Abstract:
A modular lower coil provides for a head, a vascular and, a thoracic/lumbar element that may interfit or be separated to provide individual imaging of these areas or a combined imaging region stretching from the head to the lower spine. A switch box allows individual ones of these elements to be connected to the MRI machine if necessary because of a limitation of inputs to the machine and provides for decoupling currents to minimize interference between the coil elements when one element is being used.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is based on U.S. Provisional application No. 60/429,878 filed Nov. 27, 2002 and hereby claims the benefit of the application. This application is also a continuation in part of U.S. application Ser. No. 10/303,582 filed Nov. 22, 2002 now U.S. Pat. No. 6,762,606, and hereby incorporated by reference. 

   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   BACKGROUND OF THE INVENTION 
   The present invention relates to magnetic resonance imaging and, in particular, to local coils for use in magnetic resonance imaging. 
   Magnetic resonance imaging (MRI) detects the faint nuclear magnetic resonance (NMR) signals given off by protons in the presence of a strong magnetic field after excitation of the protons with a radio frequency signal. The NMR signals are detected using loop antennas termed “coils”. 
   NMR signals are extremely faint and therefore “local coil” or “surface coils” may be designed such as can be placed in close proximity to the region of interest of the imaged object. The size of the local coils is kept small to allow them to be easily fit to the patient on the MRI patient table. Importantly, the small area of loops of the local coil provides improved signal strength relative to received noise. The local coils are in contrast to the whole body coil typically present in an MRI machine and useful for obtaining broad “survey” scans of the patient. 
   The small size of a local coil generally limits the volume over which the coil is sensitive. For imaging large areas of the body, for example, neurovascular imaging of the head, neck, and lower spine, the whole body coil with its lower signal to noise ratio must be used. Alternatively, coverage of this region can be obtained by using several local coils and taking multiple images of the patient and changing or repositioning the local coil in between images. This latter approach is time consuming and impractical in many situations. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention provides a modular local coil whose elements may be assembled together on the patient table to provide large area coverage or may be used individually for small area coverage. Because the coil elements are designed to be modular when they are assembled, the elements provide a continuous volume of sensitivity and adverse interference or coupling between the elements can be avoided. A switchbox allows each coil element to employ up to the maximum number of loops that can be accommodated by the MRI machine allowing the individual coil elements to provide comparable performance to other non-modular local coils. 
   Specifically, the present invention provides a modular local coil system for magnetic resonance imaging having at least three electrically independent multiple loop coil elements sized to assemble along an axis so as to provide a substantially continuous field coverage of a patient along that axis. Each of the coil elements has a cable and electrical connector providing communications with the multiple loops of the coil element, where the connectors are receivable by a connector receptacle on an MRI machine. A switch box provides coil-side connector receptacles receiving the connectors and an MRI machine-side cable and electrical connector receivable by the connector receptacle of the MRI machine selectively connecting coil elements to the MRI machine. 
   Thus, it is one object of the invention to provide a modular coil system where the coil elements may be used alone or in combination for different imaging requirements and for ease of positioning and assembly. 
   It is another object of the invention to provide a set of individually usable coils that are designed to work together to provide a field pattern when assembled. 
   It is another object of the invention to provide a modular coil system that provides coverage of a large region but wherein the individual elements employ many loops to provide signal-to-noise ratios comparable to other local coils. 
   The switch box may include circuit paths connecting disabling signals to the coil elements that are not selectively connected to the MRI machine. 
   Thus, it is another object of the invention to provide a set of coils that may be used individually or together without interference. The disabling of the coilsdetunes the unused coils so that they do not couple with the active coil. 
   The coil elements may include bases fitting against the upper surface of a patient table and abutting to align and space the coil elements from each other when assembled along the axis. 
   Thus, it is another object of the invention to provide a simple and intuitive method of assembling and holding the coil sections together. 
   At least two of the coil elements, when assembled together, may include antenna structure from one coil fitting within a volume defined by antenna structure of a second coil. 
   Thus, it is another object of the invention to provide a mechanism for providing substantially continuous field coverage between coil elements. 
   At least one coil element may be a head coil providing a volume for receiving a patient&#39;s head; one coil element may be a planar anterior coil fitting beneath the patient against a patient table; and at least one coil element may be a pair of opposed anterior and posterior coil fitting about the patient&#39;s upper torso. 
   Thus, it is one object of the invention to provide a modular coil having a range of flexible coil types that may be used individually. 
