Edwin Versteeg1, Tijl Van der Velden1, Jeroen Hendrikse1, Dennis Klomp1, and Jeroen Siero1,2
1Radiology, University Medical Center Utrecht, Utrecht, Netherlands, 2Spinoza Centre for Neuroimaging Amsterdam, Amsterdam, Netherlands
Synopsis
A silent gradient
axis can be achieved by driving a gradient insert above 20 kHz. In this work,
we investigate a prototype silent gradient insert that features two axes. Such
a setup would enable both silent and fast imaging. The two axes were driven with
an audio amplifier at 20 kHz and 22 kHz, and produced gradient amplitudes of 20.8
and 22 mT/m. We simulated the acceleration potential to be a factor of 9 and showed
the feasibility of imaging with this setup on a phantom.
Introduction
Rapidly
oscillating gradients can be used to accelerate imaging and allow for more
efficient imaging. Encoding methods such as Wave-CAIPI, bunched phase encoding
or FRONSAC utilize such oscillating gradients to reach up to 9-fold
acceleration with only a limited noise penalty.1–3 Previously, we introduced a method for
silent imaging that incorporates a single-axis gradient insert driven at 20 kHz
while remaining insensitive to peripheral nerve stimulation. This is an order
of magnitude faster oscillation than used in the aforementioned methods.4 Adding
an extra oscillating axis to our previous setup would enable substantial improvement
in acceleration potential, but also simultaneous silent and fast imaging. Here,
we present a prototype setup that features two axes oscillating around 20 kHz
and assess its feasibility for speeding up imaging for 3D gradient-echo
applications. Methods
A dual-axis
gradient insert was designed and produced in-house (Figure 1). The resulting
setup featured an x-and z-axis, which were made resonant at 22.0 kHz (x) and
19.9 kHz (z) using capacitors. A two-channel audio amplifier (18 kW) was used
to drive both axes, so far without gradient filters. Two external waveform
generators synthesized the gradient waveforms and were triggered by a TTL-pulse
send out by our 7T MR-system (Philips Achieva). A dynamic field camera (Skope)
setup was used to measure the achievable gradient amplitudes.5 An 8-channel
dipole array was added for RF transmit and receive.
A modified 3D
gradient-echo sequence was used for image acquisition. This sequence consisted
of a readout gradient in one direction with concurrent oscillating gradient on
the other two physical axes. Here, the whole-body gradients (GX, GY,
GZ) were used in synergy with the gradient insert (GX,insert,
GZ,insert). The frequency difference between the two oscillating gradient
axes resulted in a Lissajous-like k-space trajectory, filling a cuboid in
k-space at each TR as measured by the field cameras (Figure 2).
The imaging
result of this sequence was simulated using a 3D Shepp-Logan phantom, a coil
array of 16 channels (two rows of 8 channels), a 112x112x112 matrix size, and
by using a generalized conjugate gradient (CG) SENSE reconstruction. Two cases were considered: one sequence which
featured an overlap in the k-space sampling and one without overlap (see Figures
3 and 4). An SNR of 15 was imposed on the signal.
The
aforementioned setup and sequence were used to image a water-filled phantom.
This acquisition featured overlap in the k-space sampling and the following
imaging parameters: FOV = 224x224x112 mm3, voxel size = 2 mm
isotropic, TR/TE = 61/8.1 ms and flip angle = 25 degrees. Reconstruction was
performed using a Non-Uniform Fast Fourier Transform (NUFFT) (CG-SENSE recon
was not yet possible due to unavailable coil sensitivity data).Results and Discussion
The gradient
amplitude was measured to be 22.2 mT/m (@22 kHz) for the x-gradient and 20.8
mT/m (@19.9 kHz) for the z-gradient. Figures 3 and 4 show the results of the
simulated imaging experiments using these measured amplitudes. Here, the NUFFT
reconstruction showed aliasing artifacts while no artifacts were visible in the
CG-SENSE reconstruction (see 2nd and 3rd column Figures 3
and 4).
The
reconstruction errors of both methods showed the same behavior as the images.
Here, the residual of the CG-SENSE reconstruction featured unstructured noise,
while the NUFFT reconstruction showed structural patterns from the aliasing
(see 4th column Figure 3 and 4). Removing the overlap increased the
noise in the images (see 5th column Figures 3 and 4), because of the
reduced number of samples used for reconstruction.
In terms of
acceleration, the simulations showed that artifact free images could be
reconstructed when using only 37x37 = 1369 phase encoding steps. This equates
to an acceleration factor of (112*112)/(37*37) = 9.1 when compared to the fully
sampled image. Note, that the acceleration potential of this setup is limited
by the amplitude of the oscillating gradient and can be increased by having
separate amplifiers for each axis. In a recent report, a single-axis with one amplifier
was able to produce gradient amplitudes reaching 40 mT/m.4
Figure 5 shows multiple
slices through the water-filled phantom. Here, we see that almost no aliasing
artifacts were visible in the coronal slices. Residual aliasing was visible in
the sagittal slices as the NUFFT reconstruction cannot unfold the aliasing
pattern from the oscillating gradients without coil sensitivity data. A CG-SENSE
reconstruction, as was used in the simulation, could then be used to eliminate
these artifacts. The noise in the images
stemmed from an open RF-cage caused by the gradient inserts cable routing.
The
presented setup did produce an audible sound at a beat frequency of 2.1 kHz when
both axes were driven simultaneously. A completely silent dual axes setup may be
achieved by embedding the coils in epoxy or retuning the dual axis setup with
different capacitor values to decrease the frequency difference to an inaudible
frequency. Conclusion
We have demonstrated
a setup for spatial encoding of 3D-MRI using two rapidly oscillating gradient
axes driven around 20 kHz. The acceleration potential of this setup was
simulated and found to be a factor of 9. First imaging experiments showed that
imaging with such a setup is feasible. Acknowledgements
No acknowledgement found.References
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