Abstract:
A head coil for magnetic resonance imaging may be shortened longitudinally to provide for better access by a patient. Extension of the coil may be performed after the positioning of the patient&#39;s head significantly simplifying patient positioning with respect to the coil. A portion of a conductor support sliding over the patient&#39;s head removable to allow endotracheal tubes or the like to remain undisturbed. 
     CROSS-REFERENCE TO RELATED APPLICATIONS

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     BACKGROUND OF THE INVENTION 
     The field of the invention is magnetic resonance imaging, and in particular local coils for using in magnetic resonance imaging of the head. 
     Magnetic resonance imaging (MRI) provides images, for example, of a human patient, by detecting faint signals from precessing hydrogen protons under the influence of a strong magnetic field and after a radio frequency excitation. 
     The quality of the image produced by MRI is strongly dependent on the strength of the received signal. For this reason, it is known to use radio frequency receiving coils placed in close proximity to the area being imaged. Such coils are called local coils. 
     A common local coil used for head or brain imaging is the so-called “bird cage” design providing a set of longitudinal conductors arrayed about the periphery of a cylindrical volume and terminated by conductor supports aligned with the bases of the cylindrical volume. The patient&#39;s head fits through one conductor support into the enclosed volume. The coil may operate with the longitudinal conductors joined into a unitary resonant structure or with the longitudinal conductors isolated to provide a phased array of separate antennas. 
     Birdcage coils can be difficult to position on patients, often requiring one person to lift the patient&#39;s head while the conductor support of the coil is positioned over the patient&#39;s head. For this reason, split birdcage coils have been developed where the coil is separable along a horizontal plane bisecting the cylindrical volume. The lower portion of the coil forming a shallow trough may be pre-positioned on the table and the patient&#39;s head located in this trough when the patient is positioned on the table. After the patient is in position, the upper portion of the coil may be fitted onto the lower portion to complete the coil. 
     A disadvantage of this design is that it requires two parts, one of which must be momentarily stored in another location while the patient is positioned. Generally, two hands are required to lift the top part of the coil into position. 
     In an alternative design, the patient&#39;s head is positioned on a support held in cantilevered fashion so that the coil may slide around the support after the patient is in position. A drawback to this design is that the coil and support occupy a much greater space along the length of the table when they are separated, and in some cases, the coil may extend over the end of the table where it may catch on the bore of the magnet when the patient table is raised or lowered. 
     When a patient is intubated, for example, with an endotracheal tube, the tube may interfere with positioning of the patient in the coil or the coil about the patient. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a head coil that may be shortened along its longitudinal axis to allow easy placement of the patient onto a contained head rest. The coil is then lengthened over the patient for imaging. The resulting coil may be operated with one hand and does not require the storage or movement of separable parts. Because the coil is shortened, the end of the coil need not extend off the end of the table to interfere with the operation of the table and the magnet. 
     One conductor support of the coil may be equipped with a removable bridge so when the coil is lengthened, the conductor support may pass freely by an endotracheal tube or the like. 
     Specifically, the present invention provides an MRI head coil having a longitudinally extending head support sized to support the head of a supine patient. A first conductor support is positioned superior to the head support. A second conductor support having an opening for receiving the head support therethrough is mounted to move longitudinally with respect to the head support and the first conductor support. The second conductor support moves between a first position toward the first conductor support and a second position away from the first conductor support. A set of conductors extends longitudinally between the first and second conductor supports when the second conductor support is in the second position to define together with the first and second conductor supports, a volume substantially enclosing the patient&#39;s head. 
     It is thus one object of the invention to produce a coil that may be shortened to allow access to the imaging volume for positioning and attending to the patient. 
     It is another object of the invention to provide a high degree of access to the patient prior to imaging without unduly increasing the footprint for supporting the head coil. 
     It is another object of the invention to provide a coil that allows easy access to the patient prior to imaging that may be operated with one hand and does not require the operator to leave the patient&#39;s side in order to obtain another coil component. 