   These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an exploded perspective view of the components of the modular coil of the present invention showing the head imaging element, anterior and posterior portions of the vascular imaging element, and the thoracic/lumbar imaging element above a portion of a patient table on which they normally rest; 
       FIG. 2  is a side elevational view of the coil elements interconnected and positioned about a patient to provide imaging from the patient&#39;s head to the patient&#39;s lower spine; and 
       FIG. 3  is a schematic version of the coil of  FIG. 2  showing the continuous regions of field sensitivity provided by the assembled modular elements and their connection via a switching system to MRI inputs and decoupling signals. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to  FIGS. 1 and 2 , a modular coil  10  of the present invention includes a head imaging element  12 , a vascular imaging element  14 , including an anterior portion  16 , a posterior portion  18 , and a thoracic/lumbar imaging element  20 . Each of these coil elements  12 ,  14 , and  20  may be receive only or receive/transmit coils. 
   Each of the coil elements  12 ,  14 , and  20  includes a lower surface conforming generally to the upper surface of a patient table  22  and has dimensions allowing them to fit within the bore  24  of an MRI magnet  26  as part of an MRI machine  27  when supported on a patient table  22 . 
   The head imaging element  12  provides a base  28  and a lower surface conforming generally to the upper surface of a patient table  22 . The base  28  includes laterally extending handles  30  allowing it to be easily lifted onto or off the patient table  22  and supports on its upper surface a “bird cage” style head coil  29 . The head coil  29  includes a series of longitudinally extending rungs  32  spanning the distance between an upright superior end ring  34  and upright inferior end ring  36 . Together, the rungs  23  and the end rings  34  and  36  define a generally cylindrical volume into which the head of a patient may fit. 
   In the preferred embodiment, conductors internal to the head coil  29  are connected to provide eight separate phased array coils arranged around the circumference of the volume, the coil elements being supported by the longitudinally extending rungs  32  according to methods well known in the art. This coil may be of a collapsible design described in U.S. application Ser. No. 10/303,582 entitled “Retracting MRI Head Coil”, filed Nov. 22, 2002, and assigned to the assignee of the present invention and incorporated by reference. Alternatively, the head coil may be a quadrature style “bird cage” coil of a type well known in the art. 
   Collapsibility allows the inferior end ring  36  to move toward the superior end ring  34  allowing improved access to the volume of the coil. A cantilevered headrest  38  extends from an inferior edge of the base  28  upward then into the volume of the head coil  29 . The headrest  38  holds the patient&#39;s head above the inferior end ring  36  to prevent interference between the movement of the inferior end ring  36  and the patient. 
   A cable  40  extending from the base  28  and terminating in connector  41  provides eight independent conductor sets communicating with the loops contained within the head coil  29  to provide received NMR signals from the head imaging element  12  to a switch box  42  as will be described. The connector  41  is selected to be compatible with a connector receptacle (not shown) on the MRI machine  27  so that the cable  40  may be connected directly to the MRI machine  27 . 
   The posterior portion  18  of the vascular imaging element  14  has a lower surface fitting to the upper surface of the patient table  22  and a superior edge that abuts the inferior edge of the base  28  of the head imaging element  12  to fit closely thereto. The posterior portion  18  has a height comparable to that of the base  28  to provide continuous support for the patient  60  supine over the table  22  with the patient&#39;s head within the head imaging element  12 . A notch  44  may be included in the superior edge of the posterior portion  18  to fit around the cantilevered headrest  38 . Mechanical coupling between the coil element  12  and posterior portion  18  is not required but may be provided in the form of, for example, hook and loop fasteners, clips, or other attachment devices known in the art. Generally, the upper surface of the posterior portion  18  of vascular imaging element  14  is padded for patient comfort. 
   The posterior portion  18  may contain four independent phased array loops to provide four channels of information out of cable  46  having a connector  47  also communicating with switch box  42 . The connector  47  is also selected to be compatible with a connector receptacle (not shown) on the MRI machine  27  so that the cable  46  may be connected directly to the MRI machine  27 . 
   The phased array loops within the posterior portion  18  work in conjunction with four independent phased array loops in the anterior portion  16  positioned on the patient  60 , above and generally parallel, to the posterior portion  18  over the patients chest. The anterior portion  16  is sized to fit about the chest of the patient  60  and therefore has a slight curvature to it or is flexible to conform thereto. Anterior portion  16  of the vascular imaging element  14  includes a projecting face mask portion  50  supporting loops or portions of loops that may fit about the jaw and lower neck of the patient  60  within the head imaging element  12  for improved imaging of this vascular tissue. The face mask portion  50  is sized to fit without interference within the head imaging element  12 , and includes a nose bridge  52  fitting about the patient&#39;s nose, and neck extension wings  54  extending downward on the left and right sides of the patient&#39;s neck as shown in FIG.  2 . 
