3671

Fatty acid characterization of vertebral body marrow using MR Z-spectral imaging at 3 T
Junfeng Kuang1, Yulong Qi2, Qiting Wu1, Yang Zhou1, Guanxun Cheng2, Hairong Zheng1, and Yin Wu1
1Paul C. Lauterbur Research Center for Biomedical Imaging, Shenzhen Institute of Advanced Technology, Chinese Academy of Sciences, Shenzhen, China, 2Department of Medical Imaging, Peking University Shenzhen Hospital, Shenzhen, China

Synopsis

Keywords: Fat & Fat/Water Separation, Contrast Mechanisms

Motivation: MR Z-spectral imaging (ZSI) offers a new way to generate fat spectrum. However, its feasibility for fatty acid characterization remains to be elucidated.

Goal(s): This study designed a ZSI protocol and investigated its capability in fat measurement.

Approach: The designed ZSI protocol was tested on a fat-water phantom and vertebral body marrows in healthy volunteers and osteoporosis patients at 3 T.

Results: ZSI-measured fat fraction (FF) significantly correlated with oil volumes in the phantom. Moreover, osteoporosis patients exhibited significantly higher normalized fat peak amplitudes and FF than healthy volunteers, indicating the ability of ZSI in revealing fatty acid differences under different pathological states.

Impact: The designed ZSI protocol was feasible for fatty acid characterization. Significant differences of fatty acid metrics were detected between osteoporosis patients and healthy volunteers, suggesting the potentials of the designed ZSI protocol in facilitating fat-related disease diagnosis and evaluation.

Introduction

Bone marrow fat is involved in many bone-related and non-osseous diseases [1, 2]. MR spectroscopy and chemical shift encoding-based water-fat MRI have been intensively employed to monitor bone marrow alterations in pathological states [3]. However, these techniques either suffer from relatively low spatiotemporal resolution and complicated postprocessing strategies [4] or are prone to errors from phase wrapping, field inhomogeneity, T2* bias, etc. [5, 6]. Recently, a Z-spectrum imaging (ZSI) technique was proposed, which applies RF irradiation pulses sweeping over a range of frequencies to directly saturate signals at their resonance frequencies [7, 8]. The method is insensitive to phase artifacts and field inhomogeneity, offering a novel way for fat fraction (FF) quantification and brown adipose tissue detection [7, 8]. However, only the intense fat peak from bulk methylene protons was accounted for fat measurement with other fat signals usually ignored, making the feasibility of ZSI for fatty acid characterization remains unclear.

Materials and methods

MRI study: An agar-based fat-water phantom consisting of six 20-mL vials with oil volumes of 0, 5%, 10%, 20%, 30%, and 40% was constructed. The local Institutional Review Board approved the human study. Fourteen healthy volunteers and twelve osteoporosis patients with T-score<-2.5 were prospectively enrolled. Written informed consents were obtained from all participants. MRI scans were conducted on a 3 T scanner (uMR790, Shanghai UIH, China). ZSI images were acquired using a single-shot FSE on a single slice (TR/saturation time/TE=1600 ms/100 ms/39 ms, B1=0.25 μT, frequency offsets from -5 to 2 ppm with intervals of 0.05 ppm), including an unsaturated scan. Parameters for phantom experiments included: matrix size=96×96, in-plane resolution=1.6×1.6 mm2, slice thickness=10 mm, and NEX=1. For the human study, ZSI data was acquired on a transverse slice at the L4 vertebral body (spatial resolution=2.4×2.4×8 mm3, FOV=270-325 × 194-230 mm2 adjusted according to the body size of participants, and NEX=1). The imaging time was 3.8 minutes.
Data analysis: The acquired saturated scans (S) were normalized by the control scan without RF saturation (S0) as Z=S/S0, interpolated by smoothing splines, and corrected B0 field inhomogeneity by shifting the water peak to 0 ppm. Multiple Gaussian-Lorentzian sum functions were employed to resolve water and four fat signals with frequency offsets at -3.8, -3.4, -2.7, and -1.9 ppm [9]. The fitting goodness of R2 and fitting residue were measured to evaluate the fitting performance. Amplitudes of fat peaks were normalized to that of the water peak. FF=Afat/(Afat+Awater), where Awater and Afat are peak amplitudes of water and total fat signals, respectively. Normalized fat peak amplitudes and FF were measured in the L4 vertebral body and averaged across all subjects in each group.

