Measurement and Compensation of Respiration-Induced B0 Variations for Bone Marrow Fat Quantification in Lumbar Spine
Yoonho Nam1, Joon-Yong Jung1, Hyun Seok Choi1, Eojin Hwang1, Hongpyo Lee2, and Dong-Hyun Kim2

1Department of Radiology, Seoul St. Mary's Hospital, College of Medicine, The Catholic University of Korea, Seoul, Korea, Republic of, 2Department of Electrical and Electronic Engineering, Yonsei University, Seoul, Korea, Republic of

Synopsis

Fat fraction of the bone marrow has been suggested as an important quantitative parameter in the assessment of treatment response and determination of the benignity in oncologic imaging. Therefore, accurate fat quantification is a prerequisite for the fat fraction to be established as a reliable imaging biomarker. For this purpose, spoiled gradient echo sequences have been commonly used. However, gradient echo imaging is susceptible to B0 variations from various sources such as respiration, cardiac pulsation. In this study, we investigate and compensate the effects of respiration-induced B0 variations on fat quantification of the bone marrow in the lumbar spine.

Purpose

Fat fraction of the bone marrow has been suggested as an important quantitative parameter in the assessment of treatment response and determination of the benignity in oncologic imaging.1,2 Therefore, accurate fat quantification is a prerequisite for the fat fraction to be established as a reliable imaging biomarker. For this purpose, spoiled gradient echo sequences have been commonly used. However, gradient echo imaging is susceptible to spatial and temporal B0 variations from various sources such as respiration, cardiac pulsation, and body motion. Among them, a respiratory motion is one potential obstacle to estimating reliable fat fraction in the spine region. A recent 3T study has reported that B0 variations between inspired and expired conditions were measured up to 74 Hz in the cervical spine.3 In this study, we address the effects of respiration-induced B0 variations on fat quantification of the bone marrow in the lumbar spine. Furthermore, we compensate these effects using simultaneously acquired navigator echoes.

Methods

Eight healthy volunteers (IRB-approved) were scanned at 3T MRI. For bone marrow fat quantification, 3D multi-echo GRE was acquired with following parameters: TR = 17 ms, # of echoes = 7, TE1 = 1.7 ms, ΔTE = 1.7 ms, 7th echo was used as a 1D (SI direction) navigator without phase encoding,4 flip angle = 10 °, voxel size = 1.2×1.2×2.4 mm3, 48 sagittal slices, scan time = 2 min. Bipolar readout gradients were used for acquisition efficiency and phase/magnitude errors due to bipolar acquisitions were corrected before fat quantification.5
Temporal B0 variations in the readout direction (superior to inferior) were estimated by calculating phase differences between navigator echoes after 1D Fourier transform. Before calculation, navigator echo data were low-pass filtered (0 to 2 Hz) temporally to reduce high-frequency variations (not related to respiration). By the aid of the reconstructed images (1st to 6th echoes), each vertebral level (from L1 to L5) was manually defined in the navigator echo data (Fig. 1A). For each vertebral level, standard deviation of temporal B0 variations was calculated for comparison. By utilizing the estimated B0 variations in the readout direction, each k-space line was compensated before 3D FFT.4
For both uncompensated and compensated complex images, fat quantification was performed using T2*-IDEAL technique with 6 echoes.6 Vertebral ROIs were manually drawn on the magnitude images, and then ROI-averaged fat fractions were calculated for five vertebral levels. For results of 8 subjects, a pair-wise Student’s t-test was performed between the uncompensated and compensated.

Results

Figure 1 shows the estimated B0 variations induced from navigator echo data. An increase of B0 variations from L5 to L1 was observed for all subjects. These B0 variations result in signal dispersions in gradient echo images, particularly, in late TE images. After compensation using navigator echo data, the effects of the B0 variations were remarkably reduced in magnitude images (Fig. 2). The calculated fat fractions show significant differences (p<.05) in L1 and L3 (Fig. 3).

Discussion

We have investigated the effects of respiration-induced B0 variations on fat quantification in the lumbar spine. The results of this study raise the need for considering respiration-induced B0 variations for accurate fat quantification in the lumbar spine. The use of navigator echo data can be an effective way for the reduction of the effects of respiratory motion on the quantification because it just requires an TR increase of a few milliseconds.

Acknowledgements

No acknowledgement found.

References

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Figures

(A) The combined magnitude image of the obtained six echo images (root sum of squares). (B) The estimated B0 variations for 5 vertebral levels from a healthy subject. (C) Standard deviations of B0 variations for all 8 subjects.

Comparison of the uncompensated and compensated magnitude images at TE = 10.3 ms.

The estimated fat fractions of the uncompensated and compensated data for all 8 subjects (*: p<.05).



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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