Reduced acoustic noise in diffusion tensor imaging on a compact MRI system

Abstract

Purpose: To investigate the feasibility of substantially reducing acoustic noise while performing diffusion tensor imaging (DTI) on a compact 3T (C3T) MRI scanner equipped with a 42-cm inner-diameter asymmetric gradient.

Methods: A-weighted acoustic measurements were made using 10 mT/m-amplitude sinusoidal waveforms, corresponding to echo-planar imaging (EPI) echo spacing of 0.25 to 5.0 ms, on a conventional, whole-body 3T MRI and on the C3T. Acoustic measurements of DTI with trapezoidal EPI waveforms were then made at peak gradient performance on the C3T (80 mT/m amplitude, 700 T/m/s slew rate) and at derated performance (33 mT/m, 10 to 50 T/m/s) for acoustic noise reduction. DTI was acquired in two different phantoms and in seven human subjects, with and without gradient-derating corresponding to multi- and single-shot acquisitions, respectively.

Results: Sinusoidal waveforms on the C3T were quieter by 8.5 to 15.6 A-weighted decibels (dBA) on average as compared to the whole-body MRI. The derated multishot DTI acquisition noise level was only 8.7 dBA (at 13 T/m/s slew rate) above ambient, and was quieter than non-derated, single-shot DTI by 22.3 dBA; however, the scan time was almost quadrupled. Although derating resulted in negligible diffusivity differences in the phantoms, small biases in diffusivity measurements were observed in human subjects (apparent diffusion coefficient = +9.3 ± 8.8%, fractional anisotropy = +3.2 ± 11.2%, radial diffusivity = +9.4 ± 16.8%, parallel diffusivity = +10.3 ± 8.4%).

Conclusion: The feasibility of achieving reduced acoustic noise levels with whole-brain DTI on the C3T MRI was demonstrated. Magn Reson Med 79:2902-2911, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Keywords: compact 3T; diffusion tensor imaging; head-only gradient; silent MRI.

Figure 1

Gradient waveforms used in phantom and in vivo imaging for (a) underated, single-shot DTI (diffusion amplitude = 80 mT/m, EPI amplitude = 35 mT/m, ESP = 0.33 ms) and (b) derated, single-shot DTI (diffusion amplitude = 33 mT/m, EPI amplitude = 6.4 mT/m, ESP = 2.0 ms). The echo times (TE) of both sequences were matched to the minimum TE of the derated sequence.

Figure 2

Acoustic noise measurements on a whole-body 3T scanner vs. the compact 3T scanner using 10 mT/m amplitude sinusoidal gradient waveforms, of half-periodicity matching the EPI-relevant echo spacing (ESP), on the (a) X (left-right), (b) Y (anterior-posterior), and (c) Z (superior-inferior) axes.

Figure 3

Effects on EPI echo spacings (ESP) from (a) varying gradient slew rates via derating, and in turn (b) on the measured acoustic noise (in dBA). The ambient noise level measured on the compact 3T scanner (dashed) and the selected ESP for imaging (arrow) are shown.

Figure 4

Difference between diffusion metrics obtained from derated multi-shot vs. standard (non-derated) single-shot DTI in an MRS sphere phantom for various scans, with standard non-iterative reconstruction and with iterative reconstruction (indicated as “Recon”). The differences in (a) ADC, (b) FA, (c) radial diffusivity and (d) parallel diffusivity are shown, normalized as percentage differences with the exception of FA.

Figure 5

Plots of diffusivity measurements in a multi-compartment, diffusion ice phantom between derated multi-shot and standard (non-derated) single-shot DTI for standard (non-iterative) reconstruction and iterative multi-shot reconstruction. Plots for (a) ADC, (b) FA, (c) radial diffusivity and (d) parallel diffusivity are shown, with the line of proportionality also shown (dashed).

Figure 6

Axial maps (trace diffusivity, ADC and color-encoded FA) from derated multi-shot DTI from subject #1, with selection ROI regions for motor, corpus callosum frontal/occipital (CCF/CCO), and brain stem (BS) indicated on the color-encoded FA maps.

Figure 7

Sagittal reformat of axial DTI maps from subject #5, showing comparable diffusion maps between derated multi-shot DTI and standard single-shot DTI and increased spatial distortion in the derated DTI in the phase-encoding direction (anterior-posterior, horizontal direction in this figure).

Figure 8

Bland-Altman plots of diffusivity measurements across seven normal subjects in various brain ROI regions, with differences for the metrics obtained from derated DTI minus that from standard DTI shown. Plots are shown for (a) ADC, (b) FA, (c) RD and (d) PD. The vertical axes are normalized to show percentage differences.