Fast 3D UTE in vivo T₁ and T 2 * $$ {\mathrm{T}}_2^{\ast } $$ mapping of fast relaxing knee tissues at 3 T

UTE MR imaging captures quantitative signals in fast-relaxing tissues, enabling anatomical visualization and quantitative assessment of T₁ and $T_2^{*}$ relaxation times. However, the clinical application of quantitative UTE MRI is limited by long acquisition times. Therefore, this study introduces a novel UTE-based method for T₁ and $T_2^{*}$ mapping, achieving submillimeter resolution in less than 10 min.

The method employs a dual-echo acquisition for fast $T_2^{*}$ mapping, augmented by an additional acquisition with different T₁ weighting. This second scan enables the computation of signal ratios between scans with different T₁-weighting. These measured signal ratios are then compared to a lookup table containing distinct ratios, corresponding to discrete T₁ values. The approach was validated in phantom solutions mimicking various T₁ and $T_2^{*}$ times and applied in vivo to quantify relaxation times across different knee tissue compartments in healthy individuals.

The method demonstrated its reliability for T₁ and $T_2^{*}$ quantification in rapidly relaxing tissues (1-11 ms). However, it exhibited a tendency to underestimate $T_2^{*}$ in skeletal muscle. This limitation arises from the chosen TEs being inadequate to capture slow signal decays. In accordance with the findings of preceding studies, this in vivo study identified three distinct T₁ categories of tissue characterized by short (adipose tissue), moderate (ligaments, tendons, and menisci), and long (skeletal muscle) T₁ values.

The presented technique for combined T₁ and $T_2^{*}$ mapping enables relaxometry in rapidly relaxing tissues, indicating potential for advanced tissue characterization in clinical settings.