Abstract:
A radio frequency (RF) coil system for resonance imaging/analysis comprising a primary coil element having a plurality of axial conductors spaced to form a generally tubular structure having two ends and defining a coil volume, and a first pair of spoiler coils. The first pair of spoiler coils each comprising a plurality of axial conductors spaced to form a generally tubular structure and defining a coil volume. Each of the spoiler coils is positioned adjacent to and overlapping an end of the primary coil. Each of the primary and spoiler coils is also adapted to carry an RF signal, wherein the signal in the spoiler coils is 180 degrees out of phase with the signal in the primary coil. The counter-phased spoiler coils act to rapidly drive down the RF magnetic field generated by the primary coil in the region of the ends of the primary coil to reduce the occurrence of aliasing artifacts from outside the imaging field of view.

Description:
BACKGROUND OF INVENTION 
     The present invention relates to the field of magnetic resonance imaging (MRI) systems and, more particularly, concerns radio frequency (RF) coils for use in such systems. 
     In MRI systems or nuclear magnetic resonance (NMR) systems, radio frequency signals are provided in the form of circularly polarized or rotating magnetic fields having an axis of rotation aligned with a main magnetic field. An RF field is then applied in the region being examined in a direction orthogonal to the static field direction, to excite magnetic resonance in the region, and resulting RF signals are detected and processed. Receiving coils intercept the radio frequency magnetic field generated by the subject under investigation in the presence of the main magnetic field in order to provide an image of the subject. Typically, such RF coils are either surface-type coils or volume-type coils, depending upon the particular application. Normally, separate RF coils are used for excitation and detection, but the same coil or array of coils may be used for both purposes. 
     Conventional MRI systems have a number of artifact problems. For example, aliasing of unwanted signals into the resonance object image is a common problem in MRI applications. A particular form of artifact, sometimes referred to as an aliasing artifact, can occur in the either the frequency direction or the phase direction within MRI systems. In this type of artifact, an area of anatomy that is at least partially within the excitation field of the body coil has a local Larmor frequency identical to a pixel within the imaging field of view. This phenomenon typically originates from areas outside the field of view, but causes artifacts inside the image. It often arises as a result of the non-linearity of the gradient fields and/or non-homogeneity of the DC magnetic fields. 
     Accordingly, to reduce the occurrences of unwanted artifacts, there exists a need for MRI systems having improved linearity of gradient fields and homogeneity of DC magnetic fields in RF transmit coils with zero sensitivity outside the imaging field of view. 
     SUMMARY OF INVENTION 
     In the present invention, the aforementioned problem is solved through the provision of a novel transmit coil or array of coils that is sensitive in the imaging volume, but radically drops off in sensitivity outside of the image field of view. An advantage of the present invention is that it does not excite the spin system in areas where the aliasing artifact originates. 
     In particular, the present invention provides a radio frequency (RF) coil system for magnetic resonance imaging/analysis comprising a primary coil element having a plurality of axial conductors spaced to form a generally tubular structure having two ends and defining a coil volume, and a first pair of spoiler coils. The spoiler coils each comprise a plurality of axial conductors spaced to form a generally tubular structure and define a coil volume. Each of the spoiler coils is positioned adjacent to and slightly overlapping an end of the primary coil. Each of the primary and spoiler coils is also adapted to carry an RF signal, wherein the signal in the rungs of the spoiler coils is 180 degrees out of phase with the signal in the rungs of the primary coil. 
     In another aspect of the invention, an additional pair of spoiler coils are added adjacent to and outside of the first pair of spoiler coils with the signal carried in the additional pair of spoiler coils being 180 degrees out of phase with the signal carried in the first pair of spoiler coils. 
     In a further aspect of the invention, a RF apparatus for use in a nuclear magnetic resonance (NMR) system is provided. The RF coil has a generally tubular structure defined by an inner wall and an outer wall, the inner wall defining an imaging volume. The RF coil also comprises a plurality of discrete electrically conductive members positioned between the inner wall and the outer wall which are equally circumferentially spaced around the tubular structure so as to form opposing pairs of conductive members. Each of the conductive members comprises a conductive loop having a primary coil section and a spoiler coil section at each end of the primary coil section configured such that a current flow of a signal on the loop in a cross-over region between each of the spoiler coils and the primary coil is in opposite directions. 
     An advantage of the present invention is that it provides homogeneity of the magnetic field throughout the image field of view, but radically drops off in sensitivity outside the field of view to reduce the likelihood of aliasing artifacts. Other objects and advantages of the invention will become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     For a more complete understanding of this invention, reference should now be made to the embodiments illustrated in greater detail in the accompanying drawings and described below by way of examples of the invention. 
     In the drawings: 
     FIG. 1 is a perspective view of a single quadrature birdcage coil according to the prior art. 
