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Free-Breathing Simultaneous Quantification of Liver T1, Fat and R2* with Variable Flip Angle Golden-Angle-Ordered 3D Stack-of-Radial MRI
Le Zhang1, Shu-Fu Shih1,2, Tess Armstrong1,3, and Holden H. Wu1,2,3
1Radiological Sciences, University of California, Los Angeles, Los Angeles, CA, United States, 2Bioengineering, University of California, Los Angeles, Los Angeles, CA, United States, 3Physics and Biology in Medicine, University of California, Los Angeles, Los Angeles, CA, United States

Synopsis

Quantification of T1, proton-density fat fraction (PDFF), and R2* in the liver can provide information about a range of diseases. Existing Cartesian acquisition schemes generally require breath-holding, which limits spatial coverage and may be difficult for sick, elderly or pediatric patients. In this study, we propose a variable-flip-angle (VFA) golden-angle-ordered (GA) 3D stack-of-radial sequence that can provide multiparametric mapping with volumetric liver coverage in three minutes during free-breathing and with intrinsic motion compensation capability. Pilot studies in healthy subjects demonstrate agreement with reference breath-held scans and good measurement repeatability.

Introduction

Quantitative MR parameters can characterize liver tissue status in several disease processes, such as T1 for fibrosis1‑3, proton-density fat fraction (PDFF) for steatosis4‑6 and R2* for iron overload7. Simultaneous multiparametric mapping in the liver can be achieved with variable-flip-angle (VFA) multi-echo Cartesian techniques8, but it requires multiple breath-holds with different flip angles, which limits spatial coverage and may be challenging for certain patients. VFA T1 calculations also require separate B1+ field maps for flip angle calibration, which may have errors due to potential image mismatch. Moreover, respiratory motion can cause biases in R2* quantification9. Golden-angle-ordered stack-of-radial MRI offers self-navigating capabilities to reconstruct images at a particular respiratory state for accurate VFA T1 and R2* quantification. Acquisition during free-breathing provides opportunities for larger spatial coverage. Here we propose a 3D multi-echo gradient-echo golden-angle-ordered radial VFA sequence10 that takes advantage of these properties to simultaneously quantify liver T1, PDFF, and R2* accurately and repeatably in a single 3-minute free-breathing scan.

Methods

Imaging Sequences
A 3D stack-of-radial trajectory with GA-ordered multi-echo readouts was used (Fig.1A) and repeated with two flip angles (“radial VFA”) for T1 quantification. Radial spokes with the same radial angle were collected along kz, and their (kx,ky)=0 points were Fourier-transformed to obtain projection profiles in z for all channels, which captured motion for both flip angles. Principal component analysis (PCA) was then performed to extract the first two principal components. Finally, channels exhibiting similar results (95% correlation threshold) were clustered to generate motion signal11,12 (Fig.1B). Radial data were assigned linear weights according to their motion signal amplitude, i.e. soft gating13, and was then re-gridded and channel-combined14 to produce complex images with the liver at end-expiration (Fig.1C). Magnitude images of all echoes were combined via sum-of-squares for VFA T1 fitting15. PDFF and R2* were calculated with the complex multi-echo images acquired with 3° flip angle to avoid T1-bias, using a seven-peak fat model16 and single effective R2* per voxel17.

Experiments
In an IRB-approved study, n=12 healthy adult subjects (6 males, 6 females) aged 31±10 years with body mass index of 23.2±4.1kg/m2 provided written informed consent and underwent two free-breathing radial VFA scans (acquired >10 minutes apart) to assess its repeatability. As references, one Cartesian gradient-echo VFA scan and one Cartesian modified Look-Locker inversion recovery (MOLLI)18 scan were acquired during breath-hold at end-expiration. To calibrate flip angles for VFA T1 fitting, B1+ maps were acquired during breath-holding at end-expiration19. All scans were conducted on a 3T scanner (Prisma, Siemens) with body and spine arrays. Table 1 lists scan parameters.

Data Analysis
In each subject, regions of interest (ROI) of 5cm2 were drawn in 3 slices (near liver dome, mid-section and lower-section) near corresponding landmarks in radial VFA and Cartesian VFA acquisitions to evaluate agreement in T1, PDFF and R2*. The repeatability of radial VFA T1, PDFF, and R2* was assessed using the intraclass correlation coefficient (ICC) in the same ROIs. 12 additional ROIs were drawn in 1 slice imaged by both radial VFA and Cartesian MOLLI acquisitions to evaluate T1 agreement.

