4. Discussion of the obtained results

The performed calibration and frequency response linearization of the piezoelectric vibration sensor enables precise pick-up of vibration signals in the environment of a weak stationary magnetic field and a high-voltage RF signal disturbance that is observed in the scanning area of the MRI device.

Our measurements have shown an inverse relationship between the diameter of the used sensor and the minimum frequency of the vibration picked up from the measured surface. The sensor HM692 with a massive aluminum microphone capsule used in phonocardiography had the lowest sensitivity and caused the greatest decrease of the maximum frequency. The calibration of the SB2 sensor was carried out in parallel for both pickup elements. The measured frequency responses SB2a,b are practically identical with nonlinear decrease in the range of low frequencies from 35 to 100 Hz—see the frequency responses in Figure 3a. In 3D scanning of the human vocal tract [4, 5, 19], the MRI device generates the acoustic noise of frequencies in the range from 25 Hz to 3.5 kHz that is similar to the basic frequency range of speech signals. For this reason, the SB-1 sensor was chosen for its greatest size allowing the best low-frequency sensitivity.

Comparison of noise spectral properties recorded for different types of directional patterns of the pickup microphone yields the best recording conditions for the cardioid pattern (minimum spectral decrease as shown by the obtained results in Table 1). On the other hand, dispersion of the spectral envelope values is similar for all three analyzed pattern types as can be seen in histograms in Figure 7a. Comparison of different microphone positions has shown that at 30°, the background noise from the MRI temperature stabilizer degrades the recording (see the signal RMS values in Table 1) and the direction of 150° is a bit unnatural from the point of view of an examined person lying in the MRI scanning area. Therefore, the direction chosen as the best for noise and speech signal recording was in the main horizontal axis of the MRI device (at 90°). In addition, at this position, the lowest values of the noise signal RMS were measured and the smallest dispersion of the spectral envelopes was observed—see the green dash-dot line in Figure 7b.

The results of a detailed measurement of the acoustic noise intensity at different distances from the central point of the scanning area for the SE and GE "Hi-Res" sequences are presented in Figure 10. The GE sequence produces noise with a slightly higher intensity, then the SE one (approx. 3-dB difference in the nearest location of 45 cm from the center of the scanning area) and variation of the SPL values depending on the measuring distance is also greater as seen in the box-plot graph in Figure 10b. The minimum distance was set to 45 cm in order to eliminate interaction of metal parts of the SPL meter with the static magnetic field of the MRI device. If the SPL meter was placed near the center, the field homogeneity would be disrupted and the warning message on the MRI control console would be followed by disabling to run any scan sequence by the software system [14]. The maximum measuring distance was set to 90 cm where the measured MRI noise was masked by the background noise originating from the temperature stabilizer. In the middle of the investigated measuring distances, the SPL values were similar for both types of MR scan sequences, so the working distance of 60 cm was used for all further measurements.

Next investigation of the recorded vibration and noise signals was aimed at the influence of the choice of the slice orientation on the energy of the produced vibration and noise signals. This effect is large—the maximum can be found in the sagittal plane and the minimum in the transversal plane for the vibration signals,

2. Comparison of the predicted Q<sup>F</sup> and TDUR values for "3D" types of MR scan sequences—numerical matching of the results for the changed number of FID

Influence of the number of FID signal accumulations on the predicted quality factor of the MR image and on the time duration for the scan sequence 3D-CE (TE = 30 ms and TR = 40 ms) and 3D phases = 8 (for 72 phases

• the SS-3D-balanced 10 sequences—see the values in Table 6,

signal accumulations and different number of 3D phases using:

• the 3D-CE 30 scan sequence (see Table 7).

NACC [] TR [ms]

Noise and Vibration Control - From Theory to Practice

both Hi-Res sequences of SE and GE types; slice thickness = 4.5 mm.

Parameters NACC []

Parameters NACC []

Parameters NACC []

Table 4.

TDUR [min:sec]

TDUR [min:sec]

TDUR [min:sec]

the values are in parentheses).

Table 7.

110

42 phases, the values are in parentheses).

Table 6.

Table 5.