     The first conductor support may be mounted relative to the head support to also move longitudinally with respect to the head support. 
     Thus it is another object of the invention to provide greater access to the patient than can be obtained by shortening the coil alone. 
     The first and second conductor supports and head support can be mounted on a base sized to be received on a patient table. 
     Thus it is another object of the invention to provide a simple and stable mounting of the moveable components of the present design. 
     The second conductor support may be a ring surrounding the head support or an inverted U having arms flanking a stationary base wherein the base includes longitudinal conductors. 
     Thus it is another object of the invention to provide for a reduction in the need for collapsible conductors in the base portion which may remain uncompressed. 
     The longitudinal conductors may be connected to form a single resonant structure or phased array of separate antennas. 
     Thus it is another object of the invention to provide for a coil design and accommodates two principal types of head coils. 
     The longitudinal conductors may form a resonant circuit tuned to the NMR frequency when the second conductor support is in the second position and detuned when the second conductor is in the first position. 
     Thus it is another object of the invention to provide a coil that is automatically detuned and thus decoupled from RF fields when it is in the collapsed condition. 
     The second conductor support may include a removable bridge portion over a portion of the opening passing by the patient&#39;s nose and mouth when the patient&#39;s head is on the patient support and the second conductor support is moved from the first position to the second position. 
     Thus it is another object of the invention to provide a coil that does not interfere with endotracheal tubes and the like. 
     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. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a head coil of the present invention showing positioning of the coil on a patient table in the lengthened form normally used after the patient&#39;s head is positioned in the head coil; 
     FIG. 2 a  is a side elevational view of the coil of FIG. 1 in the shortened position showing accessibility to an intubated patient from the sides or top; 
     FIG. 2 b  is a figure similar to that of FIG. 2 a  showing the coil in the lengthened position about the patient; 
     FIG. 3 is a detailed fragmentary view of the front conductor support of the coil showing a removable bridge section allowing passage of the front conductor support past the endotracheal tube in FIG. 2 when the coil is lengthened; 
     FIG. 4 is a schematic diagram of interconnected coil elements contained in the coils of FIGS. 1 through 3 when implemented as a single resonant structure; 
     FIG. 5 is a figure similar to that of FIG. 4 showing a schematic when the coils are implemented in a phased array of separate coils; 
     FIG. 6 is a side elevational cross-section of the coil of FIG. 1 showing the placement of flexible conductors extending between the conductor supports within telescoping insulating tubes and showing a take-up reel positioned in the superior conductor support to wind the flexible conductors when the coil is shortened; 
     FIG. 7 is a perspective fragmentary view of the interior of the conductor support of FIG. 6 showing guide wheels guiding the flexible conductors onto a central take-up reel; 
     FIG. 8 is a detailed cross-section of a telescoping tube joint of FIG. 6 showing the contained flexible conductor; 
     FIG. 9 a  and  9   b  are top plan views of a self-winder that may be used instead or in addition to the take-up reel of FIG. 7 to wind the flexible conductor when the coil is shortened; 
     FIG. 10 a  and  10   b  are fragmentary views of a longitudinal conductor according to a second embodiment in which an outer insulated sheath is a jointed tube that may articulate with shortening of the coil; 
     FIGS. 11 a  and  11   b  are an alternative embodiment in which relative rotation of the conductor supports is used to take up slack in the longitudinal conductors on shortening of the coil; 
     FIG. 12 is a fragmentary view of one longitudinal conductor in yet another alternative embodiment in which outer telescoping insulating tubes are positioned about inner conductive telescoping tubes serving as longitudinal conductors; 
     FIGS. 13 a  and  13   b  are front and side elevational views of an alternative longitudinal conductor using interengaging tracks providing rigid conductors that may interconnect; 
     FIGS. 14 a  and  14   b  are yet an alternative embodiment using a braided conductor whose diameter shrinks about a rigid telescoping form with extension of the coil; 
