Simultaneous Myocardial T1, T2, T1ρ and Fat Fraction Mapping with Hybrid Dual-echo Cartesian Acquisition and Dictionary Matching

Fig. 1. Sequence diagram (A) and post-processing steps (B) of a free-breathing simultaneous cardiac fat fraction (FF) and multi-relaxation parameter mapping technique. Single-shot dual-echo Cartesian acquisitions are performed over 14 consecutive cardiac cycles, with inversion recovery (IR), T2 preparation (T2-prep) and T1ρ preparation (T1ρ-prep) pulses applied to achieve T1, T2 and T1ρ sensitization, respectively. Interleaved dual-echo acquisitions are performed in the first two cardiac cycles without any preparation pulses, resulting in four-echo images for water-fat separation to obtain FF and B0 field maps. The derived B0 map was used as prior information for dual-echo water-fat separation of the following cardiac cycles, yielding multi-contrast water-only images. Subsequently, dictionary matching was performed to obtain water-only T1, T2, T1ρ and B1+ maps.

Fig. 2. (A) Illustration of two variable flip angle strategies. The first strategy changes the flip angle after the second IR, resulting in two flip angles (FA1 and FA2). The second strategy changes the flip angle after the first T2-prep, second IR, and first T1ρ-prep, resulting in a total of four flip angles (FA1–1, FA1–2, FA2–1, and FA2–2). In both strategies, the flip angle for the first two cardiac cycles (FA0) remains unchanged. (B) The relative root mean-squared error (RRMSE) from numerical simulations of the dual flip angle scheme with different combinations of FA1 and FA2.

Fig. 3. Phantom results. (A) Example phantom T1, T2, T1ρ and fat fraction maps estimated by Dixon-MultiMap with simulated heart rate of 80 bpm. (B) Correlation of Dixon-MultiMap and sDixon-MultiMap (simplified post-processing without B1+ correction) T1, T2 and T1ρ measured at heart rate of 80 bpm with the reference values. (C) T1, T2 and T1ρ estimated with Dixon-MultiMap at simulated heart rates from 40 to 120 bpm with the reference values showing in the legend.

Fig. 4. Water images, fat images and fat fraction (FF) maps on the full FOV obtained with the conventional six-echo gradient echo (GRE) method (top row) and the Dixon-MultiMap technique (bottom row).

Fig. 5. (A) Illustration of the four regions of interest (ROIs) for measuring fat fraction (FF), including the septum (red), two separate pericardial (green and blue), and subcutaneous fat regions (yellow). The septum ROI is drawn on the water image, while the other ROIs are drawn on the fat image. (B) Comparison between Dixon-MultiMap FF and 6-echo gradient echo (GRE) reference FF measured in the four ROIs for all 13 healthy subjects, showing a high determination coefficient (R² = 0.9605). (C) Violin plots comparing FF measurements between Dixon-MultiMap and 6-echo GRE in three tissue regions with varying fat contents for all 13 healthy subjects. The median and quartiles are shown as dashed and dotted lines, respectively. *** indicate p < 0.001.

Fig. 6. T1, T2, T1ρ and Fat Fraction maps at two short-axis slices of a representative healthy subject, obtained using traditional separate breath-hold mapping techniques, breath-hold (BH)-Dixon-MultiMap with the first echo image, BH-Dixon-MultiMap with water images, and free-breathing (FB)-Dixon-MultiMap with water images. Zoomed-in views (shown on the left side of each map) highlight regions susceptible to water-fat partial volume effects, illustrating the superior image quality of water-only Dixon-MultiMap compared to first-echo-only acquisition. Mean ± SD values across the entire left ventricular myocardium are displayed on each parametric map.

Fig. 7. Bullseye plots of different mapping methods, including conventional separate breath-hold mapping techniques, breath-hold (BH)-Dixon-MultiMap with the first echo, BH-Dixon-MultiMap with water images and free-breathing (FB)-Dixon-MultiMap with water images. The mean and standard deviation (SD) of T1, T2 and T1ρ averaged across all healthy subjects are shown for the 16 segments in the three short-axis slices. The overall mean values across all segments are displayed below each bullseye plot.

Fig. 8. Violin plots showing the mean and standard deviation (SD) of myocardial T1, T2 and T1ρ values estimated using conventional separate mapping techniques, breath-hold (BH)-Dixon-MultiMap with the first echo, BH-Dixon-MultiMap with water images, and free-breathing (FB)-Dixon-MultiMap with water images measured from all healthy subjects. The black dashed lines indicate the median values, and the dotted lines represent the first and third quartiles. Mean ± SD values are displayed below each violin plot. * indicates statistically significant differences (p < 0.05); ** and *** indicate p < 0.01 and p < 0.001, respectively.

Fig. 9. Parameter maps at two slices obtained with the proposed Dixon-MultiMap method (water and first echo, respectively) and conventional techniques including MOLLI, T2-prep bSSFP, T1ρ-prep bSSFP, and 6-echo GRE, along with slice-matched LGE images in one HCM patient (Patient #1). Water-only parametric values and first echo parametric values are in good agreement with conventional measurements. The water-only T1 map demonstrates more uniform signals in the right ventricle compared to the first echo T1 map, indicating effective suppression of partial volume effects. Quantitative values (mean±SD) measured in the septum are displayed on each parametric map.

Fig. 10. Parameter maps obtained with the proposed Dixon-MultiMap method (water) and conventional techniques including MOLLI, T2-prep bSSFP, T1ρ-prep bSSFP, and 6-echo GRE, along with slice-matched LGE images in two HCM patients (Patient #2 and #3). Example ROIs drawn in the scar (red) areas are shown on the conventional T2 maps, and the mean ±SD values measured in these ROIs are displayed on each parametric map.