Robust Cardiac-gated MRI using Ultrasound Sensors

项目总结 为了使心脏MRI脉冲序列正常工作，通常必须获得信号或触发器以帮助 使采集过程与心脏运动同步。作为这种同步的结果，重建 图像代表给定的心脏时相，大部分没有心脏相关的运动。不适当地检测到此类 触发器会导致来自不同心脏时相的数据混合到同一图像中，从而导致运动伪影 和图像质量的损失。最常见的是利用心电图导联来检测 心脏的电活动，并产生这样的触发，允许磁共振成像采集和心脏运动 保持同步。除了皮肤灼伤的小风险外，使用心电图导联治疗烧伤的主要挑战之一 心脏磁共振成像是由磁流体效应引起的，容易使心电信号失真。 由于洛伦兹力，以类似霍尔效应的方式，血液中的正离子和负离子 当它们在磁场中移动时倾向于沿不同的方向弯曲，从而产生电荷分离和 容易损坏心电信号的电场。因为大动脉携带着大量快速移动的 血液，它往往是腐蚀电场的主要来源，这个问题变得更加严重 在较高的场强下。扫描仪操作，尤其是梯度切换，也会进一步破坏心电信号。 与心脏门控成像相反，实时心脏成像不一定需要可靠的 触发因素，但通常存在空间和时间分辨率较低的问题。大概是因为这个原因，心脏- 门控成像占临床心脏磁共振(CMR)扫描的绝大多数。心电的替代品 CMR门控的引线包括脉搏血氧计，它通常采用夹在手指上的传感器的形式。 虽然它的优点是对磁流体效应不敏感，但脉搏血氧仪等形式的外周 门控检测带有延迟的心跳，即压力波到达检测点的时间，而不是 对于心电信号的检测，这更加直接和即时。如果有一个完全正常的心脏，这样的延迟不会有多大影响。 没有任何后果，但心脏病患者经常有不规则的心跳和立即检测到 收缩可以使MRI采集以更合理的方式做出反应，获得更好的图像质量。 本项目介绍了一种心门磁共振心电监测的替代方案，该方案检测到 心脏直接运动。提出了一种基于超声波的传感器，以及3D打印胶囊，用于快速 适用于皮肤和现实的临床工作流程。在患者表滑入扫描仪之前，因为 磁场很弱，磁流体效应很小，我们将通过心电检测来验证我们的方法 参考标准。当病人进入扫描仪时，心电轨迹会退化，但我们的磁共振- 兼容传感器将保持不受影响。对于扫描仪中的患者，我们将展示改进的 与使用心电导联相比，使用我们的传感器可以检测心脏触发并改善图像质量。

Project summary For cardiac MRI pulse sequences to function properly, a signal or trigger must typically be obtained to help synchronize the acquisition process with the cardiac motion. As a result of such synchronization, reconstructed images represent a given cardiac phase, mostly free of cardiac-related motion. Improper detection of such triggers would cause data from different cardiac phases to mix into a same image, leading to motion artifacts and loss of image quality. Most commonly, electrocardiogram (ECG) leads are employed to detect the electrical activity of the heart and to generate such triggers, allowing the MRI acquisition and the heart motion to remain in sync. Besides a small risk for skin burns, one of the main challenges with using ECG leads for cardiac MRI comes from the magnetohydrodynamic (MHD) effect, which tends to distort the ECG signal. Because of the Lorenz force, and in a manner similar to the Hall effect, the positive and negative ions in blood tend to curve in different directions as they move through a magnetic field, creating a charge separation and an electric field that tends to corrupts the ECG signal. Because the aorta carries a large amount of fast-moving blood, it tends to be the main source of corrupting electrical fields, and the problem becomes more significant at higher field strength. Scanner operation, especially gradient switching, can also further corrupt ECG signals. Real-time cardiac imaging, as opposed to cardiac-gated imaging, does not necessarily require reliable triggers, but typically suffers from lower spatial and temporal resolution. Presumably for this reason, cardiac- gated imaging represents the overwhelming bulk of clinical cardiac MR (CMR) scans. Alternatives to ECG leads for CMR gating include pulse oximetry, which typically takes the form of a sensor clipped around a finger. While it has the advantage of being insensitive to the MHD effect, pulse oximetry and other forms of peripheral gating detect heartbeats with a delay, the time for the pressure wave to reach the detection point, as opposed to ECG detection that is more direct and immediate. With a perfectly regular heart, such delay would be of little to no consequence, but cardiac patients often have irregular heartbeats and immediate detection of a contraction allows the MRI acquisition to react in a more sensible way, with better outcomes on image quality. The present project introduces an alternative to ECG monitoring for cardiac-gated MRI which detects cardiac motion directly. An ultrasound-based sensor is proposed, along with 3D-printed capsule for quick application to the skin and realistic clinical workflow. Before the patient table slides into the scanner, as the magnetic field is weak and the MHD effect is minimal, we will validate our approach with ECG detection as the reference standard. As the patient goes into the scanner, the ECG trace will be degraded but our MR- compatible sensor will remain unaffected. With patients in the scanner, we will demonstrate an improved detection of cardiac triggers and improved image quality using our sensors as compared to using ECG leads.