Results

Fig.2 shows T1 maps obtained on a slice imaged by all protocols in one subject. Fig.3 shows PDFF and R2* maps acquired in the same subject using Cartesian VFA and radial VFA. T1 from Cartesian VFA and radial VFA had a mean difference (MD) of 35ms and 95% limits of agreement (LoA) of [-160ms, 90ms] (Fig.4A). T1 from Cartesian MOLLI and radial VFA had a MD of 40ms and LoA of [-122ms, 52ms] (Fig.4B). Cartesian VFA and radial VFA had a MD of 0.13% and LoA of [-1.30%, 1.56%] in PDFF (Fig.4C), while R2* measurements had a MD of 1.1s-1 and LoA of [-22.5s-1, 24.7s-1] (Fig.4D). In repeated radial VFA scans, ICC for T1, PDFF and R2* were 0.896, 0.843 and 0.771, respectively.

Discussion

The proposed radial VFA sequence can simultaneous quantify T1, PDFF and R2* in the liver with 3D coverage in a 3-minute free-breathing scan. The built-in motion tracking capability enables reconstructing images at end-expiration to align images with B1+ maps for accurate VFA T1 quantification, and to reduce the effect of motion on R2* mapping. In vivo liver scans demonstrated good repeatability of radial VFA for all parameters despite respiratory motion and potential bulk motion. T1 measured by all three protocols showed good agreement. PDFF showed good agreement between radial and Cartesian VFA. R2* had relatively wide LoA between radial and Cartesian VFA protocols because Cartesian R2* maps had higher levels of noise, likely due to residual motion and parallel imaging artifacts. The radial VFA acquisition time could be further reduced by incorporating parallel imaging techniques20. The new technique can be potentially used as a multiparametric diagnostic tool for liver diseases, and more evaluation is needed in patients with a wider range of liver T1, PDFF, and R2*.

Conclusion

We have demonstrated that a new VFA golden-angle-ordered stack-of-radial free-breathing liver MRI method can achieve simultaneous T1, PDFF, and R2* quantification in 3 minutes, with good intra-session repeatability and agreement with reference breath-holding methods.

Acknowledgements

This study was supported in part by Siemens Healthineers and UCLA Radiological Sciences. The authors thank the study coordinators at UCLA.

References

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9. Zhong X, Armstrong T, Nickel MD et al. Effect of respiratory motion on free-breathing three-dimensional stack-of-radial liver R2* relaxometry and improved quantification accuracy using self-gating. Magn Reson Med. 2019

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Figures

Figure 1: (A) The stack-of-radial trajectory was acquired with golden angle (GA) ordering in kx-ky plane and Cartesian encoding along kz and repeated with same angle ordering with two flip angles (α1 and α2). (B) The (kx,ky=0) points along kz of each radial angle were Fourier-transformed to obtain projection profiles in z for all N channels, where principle component analysis (PCA) was used to extract motion signals. (C) Motion signals from channels with similar patterns were combined to form the final motion signal. Soft gating was applied to reconstruct images at end-expiration.

Figure 2: Axial and reformatted sagittal/coronal T1 maps acquired in one healthy adult male subject (25 years old, BMI=23.3kg/m2) with free-breathing (FB) Radial VFA (left) and breath-holding (BH) Cartesian VFA (middle) are shown. The axial T1 map obtained with BH Cartesian MOLLI in the same subject is shown on the right. The dashed red boxes indicate the slice position of the Cartesian MOLLI T1 map. Red circles show example regions of interest.

Figure 3: (A) Axial and reformatted sagittal/coronal PDFF maps acquired in the same subject in Figure 2 with free-breathing (FB) Radial VFA (left) and breath-hold (BH) Cartesian VFA (right). (B) Axial and reformatted sagittal/coronal R2* maps acquired with FB Radial VFA and BH Cartesian VFA are shown on the left and right, respectively. White circles show example regions of interest.

Figure 4: Bland-Altman analysis of the agreement between (A) T1 measured by the radial VFA and Cartesian VFA protocols; (B) T1 measured by the radial VFA and Cartesian MOLLI protocols; (C) PDFF measured by the radial VFA and Cartesian VFA protocols using the lower flip angle; and (D) R2* measured by the radial VFA and Cartesian VFA protocols using the lower flip angle. MD: mean difference. LoA: 95% limits of agreement.

Table 1: Scanning parameters for different techniques. Radial acquisitions used golden-angle ordering. PE stands for phase encoding direction. FB: free-breathing; BH: breath-hold; VFA: variable flip angle; MOLLI: modified Look-Locker inversion recovery; FOV: field of view; TE: echo time; TR: repetition time; PE GRAPPA: phase-encoding direction GRAPPA acceleration factor.

Proc. Intl. Soc. Mag. Reson. Med. 28 (2020)
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