     FIG. 15 is a side elevational diagram of yet another embodiment of the coil in which rigid conductors pass through the second end plate to allow shortening of the coil; 
     FIGS. 16 a  and  16   b  are fragmentary views of a flexible conductor pre-stressed to fold into an accordion to take up slack when the coil is shortened; 
     FIGS. 17 a  and  17   b  are a simplified representation of a coil in which flexible conductors are drawn to the side when the coil is shortened; 
     FIG. 18 is a elevational view of a retractor mechanism using individual retractor coils in each of the flexible conductors such as may be contained in one conductor support; 
     FIG. 19 shows a connector system allowing electrical connection of the longitudinal conductors of FIG. 18 when they are fully extended; 
     FIG. 20 is a cross-sectional view of the braid for FIGS. 14 and 15 in which internal convolution of the braid serves to collect excess conductor length; 
     FIG. 21 is a collapsed view of a spiral conductor that may be extended and collapsed to accommodate the lengthening and shortening of the coil; 
     FIG. 22 shows yet further embodiment of a retractor mechanism that may be held in the second conductor support using a cam follower that draws the ends of the longitudinal conductors inward upon shortening of the coil; and 
     FIG. 23 is a perspective view of an alternative embodiment in which one or more longitudinal conductors are embedded in the base so as to not require collapsing when the coil conductor supports are moved into an adjacent configuration. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1, a head coil  10  of the present invention, in a lengthened state, detects signals for MRI imaging from a generally cylindrical volume  12  defined by a first and second conductor support  14  and  16  spanned by a series of longitudinally extending conductors  18  arranged about the periphery of the conductor supports  14  and  16 . The conductor supports  14  and  16  are sized to fit within the bore  31  of an MRI magnet with the axis of the cylindrical volume  12  aligned with the longitudinal axis  28  of the bore  31 . 
     Generally, the volume  12  need not be a perfect cylinder but may be flattened as is understood in the art. The longitudinal conductors  18  need not be parallel to the longitudinal axis so long as they extend longitudinally, that is, have a component of longitudinal extension. Thus the present invention is suitable for SENSE type coils such as are described in co-pending application Ser. No. 10/303,582 using triangular or other loop types. 
     The conductor support  14  includes an opening  20  sized to receive a patient&#39;s head. A head support  22  extends into the opening  20  to lie near the lower periphery of the cylindrical volume  12  and is attached outside of the opening  20  to a base  24 . The base  24  is sized to fit on top of a patient table  26  extending along the longitudinal axis  28  and also holds the conductor supports  14  and  16  as will be described. 
     The head coil  10  in FIG. 1 is shown in its lengthened state. Referring to FIG. 2 a , the head coil  10  of FIG. 1 may be moved to a shortened state in which conductor supports  14  and  16  slide together along longitudinal axis  28 . During the shortening, conductor support  16  may also move slightly in the superior direction. The combined shortening and movement of conductive support  16  opens access region  30  allowing positioning of the head of a patient  32  laterally or vertically onto head support  22 . The access region  30  also provides access to the patient&#39;s head prior to imaging. 
     Referring now to FIG. 2 b , conductor support  14  may be shifted as indicated by arrow  36  in an inferior direction over the head of the patient  32  lengthening the head coil  10 . This shifting may optionally include a slight amount of sliding of conductor support  16  in the inferior direction as well as indicated by arrow  38 . After the lengthening of the head coil  10 , the head of the patient  32  is contained within the volume  12 . 
     Referring momentarily to FIGS. 3 and 1, the conductor support  14  may include a bridge  40  positioned at the top of the opening  20 . The bridge  40  may be removed providing a passage  42  in the wall of opening  20  through which an endotracheal tube  34  may pass. Thus, if the patient  32  is intubated with an endotracheal tube  34 , this tube or similar devices need not be removed during the lengthening of the head coil  10  or the imaging. The bridge  40  may have key sections locking it to the conductor support  14  when it is in place and provides electrical connectors  44  that join with corresponding connectors  46  on the open ends of the passage  42 . In a preferred embodiment, the bridge  40  is hinged to the conductor support  14  (not shown) so as to not be misplaced. 
