Hendrick R E, Nelson T R, Hendee W R
Magn Reson Imaging. 1984;2(4):279-83. doi: 10.1016/0730-725x(84)90193-0.
Several recent articles have assessed the relative efficiency of nuclear magnetic resonance (NMR) pulse sequences. One consideration that has received little attention is the effect on image contrast of displaying images without information on the sign of the reconstructed signals. The radiofrequency receivers currently used on most NMR imaging systems are quadrature detectors that preserve both the magnitude and sign of the NMR signal. Usually, however, sign or phase information is not used in the final image presentation. We point out that in imaging sequences that may have negative signals, such as inversion recovery, this loss of sign information produces a reduction in contrast between some tissues in an NMR image. We discuss the tissue parameters and interpulse delay times that result in contrast loss in inversion recovery and indicate the extent of contrast loss. We point out that for some tissues with unequal hydrogen spin densities, the region of contrast loss coincides with the region where maximum contrast would occur if sign information were preserved.
最近有几篇文章评估了核磁共振（NMR）脉冲序列的相对效率。一个很少受到关注的问题是，在显示重建信号符号信息缺失的图像时，对图像对比度的影响。目前大多数NMR成像系统使用的射频接收器是正交探测器，可保留NMR信号的幅度和符号。然而，通常在最终图像呈现中并不使用符号或相位信息。我们指出，在可能存在负信号的成像序列中，如反转恢复序列，这种符号信息的丢失会导致NMR图像中某些组织之间的对比度降低。我们讨论了导致反转恢复中对比度损失的组织参数和脉冲间延迟时间，并指出了对比度损失的程度。我们指出，对于一些氢自旋密度不相等的组织，对比度损失区域与保留符号信息时将出现最大对比度的区域重合。
