Abstract:
An RF coil assembly is presented that incorporates balun networks to eliminate standing waves from cables used to apply a voltage to multiple drive ports of the coil. Each drive port is driven by an applied voltage that is shifted 90 degrees in the tangential direction. Further, all drive ports are located on one end-ring of the coil, e.g. the superior end-ring. The inequality of the efficiency of the drive ports is reduced such that a substantially circular polarization in the volume of the coil is maintained. The present invention reduces asymmetrical loading by a patient as a result of patient asymmetry and patient contact with the opposite end-ring of the coil that normally negatively affects circular polarization and conventional coil configurations.

Description:
BACKGROUND OF INVENTION 
   The present invention relates generally to MR imaging and, more particularly, to a method and apparatus to generate a substantially circular polarized RF field about a subject independent of subject asymmetry. 
   When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B 0 ), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B 1 ) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, M Z , may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment M t . A signal is emitted by the excited spins after the excitation signal B 1  is terminated and this signal may be received and processed to form an image. 
   When utilizing these signals to produce images, magnetic field gradients (G x , G y , and G z ) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. 
   The use of RF coils to generate an RF field about the bore of a magnet for imaging is known in the art of nuclear magnetic resonance imaging. Generally, a patient or other imaging subject is positioned on an examination table and inserted into a coil arrangement having a cylindrical bore therethrough. The RF coils extend around the bore and when energized, transmits and/or receives RF energy. In addition to whole body coils, it is known to utilize anatomically directed coils for imaging targeted anatomical regions of a patient. For instance, head coils have been developed and are specifically designed to image the head of a patient. 
   Head coils, as well as other coils in which the coil elements are arranged in a birdcage arrangement, are generally cylindrical and are designed to generate a substantially circular polarized RF field inside the volume of the coil. With a symmetrical polarized field, the center of the coil, perpendicular to the axis of the coil elements, is typically considered as a virtual electrical ground plane. However, when a patient is placed in the volume of the coil, the asymmetry of the patient will shift the ground plane of the coil and therefore the center of the coil may no longer be relied upon as a good grounding location. That is, the human body is asymmetric and generally distorts the symmetry of the coil especially when only a portion of the patient is positioned within the coil. 
   Further, in the context of head coils, the shoulders of a patient are placed in contact with the head coil assembly. Such contact may also affect the symmetry created within the volume of the head coil. It is also recognized that at higher frequency imaging, the asymmetries inherent in the human body impact the symmetry of the polarized field within the coil and will worsen since with higher frequency imaging coupling to the patient increases. Moreover, the otherwise substantially circular symmetry of the RF field created within the volume of the coil may also become distorted, and drive ports connected to the coil that are driven at voltages out of phase to one another by 90 degrees, are no longer shifted properly when the coil is loaded with a patient. Hence, quadrature isolation is destroyed as is efficiency resulting in a decrease in SNR as well as an increase in SAR. 
   A volume coil having sixteen coil elements with a center ground plane is illustrated in  FIG. 1 . As schematically shown, coil  2  includes an array of coil elements  3  that are uniformly spaced from one another and designed to create a substantially circular polarized field when oriented in a cylindrical arrangement. As mentioned above, with a conventional birdcage coil  2 , a center of the coil  4  is considered as the virtual ground. The coils are driven through the application of voltages at two tangentially 90 degree apart drive ports  5 ,  6 . Extending from each drive port  5 ,  6  is a drive cable  7 , 8 , respectively, The drive cable  7  connected to drive port  5  extends to the superior end-ring  9  of the coil  2 , and drive cable  8  extends from drive port  6  to the inferior end-ring  10 . The drive voltage applied at drive port  6  is 90 degrees shifted in phase from the voltage applied at drive port  5  so as to set up, absent patient induced asymmetry, the generally circular polarized field inside the volume of the coil. Each drive cable  7 ,  8  is physically soldered to the substrate of the coil  2  with their shields contacting the center of the coil. The center of the coil  2  is considered the virtual ground, thus killing all standing waves on the cable shield. Notwithstanding the benefits of such a coil design, as noted above, the patient asymmetry may impact the symmetry of the circularized polarized field otherwise created within coil  2 . As a result, the field may become more linear than circular thereby introducing shading to reconstructed images. Moreover, with decreased circularity in the polarized field, the power requirements of the coil also increase. Additionally, the linearity resulting in the volume of the coil creates localized high energy fields within the coil volume thereby increasing temperature differentiation across the coil volume. 
   It would therefore be desirable to have a system and method capable of generating a substantially circular polarized RF field independent of subject asymmetry or incidental subject contact with an RF coil assembly during data acquisition. 
