Truncated sinc slice excitation for 31P spectroscopic imaging

The shortest possible delay (Td) between slice selection and data acquisition is important for producing high quality 31P spectra. In single slice multivoxel spectroscopic imaging, conventional excitation using sinc-shaped rf pulses within typical gradient limitations can have values of Td that lead to significant spectral distortion and loss of signal. Truncated sinc excitation, which ends the excitation close to the center of the main rf lobe has been suggested for MR angiographic applications to produce short values of Td. In this work, the slice profiles, spectral signal-to-noise ratio (SNR) and spectral distortions are compared using the minimum delay achievable on a commercial MRI system for conventional 'sinc' rf excitation and truncated sinc excitation. Slice profiles are calculated using the Bloch equations and measured with a phantom. SNR and spectral distortions are evaluated from whole slice spectra on a human volunteer. On an MRI system with 1 G/cm gradients (0.5 msec risetime), for a 2.5-cm slice at 31P frequencies, conventional excitation can be adjusted to achieve Td = 2.5 msec while truncated sinc excitation yields Td = 1.5 msec. The truncated sinc excitation's shorter value of Td leads to much smaller spectral distortions, but its slice profile has "dispersive tails" which increase as more of the rf is truncated. Slice profile corrected SNR for the beta-ATP peak of 31P on a human volunteer is equivalent for both sequences while, qualitatively, in the PDE region the truncated sinc approach has improved SNR.