Research On Fat Quantification Based On Z-Spectrum At Ultra-High-Field Magnetic Resonance Imaging

Nowadays,the drive towards high-field MRI is fueled by the pursuit of higher signal-to-noise ratio(SNR),spatial resolution,and imaging speed.However,high field strength is associated with field inhomogeneity,acceleration of T2*decay,and increased chemical shift,which may pose challenges to conventional MRI for fat quantification in complex tissues such as bone marrow adipose tissue(MAT)and brown adipose tissue(BAT).As a novel alternative fat quantification method,Z-Spectral MRI(ZS-MRI)can achieve fat quantification by acquiring direct saturated images of both fat and water under the same TE,which is not affected by T2*decay and field inhomogeneity.Compared with the commonly used fat quantitative techniques,ZS-MRI has higher spatial resolution and scanning efficiency than 1H-magnetic resonance spectroscopy(1H-MRS)and has better spectral resolution than Dixon’s fat-water separation.Based on the ultra-high-field magnetic resonance,we did three main works around the validation of fat quantification by ZS-MRI,the MAT quantification of rat’s lumbar by ZS-MRI,and the BAT quantification of rat’s by ZS-MRI in this paper:(1)The validation of fat quantification based on ZS-MRI.To investigate the accuracy and reproducibility of ZS-MRI in fat quantification,13 phantoms with various fat fractions were performed.Compared with the actual fat content,our results showed that ZS-MRI can achieve similar fat quantitative results to Dixon’s fat-water separation with good consistency(slope ≥ 0.88,R2>0.98)and reproducibility(slope ≥0.92,R2>0.99).(2)Fat quantification of rat’s lumbar MAT based on ZS-MRI.MAT,the content of which is associated with bone mineral density,osteoporosis,and fracture risk,serves as a relevant marker of impaired bone integrity.Since MAT fills between the trabeculae bone in the bone cavity,the interface between the bone and the MAT experiences a stark local magnetic field inhomogeneity,which greatly influences the accuracy of Dixon’s fat-water separation on the quantification of MAT.To achieve the precise quantification of MAT content on ultra-high-field and longitudinally monitor the relationship between MAT and age,MRI data of 1H-MRS,Dixon’s fat-water separation,and ZS-MRI of seven female rats at 13,17,and 21 weeks were performed in this study.Hematoxylin-eosin(HE)staining of lumbar spines from separated rat groups was performed for validation.Compared with 1H-MRS,our results showed that ZS-MRI outperformed Dixon’s fat-water separation in the MAT quantitative consistency(Dixon:R2=0.105,slope=0.49;ZS-MRI:R2=0.776,slope=0.78);and ZS-MRI can obtain the quantitative results of MAT which consistent with the histological results,can monitor the subtle changes of MAT content caused by aging.(3)Fat quantification of rat’s BAT based on ZS-MRI.As the main source of non-shivering thermogenesis in mammals,BAT plays an important role in maintaining body temperature and energy balance.Since BAT is closely innervated by the sympathetic nerve and has the characteristics of dense blood vessels and rich iron-containing mitochondria,the activity of BAT tissue will change the uniformity of the local magnetic field,thus influencing the fat quantitative results of Dixon’s fat-water separation.To achieve the precise quantification of BAT content on ultra-high-field,MRI data of 1H-MRS,Dixon’s fat-water separation,and ZS-MRI were acquired simultaneously in this study to compare the pros and cons of ZS-MRI and Dixon’s fat-water separation in the fat quantification of BAT.Our results of fat quantification of BAT showed that Dixon’s fat-water separation occasionally rendered fat-water switch artifacts,while ZS-MRI is not affected by field inhomogeneity and T2*factors,and can generate artifact-free fat distribution maps in areas with complex structures.