3.1 Calibration of vibration sensors suitable for measurement in the low magnetic field environment

The calibration and measurement experiments were realized with the help of the main devices: the Audio Precision System One including two programmable input and output channels for simultaneous measurement of electrical signals from the vibration sensors mounted on the Vibration Exciter ESE 201 located at the Institute of Electronics and Photonics, FEE&IT SUT, Bratislava. As a reference sensor, the accelerometer KD35a from the company Metra Mess- und Frequenztechnik was used. The sensor sensitivity of this standardized accelerometer is guaranteed, and it operates over a frequency range from 50 Hz to 10 kHz. Three types of vibration sensors having good response in the lower audio frequency range up to 2 kHz were tested within this work:


The sensors were mounted on the plate of the vibration exciter as shown in the detailed photo of the arrangement of the sensors in the right part of Figure 2. The output voltage for supply of this exciter and the signal from the calibrated sensors were checked parallel by the digital oscilloscope Rigol DS1102E. Two types of the parameters of the vibration sensors were measured and compared in our experiment:

sensor was determined from this graph. Comparison in Figure 3b shows that the measured frequency responses of SB-1, SB2ab, and HM692 are rotated by a slope of about 20 dB per decade with respect to the frequency response of KD35a. As the reference KD35a is an acceleration sensor, it emerges that the remaining three sensors are velocity ones. The calculated inverse frequency response of the SB-1 is drawn by the magenta dashed line together with the correction frequency response obtained by shelving equalization that is plotted by the cyan dot-dash line in Figure 3c. The effect of this shelving filter on the time-domain vibration signal, its frequency-domain periodogram with chosen spectral features, and the spectrogram

Graph of: (a) measured sensors' sensitivities, (b) frequency responses in the range 20 Hz to 2 kHz measured and recalculated in [dB], and (c) correction frequency response for the SB-1 sensor linearization using the shelving filter (b): fref = 125 Hz, UexcBa0 = 360 mV, Ba0 = {3.69 (KD35a), 12.9 (SB-1), 5.65 (SB2ab), and 2.45

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

3.2 Analysis of the influence of the directional pattern of the pickup microphone on the spectral properties of the recorded noise signal

Acoustic noise measurement in the MRI neighborhood was realized in the direc-

Subsequently, the spectral properties of the recorded noise signals were analyzed using the mentioned three microphone pickup patterns. The obtained results are presented for visual comparison in Figure 7 and summarized in numerical form in Table 1; the output statistical parameters of the supplementary spectral features are

tions of 30, 90, and 150°, at the distance of 60 cm from the central point of the scanning area, and at the height of 85 cm from the floor—see the principal arrangement photo in Figure 5. In this noise recording part of the experiment, the pick-up

Behringer dual-diaphragm condenser microphone B-2 PRO with switchable cardioid, omnidirectional, or figure-of-eight pickup patterns was used—see the directional patterns from the manufacturer's specification sheet in Figure 6.

can be seen in Figure 4.

Figure 3.

(HM692)} mV/m s<sup>2</sup>

.

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

shown in Figure 8.

103


Dependence of the sensor's sensitivity on the excitation voltage for all three sensors is presented in Figure 3a. The reference voltage sensitivity Ba0 of the SB-1

#### Figure 2.

Principle block diagram of the used calibration and measurement method together with a detailed photo of practical mounting of the sensors on the plate of the vibration exciter.

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 3.

3.1 Calibration of vibration sensors suitable for measurement in the low

The calibration and measurement experiments were realized with the help of the main devices: the Audio Precision System One including two programmable input and output channels for simultaneous measurement of electrical signals from the vibration sensors mounted on the Vibration Exciter ESE 201 located at the Institute of Electronics and Photonics, FEE&IT SUT, Bratislava. As a reference sensor, the accelerometer KD35a from the company Metra Mess- und Frequenztechnik was used. The sensor sensitivity of this standardized accelerometer is guaranteed, and it operates over a frequency range from 50 Hz to 10 kHz. Three types of vibration sensors having good response in the lower audio frequency range up to 2 kHz were

• Cejpek SB-1 with the thin circular brass disc of 0.25-mm thickness and 27.5 mm diameter designed primarily for pickup of a musical sound of a contrabass