     Referring now to FIGS. 1 and 4, each of the longitudinal conductors  18  may be joined as a single resonant structure in which their ends are joined by segments of conductive rings  48  which may be positioned within the conductor supports  14  and  16  and of which bridge  40  provides a part. The rings are tuned to provide a standing wave at the resonant frequency of interest as is well understood in the art. One or two quadrature leads  49  may extend form the conductor support  16  where they attach to the internal loop as is well understood in the art. 
     Alternatively, as shown in FIG. 5, each of the longitudinal conductors  18  may carry two conductors such as forms opposite sides of independent phased array coils  50 . In this case, the conductor supports  14  and  16  do not contain a continuous ring but contain short segments making up parts of the separate phased array coils  50 . Separate conduction leads for each of the coils  52  may be routed typically from conductor support  16  (not shown). 
     Referring now to FIG. 6, each of the conductor supports  14  and  16  may be mounted on carriers  54  and  56 , respectively, riding on tracks  58 , for example, a rod held by supports  62 , to slide longitudinally thereupon. Carrier  56  of conductor support  16  may be biased by spring  60  to support  62  at the superior end of the head coil  10  so as to naturally be drawn to that end except when pulled by tension on the longitudinal conductors  18  excreted by conductor support  14 . Conductor support  14  may be pulled manually to the inferior end of the head coil  10  and held by a catch  64  engaging the track  58  as activated by a user accessible button  66 . The tracks  58  may provide resistance to twisting and allow single-handed operation of the carriers  54  in lengthening the head coil  10 . 
     To accommodate the varying separation between the conductor supports  14  and  16 , longitudinal conductors  18  may include an outer sheath of insulating telescoping tubes  68 . Referring also to FIG. 8, each tube  68  fits within the preceding tube  68  to be retained from separation by interengaging flanges  70 . A sufficient number of tube segments is used so as to provide for a compliance or lapse distance of approximately eighty percent. 
     The tubes  68  retain in their center a flexible conductor  72  that provides for the conduction of the longitudinal conductor. The conductor  72  may be a flexible wire, metal braid, foil or other flexible conductive material. 
     Referring now to FIGS. 6 and 7, one end of the flexible conductors  72  may be attached to a generally disk-shaped printed circuit board  74  held within the conductor support  14  to provide for the electrical interconnection and tuning circuitry and connecting cable interfaces. The length of the conductors  72  then extend through the insulating tubes  68  as part of the longitudinal conductors  18  to the conductor support  16  and within its housing to be received by idler pulleys  76 . Idler pulleys  76  have radial axes and direct the conductors  72  along the circumference of a take-up reel  78 , the take-up reel  78  rotatable about a longitudinal axis and fitting within the housing of the conductor support  16 : 
     The take-up reel  78  may be rotationally spring biased by a constant force spring  81  so that with collapse of the coil  10 , moving conductor supports  14  and  16  together, conductors  72  are directed around radial pulleys  76  and are wrapped about an outer circumference of the take-up reel  78  as it turns. Attached to a back surface of the take-up reel  78  to rotate with it is a printed circuit board  80  (corresponding electrically to printed circuit board  74 ) to which conductors  72  may be terminated within the conductor support  16 . Bending of the conductor  72  is minimized and points of electrical connection and disconnection, which may have varying resistances and/or capacitances, are avoided by having ends of conductor  72  always permanently affixed to their respective circuit boards  80  and  74 . 