Results

Figure 1 shows the Z spectra of the six fat-water vials with different oil volumes. Z spectra were well fitted with an average fitting goodness of R2>0.99 and residues<0.1%. The water peak amplitude decreased, and fat peak amplitudes increased with oil volumes.
Figure 2 displays maps of normalized fat peak amplitudes at frequency offsets of -3.8, -3.4, -2.7, and -1.9 ppm, respectively. The normalized fat peak amplitudes increased with oil volumes. Moreover, FF measured with the ZSI method significantly correlated with prepared oil volumes (r=0.996, P<0.001).
Figures 3 and 4 show multiparametric images from a representative healthy volunteer and an osteoporosis patient, including Z spectra of L4 vertebral body marrows with resolved water and fat signals and respective normalized fat peak amplitude maps. Compared to the healthy volunteer, the osteoporosis patient exhibited conspicuously elevated fat signals and reduced water signals in vertebral body marrows, resulting in noticeable hyperintense in the normalized fat peak amplitude and FF maps.
Table 1 summarizes the demographic information of the participants and quantitatively compares fatty acid metrics, including the normalized fat peak amplitudes and FF, between the healthy volunteers and the osteoporosis patients. The osteoporosis patients exhibited significantly higher normalized fat peak amplitudes and FF compared to the healthy volunteers (all P<0.01).

Discussion and Conclusion

This study designed a ZSI protocol and demonstrated its feasibility in fatty acid characterization in both fat-water phantom and human experiments. The developed ZSI technique revealed significantly higher normalized fat peak amplitudes and FF in vertebral bone marrows of osteoporosis patients than that of healthy volunteers, likely due to a shift of differentiation of mesenchymal stem cells to adipocytes [10]. To our best knowledge, this was the first study to report differences in four main fat components in vertebral body marrows between healthy volunteers and osteoporosis patients. The ZSI technique is promising to facilitate the diagnosis and evaluation of fat-altered diseases by providing complementary information.

Acknowledgements

National Natural Science Foundation of China (92259203 and 82271976), and the Outstanding Scientific and Technological Innovation Talent Training Program of Shenzhen (RCJC20221008092809018).

References

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Figures

Figure 1. Spectra of the six vials in the fat-water phantom, with the oil volumes of (a) 0, (b) 5%, (c) 10%, (d) 20%, (e) 30%, and (f) 40% acquired with ZSI, from which water and fat signals with frequency offsets at 0, -3.8, -3.4, -2.7 and -1.9 ppm were resolved. Note that fat signals were magnified two times for visualization.

Figure 2. Maps of the six-vial fat-water phantom, including maps of normalized fat peak amplitudes with frequency offsets at -3.8, -3.4, -2.7 and-1.9 ppm, respectively, and ZSI-measured FF map and its correlation with prepared oil volumes.

Figure 3. Multiparametric maps of a representative healthy volunteer, including average Z spectrum and resolved water and fat signals in L4 vertebral bone marrow, and maps of normalized fat peak amplitudes with frequency offsets at -3.8, -3.4, -2.7, and -1.9 ppm, respectively, and corresponding FF map overlaid on an unsaturated S0 image.

Figure 4. Multiparametric maps of a representative osteoporosis patient, including average Z spectrum and resolved water and fat signals in L4 vertebral bone marrow, and maps of normalized fat peak amplitudes with frequency offsets at -3.8, -3.4, -2.7, and -1.9 ppm, respectively, and corresponding FF map overlaid on an unsaturated S0 image.

Table 1. Demographics of the healthy volunteers and osteoporosis patients, and their normalized fat peak amplitudes and FF values.

Proc. Intl. Soc. Mag. Reson. Med. 32 (2024)
3671
DOI: https://doi.org/10.58530/2024/3671