QUANTITATIVE SODIUM MRI AT 7T WITH 12 CHANNEL PHASED-ARRAY COIL

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. To maximize signal-to-noise ratio (SNR) and minimize relaxation weighting, ultra-short echo (UTE) readouts have been generally been favored for sodium MRI. UTE methods have also been used to measure total sodium concentration (TSC) in tissues, although this typically requires accurate mapping of the B1 field. B1 measurements are typically noisy and error prone, making good TSC measurements difficult unless the B1 is highly homogeneous. The multi-channel phase coils that are preferred in sodium MRI studies because of the increase SNR are more inhomogeneous that birdcage coils, making good B1 measurement important in most sodium MRI applications. UTE methods also suffer from off-resonance and T2*-associated blurring in the readout direction, further making quantitative measurements with these techniques difficult. A more reliable quantitative parameter for sodium MRI is the measurement of the long component of the transverse relaxation time, T2L*. By collecting several echoes after the short transverse relaxation component has decayed, the resulting decay curve can be accurately fitted to a monoexponential curve. UTE methods typically use a radial or spiral readout in two or three- dimensions, followed by gridding onto Cartesian k-space before Fourier transforming to image space. We have found that we can obtain very similar results using an acquisition weighted half-echo readout directly on a Cartesian grid. We then followed this half-echo readout with five full-echo standard Cartesian readouts. This imaging sequence has the benefit of producing an image with the highest possible SNR and lowest possible relaxation weighting while also providing a robust quantitative parameter.