Abstract:
A wireless magnetic resonance imaging (MRI) scanner bore tube assembly has a radio frequency (RF) antenna, a microwave antenna array and an electrical screen. The RF antenna is formed of a series of RF antenna elements, each comprising a rung. The rungs are spaced at intervals of substantially half of the wavelength of the frequency of operation of the microwave antenna array. The microwave antenna array is formed by a series of microwave antenna elements interleaved between the rungs and the screen acts as a reflector to reflect signals from the microwave antenna elements towards the center of the bore tube.

Description:
RELATED APPLICATIONS 
     The present application is related to the following applications filed simultaneously herewith and respectively having Ser. No. 12/612,831, Ser. No. 12/612,842, Ser. No. 12/612,856, Ser. No. 12/613,033 and Ser. No. 12/613,082. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a wireless magnetic resonance imaging (MRI) scanner bore tube assembly and in particular to an arrangement of microwave dipole antennas of a magnetic resonance imaging (MRI) birdcage antenna using an electrical screen as a reflector. 
     2. Description of the Prior Art 
     MRI scanners use a combination of a strong constant magnetic field (B 0 ) from a superconducting magnet which is modified by gradient fields generated by gradient coils, together with a rotating magnetic field (B 1 ) from a radio frequency (RF) antenna to excite nuclear magnetic resonances in the body that generate short term RF signals that are received to build up a tomographic image. 
     All current-generation MRI scanners employ arrays of local coils mounted in close proximity to the scanned patient to receive the RF with maximum possible signal to noise ratio (SNR). The local coils that receive signals from the back of the patient are mounted in the patient table. Local coils that receive signals from the front of the patient are arranged into ‘mats’ that are carefully placed over the patient. Associated with each mat is a flexible cable typically containing one co-axial line for each local coil. The cables interact with the B 1  field and with the signals generated from the patient so ‘traps’ (high impedance sections) must be included at regular (typically λ/8) intervals. These add cost and inconvenience to the structure. 
     In use, the requirement to connect the cables and sterilize them between scanning one patient and the next leads to increased down-time between scans. It is therefore desirable that the cables be eliminated. 
     SUMMARY OF THE INVENTION 
     In accordance with a first aspect of the present invention a wireless magnetic resonance imaging (MRI) scanner bore tube assembly, the bore tube assembly has a radio frequency (RF) antenna, a microwave antenna array and an electrical screen. The RF antenna is formed by a series of RF antenna elements, each having a rung, with the rungs being spaced at intervals of substantially half of the wavelength of the frequency of operation of the microwave antenna array. The microwave antenna array is formed by a series of microwave antenna elements interleaved between the rungs; and the screen acts as a reflector to reflect signals from the microwave antenna elements toward the center of the bore tube. 
     The present invention enables a wireless solution to this requirement. Ideally, the wireless solution substantially satisfies the requirements of the existing wired system, maintaining in particular the low noise figure and the dynamic range. 
     Preferably, the RF antenna is implemented as etched copper structures on a thin printed circuit board (PCB) mounted on a support tube. 
     Preferably, the microwave antenna array is implemented as etched copper structures on a thin PCB and mounted on the support tube. 
     The RF antenna, microwave antenna array and additional microwave antenna elements are implemented as different layers on the PCB. 
     Preferably, the microwave antenna elements comprise one of dipole antennas, folded dipole antennas, inductively loaded dipole antennas or capacitively loaded dipole antennas. 
     Preferably, the microwave antenna elements are fed by a microstrip or stripline transmission line structure. 
     Preferably, filters are provided at ends of transmission lines, of the transmission line structure, remote from the microwave antenna elements. 
     Preferably, the RF antenna array further comprises an end ring segment at each end of each rung, forming end rings. 
     Preferably, the end rings further comprise slot antennas. 
     Preferably, the slot antennas are aligned substantially parallel with a dominant current flow direction in the end rings. 
     Preferably, additional microwave antenna elements are provided beyond the end rings of the RF antenna array. 
     Preferably, the additional microwave antenna elements comprise one of dipole antennas or patch antennas. 
     Preferably, the additional microwave antenna elements are implemented as etched copper structures on a thin PCB and mounted on the support tube. 
     Preferably, the RF antenna is a birdcage antenna. 
     Preferably, the support tube is substantially electrically insulating. 
     Preferably, the electrical screen is radially outward of the insulating mount. 
     Preferably, the electrical screen comprises a copper screen. 
     In accordance with a second aspect of the present invention, a wireless MRI scanner has a magnet, gradient coils, a number of local coils; and a bore tube according to the first aspect. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a wireless MRI system incorporating a bore tube assembly in accordance with the present invention. 
         FIG. 2  illustrates a schematic outline of a simplified conventional birdcage type RF antenna. 
         FIG. 3  illustrates the integration of microwave antenna elements into a birdcage RF antenna with a bore tube assembly in accordance with the present invention. 
         FIG. 4  illustrates the arrangement of  FIG. 3 , with an example of how feed lines to the microwave antenna elements are provided. 
