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MR Imaging of the Lumbar Spine: Relation of “Back Fat Index (BFI)” and Body Mass Index (BMI) 2011

General Spine

Matthew, J, DeVries, M.D.
Francis, J, Hahn, M.D., Non ASSR Member
Theresa, J, Hahn, B.S., M.A., Non ASSR Member

Poster

Purpose

Our purpose was to determine if Body Mass Index (BMI) correlated with the amount of posterior subcutaneous adipose tissue identified on lumbar spine MR imaging. We sought to create a Back Fat Index (BFI) that could serve as a substitute for anthropomorphic data.

Methods & Materials

In a review of the medical records, fifty consecutive patients undergoing lumbar spine MR filled out standard protocol sheets indicating height, weight, sex, and age. Using the midline sagittal T1 images (Fig.1), four measurements were obtained: 1) AP dimension of the L1 vertebral body (L1), 2) AP dimension of the subcutaneous fat posterior to L1 (L1 Fat), 3) Widest AP dimension of posterior subcutaneous fat within the lumbosacral spine (LS Fat), and 4) Average of L1 Fat and LS Fat (Ave Fat). Three separate BFIs were calculated: 1) L1 Fat / L1, 2) LS Fat / L1 and 3) Ave Fat / L1. Patient BFIs were then correlated with their BMI using linear regression analysis. Sensitivities and specificities for various threshold levels of BFI were calculated.

Results

Linear regression correlation coefficients (r) for L1 Fat / L1, LS Fat / L1, and Ave Fat / L1 were r=0.71, 0.72, and 0.74 respectively. At a threshold value of 1.0 for predicting BMI > 25: L1 Fat / L1 sens 68% and spec 100%, LS Fat / L1 sens 100% and spec 80%, and Ave Fat / L1 sens 79% and spec 80%. At a threshold value of 1.0 for predicting BMI > 30: L1 Fat / L1 sens 73% and spec 78%, LS Fat / L1 sens 100% and spec 44%, and Ave Fat / L1 sens 80% and spec 56%. Research Reproduced using Lumbar Spine MRI on other Sixty Patients 1. Nine patients show BFI on L1 level, greater than 1.5 correlated with BMI greater than 30 (Obese). Specificity is 100%. 2. 28 patients > 1.0 BFI correlated BMI > 25 (overweight). Specificity is 100%. 3. Our study proved these results are reproducible.

Conclusion

1. All three Back Fat Indices (BFI) obtained from Lumbar Spine MRI could serve as additional indices for obesity. 2. All three BFIâ