Free-Breathing Liver Fat, R₂* and B₀ Field Mapping Using Multi-Echo Radial FLASH and Regularized Model-Based Reconstruction

Abstract

This work introduced a stack-of-radial multi-echo asymmetric-echo MRI sequence for free-breathing liver volumetric acquisition. Regularized model-based reconstruction was implemented in Berkeley Advanced Reconstruction Toolbox (BART) to jointly estimate all physical parameter maps (water, fat, R₂^∗ , and B₀ field inhomogeneity maps) and coil sensitivity maps from self-gated k -space data. Specifically, locally low rank and temporal total variation regularization were employed directly on physical parameter maps. The proposed free-breathing radial technique was tested on a water/fat & iron phantom, a young volunteer, and obesity/diabetes/hepatic steatosis patients. Quantitative fat fraction and R₂^∗ accuracy were confirmed by comparing our technique with the reference breath-hold Cartesian scan. The multi-echo radial sampling sequence achieves fast k -space coverage and is robust to motion. Moreover, the proposed motion-resolved model-based reconstruction allows for free-breathing liver fat and R₂^∗ quantification in multiple motion states. Overall, our proposed technique offers a convenient tool for non-invasive liver assessment with no breath holding requirement.

Fig. 1.

(Left) One representative repetition time $\left(\text{TR}\right)$ block of the proposed multi-echo asymmetric-echo radial sequence. (Right) The corresponding $k$-space trajectory. The echoes are color coded, indicating the period when ADC is switched on, while the dark solid lines indicate either the ramp or the blip gradients.

Fig. 2.

Photos of the constructed water/fat & iron phantom. (A) 34.5 mg iron nano particle diluted in 100 mL distilled water. (B) Phantom layout with eight tubes as listed in (C).

Fig. 3.

Multi-echo radial FLASH acquisition and model-based reconstruction results of the water/fat & iron phantom built in-house. Displayed images are $\text{FF}$, $R_2^{\ast }$, and $B_0$ maps, respectively.

Fig. 4.

Quantitative analysis of $\text{FF}$ values for the eights tubes via linear regression between the standard MR Spectroscopy (MRS) and the proposed multi-echo radial acquisition with model-based reconstruction.

Fig. 5.

Comparison of (A) the reference breath-hold Cartesian scan and (B) the proposed free-breathing radial scan on Patients #1 and #2. Furthermore, for the radial data in (B), we compared three different regularizations: L2 regularization, temporal TV regularization without $B_0$ update, and the proposed spatial LLR and temporal TV regularization with $B_0$ update.

Fig. 6.

(A) Initial $B_0$ maps of four respiratory bins for the model-based reconstruction without $B_0$ update in Fig. 5. (B) Jointly updated $B_0$ maps of four respiratory bins. Joint update using seven echoes helps to recover more details in the $B_0$ maps.

Fig. 7.

(A) One coronal-view $\text{FF}$ and $R_2^{\ast }$ map from breath-hold Cartesian scan of Patient #1. (B) Plots of self-gating signal and phase portraits. (C) Four coronal-view $\text{FF}$ and $R_2^{\ast }$ maps from free-breathing radial scan with self-gated motion-resolved model-based reconstruction. The $R_2^{\ast }$ map in (A) suffers from hyper intensities in the liver dome region (yellow ellipse), whereas the proposed radial scan shows consistent $R_2^{\ast }$ values.

Fig. 8.

Quantitative analysis of reconstructed $\text{FF}$ and $R_2^{\ast }$ maps for all subjects comparing the reference breath-hold Cartesian scan and the proposed free-breathing radial scan, respectively.

Fig. 9.

(A) Transversal view and (B) sagittal view of reconstructed $\text{FF}$ and $R_2^{\ast }$ maps in Patient #6 comparing the reference breath-hold Cartesian scan and the proposed radial scan, respectively. This patient shows definite obesity symptom (see Table III), but has no fatty liver. In fact, the $\text{FF}$ values of this patient is the lowest among all subjects (see also Fig. 8). Similar to the results of Patient #1 in Fig. 5, the Cartesian scan suffers from fold-in artifacts in the $R_2^{\ast }$ maps (yellow arrows), while the proposed radial scan shows consistent $R_2^{\ast }$ values.

Fig. 10.

(A) Volunteer #1 and (B) Patient #9 with elevated $\text{FF}$ values, but no clear symptom of obesity. In particular, Patient #9 was diagnosed with hepatic steatosis by the standard liver biopsy and ultrasound.

Fig. 11.

Comparison of $\text{FF}$ and $R_2^{\ast }$ maps for Patients (A) #3 and (B) #8 among the reference Cartesian scan, the free-breathing radial 2:47 min scan, and the free-breathing radial scan with retrospective two-fold undersampling, corresponding to a scan time of 1:24 min. Note the ripple-like artifact in the $R_2^{\ast }$ map from the Cartesian scans due to incomplete breath hold (yellow arrows in the 1st row).

Fig. 12.

Quantitative analysis of the reconstructed $\text{FF}$ and $R_2^{\ast }$ maps from radial scans: actual acquisition (2:47 min) and retrospectively undersampled acquisition (1:24 min), respectively.

Fig. 13.

Magnitude of coil sensitivity maps for Patient #1 from (top) the proposed model-based method and (bottom) the ESPIRiT method.