Three-dimensional biexponential weighted (23)Na imaging of the human brain with higher SNR and shorter acquisition time.

A new method is presented for acquiring 3D biexponential weighted sodium images of the in vivo human brain with up to three times higher signal-to-noise ratio compared with conventional six-step phase-cycling triple-quantum-filtered imaging. To excite and detect multiple-quantum coherences, a three-pulse preparation is used. During the pulse train, two images are obtained. The first image is acquired with ultrashort echo time (0.3 ms) during preparation between the first two pulses to yield a spin-density-weighted image. After the last pulse, a single-quantum-filtered image is acquired with an echo time of 11 ms that maximizes the resulting signal. The biexponential weighted image is calculated by subtracting the single-quantum-filtered image from the spin-density-weighted image. The resulting image mainly shows signal from sodium ions with biexponential quadrupolar relaxation behavior. In isotropic environments, the resulting image mainly contains triple-quantum-filtered signal. The four-step phase cycling yields similar signal-to-noise ratio in shorter acquisition time compared with six-step phase-cycling biexponential weighted imaging.