Water-fat Dixon cardiac magnetic resonance fingerprinting

Abstract

Purpose: Cardiac magnetic resonance fingerprinting (cMRF) has been recently introduced to simultaneously provide T₁ , T₂ , and M₀ maps. Here, we develop a 3-point Dixon-cMRF approach to enable simultaneous water specific T₁ , T₂ , and M₀ mapping of the heart and fat fraction (FF) estimation in a single breath-hold scan.

Methods: Dixon-cMRF is achieved by combining cMRF with several innovations that were previously introduced for other applications, including a 3-echo GRE acquisition with golden angle radial readout and a high-dimensional low-rank tensor constrained reconstruction to recover the highly undersampled time series images for each echo. Water-fat separation of the Dixon-cMRF time series is performed to allow for water- and fat-specific T₁ , T₂ , and M₀ estimation, whereas FF estimation is extracted from the M₀ maps. Dixon-cMRF was evaluated in a standardized T₁ -T₂ phantom, in a water-fat phantom, and in healthy subjects in comparison to current clinical standards: MOLLI, SASHA, T₂ -GRASE, and 6-point Dixon proton density FF (PDFF) mapping.

Results: Dixon-cMRF water T₁ and T₂ maps showed good agreement with reference T₁ and T₂ mapping techniques (R² > 0.99 and maximum normalized RMSE ~5%) in a standardized phantom. Good agreement was also observed between Dixon-cMRF FF and reference PDFF (R² > 0.99) and between Dixon-cMRF water T₁ and T₂ and water selective T₁ and T₂ maps (R² > 0.99) in a water-fat phantom. In vivo Dixon-cMRF water T₁ values were in good agreement with MOLLI and water T₂ values were slightly underestimated when compared to T₂ -GRASE. Average myocardium septal T₁ values were 1129 ± 38 ms, 1026 ± 28 ms, and 1045 ± 32 ms for SASHA, MOLLI, and the proposed water Dixon-cMRF. Average T₂ values were 51.7 ± 2.2 ms and 42.8 ± 2.6 ms for T₂ -GRASE and water Dixon-cMRF, respectively. Dixon-cMRF FF maps showed good agreement with in vivo PDFF measurements (R² > 0.98) and average FF in the septum was measured at 1.3%.

Conclusion: The proposed Dixon-cMRF allows to simultaneously quantify myocardial water T₁ , water T₂ , and FF in a single breath-hold scan, enabling multi-parametric T₁ , T₂ , and fat characterization. Moreover, reduced T₁ and T₂ quantification bias caused by water-fat partial volume was demonstrated in phantom experiments.

Keywords: MR fingerprinting; T1 mapping; T2 mapping; cardiac MRI; fat fraction; water-fat DIXON.

Figure 1

Proposed Dixon‐cMRF framework. (A) A 3‐echo gradient echo golden radial acquisition is ECG‐triggered to acquire k‐space data at the mid‐diastolic cardiac phase for 15 heartbeats (i.e., ~15 s). Magnetization preparation includes inversion pulses every 5 heartbeats (with inversion delays of 10, 300, and 10 ms, respectively) and variable T₂ preparation modules (no T₂ preparation, no T₂ preparation, 40, 80, and 160 ms repeated over 15 heartbeats). (B) The 3 echoes are reconstructed as separate time series from which a B₀ map estimate and water–fat separated time series are obtained. (C) After a matching step, water‐ and fat‐specific T₁, T₂, and M₀ maps and fat fraction can be extracted

Figure 2

Dixon‐cMRF T₁/T₂ phantom experiment. (A) Single echo cMRF (echo 1) and Dixon‐cMRF water‐specific T₁ and T₂ maps. (B) T₁ and T₂ values for all vials measured with water Dixon‐cMRF and single echo cMRF (echo 1) in comparison to reference spin echo values. For both single echo cMRF and water Dixon‐cMRF high determination coefficient (R² ≥ 0.99) and low NRMSE (~5%) were obtained when compared with reference values

