R2* estimation using "in-phase" echoes in the presence of fat: the effects of complex spectrum of fat.

PURPOSE: To investigate R2* mapping robustness in the presence of fat using in-phase echoes, without and with spectral modeling of fat (single-peak and multipeak models, respectively), using varying numbers of echoes.
MATERIALS AND METHODS: Data from 88 volunteers (men/women: 52/36, ages: 55.4 ± 12.2) were randomly chosen according to magnetic resonance imaging (MRI) liver fat-fraction (%), and classified into six fat-fraction groups (1: 20 cases, 0%-<10%; 2: 20 cases, 10%-<20%; 3: 20 cases, 20%-<30%; 4: 20 cases, 30%-<40%; 5: 8 cases >40% liver fat; 6: subcutaneous fat from all cases). R2* maps obtained from five in-phase echoes (echo times: 4.8-23.8 msec) were retrospectively reconstructed using single-peak and multipeak fat modeling. R2* maps were also calculated using different numbers (2-5) of echoes.
RESULTS: Multipeak fat corrected R2* mapping is feasible from in-phase echoes, with noise performance comparable to single-peak R2* when using ≥ 4 echoes. Single-peak R2* showed poor robustness to varying echo time combinations in the presence of fat, where using few echoes resulted in large errors. These errors can be reduced using more echoes, or fully corrected using multipeak fat modeling. The mean R2* increased significantly with increasing fat-fraction when using single-peak R2* for any TE combination (P < 0.001), but did not vary when using multipeak R2* for any TE combination (P ≥ 0.158).
CONCLUSION: R2* mapping uncorrected for spectral complexity of fat contains protocol and fat-dependent errors (lack of robustness) in tissues with high fat content. Accounting for complex fat spectrum improves robustness and accuracy of signal fitting, with modest noise performance loss.