Microstrip radio frequency (RF) transmission line coil technology for magnetic resonance (MR) imaging developed by Insight Neuroimaging Systems, LLC (the assignee of the present application) has been successfully applied to a diverse set of magnet systems, ranging from 3 T human to 11.7 T animal scanners. The coils have been developed for linear and quadrature mode of operation and can function in combination with separate receiver coils.
In conventional MR imaging applications, the RF coils are tuned to the hydrogen (1H) resonance frequency determined by the main magnet field strength. Depending on the biomedical application, however, it may prove desirable to extend the RF coil capabilities beyond proton resonance imaging. For instance, by tuning to other atom resonance frequencies such as phosphorous, fluoride, or carbon, the range of applications for the RF coils can be significantly extended.
A dual-tuned volume coil for performing MR imaging according to one embodiment includes an inner cylinder having a first coil structure disposed on an inner surface thereof and a second coil structure disposed on an outer surface thereof. The first coil structure operates at a first resonance frequency and the second coil structure operates at a second resonance frequency. The volume coil includes an outer shield disposed about the inner cylinder, with the first and second coil structure being connected to the outer shield by means of a plurality of capacitors.
A dual-tuned volume coil includes a single inner coil structure having a plurality of microstrips that operate at two different resonance frequencies to provide two imaging modes and an outer shield that is electrically coupled to the inner coil structure. The widths of the microstrips vary as a function of their location about the coil.
The foregoing and other features of the present invention will be more readily apparent from the following detailed description and drawings of the illustrative embodiments of the invention where like reference numbers refer to similar elements and in which:
In an exemplary embodiment, the present invention provides a dual-tuned RF transmit/receive coil 100 for MR imaging that has the ability to tune selectively to two separate resonance frequencies.
The present invention relates to resonator volume coils 110, 120 that enable imaging at two different resonance frequencies. In a first exemplary embodiment, two widely spaced resonance frequencies, such as 170 MHz (1H imaging at 4 Tesla) and 40 MHz (13C imaging at 4 Tesla) are achieved using two separate coil structures 110, 120 placed on the inner and outer surfaces, respectively, of an inner cylinder and connected to an outer cylindrical shield 130 via fixed and/or variable tuning capacitors 140.
Several variants of the exemplary two-layer coil 100 of
Simulations were conducted to compare the field linearity performance of the exemplary, two-layer coil 100 of the present invention to that of a conventional, single-layer microstrip coil, referred to as the “reference coil” and shown schematically in cross-section in
An exemplary embodiment of coil 100 according to the present invention has the following parameters:
Shield Support Cylinder
Relative strip width=66.7% of center-to-center spacing
Inner coil strip width=1.701 in (43 mm)
Outer coil strips width=1.833 in (47 mm)
Outer coil is rotated 22.5° with respect to the inner coil.
Operating frequencies for the coil 100 in accordance with one embodiment of the present invention:
flow=40.8 MHz
fhigh=170.2 MHz
Table I summarizes the results of the simulations for the three variants of coil 100 and the reference coil implementations.
Exemplary embodiments of the aforementioned variants will now be described in greater detail.
An exemplary embodiment of Variant I, in which the lower frequency coil is on the inside 110, has the following capacitor values:
For the lower frequency (e.g., 40.8 MHz) operation of this embodiment:
For the higher frequency (e.g., 170.2 MHz) of operation of this embodiment:
An exemplary embodiment of Variant II, in which the higher frequency coil is on the inside 110, has the following capacitor values:
For the lower frequency (e.g., 40.8 MHz) of operation of this embodiment:
For the higher frequency (e.g., 170.2 MHz) of operation of this embodiment:
An exemplary embodiment of Variant III, in which the lower frequency coil is on the inside 110 and the inner coil 110 extends beyond the outer coil 120, has the following capacitor values:
In this exemplary embodiment, the microstrips of the inner coil 110 extend by 0.5″ beyond both ends of the outer coil 120 for a total length of 6″.
For the lower frequency (e.g., 40.8 MHz) of operation of this embodiment:
For the higher frequency (e.g., 170.2 MHz) of operation of this embodiment:
For comparison, the performance of the reference coil will now be described.
For the lower frequency (e.g., 40.8 MHz) of operation, the value of the capacitors used is 598 pF, and the resultant parameters are:
For the higher frequency (e.g., 170.2 MHz) of operation, the value of the capacitors used is 33.3 pF, and the resultant parameters are:
In yet a further exemplary embodiment in accordance with the present invention, a single coil is provided having relatively closely spaced resonances frequencies, such as, for example 170 MHz (1H at 4 T) and 160 MHz (19F at 4 T). In this embodiment, the two imaging modes (typically degenerate, used in quadrature) are moved to two different frequencies. An advantage is that the modes are orthogonal, and thus decoupled, while homogeneity and efficiency are preserved. It is noted that this coil configuration can be used at linear-only modes of operation for both frequencies.
As in the above described embodiments, capacitors (not shown) are coupled between each strip of the coil and an outer cylindrical shield. In an exemplary embodiment, the values of the capacitors vary sinusoidally as a function of the position of their corresponding strip. In an exemplary embodiment, the capacitors of the strips at the 0° and 180° positions are −17.6% of the nominal capacitance, and the capacitors of the strips at the 90° and 270° positions are +17.6% of the nominal capacitance.
An exemplary embodiment of the coil has the following parameters:
Shield Support Cylinder
The operating frequencies are as follows:
Table II summarizes the results of simulations for the aforementioned embodiment and the reference coil.
The performance of an exemplary embodiment of the dual-tuned, single layer coil of the present invention will now be described.
In this exemplary embodiment, the capacitor values and strip widths are as follows:
For the lower frequency (e.g., 160.8 MHz) of operation of this embodiment:
For the higher frequency (e.g., 170.2 MHz) of operation of this embodiment:
For comparison, the performance of the single-tuned reference coil will now be described.
For the lower frequency (e.g., 160.8 MHz) of operation, the value of the capacitors used is 30.0 pF, and the resultant parameters are:
For the higher frequency (e.g., 170.2 MHz) of operation, the value of the capacitors used is 24.9 pF, and the resultant parameters are:
Having described embodiments of the invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention as defined in the appended claims.
The present application claims the benefit of U.S. patent application Ser. No. 60/670,605, filed Apr. 11, 2005, which is hereby incorporated by reference in its entirety. The present application is also related to and incorporates by reference in its entirety, International Patent Application No. PCT/US2004/027532, entitled MICROSTRIP COIL DESIGN FOR MRI APPARATUS, filed on Aug. 23, 2004 and published as WO 2005/020793 A2 on Mar. 10, 2005.
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