High-speed spectroscopic imaging for cancellous bone marrow R(2)* mapping and lipid quantification.

In this work an interleaved multiple-gradient-echo chemical shift imaging (IMGE-CSI) technique was designed, implemented and evaluated at 1.5 and 4T for high-resolution lipid quantification and R(2)* measurement in-vivo. The method is analogous to echo planar CSI but utilizes conventional gradient echoes, exploiting the principle of spectroscopic bandwidth extension by interleaving temporally offset gradient-echo trains. It is shown that IMGE-CSI is able to measure true fat volume fraction in oil/water mixtures with high accuracy, not possible with Dixon-type methods which approximate the spectrum as consisting of only two spectral components. Correlation of the CSI- derived volume fractions with volumetry afforded r(2) > 0.99 with a slope of 0.98. The method is shown to be able to quantify regional variations in bone marrow composition in vivo with a spatial resolution of 2.5 x 2.5 x 5 mm(3.) R(2)* was obtained by multi-line spectral curve fitting. For the measurement of R(2)* in cancellous bone marrow the method is shown to agree well with time-domain fitting techniques but is superior in instances where the marrow has both hematopoietic and fatty constituents. Finally, excellent inter-scan reproducibility (1% coefficient of variation for global means and medians) was achieved, yielding r(2) = 0.98 of the test-retest correlation for three scans in four test subjects. In conclusion, IMGE-CSI is found to enable highly accurate lipid quantification and measurement of cancellous bone marrow R(2)* at spatial resolutions and scan times typical of standard clinical protocols.