基于分段式长码的超声粘弹性成像方法研究

Tissue elasticity and viscosity are closely related to pathological changes. Quantitative measurement of tissue viscoelasticity has important medical applications. Ultrasound-based elastography, which has the advantages of real-time, noninvasive, low-cost, et al, has been investigated by a number of teams. However, the challenges are great before it gets widely clinical applications. Most of the ultrasound elasticity imaging techniques are limited by the resolution, penetration, clinical safety and integration with conventional imaging system, which require dedicated effort to strengthen the foundations of elastography. In this study, a new elastography imaging method is proposed, in which the truncated long code modulated pulse excitation takes the place of conventional pulse-echo ultrasound detection and the adaptive coded phase-locked loop technology is used to estimate the phase of the harmonic motion, the harmonically related low frequency sinusoidal vibration takes the place of single sinusoidal vibration to generate multiple frequencies shear waves. The idea of a truncated long code is that the coding time is extended to as long as the parameter estimation time window allows. The transmitted pulses are coded using different short codes from burst to burst and all the echoes are organized together coherently in the later estimation process to form a long code modulation. With this coded excitation, very good correlation property can be achieved, high resolution and deep penetration can be obtained, signal-to-noise ratio of the backscattered signal can be improved. The phases of the harmonically related shear waves will be estimated by adaptive coded phase-locked loop technology, which can improve the accuracy of phases estimation greatly. Neglecting the tissue viscosity can cause bias in the estimation of tissue elasticity. With the harmonic vibration, the elasticity and viscosity of tissue can be measured using the same detection data in our project. Strong frequency-dependent attenuation and nonlinear propagation in tissue distort the received coded waveform and degrade code performance. An adaptive compensation filter will be developed for given coded excitation based on the mathematical model of the backscattered signal.Theoretical research, phantom and in vitro experiments will be carried out to characterize the performance of our elastographic imaging method.

超声弹性成像技术对生物组织粘弹性的定量在临床诊断上具有重要意义,但在迈向临床实用化的过程中还面临诸多挑战。目前的超声弹性成像方法在检测分辨率、检测深度、与现有超声影像系统的集成、临床安全性等方面还存在问题亟待解决。本项目拟将分段式长码检测、自适应数字编码锁相技术、低频谐波激励引入到外加振子的超声弹性成像方法中。首先，利用分段式长码的编码增益和良好的相关特性，以期提高弹性成像系统的穿透力和空间分辨率；其次，采用基于自适应数字编码锁相技术的方法来提取剪切波相位，可以提高相位检测的精度；再次，采用低频谐波信号代替单一的正弦信号激励组织振动，实现不同频率的剪切波波速测量，从而实现粘性、弹性参数的测量。另外，我们还将根据回波信号对声场进行估计，采用自适应算法对回波信号进行补偿和校正，以减少组织的频率相关衰减、非线性效应对编码的影响。通过理论仿真分析、仿体、离体实验来检验该成像方法的关键性能指标。

通常组织发生病变会导致组织粘弹性的变化。因此，对组织的弹性、粘性进行定量的测量对疾病的临床诊断、治疗效果的评价等有一定的参考价值。基于外加振子的瞬态弹性成像技术应用机械振动在组织中产生剪切波，应用脉冲回波检测剪切波信息，进而估算组织弹性。该方法安全、结构简单、易于集成到现有的商业超声系统等优点，但该方法忽略了组织的粘性，且检测深度受限。. 本项目提出将通讯领域的扩谱编码技术应用到外加机械振子的瞬时弹性成像中，以增加瞬时弹性成像技术的检测深度并且使其同时测量组织的粘弹性。通过编码方法设计具有特定频谱分布的激励信号振动组织，在组织中产生包含多个频率成分的剪切波，且保证高频剪切波具有较高的能量，不仅可以同时检测多个频率剪切波波速，还可以提高高频剪切波的检测能力；剪切波波速检测方面，应用编码检测代替传统的脉冲回波检测，切实可以增加检测深度。理论、仿体、离体实验研究结果表明，本项目的研究给肥胖病人以及深层组织的粘弹性测量提供了一种可行的方法。. 项目执行期间所取得的研究成果主要集中在以下几个方面：（1）实验平台的建设和系统开发；（2）超声瞬时弹性成像回波信号处理算法；（3）谐波振动信号的编码方法研究；（4）组织振动信号的编码检测方法研究；（5）大鼠非酒精性脂肪性肝炎分期实验；（6）其他研究成果。. 本项目的研究基本完成了计划书的内容，取得了优秀的成绩。该项目的研究成果，项目组发表了1篇SCI 论文， 5篇EI 论文，1篇中文期刊，1项专利。参加国际国内会议5次，并且在2015 的IEEE International Ultrasonics Symposium (IUS)会议上以口头报告的形式展示了我们的研究成果。