     Referring now to FIGS. 9 a  and  9   b , in an alternative embodiment, the take-up reel  78  is eliminated and modular self-winders  82  are placed within the insulating tubes  68 . The self-winders include a base  84  holding the self-winder  82  against rotation within the tube  68  and a jaw  86  through which the conductor  72  may pass. The jaw  86  is biased by a constant force spring as indicated by arrows  90  so that when slack is obtained in the conductor  72 , as shown in FIG. 9 b , the jaw  86  rotates and wraps the conductor  72  about its outer circumference. One or more self-winders  82  may be used on each longitudinal conductor  18  and enlarged sections of the tubes  68  may be provided for the purpose of holding these self-winders  82 . Again, the conductors  72  remain permanently attached to the respective circuit boards  74  and  80 . 
     Referring now to FIGS. 10 a  and  10   b , the flexible conductors  72  may in an alternative embodiment be contained in tubes  92  connected by means of joints  94  so as to articulate or bend about radial or circumferential axes when conductor supports  14  and  16  are moved together. The bending may be in an accordion fashion to reduce possible interference with the patient and/or bore. Again the internal conductor  72  may be a flexible metallic member or the like. 
     Referring now to FIGS. 11 a  and  11   b , in yet another alternative embodiment, the longitudinal conductors  18  may be flexible conductors with flexible insulating sheaths or alternatively stiff metal members with insulating coverings mounted for rotation and angulation at the conductor supports  14  and  16 . In this embodiment, the one or both of the conductor supports  14  and  16  may be mounted so as to twist about the longitudinal axis  28  as conductor supports  14  and  16  move together about the longitudinal axis  28  as indicated by arrow  98 . This relative rotation between conductor supports  14  and  16  causes angulation of the longitudinal conductors  18  from parallel to longitudinal axis  28  decreasing their length along the longitudinal axis  28  without the need to bend the longitudinal conductors  18  except at their junctions to the conductor supports  14  and  16 . As before, the ends of the longitudinal conductors may be permanently affixed to the respective circuit boards  74  and  80 . 
     Referring now to FIG. 12 in an alternative embodiment, the tubes  68  that telescope to provide an insulating sheath may have corresponding conductive tubes  100  within them. Each conductive tube  100  is assembled in telescoping fashion and has a diameter to fit within the preceding conductive tube  100  if any and to be retained by opposing flanges  102  on each end of the two conductive tubes  100 . The tubes themselves may be metallic so as to provide conduction and their junctures may be enhanced by spring-biased brushes  104  according to techniques well known in the art of radio antennas and the like. 
     Referring now to FIGS. 13 a  and  13   b  in a variation on FIG. 12, the longitudinal conductors  18  may be composed of interengaging tracks  106  having insulating and conductive portions. The tracks may interengage by means of a dovetail joint formed between the tracks, one side of which holds a metallic conductor  108  and the other side of which holds a spring conductor  110  that provides a continuous path of conduction between the conductor  108  and a conductor  112  as the tracks  106  slide with respect to each other. A set of several such interengaging tracks  106  provides the longitudinal conductors  18 . 
     Referring now to FIGS. 14 a  and  14   b , in yet a further embodiment, the longitudinal conductors  18  may be composed of metal and possibly plastic fibers in a braided sheath  120 . The braided sheath  120  may be fit about a telescoping mandrel  122  providing it with support and precise location when it is in an extended configuration. Compression of the braided sheath  120  shown in FIG. 14 b  causes it to expand in diameter  123  to take up the slack caused by the conductor supports  14  and  16  moving together. An insulating braid (not shown) may be placed about the surface of the conductive braided sheath  120 . 
     Referring now to FIG. 15, in an embodiment that does not provide some of the advantages of the foregoing embodiments, the longitudinal conductors  18  and their sheathing may be rigid and may pass through holes in the conductor support  16  with movement of the conductor support  14  longitudinally as indicated by arrow  124 . The longitudinal conductors  18  are permanently attached to a circuit board  74  (not shown) in the conductor support  14 . Electrical connectors  126  on the ends of the longitudinal conductors  18  allow the longitudinal conductors  18  to connect electrical connectors  128  on circuit board  80  (not shown) in the conductor support  16  when the conductor supports  14  and  16  are fully separated. 
