Abstract:
A MRI coil having an axis and a first end and an opposite second end with respect to said axis includes a first ring element at the first end, a second ring element, a third ring element, a fourth ring element at the second end where the first ring element encompasses a smaller area than each of the second, third, and fourth ring elements. The coil also includes a plurality of axial elements connected between the first, second, third and fourth ring elements. The third and fourth ring elements are axially closer than the first and second ring elements.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. provisional patent application Ser. No. 60/270,660 filed Mar. 8, 2001. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to magnetic resonance imaging and, in particular, to a dome-like radio frequency coil. 
     Radio frequency (RF) coils are used to excite and/or sense magnetic resonance (MR) signals for magnetic resonance imaging (MRI). 
     Birdcage resonators are a commonly used form of volume RF coil. A birdcage coil is typically cylindrically shaped with a ring element at each end connected with multiple axial elements. Ellipse-like and other shapes have also been used for the rings. Good radial RF field uniformity is achieved, but towards the ends of the cylinder, the RF field in the axial direction drops off substantially (e.g., 50 percent). 
     Improvements in uniformity of the RF field in the axial direction can be achieved by lengthening the coil. Unfortunately, this may be impractical for various reasons, including: the necessary space may not be available, particularly in the case of a head coil where the patient&#39;s shoulders are in the way; or a larger coil may require too much power to be coupled into the patient. 
     Improved axial uniformity has also been produced by symmetrically locating a second ring near to each of the end rings. This structure does not however take advantage of the signal to noise ratio (SNR) improvements that may be achieved by having the coil conform better to the patient, for example, by getting close to the top of a patient&#39;s head in a head coil. 
     Improved axial uniformity has also been achieved at one end of the coil by creating a dome-like structure by joining the axial elements together at that end instead of terminating them on an end ring. The dome may, for example, follow the general shape of a patient&#39;s head and increase the RF field as the dome narrows, helping to compensate for the axial drop off in field that would occur otherwise. Unfortunately this can only improve the performance of one end of the coil. In addition, this convergence to a single point generates a very intense RF “hot spot”. This hot spot can over rotate the local nuclei spins, resulting in the presence of “black” spots in the resulting image. 
     SUMMARY OF THE INVENTION 
     A MRI coil having an axis and a first end and an opposite second end with respect to said axis includes a first ring element at the first end, a second ring element, a third ring element, a fourth ring element at the second end where the first ring element encompasses a smaller area than each of the second, third, and fourth ring elements. The coil also includes a plurality of axial elements connected between the first, second, third and fourth ring elements. The third and fourth ring elements are axially closer than the first and second ring elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view schematic diagram of a coil according to the present invention. 
     FIG. 2 is a perspective view schematic diagram of the coil of FIG. 1 illustrating one mode of operation. 
     FIG. 3 is a perspective view schematic diagram of the coil of FIG. 1 illustrating another mode of operation. 
     FIG. 4 is a graphically representation of the flux lines corresponding to FIG.  2 . 
     FIG. 5 is a block diagram of one configuration for operating a coil according to the present invention. 
     FIG. 6 is an exemplary graph of the axial field uniformity of a coil according to the invention and an exemplary graph of the axial field uniformity of a prior art dome coil. 
     FIG. 7 is a perspective view schematic diagram of another coil according to the present invention. 
     FIG. 8 is a perspective view schematic diagram of still is another coil according to the present invention. 
     FIG. 9 is a perspective view schematic diagram of an additional coil according to the present invention. 
     FIG. 10 is a perspective view schematic diagram of another additional coil according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a MRI coil  10  includes a first ring element  12 , a second ring element  14 , a third ring element  16  and a fourth ring element  18 . The elements  14 ,  16  and  18  have a diameter D open . The element  12  has a diameter D cone  that is less than D open . The coil  10  has an axial length A. The element  16  is spaced a distance A R1  from the element  18  and the element  14  is spaced a distance A R2  from the element  18 . The element  16  and the element  18  are axially closer than the element  12  is to the element  14 . For example, D open  may be 267 mm, D cone  may be 180 mm, A may be 245 mm, A R1  may be 50 mm, and A R2  may be 120 mm. 
     Axial elements  20  connect the ring elements  12 ,  14 ,  16 ,  18 . The elements  20  radially curve from the element  14  to the element  12  to provide a dome-like shape to the coil  10 . There may be, for example, sixteen elements  20 . The elements  20  are largely responsible for transverse RF field uniformity. 
