Measurement ofR′2in the Presence of Multiple Spectral Components Using Reference Spectrum Deconvolution

A method is described for measuringR′2, the RF reversible contribution to the effective transverse relaxation rate in yellow trabecular marrow, as a means to evaluate trabecular bone structure and density. The method exploits the similarity in spectral composition of the marrow and fat in subcutaneous tissue. Under these conditions the gradient echo envelope of the marrow signal can be regarded as a convolution of a function describing the bone marrow intravoxel line broadening (R′2) with a function expressing chemical shift modulation, which is obtained from the echo envelope of the subcutaneous fat signal in a reference region. Simple division of each of a series of echoes by the reference signal is shown to afford a smooth decay which can be fitted to a model to extractR′2. The method has been evaluated in the upper femur of test subjects and a strong correlation of the thus derivedR′2values with those obtained by the GESFIDE technique is demonstrated. The close correspondence in spectral composition of proximal femur marrow and subcutaneous fat is further illustrated by means of localized spectroscopy. The major potential error source is global inhomogeneity in the reference region which can lead to an underestimation of the demodulation-derivedR′2.