   The anterior portion  16  may include cable  49  containing four independent conductors, one associated with each of the contained loops, the cable  49  also extending to switch box  42  to connect thereto by connector  53 . The connector  53  is selected to be compatible with a connector receptacle (not shown) on the MRI machine  27  so that the cable  49  may be alternatively connected directly to the MRI machine  27 . 
   Abutting the inferior edge of the posterior portion  18  is the superior edge of thoracic/lumbar imaging element  20  presenting a generally upwardly concave surface and a lower surface fitting against the top of the patient table  22  and under the back of the patient  60  when the patient  60  is supine on the patient table  22 . The thickness of the thoracic/lumbar imaging element  20  is such as to provide continued support to the patient  60  even with the posterior portion  18 . The thoracic/lumbar imaging element  20  provides laterally extending handles  56  allowing it to be easily placed on the table  22 . Again, mechanical connection between the thoracic/lumbar imaging element  20  and the vascular imaging element  14  is not required, but may be provided in the form of hook and loop fasteners or other mechanical attachment mechanisms. The thoracic/lumbar imaging element  20  includes eight independent phased array coils providing eight independent signals out of cable  55  also connected to switch box  42  by means of connector  57 . The connector  57  is selected to be compatible with a connector receptacle (not shown) on the MRI machine  27  so that the cable  55  may be connected directly to the MRI machine  27 . 
   The head imaging element  12  may be used independently of the other coil elements  14  and  20  or may be selectively combined with these coil elements  14  and  20  by adding the vascular imaging element  14  and/or the vascular imaging element  14  and the thoracic/lumbar imaging element  20  to the head imaging element  12  as desired. Once assembled together, the elements may be connected one at a time via the switch box  42  to the MRI machine to provide a broad area of imaging. 
   Referring to  FIG. 3 , the region of sensitivity  68  of head imaging element  12  generally includes the volume defined by the superior end ring  34 , the inferior end ring  36 , and the longitudinally extending rungs  32 . The vascular imaging element  14  provides a region of sensitivity  70  generally between the anterior portion  16  and posterior portion  18 , but includes a region  62  within the volume defined by the face mask portion  50 . Finally, the region of sensitivity  72  of the thoracic/lumbar imaging element  20  is generally an area above that element. As such, a continuous volume of sensitivity is provided from the patient&#39;s head to the lumbar spine. 
   As shown in  FIG. 3 , switch box  42  may receive cables  55 ,  49 , and  43  (joined as internal cable  63 ), and cable  40 , each providing eight separate conductors associated with individual phased array coils in the various coil elements  12 ,  14 , and  20 . In this capacity, the switch box  42  includes connector receptacles  43  similar to those on the MRI machine  27 . Switch box  42  implements a three-throw, three-pole switch, either mechanically or by solid-state electronics. By means of this switch, each of the eight separate conductors of cables  55 ,  63 , and  40  may be alternately connected to an eight conductor cable  65  (shown in  FIG. 1 ) leading to the MRI machine  27 . The cable  65  may include a connector  45  (shown in  FIG. 1 ) compatible with the connector receptacle of the MRI machine  27 . 
   The connection of this cable  65  to the other cables  55 ,  63 , and  40  is according to switch positions designated generally as (A), (B), and (C). In the A position, the conductors of cable  55  are connected to the corresponding conductors leading to input/output  64  of the MRI machine  27  whereas the conductors of cables  63  and  40  are connected to a decoupling current source  66  providing a current for decoupling. The decoupling circuit in each of the coil elements  12 ,  14 , and  20  may, for example, use pin diodes within the coil elements  12 ,  14  and  20  to detune each of the coils when the decoupling current is received according to methods well known in the art. Thus in position A, thoracic/lumbar imaging element  20  may be activated for transmission and reception with the MRI machine  27  while coil elements  14  and  12  are decoupled. 
   Correspondingly, when the switch is in the B position, thoracic/lumbar imaging element  20  is connected to the decoupling current source  66  along with the head imaging element  12  and, the vascular imaging element  14  is connected to the input/output  64  of the MRI machine. 
   Finally, in the C position, only the cable  40  is connected to the input/output  64  of the MRI system and the other two coil elements  14  and  20  are connected to the decoupling current source  66 . 
   The certainty in assembly provided by the modularity of coil elements  12 ,  14 , and  20 , and the electrical decoupling, allow cross coupling between the loops of these elements such as might affect their regions of sensitivity  68 ,  70 , and  72  to be minimized. The designed modularity of the coil elements  12 ,  14 , and  20  further assures an unbroken region of sensitivity for an area of imaging extending from the head to lower spine. If three general purpose local coils were collected at one time to provide a similar coverage region, sensitivity of the coils within the coverage regions would be unpredictable. 
   It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.