Plewes D B
Department of Medical Biophysics, University of Toronto Sunnybrook Health Science Centre, Ontario, Canada.
Radiographics. 1994 Nov;14(6):1389-404; quiz 1405-6. doi: 10.1148/radiographics.14.6.7855348.
The majority of sequences used in routine clinical magnetic resonance imaging rely on the concepts involving the spin echo. Spin-echo sequences require long acquisition times (1-10 minutes), but compared with faster gradient-recalled echo methods, spin-echo methods are relatively immune to signal loss and distortions from field inhomogeneity and tissue-induced susceptibility variations. Through modifications of intersequence repetition time (TR), echo formation interval (echo time [TE]), and various gradient moments, image contrast can be altered to emphasize tissue relaxation times T1, T2, or proton density. The TR and TE values control the amount of T1 weighting and T2 weighting, respectively. At long TR intervals (approximately 10 x tissue T1 values) and minimum TE values, the difference in signal intensity arising from relaxation vanishes, and contrast arises solely from the differences in proton density between the two tissues. Images formed with short TR intervals and long TE values exhibit very low signal-to-noise ratio and negligible contrast and should be avoided. Recently, fast spin-echo sequences have partially overcome the limitation of long acquisition times, with up to 16-fold reduction, by acquiring multiple lines in k space with multiecho sequences.
常规临床磁共振成像中使用的大多数序列都基于涉及自旋回波的概念。自旋回波序列需要较长的采集时间（1 - 10分钟），但与更快的梯度回波方法相比，自旋回波方法相对不易受到磁场不均匀性和组织诱导的磁化率变化导致的信号丢失和失真的影响。通过改变序列间重复时间（TR）、回波形成间隔（回波时间[TE]）以及各种梯度矩，可以改变图像对比度以突出组织弛豫时间T1、T2或质子密度。TR和TE值分别控制T1加权和T2加权的量。在长TR间隔（约为组织T1值的10倍）和最小TE值时，由弛豫引起的信号强度差异消失，对比度仅源于两种组织之间质子密度的差异。用短TR间隔和长TE值形成的图像具有非常低的信噪比和可忽略不计的对比度，应予以避免。最近，快速自旋回波序列通过使用多回波序列在k空间中采集多条线，部分克服了长采集时间的限制，采集时间最多可减少16倍。
