Kjølby B F, Østergaard L, Kiselev V G
CFIN, Department of Neuroradiology, Aarhus University Hospital, Aarhus, Denmark.
Magn Reson Med. 2006 Jul;56(1):187-97. doi: 10.1002/mrm.20920.
The concentration of MRI tracers cannot be measured directly by MRI and is commonly evaluated indirectly using their relaxation effect. This study develops a comprehensive theoretical model to describe the transverse relaxation in perfused tissue caused by intravascular tracers. The model takes into account a number of individual compartments. The signal dephasing is simulated in a semianalytical way by embedding Monte Carlo simulations in the framework of analytical theory. This approach yields a tool for fast, realistic simulation of the change in the transverse relaxation. The results indicate that the relaxivity of intravascular contrast agents depends significantly on the host tissue. This agrees with experimental data by Johnson et al. (Magn Reson Med 2000;44:909). In particular, the present results suggest a several-fold increase in the relaxivity of Gd-based contrast agents in brain tissue compared with bulk blood. The enhancement of relaxation in tissue is due to the contrast in magnetic susceptibility between blood vessels and parenchyma induced by the presence of paramagnetic tracer. Beyond the perfusion measurements, the results can be applied to quantitation of functional MRI and to vessel size imaging.
磁共振成像（MRI）示踪剂的浓度无法通过MRI直接测量，通常利用其弛豫效应进行间接评估。本研究建立了一个综合理论模型，用于描述血管内示踪剂引起的灌注组织中的横向弛豫。该模型考虑了多个独立的隔室。通过将蒙特卡罗模拟嵌入解析理论框架中，以半解析方式模拟信号去相位。这种方法产生了一种用于快速、逼真地模拟横向弛豫变化的工具。结果表明，血管内造影剂的弛豫率显著取决于宿主组织。这与Johnson等人的实验数据一致（《磁共振医学》2000年；44:909）。特别是，目前的结果表明，与全血相比，基于钆的造影剂在脑组织中的弛豫率提高了几倍。组织中弛豫的增强是由于顺磁性示踪剂的存在引起血管与实质之间的磁化率差异。除了灌注测量外，这些结果还可应用于功能MRI的定量分析和血管大小成像。
