SLOTTED RADIOFREQUENCY (RF) FRIENDLY RF SHIELD FOR GRADIENT EDDY CURRENT HEAT REDUCTION
Publication Date: 2014-Jul-28
The IP.com Prior Art Database
The invention proposes a radiofrequency (RF) shield structure that is slotted to prevent or impose high impedance for gradient eddy currents. The slits have no end. As a result, current crowding at the slit end and local hotspots are prevented. All cuts are made in the direction of RF current. Consequently, the RF losses are low, especially over the birdcage endring locations. The RF shield is further provided with slotted copper rings which are added in a tangential direction over the endring locations.
FIELD OF INVENTION
The invention generally relates toradiofrequency (RF) shieldpresent in a magnetic resonance imaging (MRI) system and more particularly to a technique for reducing gradient eddy currents and associated heatthat are provided by RF shield.
BACKGROUND OF THE INVENTION
Generally in a magnetic resonance (MR) system, RF shield is placed between a gradient coil and a radiofrequency (RF) body coil. The RF shield keeps gradient electric fields out of a patient space to avoid interference with electrocardiogram (ECG) and electroencephalogram (EEG) leads. The RF shield also keeps RF fields out of the gradient coil environment, which is a very lossy environment at the RF frequencies. Eddy currents in the RF shield due to switching gradient fields produce lots of heat, which we try to prevent by slitting the RF shield in a certain way to produce more impedance for the gradient eddy currents. The RF current density in the RF shield is high over the endrings of a birdcage. This is where most of the RF losses occur, and where we can expect the highest voltages, currents and potential for arcing
A conventional technique utilizes slitted RF shield as depicted in FIG. 1 below.
Slitting the shield reduces gradient eddy currents. However, at the end of a slit there is a lot of eddy current crowding which generates many thermal hotspots.
Another conventional technique includes a slotted RF shield as in fig 2.This type of slotted RF shield has lowloss associated with RF current density in the Z direction. It is therefore regarded as an ideal shield for TEM like structures which have no endring and therefore no endring current. The slots go all the way to the end of the structure.As a result, there is no local hotspot at the end of the slits. FIG. 2 depicts this type of conventional slotted RF shield.
In case such RF shield is applied to a birdcage, capacitance between orange and purple layers is used to guarantee RF continuity. However, the above mentioned conventional technique does not provide sufficient RF coil efficiency.
Hence there exists a need for an efficient technique toreduce RF losses in high RF current density areas over the birdcage endring.
BRIEF DESCRIPTION OF THE INVENTION
The invention proposes a radiofrequency (RF) shield structure that is slotted to prevent or impose high impedance for gradient eddy currents. The slits have no end. As a result, current crowding at the slit end and local hotspots are prevented. All cuts are made in the direction of RF current density.Consequently, the RF losses are low, e...