T₂ relaxation‐time mapping in healthy and diseased skeletal muscle using extended phase graph algorithms

FIGURE 1. A, Signal profiles for a B1 of 70%, 100%, and 130%, from left to right. Noticeable is the asymmetry around a B1 of 100%. The right dotted line in each signal profile represents the signal in the center of the slice, and the right colored line depicts the final signal after averaging all signal contributions. B, Example flip‐angle profile of the pulses throughout the slice. The left side shows an example of a flip‐angle profile with a minor shift (1.1 mm) between the excitation and refocusing pulse as experienced by the fat, and the right side shows an example of a major shift (5.1 mm)

FIGURE 2. Example raw data and maps from 1 patient in all four cohorts. Left to right: the signal for the first echo, the estimated fat map with the T2fat calibration mask, the estimated water map with the region of interest, and the T2water map

FIGURE 3. Simulation 1A shows the relationship between the simulated fat fraction and the fitted T2water for simulation without incorporation of the flip‐angle profile (T2water: median: 40 ms [34‐57 ms], on dictionary boundaries: 30%) and fitted by incorporating the flip‐angle profile (top row) (T2water: median: 30 ms [26‐35 ms], on dictionary boundaries: 1.5%). In the bottom row, the relation between the simulated fat fraction and the fitted fat fraction is shown without and with the flip‐angle profile. The values fitted on dictionary boundaries are depicted in red, and the correctly fitted values are depicted in blue. A reference line is shown at a T2water of 30 ms, and a unity line is shown for the fat fractions

FIGURE 4. Simulation 2 shows the effect of simulating data with a chemical shift in the location of the pulses and fitting it with or without incorporating the chemical shift of the slice. The four figures on top show the simulation with a small shift (T2water: median: 32 ms (28‐51 ms), on dictionary boundaries: 6.8%; and fat fraction: intercept: 0.72%, slope: 0.92), and the four figures on bottom show the simulation with a large shift (T2water: median: 34 ms (30‐53 ms), on dictionary boundaries: 22%; and fat fraction: intercept: −1.28%, slope: 0.79). The values fitted on dictionary boundaries are depicted in red, and the correctly fitted values are depicted in blue. A reference line is shown at a T2water of 30 ms, and a unity line is shown for the fat fractions

FIGURE 5. Simulation 2 shows the effect of fitting simulated data with different assumed T2fat values. A, Top row shows the T2water for assuming the correct T2fat (T2water: median: 30 ms [23‐38 ms], on dictionary boundaries: 4%. B, Underestimation of T2fat (T2water: median: 26 ms [14‐30 ms], on dictionary boundaries: 26%. C, Overestimation of T2fat (T2water: median: 33 ms [30‐51 ms], on dictionary boundaries: 9%). D‐F, Relationship between the simulated fat fraction and fitted fat fraction. The values fitted on dictionary boundaries are depicted in red, and the correctly fitted values are depicted in blue. A reference line is shown at a T2water of 30 ms, and a unity line is shown for the fat fractions

FIGURE 6. Outcome of the calibration of the T2fat on subcutaneous fat for methods A, B, and C. The values for the patients in the cohorts are shown as a solid line, and the values for the healthy controls are shown as a dashed line. Method A with one‐component calibration leads to a lower T2fat (137 ms, 132 ms, 131 ms, and 144 ms for cohorts 1‐4, respectively) than method B with two‐component calibration (149 ms, 141 ms, 140 ms, and 158 ms for cohorts 1‐4, respectively). The calibrated T2fat is comparable for method B without a fixed T2water and method C with a fixed T2water (150 ms, 142 ms, 140 ms, and 159 ms for cohorts 1‐4, respectively)

FIGURE 7. Association between the fat fraction and the T2 of the water component in vivo in four cohorts (cohorts 1‐4 from left to right) for the two calibration methods: A (top row), B (middle row), and C (bottom row). Healthy controls are depicted in green, and patients are depicted in blue. Values are shown for each of the five slices per individual