Jennifer Nussbaum1, Maria Engel1, and Klaas Paul Pruessmann1
1Institute for Biomedical Engineering, ETH Zurich and University of Zurich, Zurich, Switzerland
Synopsis
Concurrent acquisition of dynamic fields has become a prominent
way to correct for field imperfections. However, when using field probes in
close proximity to a head coil, the field measurement can be corrupted by eddy
currents on the head coil that are induced by the gradient fields. In this
work, we provide a one-time calibration solution to correct for this issue.
Introduction
Concurrent acquisition of dynamic fields
[1] has become a prominent way to correct for field imperfections. In most
cases it is needed when the MRI hardware is at driven at its limits, as it is
required for fast imaging techniques as EPI and Spiral. However, when using
field probes in close proximity to conductors, the field measurement can be
corrupted by eddy currents on the conductors that are induced by the gradient
fields. This was observed when using 16 transmit/receive 19F NMR probes [2,3] together with a quadrature-transmit and 32-channel
head receive array (Nova
Medical,Wilmington, MA). Using the disturbed probe data for reconstruction resulted
in ghost artefacts in EPIs and blurring in Spirals.
In this work, we show the effect of the
eddy currents induced by a head coil on nearby field probes and provide a one
time calibration solution to correct for this issue.Methods
The experiments were carried out on a 7T
Achieva system (Philips Healthcare, Best, Netherlands) using a quadrature-transmit
and 32-channel head receive array (Nova Medical,Wilmington, MA). The field
dynamics were acquired with 16 fluorine NMR field sensors mounted on a laser-sintered
nylon frame installed between the transmit coil and the receive array as
described in [4].
To capture the whole spectrum of the
effect of the head coil on the probe signal, 40 ms long
frequency swept pulses (0-30kHz, slew rate = 200mT/m/s, maximal amplitude =
30mT/m or 20mT/m) were played out and acquired, once in presence and once in
absence of the head coil. To obtain high
spectral resolution, ten partial measurements were concatenated
to a single 200ms readout and 50 averages were acquired.
To calculate the removal of the head coil
induced eddy currents on the field probe data, we state three deliberate
assumptions: First, the eddy currents
are local and only disturb the probe signal but not the actual MR measurement.
Second, the eddy currents are (mainly) driven by the 3 gradients and superpose
linearly. And third, the signal in presence of the head coil is related by a
linear and time invariant function to the signal in absence of the same.
Then, the spectra of the gradient fields g(ω) at probe positions fulfil: gpres(ω)=A(ω) gabs(ω) where gpres(ω) is the field in presence and gabs(ω) in absence of the head coil. For each frequency ω, A(ω) is a 16 x 16 -transfer-matrix and can therefore not be fully measured by using the first order gradients only. However, based on our assumptions, we can get a very good estimate for A(ω) :
A(ω)=1+( gpres(ω)-gabs(ω) )(gabsT(ω) gabs(ω))-1 gabsT(ω) ,
where gabsT(ω) is the Hermitian transpose of gabs(ω). Monitored field data hpres(t) can then be corrected by a convolution, which in frequency domain is a multiplication: hcorr(ω)=A(ω)-1 hpres(ω).Results
In Figure 1 the differences of the spectra of
the gradient fields at the probe positions measured in presence and in absence
of the head coil are shown. The effect amounts to several per mill of the
maximal probe amplitude and concerns all probes. Figure 2 shows an example of a
correction: the probe phase under a different frequency swept x-gradient was
corrected. The result is summarized in Figure 3, which shows the mean remaining
error of all corrections (with the mean taken over all three gradients and all
probes), which is a lot smaller than the mean error without correction.
Finally, Figure 4 shows that the ghost in an EPI reconstructed with a
concurrently monitored trajectory vanishes when applying the correction.Discussion and Conclusion
Erroneous probe signal due to local eddy
currents induced by a head coil close to the probes can be effectively removed
by convolving the signal with a transfer matrix. This one-time measurement correction
enables now concurrent field monitoring of gradient sequences that were avoided
because of this issue.Acknowledgements
The authors thank Thomas Schmid for his support in the setup construction.References
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