Dan Ma1, Eric Y. Pierre2, Yun Jiang 2, Mark D. Schluchter3, Kawin Setsompop4, Vikas Gulani1, and Mark Griswold1
1Radiology, Case Western Reserve University, Cleveland, OH, United States, 2Biomedical Engineering, Case Western Reserve University, Cleveland, OH, United States, 3Epidemiology & Biostatistics, Case Western Reserve University, Cleveland, OH, United States, 4A.A Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Boston, MA, United States
Synopsis
An acquisition method named MRF-Music is proposed to mitigate
the acoustic noise during MRI scans by producing musical sounds directly from
the switching magnetic fields while simultaneously quantifying multiple
important tissue properties. MP3 music files were converted to arbitrary
encoding gradients, which were then used with varying flip angles and TRs in
both 2D and 3D MRF exam to generate T1, T2 and proton
density maps. The MRF-Music scans were shown to significantly improve patients’
comfort. T1 and T2 measured from phantom and in vivo scans
were also in good agreement with those from the standard measurements and
reported values. Introduction
Acoustic noise during MR scans can cause
discomfort for both patients and technicians. Instead of trying to eliminate
the noise from the scanner, we hypothesize that making the sounds pleasant
would also increase patient comfort. In this study, we take advantage of the
flexibility of Magnetic Resonance Fingerprinting (MRF)(1,2) to directly convert music to encoding gradients,
which were then used with varying flip angles and TRs to quantify multiple
tissue parameters simultaneously. This MRF-Music acquisition will significantly
improve patients’ experience during MR scans while maintaining high scan
efficiency and diagnostic quality.
Theory
The main considerations of
converting music to gradients were to 1) meet the hardware requirements of the
scanner system, 2) preserve the sound of the music, 3) combine gradient design
with basic imaging criteria such as sequence timing, image resolution and field
of view, and 4) have sufficient sampling density to generate accurate maps. Figure
1 illustrates the flow chart of the music conversion. The audio waveform (Fig
1a) was first pre-processed with a conventional audio compression filter that
constrains the waveform to a band of amplitude, and then low pass filtered to
remove high frequency oscillation and resampled to match the gradient raster
time(Fig 1b).
We have observed that one
can maintain sound quality while making relatively significant deviations from
the actual music waveform as long as the basic timing of the music is
preserved. In particular, it is important to preserve the inherent zero
crossings of the music waveform. Therefore, acquisition blocks (TR) were defined
in time by the zero crossings. These were then grouped such that each TR had a
fixed number of music segments, each of sufficient duration that starts and ends
at zero. Fig 1c shows an example of a SSFP-based sequence with four segments in
each TR, with each segment being assigned to a specific encoding gradient. In
Figure 2, half-sine waves were used to design slice selection gradients in
order to make RF pulse design easy and robust. For the RF excitation, VERSE RF
pulses(3,4) were designed to achieve the same slice profile for
all TRs. A scaled music waveform formed the readout segment in Gx and a
three-lobed balanced waveform was added in Gy to increase k-space coverage. The
amplitudes of these waveforms were derived from Table 1. Figure 2b shows
example k-space trajectories from ‘Bach Cello Suite No.1’ as performed by YoYo
Ma. MRF-Music has also been extended to 3D acquisition with isotropic image
resolution(5).
Methods
The total conversion time for a 2D MRF-Music
sequence was 2 minutes. All scans were acquired on a 3T scanner (Siemens
Skyra). Acquisition parameters were: FOV of 300x300 mm2, image
resolution of 1.2x1.2 mm2, slice thickness of 5 mm, and 4000 undersampled
images were acquired in 68 seconds. The resultant time series of images were
used to determine the values for T1, T2, and M0. A multi-scale reconstruction(6) was used to iteratively improve map quality.
The experiments included phantom studies to estimate accuracy and scan efficiency,
2D and 3D in vivo studies to evaluate image quality. Additionally we performed
a comfort level survey in 10 volunteers who haven’t had MRI before. These volunteers
were asked to rated their comfort levels immediately after a one-minute
acquisition from 1-10 (1=most uncomfortable and 10=most comfortable) for each
of four different acquisitions (TSE, EPI, MRF-Music and no scan), presented in
random order with 3 repetitions.
Results
The ANOVA analysis of the survey results showed
a significant improvement of the comfort level from MRF-Music scan in
comparison to both EPI and TSE, with an average scores of 7.2±1.2 from
MRF-Music and 4.5±1.4 and 4.9±1.5 from EPI and TSE, respectively. Figure 3a,b
compare T1 and T2 values of a phantom study from MRF-Music with those from MRF
and standard spin-echo methods. The concordance correlation coefficients (CCC)
indicated that both methods were in good agreement with the standard methods. Figure
3c,d compare the scan efficiency between MRF and MRF-Music. Both methods
achieve high precision in a short acquisition time, with conventional MRF only outperforming
MRF-Music by an average factor of 1.11 and 1.32 for T1 and T2, respectively. Figure
4 compares the maps obtained from MRF-Music with those from MRF scan. Both
quantitative values and tissue appearance are in good agreement.
Conclusion
This study demonstrated that MRF-Music can
combine data acquisition with a musical sound to quantify multiple tissue parameters
in a clinically acceptable time. Thus we believe that MRF-Music represents a
broadly applicable, flexible framework to improve patient care in MRI.
Acknowledgements
The authors would like to acknowledge funding from Siemens Healthcare and NIH grants NIH 1R01EB016728-01A1 and NIH 5R01EB017219-02References
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