• Shadow SH-SB2 double bass pickup with two disc transducers of 0.5-mm

• RFT heart microphone device HM 692 comprising a piezo-electric element integrated in the 1-mm thin aluminum metal cover with 30-mm diameter

The sensors were mounted on the plate of the vibration exciter as shown in the

detailed photo of the arrangement of the sensors in the right part of Figure 2. The output voltage for supply of this exciter and the signal from the calibrated sensors were checked parallel by the digital oscilloscope Rigol DS1102E. Two types of the parameters of the vibration sensors were measured and compared in our

• frequency response in the range from 20 Hz to 2 kHz at the chosen output

Dependence of the sensor's sensitivity on the excitation voltage for all three sensors is presented in Figure 3a. The reference voltage sensitivity Ba0 of the SB-1

Principle block diagram of the used calibration and measurement method together with a detailed photo of

• relative sensitivity at the reference frequency fref = 125 Hz,

voltage of the vibration exciter (UexcBa0 = 360 mV).

practical mounting of the sensors on the plate of the vibration exciter.

thickness and 22.5-mm diameter (further called as "SB2a,b"),

magnetic field environment

Noise and Vibration Control - From Theory to Practice

tested within this work:

experiment:

Figure 2.

102

(further called as "SB-1"),

(further called as "HM692").

Graph of: (a) measured sensors' sensitivities, (b) frequency responses in the range 20 Hz to 2 kHz measured and recalculated in [dB], and (c) correction frequency response for the SB-1 sensor linearization using the shelving filter (b): fref = 125 Hz, UexcBa0 = 360 mV, Ba0 = {3.69 (KD35a), 12.9 (SB-1), 5.65 (SB2ab), and 2.45 (HM692)} mV/m s<sup>2</sup> .

sensor was determined from this graph. Comparison in Figure 3b shows that the measured frequency responses of SB-1, SB2ab, and HM692 are rotated by a slope of about 20 dB per decade with respect to the frequency response of KD35a. As the reference KD35a is an acceleration sensor, it emerges that the remaining three sensors are velocity ones. The calculated inverse frequency response of the SB-1 is drawn by the magenta dashed line together with the correction frequency response obtained by shelving equalization that is plotted by the cyan dot-dash line in Figure 3c. The effect of this shelving filter on the time-domain vibration signal, its frequency-domain periodogram with chosen spectral features, and the spectrogram can be seen in Figure 4.

## 3.2 Analysis of the influence of the directional pattern of the pickup microphone on the spectral properties of the recorded noise signal

Acoustic noise measurement in the MRI neighborhood was realized in the directions of 30, 90, and 150°, at the distance of 60 cm from the central point of the scanning area, and at the height of 85 cm from the floor—see the principal arrangement photo in Figure 5. In this noise recording part of the experiment, the pick-up Behringer dual-diaphragm condenser microphone B-2 PRO with switchable cardioid, omnidirectional, or figure-of-eight pickup patterns was used—see the directional patterns from the manufacturer's specification sheet in Figure 6.

Subsequently, the spectral properties of the recorded noise signals were analyzed using the mentioned three microphone pickup patterns. The obtained results are presented for visual comparison in Figure 7 and summarized in numerical form in Table 1; the output statistical parameters of the supplementary spectral features are shown in Figure 8.

#### Figure 4.

The vibration signal picked up by the SB-1 sensor without/with the applied shelving filter (left/right set of graphs): selected 150-ms ROI of the signal together with the calculated RMS value (a), corresponding periodogram including the spectral decrease-tilt (b), and spectrogram calculated from the whole 8-s duration of the vibration signal (c); Q = 0.115, f<sup>c</sup> = 120, G = 30, and fs = 16 kHz.

Figure 6.

Figure 7.

position

Table 1.

105

condenser microphone B-2 PRO.

cardioid directional pattern (b).

Microphone pickup pattern/

directional patterns placed at different positions.

