Innovative Technology for MRI Guided Procedures

背景 影像导引是心脏病微创诊断和治疗的重要工具。许多过去需要开胸手术的手术现在可以经皮进行。目前的做法是使用X射线透视进行图像引导。该技术提供了适用于程序性指导的高空间和时间分辨率。然而，X射线透视有一些明显的缺点。软组织(如(心肌)在X射线图像上显示不好，这阻碍了需要精确组织定位的程序的指导，例如心肌活组织检查。它还会使患者和操作员暴露在电离辐射中。结构性心脏病患者一生中要经历许多手术，累积的电离辐射剂量以及随之而来的癌症风险可能是巨大的。为了克服X-射线引导的局限性，人们对转移到MRI引导的操作非常感兴趣。 MRI提供卓越的软组织可视化、灵活的图像对比度，并且不会使患者暴露在电离辐射中，但使用MRI进行程序指导还存在其他挑战。在磁共振成像技术计划中，我们专注于两个主要挑战：成像速度和成像安全。传统的核磁共振成像可能需要几秒钟才能获得一张图像，对于依赖高帧速率成像(每秒几帧)的程序性指导来说，这太慢了。为了弥补这一点，我们获取欠采样数据集，并实时应用新的重建技术来获得足够的帧速率。我们开发了专门的成像序列，允许交互控制成像参数，如图像方向、帧速率和图像对比度。标准的导尿术实验室程序依赖于长的金属装置(例如。导丝和导管)以达到血管系统或心脏中的特定靶点。由于核磁共振期间沉积的射频能量会造成组织损伤，这些长金属设备很容易受到显著的加热。无法获得安全和可见的设备是MRI引导干预领域的一个限制。我们的目标是通过开发在患者体内沉积较少射频能量的成像技术来缓解设备发热问题。 2019财年的进展情况 我们继续开发MRI技术，以便能够在MRI指导下进行心血管插管手术。我们已经开发了低能量成像方法，以提高标准的商用介入设备在MRI成像过程中的安全性。这些方法降低了射频占空比，从而限制了实时成像期间的沉积能量和金属设备加热。通过这种方法，我们完成了一项首次人体研究，使用了一种商用金属导丝和一种安全的成像序列，用于MRI引导的右心导管术。 为了进一步提高设备的安全性，我们对我们的MRI系统进行了修改，使其在0.55T下运行，同时保留了能够实时成像的当代硬件。改进的低场安全配置扩大了我们对具有标准成像序列的金属设备的使用。自从改进我们的MRI系统以来，我们定期使用金属导丝在MRI引导下对转诊为临床右心导管术的患者进行心血管插管。这种高性能低场磁共振成像系统的开发可能会使使用标准金属设备进行更复杂的程序成为可能。 我们继续开发具有在线图像失真校正的实时螺旋成像。我们已经提出了这些方法，包括螺旋平衡稳态自由进动成像用于诊断心血管成像。这些方法是顺从的，因为它们利用了低场下的物理性质。这些螺旋成像技术已被应用于低场图像信号的恢复和介入环境下的快速采集。我们还进行了对比研究，以验证0.55T定量心脏MRI在健康志愿者和转诊为临床诊断心脏MRI的已知心血管疾病患者中的准确性。 我们参与了介入性MRI辅助设备和软件的改进。我们设计并实现了一个用于实时血流定量的交互式前端软件，它包括图像数据的连续传输和每搏心输出量的计算。 低场磁共振也为肺功能成像提供了重要的机会。众所周知，由于空气-组织界面造成的扭曲，肺部MRI具有挑战性，而低场强提供了更高质量的肺实质成像，使MRI能够应用于肺功能的测量。核磁共振技术计划已经开发出使用低场磁共振成像肺的方法。这项新技术允许在MRI插管环境中进行全面的心肺评估。 我们在MRI引导的干预方面的工作是与Robert Lederman博士和心血管干预实验室合作完成的。

Background Image guidance is an important tool used for minimally invasive diagnostic and therapeutic procedures in cardiology. Many procedures that required open chest surgery in the past can now be performed percutaneously. Current practice uses X-Ray fluoroscopy for image guidance. This technique provides high spatial and temporal resolution suitable for procedural guidance. However, X-Ray fluoroscopy has some significant drawbacks. Soft tissues (eg. cardiac muscle) are not well visualized on X-Ray images, which hampers the guidance of procedures that require precise tissue localization such as myocardial biopsy. It also exposes the patients and operators to ionizing radiation. Patients with structural heart disease undergo many procedures throughout their lifetime and the cumulative ionizing radiation dose, and ensuing risk of developing cancer, can be substantial. To overcome the limitations of X-Ray guidance, there is a great interest in moving to MRI guidance of procedures. MRI provides superior soft tissue visualization, flexible image contrast, and does not expose the patient to ionizing radiation, but there are other challenges associated with the use of MRI for procedural guidance. In the MRI Technology Program, we are focused on two main challenges: imaging speed and imaging safety. Conventional MRI imaging can take seconds to acquire a single image, which is too slow for procedural guidance which depends on high frame rate imaging (several frames per second). To compensate for this, we acquire undersampled data sets and apply novel reconstruction techniques in real-time to achieve sufficient frame rates. We develop specialized imaging sequences that allow interactive control of imaging parameters, such as image orientation, frame rate and image contrast. Standard catheterization lab