High-resolution multi-T1 -weighted contrast and T1 mapping with low B1>+ sensitivity using the fluid and white matter suppression (FLAWS) sequence at 7T

Abstract

Purpose: To demonstrate that fluid and white matter suppression (FLAWS) imaging can be used for high-resolution T₁ mapping with low transmitted bias field ( $B_1^{+}$ ) sensitivity at 7T.

Methods: The FLAWS sequence was optimized for 0.8-mm isotropic resolution imaging. The theoretical accuracy and precision of the FLAWS T₁ mapping was compared with the one of the magnetization-prepared two rapid gradient echoes (MP2RAGE) sequence optimized for low $B_1^{+}$ sensitivity. FLAWS images were acquired at 7T on six healthy volunteers (21 to 48 years old; two women). MP2RAGE and saturation-prepared with two rapid gradient echoes (SA2RAGE) datasets were also acquired to obtain T₁ mapping references and $B_1^{+}$ maps. The contrast-to-noise ratio (CNR) between brain tissues was measured in the FLAWS-hco and MP2RAGE-uni images. The Pearson correlation was measured between the MP2RAGE and FLAWS T₁ maps. The effect of $B_1^{+}$ on FLAWS T₁ mapping was assessed using the Pearson correlation.

Results: The FLAWS-hco images were characterized by a higher brain tissue CNR ( ${\text{CNR}}_{\text{WM}/\text{GM}}=5.5$ , ${\text{CNR}}_{\text{WM}/\text{CSF}}=14.7$ , ${\text{CNR}}_{\text{GM}/\text{CSF}}=10.3$ ) than the MP2RAGE-uni images ( ${\text{CNR}}_{\text{WM}/\text{GM}}=4.9$ , ${\text{CNR}}_{\text{WM}/\text{CSF}}=6.6$ , ${\text{CNR}}_{\text{GM}/\text{CSF}}=3.7$ ). The theoretical accuracy and precision of the FLAWS T₁ mapping ( $\text{acc}=91.9\%;\text{prec}=90.2\%$ ) were in agreement with those provided by the MP2RAGE T₁ mapping ( $\text{acc}=90.0\%;\text{prec}=86.8\%$ ). A good agreement was found between in vivo T₁ values measured with the MP2RAGE and FLAWS sequences (r = 0.91). A weak correlation was found between the FLAWS T₁ map and the $B_1^{+}$ map within cortical gray matter and white matter segmentations ( $r_{\text{WM}}=-0.026$ ; $r_{\text{GM}}=0.081$ ).

Conclusion: The results from this study suggest that FLAWS is a good candidate for high-resolution T₁ -weighted imaging and T₁ mapping at the field strength of 7T.

Keywords: Brain; FLAWS; MP2RAGE; MRI; T1 mapping.