     The coil  10  has capacitances (e.g., C 1  and C 4 ) in the ring elements  12 ,  18  and no capacitances in the axial elements  20 . The coil  10  is a tuned circuit resonator which is in a high pass configuration. The capacitances are chosen to provide resonance at the desired frequency (e.g., 127.7 MHz). 
     The coil  10  can support the modes shown in FIGS. 2 and 3. FIG. 2 shows a co-rotating mode for the currents in the coil  10 . FIG. 3 shows a counter-rotating mode for the currents in the coil  10 . For producing a uniform field at the element  18  end of the coil  10 , the co-rotating mode is used. 
     FIG. 4 illustrates the very uniform flux lines produced in the coil  10 . FIG. 6 illustrates a comparison of the axial RF uniformity of the coil  10  compared to a prior art dome coil without a ring element  12  at the top of the dome and without asymmetrically located intermediate ring elements  14 ,  16 . The open end of the coils are at the origin. It is clear that the coil  10  provides as much as  25  percent improvement over the prior art coil. 
     The coil  10  can be used as a transmit, receive or transceiver coil. The ring element  12  provides for the adjustment of the RF field in that area to correspond to that of the center of the coil without “hot spots” that degrade the resulting images. The two asymmetrically located ring element  14 ,  16  operate to enhance the uniformity of the open end of the coil in the area of the ring element  18 . 
     In addition to being circular, just as in prior art coils, the ring elements of the coil  10  can be ellipse-like or other non-circular shapes appropriate for minimizing the distance between the coil and the subject to be imaged. In some cases, it may also be desirable to have differing numbers of the axial elements  20  between the ring elements  12 ,  14 ,  16  and  18 . For example, sixteen elements  20  between the ring elements  14 ,  16  and twelve elements  20  between the ring elements  12 ,  14 . 
     The coil  10  may be mounted in any appropriate manner. For example, the coil  10  may be within plastic or foam in such configurations as clamshell, split-top, solid configuration or split into more than two pieces. 
     Referring to FIG. 5, a possible coupling circuit  100  for coupling the coil  10  to a MR scanner system is illustrated. The coil is matched to the load via tuning points and associated isolation points utilizing inductive matching. Capacitive matching on the coil&#39;s feed ports can also be used. The feeding coil ports of the coil can be located, for example, at the dome end of the coil, the patient end of the coil, or along the legs of the coil or any combination of these locations. In addition, inductive matching techniques can be utilized to drive both ports of the coil in a quadrature fashion. In the case that a linear coil is desired, the matching of the coil can be done with one port. In FIG. 5 a balun is used after the matching network of the coil. It would also be possible to use other configurations, or perhaps a balun could be used at a cable location further removed from the coil. The coil can also be driven in a transmit/receive configuration. In this case a hybrid network can be used. When the coil is used in a phased array configuration, the hybrid network may be omitted. Also pre-amplifiers can be located in the coil design in order to eliminate cable and/or system losses. 
     Referring to FIG. 7, a coil  20  similar to the coil  10  is illustrated. The coil  20  has capacitors C 6 , C 7  in the axial elements instead of capacitors in the ring elements. The coil  20  is a tuned circuit resonator which is in a low pass configuration. The capacitances are chosen to provide resonance at the desired frequency (e.g., 127.7 MHz). 
     Referring to FIG. 8, a coil  30  similar to the coil  10  is illustrated. The coil  30  has capacitors C 6 , C 7  in the axial elements in addition to the capacitors C 1 , C 4  in the end ring elements. The coil  20  is a tuned circuit resonator which is in a band pass configuration. The capacitances are chosen to provide resonance at the desired frequency (e.g., 127.7 MHz). 
     Referring to FIG. 9, a coil  40  similar to the coil  20  is illustrated. The coil  40  has capacitors C 6 , C 7  plus capacitors C 5  in the axial elements. The coil  40  is a tuned circuit resonator which is in a modified low pass configuration, that may be tuned to dual resonant frequencies. 
     Referring to FIG. 10, a coil  50  similar to the coil  40  is illustrated. The coil  50  has the capacitors C 5 , C 6 , C 7  in the axial elements and the capacitors C 1 , C 4  in the end ring elements. The coil  50  is a tuned circuit resonator which is in a modified band pass configuration, that may be tuned to dual resonant frequencies. 
     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.