The MRI system – Magnetic Resonance scanners

The MRI system has prompted a revolution in the field of cognitive neuroscience, since it has allowed researchers to map the brain function noninvasively in response to task demands. However, acquiring images from this system generates some substantial acoustic noise, which creates a problem to the imaging studies of speech, hearing and the language (Chambers, Bullock, Kahana, Kots & Palmer, 2007). On the other hand, there is only on criteria for reducing the generated acoustic noise within this system, and that is using micro-perforated panels that are placed in the bore of the MRI scanner. These panels are able to be used onto the existing scanners at a minimum cost and they are also suitable for sterile surroundings. Although this method may result into quantifiable lower levels of noise, measured with the microphones in an empty MRI system, this method has not been tried with a patient in the scanner bore, which normally affects the acoustic noise field. A Magnetic Resonance Imaging scanner usually produce strong stationary magnetic field through the aid of a superconducting coil that is bathed in liquid helium (Katsunuma, 2002). When an object is introduced in this field, hydrogen atoms in this object orient antiparallel or parallel to the magnetic field pattern. The favorable energy state make the hydrogen atoms in the object to align parallel thus creating a net of magnetization Mo parallel as compared to the static field Bo. The hydrogen atoms in the object are mainly found in fat and water, which are in abundance within the human brain.

The main source of noise during Magnetic Resonance imaging process is the gradient coil and the surrounding tools (Katsunuma, 2002). During the process of imaging, a series of gradient fields are applied transiently so as to provide spatial data to the measured signal. For a researcher to produce these gradient fields, electric current is passed through the coil that is located in the strong stationary magnetic field. In this case, Lorentz forces are produced due to the current in the magnetic field hence, resulting in the vibrations of the equipment and the acoustic noise (Katsunuma, 2002). As the amount of current is increased in the coils, the amount of Lorentz forces acting on the coil also increases and hence results in creation of louder noise. The Lorentz forces are always turned on and off as the current transmitted in the coils is switched on and off, hence causing the coils and surrounding equipment to vibrate. This current habit on higher field strengths indicate just how the acoustic noise concerns will always be on the rise.

Although Magnetic Resonance scanners allow researchers and medical practitioners to non-invasively acquire information concerning the human body, the noise generated by these scanners during the scanning process poses a great cause of concern for the patients undergoing the imaging process and to the researchers carrying out the experiments. These amounts of noise levels generated during imaging process are estimated to be over 100 dB and they are as loud as 138 dB. At such levels they are said to be posing a great health and safety risk towards those individuals that are within the surrounding of the MRI system (Raylman, 2006). For one to prevent hearing damage, he or she is required to wear some personal protective equipment, which might include ear plugs or defenders, whenever they are inside the scanner room. Moreover, research has shown that this noise may lead to an increase in the levels of anxiety to the patient during imaging. On the other hand, the increased anxiety may also result from the fact that the mode of communication at the time for scanning is impaired as a result of the high noise levels (Raylman, 2006). This communication barrier might be problematic during Imaging since providing the task instructions and giving feedbacks are considered to be more difficult during the scanning process (Chambers, Bullock, Kahana, Kots & Palmer, 2007).

Researchers argue that the noise produced by the MRI scanner can be substantially decreased through sealing the gradient coil in a vacuum area so as to prevent the airborne vibration propagation, by supporting the gradient coil separately so as to prevent the solid vibration propagation and by reducing the eddy currents produced in the Radiofrequency coils, the Radiofrequency shield and the static-field-magnet cryostat (Stipsitz, Koerber, Stampfl & Schoenhuber, 2014). When a patient or a researchers head is subjected to a pulsed radiofrequency radiation under certain frequencies, he or she gets to hear an audible sound that is perceived as a knocking noise, buzz, or a click in his or her head. This type of acoustic phenomenon is described as RF hearing or a microwave hearing. Scientists argue that thermoelastic expansions are the ones responsible for production of the RF hearing, since there is an absorption of the RF energy, which generates a minute temperature elevation within a short period of time within the tissues in the human head (Berger, Kieper & Gauger, 2003). Most of the RF hearing has been associated with frequencies that range from 2.4MHz to 170MHz. However, the gradient magnetic field-induced acoustic noise is considered to be louder as compared to the sounds that associate with RF hearing. In this case, the noises generated by the RF auditory phenomenon are masked effectively hence they cannot be perceived by the researcher or patient (Berger, Kieper & Gauger, 2003).

Noise from the subsidiary system may also be considered as a source of acoustic noise within the MRI scanner. The comfort fans and the cryogen reclamation system that is associated with the superconducting magnet of the scanner are seen as sources of ambient acoustic noise that may be found in the surrounding of the MRI system. However, the acoustic noise caused by these subsidiary systems is considered to be much less compared to the one generated by the gradient magnetic fields. In general, the United States Occupational Safety and Health Administration point out that the levels of acoustic noise that they have recorded are below the permitted maximum limits, especially when one takes into consideration the duration that a patient is exposed in the scanner also determines the impact of the noise on hearing (Berger, Kieper & Gauger, 2003). Moreover, there has been a significant noise reduction in the acoustic noise levels through the use of an active noise cancellation. In 1989, Goldman et al, combined an active noise control system with a passive noise control system so as to achieve an average reduction in the level of noise by approximately 14 dB. In addition, the advancement in technology has led to introduction of a digital signal processor, which has helped efficiently the active noise control systems to be implemented at a cheaper cost (Berger, Kieper & Gauger, 2003). This anti-noise system works on the principle of a continuous feedback loop and continuously sampling the noise produced so that the gradient magnetic field induced noise can be attenuated.

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