procedures rely on long metallic devices (eg. guidewires and catheters) to reach a particular target in the vasculature or heart. These long metallic devices are susceptible to significant heating due to the radiofrequency energy deposited during MRI causing tissue damage. The unavailability of safe and visible devices is a limitation in the field of MRI-guided interventions. We aim to mitigate the device heating problem by developing imaging technologies that deposit less radiofrequency energy in the patient. Progress in fiscal year 2019 We continue to develop MRI technology to enable MRI-guidance of cardiovascular catheterization procedures. We have developed lower energy imaging methods to improve safety of standard commercially available interventional devices during MRI imaging. These methods reduce the radiofrequency duty cycle, thereby limiting deposited energy and metallic device heating during real-time imaging. With this approach, we completed a first-in-human study using one commercial metallic guidewire with a single safe imaging sequence for MRI-guided right heart catheterization. To improve device safety even further, we modified our MRI system to operate at 0.55T while retaining the contemporary hardware capable of real-time imaging. The improved safety profile of the lower field has expanded our use of metallic devices with standard imaging sequences. Since modifying our MRI system, we regularly perform MRI-guided cardiovascular catheterizations with metallic guidewires in patients referred for clinical right heart catheterization. This development of a high-performance low field MRI system can potentially enable more complex procedures with standard metallic devices. We continue our development of real-time spiral imaging with inline image distortion correction. We have advanced these methods to include spiral balanced steady-state free precession imaging for diagnostic cardiovascular imaging. These methods are amenable because they exploit the physical properties at low field. These spiral imaging techniques have been applied to recover image signal at low field and for fast acquisition within the interventional setting. We have also performed comparison studies to validate the accuracy of quantitative cardiac MRI at 0.55T in healthy volunteers and patients with known cardiovascular disease referred for clinical diagnostic cardiac MRI. We participate in the improvement of accessory devices and software for interventional MRI. We have designed and implemented an interactive front end software for real-time flow quantification, which includes the continuous streaming of imaging data and computation using of beat-to-beat cardiac output. Low field MRI also offers significant opportunities for functional lung imaging. Lung MRI is notoriously challenging due to distortions caused by air-tissue interfaces, and low field provides higher quality imaging of lung parenchyma, enabling the application of MRI for measurement of lung function. The MRI Technology program has developed methods to image the lung using low field MRI. This new technology allows comprehensive cardiopulmonary evaluation in the MRI catheterization environment. Our work in MRI guided interventions is done in collaboration with Dr. Robert Lederman and the Laboratory of Cardiovascular Interventions.