The paper focuses on describing three methods of noise reduction in the speech signal recorded in an open-air magnetic resonance imager (MRI) working in a weak magnetic field during human phonation for the vocal tract modelling. This paper also analyses and compares spectral properties of the acoustic noise produced by mechanical vibration of the MRI device gradient coils. Then, the experiment with mapping of noise sound pressure level (SPL) in the MRI neighbourhood is described. The changes in acoustic noise spectral properties caused by loading of the holder of the lower gradient coils by the weight of the examined person lying in the scanning area of the MRI device is evaluated too. The influence of setting of the basic scan parameters of the used MR sequence (TR and TE times) on the spectral properties of the generated acoustic noise is also analysed. The results achieved are used to create a database of initial MR scan parameters such as the filter bank for noise signal pre-processing and to design a correction filter for noise suppression in the speech signal recorded simultaneously with three-dimensional (3D) human vocal tract scanning.
Part of the book: Advances in Noise Analysis, Mitigation and Control
Magnetic resonance imaging (MRI) tomography is often used for noninvasive scanning of various parts of a human body without undesirable effects present in X-ray computed tomography. In MRI devices, slices of a tested subject are selected in 3D coordinates by a system of gradient coils. The current flowing through these coils changes rapidly, which results in mechanical vibration. This vibration is significant also in the equipment working with a low magnetic field, and it causes image blurring of thin layer samples and acoustic noise significantly degrading a speech signal recorded simultaneously during MR scanning of the vocal tract. There are always negative physiological and psychological effects on a person exposed to vibration and acoustic noise. In order to minimize these negative impacts depending on intensity and time duration of exposition, we mapped relationship between energy of vibration and noise signals measured in the MRI scanning area and its vicinity.
Part of the book: Noise and Vibration Control