Signal-to-noise ratio comparison of encoding methods for hyperpolarized noble gas MRI

Some non-Fourier encoding methods such as wavelet and direct encoding use s patially localized bases. The spatial localization feature of these methods enables optimized encoding for improved spatial and temporal resolution du ring dynamically adaptive MR imaging. These spatially localized bases, howe ver, have inherently reduced image signal-to-noise ratio compared with Four ier or Hadamad encoding for proton imaging. Hyperpolarized noble gases, on the other hand, have quite different MR properties compared to proton, prim arily the nonrenewability of the signal. It could be expected, therefore, t hat the characteristics of image SNR with respect to encoding method will a lso be very different from hyperpolarized noble gas MRI compared to proton MRI. In this article, hyperpolarized noble gas image SNRs of different enco ding methods are compared theoretically using a matrix description of the e ncoding process. It is shown that image SNR for hyperpolarized noble gas im aging is maximized for any orthonormal encoding method. Methods are then pr oposed for designing RF pulses to achieve normalized encoding profiles usin g Fourier, Hadamard, wavelet, and direct encoding methods for hyperpolarize d noble gases. Theoretical results are confirmed with hyperpolarized noble gas MRI experiments. (C) 2001 Academic Press.