Example of directional patterns: cardioid (a), omnidirectional (b), and figure-of-eight (c) for the Behringer

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

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

Comparison of spectral envelope values in [dB] of the noise signals with different directional patterns of the pickup microphone placed at different positions: histograms for omnidirectional, cardioid, and figure-of-eight patterns—signals recorded at 90° (a) and histograms for signals recorded at 30, 90, and 150°—with the

> Stilt [deg]

Omnidirectional 15.3 16 13.5 15 14.2 13 Cardioid 15.2 11 13.3 10 14.0 4 Figure-of-eight 14.1 18 13.1 13 13.0 9

Comparison of the noise spectral parameters of the recordings picked up by the microphone with different

Signal RMS []

At 30° At 90° At 150°

Signal RMS []

Stilt [deg] Signal RMS []

Stilt [deg]

#### Figure 5.

Principal arrangement of acoustic noise recording in the vicinity of the scanning area of the open-air MRI device Opera: the pickup microphone situated at 30, 90, and 150°.

#### 3.3 Mapping of the acoustic noise SPL in the MRI device vicinity

The acoustic noise SPL was measured using the multifunction environment meter Lafayette DT 8820. In the first step, the dependence of the SPL noise values on the distances D<sup>X</sup> was mapped. The measuring device was located successively at the distances of {45, 50, 55, 60, 70, 80, 90} cm from the central point of the scanning area, at the height of 85 cm from the floor (between both gradient coils), and in the direction of 30° from the left corner near the temperature stabilizer, producing majority of the background noise SPL0—see the experiment

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 6.

Example of directional patterns: cardioid (a), omnidirectional (b), and figure-of-eight (c) for the Behringer condenser microphone B-2 PRO.

#### Figure 7.

Comparison of spectral envelope values in [dB] of the noise signals with different directional patterns of the pickup microphone placed at different positions: histograms for omnidirectional, cardioid, and figure-of-eight patterns—signals recorded at 90° (a) and histograms for signals recorded at 30, 90, and 150°—with the cardioid directional pattern (b).


#### Table 1.

3.3 Mapping of the acoustic noise SPL in the MRI device vicinity

Opera: the pickup microphone situated at 30, 90, and 150°.

Figure 4.

Figure 5.

104

producing majority of the background noise SPL0—see the experiment

The acoustic noise SPL was measured using the multifunction environment meter Lafayette DT 8820. In the first step, the dependence of the SPL noise values on the distances D<sup>X</sup> was mapped. The measuring device was located successively at the distances of {45, 50, 55, 60, 70, 80, 90} cm from the central point of the scanning area, at the height of 85 cm from the floor (between both gradient coils), and in the direction of 30° from the left corner near the temperature stabilizer,

Principal arrangement of acoustic noise recording in the vicinity of the scanning area of the open-air MRI device

The vibration signal picked up by the SB-1 sensor without/with the applied shelving filter (left/right set of graphs): selected 150-ms ROI of the signal together with the calculated RMS value (a), corresponding periodogram including the spectral decrease-tilt (b), and spectrogram calculated from the whole 8-s duration of

the vibration signal (c); Q = 0.115, f<sup>c</sup> = 120, G = 30, and fs = 16 kHz.

Noise and Vibration Control - From Theory to Practice

Comparison of the noise spectral parameters of the recordings picked up by the microphone with different directional patterns placed at different positions.

Figure 8.

Supplementary spectral properties of the recorded noise signals with different directional patterns of the pickup microphone—(a) omnidirectional, (b) cardioid, and (c) figure-of-eight; box-plots of the basic statistical parameters in the upper graphs, corresponding histograms of values of the spectral centroid, flatness, and Shannon entropy (in the lower set of graphs); signal recorded at 90°.

• high-resolution (Hi-Res) sequences using the basic SE/GE MRI scan methods

Mapping of the acoustic noise SPL at different distances D<sup>X</sup> = {45, 50, 55, 60, 70, 80, 90} cm from the middle of the scanning area of the MRI device for SE/GE sequences: (a) comparison of the SPL values with those of the

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

• special 3D sequences used for building or reconstruction of 3D models of

Five types of MR scan sequences were tested in total in the investigated MRI device Opera: SE 18 HF, SE 26 HF, GE T2 (as a typical representative of the "Hi-Res" class), SS-3Dbalanced, and 3D-CE [15]. For each of these scan sequences,

• orientation of scan slices TORIENT = {Coronal, Sagittal, Transversal}—see visualization of the energy features of the vibration and noise signals in

Visualization of energy features of the vibration and noise signals for different slice orientations: {coronal, sagittal, transversal}: (a) signal RMS together with noise SPL values, (b) mean Enc0, (c) mean Enr0, and

(d) mean EnTK; used Hi-Res SE scan sequence with TE = 18 ms and TR = 500 ms.

