Detre J A, Alsop D C
Department of Neurology, University of Pennsylvania, Philadelphia 19104, USA.
Eur J Radiol. 1999 May;30(2):115-24. doi: 10.1016/s0720-048x(99)00050-9.
Several methods are now available for measuring cerebral perfusion and related hemodynamic parameters using magnetic resonance imaging (MRI). One class of techniques utilizes electromagnetically labeled arterial blood water as a noninvasive diffusible tracer for blood flow measurements. The electromagnetically labeled tracer has a decay rate of T1, which is sufficiently long to allow perfusion of the tissue and microvasculature to be detected. Alternatively, electromagnetic arterial spin labeling (ASL) may be used to obtain qualitative perfusion contrast for detecting changes in blood flow, similar to the use of susceptibility contrast in blood oxygenation level dependent functional MRI (BOLD fMRI) to detect functional activation in the brain. The ability to obtain blood flow maps using a non-invasive and widely available modality such as MRI should greatly enhance the utility of blood flow measurement as a means of gaining further insight into the broad range of hemodynamically related physiology and pathophysiology. This article describes the biophysical considerations pertaining to the generation of quantitative blood flow maps using a particular form of ASL in which arterial blood water is continuously labeled, termed continuous arterial spin labeling (CASL). Technical advances permit multislice perfusion imaging using CASL with reduced sensitivity to motion and transit time effects. Interpretable cerebral perfusion images can now be reliably obtained in a variety of clinical settings including acute stroke, chronic cerebrovascular disease, degenerative diseases and epilepsy. Over the past several years, the technical and theoretical foundations of CASL perfusion MRI techniques have evolved from feasibility studies into practical usage. Currently existing methodologies are sufficient to make reliable and clinically relevant observations which complement structural assessment using MRI. Future technical improvements should further reduce the acquisition times for CASL perfusion MRI, while increasing the slice coverage, resolution and stability of the images. These techniques have a broad range of potential applications in clinical and basic research of brain physiology, as well as in other organs.
目前有几种方法可用于利用磁共振成像（MRI）测量脑灌注及相关血流动力学参数。一类技术利用电磁标记的动脉血中的水作为一种用于血流测量的非侵入性可扩散示踪剂。这种电磁标记的示踪剂具有T1衰减率，其足够长，以便能够检测到组织和微血管的灌注情况。另外，电磁动脉自旋标记（ASL）可用于获得定性的灌注对比，以检测血流变化，这类似于在血氧水平依赖性功能磁共振成像（BOLD fMRI）中使用磁化率对比来检测大脑中的功能激活。利用MRI这种非侵入性且广泛可用的方式来获取血流图的能力，应能极大地提高血流测量作为一种深入了解广泛的血流动力学相关生理学和病理生理学手段的实用性。本文描述了与使用一种特定形式的ASL（其中动脉血中的水被连续标记，称为连续动脉自旋标记（CASL））生成定量血流图相关的生物物理学考量。技术进步使得使用CASL进行多层灌注成像成为可能，且对运动和通过时间效应的敏感性降低。现在，在包括急性中风、慢性脑血管疾病、退行性疾病和癫痫等多种临床环境中，都能可靠地获得可解读的脑灌注图像。在过去几年中，CASL灌注MRI技术的技术和理论基础已从可行性研究发展到实际应用。目前现有的方法足以做出可靠且与临床相关的观察结果，这些结果可补充使用MRI进行的结构评估。未来的技术改进应能进一步缩短CASL灌注MRI的采集时间，同时增加图像的切片覆盖范围、分辨率和稳定性。这些技术在脑生理学的临床和基础研究以及其他器官中具有广泛的潜在应用。
