Optimized RF shield design

Patent metadata

Field	Value
Publication number	US-10557902-B2
Application number	US-201514685021-A
Country	US
Kind code	B2
Filing date	Apr 13, 2015
Priority date	Mar 7, 2011
Publication date	Feb 11, 2020
Grant date	Feb 11, 2020

Abstract

Radio frequency (RF) shields used with magnetic resonance imaging (MRI) apparatus may experience gradient field induced eddy currents and RF field induced eddy currents. These eddy currents can cause the RF shield to heat up at an undesirable rate. RF shields are designed to have a desired degree of RF shielding and a desired heating attribute. Design goals for RF shields include gradient field transparency and RF field opacity, both of which can be influenced by eddy currents. Example methods identify a gradient field that will induce eddy currents and identify an RF field that will induce eddy currents. If a region on the RF shield is identified where the desired heating attribute will not be achieved, then a pattern of axial cuts and azimuthal cuts can be made in the RF shield to reduce gradient eddy current heating in the RF shield while maintaining desired RF shielding.

First claim

What is claimed is:

1. A radio frequency (RF) shield for use with a split magnetic resonance imaging (MRI) apparatus, comprising:
a dielectric material layer;
a first sheet attached to a first side of the dielectric material layer; and
a second sheet attached to a second, different side of the dielectric material layer,
where the first sheet includes a first set of axial cuts sized and positioned to reduce ohmic heating due to eddy currents induced in the RF shield by a split X-gradient coil in the split MRI apparatus in the split MRI apparatus,
where the first sheet includes a first set of azimuthal cuts sized and positioned to reduce ohmic heating due to eddy currents induced in the RF shield by the split X-gradient coil,
where the second sheet includes a second set of axial cuts sized and positioned to reduce ohmic heating due to eddy currents induced in the RF shield by a split Y-gradient coil in the split MRI apparatus,
where the second sheet includes a second set of azimuthal cuts sized and positioned to reduce ohmic heating due to eddy currents induced in the RF shield by the split Y-gradient coil.

2. The RF shield of claim 1 , where the first set of axial cuts and the first set of azimuthal cuts are positioned with respect to the second set of axial cuts and the second set of azimuthal cuts to account for rotating fields produced by quadrature operation of the split MRI system.

3. The RF shield of claim 1 , where at least one of, a length, a width, and a position of a member of the first set of axial cuts, a member of the second set of axial cuts, a member of the first set of azimuthal cuts, and a member of the second set of azimuthal cuts are determined, at least in part, by eddy current pattern distributions on the first sheet and the second sheet.

4. A printed circuit board radio frequency (RF) shield for use with a split magnetic resonance imaging (MRI) apparatus having a split x-gradient coil, a split y-gradient coil, and an RF transmission coil, comprising:
a dielectric material layer;
a first metallic sheet attached to or in contact with a first side of the dielectric material layer; and
a second metallic sheet attached to or in contact with a second, different side of the dielectric material layer,
where the first metallic sheet includes a first set of axial cuts positioned in a region where an x-gradient coil eddy current induced in the printed circuit board RF shield by the split x-gradient coil exceeds an x-gradient coil threshold density level, and where an RF transmission coil eddy current induced in the printed circuit board RF shield by the RF transmission coil is less than a first RF transmission coil eddy current density level;
where the first metallic sheet includes a first set of azimuthal cuts positioned in a region where the RF transmission coil eddy currents induced in the printed circuit board RF shield by the RF transmission coil are greater than a second RF transmission coil eddy current density level,
where the second metallic sheet includes a second set of axial cuts positioned in a region where a y-gradient coil eddy current induced in the printed circuit board RF shield by the split y-gradient coil exceeds a y-gradient coil threshold density level, and where the RF transmission coil eddy current induced in the printed circuit board RF shield by the RF transmission coil are less than a third RF transmission coil eddy current density level,
where the second metallic sheet includes a second set of azimuthal cuts positioned in a region where the RF transmission coil eddy current induced in the printed circuit board RF shield by the RF transmission coil is greater than a fourth RF transmission coil eddy current density level,
where the first set of axial cuts and the first set of azimuthal cuts reduce ohmic heating due to the x-gradient coil eddy current without reducing RF shielding, and
where the second set of axial cuts and the second set of azimuthal cuts reduce ohmic heating due to the y-gradient coil eddy current without reducing RF shielding.

5. The printed circuit board RF shield of claim 4 , where the x-gradient coil eddy current has a density described by:

J
ϕ

⁡

(

ϕ
,
z

)

=

∂

∂
z

⁢

S
⁡

(

ϕ
,
z

)

;

J
z

⁡

(

ϕ
,
z

)

=

-

1

R
s

⁢

∂

∂
ϕ

⁢

S
⁡

(

ϕ
,
z

)

where J is the eddy current density (A/m 2 ).

6. The printed circuit board RF shield of claim 4 , where the y-gradient coil eddy current has a density described by:

J
ϕ

⁡

(

ϕ
,
z

)

=

∂

∂
z

⁢

S
⁡

(

ϕ
,
z

)

;

J
z

⁡

(

ϕ

Assignees

• Univ Case Western Reserve

Inventors

• Brown Robert W
• Wu Yong
• Yao Zhen
• Shvartsman Shmaryu
• Chmielewski Thomas
• Eagan Timothy

Classifications

• G01R33/385

  using gradient magnetic field coils · CPC title

• G01R33/56518

  due to eddy currents, e.g. caused by switching of the gradient magnetic field · CPC title

• G01R33/4822

  in three dimensions · CPC title

• G01R33/422 Primary

  of the radio frequency field · CPC title

• Y10T83/505

  Operation controlled by means responsive to product · CPC title