[16],

Figure 10.

Figure 11,

Figure 11.

107

biological or botanical issues [17].

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

different settings of the scan parameters are analyzed:

background noise (SPL0) and (b) box-plot of their basic statistical parameters.

arrangement photo in Figure 9. Comparison of the resulting SPL values obtained during execution of two basic SE and GE types of the MR scan sequences with the background noise SPL (with no sequence running) is presented in the graphs of Figure 10.

#### 3.4 Main measurement experiments with the open-air MRI device

Within the scope of our main experiments, the baseline measurement and recording of the vibration and noise signals were carried out during the execution of the MR scan sequences. For noninvasive testing of the subject/object, usually two basic classes of scan sequences are used to take MR images of human body parts with high quality:

#### Figure 9.

Arrangement photo of SPL noise measurement and parallel recording of noise and vibration signals of the openair MRI device Opera: (1) RF knee coil with a spherical water phantom, (2) vibration sensor, (3) pick-up microphone, (4) SPL noise meter, and (5) principal angle diagram of the scanning area.

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 10.

arrangement photo in Figure 9. Comparison of the resulting SPL values obtained during execution of two basic SE and GE types of the MR scan sequences with the background noise SPL (with no sequence running) is presented in the graphs of

Supplementary spectral properties of the recorded noise signals with different directional patterns of the pickup microphone—(a) omnidirectional, (b) cardioid, and (c) figure-of-eight; box-plots of the basic statistical parameters in the upper graphs, corresponding histograms of values of the spectral centroid, flatness, and

Within the scope of our main experiments, the baseline measurement and recording of the vibration and noise signals were carried out during the execution of the MR scan sequences. For noninvasive testing of the subject/object, usually two basic classes of scan sequences are used to take MR images of human body parts

Arrangement photo of SPL noise measurement and parallel recording of noise and vibration signals of the openair MRI device Opera: (1) RF knee coil with a spherical water phantom, (2) vibration sensor, (3) pick-up

microphone, (4) SPL noise meter, and (5) principal angle diagram of the scanning area.

3.4 Main measurement experiments with the open-air MRI device

Shannon entropy (in the lower set of graphs); signal recorded at 90°.

Noise and Vibration Control - From Theory to Practice

Figure 10.

Figure 8.

with high quality:

Figure 9.

106

Mapping of the acoustic noise SPL at different distances D<sup>X</sup> = {45, 50, 55, 60, 70, 80, 90} cm from the middle of the scanning area of the MRI device for SE/GE sequences: (a) comparison of the SPL values with those of the background noise (SPL0) and (b) box-plot of their basic statistical parameters.


Five types of MR scan sequences were tested in total in the investigated MRI device Opera: SE 18 HF, SE 26 HF, GE T2 (as a typical representative of the "Hi-Res" class), SS-3Dbalanced, and 3D-CE [15]. For each of these scan sequences, different settings of the scan parameters are analyzed:

• orientation of scan slices TORIENT = {Coronal, Sagittal, Transversal}—see visualization of the energy features of the vibration and noise signals in Figure 11,

#### Figure 11.

Visualization of energy features of the vibration and noise signals for different slice orientations: {coronal, sagittal, transversal}: (a) signal RMS together with noise SPL values, (b) mean Enc0, (c) mean Enr0, and (d) mean EnTK; used Hi-Res SE scan sequence with TE = 18 ms and TR = 500 ms.

• echo times TTE = {18, 22, 26} ms—compare the numerical results in Table 2,

MRI device and of the acoustic noise signal using the microphone in its proximity, and the additional measurement to check the noise SPL. In this part of the measurement, the microphone stand with the Behringer dual-diaphragm condenser microphone B-2 PRO was placed together with the SPL meter at the distance of D<sup>X</sup> = 60 cm, and the 140-mm diameter spherical testing phantom filled with doped water [15] was placed inside the knee RF coil. The SB-1 sensor [12, 18] was used to pick up the vibration signal inside the scanning area of the MRI Opera device. Practical position of the sensing disc was on the surface of the plastic holder of the bottom gradient coils, as can be seen in the arrangement photo in Figure 9. The stored recordings were further processed in order to evaluate and compare the measured signal properties. All the noise and vibration signals were recorded with the help of the Behringer Podcast Studio equipment. The signals with duration of about 15 s sampled at 32 kHz were next processed in the sound editor program

