment, aging, and a variety of neurodegenerative diseases. However, although it is self-evident that T₁w/T₂w ratios increase with the amount of T₂-weighting in the T₂w image - which is determined by the echo time (TE), all else being equal - longer TEs also reduce the signal-to-noise ratio (SNR) of the T₂w images, and it is not clear how these SNR characteristics affect the reliability of T₁w/T₂w measurements. Therefore, the current study systematically investigated how different amounts of T₂-weighting affected T₁w/T₂w measurements in order to determine whether there is an optimal amount of T₂-weighting. T₁w images were acquired from 10 neurologically healthy adults using a 3D turbo field echo (TFE) sequence, and a series of T₂-weighted images were extracted from a multi-echo 3D combined gradient- and spin-echo (GRASE) sequence. Analyses of 12 anatomically defined brain regions revealed that both the mean and standard deviation of the T₁w/T₂w measurements increased exponentially with TE of the T₂w images, and that T₂w images with TE ≈ 120-160 ms yielded the most consistent/reproducible contrast between white matter ROIs and the whole-brain T₁w/T₂w signal. Furthermore, comparisons between T₁w/T₂w measurements and multi-component T₂-relaxation myelin water fractions (MWFs) in the same brain regions revealed that T₂w images with TE ≥ 160 ms drastically reduced the degree of correlation between T₁w/T₂w measurements and MWF. Overall, these findings suggest that: 1) there is a substantial trade-off between increased T₁w/T₂w contrast (based on longer TEs for the T₂w images) and the reliability of quantitative T₁w/T₂w signals; and 2) the optimal TE for T₂w GRASE scans is between 120 ms and 160 ms for calculating T₁w/T₂w ratios.

Keywords:

Echo time GRASE Human brain MRI Myelin water imaging T(1)w/T(2)w T(2)-weighting

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