60 100 200 300 400 500

1 2 3 4 5 6 7 8 10 16

0:14 0:26 0:37 0:49 1:00 1:12 1:24 1:35 1:58 3:08

1 2 3 4 8 16

3:14 (5:36) 6:25 (11:04) 9:37 12:48 (22:00) 25:34 51:05

1 2 3 4 8 16

1:04 (9:53) 2:00 (19:44) 2:56 3:52 (29:35) 7:36 15:04

1 0:14 0:22 0:41 1:09 1:20 1:39 8 1:35 2:37 5:12 7:46 10:20 12:55 16 3:08 5:11 10:20 15:29 20:38 25:47

Dependence of the time duration TDUR [min:sec] on setting of TR and NACC parameters—merged values for

Q <sup>F</sup> [] 14 20 24 28 31 34 37 40 44 56

Influence of the number of FID signal accumulations on the predicted quality factor of the MR image and on the

Q <sup>F</sup> [] 59 (102) 84 (144) 103 118 (204) 167 237

Influence of the number of FID signal accumulations on the predicted quality factor of the MR image and on the time duration for the scan sequence SS-3D balanced (TE = 10 ms and TR = 20 ms) and 3D phases = 24 (for

Q <sup>F</sup> [] 134 (79) 189 (122) 231 267 (137) 378 534

time duration for the scan sequence Hi-Res SE18 HE (TR = 60 ms and slice thickness = 10 mm).

and in the coronal plane for the noise signals—see the column charts in Figure 11. Therefore, the remaining experiments used only the sagittal orientation.

In accordance with our previous research [12, 18] the current experiments confirm the influence of the TE and TR times on the vibration and acoustic noise properties. The TE time extension causes fall of the final signal energy as documented by raised all the four determined vibration energetic parameters as well as the achieved SPL noise values in Table 2. The influence of the TR time determining the basic dominant frequency can be seen in box-plot graphs in Figure 12. This visualization of the basic statistical parameters obtained from analysis of vibration and noise signals shows the highest values of all energetic parameters for the shortest TR times (60 or 100 ms).

Comparison of energetic relations of the vibration and noise signals for different sequence types brings ambiguous results and shows only small differences—see three bar-graphs in Figure 13. The 3D sequence "SS-3Dbalanced" differs from the remaining sequence types by reverse behavior: while the Enc0 and Enr0 parameters indicate the minimum values, the EnTK achieves the maximum ones (see the graph in Figure 13c). This situation can be caused by the minimum settings of the TE and TR times that were used for the "Hi-Res" types to be comparable with the "3D" types with slightly atypical values being out of the normal range of use although the control software enables their setting [15].

Next comparison of energetic relations of the vibration and noise signals for different objects placed in the scanning area of the MRI device shows a relatively high effect of the mass put upon the bottom plastic holder of the gradient coils. The effective weight of the person exerting a pressure on the bottom plastic holder of the gradient coils attenuates the vibration pulses partially. The mass effect is demonstrated by increase of the vibration signal energy based on Enc0 parameter with its maximum for the lying male person with the weight of 80 kg (see the bar-graph in Figure 14a). It is also demonstrated in the spectral properties of the vibration

#### Figure 13.

Comparison of energetic relations of vibration and noise signals for different sequence types: {Hi-Res SE-HE, Hi-Res SE-HF, Hi-Res GE-T2, SS-3Dbal, 3D-CE}: (a) mean Enc0, (b) mean Enr0, and (c) mean EnTK; in all cases, the sagittal slice orientation was used.

signal as shown by lower spectral decrease in Figure 14a and by shift of the first two dominant frequencies toward higher values—see the mutual FV1,2 position in

Influence of the TR time and the number of FID signal accumulations on the predicted image quality factor for

the Hi-Res sequences of—(a) SE and (b) GE type; slice thickness = 4.5 mm.

Examples of MR images of the human vocal tract obtained with different values of the quality factor: (a) scan sequence Hi-Res SE26 HF (TR = 500), slice thickness = 4.5 mm, Q<sup>F</sup> = 100, (b) scan sequence Hi-Res SE26 HF (TR = 500), slice thickness = 7.5 mm, Q<sup>F</sup> = 196, and (c) scan sequence 3D SSF 30 (TR = 10), slice

Analysis of Energy Relations between Noise and Vibration Produced by a Low-Field MRI Device

DOI: http://dx.doi.org/10.5772/intechopen.85275

Results of the preliminary analysis of influence of the slice thickness document that its increase has a positive effect on the predicted quality factor of MR images compare the values in Table 3. Next comparison shows a positive influence of increase in the TR time on the quality factor, and this effect is more pronounced when using the SE sequence type—see the left graph in Figure 16. This figure also documents significant dependence between the applied number of FID signal

Figure 14c.

113

Figure 16.

Figure 15.

thickness = 9.4 mm, and Q<sup>F</sup> = 398.

#### Figure 14.

Comparison of mean values of the energy and basic spectral properties of the vibration signal for different objects placed in the scanning area of the MRI device: (a) energy Enc0, (b) box-plot of basic statistical properties for the spectral decrease values, and (c) mutual values of the frequencies FV1 and FV2; used Hi-Res SE-HF scan sequences with TE = 18 ms,TR = 400 ms, and sagittal orientation.

