Abstract:
Two halves of a separable wrist coil provide a coil volume closely conforming to a patient&#39;s hand yet separable for insertion and removal of the hand. The halves are hinged allowing easy one-handed operation of the coil in which an operator may stabilize the patient&#39;s hand within a propositioned coil with one hand while closing the coil and locking it in position with the other. Detents and a manual lock assist in this process.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims the benefit of U.S. Provisional Application No. 60/739,553 filed Nov. 23, 2005 and hereby incorporated by reference. 

   BACKGROUND OF THE INVENTION 
   The field of the invention is magnetic resonance imaging (MRI) and, in particular, local coils for use in MRI of the wrist. 
   In MRI, a uniform magnetic field B 0  is applied to an imaged object to align the object&#39;s nuclear spins along a z-axis. In response to a radio frequency (RF) excitation signal of the proper frequency oriented within the x-y plane, the nuclei precess about the z-axis. 
   In a typical imaging sequence for magnetic resonance imaging, the precessing nuclear spins are then phase and frequency encoded according to their positions, using magnetic gradient fields. A weak nuclear magnetic resonance generated by the precessing nuclei may then be sensed by the RF coil and recorded as an NMR signal. From this NMR signal for a series of such signal acquisitions with different phase encodings, a slice image may be derived according to well-known reconstruction techniques in which the phase and frequency encoding is used to map NMR signal strength to different locations in the image. 
   The RF excitation and the NMR signal may be transmitted and received respectively by means of one or more RF coils. Improvements in the signal to noise ratio of the received NMR signal can be obtained by placing “local coils” on the patient. The local coil having a smaller reception pattern can focus in on the region of interest to obtain a stronger signal and to receive less noise. Phased array RF coils are multiple loop local coils whose outputs are processed separately and are electrically independent. 
   Desirably, a local coil may be “wrapped” about a portion of the body so that the volume within the local coil conforms closely to the imaged part. One way of accomplishing this is the use of flexible coils having thin copper conductors held in fabric-like panels that may be literally wrapped about the patient. 
   Coils with rigid structure can also be used but for such coils the ability to have the local coil closely conform to the patient is limited by the ability of the patient to thread his or her limb into the coils structure. For this reason, it is known to construct coils, for example, for imaging the hand or wrist, in two parts that may be separated from each other along a seam line so that the hand may be inserted, and then reassembled using several clamps holding the two parts together about the limb. Such coils provide good support for the antenna structure and the patient&#39;s limb but can be cumbersome for the MRI operator who must contend with positioning the patient and manipulating the loose part of the coil into alignment and managing the clamps necessary to hold the pieces together. 
   SUMMARY OF THE INVENTION 
   The present invention provides a two part rigid coil for imaging the wrist or other body member in which the two parts are held by a hinge structure allowing the coil to be opened for insertion of a limb and then to be hinged closed. The hinge, by limiting the freedom of motion of the moving portion of the coil, greatly simplifies the positioning of the patient and assembly of the parts, particularly when the coil is positioned with the hinge axis vertically. An automatically operating detent and lock can allow the operator to avoid the need to contend with cumbersome mechanical clasps and the like normally used in two-part designs. 
   Specifically, then, the present invention provides an MRI local coil suitable for imaging portions of a human limb having a pair of shells interfitting along a seam to define a volume closely conforming to the human limb, and a hinge attaching the shells so that the shells may hinge apart about the seam to permit the limb to be inserted into or removed from between the shells and hinge together along the seam. Antenna conductors are held by the shells to provide detection of NMR signals from the wrist when the shells are hinged together. 
   It is thus one aspect of at least one embodiment of the invention to provide a two-part wrist coil allowing close conformance to the wrist and where the parts remain connected for rapid set up by an operator 
   The volume may allow insertion of the human limb into the volume to extend along a first axis and wherein the hinge provides rotation of one shell with respect to the other shell along a second axis perpendicular or parallel to the first axis. 
   It is another object of the invention to provide a hinge axis that provides a clear view of the patient&#39;s hand when used on both the left and right patient&#39;s hand during set up. 
   The hinge axis may be displaced with respect to the volume to not intersect the volume. 
   It is another aspect of at least one embodiment of the invention to provide a location of the hinge axis so that opening of the parts does not cause a compression of the hand located between the shells. 
   The volume may be open at opposite ends of the local coil along the axis when the shells are hinged together and when the shells are hinged apart. 
   It is another object invention to provide a coil that allows free extension of the fingers for different sized patients and for increased patient comfort. 
