Overhauser Enhanced Magnetic Resonance Imaging (OMRI)

奥豪瑟增强磁共振成像 (OMRI)

• 批准号: 7292183
• 负责人: murali cherukuri
• 金额: --
• 依托单位: DIVISION OF BASIC SCIENCES - NCI
• 依托单位国家: 美国
• 资助国家: 美国
• 起止时间: 至
• 项目状态: 未结题
• 来源: https://reporter.nih.gov/project-details/7292183
• 关键词: Overhauser; Enhanced; Magnetic; Resonance; Imaging

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.

项目2：Overhauser增强磁共振成像。(OMRI)这种方法通过结合低场EPR检测的敏感性和动态核极化(DNP)来显著增强低磁场下的MR图像强度，提供解剖联合配准的pO2图像。因此，该扫描仪是EPR成像和MR成像的组合，我们使用的造影剂的松弛特性与pO2线性相关。早期的研究证明了这种技术的定量pO2成像能力。然而，要使这项技术在活体成像应用中取得进展，需要解决两个方面的问题，即对质子T1的变化进行归一化的pO2图像。来自OMRI研究的基于水的1H磁共振图像的增强取决于：a)造影剂的浓度；b)激发造影剂的射频功率；c)组织pO2；以及d)固有的1H T1弛豫时间。其中，组织中质子的T1随检查区域的不同而不同，因此可能导致估计pO2的重大误差。我们已经开发了图像数据收集策略来纠正这种伪影，通过使用新的脉冲序列，可以独立地估计基于组织的T1值的变化。使用感应环在OMRI中进行RF聚焦：OMRI成像的一个独特特征是，照射未配对的自旋以产生所需的Ovehauser增强，在占空比接近总测量时间的50%的情况下使用高RF功率。这导致了高的SAR(特定吸收率)，远远高于FDA规定的人类使用的吸收率。因此，任何预期的人类应用都需要努力减少OMRI中的SAR。在我们正在规划的几种策略中，我们最近成功测试的一种是使用局部感应环路，这是一种被动调谐谐振器(如表面环路)，它被调到与主体积谐振器相同的频率，保持在感兴趣区域(ROI)附近。这种被动调谐元件以牺牲其他地方的射频磁通为代价，将射频磁通吸引到接近其自身的位置，并且与环路外的位置相比，产生增强的图像强度。这一策略已经在模体和活体上进行了测试，并产生了射频聚焦，从而提高了局部区域的图像灵敏度。这将允许使用比正常低得多的射频功率，但在ROI处产生更高的图像强度。射频的这种聚焦效应将使动物模型中局部肿瘤的有效OMRI成像和血氧测量能够在大大降低的SAR下进行。我们还在试验使用正交发射和接收线圈，将功率需求进一步降低2倍，并将有效的合成孔径雷达很好地控制在FDA的限制之内。