Electricity: conductors and insulators – Anti-inductive structures – Conductor transposition
Reexamination Certificate
1999-09-22
2001-05-15
Han, Jessica (Department: 2838)
Electricity: conductors and insulators
Anti-inductive structures
Conductor transposition
C174S036000, C333S222000
Reexamination Certificate
active
06232548
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a radio-frequency shield for a diagnostic magnetic resonance apparatus, the shield being of the type having a hollow-cylindrical carrier composed of a dielectric, having a first, electrically conductive coating arranged at an inside of the carrier and provided with axially aligned separating slots, a second electrically conductive coating at an exterior of the carrier and provided with axially aligned separating slots, with the separating slots in the first coating and in the second coating being offset relative to one another in a circumferential direction, and wherein the first coating has slotted end regions at the ends of the carrying member and at least one slotted middle region between the end regions.
2. Description of the Prior Art
A radio-frequency shield of this type is disclosed in U.S. Pat. No. 5,574,372. The radio frequency shield is arranged between a gradient coil system and a whole body radio-frequency antenna. Its function is to shield the radio-frequency field of the whole body radio-frequency antenna from the exterior and to keep disturbances originating from the exterior out of the imaging region of the magnetic resonance apparatus. A completely closed, conductive envelope would in fact be optimum for the radio-frequency shielding effect, but is unusable because the radio-frequency shield must be transmissive for the low-frequency gradient fields on the order of magnitude of up to 20 kHz. In order for the low-frequency gradient fields to be built up substantially undistorted in the imaging volume, it is necessary to substantially suppress eddy currents induced by the fields in the radio-frequency shield. To that end, the conductive layer acting as radio-frequency shield is slotted in the longitudinal direction, i.e., in a principal direction of the mirror currents generated by the antenna in the radio-frequency shield. The leaks in the radio-frequency shield which arise due to the slots are shielded by a second conductive layer wherein the separating slots are arranged offset in the circumferential direction from the slots in the first coating. The two layers or coatings are applied on opposite sides of a tubular carrier composed of a dielectric material. Radio-frequency currents can flow across the slots due to the capacitive coupling. Since the gradient coils extend beyond the exterior dimensions of the whole body antenna in the axial direction, the longitudinal dimensions of the radio-frequency shield are also basically adapted to those of the gradient coils. The design of the separating slots is essentially dependent on whether a linearly or a circularly polarizing whole body antenna is to be shielded. A disadvantage, however, is that local heating can occur in the radio-frequency shield, particularly given rapid imaging or given high gradient field strengths.
Another possibility for the design of the radio-frequency shield is disclosed in U.S. Pat. No. 5,680,046. C-shaped structures are applied double-sided on a carrier in order to simulate the radio-frequency mirror current paths as exactly as possible. A good radio-frequency shielding with a simultaneously good suppression of the eddy currents thus is achieved. A disadvantage, however, is that the shielding effect is only effective for a specific radio-frequency antenna. Thus, the shield design must be implemented differently when a circularly polarizing antenna is to be shielded instead of a linearly polarizing antenna.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a radio-frequency shield for a diagnostic magnetic resonance apparatus, wherein the shielding effect is largely independent of whether a linearly or a circularly polarizing whole body antenna is to be shielded, and wherein local heating due to increased eddy current densities is largely reduced in the radio-frequency shield.
This object is achieved in accordance with the principles of the present invention in a radio-frequency shield of the type initially described, having the additional feature of separating slots introduced into the end regions being transverse slots that are arranged at the axially inwardly disposed end of the separating slots. The separating slots extending essentially only in the axial direction and the capacitive coupling of the micro-strips separated by the separating slots relative to one another causes the shielding effect to remain constantly good regardless of the type of radio-frequency antenna utilized. As a result of the transverse slots, eddy currents flowing in the circumferential direction can no longer penetrate as far into the edge-side micro-strips. The flow field becomes more uniform. Local increases of the eddy current density at the end of the outer longitudinal slots, and thus local heating are greatly reduced.
In an embodiment, the inner coating has a continuous separating slot in the middle proceeding in the circumferential direction that is bridged with a resistor. Due to the interruption in the middle of the shield, eddy currents that are induced by a gradient (z-gradient) rising in the axial direction cannot flow. The interruption does not represent a resistance for the radio-frequency currents since both shield halves are connected to the shield structure at the exterior via a capacitive coupling. Static charges of the two shield halves that could lead to partial discharges (spikes) are dismantled by the resistor.
In another embodiment, the spacing of the separating slots from one another in the circumferential direction is lower in the end regions than a corresponding spacing of the separating slots in the middle region. At the expense of the capacitive coupling between neighboring micro-strips, the eddy currents induced by the gradient fields are reduced in this area. Since, however, the mirror currents induced by the radio-frequency antenna in the end regions of the radio-frequency shield are significantly lower in the middle region because of the greater distance of the antenna, the capacitive coupling between neighboring micro-strips can also be reduced in order to farther reduce the eddy currents in this region.
REFERENCES:
patent: 5247270 (1993-09-01), Harman et al.
patent: 5574372 (1996-11-01), Moritz et al.
patent: 5680046 (1997-10-01), Frederick et al.
Eberler Ludwig
Nistler Juergen
Han Jessica
Schiff & Hardin & Waite
Siemens Aktiengesellschaft
LandOfFree
Radio-frequency shield for a diagnostic magnetic resonance... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Radio-frequency shield for a diagnostic magnetic resonance..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Radio-frequency shield for a diagnostic magnetic resonance... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2481543