   The volume provides a first channel for receiving a wrist connecting to a second broader channel for receiving a hand. A wall of the volume adjacent to a palm of the hand when the hand is positioned in the volume may present a surface that curves along an axis of the hand to conform to the palm. 
   It is another aspect of at least one embodiment of the invention to provide a coil that may closely conform to the wrist area for improved imaging of the wrist bones. 
   The seam may provide a plurality of electrical connectors joining when the shells are hinged together to provide electrical conduction paths for the antenna conductors. 
   It is another aspect of at least one embodiment of the invention to provide a hinge axis that provides a clean electrical connection and disconnection of coil portions in each of the wrist shells. 
   The antenna conductors may be conductors of a phased array coil. 
   It is another aspect of at least one embodiment of the invention to provide a support for a coil allowing the coils to be electrically continuous about the hand when the shells are closed, providing for improved imaging. 
   The coils may include a mechanical detent biasing the hinge toward a closed position when the shells are proximate to each other about the seam and/or biasing the hinge away from a closed position when the shells are not proximate to each other about the seam. 
   It is another aspect of at least one embodiment of the invention to provide stability in the hinged halves of the shell to assist the operator in positioning the patient in the shell. 
   The coil may further include a manually actuated lock, the lock locking the shells together when the shells are in a closed position. 
   It is another aspect of at least one embodiment of the invention to provide for a positive locking of the shells after initial location by detent. 
   The invention may further include a stand fitting against a patient table and having engagement means engaging with an edge of at least one shell to hold the shell so that a plane of the seam extends generally vertically or horizontally. 
   It is another aspect of at least one embodiment of the invention to allow the shells to be pre-positioned before insertion of the patient&#39;s hand for multiple orientations of the wrist coil for different imaging purposes. 
   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 a perspective view of a wrist coil of the present invention having hinged shells in an open position; 
       FIG. 2  is a perspective view similar to that of  FIG. 1  of the wrist coil assembled on a base unit for use on a patient&#39;s right hand in a palm-vertical orientation; 
       FIG. 3  is an elevational cross-sectional view through the coil of  FIG. 1  along lines  3 - 3  of  FIG. 1  showing a curved support surface position near the palm of the user and showing a position of the hinge line preventing entrapment of the patient&#39;s hand as the shells are opened; 
       FIGS. 4 and 5  are detailed views all a hinge of  FIG. 3  showing a spring detent that biases the shell&#39;s bistability in the open and closed positions; 
       FIG. 6  is a cross-sectional view along line  6 - 6  of  FIG. 1  showing a second detent/lock mechanism for holding the shells in a closed position and locking the shells closed; 
       FIG. 7  is a top plan view in phantom of a lock button of the detent/lock of  FIG. 6  moving to slide a peg between two arms of the detent/lock preventing the release of the door; 
       FIG. 8  is a schematic unrolled representation of the coil assembly of the wrist coil of  FIG. 1  showing the capacitive decoupling network used to isolate the individual phased array coil; and 
       FIG. 9  is a figure showing the coil assembly as rolled inside the shells of the wrist coil of  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to  FIGS. 1 and 2 , a wrist coil  10  of the present invention may include a first shell  12  and a second hingable shell  14  hinging with respect to shell  12  along a hinge axis  16 , as shown in  FIGS. 3-5 . The shells may be constructed of a rigid polymer material to be compatible with the magnetic and radiofrequency fields of a MRJ machine (not shown) in which the wrist coil  10  will be used. 
   The shells  12  and  14  are connected together to hinge about a hinge axis  16  between an open position (as shown) and a closed position in which a generally planar seam surface  18  of shell  12  abuts generally planar seams surface  20  of shell  14 , and the shells together define an internal hand volume  24  extending along axis  34  and are sized to receive a human hand  26 . The hinge axis  16  is generally perpendicular to the axis  34  of the hand volume  24 . 
   A seam interface  22 , generally dividing the shells  12  and  14  along their respective seam surfaces  18  and  20  when the shells  12  and  14  are closed, approximately bisects the hand volume  24 , so that when the shells  12  and  14  are opened with respect to each other, a hand  26  may be inserted between the shells  12  and  14  with axial motion along axis  34  and then moved laterally into the portion of the hand volume  24  defined by shell  12 . 
   The hand volume  2  includes a narrow wrist portion  28  for receiving the wrist of the hand  26  which joins with an expanded hand portion  30  receiving the fingers of the hand  26 . The hand volume  24  is open at opposite ends, at the wrist portion  28  and at the hand portion  30 , so that the fingers  32  may extend out of the hand portion  30  as shown in  FIG. 2 . The hand portion  30  expands laterally and provides a conformal palm surface  35  properly positioning the hand as may be desired for scanning. In addition radiofrequency transparent foam cushions (not shown) may be placed within the hand volume  24  to provide for additional support and stabilization of the hand  26  in the shells  12  and  14  when they are closed. Generally, the close conformance of the shells  12  and  14  to the hand  26  prevent withdrawal of the hand  26  when the shells  12  and  14  are closed. 