     FIG. 2 is a schematic view of a receiver/transmitter coil system according to one embodiment of the present invention. 
     FIG. 2A is a perspective view of the primary and spoiler birdcage coils of FIG.  2 . 
     FIG. 3 is a side view of a RF coil according to another embodiment of the present invention. 
     FIG. 4 is a sectional view of the RF coil of FIG. 3 taken along line  4 — 4 . 
     FIG. 5 is an isocontour plot of the RF field amplitudes projected in the x-y plane for the receiver/transmitter system of FIG.  2 . 
     FIG. 6 is an isocontour plot of the RF field amplitudes projected in the x-y plane for the RF coil of FIG.  3 . 
    
    
     DETAILED DESCRIPTION 
     It is important in nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) applications to maximize the signal-to-noise ratio of the system, and to irradiate all parts of the object under consideration with the same strength RF field. In this regard, an important characteristic of an RF transmit coil is to provide a homogeneous magnetic field in the volume of the RF coil. Conversely, if a coil provides homogeneous excitation, it will also receive a NMR signals in a homogeneous fashion. Accordingly, in the discussion which follows, references to excitation distributions of the coils of the present invention apply with equal relevance to their use as a NMR receiver. In addition, although the present invention will be described with reference to a “birdcage” whole body transmit coil and an RF whole body transmit coil, the teachings herein are equally applicable to other types of volume coils used in a NMR imaging applications. 
     Referring now to FIG. 1, there is shown a perspective view of a single quadrature birdcage coil  10  according to the prior art. The birdcage coil  10  consists of two rings  12 ,  14  which form circular conductive loops which are connected to each other and spaced apart from each other by conductive connection members or rungs  16 . Typically, there are eight, twelve or sixteen electrically conductive connecting members  16  joining the circular conductive rings  12 ,  14  and each rung is equally circumferentially spaced. Such quadrature transmit and receiving coils  10  are designed for receiving a variety of anatomical regions of the body such as the knee, leg, arm, or the entire body and are thus referred to as volume coils. The coils  10  are typically disposed around a hollow cylindrical drum (not shown) to provide structural support for the coil  10 . The primary RF magnetic field of the coil  10  is perpendicular to the direction of the z-axis shown in FIG.  1 . 
     For transmission, a waveform generator and power amplifier communicate RF waveforms to the conductive members to generate the RF magnetic field. For reception, electrical leads (not shown) are connected to the coil  10  to communicate the received signals to a data acquisition system as is known in the art. Such data processing systems typical comprise a data processing channel including an individual amplifier, filter, and A/D converter for processing the image signals received by a corresponding coaxial lead connected to the birdcage coil  10 . The outputs of the data processing channels are then multiplexed and combined by a microprocessor according to a processing algorithm to produce and display an overall image signal. 
     An advantage of the birdcage coil design is that it creates a homogeneous RF field in the x-y plane and to a lesser extent, along the z-axis direction. A disadvantage of the coil, however, is that it has a significant amount of stray magnetic field beyond the endings of the coil. Preferably, the magnetic field should fall off rapidly outside of the imaging field of view to prevent the occurrence of image artifacts. 
     Referring now to FIG. 2 there is shown a schematic diagram of a transmitter/receiver system according to one embodiment of the present invention. The coil system of FIG. 2 comprises three birdcage coils, a primary birdcage coil  50  and two small birdcage coils referred to spoilers  52 ,  54  on either end of the primary birdcage  50 . The birdcages are arranged such that the rung current in the spoilers  52 ,  54  is 180° out of phase with the rung current flowing through the primary birdcage  50 . This creates a counter field at each end  56 ,  58  of the primary birdcage coil  50  which drives the field amplitude down faster in those areas and, as a result, significantly reduces any stray magnetic field outside the field of view of the primary birdcage coil  50 . An unequal power splitter  60  is used to shift the current phase of the signal received from amplifier  62  by 180° before it is split by an equal power splitter  64  and transmitted to each of the spoilers  52 ,  54 . Alternatively, the unequal power splitter  60  can comprise two amplifiers to accomplish the desired current phase shift. In such a case, unequal power splitter  60  comprises two amplifiers, one feeding the equal power splitter  64  and one feeding the primary birdcage  50  with a current 180° out of phase with the signal transmitted to each of the spoiler birdcages  52 ,  54 . 
     Each of the primary birdcage  50  and spoilers  52 ,  54  can be linear birdcages as shown or quadrature birdcages as described above. The amount of overlap between the primary birdcage  50  and each respective spoiler  52 ,  54  is on the order of approximately 2 to 10 mm. Furthermore, due to the current phase shift, the current in end ring  56  of primary birdcage  50  will be in the same direction as the current flow of end ring  70  of spoiler  52 . Similarly, the current flow of end ring  58  of the primary birdcage  50  will be in the same direction as that of end ring  72  of spoiler  54 . The current flowing in the rungs of each of the primary birdcage  50  and spoilers  52 ,  54 , however, are opposite for the same as azimuthal angle. 
