Overhauser Enhanced Magnetic Resonance Imaging (OMRI)

项目2：Overhauser增强磁共振成像。（OMRI）该方法通过结合低磁场下EPR检测的灵敏度和动态核极化（DNP）来显著增强低磁场下的MR图像强度，从而提供解剖学上共配准的pO 2图像。因此，扫描仪是EPR成像和MR成像的组合，我们使用的造影剂的弛豫特性与pO 2呈线性相关。早期的研究证明了该技术的定量pO 2成像能力。然而，需要解决两个方面的这种技术在体内成像应用的进展与最小的伪影。pO 2图像质子T1的变化归一化。来自OMRI研究的水基1H MR图像的增强取决于：a）造影剂的浓度; B）用于激发造影剂的RF功率; c）组织pO 2;和d）固有1H T1弛豫时间。其中，组织中质子的T1随检查区域而变化，因此可能导致pO 2估计的显著误差。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校正这种伪影，从该脉冲序列可以独立估计基于组织的T1值的变化。OMRI成像中的一个独特特征是，照射未成对自旋以产生所需的Ovehauser增强使用占空比接近50%的高RF功率总测量时间。这导致SAR（比吸收率）远高于FDA规定的人类使用SAR。因此，任何预期的人体应用都需要努力降低OMRI中的SAR。在我们正在计划的几种策略中，我们最近成功测试的一种是使用局部感应回路，这是一种无源调谐谐振器（例如表面回路），其调谐到与主体积谐振器相同的频率，保持靠近感兴趣区域（ROI）。这样的无源调谐元件以其他地方的RF通量为代价将RF通量吸引到靠近其自身，并且与在环外部的位置相比产生增强的图像强度。该策略已在体模和体内进行了测试，并产生RF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RF功率，但在ROI处产生增强的图像强度。RF的这种聚焦效应将允许以大大降低的SAR对动物模型中的局部肿瘤进行有效的OMRI成像和血氧测定。我们还在试验使用正交发射和接收线圈，以将功率要求进一步降低2倍，并使有效SAR远低于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.