Abstract:
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Purpose: The extent of fat brightening in fast spin echo (FSE) imaging is a function of the echo spacing. The purpose of this study is to quantify the bright fat effect (Henkelman et al, JMRI 2:533-540, 1992) in normal vertebral marrow using the brightening factor that is defined as the ratio of signal intensities from short versus long FSE echo spacing acquisitions.
Methods and Materials: Thirteen patients with low back pain and no know marrow abnormality referred for lumbar spine MRI (1.5T) were examined with two separate sagittal T2-weighted FSE imaging sequences. Sequence 1 used 16 echoes and 11 ms echo spacing with a 41.7 kHz receiver bandwidth (RB). Sequence 2 used 8 echoes and 22 ms echo spacing with a 15.6 kHz RB. For both sequences, TR=3200 ms and the effective TE was approximately 110 ms. Identical 4 mm slices were acquired with a 512x256 matrix and a 28 cm FOV. Region of interest signal intensity measurements were performed for vertebral marrow (L2, L3, L4), subcutaneous fat, spinal cord, CSF, and noise. Brightening factors for each of the tissues sampled and the noise were evaluated. The significance of the differences between the means of the brightening factors for the tissues sampled was determined by a two-tailed t-test. For the marrow, the overall mean (including all vertebral levels measured) was used. The noise brightening factor was compared with the expected result from MR noise theory.
Results: The mean ±SD brightening factors were marrow 1.56 ±0.17, fat 1.97 ±0.47, cord 1.01 ±0.16, CSF 0.96 ±0.06. The marrow brightening factor was significantly smaller than that for subcutatneous fat (p < 0.01) but significantly larger than that of CSF (p < 0.001) and spinal cord (p < 0.001). The noise brightening factor (1.64 ±0.07) proved consitent with the ratio of the square root of the RB (1.63).
Conclusion: The bright fat effect in FSE imaging may serve as a non-chemical shift, non-relaxation based means to generate pure fat and water images by using the brightening factor as a tissue classifier. Brightening factors of pure fat (~2) and pure water (~1) allow for clear differentiation. The marrow brightening factor is statistically different than either pure fat or water. The brightening factor of marrow is closer to fat than water, at least in part due to the relatively long T2 of fat and the 110 ms effective echo time.
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MacKenzie MD, J,
Fat Brightening Factors in FSE Imaging of Vertebral Bone Marrow. Radiological Society of North America 2003 Scientific Assembly and Annual Meeting, November 30 - December 5, 2003 ,Chicago IL.
http://archive.rsna.org/2003/3102445.html
