Maspes F, Apruzzese A, Squillaci E, Floris R, Santino P, Simonetti G
Istituto di Radiologia, Università degli Studi di Roma Tor Vergata.
Radiol Med. 1994 Sep;88(3):249-58.
The main limitations of magnetic resonance imaging (MRI) are due to the long acquisition time needed for data sampling. Fast and ultrafast sequences, thanks to progress in hardware and software technology, allow the acquisition times to be dramatically reduced. Such improvements will definitely increase MR capabilities and make MRI a fast and dynamic technique no longer depending on patients' cooperation and limited by patients' conditions--e.g., trauma, pediatric, or claustrophobic patients--or even by the anatomic features of the region of interest. The latest experimental studies are aimed at increasing the clinical applications and the perspectives of clinical MRI and at such ambitious goals as MR-fluoroscopy, interventional MRI, total body examination in less than one minute and functional MRI. In this paper the technical principles and the main indications of this new kind of sequences are reported, with a special emphasis on Gradient-Echo (GE), Turbo Spin-Echo (turbo SE) or Fast Spin-Echo (fast SE), Turbo FLASH and Echo Planar (EP) sequence. A smaller flip angle (FA) and the substitution of the 180 degrees RF pulse with a gradient reversal are the main differences of GE from SE sequences. From GE sequences, several fast sequences are derived, such as angiographic and turbo FLASH sequences. The latter, thanks to 180 degrees preinversion RF pulse, using idoneous inversion times (IT), provide flexible contrast. Turbo SE sequences, which are directly derived from conventional SE sequence, allow the acquisition time to be markedly shortened, by acquiring several Fourier lines of K-space for each TR. The repetition of several 180 degrees pulses following the excitatory 90 degrees RF pulse does the trick through the creation of multiple echoes for a single TR. EP sequences are the fastest ones currently available: with them, an image can be acquired in 30-100 ms. However, the limited availability of the relative hardware and the need of both quality implementation and the definition of clinical indications are major obstacles to the widespread use of these sequences. The SE T2-weighted sequence, the main responsible for the long examination time, will be replaced soon. Further implementation of these sequences will make MRI a fast, flexible and adaptable method to any exam and patient disability.
磁共振成像（MRI）的主要局限性在于数据采样所需的采集时间较长。得益于硬件和软件技术的进步，快速和超快序列能够显著缩短采集时间。这些改进必将增强磁共振成像的能力，使MRI成为一种快速且动态的技术，不再依赖患者的配合，也不受患者病情（如外伤、儿科或幽闭恐惧症患者）甚至感兴趣区域的解剖特征的限制。最新的实验研究旨在增加临床MRI的临床应用和前景，并朝着诸如磁共振荧光透视、介入性MRI、不到一分钟的全身检查以及功能MRI等宏伟目标迈进。本文报道了这类新序列的技术原理和主要适应证，特别强调了梯度回波（GE）、快速自旋回波（turbo SE）或快速自旋回波（fast SE）、快速小角度激发（Turbo FLASH）和回波平面（EP）序列。较小的翻转角（FA）以及用梯度反转替代180度射频脉冲是GE序列与SE序列的主要区别。从GE序列衍生出了几种快速序列，如血管造影和快速小角度激发序列。后者由于采用180度预反转射频脉冲并使用合适的反转时间（IT），提供了灵活的对比度。直接从传统SE序列衍生而来的Turbo SE序列，通过为每个TR采集多条K空间的傅里叶线，可显著缩短采集时间。在激发性90度射频脉冲之后重复多个180度脉冲，通过为单个TR创建多个回波来实现这一点。EP序列是目前可用的最快序列：使用它们，一幅图像可在30 - 100毫秒内采集完成。然而，相关硬件的可用性有限以及高质量实施和临床适应证定义的需求是这些序列广泛应用的主要障碍。主要导致检查时间较长的SE T2加权序列很快将被取代。这些序列的进一步实施将使MRI成为一种快速、灵活且适用于任何检查和患者残疾情况的方法。