         FIG. 5  is a cross-section showing the relative arrangements of the RF antenna elements and microwave antenna elements in a bore tube in accordance with the present invention. 
         FIG. 6  shows an embodiment of the bore tube assembly of the present invention, in which slot antenna elements are provided in end rings. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The wireless concept to which the features of the present invention apply is based on upconversion of the RF (Larmor) frequency signals to microwave frequencies and transmission from local coils located in the patient mat to microwave antennas located on the bore of the scanner. The combination of transmit and receive antennas on the patient and bore respectively constitutes a MIMO (Multiple Input/Multiple Output) system. The greater multiplicity of receive antennas in the bore array allows individual signals from the patient antennas to be resolved. The present invention relates to the bore tube for use in the system described above and in particular to an arrangement of the microwave antenna arrays. 
     An example of an MRI system using a MIMO microwave link, suitable for using a bore tube according to the present invention will now be described. However, other architectures are possible and the invention is not limited to the one described below.  FIG. 1  shows a patient  1  within an MRI scanner bore tube  2 . A mat covers the part of the patient for imaging and embedded in the mat are a plurality of local coils  3 . Associated with each local coil  3  is an upconverter  4  and microwave antenna  5 . Transceivers  9  connected to an array  6  of antennas  12  are integrated into the scanner bore  2 . The frequency upconverter  4  for each patient mat coil  3  produces signals for transmission to the array of transceivers in the scanner bore  2 . A signal generator  10  generates a local oscillator (LO) signal at 2.44 GHz, or other chosen microwave frequency, which feeds the transceivers connected to the antenna array  6  to illuminate the patient coil electronics  14  with a signal  7  at the local oscillator frequency. The same LO signal in the transceivers converts the microwave signals  8 , received from the patient coils  3  at the LO frequency ±63 MHz, back to the original magnetic resonance (MR) frequency of 63 MHz for input  11  to MR receivers in an image processing system (not shown). The local coil upconverters  4  are based on parametric amplifiers and implement low noise frequency conversion and amplification in simple, low cost circuitry. The parametric amplifiers use the incident local oscillator signal  7  to provide the frequency reference and the power for the upconversion. MR signals  13  from the coils are thereby converted to microwave frequency and transmitted to the bore transceiver antenna array  6 . In one embodiment of the present invention, an arrangement of microwave dipole antennas is fabricated on the same printed circuit board as a magnetic resonance imaging (MRI) birdcage antenna and located between the rungs. The radio frequency (RF) screen, located on the inner surface of the gradient coil acts as a reflector for the microwave antennas 
     In order for the wireless system to operate with minimal or no degradation in received signal to noise ratio the array  6  of microwave antennas  12  around the bore  2  must have element centre to element centre spacing in both directions, around and along the bore, of the order of half the wavelength of the operational microwave frequency. Patient coil spacing influences the choice of microwave link frequency but in general, any microwave frequency may be chosen providing that it is high enough to provide the spatial resolution and low enough that the number of required bore antenna elements and associated electronics is practical. Operation at 2.44 GHz gives an antenna spacing of about 6 cm. 
     As described earlier, MRI scanners necessarily incorporate an RF antenna that generates the B 1  field at the Larmor frequency, which is dependent upon the B 0  field strength, so is 63.6 MHz for a typical 1.5 Tesla B 0  magnetic field, or 123 MHz for a 3 T system. These values are commonly used magnet and MR frequencies, but magnets ranging from 0.5 T to 11 T could be used and the MR and local oscillator microwave frequencies may also be chosen from a much wider band. For example, dependent upon the nucleus type, the MR frequencies may range from 20 MHz to 500 MHz and the LO frequency might be chosen in the range of 1 GHz to 5 GHz. Commonly the construction of the B 1  field antenna  24 , also known as the ‘body coil’, takes the form of a ‘birdcage’ with two separated rings  20 ,  21  printed around the bore  2  and connected by a number of rungs  22  regularly spaced around the bore, as shown in  FIG. 2 . Each element  23  of the antenna is formed of a rung and a part of each ring. The width and spacing of the rungs around the circumference of the bore are chosen to maintain half wavelength spacing of the microwave elements and provide sufficient current handling capability for the body coil. The minimum gap between end ring segments that form the end rings is determined by the RF voltage present between adjacent segments during the excitation phases of a scan. The length of the rungs and the dimensions of the end ring segments are dependent upon the volume that needs to be imaged and the constraints imposed by specific absorption rate (SAR) limits. Capacitors and diodes (not shown) are incorporated in the design of the body coil at appropriate positions for tuning and switching purposes. A screen  34  is printed on the inside of the gradient coil (not shown) to minimize undesired egress of B 1  field energy outside the bore imaging region. A gap of 1 to 2 cm of air or glass reinforced plastic (GRP) between the screen and the body coil allows a flux return path for the magnetic fields generated by the body coil rung currents. 