Figure 3

(A) Reference in‐house phantom values and map measured with proton density fat fraction (PDFF). (B) Dixon‐cMRF fat fraction (FF) measurement compared to the reference PDFF for all phantom vials. (C) T₁ estimation using single echo cMRF (echo 1) and Dixon‐cMRF (water‐ and fat‐specific) in comparison to the reference water selective IRSE acquisition. (D) T₂ estimation using single echo cMRF (echo 1) and Dixon‐cMRF (water‐ and fat‐specific) in comparison to the reference water‐selective T₂ MESE measurement

Figure 4

Water–fat separated Dixon‐cMRF singular images for a representative healthy subject

Figure 5

Dixon‐cMRF in a representative healthy subject. (A) Good image quality and water–fat separation is seen on the whole FOV of the 1st singular images leading to good quality water and fat T₁ and T₂ maps. (B) Comparison of Dixon‐cMRF and single echo cMRF (echo 1) in a zoomed region around the heart. Myocardium wall recovery with Dixon‐cMRF in both T₁ and T₂ maps in regions with water–fat partial volume is indicated by the white arrows. Additional water‐ and fat‐specific M₀ can be estimated with Dixon‐cMRF to obtain a fat fraction map

Figure 6

Comparison of water Dixon‐cMRF, single echo cMRF (echo 1), and conventional MOLLI, SASHA, and T₂‐GRASE maps for a representative healthy subject

Figure 7

(A) Water and fat magnitude images and reference proton density fat fraction (PDFF) map obtained from a 6 echo Dixon GRE Cartesian scan. (B) Dixon‐cMRF water and fat M₀ images as well as the resulting fat fraction (FF) estimate. The ROIs used for the analysis in Supporting Information Table S1 and Figure S9 are shown for this particular volunteer superimposed on the water M₀ and fat M₀ images. (C) Absolute difference image between the reference PDFF and Dixon‐cMRF FF maps

Figure 8

Top: T₁, T₂, and FF septum measurements in 10 healthy subjects for the different mapping techniques. Average across subjects ± SD (ms) are reported for each technique on the top of each plot. Bottom: spatial variability on the T₁, T₂, and FF septum measurements for the different mapping techniques. Average spatial variability (ms) of the measurements are reported for each technique on the top of each plot. Differences with statistical significance are identified by * (P < 0.025 for T₁ and P < 0.05 for T₂)

Figure 9

Regional T₁ and T₂ assessment of the different mapping techniques. T₁ (top) and T₂ (bottom) mean values and spatial variability reported for each segment as average measurements over 10 subjects. The value in the central segment represents the average over all segments. Average measurements/spatial variability for Dixon‐cMRF water T₁ and T₂ were 1032/55 ms and 42.1/5.5 ms, respectively. Overall Dixon‐cMRF presented lower spatial variability than MOLLI and SASHA measurements but higher than T₂‐GRASE measurements

Figure 10

(A) Single echo (echo 1) cMRF T₁, Dixon‐cMRF water T₁ and water–fat partial volume mask (with FF ϵ [0.3; 0.7]). The mask was used to obtain the T₁ and T₂ values of single echo cMRF for echoes 1, 2, 3, and Dixon‐cMRF water and fat maps that are shown in (B) as point clouds (top) and ellipses (bottom). The ellipse is centered on the mean and the horizontal and vertical radius represent the SD over T₁ and T₂ measurements, respectively. (C) Same plots in (B) using the values obtained from all the subjects included in the study. The 2 compartments of water and fat are clearly separated in (B) and (C). Mean T₁ and T₂ measurements in the masked pixels for water are 1174 ms and 60.9 ms, respectively, whereas mean values for single echo cMRF for each of the 3 echoes are: echo 1 T₁/T₂ = 537/80.1 ms, echo 2 T₁/T₂ = 549/89.3 ms, and echo 3 T₁/T₂ = 541/90.6 ms