3.5 Analysis of the influence of the scan parameters on the time duration and the quality factor of the MR images

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 scans of the human vocal tract in Figure 15.

The console program "ESAMRI" of the MRI device control software [15] was used to carry out the following two parts of the analysis and comparison:

	- 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 for both Hi-Res sequences types,
	- 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 numerically in Table 5 for the scan sequence Hi-Res SE18 HE.


#### Table 3.

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

• mass of the object inserted in the MRI device scanning area {testing phantom/ lying person}—see graphical comparison of the mean values of the energy and

The slice orientations as well as the TE and TR parameters were set manually to

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

Sequence<sup>1</sup> Vibrations (SB-1) Noise<sup>2</sup> SPL (C) [dB]

Comparison of the mean energetic parameters of the vibration signal and the acoustic noise SPL (together with

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.

Signal RMS[] EnTK [] Enc0[] Enr0[] TE = 18 ms 31.5 (1.53) 4.32 (0.67) 0.044 (0.002) 23.04 (4.7) 61.5 TE = 22 ms 34.6 (2.11) 4.96 (1.02) 0.040 (0.003) 24.03 (8.5) 62.5 TE = 26 ms 36.0 (2.27) 5.75 (0.85) 0.055 (0.004) 24.40 (9.3) 63.0

Used Hi-Res SE-HF scan sequences with TR = 500 ms and sagittal orientation.

Measured at the distance of DX = 60 cm and the angle of 30°, SPL0 = 56 dB.

std. values in parentheses) for different settings of the TE time.

• repetition time TTR = {60, 100, 200, 300, 400, 500} ms—documented by comparison of the basic statistical parameters calculated from the vibration

basic spectral properties of the vibration signal in Figure 14.

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

and noise signals in Figure 12,

Noise and Vibration Control - From Theory to Practice

the MRI device were approx. 80 and 55 kg.

1

2

Table 2.

Figure 12.

108

Influence of the slice thickness on the predicted quality factor of the MR image and on the time duration for the scan sequence Hi-Res SE18 HE (TR = 500 ms, NACC = 1).


4. Discussion of the obtained results

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

that is observed in the scanning area of the MRI device.

size allowing the best low-frequency sensitivity.

measurements.

111

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

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

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

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,

Table 4.

Dependence of the time duration TDUR [min:sec] on setting of TR and NACC parameters—merged values for both Hi-Res sequences of SE and GE types; slice thickness = 4.5 mm.


Table 5.

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 Hi-Res SE18 HE (TR = 60 ms and slice thickness = 10 mm).


#### Table 6.

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 42 phases, the values are in parentheses).


#### Table 7.

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 values are in parentheses).

	- the SS-3D-balanced 10 sequences—see the values in Table 6,
	- the 3D-CE 30 scan sequence (see Table 7).

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