   BRIEF DESCRIPTION OF INVENTION 
   The present invention is directed to a method and apparatus of generating a substantially circular polarized RF field independent of subject symmetry or incidental subject contact with an RF coil assembly that overcomes the aforementioned drawbacks. 
   An RF coil assembly is presented that incorporates balun networks to eliminate standing waves from cables used to apply a voltage to multiple drive ports of the coil. Each drive port is driven by an applied voltage that is shifted 90 degrees in the tangential direction. Further, all drive ports are located on one end-ring of the coil, e.g. the superior end-ring. As a result, the coil will be asymmetrically loaded by a patient as a result of patient asymmetry and patient contact with the opposite end-ring of the coil. This asymmetry negatively affects circular polarization and conventional coil configurations, but with the present invention, the inequality of the efficiency of the drive ports is reduced such that a substantially circular polarization in the volume of the coil is maintained independent of the asymmetry. 
   Therefore, in accordance with one aspect of the present invention, an MR coil assembly is provided that includes a volume coil arrangement situated to generate a polarized field about a subject to be imaged. The coil assembly further includes multiple drive ports connected to a common end of the volume coil arrangement as well as multiple drive cables connected to a voltage source at one end and connected to the multiple drive ports at another end to apply voltages to the multiple drive ports. In this manner, the volume coil arrangement generates a substantially circular polarized field independent of subject asymmetry. 
   In accordance with another aspect of the invention, an MRI apparatus includes an MRI system having a plurality of gradient coils positioned about a bore of a magnet to impress a polarizing magnetic field. An RF transceiver system and an RF switch are controlled by a pulse module to transmit and receive RF signals to and from an RF coil assembly to acquire MR images. The RF coil assembly includes a plurality of RF coils arranged in a birdcage arrangement to acquire MR data from the subject at least partially positioned in a volume-of-interest. The coil assembly further includes a number of drive ports to receive an applied voltage to drive the plurality of RF coils to maintain a substantially circular polarized field about the volume-of-interest irrespective of possible subject contact with the RF coil assembly. 
   According to another aspect, the present invention includes a method of driving coils of an MR coil assembly to maintain a polarized RF field independent of subject asymmetry. The method includes the steps of providing a pair of voltage inputs and splitting each voltage input into a pair of driving inputs. The method also includes the steps of inputting each driving input into a balun and inputting an output of each balun to a respective MR coil drive port of an MR coil assembly for generation of an RF field about a volume-of-interest. 
   Various other features, objects, and advantages of the present invention will be made apparent from the following detailed description and the drawings. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     The drawings illustrate one preferred embodiment presently contemplated for carrying out the invention. 
     In the drawings: 
       FIG. 1  is a schematic of a known MR coil assembly. 
       FIG. 2  is a schematic block diagram of an MR imaging system for use with the present invention. 
       FIG. 3  is a schematic of an RF coil assembly in accordance with the present invention and applicable with the MR imaging system shown in  FIG. 2 . 
   

   DETAILED DESCRIPTION 
   Referring to  FIG. 2 , the major components of a preferred magnetic resonance imaging (MRI) system  11  incorporating the present invention are shown. The operation of the system is controlled from an operator console  12  which includes a keyboard or other input device  13 , a control panel  14 , and a display screen  16 . The console  12  communicates through a link  18  with a separate computer system  20  that enables an operator to control the production and display of images on the display screen  16 . The computer system  20  includes a number of modules which communicate with each other through a backplane  20   a . These include an image processor module  22 , a CPU module  24  and a memory module  26 , known in the art as a frame buffer for storing image data arrays. The computer system  20  is linked to disk storage  28  and tape drive  30  for storage of image data and programs, and communicates with a separate system control  32  through a high speed serial link  34 . The input device  13  can include a mouse, joystick, keyboard, track ball, touch activated screen, light wand, voice control, or any similar or equivalent input device, and may be used for interactive geometry prescription. 
   The system control  32  includes a set of modules connected together by a backplane  32   a . These include a CPU module  36  and a pulse generator module  38  which connects to the operator console  12  through a serial link  40 . It is through link  40  that the system control  32  receives commands from the operator to indicate the scan sequence that is to be performed. The pulse generator module  38  operates the system components to carry out the desired scan sequence and produces data which indicates the timing, strength and shape of the RF pulses produced, and the timing and length of the data acquisition window. The pulse generator module  38  connects to a set of gradient amplifiers  42 , to indicate the timing and shape of the gradient pulses that are produced during the scan. The pulse generator module  38  can also receive patient data from a physiological acquisition controller  44  that receives signals from a number of different sensors connected to the patient, such as ECG signals from electrodes attached to the patient. And finally, the pulse generator module  38  connects to a scan room interface circuit  46  which receives signals from various sensors associated with the condition of the patient and the magnet system. It is also through the scan room interface circuit  46  that a patient positioning system  48  receives commands to move the patient to the desired position for the scan. 
