Vocal Tract Area Function for Vowels Using Three-Dimensional Magnetic Resonance Imaging. A Preliminary Study

OBJECTIVE: To assess whether magnetic resonance imaging (MRI) allows the vocal tract (VT) area function to be determined for a normal male speaker. METHOD: VT shapes were acquired using MRI during sustained production of French points vowels: /i/, /a/, /u/. Cross-sectional areas were measured from a series of planes spaced at intervals of 1 cm along the length of the VT and were used as input in a previously described VT model to simulate the vowels. The first three formant frequencies, F1, F2, and F3, computed from the MRI-measured VT model were compared with subject's natural formant frequencies. RESULTS: Including piriform sinuses, calculated formants differed from measured formants F1, F2, and F3, respectively, for /i/ by -3.5%, +7.7%, and +27.5%; for /a/ by +11% +19.5%, and -4.3%; and for /u/ by +.9%, +23.4%, and +9.6%. Excluding piriform sinuses, calculated formants differed from measured formants F1, F2, and F3, respectively, for /i/ by -3.5%, +12%, and +28%, and for /u/ by +10.1%, +26.8%, and +13.7% The piriform sinuses were not visualized for /a/ on MRI. CONCLUSIONS: MRI is a noninvasive technique that allows VT imaging and determination of VT area function for a normal male speaker. Several possible sources of discrepancies are as follows: variability of the articulation, difficulties in assessment of VT wall boundaries, role of the piriform sinuses, and VT length.     

Towards the Automatic Study of the Vocal Tract From Magnetic Resonance Images

Over the last few decades, researchers have been investigating the mechanisms involved in speech production. Image analysis can be a valuable aid in the understanding of the morphology of the vocal tract. The application of magnetic resonance imaging to study these mechanisms has been proven to be reliable and safe. We have applied deformable models in magnetic resonance images to conduct an automatic study of the vocal tract; mainly, to evaluate the shape of the vocal tract in the articulation of some European Portuguese sounds, and then to successfully automatically segment the vocal tract's shape in new images. Thus, a point distribution model has been built from a set of magnetic resonance images acquired during artificially sustained articulations of 21 sounds, which successfully extracts the main characteristics of the movements of the vocal tract. The combination of that statistical shape model with the gray levels of its points is subsequently used to build active shape models and active appearance models. Those models have then been used to segment the modeled vocal tract into new images in a successful and automatic manner. The computational models have thus been revealed to be useful for the specific area of speech simulation and rehabilitation, namely to simulate and recognize the compensatory movements of the articulators during speech production.     

Vocal Tract Resonance Analysis of Aging Voice Using Long-Term Average Spectra

This study is the first to use long-term average spectra (LTAS) to investigate resonance characteristics of dynamic speech in young adulthood and old age. A total of 80 speakers participated, divided equally by age group and gender. All elderly speakers were healthy, active members of the community. Measurement of the first three spectral peaks in LTAS from the first paragraph of the Rainbow Passage revealed significant lowering of peak 1 from young adulthood to old age in both men and women. Peaks 2 and 3 also lowered significantly across the adult lifespan in women and showed a tendency to lower in men. These acoustic findings are consistent with anatomic data suggesting that aging results in lengthening of the supraglottic vocal tract. Findings that women demonstrate more substantial lowering of spectral peaks with aging than men suggest that women may undergo more pronounced age-related lengthening of the supraglottic vocal tract. Alternatively, it is possible that elderly men systematically alter tongue position during vowel articulation while elderly women are less inclined to do so. Taken in conjunction with previous research, these findings suggest a "mixed model" of vocal tract resonance changes with aging in which an interaction exists between gender, the resonance effects of laryngeal lowering, and vowel articulatory patterns.     

Magnetic Resonance Imaging Measurement of Volume Magnetic Susceptibility Using a Boundary Condition

A magnetic resonance imaging method is described for measuring the magnetic susceptibility difference between two homogeneous macroscopic compartments in contact with each other. A boundary condition is derived for the interface of the two compartments. This boundary condition predicts that across the interface there is a resonant frequency jump, which is a function of interfacial orientation relative to B0 field and the difference in susceptibility of the two sides. Based on this relationship, the magnetic susceptibility difference between two materials can be obtained from MR gradient echo imaging using signals from both sides in the vicinity of the boundary. This method is demonstrated by solution phantom experiments.     

