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3T-Chemical Shift Encoded MRI with Ultra-Short Echo Time Acquisition for Bone Quality Assessment: Preliminary Results in the Hip.
Dimitri MARTEL1, Benjamin LEPORQ2, Stephen HONIG3, and Gregory CHANG1
1Radiology, NYU Langone Health, New york, NY, United States, 2Université de Lyon; CREATIS CNRS UMR 5220, Inserm U1206, INSA-Lyon, UCBL Lyon 1, Villeurbanne, France, 3Osteoporosis Center, Hospital for Joint Diseases, NYU Langone Health, New york, NY, United States

Synopsis

Osteoporosis (OP) is a disease associated with low bone mass and deterioration of bone microarchitecture leading to bone fragility and increased fracture risk, especially in the proximal femur. Therefore, we have developed a chemical-shift encoded acquisition performed with a spiral k-space sampling to acquire ultrashort echo-time and longer echo time in the echo train. This study aims to determine if uTE acquisition can be performed for fat/water separation and if additional information can be provided through cortical bone imaging.

Introduction

Osteoporosis (OP) is a disease associated with low bone mass and deterioration of bone microarchitecture leading to bone fragility and increased fracture risk, especially in the proximal femur. OP is a severe public health problem, with over 200 million worldwide suffering from this disease. The main imaging method to assess OP includes Dual Energy X-ray absorptiometry (DXA) for measuring areal bone mineral density (BMD). DXA is relatively easy to perform in the clinic and the amount of radiation exposure is low. Recent studies using MRI to assess bone quality and health have focused on bone marrow adipose tissue to assess its composition and quantity1,2. While early MRI methods predominantly exploited the signal arising from MAT through Chemical Shift Encoded (CSE) MRI, more recent approaches like ultrashort echo time (uTE) imaging enable more direct imaging of bone tissue and have only recently been applied notably to image the cortical bone in the spine and hip3-6. Usually, CSE-MRI is achieved with a Cartesian k space-filling multiple gradient echo acquisitions and cannot sample enough short echo time values to recover information about cortical bone, mainly composed of short T2 species. Therefore, we have developed a chemical-shift encoded acquisition performed with a spiral k-space sampling to acquire ultrashort echo-time and longer echo time in the echo train7,8. This study aims to determine if uTE acquisition can be performed for fat/water separation and if additional information can be provided through cortical bone imaging.

Materials/methods

This study had institutional review board approval, and written informed consent was obtained from all subjects. MRI acquisitions were performed on n=5 subjects with osteoporosis (BMI= 19.5+/- 2.4 kg/m2; 48.41+/-7.7 years ) using a 3T system (Siemens Healthcare, Erlangen, Germany). First, we used a 3D spoiled gradient-echo sequence with an n=12 echoes train length (n ×1.2ms) with flyback readout gradient with the following parameters: TR/FA/NA = 16ms/5°/4 and BW= 2000 Hz/px; 32 coronal slices were acquired; Acquisition time =4 min. Then we acquired 2 interleaved 3D uTE stack of spirals with following parameters: TEs=0.08, 0.22, 2.4, 3.6, 4.8, 6.0, 7.2, 8.4, 9.6, 10.8, 12.0, 13.2ms; TR/FA: 16.2 ms / 5° ; BW= 975 Hz/px; Spiral duration= 800 µs ; Number of interleaved spirals= 800; Resolution= 1.4x1.4x1.7 mm3; SPIRIT reconstruction; Acquisition time =5min. Native time-series magnitude and phase images were saved from the scanner and processed using a homemade MATLAB script. We use the method previously described2 to compute Proton Density Fat Fraction (PDFF). For uTE acquisition, we used uTE echoes to compute porosity index9, weighted subtraction of long and short echo image10, and a short T2 component was fitted using a mono-exponential fit using the two first echo, (0.08 and 0.22 ms) assuming a similar long species contribution. For each reconstructed maps, the proximal femur was segmented and values integrated for the whole proximal femur, femoral head, femoral neck, and shaft. Linear regression between T-scores and uTE parameters was performed. PDFF values obtained using uTE and CSE were compared using Pearson's correlation coefficient.

Results

Typical acquired and reconstructed maps are shown in Fig.1. The regression plot between T-scores and uTE parameters are shown in Fig.2. Correlation between CSE and uTE PDFF is shown in Fig.3. We find that the T-score correlates with the uTE method's parameters in the whole hip bone and the shaft (Fig.2). T-score correlates with T2* short (r=0.71 in whole hip; r=0.90 in shaft), porosity index (r=0.95 in whole hip; r=0.95 in shaft) and PDFF (r=0.57 in whole hip; r=0.68 in shaft). We compared the PDFF value obtained with uTE with our standard CSE method and found good agreement between the two, notably in the femoral region (r=0.80 in femoral head and r=0.85 in femoral neck) (Fig.3)

Conclusion

In this preliminary study, we demonstrate the feasibility of performing a single 3T uTE acquisition in vivo to assess metrics of bone health including T2*, cortical bone porosity, and marrow fat. The next step would be to compare the results in healthy vs. osteoporotic subjects.

Acknowledgements

No acknowledgement found.

References

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Figures

Figure 1: A) Coronal slices at different echo time extracted from the 3D uTE. B) Parametric maps from a uTE-CSE acquisition: Fat-water decomposition allowed to reconstruct adiposity map (PDFF) and long T2 species T2* map. C) uTE Sub (weighted substraction of long and short echo image) and porosity maps which respectively indicate average pore size volume and bone porosity, and short T2* map of cortical bone.

Figure 2: uTE parameters v. T-score in the whole hip bone and shaft. Excellent correlation is found between parameters.

Figure 3: Comparison between PDFF measurements obtained by uTE and CSE in different hip bone subregions.

Proc. Intl. Soc. Mag. Reson. Med. 29 (2021)
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