Liu Yu, Liu Jingfei, Fite Brett Z, Foiret Josquin, Ilovitsh Asaf, Leach J Kent, Dumont Erik, Caskey Charles F, Ferrara Katherine W
Department of Biomedical Engineering, University of California, Davis, CA 95616, United States of America.
Phys Med Biol. 2017 May 21;62(10):4083-4106. doi: 10.1088/1361-6560/aa6674. Epub 2017 Apr 20.
Non-invasive, quantitative methods to assess the properties of biological tissues are needed for many therapeutic and tissue engineering applications. Magnetic resonance elastography (MRE) has historically relied on external vibration to generate periodic shear waves. In order to focally assess a biomaterial or to monitor the response to ablative therapy, the interrogation of a specific region of interest by a focused beam is desirable and transient MRE (t-MRE) techniques have previously been developed to accomplish this goal. Also, strategies employing a series of discrete ultrasound pulses directed to increasing depths along a single line-of-sight have been designed to generate a quasi-planar shear wave. Such 'supersonic' excitations have been applied for ultrasound elasticity measurements. The resulting shear wave is higher in amplitude than that generated from a single excitation and the properties of the media are simply visualized and quantified due to the quasi-planar wave geometry and the opportunity to generate the wave at the site of interest. Here for the first time, we extend the application of supersonic methods by developing a protocol for supersonic transient magnetic resonance elastography (sst-MRE) using an MR-guided focused ultrasound system capable of therapeutic ablation. We apply the new protocol to quantify tissue elasticity in vitro using biologically-relevant inclusions and tissue-mimicking phantoms, compare the results with elasticity maps acquired with ultrasound shear wave elasticity imaging (US-SWEI), and validate both methods with mechanical testing. We found that a modified time-of-flight (TOF) method efficiently quantified shear modulus from sst-MRE data, and both the TOF and local inversion methods result in similar maps based on US-SWEI. With a three-pulse excitation, the proposed sst-MRE protocol was capable of visualizing quasi-planar shear waves propagating away from the excitation location and detecting differences in shear modulus of 1 kPa. The techniques demonstrated here have potential application in real-time in vivo lesion detection and monitoring, with particular significance for image-guided interventions.
许多治疗和组织工程应用都需要非侵入性的定量方法来评估生物组织的特性。磁共振弹性成像（MRE）历来依赖外部振动来产生周期性剪切波。为了局部评估生物材料或监测对消融治疗的反应，利用聚焦束对特定感兴趣区域进行询问是很有必要的，并且之前已经开发了瞬态MRE（t-MRE）技术来实现这一目标。此外，还设计了一系列沿单一视线方向指向增加深度的离散超声脉冲的策略，以产生准平面剪切波。这种“超音速”激发已应用于超声弹性测量。由于准平面波几何形状以及在感兴趣部位产生波的机会，所产生的剪切波幅度高于单次激发产生的剪切波，并且介质的特性可以简单地可视化和量化。在此，我们首次通过开发一种使用能够进行治疗性消融的磁共振引导聚焦超声系统的超音速瞬态磁共振弹性成像（sst-MRE）方案，扩展了超音速方法的应用。我们应用新方案，使用生物学相关内含物和组织模拟体模在体外量化组织弹性，将结果与通过超声剪切波弹性成像（US-SWEI）获得的弹性图进行比较，并用机械测试验证这两种方法。我们发现，一种改进的飞行时间（TOF）方法能够有效地从sst-MRE数据中量化剪切模量，并且基于US-SWEI，TOF方法和局部反演方法都能得到相似的弹性图。通过三脉冲激发，所提出的sst-MRE方案能够可视化从激发位置传播开的准平面剪切波，并检测到1 kPa的剪切模量差异。这里展示的技术在实时体内病变检测和监测方面具有潜在应用，对图像引导干预具有特别重要的意义。