     In a preferred embodiment, the primary birdcage  50  is approximately 40 cm long in the z-axis direction and each of the spoiler birdcages  52 ,  54  are 12 cm in length in the z-axis direction. 
     Additional birdcage spoilers  53 ,  55  may also be added, each having a rung current flow 180° out of phase with respect to the rungs of its adjacent birdcage  52 ,  54 . The amount of the magnetic field drop off and the magnitude of the field recovery after going through the zero crossing can be adjusted by varying the length of the primary birdcage  50 , the length of the spoilers  52 ,  54 , the current ratio between the primary birdcage  50  and its respective spoilers  52 ,  54 , the angle between the conductors and the z-axis or RF shield, as well as the number of sets of spoilers surrounding the primary birdcage  50 . 
     FIG. 2A is a perspective view of one embodiment of the primary birdcage oil  50  and .spoiler birdcage coils  52 ,  54  of FIG.  1 . The rung and ladder configuration of the primary birdcage  50  and spoiler birdcages  52 ,  54  as well as the overlap of the end rings  70 ,  56  and  72 ,  58 , respectively, can be clearly seen. 
     Referring now to FIG. 6, there is shown an isocontour plot of equal RF field amplitudes in a sagittal plane for a 40 cm primary birdcage  50  with 12 cm spoilers  52 ,  54  carrying equal but opposite current amplitudes. From the plot shown in FIG. 5, it is clear that at approximately +/−20 cm from the center of the primary birdcage  50  along the z-axis, the magnetic field drops to zero. This occurs much closer to the end rings  56 ,  58  of the primary birdcage  50  than would otherwise be the case for an unspoiled birdcage. In operation, this area of low sensitivity can be positioned over the areas where aliasing artifacts would otherwise originate thereby eliminating the aliasing artifact by not exciting the spin system in the region of concern. 
     Referring now to FIG. 4, there is shown a side sectional view of the RF coil arrangement of FIG. 3 taken along line  4 — 4  to show an opposing pair of conductors  82 . As shown in FIG. 4, each conductor  82  of the RF coil comprises a primary coil section  84  having counter coil sections  86 ,  88  at each end thereof. In a preferred embodiment, the shield diameter D 4  of the RF coil  80  is equal to 60 cm, the length L 1  of the primary coil  84  is equal to 45 cm and the lengths L 2 , L 3  of the counter coils  86 ,  88  are equal to 15 cm. 
     As can be seen in FIG. 4, in the regions  90  where the counter coils  86 ,  88  are connected to the primary coil  84 , the current flow as indicated by arrows  92  is in opposite directions through the conductive element. In this way, the RF magnetic field generated by the RF coil is driven down towards zero in the regions  90  thereby having the same effect as the overlapping birdcage design of FIG.  2 . In a preferred embodiment, the coil sections  94 ,  96  are set at 45° with respect to the horizontal such that they are orthogonal to each other. In addition, it is preferable to have the return current path  83  close to the exterior shield  79 . A preferred arrangement comprises a shield diameter D 4  of 60 cm and a return current path diameter D 3  of approximately 58 cm. 
     In a further preferred embodiment, the primary coil  84  is tapered from approximately a diameter of 55 cm at D 2  to approximately 56.6 cm diameter at its center D 1 . 
     Referring now to FIG. 6 there is shown an isocontour plot of equal RF field amplitudes in a sagittal plane for a RF transmit coil according to the present invention having a primary coil length of 40 cm and counter coil lengths of 12 cm. As can be seen in FIG. 6, there is pronounced zero crossing for the RF magnetic field at approximately the end regions of the primary coil. In this regard, the RF coil of the present invention can effectively eliminate aliasing artifacts by not exciting the spin system in the aliasing artifact region. 
     Similar to the birdcage arrangement as shown in FIG. 2, the RF transmit coil arrangement of FIG. 4 can be configured to optimize the amount of drop off at the end regions  90  of the primary coil  84  as well as the amount by which the RF magnetic field recovers after going through the zero crossing by varying the length L 1  of the primary coil  84 , the lengths L 2 , L 3  of the counter coils  86 ,  88 , the distances to the RF shield  79  for the counter coils  86 ,  88  and the primary coil  84 , the angle between the conductors in the region  90  between the primary coil  84  and counter coils  86 ,  88 , and the number of counter coils at each end of the primary coil  84 . 
     From the foregoing, it can be seen that there has been brought to the art a new and improved transmission coil for MRI applications which provides advantages over conventional transmission coils. While the invention has been described in connection with one or more embodiments, it should be understood that the invention is not limited to those embodiments. On the contrary, the invention covers all alternatives, modifications, and equivalents, as may be included within the spirit and scope of the appended claims.