ABSTRACT:In some MRI-LINAC (Magnetic Resonance Imaging and Linear Accelerator) hybrid systems, the MRI scanner is split into two parts to form a central gap for the accommodation of the patient or a LINAC. Little is known about the acoustic characteristics of the split gradient coil structure needed for this system; however, it is believed to be very different from its typical configurations. It is important to develop dedicated numerical methods for the characterization of the unique acoustic properties, to provide engineering solutions for the noise attenuation for such a new system. In this article, we modeled the acoustic fields of a split MRI system and traditional gradient structures using the finite element method. The models were validated against acoustic experimental results obtained from a conventional MRI scanner. The acoustic field distribution analysis showed that the average sound pressure levels in the central gap were lower than those in the cylindrical tunnels of the split MRI system at most frequencies. This was also true when both the x coils or z coils were energized independently. Thus, if the patient bed is placed perpendicular to the axis of the main magnet of the split MRI system, the patient will be subjected to relatively lower acoustic intensities compared with that if the patient bed is placed parallel to the axis of the main magnet. Further work is planned to reduce the sound level in the central gap where the patient bed may be placed for this split system.

For head magnetic resonance imaging, local gradient coils are often used to achieve high solution images. To accommodate the human head and shoulder, the head gradient coils are usually designed in an asymmetric configuration, allowing the region-of-uniformity (ROU) close to the coil's patient end. However, the asymmetric configuration leads to technical difficulties in maintaining a high gradient performance for the insertable head coil with very limited space. In this work, we present a practical design configuration of an asymmetric insertable gradient head coil offering an improved performance. In the proposed design, at the patient end, the primary and secondary coils are connected using an additional radial surface, thus allowing the coil conductors distributed on the flange to ensure an improvement in the coil performance. At the service end, the primary and shielding coils are not connected, to permit access to shim trays, cooling system piping, cabling, and so on. The new designs are compared with conventional coil configurations and the simulation results show that, with a similar field quality in the ROU, the proposed coil pattern has improved construction characteristics (open service end, well-distributed wire pattern) and offers a better coil performance (lower inductance, higher efficiency, etc) than conventional head coil configurations.

The authors' FE results showed that, for the cases of transverse coil and longitudinal coil switching, the overall average-SPL reduction quantities amounted to around 20 dB by applying the proposed noise reduction scheme, resulting in lower SPLs than the human hearing threshold.

In this paper, we report the design and fabrication of a novel micromachined electro-magnetically driven tuning fork type gyroscope with bar structure proof masses working at atmospheric pressure. The applied angular rate is sensed by detecting the differential change of capacitance between the bar structure electrodes and the fixed electrodes on the glass substrate. Instead of common squeeze-film damping, slide-film damping in the gap between proof masses and glass substrate plays a dominant role, which enables it to achieve high Q-factors and thus eliminate vacuum packaging. The measured Q-factors for driving and sensing modes are 965 and 716, respectively. The sensor obtained a sensitivity of 6 mV/°/s and a non-linearity of less than 0.5%. IntroductionIn automotive industry, the growing needs for micromachined gyroscopes push the researchers to produce even smaller, cheaper and better performing devices. A variety of prototypes have been developed in past years, most of which are of tuning fork vibratory type employing different excitation and detection mechanisms [1]. Electrostatic excitation and capacitive detection mechanism [2, 3, 4] is the most preferred, while others, such as electromagnetic excitation mechanism [5] and piezoelectric drive and piezoresistive read-out [6], have also been reported.Air damping effect is one critical issue that is worth taking into account when designing a gyroscope, which determines the Q-factors for both driving and sensing modes and thus the sensors' performance. For example, in a typical capacitive detection gyroscope where comb-type detection structure is commonly used [7], the squeeze-film damping between comb fingers plays an important role, which usually results in a low Q-factor and requires costly vacuum packaging solution in some cases to achieve better sensitivity. In our previous work [8], slide-film damping effect was introduced to a vibratory gyroscope for achieve good Q-factor, but the electrostatic driving mode had a limited oscillating amplitude.In this paper, we report the design and fabrication of a novel micromachined electro-magnetically driven tuning fork type gyroscope with bar structure proof masses for capacitive detection. The great enough driving amplitude under electro-magnetical excitation, along with high Qfactor for detection mode due to the slide-film damping effect, enable it a good performance at atmospheric pressure. Working principleAs shown schematically in Fig. 1, the gyroscope consists of two silicon oscillating frames, each of which is anchored on glass substrate by four spring beams and is connected each other through a connection ring. Each proof mass with bar structure detection electrodes is connected to the surrounding oscillating frame by two suspension beams. The oscillating frames as well as the proof masses with bar structure can move above the glass substrate along X or Ydirection. The bar structure electrodes and fixed interdigitated electrodes on glass substrate form the detection capacitors. The silicon surface is covered...