Asymmetric Spin Echo Sequences A Simple New Method for Obtaining NMR !H Spectral Images

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Abstract
The nuclear magnetic resonance (NMR) signal decay produced by reversible tissue-induced dephasing of the magnetization components in the transverse plane (reversible tissue-induced dephasing) was measured and expressed as a function of a new transverse relaxation time T2'' (T2 prime) for samples of rat liver, retroperitoneal fat, inflated lung, and corn oil. Simple exponentials did not adequately describe the observed NMR signal decay. Inflated lung demonstrated the most rapid signal decay (T2'' = 4.8 ms) followed by retroperitoneal fat (T2'' = 16 ms). No reversible tissue-induced dephasing was observed in liver (T2'' immeasurably long). In tissues which contain both fat and water, the chemically shifted 1H resonance peaks from -OH and -CH- are in phase with symmetric spin echo sequences but out of phase with asymmetric sequences. The interference of these two peaks produces a beat pattern with asymmetric sequences. Subtraction images obtained from paired symmetric- and asymmetric-sequence images accurately (r = .96) reflect T2'' and can be used to indicate the presence of fat. In vivo subtraction images of ethionine-induced fatty rat livers were significantly different from similar in vivo images of normal rat livers (P < .0005). Since for each pixel of a subtraction image, the magnitude of the difference signal should be approximately proportional to the ratio of hydroxyl and alkyl protons, this simple spin echo sequence modification may obviate the need for more time-consuming 3-dimensional Fourier transform proton chemical shift images.