Boxerman J L, Hamberg L M, Rosen B R, Weisskoff R M
NMR Center, Massachusetts General Hospital, Department of Radiology, USA.
Magn Reson Med. 1995 Oct;34(4):555-66. doi: 10.1002/mrm.1910340412.
A particularly powerful paradigm for functional MR imaging of microvascular hemodynamics incorporates paramagnetic materials that create significant image contrast. These include exogenous (lanthanide chelates) and endogenous (deoxygenated hemoglobin) agents for mapping cerebral blood volume and neuronal activity, respectively. Accurate interpretation of these maps requires an understanding of the biophysics of susceptibility-based image contrast. The authors developed a novel Monte Carlo model with which the authors quantified the relationship between microscopic tissue parameters, NMR imaging parameters, and susceptibility contrast in vivo. The authors found vascular permeability to water and the flow of erythrocytes to be relatively unimportant contributors to susceptibility-induced delta R2. However, pulse sequence, echo time, and concentration of contrast agent have profound effects on the vessel size dependence of delta R2. For a model vasculature containing both capillaries and venules, the authors predicted a linear volume fraction dependence for physiological volume changes based on recruitment and dilation, and a concentration dependence that is nonlinear and pulse sequence dependent. Using the model, the authors demonstrated that spin echo functional images have greater microvascular sensitivity than gradient echo images, and that the specifies of the volume fraction and concentration dependence of transverse relaxivity change should allow for robust mapping of relative blood volume. The authors also demonstrated excellent agreement between the predictions of their model and experimental data obtained from the serial injection of superparamagnetic contrast agent in a rat model.
一种用于微血管血流动力学功能磁共振成像的特别强大的范例，纳入了能产生显著图像对比度的顺磁性材料。这些材料包括外源性（镧系螯合物）和内源性（脱氧血红蛋白）试剂，分别用于绘制脑血容量和神经元活动图。准确解读这些图谱需要了解基于磁化率的图像对比度的生物物理学。作者开发了一种新型蒙特卡洛模型，通过该模型量化了微观组织参数、核磁共振成像参数和体内磁化率对比度之间的关系。作者发现水的血管渗透性和红细胞的流动对磁化率诱导的ΔR2贡献相对较小。然而，脉冲序列、回波时间和造影剂浓度对ΔR2的血管大小依赖性有深远影响。对于包含毛细血管和小静脉的模型脉管系统，作者基于募集和扩张预测了生理体积变化的线性体积分数依赖性，以及非线性且依赖脉冲序列的浓度依赖性。使用该模型，作者证明自旋回波功能图像比梯度回波图像具有更高的微血管敏感性，并且横向弛豫率变化的体积分数和浓度依赖性的具体情况应允许对相对血容量进行稳健的映射。作者还证明了他们模型的预测与在大鼠模型中通过连续注射超顺磁性造影剂获得的实验数据之间具有极好的一致性。