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

3.5 Analysis of the influence of the scan parameters on the time duration and

The chosen type of the scanning sequence and the values of the resulting basic scan parameters (TR and TE) have significant influence on the scanning time. These parameters can also be changed manually, but their final values depend on the setting of the other scan parameters—number of slices, slice thickness, number of used accumulations NACC of the free induction decay (FID) signal [8, 16], etc. Practical demonstration of the acquired MR images with increasing quality factor (QF) shows greater range of visible details in the images for three different MR

The console program "ESAMRI" of the MRI device control software [15] was

1. Influence of the basic setting of scan parameters on the final quality factor of MR images and on the time duration TDUR of the scan sequence execution for:

• different slice thickness of {2, 2.5, 3, 4, 4.5, 5, 10} mm—the predicted Q<sup>F</sup> values are presented in Table 3 for the scan sequence Hi-Res SE18 HE,

• different repetition times of {60, 100, 200, 300, 400, 500} ms together with NACC—see visualization of the graphical results using the "Hi-Res" sequences of SE and GE types in Figure 16, and TDUR values in Table 4

• increased number of applied accumulations of the FID signal: NACC = {1, 2, 3, 4, 5, 6, 7, 8, 10, 16}—the predicted values of Q<sup>F</sup> and TDUR are shown

2 2.5 3 4 4.5 5 10

numerically in Table 5 for the scan sequence Hi-Res SE18 HE.

Q <sup>F</sup> [] 17 21 26 34 38 43 85

Influence of the slice thickness on the predicted quality factor of the MR image and on the time duration for the

used to carry out the following two parts of the analysis and comparison:

Sound Forge 9.0a.

1

109

Table 3.

TDUR = 1 min 39 sec in all cases.

the quality factor of the MR images

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

scans of the human vocal tract in Figure 15.

for both Hi-Res sequences types,

scan sequence Hi-Res SE18 HE (TR = 500 ms, NACC = 1).

Parameters<sup>1</sup> Slice thickness [mm]


The slice orientations as well as the TE and TR parameters were set manually to perform measurement and comparison in the range enabled by the current sequence [15]. Practical realization of the last part of the experiment consists in placing a testing phantom or a head and a neck of a lying person in the RF scan coil between the upper and lower gradient coils of the MRI device. While the total weight of the used testing phantom in the first part of the experiment was 0.75 kg, the weighs of one male and one female voluntary person lying on the patient bed of the MRI device were approx. 80 and 55 kg.

The multisignal measurement comprised real-time recording of the vibration signal by the piezoelectric sensor located inside the scanning area of the investigated


1 Used Hi-Res SE-HF scan sequences with TR = 500 ms and sagittal orientation. 2 Measured at the distance of DX = 60 cm and the angle of 30°, SPL0 = 56 dB.

#### Table 2.

Comparison of the mean energetic parameters of the vibration signal and the acoustic noise SPL (together with std. values in parentheses) for different settings of the TE time.

#### Figure 12.

Visualization of energetic relations of the vibration (upper set of graphs) and noise (lower set) signals for different TR times; {60, 100, 200, 300, 400, 500} ms—basic statistical parameters of: (a) Enc0, (b) Enr0, and (c) EnTK; used Hi-Res GE-T2 sequences with TE = 22 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

MRI device and of the acoustic noise signal using the microphone in its proximity, and the additional measurement to check the noise SPL. In this part of the measurement, the microphone stand with the Behringer dual-diaphragm condenser microphone B-2 PRO was placed together with the SPL meter at the distance of D<sup>X</sup> = 60 cm, and the 140-mm diameter spherical testing phantom filled with doped water [15] was placed inside the knee RF coil. The SB-1 sensor [12, 18] was used to pick up the vibration signal inside the scanning area of the MRI Opera device. Practical position of the sensing disc was on the surface of the plastic holder of the bottom gradient coils, as can be seen in the arrangement photo in Figure 9. The stored recordings were further processed in order to evaluate and compare the measured signal properties. All the noise and vibration signals were recorded with the help of the Behringer Podcast Studio equipment. The signals with duration of about 15 s sampled at 32 kHz were next processed in the sound editor program Sound Forge 9.0a.
