Taheri Saeid, Sood Rohit
Department of Neurology, Health Sciences Center and BRaIN center, University of New Mexico, Albuquerque, NM 87131, USA.
Magn Reson Imaging. 2007 Jun;25(5):613-25. doi: 10.1016/j.mri.2006.09.029. Epub 2006 Nov 28.
Blood-brain barrier (BBB) plays an important role in the pathophysiology of many central nervous system disorders. In the past, a number of laboratory techniques have been proposed to quantify permeability coefficient, k(i), an important index of barrier function. Recently, MRI has been used to estimate k(i) based on the unidirectional tracer kinetics model in one compartment as proposed by Patlak et al. and has been found to be in good agreement with the gold standard quantitative autoradiography technique. Rapid data acquisition, a prerequisite of this MRI-based technique, causes a compromise in spatial resolution resulting in partial volume (PV) averaging, an effect that is seldom explicitly compensated for in quantitative neuroimaging studies. This may have profound effect on the reliability of estimates obtained using quantitative methods. Existing PV compensation techniques that use complex statistical algorithms perform corrections on stationary images. In this proof-of-principle study, the effect of PV averaging on BBB permeability coefficient has been evaluated using a simulation model, and a postprocessing technique that makes use of dynamic information has been proposed for PV compensation in order to improve the reliability of this quantitative method.
A computer simulation model is presented, which evaluates the effect of PV averaging on permeability coefficient estimates. Beginning with a known k(i), a PV compensation technique is proposed, which aims at correcting calculated k(i) to obtain the original estimate. The application of the PV compensation technique is demonstrated in a rat stroke brain model. Magnetic resonance imaging experiments were performed in Wistar rats (n=2) on a 4.7-T scanner. After acquiring localizer, T2-weighted and diffusion-weighted images, a rapid T1 mapping protocol was implemented to acquire one pre-gadolinium-diethylenetriaminepentaacetic acid baseline data set followed by a series of postinjection data sets. The data were postprocessed without and with application of PV compensation technique to obtain a k(i) estimate.
The issue of PV averaging as a result of limited spatial resolution is often not addressed in quantitative MRI studies. In this work, simulation experiments have provided useful insight into the PV effects on permeability coefficient estimate. The findings of the simulation experiments agree well with the results obtained from MR experiments. Results from the MR experiments suggest that it may be important to perform PV compensation in order to improve the reliability of permeability coefficient estimates. Future work involves classification of tissue component into gray and white matter and CSF to improve the accuracy of the compensation technique and to investigate repeatability of the technique in a larger group of animals.
血脑屏障（BBB）在许多中枢神经系统疾病的病理生理学中起着重要作用。过去，已经提出了许多实验室技术来量化通透系数k(i)，这是屏障功能的一个重要指标。最近，磁共振成像（MRI）已被用于根据Patlak等人提出的单室单向示踪剂动力学模型来估计k(i)，并且已发现其与金标准定量放射自显影技术高度一致。快速数据采集是这种基于MRI的技术的一个先决条件，这会导致空间分辨率下降，从而产生部分容积（PV）平均效应，而在定量神经成像研究中很少明确补偿这种效应。这可能会对使用定量方法获得的估计值的可靠性产生深远影响。现有的使用复杂统计算法的PV补偿技术对静态图像进行校正。在这项原理验证研究中，使用模拟模型评估了PV平均对BBB通透系数的影响，并提出了一种利用动态信息的后处理技术进行PV补偿，以提高这种定量方法的可靠性。
提出了一个计算机模拟模型，用于评估PV平均对通透系数估计值的影响。从已知的k(i)开始，提出了一种PV补偿技术，旨在校正计算得到的k(i)以获得原始估计值。在大鼠中风脑模型中展示了PV补偿技术的应用。在4.7-T扫描仪上对Wistar大鼠（n = 2）进行磁共振成像实验。在获取定位像、T2加权像和扩散加权像后，实施快速T1映射协议以获取一组注射钆喷酸葡胺前的基线数据集，随后是一系列注射后的数据集。对数据进行后处理，分别在不应用和应用PV补偿技术的情况下获得k(i)估计值。
在定量MRI研究中，由于空间分辨率有限导致的PV平均问题常常未得到解决。在这项工作中，模拟实验为PV对通透系数估计值的影响提供了有用的见解。模拟实验的结果与MR实验获得的结果非常吻合。MR实验结果表明，进行PV补偿对于提高通透系数估计值的可靠性可能很重要。未来的工作包括将组织成分分类为灰质、白质和脑脊液，以提高补偿技术的准确性，并在更大的动物群体中研究该技术的可重复性。