   The gradient waveforms produced by the pulse generator module  38  are applied to the gradient amplifier system  42  having G x , G y , and G z  amplifiers. Each gradient amplifier excites a corresponding physical gradient coil in a gradient coil assembly generally designated  50  to produce the magnetic field gradients used for spatially encoding acquired signals. The gradient coil assembly  50  forms part of a magnet assembly  52  which includes a polarizing magnet  54  and a whole-body RF coil  56 . One skilled in the art will appreciate that anatomically targeted coils such as head coils are also applicable with the illustrated MRI system  11 . A transceiver module  58  in the system control  32  produces pulses which are amplified by an RF amplifier  60  and coupled to the RF coil  56  by a transmit/receive switch  62 . The resulting signals emitted by the excited nuclei in the patient may be sensed by the same RF coil  56  and coupled through the transmit/receive switch  62  to a preamplifier  64 . The amplified MR signals are demodulated, filtered, and digitized in the receiver section of the transceiver  58 . The transmit/receive switch  62  is controlled by a signal from the pulse generator module  38  to electrically connect the RF amplifier  60  to the coil  56  during the transmit mode and to connect the preamplifier  64  to the coil  56  during the receive mode. The transmit/receive switch  62  can also enable a separate RF coil (for example, a surface coil) to be used in either the transmit or receive mode. 
   The MR signals picked up by the RF coil  56  are digitized by the transceiver module  58  and transferred to a memory module  66  in the system control  32 . A scan is complete when an array of raw k-space data has been acquired in the memory module  66 . This raw k-space data is rearranged into separate k-space data arrays for each image to be reconstructed, and each of these is input to an array processor  68  which operates to Fourier transform the data into an array of image data. This image data is conveyed through the serial link  34  to the computer system  20  where it is stored in memory, such as disk storage  28 . In response to commands received from the operator console  12 , this image data may be archived in long term storage, such as on the tape drive  30 , or it may be further processed by the image processor  22  and conveyed to the operator console  12  and presented on the display  16 . 
   As will be described in greater detail below, the present invention is directed to an MR coil arrangement that reduces standing waves in cable shields connected to drive ports of the coil using balun networks. As a result, the efficiency of the coil is enhanced. Additionally, the coil arrangement includes a multi-point drive network whereupon circular polarization in the coil volume is maintained independent of patient or subject geometry or position. Maintaining of a circular polarized field is desirable as it has been shown that some circular polarized coils constructed as claimed can have twice the efficiency and forty percent better SNR than linear polarized coils. 
   Referring now to  FIG. 3 , a birdcage coil arrangement  68  is shown as having an exemplary sixteen coil elements  70 . It should be noted that for purposes of illustration, the coil arrangement is being shown as a schematic representation of a plan view of an outstretched coil. That is, coil elements  70 , in implementation, are collectively arranged in a cylindrical arrangement so as to form an imaging volume therein. Further, one skilled in the art will appreciate that fewer or more coil elements may be used depending on the field-of-view to be imaged. Coil  68  includes an inferior end-ring  72  and a superior end-ring  74 . Associated with each coil element  70  is a pair of capacitors with one capacitor being connected at the inferior end-ring and the other capacitor being connected at the superior end-ring. As described above, coil  68  is designed to have a cylindrical shape and coil elements  70  are designed to generate a substantially circular polarized RF field in the interior volume of the coil when appropriately driven. 
   Coil  68  is driven through multiple drive ports  80 . Further, in the illustrated example, coil  68  includes four drive ports  80 . Each drive port  80  is designed to be driven at a voltage having a phase that is shifted from the phase of the voltage applied to a neighboring drive port. In a preferred embodiment, each of the drive ports is driven by a voltage with a phase that is shifted 90 degrees from the phase of the voltage applied to a neighboring drive port. For example, drive port  80 A may have an applied voltage with a phase of zero degrees whereas the phase of the voltage applied to drive port  80 B may then be 90 degrees. Further to the above example, the phases of the voltages applied to drive ports  80 C and  80 D may be 180 degrees and 270 degrees, respectively. The drive voltages are 90 degrees shifted in phase so as to set up a circular polarized field inside the interior volume of coil  68 . Further, by implementing more than two drive ports, the asymmetry that typically results in a two drive port coil is greatly diminished. Therefore, with the coil design of the present invention, a substantially circular polarized RF field is maintained independent of patient asymmetry or field inequality or non-uniformity that results with subject contact with the coil when placed in the interior volume of the coil. 
