Background: Safe robot-assisted intervention using magnetic resonance imaging\n(MRI) guidance requires the precise control of assistive devices, and most currently\navailable tools are rarely MRI-compatible. To obtain high precision, it is necessary to\ncharacterize and develop existing MRI-safe actuators for use in a high magnetic field\n(ââ?°Â¥3 T). Although an ultrasonic motor (USM) is considered to be an MRI-safe actuator,\nand can be used in the vicinity of a high field scanner, its presence interferes with MR\nimages. Although an MR image provides valuable information regarding the pathology\nof a patientââ?¬â?¢s body, noise, generally of a granular type, decreases the quality of the\nimage and jeopardizes the true evaluation of any existing pathological issues. An eddy\ncurrent induced in the conductor material of the motor structure can be a source of\nnoise when the motor is close to the isocenter of the image. We aimed to assess the\neffects of a USM on the signal-to-noise ratio (SNR) of MR images in a 3-T scanner. The\nSNR was compared for four image sequences in transverse directions for three orientations\nof the motor (x, y, and z) when the motor was in the ââ?¬Å?offââ?¬Â state. The SNR was evaluated\nto assess three artifact reduction methods used to minimize the motor-induced\nartifacts.\nResults: The SNR had a range of 5ââ?¬â??10 dB for slices close to the motor in the x and y\norientations, and increased to 15ââ?¬â??20 dB for slices far from the motor. Averaging the\nSNR for slices in all cases gave an SNR loss of about 10 dB. The maximum SNR was\nmeasured in the z orientation. In this case, the SNR loss was almost the same as that of\nother motor orientations, approximately 10 dB, but with a higher range, approximately\n20ââ?¬â??40 dB.\nConclusions: The selection of certain scanning parameters is necessary for reducing\nmotor-generated artifacts. These parameters include slice selection and bandwidth. In\ndeveloping any MRI-compatible assisted device actuated by a USM, this study recommends\nthe use of an approximately 3-mm slice thickness with minimum bandwidth to\nachieve optimized SNR values when a USM is operating close to (within approximately\n40 mm) the region being imaged. The SNR can be further enhanced by increasing\nthe number of signal averages, but this is achieved only at the cost of increased scan\nduration.
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