     Referring now to FIGS. 16 a  and  16   b , in yet an alternative embodiment, longitudinal conductors  18  may be formed from a flexible conductor  132 , for example, metal foil, laminated to a springy support  130  having memory holding it in a reduced length configurations, for example, as shown here in a corrugated fold. Extension of the conductor supports  14  and  16  stretches out the springy support  130  and flexible conductor  132  as shown in FIG. 16 b . Such an approach may be used within a separate sheath or independently with flexible insulation placed on both sides of the flexible conductor  132 . 
     Referring now to FIGS. 17 a  and  17   b , in an alternative embodiment, flexible conductors and insulators of longitudinal conductors  18  may simply be draped to the left and right of the conductor supports  14  and  16  possibly assisted by retractor elastic  133  or the like, when the conductor supports  14  and  16  are moved together. 
     FIG. 18 shows a variation on the retractor reel of FIG. 6 in which each conductor  72  is associated with a separate spring driven reel  134  mounted on the circuit board  80  at the point where the conductor  72  enters the housing of the conductor support  16 . The reels  134  take up the conductor  172  as the conductor supports  14  and  16  (not shown) are moved together. Electrical conduction to the end of the conductor  72  at the conductor support  16  may be obtained by a slip ring system or by a connector system shown in FIG. 9 in which conductor  72  at fill extension has a conductive collar  136  that engages a corresponding electrical connector  138  attached to the circuit board  80  within the conductor support  16 . In this case, it is noted that no electrical connection is required between conductor  72  and the remainder of the coil structure held by the printed circuit board  80  except during full extension. 
     It will be noted in this regard that the drastic change in geometry of the coil  10  in its compressed and extended form ensures that the coil  10  is detuned when in its shortened form preventing incidental and possibly damaging resonance currents from forming. 
     Referring now to FIG. 20, the braid system shown in FIGS. 14 a  and  14   b  may also provide for collapsing about a central mandrel by a convolution of the braided sheath  120  folding in among itself in concentric fashion. 
     Referring to FIG. 21, an alternative method of creating the longitudinal conductors  18  includes a planer spiral  140  whose center  142  may be attached to one circuit board  74  shown in FIG. 6, and whose outer end  144  may be attached to circuit board  80  in conductor support  16 . When the conductor supports  14  and  16  are close together, the spiral  140  assumes a more planar configuration and while the conductor supports  14  and  16  are fully extended, the spiral  140  assumes a twisted linear form. 
     Referring to FIG. 22, an alternative mechanism to the reel structure of FIG. 6 provides for a rotating cam plate  146  rotating about a longitudinal axis and held within the housing of conductor support  16 . Cam followers  148  follow cam surfaces on the cam plate  146  which spiral inward. The flexible conductors  72  are diverted along the cam plate  146  by idler pulleys  150  being similar to pulleys  76  of FIG. 7 but having circumferential axes of rotations to direct the conductor  72  in a radial fashion along cam plate  146 . Rotation of the cam plate  146  causes pulling inward of the cam followers  148  toward the center of the cam plate  146  taking up the requisite slack. 
     Referring now to FIG. 23, the longitudinal conductors  18  beneath the head support  22  need not be so shortened as they do not interfere with access by the patient&#39;s head to the cylindrical volume  12 . Accordingly, two longitudinal conductors  18 ′ beneath or within the head support  22  may be permanently in extended form and may electrically join to the other longitudinal conductors  18  when the conductor support  14  is in full extension position. Alternatively, in the phased array configuration, such joining may not be required. In this way, the conductor support  14  need not be a continuous ring that surrounds the head support  22  but may be U-shaped. 
     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 different combinations of elements of different embodiments as come within the scope of the following claims.