【作者】 宁瑞鹏；

【导师】 李鲠颖；

【作者基本信息】 华东师范大学 ， 无线电物理， 2011， 博士

【摘要】 目前,永磁开放式MRI在HIFU治疗中的应用仍处在发展阶段,在系统软、硬件和成像方法等方面还有许多技术问题急需解决。本论文对术前、术中和术后等阶段存在的一些技术问题进行了研究,主要内容如下：1.具有快速增益切换功能的数字接收机在HIFU治疗的术前诊断中,需要尽可能精确地对病灶进行定位和边界勾勒。这就需要获得高分辨的MRI诊断图像,而常规的数字接收机将会面临动态范围不足的问题。本文提出了增益快速切换方法来提高接收机的动态范围,并研制出具有快速增益切换功能的数字接收机。对比实验证明,该方法可以有效提高数字接收机的动态范围。2.MRI的正交线圈数字合成方法为了对病灶进行精确定位和边界勾勒,需要高质量的MRI图像。永磁MRI的一个不足之处在于图像信噪比比较低。提高接收通道的灵敏度对解决这一问题是十分关键的。本文针对现有正交合成技术的不足,提出了正交线圈数字合成方法来提高永磁MRI图像信噪比,最后进行了实验验证。3.梯度系统延时的校正在HIFU术前诊断中,需要利用T2加权像对病灶进行精确定位和边界勾勒。获得T2加权像一般采用的FSE成像序列。FSE是一种比较成熟的成像序列,随着其自身的不断完善,一些仪器上的非理想因素逐渐成为影响FSE图像质量的关键。本文针对梯度系统延时对FSE成像的影响,提出了多通道独立延时触发方法来校正梯度延时的方法,并研制了具有多通道独立延时触发功能的脉冲序列发生器。最后对梯度延时校正效果进行了验证。4.多层同步成像技术研究在临床诊断中,特别是在肝脏病变的诊断中,T1加权像可以提供重要的诊断信息,因此被作为常规扫描之一。在HIFU治疗中,T1加权像不但可以提供术前诊断信息,还可用于术后对治疗效果进行评估。然而,在永磁MRI系统中进行T1加权成像需要较短的TR,这将导致在一个TR时间内扫描的层数受到限制。本文提出了采用多层同步成像技术来解决这一问题,并对现有的多层同步成像技术进行了必要的改进。论文介绍了多层同步成像的原理及实现方法,并给出计算机模拟和成像实验结果。5.MRI在HIFU术中监控的应用研究HIFU治疗过程中,医生需要根据靶点及其周围组织的温度来决定所施加的热剂量。本文在永磁MRI系统上,对质子共振频率测温方法进行了研究；并提出采用导航回波技术和动态匀场方法来克服永磁系统磁场长期、短期波动,以及磁场的空间分布随时间改变等问题。在永磁MRI系统上进行测温实验,实验结果表明,采用导航回波技术和动态匀场方法可以有效提高永磁MRI测温精度。更多还原

【Abstract】 Magnetic resonance imaging (MRI) guided high intensity focused ultrasound (HIFU) treatment has attracted a lot of attention in recent years. However, there are still many key technical problems to be overcome before the MRI-guided HIFU becomes practical. The studies in this thesis will focus on MRI equipment building-up and imaging techniques for guidance of HIFU treatment. The MRI system is equipped with U-type permanent magnet to fulfill the requirements of guiding the HIFU treatments. To be specific, the following items will be covered:1. A digital receiver with fast gain switching capabilityBefore HIFU treatment, high resolution MRI images are probably required for precise localization and outline of diseases. However, a typical MRI digital receiver cannot always meet the requirement of dynamic range in high resolution imaging due to ADC resolution restriction. In order to increase the dynamic range of the digital receiver, a method of fast gain switching is proposed, and a corresponding receiver unit is implemented. The effectiveness of the fast gain switching method and the performance of the proposed digital receiver are verified and demonstrated experimentally.2. Digital Combination of signals from quadrature MRI receiver coilsIn order to ensure sufficient accuracy of lesion localization, signal-to-noise ratio (SNR) of MRI images should be taken into account. An effective method of improving the SNR is using quadrature receiver coils (I-coil and Q-coil) to detect MRI signals. However, the sensitivity imbalance between I and Q receive channels will lead to unexpected quality degradation of final images. In order to address this imbalance problem, we propose a digital combination method to optimize SNR of final images, in which the weighting coefficients of I and Q channels are determined dynamically in K-space. It is demonstrated that the digital combination method is effective to enhance SNR of the final images acquired with quadrature MRI receiver coils.3. Correction of gradient delayFast spin echo (FSE) imaging sequence is typically used to obtain T2-weighted MRI images, which is supposed to provide good contrast between tumor and normal tissues. Unfortunately the FSE sequence is rather susceptible to imperfections in equipment and configuration--gradient delay is one of them. In order to correct the effects of the gradient delay, a novel method is proposed, in which the gradient channels are always triggered earlier than the radio-frequency channels. An MRI pulse programmer providing this independent trigger delay is developed, and the correction of the gradient delay is experimentally demonstrated.4. Simultaneous multi-slice imagingT1-weighted MR images can provide important information in preoperative diagnosis and postoperative evaluation, especially for liver disease. Generally, in low-field MRI scanners, a shorter TR is used for T1-weighted imaging. As a result, the allowable slice number in one TR is limited. In this paper, simultaneous multi-slice imaging (SMI) method is utilized to overcome this difficulty. Theory and specific implementation details are described, and the results of computer simulation and MRI experiments are reported.5. MRI monitoring techniquesDuring HIFU treatment, temperal temperature evaluation of disease and surrounding tissues is important. In this thesis, MR thermometry with proton resonance frequency is investigated on a low-field MRI scanner with U-type permanent magnet. Nevertheless, high openness of the U-type construction will cause the scanner to be severely sensitive to the variation of external electromagnetic field. Another challenge induced by the high openness is significant variation of Bo field distribution. In order to reduce or eliminate these effects on thermometry imaging, a series of corrections are treated, including navigator echo and dynamic shimming. Finally, a thermometry experiment is executed with these corrections, and the results are compared with those obtained without the corrections.更多还原