Synopsis
This work present two
methods for designing simple and compact planar coils. The length of a
constant wire width is minimized using Euclidian and Manhattan distance.
Euclidian coils exhibit smooth patterns of slightly lower resistance than that
of square-shape coils obtained using Manhattan distance. Manhattan coils show straight
conductors which minimizes the force and facilitate the coil manufacturing. The
coil sensitivity increases up to 1.8 times when placed in the iron poles but in
detriment of the coil slew rate. Compact transverse coils architected using
both distance minimization induces a residual eddy current smaller than 0.05% within
the DSV. Target Audience
This work will be of
interest of those interested in the design of gradient/shim coils for
conventional and hybrid MRI scanners.
Purpose
This study aims to present two methods for
designing planar gradient coils with emphasis in the simplicity of coil
manufacturing and coil performance: $$$\eta^{2}/R$$$ and $$$\eta^{2}/L$$$; where
η is the coil sensitivity, L and R are coil
inductance and resistance, respectively.
The MRI market is broadly dominated by 1.5T
whole body scanners. Therefore, in the last decade whole-body gradient coils
had all the attention from coil designers to mitigate limitations demanded by
the high field strength and fast sequences. However, planar gradient coils have
received limited attention despite the extensive use in low field MRI scanners.
Designers dedicated to architect planar gradient coils focused their work in
variations of the target field method and sometimes the effect of the pole is not
considered due to the heavy calculation required for simulating the magnetization
effect produced by the gradient field. Moreover, shielding, force balancing and
simplicity of construction are not usually considered due to the constrained
space (see Figure A) and the limitation of the applied method. The lack of
active shielding is justified by the use of a grid of high resistivity material
(eddy device) located between the pole face and the gradient coil, leaving the
pole ring exposed to the induction of eddy currents. In addition single layer
coil create additional difficulty for an appropriate force balancing. In
this work convex optimization is used to minimize the coil conductor assuming
Euclidian distance
1 and the Manhattan distance, respectively. Different
from minimizing ||j||
11, in this work the Manhattan (l1-norm) distance of the coil conductor
is calculated assuming that the distance between two points is calculated at right
angles; which is equivalent to min||
J||
1.
J and j are the current density vector
and magnitude, respectively.
Method
Convex optimization
was used to minimize the joule loses in gradient and shims coils
1.
It was demonstrated that compact circle-shape like winding patterns of reduced conductor
length generates higher performance $$$\eta^{2}/R$$$ compared to that produced
by coils designed using minimal resistance approach
1. Compact winding
minimizes the induction of eddy currents in the pole ring as the coil most
external turn is far from the ring (see figure B for reference). However,
simplicity of manufacturing where the wires may be placed by hand is highly
desirable as one of the ways to reduce cost. Sections of straight wires
certainly facilitate the coil manufacturing. This type of current pattern are obtained
by minimizing the joule power and applying convex optimization assuming
that the coil conductor length is minimized using the Manhattan norm. The optimization
problem is stated as $$$min\left\{max\bf ||J_\it{i}||_{\it{2}}\normalsize
\sum_i^NA_{i}\bf||J_\it{i}||_{1}\right\}$$$ subject to field, force/torque, shielding
and maximal current density Jo constraints. A total of sixteen planar gradient
coils were designed using min||
J||
1
and min||
J||
2. The weighting
factor α was used to trade the minimization of the conductor
length and resistance. Coil performance and force were evaluated for each
strategy and finally two planar gradient coils were designed for each strategy considering
the magnetization effect of the iron pole
2 (the yoke and the permanent magnet were not
considered). The maximal current density was constrained to a target value for
all designs. The coil diameter was set to 540 mm, the axial position at ±150 mm;
a gradient of 40 mT/m and maximal non-linearity of 5% was set as target field within
16.5 mm DSV. Constant wire width of 4 mm
was used and Bo magnetic field profile was provided by the magnet vendor (0.35T).
Results and Discussions
Figure C shows a
current pattern corresponding to min||
J||
1;
the pattern shows a typical D-shape like winding while figure D describes a
bean-shape resulting from min||
J||
2.
The pattern showed in D is smooth and that described in figure C has sections of straight wires. Figure E, shows a
typical resistance minimization design where the coil covers the whole coil surface
domain. Inductive interactions are more likely to occur between this type of
configuration and the pole ring due to close proximity between them. However, residual eddy currents for X/Y coils
are smaller than 1%; contrary Z coil residual eddy can be as high as 30% but
this case is not presented here. Figure F, shows that coils designed using min||
J||
2 exhibits better $$$\eta^{2}/R$$$
than that using min||
J||
1,
mainly due to the shorter conductor length and lower resistance. When α->1 the coil tends
to occupy the whole surface hence increasing the wire length and as results the
resistance. This effect, however works in favour of $$$\eta^{2}/L$$$ (figure G)
and again, in detriment of the force exerted on the coil winding (figure H). Figures
I,J and K, describe the planar coils designed considering the iron pole
2.
The iron pole increases the field 1.8 but increases L as much as twice the
inductance when no iron is present. Patterns in Figures I and J, show
simplicity and compact winding compare to that of figure K. Reverse turns
appear in K to compensate the pole effect over the field linearity. The coil
performance in I and J are similar but superior to coil pattern in K; the
residual eddy in the DSV was 0.05%.
Acknowledgements
No acknowledgement found.References
[1] Michael S Poole, and N
Jon Shah, JMR, 244 (2014), 36-45.
[2] Jingguo
Wang et all, JMRI, 6 (1996), 239-43.