Overhauser Enhanced Magnetic Resonance Imaging (OMRI)

项目二：检修用户增强磁共振成像。（OMRI）该方法通过结合EPR检测在低磁场下的灵敏度和动态核极化（DNP）提供解剖共配的pO2图像，显著增强了低磁场下的MR图像强度。因此，扫描仪是EPR成像和MR成像的结合，我们使用的对比剂的弛豫特性与pO2线性相关。早期的研究证明了该技术的定量pO2成像能力。然而，为了使这项技术在体内成像应用中取得进展，并使伪影最小化，需要解决两个方面的问题。pO2图像归一化质子T1的变化。OMRI研究中基于水的1H磁共振图像的增强取决于：a)造影剂的浓度；b)用于激发造影剂的射频功率；c)组织pO2；d)本征1H T1弛豫时间。其中，组织中质子的T1随检测区域的不同而变化，因此可能导致估计pO2的显着误差。我们已经开发了图像数据收集策略，通过使用新的脉冲序列来纠正这种伪影，从中可以独立估计基于组织的T1值的变化。在OMRI中使用电感回路进行射频聚焦：OMRI成像的一个独特之处在于，在占空比接近总测量时间50%的情况下，对未配对自旋进行照射以产生所需的Ovehauser增强。这导致高SAR（特定吸收率）远远高于FDA规定的人类使用。因此，任何潜在的人类应用都需要努力降低OMRI中的SAR。在我们计划的几种策略中，我们最近成功测试的一种是使用本地感应回路，这是一种被动调谐谐振器（如表面回路），其调谐到与主体积谐振器相同的频率，保持在感兴趣区域（ROI）附近。这种无源调谐元件以牺牲其他地方的RF通量为代价将RF通量吸引到自己附近，并且与环路外的位置相比，产生增强的图像强度。该策略已在幻影和体内进行了测试，并产生射频聚焦，从而提高了局部区域的图像灵敏度。这将允许使用显著低于正常的射频功率，但在ROI产生增强的图像强度。RF的这种聚焦效应将允许在大大降低的SAR下对动物模型中的局部肿瘤进行有效的OMRI成像和氧测量。我们还在试验使用正交发射和接收线圈，以进一步将功率要求降低2倍，并使有效SAR低于FDA限制。

Project 2: Overhauser enhanced Magnetic Resonance Imaging. (OMRI)This methodology provides anatomically co-registered pO2 images by combining the sensitivity of EPR detection at low fields and the dynamic nuclear polarization (DNP) to enhance the MR image intensity significantly at low magnetic fields. Therefore, the scanner is a combination of EPR Imaging and MR imaging and we use a contrast agent whose relaxation properties are linearly dependent on pO2. Earlier studies demonstrated the quantitative pO2 imaging capabilities of this technique. However, two aspects needed to be addressed for this technique to progress for in vivo imaging applications with minimal artifacts.pO2 images normalized for changes in proton T1. The enhancement of the water 1H based MR images from OMRI studies depends upon: a) concentration of the contrast agent; b) RF power deposited to excite the contrast agent; c) tissue pO2; and d) intrinsic 1H T1 relaxation times. Of these, the T1 of the protons in tissue varies with the regions examined and hence may lead to significant errors in estimating pO2. We have developed image data collection strategies to correct for this artifact by using novel pulse sequences from which the changes in the tissue based T1 values can be independently estimated.Use of Inductive loop for RF Focusing in OMRI: One unique feature in OMRI imaging is that the irradiation of the unpaired spin to generate the required Ovehauser enhancement uses high RF power at a duty cycle of nearly 50% of the total measurement time. This leads to a high SAR (Specific Absorption Rate) well above the one prescribed for human use by the FDA. Any prospective human application therefore necessitates efforts to reduce SAR in OMRI. Among several strategies that we are planning, the one we have recently successfully tested is the use of local inductive loop, which is a passive tuned resonator (such as a surface loop) that is tuned to the same frequency as the main volume resonator, kept close to the region of interest (ROI). Such a passive tuned element draws RF flux close to itself at the expense of RF flux elsewhere, and produces enhanced image intensity compared to locations which are outside the loop. This strategy has been tested both on phantoms and in vivo, and produces RF focusing leading to improved image sensitivity at the localized region. This will allow the use of significantly lower RF power than normal, and yet produce enhanced image intensity at the ROI. Such a focusing effect of RF will allow effective OMRI imaging and oxymetry of topical tumors in animal models at much reduced SAR. We are also experimenting on the use of quadrature transmit and receive coils to reduce the power requirements further by factor of 2, and bring the effective SAR well under FDA limits.