   Coil  68  further includes a balun network  82  that includes a balun  84 A–D for the drive ports  80 A–D. More particularly, each drive port  80  is connected to a dedicated balun  84 . Balun network  82  is constructed to eliminate standing waves that result from the voltage cables that would otherwise be directly connected to the drive ports  80 A–D in a conventional birdcage coil. 
   Still referring to  FIG. 3 , each balun  84 A–D is connected to receive an input from a splitter network  86 . Each splitter  88 A,  88 B is constructed to receive a voltage input  90 A and  90 B, respectively, and split the phase of the voltage input. For example, each splitter  88 A,  88 B is designed to perform a 90 degree split of the phase of a voltage input  90 A,  90 B, respectively. Moreover, each splitter  88 A,  88 B is designed to not only split the single voltage input and provide two voltage outputs, it is also designed to perform a phase shift of 90 degrees. In this regard, splitter  88 A, in one embodiment, will receive a voltage input with a phase of zero degrees and will provide two out-of-phase voltage outputs; one voltage output having a phase of 90 degrees and a second output having a phase of 270 degrees. On the other hand, splitter  88 B will receive a voltage input having a phase of 90 degrees and will split that input into two out-of-phase voltage outputs. One voltage output having a phase of zero degrees and a second voltage output having a phase of 180 degrees. That is, each splitter performs a ±90 degrees phase shift of its input. The outputs of splitters  88 A and  88 B are then appropriately matched. As further shown in  FIG. 3 , the outputs of each splitter  88 A,  88 B are not input to neighboring baluns. Specifically, one output of splitter  88 A is input to balun  84 A whereas the other output splitter  88 A is input to balun  84 C. The outputs of splitter  88 B are input to balun  84 B and balun  84 D, respectively. 
   Accordingly, the present invention is directed to an improved RF coil design and drive assembly that maintains a substantially circular polarized RF field independent of subject asymmetry or incidental subject contact with the coil itself. A balun network is integrated with the coil to kill standing waves that occur on the drive cables connecting drive ports of the coil to a power source. The drive cables are routed entirely along an outside of the coil volume. In addition, the coil is multi-point driven. That is, in a preferred embodiment, the coil is driven at four drive ports that are disposed along a common end ring of the coil. Each drive port is driven by a phase-shifted input such that the substantially circular polarized field is maintained. 
   The coil design of the present invention maintains quadrature isolation at a multitude of subject positions within the imaging volume. Additionally, the coil may be driven with less power than required of conventional coils without sacrificing SNR or image quality. Further, by maintaining a substantially circular polarized field, thermal uniformity is maintained across the imaging volume. 
   Therefore, in accordance with one embodiment of the present invention, an MR coil assembly is provided that includes a volume coil arrangement situated to generate a polarized field about a subject to be imaged. The coil assembly further includes multiple drive ports connected to a common end of the volume coil arrangement as well as multiple drive cables connected to a voltage source at one end and connected to the multiple drive ports at another end to apply voltages to the multiple drive ports. In this manner, the volume coil arrangement generates a substantially circular polarized field independent of subject asymmetry. 
   In accordance with another embodiment of the invention, an MRI apparatus includes an MRI system having a plurality of gradient coils positioned about a bore of a magnet to impress a polarizing magnetic field. An RF transceiver system and an RF switch are controlled by a pulse module to transmit and receive RF signals to and from an RF coil assembly to acquire MR images. The RF coil assembly includes a plurality of RF coils arranged in a birdcage arrangement to acquire MR data from the subject at least partially positioned in a volume-of-interest. The coil assembly further includes a number of drive ports to receive an applied voltage to drive the plurality of RF coils to maintain a substantially circular polarized field about the volume-of-interest irrespective of possible subject contact with the RF coil assembly. 
   According to another embodiment, the present invention includes a method of driving coils of an MR coil assembly to maintain a polarized RF field independent of subject asymmetry. The method includes the steps of providing a pair of voltage inputs and splitting each voltage input into a pair of driving inputs. The method also includes the steps of inputting each driving input into a balun and inputting an output of each balun to a respective MR coil drive port of an MR coil assembly for generation of an RF field about a volume-of-interest. 
   The present invention has been described in terms of the preferred embodiment, and it is recognized that equivalents, alternatives, and modifications, aside from those expressly stated, are possible and within the scope of the appending claims.