Abstract:
A MRI RF coil includes a first solenoidal section, a second solenoidal section, and a third solenoidal section. The first section is between the second and third sections. The first section has a counter-rotational orientation with respect to the second and third sections.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. provisional patent application Ser. No. 60/276,297 filed Mar. 16, 2001. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to magnetic resonance imaging and, in particular, to radio frequency coils. 
     Magnetic resonance imaging (MRI) relies on the detection of the magnetic resonance (MR) signal from abundant protons in the volume of interest. A radio frequency (RF) receive coil is a device to effectively “pick up” the MR signal from a background of noise for image processing. MR signals induced in a RF receive coil are weak signals due to the very small population difference between the two proton energy states at room temperature. One of the major challenges in RF coil design is to improve the MR signal detection sensitivity. 
     One of the approaches to improve signal detection sensitivity and/or field of view is to use multiple receive coils as an array. The basic idea is that instead of making a larger and less sensitive coil that covers the entire volume of interest, plural smaller and more sensitive coils are distributed over the volume of interest. Each individual coil picks up signal and noise from a localized volume. With separate detection circuitry, each coil element receives image signal simultaneously. Signals from all coil elements are finally combined and processed to reconstruct MR image for the entire volume of interest. 
     The principle of MRI involves exciting protons and detecting their free induction decay signals. Each proton possesses a tiny magnetic moment precessing about the static magnetic field. The macroscopic behavior of millions of protons can be represented by a resultant magnetization vector aligning with the static magnetic field B 0 . A strong RF excitation pulse may effectively tip the magnetization away from B 0 . The free induction decay of this magnetization is detected in a plane perpendicular to B 0 . Thus, for maximal signal induction, the normal direction of a receive coil must be perpendicular to the direction of the static magnetic field B 0 . 
     Based on the direction of static magnetic field, commercial MRI systems are either horizontal or vertical. So-called co-planar type array coils have proved to be effective for horizontal MRI systems for reasons discussed in the previous paragraph. In a co-planar array coil, surface coils are arranged in a co-planar fashion and distributed over a volume of interest. 
     In general, such co-planar type surface array coils are not very effective for a vertical system because the condition required for maximal signal induction can hardly be fulfilled. Various modifications to the co-planar designs have been proposed with limited success. 
     It is known that solenoidal type coils have several advantages for a vertical field system, including its sensitivity, uniformity and its natural fit to various body parts. To successfully implement a solenoidal array coil, one must be able to isolate solenoidal coil elements to prevent them from coupling to each other. This is required because all coil elements in an array coil are to receive signals simultaneously. “Cross-talk” between different coil elements is un-desirable. Thus effective coil isolation is a major challenge in solenoidal array coil design. 
     The so-called Sandwiched Solenoidal Array Coil (SSAC) disclosed in U.S. patent application Ser. No. 09/408,506 by Su et al. includes two solenoidal RF receive coil elements, a counter-rotational solenoidal element and a second solenoidal element sandwiched between the two counter-rotational winding sections. 
     The counter-rotational solenoidal coil element produces a gradient B 1  field that has a double-peak “M” shape sensitivity profile. The second solenoidal coil element produces a single-peak profile sandwiched between the two peaks of the “M” shape profile of the first coil element. 
     The sensitivity profile of a SSAC is determined by the summation of an “M” shape double-peak profile and a centralized single-peak profile generated by the two solenoidal coil elements. To avoid unwanted dark band artifact in the array coil sensitivity profile, the geometric parameters of both coil elements must be set properly. 
     The uneven-counter-rotational (UCR) coil and its application to a solenoidal array produces a quasi-one-peak sensitivity profile and a null-B 1  point, through uneven winding of its counter-rotational solenoidal sections. A second solenoid coil element can be placed near the null-B 1  point of the UCR coil to form an inherently decoupled solenoidal array. 
     A UCR coil based solenoidal array is more versatile than the SSAC based array due to the fact that the former is easier to implement and that an artifact free array signal summation is easier to obtain. However, it still remains difficult to build larger arrays. 
     SUMMARY OF THE INVENTION 
     A MRI RF coil includes a first solenoidal section, a second solenoidal section, and a third solenoidal section. The first section is between the second and third sections. The first section has a counter-rotational orientation with respect to the second and third sections. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is schematic diagram of a double-counter-rotational coil (DCR) according to the invention. 
     FIG. 2 is a graphical diagram of an exemplary B 1  profile of a DCR coil according to the invention. 
     FIG. 3 is a schematic diagram of an array coil employing a DCR coil according to another aspect of the invention. 
     FIG. 4 is a graphical diagram of an exemplary B 1  profile of the coil of FIG.  3 . 
     FIG. 5 is a schematic diagram of using DCR coils as multiple elements of an array coil 
     FIG. 6 is a graphical diagram illustrating the isolation between elements of the coil of FIG.  3 . 
