Validation of bone marrow fat quantification in the presence of trabecular bone using MRI

Abstract

Background: To validate six-echo, chemical-shift based MRI with T2 * correction for the quantification of bone marrow fat content in the presence of trabecular bone.

Methods: Ten bone phantoms were made using trabecular bone cores extracted from the distal femur and proximal tibia of 20 human cadaveric knees. Bone marrow was removed from the cores and the marrow spaces were filled with water-fat gelatin to mimic bone marrow of known fat fractions. A chemical-shift based water-fat separation method with T2 * correction was used to generate fat fraction maps. The proton density fat fractions (PDFF) between marrow regions with and without bone were compared with the reference standard of known fat fraction using the squared Pearson correlation coefficient and unpaired t-test.

Results: Strong correlations were found between the known fat fraction and measured PDFF in marrow without trabecular bone (R(2) = 0.99; slope = 0.99, intercept = 0.94) as well as in marrow with trabecular bone (R(2) = 0.97; slope = 1.0, intercept = -3.58). Measured PDFF between regions with and without bone were not significantly different (P = 0.5). However, PDFF was systematically underestimated by -3.2% fat fraction in regions containing trabecular bone.

Conclusion: Our implementation of a six-echo chemical-shift based MRI pulse sequence with T2 * correction provided an accurate means of determining fat content in bone marrow in the presence of trabecular bone.

Keywords: 3 Tesla; bone marrow fat content; fat quantification; validation.

Figure 1

Representative high-resolution HR-pQCT images of two phantoms acquired with isotropic voxel size of 82 µm. Left: It is clearly demonstrated that the fat solution nicely fills all the bone marrow spaces. Right: For comparison a bone core is shown with residual air. These air bubbles can be easily identified and these phantoms can be discarded.

Figure 2

Shown are the obtained IDEAL fat fraction maps. The left images show fat only in the phantom and the right images contain a bone core embedded in fat. The color bar indicates PDFF in percent fat.

Figure 3

Shown are the linear relationships between Standard Reference of fat fraction and PDFF in the presence of trabecular bone and without bone. a) Comparing PDFF without bone to the known fat fraction; b) Comparing PDFF with bone to the known fat fraction; c) Comparing measured PDFF with and without bone. The solid line represents the fitting results from linear regression analyses. The dashed line represents the unity line.

Figure 4

Shown is the Bland-Altman plot comparing the measured PDFF fat fractions with the known fat fractions. a) Comparing differences between PDFF without bone and known fat fraction; b) Comparing differences of PDFF with bone and known fat fraction; c) Comparing PDFF with and without bone. A systematic underestimation of PDFF in the presence of bone compared to PDFF without bone and known fat fraction is clearly demonstrated.