Magnetic resonance (MR) performance in MR-guided ultrasound systems
Ultrasound transducers adjust the B 1 + and/or B 1 − field distribution in an MRI apparatus to improve the signal sensitivity and homogeneity at a region of interest. Approaches employed include strategic placement of field-altering features such as slots and/or dipoles along the exterior surface or, in some cases, the interior of the transducer. In various embodiments, the field-altering features are (or behave as) passive resonators.
1. An ultrasound transducer for use with an MRI unit having at least one transmit coil and at least one receive coil, the ultrasound transducer comprising:
a dielectric layer; and
conforming to at least a portion thereof, a first conductive layer having at least one passive resonator thereon or therethrough, the passive resonator having a geometry and dimensions selected to allow RF waves to penetrate the first conductive layer and rearrange a B 1 field inside the transducer to achieve an optimized B 1 + and/or B 1 − magnetic field distribution within a field of view of the MRI transmit and/or receive coil.
2. The ultrasound transducer of claim 1 , wherein the optimized magnetic field distribution is maximal B 1 + and/or B 1 − homogeneity in a region of interest.
3. The ultrasound transducer of claim 1 , wherein the optimized magnetic field distribution is maximal B 1 + and/or B 1 − sensitivity in a region of interest.
4. The ultrasound transducer of claim 1 , wherein the at least one passive resonator is at least one elongated slot through the first conductive layer of the transducer and extending across at least a portion thereof.
5. The ultrasound transducer of claim 4 , wherein the at least one slot is a pair of slots in a cruciform configuration.
6. The ultrasound transducer of claim 1 , wherein the dielectric and first conductive layer are substantially hemispheric.
7. The ultrasound transducer of claim 1 , further comprising a second conductive layer spaced apart from the first conductive layer but also conforming to the dielectric layer, wherein the dielectric layer is disposed between the first conductive layer and the second conductive layer in a multi-layer arrangement in a single transducer.
8. The ultrasound transducer of claim 5 , wherein the at least one slot is filled with a metamaterial element.
9. The ultrasound transducer of claim 1 wherein a geometry, a location, and/or an orientation of the at least one passive resonator is adjustable to improve the B 1 + and/or B 1 − magnetic field distribution in a region of interest in response to one or more initial MRI images.
10. The ultrasound transducer of claim 9 wherein the at least one resonator is shaped as a loop, dipole, or figure-8.
11. The ultrasound transducer of claim 1 , wherein the at least one passive resonator is disposed on an interior surface of the transducer at an interface with a dielectric medium.
12. The ultrasound transducer of claim 1 , wherein the first conductive layer has at least two passive resonators thereon or therethrough.
13. The transducer of claim 12 , wherein at least one passive resonator has a dielectric constant in excess of 10 and is attached to a surface of the transducer, and at least one passive resonator is at least one elongated slot through the first conductive layer of the transducer and extending across at least a portion thereof.
14. The transducer of claim 1 , wherein the at least one passive resonator is disposed on an external area of the transducer.
15. The transducer of claim 14 , wherein the external area is a bottom ring surface configured to surround a patient's head.
16. The transducer of claim 15 , wherein the at least one passive resonator is a foil sheet or a mesh.
17. The transducer of claim 1 , wherein
the first conductive layer comprises a two-element curved hemisphere including a domed top element and a bottom skirt element separated by an annular gap; and
the at least one passive resonator is a linear slot antenna configured to shift a region of B 1 + and/or B 1 − homogeneity toward a region of interest.
18. The transducer of claim 17 , further comprising a conducting sheet slidably situated within guides to cover an adjustable amount of the linear slot antenna.
19. The transducer of claim 1 , wherein:
the first conductive layer comprises a plurality of discrete conductive elements tiled in a mosaic configuration; and
the at least one passive resonator is a dipole antenna configured to shift a region of B 1 + and/or B 1 − homogeneity toward a region of interest.
20. The transducer of claim 19 , further comprising a layer of adhesive or polymer under the dipole antenna to isolate the dipole antenna from the first conductive layer.