Low eddy current RF shielding enclosure designs for 3T MR applications
Systems that simultaneously operate within an MRI system require special design considerations to avoid the electromagnetic (EM) interference among the systems 17. Magnetic resonance images are acquired by fast‐oscillating EM fields in a strong static magnetic field environment. To avoid EM interference, the inserted systems typically encapsulate electronic components inside a shielding Faraday enclosure to preserve the performances of both MRI and the inserted device 24. Without sufficient shielding designs, intense radiofrequency (RF) fields in the megahertz range could disturb the device electronics or, reciprocally, the noise generated from the device could corrupt the MRI image 21.
The main function of a shielding enclosure is to shield the RF field, while also minimizing any secondary artifact, such as the eddy current induced from the MR gradient fields. The primary mechanisms of EM interference (EMI) shielding are reflection and absorption 30. For reflection of the incident fields, the shield must have mobile charge carriers, which interact with the fields. Electrically conducting shields are preferred but not necessarily required. In addition, connectivity in the current path enhances the shielding effectiveness. The absorption loss is described by a function known as JOURNAL/mrim/04.02/01445475-201803000-00054/math_54MM1/v/2018-01-24T161827Z/r/image-png , in which JOURNAL/mrim/04.02/01445475-201803000-00054/math_54MM2/v/2018-01-24T161827Z/r/image-png is the relative electrical conductivity and JOURNAL/mrim/04.02/01445475-201803000-00054/math_54MM3/v/2018-01-24T161827Z/r/image-png is the relative permeability. With a few assumptions, including high conductivity ( JOURNAL/mrim/04.02/01445475-201803000-00054/math_54MM4/v/2018-01-24T161827Z/r/image-png ), thick shield material ( JOURNAL/mrim/04.02/01445475-201803000-00054/math_54MM5/v/2018-01-24T161827Z/r/image-png ), and the intrinsic impedance mismatch ( JOURNAL/mrim/04.02/01445475-201803000-00054/math_54MM6/v/2018-01-24T161827Z/r/image-png ), the reflection loss is independent of the shield thickness, whereas the absorption loss is proportional to the thickness of the shield. The shielding “effectiveness” is equal to the sum of both loss mechanisms, and may be expressed in terms of material parameters as follows: JOURNAL/mrim/04.02/01445475-201803000-00054/math_54MM7/v/2018-01-24T161827Z/r/image-png where JOURNAL/mrim/04.02/01445475-201803000-00054/math_54MM8/v/2018-01-24T161827Z/r/image-png is the conductivity of the shield, JOURNAL/mrim/04.02/01445475-201803000-00054/math_54MM9/v/2018-01-24T161827Z/r/image-png is the angular frequency of the RF field, JOURNAL/mrim/04.02/01445475-201803000-00054/math_54MM10/v/2018-01-24T161827Z/r/image-png is the relative permittivity of the shield, JOURNAL/mrim/04.02/01445475-201803000-00054/math_54MM11/v/2018-01-24T161827Z/r/image-png is the intrinsic impedance of air, JOURNAL/mrim/04.02/01445475-201803000-00054/math_54MM12/v/2018-01-24T161827Z/r/image-png is the intrinsic impedance of the shield, JOURNAL/mrim/04.02/01445475-201803000-00054/math_54MM13/v/2018-01-24T161827Z/r/image-png is the thickness of the shield, and JOURNAL/mrim/04.02/01445475-201803000-00054/math_54MM14/v/2018-01-24T161827Z/r/image-png is the skin depth of the shield. From this expression it can be seen that metals with high conductivity are by far the most common materials for EM shielding. However, a changing magnetic flux from the time‐varying gradient magnetic field induces eddy currents on any conductive surfaces based on Faraday's law of induction, which in turn may cause resistive heating or ghosting artifacts in MRI images 32. The power dissipation of eddy currents can be calculated using the following equation 37: JOURNAL/mrim/04.02/01445475-201803000-00054/math_54MM15/v/2018-01-24T161827Z/r/image-png where JOURNAL/mrim/04.02/01445475-201803000-00054/math_54MM16/v/2018-01-24T161827Z/r/image-png is the power lost per unit mass (W/kg), JOURNAL/mrim/04.02/01445475-201803000-00054/math_54MM17/v/2018-01-24T161827Z/r/image-png is the peak magnetic field (T), JOURNAL/mrim/04.02/01445475-201803000-00054/math_54MM18/v/2018-01-24T161827Z/r/image-png is the thickness of the sheet (m), JOURNAL/mrim/04.02/01445475-201803000-00054/math_54MM19/v/2018-01-24T161827Z/r/image-png is the frequency (Hz), JOURNAL/mrim/04.02/01445475-201803000-00054/math_54MM20/v/2018-01-24T161827Z/r/image-png is a constant equal to 1 for a thin sheet and 2 for a thin wire, JOURNAL/mrim/04.02/01445475-201803000-00054/math_54MM21/v/2018-01-24T161827Z/r/image-png is the resistivity of the material (Ω·m), and JOURNAL/mrim/04.02/01445475-201803000-00054/math_54MM22/v/2018-01-24T161827Z/r/image-png is the density of the material (kg/m3).
In turn, the eddy current generates magnetic fields that oppose the primary gradient fields as a result of Lenz's law 38; hence, the imaging objects do not experience the gradient field magnitudes that were programmed. Accordingly, this results in a distorted MR image during gradient‐intense sequences such as echo‐planar imaging (EPI). Eddy currents can generate resistive heat and vibration as well.