The acoustic noise magnitude induced by the gradient impulse varied from 93.5 dB to 116.4 dB (3.0 T) and from 76.9 dB to 103.6 dB (0.4 T), according to the measurement positions.

Figure 8 shows the spatial distribution of GPAN-TFs of X, Y, and Z gradient coils in a superconducting MRI (3.0 T). GPAN-TF of the X coil reached its peak at ±X sides from the isocenter (figure 9). GPAN-TF of the Y coil had a maximum value at a higher position than the isocenter (figure 10a). GPAN-TF of the Z coil showed bimodal peaks on the patient axis (figure 10b).

Fig. 8: Spatial distribution of GPAN-TFs in a superconducting MRI (3.0 T).

Fig. 9: GPAN-TF of the X coil reached its peak at ±X sides from the isocenter.

Fig. 10: a) GPAN-TF of the Y coil had a maximum value at a higher position than the isocenter (Z=0). b) GPAN-TF of the Z coil showed bimodal peaks on the patient axis (Y=0).

Figure 11 shows the spatial distribution of GPAN-TFs of X, Y, and Z gradient coils in a permanent magnet open MRI (0.4 T). Maximum values of GPAN-TFs for the X and Y coils were present in ±X and ±Y directions from the isocenter, respectively (figure 12). The noise generated by the Z coil was exceedingly smaller than that generated by the X and Y coils at all the measurement positions.

Fig. 11: Spatial distribution of GPAN-TFs in a permanent magnet open MRI (0.4 T).

Fig. 12: Maximum values of GPAN-TFs for the a) X and b) Y coils were present in ±X and ±Y directions from the isocenter, respectively.

Figure 13 shows the shapes and designs of the gradient coils in the two MR systems. The spatial distribution of GPAN-TFs by multipoint measurement revealed the properties on the basis of the structural differences in each MRI scanner type and their respective gradient coils. This result shows that the “hotspot” area of GPAN-TFs may be associated with acoustic noise sources. It may be able to effectively reduce the noise because the propagating sound from the gradient coils to the “hotspot” was shielded.

Fig. 13: Shapes and designs of the gradient coils in the a) 3.0-T superconducting MRI system and b) 0.4-T permanent magnet open MRI system.

GPAN-TFs at the isocenter were not always higher than those at the other measurement positions (figure 14). A traditional measurement method for the noise during MRI was standardized only under the isocenter and the patient axis [5]. Therefore, the patient would be exposed to a more intense noise than initially planned.

Fig. 14: Comparison of boxplot distribution for GPAN-TFs at all measurement positions in the 3.0-T (left) and 0.4-T (right) MRI systems.

In particular, at a high frequency (>1000 Hz), GPAN-TF of the superconducting MRI scanner (3.0 T) was significantly higher than that of the permanent magnet open MRI scanner (0.4 T) (P < 0.001, unpaired t-test) (figure 15). The acoustic noise transfer for the high frequency content may be attributed to the increase in the uncomfortable acoustic noise of the high static field scanner.

Fig. 15: Spectra of GPAN-TFs in the a) 3.0-T and b) 0.4-T MRI scanner at isocenter.