We measured sound pressure levels induced by gradient impulses in a 3.0-T superconducting MRI system (Signa Excite HDxt,
General Electric Healthcare,
Wisconsin,
USA) and a 0.4-T permanent magnet open MRI system (APERTO Eterna,
Hitachi medical corporation,
Tokyo,
Japan) (figure 2).
Measurements were performed using a precision integrating sound level meter (ISLM)(NL-18,
RION Co.,
Ltd.,
Tokyo,
Japan) connected to an electret condenser microphone.
![](https://epos.myesr.org/posterimage/esr/ecr2014/120906/media/549031?maxheight=300&maxwidth=300)
Fig. 2: a) 3.0-T superconducting MRI system (Signa Excite HDxt, General Electric Healthcare, Wisconsin, USA),
b) 0.4-T permanent magnet open MRI system (APERTO Eterna, Hitachi medical corporation, Tokyo, Japan).
The measurement positions inside the bore in both the systems were 143 and 75 points,
respectively (figure 3).
A simple trapezoidal pulse was applied to X,
Y,
and Z gradient coils,
as shown in figure 4.
The gradient pulse amplitude,
rise and fall times,
and gradient pulse on time are summarized in Table 1.
Within the manufacturer’s pulse sequence design limitations,
the rise and fall times and gradient pulse on time were the minimum allowed,
whereas the amplitude was the maximum allowed.
Acoustic noise waveforms induced by a single gradient pulse were digitalized and sampled at 96 kHz,
following which data were acquired in the frequency domain using a computational tool,
including Fourier transforms.
To measure the peak sound pressure levels,
a microphone was directed parallel to the patient axis and in a horizontal plane,
without a patient or phantom on the patient table (figure 5).
![](https://epos.myesr.org/posterimage/esr/ecr2014/120906/media/549050?maxheight=300&maxwidth=300)
Fig. 3: The measurement positions inside the bore in the a) superconducting MRI system (143 points) and b) permanent magnet open MRI system (75 points).
![](https://epos.myesr.org/posterimage/esr/ecr2014/120906/media/549065?maxheight=300&maxwidth=300)
Fig. 4: Gradient pulse were excited for each gradient.
Pulse sequence was programmed to excite X, Y, and Z coil, respectively.
The gradient pulse amplitude, rise and fall times, and gradient pulse on time are summarized in Table 1.
![](https://epos.myesr.org/posterimage/esr/ecr2014/120906/media/549066?maxheight=300&maxwidth=300)
Table 1: Characteristic of narrow trapezoidal input waveform
![](https://epos.myesr.org/posterimage/esr/ecr2014/120906/media/549067?maxheight=300&maxwidth=300)
Fig. 5: Experimental set-up for acoustic noise measurement.
The MRI scanner was confirmed to be a linear system between acoustic noise and gradient amplitude [4].
We also conducted preliminary tests to check the linearity in both the scanners (figure 6).
As shown in figure 7,
we used the deconvolution process to calculate GPAN-TFs [μPa/(mT/m)] for the various spatial measurement positions inside each gradient coil (i.e.,
X-,
Y-,
and Z-axes).
![](https://epos.myesr.org/posterimage/esr/ecr2014/120906/media/549068?maxheight=300&maxwidth=300)
Fig. 6: Relation between gradient amplitude and acoustic noise.
A strong positive correlation was noted.
There is a linear dependency between acoustic noise and gradient amplitude.
![](https://epos.myesr.org/posterimage/esr/ecr2014/120906/media/549070?maxheight=300&maxwidth=300)
Fig. 7: Calculation of GPAN-TF in each gradient coil (X, Y, and Z-axis), by the deconvolution process.