Robust Cardiac-gated MRI using Ultrasound Sensors

项目摘要 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以帮助 使采集过程与心脏运动同步。由于这种同步，重建 图像代表给定的心脏时相，大多没有与心脏相关的运动。不正确的检测， 触发会导致来自不同心脏相位的数据混合到同一图像中，从而导致运动伪影 以及图像质量的损失。最常见的是，采用心电图（ECG）导联来检测 心脏的电活动，并产生这样的触发，允许MRI采集和心脏运动 保持同步除了皮肤烧伤的小风险外，使用ECG导联进行以下检查的主要挑战之一是 心脏MRI来自磁流体动力学（MHD）效应，其倾向于使ECG信号失真。 由于洛伦兹力，并以类似于霍尔效应的方式，血液中的正负离子 当它们通过磁场时，倾向于在不同的方向上弯曲，从而产生电荷分离， 电场会破坏ECG信号。因为主动脉携带着大量的快速移动的 血液，它往往是腐蚀电场的主要来源，问题变得更加重要 在更高的场强下。扫描仪操作，特别是梯度切换，也可能进一步破坏ECG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心脏成像。 触发，但通常遭受较低的空间和时间分辨率。可能是因为这个原因，心脏- 门控成像代表了临床心脏MR（CMR）扫描的绝大多数。ECG的替代方法 用于CMR门控的引线包括脉搏血氧仪，其通常采取夹在手指周围的传感器的形式。 虽然它具有对MHD效应不敏感的优点，但脉搏血氧仪和其他形式的外周血 门控检测具有延迟的心跳，即压力波到达检测点的时间， ECG检测更加直接和即时。有一颗完全正常的心脏，这种延迟是微不足道的。 但心脏病患者往往有不规则的心跳和立即检测到一个 收缩允许MRI采集以更合理的方式反应，具有更好的图像质量结果。 本项目介绍了一种替代心电图监测的心脏门控MRI， 心脏的运动。提出了一种基于超声的传感器，沿着3D打印胶囊， 应用于皮肤和现实的临床工作流程。在扫描床滑入扫描仪之前， 磁场很弱，MHD效应很小，我们将用ECG检测来验证我们的方法， 参比标准品当病人进入扫描仪时，心电图跟踪会被降级，但我们的MR- 兼容的传感器将不受影响。在扫描仪中的病人，我们将展示一种改进的 与使用ECG导联相比，使用我们的传感器检测心脏触发并提高图像质量。

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.