Nuclear Magnetic Resonance Imaging

Abstract

The advent of X-ray computed tomographic (CT) imaging revolutionized the evaluation of a wide range of pathological conditions by producing thin tomographic sections through the body with remarkable anatomical detail. By the early 1980s, X-ray CT was an established imaging modality, and a second computer-based form of imaging was emerging from the research laboratory into the clinic. The second wave of the imaging revolution has been the development of NMR imaging (usually referred to as magnetic resonance imaging, or MRI) and its acceptance as the preferred modality for much neurological and musculoskeletal imaging. MRI's soft-tissue contrast and resolution is superior to that of other imaging techniques, the low NMR signal from bone renders it superior to X-ray CT in many cases for images of the head and spine, it has more varied contrast possibilities than CT, and can image in any plane without repositioning the patient. In spite of the high cost of purchase and installation, MR scanners are proliferating rapidly, and techniques and clinical applications for MR imaging continue to advance at an equally rapid rate.     

利用动态接收增益提高磁共振成像信噪比的方法

提出了一种提高磁共振成像(MRI)信噪比的有效方法.该方法在每次采集回波信号前,能够快速、灵活地控制MRI接收机的增益,实现磁共振信号动态范围的压缩;在图像重建之前采用双精度浮点运算扩展动态范围压缩的磁共振信号,最终得到信噪比提高的重建图像.在1.5 T超导MRI系统上进行了自旋回波序列的水模成像,实验结果表明,相比传统的基于固定接收增益的扫描图像,利用该方法得到的T1加权图像信噪比可以提高10%.和其他提高磁共振信号动态范围的方法相比,该方法无需增加额外硬件电路,避免多次采集图像,因而具有实现成本低的优点,是一种提高MRI信噪比的有效方法.     

增强低场下脑功能磁共振成像显著性的研究

实现了基于低场0.35 T磁共振成像系统的大脑功能磁共振成像（functional Magnetic Resonance Imaging,fMRI）的研究. 基于质子密度加权的快速自旋回波(Turbo Spin Echo,TSE)图像,重点研究增强低场fMRI显著性的方法,目的在于提高低场fMRI的可用性. 结果表明：健康受试者在执行手动任务期间,大脑运动区的信号强度变化可以由基于血管外质子信号增强 (Signal Enhancement by Extravascular water Protons,EEP)的对比机制探测. 优化TSE序列参数能提高图像SNR和扫描速度,并在统计分析中增加外在屏蔽图像,可以有效地提高低场下fMRI研究结果的显著性.     

超强边缘磁场梯度场下的磁共振成像:STRAFI 实验

磁共振成像（MRI）是一个能够探测样品内部特性的有效检测手段,已被广泛应用于化学、生物研究,以及医疗诊断领域. 自约40年前发展以来, 成像方法的不断发展使得MRI的成像分辨率、实验效率和成像杂核能力得到了很大的改进. 边缘磁场成像（STRAFI）是一种很具潜力的成像方法之一,它利用了超导磁体本身具有的边缘场的强梯度场. 该综述介绍了STRAFI基础,并概括了成像的基本原理、STRAFI的实验理论和方法及其在实际研究中的应用. 由此将比较STRAFI实验相对于传统MRI方法的所具有的优势和多面可行性.     

一种以超分辨率理论为基础的磁共振眼球成像方法

磁共振成像（MRI）是一种无电离辐射的非介入性的眼内肿瘤检测方法,但分辨率和运动伪影是成像过程中不易克服的困难．以往的扫描方法或是不可避免的引入运动伪影,或是需要受试者做精确的配合,增加了成像的难度,给受试者带来不舒适的体验．本文提出了一种以超分辨率理论为基础的新的磁共振眼球成像方法,使用一种特制的眼球线圈,对眼部区域扫描一系列动态的图像,使得不同方向上的采集分辨率互补．最后经过预处理、配准、超分辨率重建等操作,得到高质量的磁共振眼球图像．实验结果表明,这种方法可以在不需要受试者做额外配合工作的情况下,得到更加清晰的磁共振眼球图像．     

大白鼠脑中风模型的核磁共振成像(英文)

核磁共振成像现今已经成为临床神经影像的常规工具.在诊断、评估和监测中风从急性到慢性的各个阶段的脑组织的变化过程中,核磁共振成像扮演了一个重要的角色.该综述提供了多种核磁共振成像方法的描述,以及选择性地展示了作者在美国亨利福特医院神经科核磁共振成像实验室所获得的大白鼠栓塞中风模型的核磁共振成像研究成果.