Introduction

Higher magnetic field, gradient strength and slew rates increase vibrations and acoustic noise according to the Lorentz force. Large acoustic noise is always detrimental to volunteer’s comfort and may become in the future one of the most severe obstacles for MRI operation at higher fields. Acoustic noise in MRI is often attributed to the strong vibrations of the gradient coil and in this context, acoustic modes in the magnet bore can play a role¹. Foam rings can then be placed around the boreliner to attenuate sound, beyond which very few means seem readily available to the MR users for additional soundproofing, besides usual hearing protection or sequences avoiding mechanical resonances. In this work, we investigated the origin of acoustic noise with vibration and sound pressure level (SPL) measurements on the NexGen 7T scanner, which is equipped with the investigational high performance, Impulse, head gradient coil². We establish for this setup that most of the noise in fact originates from the eddy-currents induced in the RF shield of the RF coil. And we investigate possible ways to affect its vibrations and decrease SPLs.

Methods

Measurements were performed on the 7T NexGen scanner equipped with the investigational Impulse head gradient (max gradient strength = 200 mT/m, max slew rate = 900 mT/m/ms) and a 8Tx-64Rx head coil² (MRCoilTech, Glasgow, UK). SPL measurements (Optoacoustics, Mazor, Isreal) were first performed with continuous wave sinusoidal gradient waveforms at constant frequency and amplitude (5 mT/m), for each gradient axis, in 25 Hz steps, with and without the RF coil and at the right ear location of an anthropomorphic phantom. Vibration measurements were performed on the external surface of the RF coil and on the patient table using mono axial accelerometers (B&K, Naerum, Denmark) with gradient sweeps over the 0-3 kHz range in 2 min and at 5 mT/m amplitude. To ascertain the possible influence of mechanical coupling between the patient table and the RF coil, three different supporting materials between the two were tried: soft Sylodyn ND pads (Getzner, Bürs, Austria), Siemens gray foam pad and hard wooden wedges. Sponge window seals (M-D building products, Oklahoma city, OK, USA) were finally stuck around the RF coil to aim at disturbing its mechanical vibration modes by making contact with the boreliner. SPL measurements were repeated in this configuration in vivo with EPI, at several echo-spacings (ES=0.56, 0.61, 0.66, 0.71, 0.76, 0.81, 0.96, 1.01 ms), at constant gradient amplitude (76 mT/m) and bandwidth, with the acoustic sensor placed at the left or right ear of the volunteer. The different setups are illustrated in Figure 1.

Results

Figure 2 reports very different SPLs with and without the RF coil, with an increasing trend towards higher frequencies in the presence of the RF coil, which is consistent with eddy-currents (Faraday’s law) induced in the shield of the RF coil. Figure 3 shows the different sensor accelerations on the RF coil aligned with the sound measurements, where reasonable correlation can be observed, suggesting again that SPL originates mostly from the RF coil. The vibration of the table also was considerably lower without the RF coil than with the RF coil (data not shown). Lastly, Figure 4 shows the vibration measurements on the RF coil with the different support scenarios (pads or wooden wedges). The different configurations made little difference, indicating that mechanical coupling between the bed and the RF coil, and thus gradient coil vibration, is likely not responsible for most of the acoustic noise. The data of sensor 3 yet revealed some differences which were attributed to the corresponding contact levels between the accelerometer and the boreliner, thereby indicating that mechanical vibrations could be altered this way. Following this observation, the adhesive sponge window seals were placed to attempt establishing contact between the RF coil and the boreliner and disturb the vibrations. The acoustic measurements performed in vivo in these conditions, with EPI and at various ES are reported in Figure 5. Although there is not a systematic gain, up to 10 dB improvement could be observed at short ES.

Discussion

Strong evidence was gathered to point towards eddy-current induced vibrations in the RF coil as the most important factor responsible for acoustic noise. Alterations of the mechanical vibrations of the RF coil were achieved by constraining its vibrations with the boreliner and allowed decreasing SPL by up to 10 dB.

Conclusion

In our setup, acoustic noise is mostly induced by eddy-currents in the RF coil. The results pave the way for more investigations to reduce further acoustic noise in MRI.

Acknowledgements

AROMA H2020 FET-Open (885876). U01-EB025162, U24-NS129949, R44-MH129278 (NIH).