Analysis of Energy Relations between Noise and Vibration Produced by a Low-Field MRI Device DOI: http://dx.doi.org/10.5772/intechopen.85275

#### Figure 15.

and in the coronal plane for the noise signals—see the column charts in Figure 11.

In accordance with our previous research [12, 18] the current experiments confirm the influence of the TE and TR times on the vibration and acoustic noise

Comparison of energetic relations of the vibration and noise signals for different sequence types brings ambiguous results and shows only small differences—see three bar-graphs in Figure 13. The 3D sequence "SS-3Dbalanced" differs from the remaining sequence types by reverse behavior: while the Enc0 and Enr0 parameters indicate the minimum values, the EnTK achieves the maximum ones (see the graph in Figure 13c). This situation can be caused by the minimum settings of the TE and TR times that were used for the "Hi-Res" types to be comparable with the "3D" types with slightly atypical values being out of the normal range of use although the

Next comparison of energetic relations of the vibration and noise signals for different objects placed in the scanning area of the MRI device shows a relatively high effect of the mass put upon the bottom plastic holder of the gradient coils. The effective weight of the person exerting a pressure on the bottom plastic holder of the gradient coils attenuates the vibration pulses partially. The mass effect is demonstrated by increase of the vibration signal energy based on Enc0 parameter with its maximum for the lying male person with the weight of 80 kg (see the bar-graph in Figure 14a). It is also demonstrated in the spectral properties of the vibration

Comparison of energetic relations of vibration and noise signals for different sequence types: {Hi-Res SE-HE, Hi-Res SE-HF, Hi-Res GE-T2, SS-3Dbal, 3D-CE}: (a) mean Enc0, (b) mean Enr0, and (c) mean EnTK; in all

Comparison of mean values of the energy and basic spectral properties of the vibration signal for different objects placed in the scanning area of the MRI device: (a) energy Enc0, (b) box-plot of basic statistical properties for the spectral decrease values, and (c) mutual values of the frequencies FV1 and FV2; used Hi-Res SE-HF scan

Therefore, the remaining experiments used only the sagittal orientation.

properties. The TE time extension causes fall of the final signal energy as documented by raised all the four determined vibration energetic parameters as well as the achieved SPL noise values in Table 2. The influence of the TR time determining the basic dominant frequency can be seen in box-plot graphs in Figure 12. This visualization of the basic statistical parameters obtained from analysis of vibration and noise signals shows the highest values of all energetic param-

eters for the shortest TR times (60 or 100 ms).

Noise and Vibration Control - From Theory to Practice

control software enables their setting [15].

Figure 13.

Figure 14.

112

cases, the sagittal slice orientation was used.

sequences with TE = 18 ms,TR = 400 ms, and sagittal orientation.

Examples of MR images of the human vocal tract obtained with different values of the quality factor: (a) scan sequence Hi-Res SE26 HF (TR = 500), slice thickness = 4.5 mm, Q<sup>F</sup> = 100, (b) scan sequence Hi-Res SE26 HF (TR = 500), slice thickness = 7.5 mm, Q<sup>F</sup> = 196, and (c) scan sequence 3D SSF 30 (TR = 10), slice thickness = 9.4 mm, and Q<sup>F</sup> = 398.

#### Figure 16.

Influence of the TR time and the number of FID signal accumulations on the predicted image quality factor for the Hi-Res sequences of—(a) SE and (b) GE type; slice thickness = 4.5 mm.

signal as shown by lower spectral decrease in Figure 14a and by shift of the first two dominant frequencies toward higher values—see the mutual FV1,2 position in Figure 14c.

Results of the preliminary analysis of influence of the slice thickness document that its increase has a positive effect on the predicted quality factor of MR images compare the values in Table 3. Next comparison shows a positive influence of increase in the TR time on the quality factor, and this effect is more pronounced when using the SE sequence type—see the left graph in Figure 16. This figure also documents significant dependence between the applied number of FID signal

accumulations and the predicted Q<sup>F</sup> value. Also, in this case, the increase of Q<sup>F</sup> is more distinctive for the SE sequences. Values in Table 4 describe the influence of TR and NACC values on the final time duration of the executed scan sequence. While the increased TR causes only moderately greater overall time duration, the changed NACC parameter has comparably higher influence on the final time duration. This effect is also shown in a detailed comparison of numerical results for different NACC values in Table 5. For the "Hi-Res" sequence types, the increase of the parameter NACC from 2 to 16 results in about 2.8 times greater value of Q<sup>F</sup> but 6 times greater than that of TDUR. For the "3D" sequence types, the increase of the resulting time duration is also affected by the choice of the number of 3D phases (equivalent to the number of slices with selection of the slice thickness for the "Hi-Res" sequences) in parallel as shown in Tables 6 and 7.