     The present invention aims to provide an arrangement of microwave antennas that satisfies a number of requirements. These include high efficiency, i.e. that gain is directed towards the centre of the bore; a consistently good impedance match over a useful range of angles of incidence; a negligible effect on B 1  field uniformity and strength; practical implementation of feeds to the microwave antennas and the creation of a microwave absorptive surface achieved by the half wavelength spacing of the elements in the array. 
     A first aspect of the present invention is that the number of body coil rungs around the bore is arranged, such that the separation between adjacent rungs is nominally equal to λ/2 at the LO frequency, which for the example given above means a separation of about 6 cm. Dipole antennas, which may take the form of ‘standard’ dipole antennas, folded dipole antennas, inductively loaded or capacitively loaded dipole antennas, are located in between and coplanar with the rungs and orientated as shown in  FIG. 3 . In a practical implementation, the birdcage rungs and antenna elements cover the entire circumference of the bore tube  2 , but for clarity, only a subset of antenna elements are shown in the Figures. 
     The bore tube of the present invention, shown in  FIG. 3  comprises a support tube  2 , with bird cage antenna elements  23  provided on the support tube and microwave dipole antenna elements  12  in parallel in gaps between the bird cage antenna elements  23 . One method of construction of the microwave array  6  is to print both the body coil  24  and microwave dipole array  6  on a thin flexible substrate material  35 , which is wrapped around the outside of the mechanically strong support tube  2  having a similar form to support tubes that are currently in use. As shown in  FIG. 5 , an RF screen  34  is provided outside the rungs  22  and dipoles  12 . This screen then also functions as a reflector for the dipole antenna elements, augmenting their gain towards the bore centre whilst reducing wasteful radiation of energy away from the bore. The bore tube is constructed with the screen layer  34 , typically copper, positioned inside the gradient coils and outside the support tube  2 , which is typically a plastic tube. The integration of the microwave antenna in the bore tube is arranged to minimize adverse interaction between the two. 
     In such a construction, the antennas may be fed by microstrip or stripline feed lines, where the rungs of the body coil form the ground plane for the feeds. The example shown in  FIG. 4  is of microstrip feed lines  31 , running along the long axis of the RF antenna elements  23 , connected to the microwave antennas  12  using Baluns  32 . The other end of each feedline is connected to suitable filters, or “traps”, (not shown) that are designed to block the transmission of signals at the Larmor frequency, but allow transmission of the microwave signals to and from the bore electronics. Additional microwave antenna elements  33  may be located beyond the ends of the body coil, but also printed on the same substrate material. These elements will radiate with the same polarization as the dipoles to provide the required microwave coverage for both LO illumination and microwave reception of the upconverted signals. These elements may take the form of dipoles or patch antennas. 
     The microwave feedline  31  and array structure  6  integrated with the body coil  24  has a number of advantages making it suitable for the wireless application. The RF screen  34  acts as a reflector directing energy towards the centre of the bore tube  2 . The structure can be fabricated using standard printed circuit board manufacturing methods, so that the structure benefits from thin metallic layers to minimize the introduction of eddy currents caused by the switching of the gradient coils; the structure is mechanically and hence electrically repeatable and so provides a good impedance match at the microwave antenna ports; the structure fits into the existing spatial constraints with no impact on bore diameter; and a single process is used to deposit the body coil  14  and the microwave array  6  on same substrate  35 . In an example of using a microstrip feed arrangement, the dipoles  12  and antenna elements  23  may be printed onto one side of a thin pcb, with the feed  31  printed onto the other side with the pcb then wrapped around the outside of the support tube  30 . The feed structure of the present invention makes use of the copper rungs in the existing designs as ground planes and therefore has minimal impact on B 1  field uniformity and strength. Low noise amplifiers, or similar electronics may also be mounted on the thin PCB within the active bore region, also using the body coil rung  22  as the ground plane. The dc power for these components may be fed via trapped dc feedlines or connected to the zero RF potential points on the body coil, or fed via bias tees and the microwave lines. 
     The structure of the present invention also lends itself to the introduction of slot antennas  25  in the end rings  20 ,  21  if space precludes the use of dipoles  12  in this region. This is illustrated in  FIG. 6 . Such slot antennas are orientated circumferentially to maintain the E-field polarization along the axis of the bore in alignment with the dipoles. Such slot antennas may be aligned parallel with the dominant current flow direction in the end rings  20 ,  21  and can thereby minimize disruption of the high RF excitation currents. The aggregate antenna structure provides a microwave absorptive surface with the chosen element spacing and feedpoint leading to realizable feedline impedances. Any microwave antennas that do not need to be fed may be terminated with resistive loads to maintain the absorptive properties of the array 
     A benefit of the present invention is that individual patient coils are only activated when they are physically in the field of view. Patient coils outside the field of view do not receive local oscillator power and so do not generate any interference. There is therefore no need for the operator to keep track of the positions of the patient coils. In principle, due to this self-selecting nature of the local coils in the field of view, it would be possible to have a patient mat extending from the neck to the feet of a patient and perform a whole-body scan by moving the patient table in stages, with no need for operator intervention. 
     Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.