*3.2. Description of Main Measurement and Auxiliary Experiments*

For all basic measurements, a plastic sphere-phantom of a 140-mm diameter filled with doped water [6] was placed inside the knee RF coil; see the arrangement photo in Figure 2. For acoustic noise SPL measurement in the MRI device vicinity, the multi-function environment meter Lafayette DT 8820 was used. Its distance from the center of the scanning area was 60 cm, its height from the floor was 75 cm (at the level of the bottom gradient coils), and it was oriented at 30 degrees from the left corner where the temperature stabilizer was placed. First, the noise SPL was mapped in the range of distances from 45 to 90 cm—the obtained values for Hi-Res SE and GE sequences are presented in Figure 3. As a result of the preliminary measurements, the SB-1 sensor was used to record the vibration signal from the solid surface of the plastic holder of the bottom gradient coils in the MRI scanning area—see

position P0 in Figure 2. This figure also shows positions of the sound level meter (Lafayette DT 8820) and the dual diaphragm condenser microphone (Behringer B-2 PRO) on a stand with shock mounting for the noise SPL and signal pick up. The signals of the vibration and noise were routed through the Behringer Podcast Studio equipment by USB connection to PC. The signals with duration of about 15 s were sampled at 32 kHz and then processed in the sound editor program Sound Forge 9.0a.

**Figure 2.** Arrangement photo of SPL noise measurement and parallel recording of noise and vibration signals of the open-air MRI device Opera using the testing phantom.

**Figure 3.** Mapping of the acoustic noise SPL at different distances DX = {45, 50, 55, 60, 70, 80, and 90} cm from the middle of the scanning area of the MRI device for SE/GE sequences: (**a**) SPL values together with the background ones (SPL0), and (**b**) box-plot of their basic statistical parameters.

The main experimental measurement was aimed at investigation of the impact of MRI scan parameters on the recorded vibration and noise. As presented in our earlier paper [17], only five types of scanning sequences are implemented in the investigated MRI device [6]:


In practice, two basic types of scan sequences are commonly used for non-invasive examination of human body parts by acquiring high-quality MR images in this type of MRI device:


The baseline measurement and recording of the vibration and noise signals were carried out during the execution of MR scan sequences typical for 3D imaging of the human vocal tract. Figure 4 shows how the energetic and spectral parameters of the picked-up vibration and noise signals are determined and processed.

**Figure 4.** Block diagram of processing and comparison of vibration and noise signals.

Scan parameters of five tested MR sequences (*T*SEQV = {Hi-Res SE 18 HF, Hi-Res SE 26 HF, Hi-Res GE T2, SS-3Dbalanced, 3D-CE}) were set as shown in Table 1. Graphical comparison of energetic relations between the measured vibration and noise signals can be seen in Figure 6. Then, analysis of the influence of scan parameters on properties of the vibration and noise signals was executed for different parameters:



**Table 1.** Basic scan parameters of used MR sequences.


**Table 2.** Comparison of mean energy values of vibration and noise signals for different objects placed in the scanning area of the MRI device.

<sup>1</sup> Used Hi-Res SE-HF scan sequences with TE = 18 ms, TR = 400 ms, and sagittal orientation.

**Figure 5.** Visualization of vibration signal features for different slice orientations: {Coronal, sagittal, transversal}; (**a**) bar-graph of signal RMS values; (**b**) histograms of *En*c0; (**c**) mutual *F*v1/*F*v2 positions for Hi-Res SE scan sequences with TE = 18 ms and TR = 500 ms.

**Figure 6.** Comparison of energetic relations of vibration and noise signals for different sequence types—Hi-Res {SE-HE, SE-HF, GE-T2} and 3D {SS-3Dbal, 3D-CE}; (**a**) signal RMS together with SPL values; (**b**) bar-graphs of basic statistical parameters of *En*c0 values; (**c**) corresponding histograms for *En*r0 parameter. In all cases, a sagittal slice orientation was used.

**Figure 7.** Visualization of vibration signal features for different TE times: {18, 22, 26} ms; (**a**) bar-graphs of signal RMS values and basic statistical parameters: (**b**) EnTK; (**c**) Enc0; (**d**) Enr0; (**e**) mean mutual Fv1/Fv2 positions for Hi-Res SE-HF sequences (TR = 500 ms, sagittal orientation).

**Figure 8.** Visualization of energetic relations of vibration and noise signals for different TR times: {60, 100, 200, 300, 400, 500} ms; (**a**) signal RMS together with noise SPL values; (**b**) mean Enc0; (**c**) mean Enr0; (**d**) mean EnTK; used Hi-Res GE-T2 sequences with TE = 22 ms, and sagittal orientation.

### *3.3. Parameters Determinig the Scan Sequence Duration and the MR Image Quality Factor*

The final scanning time is practically defined by a chosen scan sequence and basic scan parameters (TR and TE). The default configuration [6] is often modified manually by changing these two parameters; however, other scan settings (number of slices, slice thickness, number of accumulations *N*ACC of the free induction decay (FID) signal [13], etc.) also have influence on duration, as well as on the MR image quality. The main aim is to obtain final MR images with maximum quality factor (*Q*F) and minimum scan time duration (*T*DUR).

Using the operating console of the Esaote Opera MRI device [6], we analyzed how the predicted MR image *Q*<sup>F</sup> and the scan-sequence time duration are affected by the following scan parameters:


**Table 3.** Dependence of the MR image quality factor *Q*<sup>F</sup> and the scanning time duration *T*DUR [min:sec] on TR and *N*ACC parameters; used Hi-Res SE26 HF sequence, slice thickness = 4.5 mm.


**Figure 9.** Influence of the thickness of the slice sample on the predicted quality factor and the time duration for the scan sequences Hi-Res SE18 HE and GE T2 22 (TR = 500 ms, *N*ACC = 1).

**Figure 10.** Influence of the number of FID signal accumulations on (**a**) the predicted image quality factor and (**b**) the time duration; analyzed scan sequences SS-3Dbalanced (TE = 10 ms, TR=20 ms, 3D phases = 24) and 3D-CE (TE = 30 ms, TR = 40 ms, 3D phases = 8).

**Table 4.** Dependence of the MR image quality factor *Q*<sup>F</sup> and the scanning time duration *T*DUR [min:sec] on TR and *N*ACC parameters; used Hi-Res GE-T2 22 sequence, slice thickness = 4.5 mm.