     FIG. 7 is a schematic diagram of a DCR coil in combination with double saddle coils to provide a quadrature pair. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a double-counter-rotational (DCR)coil  10  includes a middle solenoid section A and two counter-rotational sections B and C. Section A includes three loops with currents flowing in the same direction. Sections B and C includes a single loop with currents flowing in the counter-rotational direction as shown by the arrows. The separation between the neighboring loops is denoted as S 12 , S 23 , S 34  and S 15 , respectively. In general, Section A has more turns than either of Sections B and C, and the loop separation and diameter parameters may have different values depending on the specific coil design needs. 
     FIG. 2 is an exemplary graph of the B 1  field profile produced by the DCR coil  10 . The B 1  field has a quasi-one-peak profile. In addition, it produces two null-B 1  points, symmetrical to the center of the coil  10 . 
     Referring to FIG. 3, a DCR coil  2  is combined with a solenoidal coil  1  and a solenoidal coil  3  as elements of a solenoidal array coil  20 . The coils are actually basically coaxial, but are shown displaced for ease of visualization. Each of the coils  1 ,  3  are located to take advantage of the respective null B 1  points of the coil  2  to minimize coupling between the coils. 
     FIG. 4 is an exemplary graph of the B 1  fields produced by each coil element of a three-solenoid array coil  20 . The central peak  22  represents the B 1  field of the coil  2  and the two side peaks  24 ,  26  represent respective B 1  fields of the coils  1 , 3 . Also shown, is the overall B 1  profile  28  of the array coil  20 . In practice, the number of turns and separations can be designed to meet the signal and filed of view (FOV) requirements. 
     As mentioned above, a DCR coil element produces two null B 1  points, one to each side, providing the possibility for the addition of solenoid coil elements near the null B 1  points without magnetic coupling between neighboring coil elements. If each added coil element is also a DCR coil, still additional solenoid coil element can be added near the new null B 1  point. In this way, more solenoid coil elements can be included coaxially to the array as needed. Therefore, the DCR coil becomes the building block of solenoidal array that can, in principle, consists of as many solenoid coil elements as desired. 
     Referring to FIG. 5, a solenoidal array using DCR coil elements  30  as building blocks and for an array coil  40 . If one considers the DCR coil  30  as a building block (FIG. 5 a ), the solenoidal array coil  40  can be built by properly overlapping the building blocks (FIG. 5 b ). 
     Magnetic coupling between next neighboring coils is much weaker than the coupling between neighboring coil elements due to their greater separation. Such coupling can be compensated using normal isolation methods. For example, the application of a 10 pre-amplifier to the coil circuit will help next neighboring coil isolation effectively in the same way as that in coplanar array coils. 
     A prototype DCR solenoidal array was built to prove the concept. The prototype solenoidal array coil included three solenoid coil elements, a DCR coil and two 2-turn solenoidal coil elements. The solenoidal array coil was built and tested at the resonance frequency of 29.8 MHz. 
     Coil traces were made of 0.2 mm thick and 10 mm wide copper strips wound on a 267 mm diameter acrylic tube. FIG. 3 shows the coil configuration and dimensional parameters. 
     The DCR coil element is inherently decoupled from neighboring solenoid coil elements. No additional decoupling circuit was needed and the isolation between the pairs were excellent. Excellent isolation was achieved between the next neighboring solenoid coil elements by using capacitive decoupling circuits. The S 21  parameter for the three coupling modes is shown in FIG.  6 . 
     The prototype solenoidal array coil test results prove the concept of this invention and the technique of making the same. 
     Various modifications can be made to the basic invention as discussed above. Orthogonal coil elements of various configurations can be added to any solenoid coil element in a DCR solenoidal array to form a quadrature pair to take advantage of the quadrature effect for signal to noise (SNR) ratio improvement. For example, A saddle coil-based element can form a quadrature pair with a DCR solenoid coil element and so can a figure-8 coil element. 
     For example, referring to FIG. 7, two saddle coils  52  can be added to a DCR solenoid coil element  54  to form a quadrature pair  50 . In this design, the DCR coil element  54  is decoupled from either of the saddle coils  52  by field orthogonality. The saddle coil elements  52  are decoupled from each other through an overlapping technique. This configuration is advantageous for large size coils where a large size wrapping-around saddle coil element does not give optimized sensitivity and SNR. One (or both) of the two saddle coil elements can also be replaced by a figure-8 coil element as needed and the above discussion regarding coil decoupling remains the same. Orthogonal coil elements in the above discussion can be added to any one or all of the solenoid coil elements in a DCR solenoidal array. 
     It should be evident that this disclosure is by way of example and that various changes may be made by adding, modifying or eliminating details without departing from the fair scope of the teaching contained in this disclosure. The invention is therefore not limited to particular details of this disclosure except to the extent that the following claims are necessarily so limited.