Zhao L, Venkatesh A K, Albert M S, Panych L P
Department of Radiology, Harvard Medical School, Brigham and Women's Hospital, 75 Francis Street, Boston, Massachusetts 02115, USA.
J Magn Reson. 2001 Feb;148(2):314-26. doi: 10.1006/jmre.2000.2253.
Some non-Fourier encoding methods such as wavelet and direct encoding use spatially localized bases. The spatial localization feature of these methods enables optimized encoding for improved spatial and temporal resolution during dynamically adaptive MR imaging. These spatially localized bases, however, have inherently reduced image signal-to-noise ratio compared with Fourier or Hadamad encoding for proton imaging. Hyperpolarized noble gases, on the other hand, have quite different MR properties compared to proton, primarily the nonrenewability of the signal. It could be expected, therefore, that the characteristics of image SNR with respect to encoding method will also be very different from hyperpolarized noble gas MRI compared to proton MRI. In this article, hyperpolarized noble gas image SNRs of different encoding methods are compared theoretically using a matrix description of the encoding process. It is shown that image SNR for hyperpolarized noble gas imaging is maximized for any orthonormal encoding method. Methods are then proposed for designing RF pulses to achieve normalized encoding profiles using Fourier, Hadamard, wavelet, and direct encoding methods for hyperpolarized noble gases. Theoretical results are confirmed with hyperpolarized noble gas MRI experiments.
一些非傅里叶编码方法，如小波编码和直接编码，使用空间局部化基。这些方法的空间局部化特性使得在动态自适应磁共振成像期间能够进行优化编码，以提高空间和时间分辨率。然而，与用于质子成像的傅里叶或哈达玛编码相比，这些空间局部化基固有地降低了图像信噪比。另一方面，超极化惰性气体与质子相比具有截然不同的磁共振特性，主要是信号的不可再生性。因此，可以预期，与质子磁共振成像相比，超极化惰性气体磁共振成像中关于编码方法的图像信噪比特性也将非常不同。在本文中，使用编码过程的矩阵描述从理论上比较了不同编码方法的超极化惰性气体图像信噪比。结果表明，对于任何正交归一编码方法，超极化惰性气体成像的图像信噪比都是最大的。然后提出了使用傅里叶、哈达玛、小波和直接编码方法为超极化惰性气体设计射频脉冲以实现归一化编码轮廓的方法。理论结果通过超极化惰性气体磁共振成像实验得到了证实。