   Referring now to  FIG. 3 , the hinge axis  16  is displaced away from the hand volume  24  so as not intersect the hand volume  24  so that the shell  14  may hinge upward (as shown by shell  14 ′) without notably decreasing the volume of the hand volume  24  such as may create a compression or pinching of the hand  26  placed there in the hand volume  24 . This may be contrasted to a location of a hinge axis  16 ′ along seam interface  22  such as would cause the leading upper edge  19  of the shell  14  to pivot down into the hand volume  24 . 
   Referring to  FIG. 4 and 5 , a cam  38  coaxial with the hinge axis  16  and turning with the shell  14  may have a first flat  40  compressed by leaf spring  42  to bias the shell  14  in a closed direction  44  when the shell  14  is proximate to the shell  12 . Conversely as shown in  FIG. 5 , the cam  38  may present a second flat  46  compressed by the leaf spring  42  when the shell  14  is in the open position tending to hold the shell  14  in the open position. Thus the shell  14  may be bi-stable, tending to remain in an open or closed position. This bi-stability keeps the shell  14  from moving accidentally and interfering with proper positioning of the patient&#39;s hand  26  by an operator. The bi-stability also holds the shell  14  closed allowing one-handed operation of a lock to be described below, so that the operator may stabilize the patient&#39;s hand  26  in the shell  12  with one hand while closing the shell  14  with the other hand, and then moving a hand from the shell  14  to the lock with certainty that the shell  14  will remain closed until the locking is complete. 
   The first shell  14  may have an observation hole  15 , as shown in  FIG. 1 , passing from an outer surface of the shell  14  and into the hand volume  24  further improving air circulation and allowing the operator to confirm the location of the patient&#39;s wrist within the wrist coil  10  when the shells are closed. 
   Referring to  FIG. 6 , shell  14  may further include a set of lock arms  50  biased outwardly by compression springs  52  perpendicular to axis  34  and generally along the seam interface  22 . Outwardly opposed teeth  54  on each of the lock arms  50  near the surface  20  engage corresponding tabs  56  extending upward from surface  18  of shell  12 . The teeth  54  are beveled at a surface of contact between the teeth  54  and the tabs  56  with closure of the shell  14  so that the teeth  54  and lock arms  50  are pressed inward against their springs  42  so that the teeth  54  pass by an upper side of a lip of the tabs  56  and then spring outward to engage a lower side of the lip of the tabs when the shells  12  and  14  are fully closed against each other. This spring action provides a second detent mechanism allowing the shell  14  to snap closed against shell  12  with slight operator pressure. 
   Conversely, the teeth  54  are also beveled at a surface of contact between the teeth  54  and the tabs  56  with the opening of the shell  14  so that the teeth  54  and lock arms  50  are also pressed inward against their springs  42  allowing the teeth  54  pass the lower side of a lip of the tabs  56  with upward force on the shell  14 . This allows the shell  14  to be opened simply by applying sufficient force to the shell  14 . 
   Referring also to  FIG. 7 , when the teeth  54  are engaged with the tabs  56  with the shell  14  in the closed position, a peg  60  extending downwardly from a lock button  62  slidable generally along axis  34  may be placed between two adjacent ends of the lock arms  50  preventing the lock arms  50  from moving inward and effectively locking the shell  14  in the closed position. The peg  60  may be so inserted or removed by an axial sliding action of lock button  62 . 
   Referring now to  FIG. 8 , a phased array coil  63  may be held within the shells  12  and  14  of the wrist coil  10  to surround the wrist of the hand  26  when the hand is inserted in the coil  10  and to provide for an eight channel phased array coil positioned generally about the wrist of the hand  26 . Phased array coil  63  provides eight isolated loops  64   a  through  64   h  formed of eight axial conductors  66  and three circumferential conductors  67 ,  68  and  70  connected by coupling and isolation capacitors  72 . A coil design suitable for this purpose is described in U.S. Pat. No. 7,091,721 issuing Aug. 15, 2006, entitled: “Phased array local coil for MRI imaging having non-overlapping regions of sensitivity” and hereby incorporated by reference. Referring to  FIG. 9  the phased array coil  63  of  FIG. 8  wraps circularly and continuously electrically about the hand  26  of the patient. Each of the loops  64   a  through  64   h  provides for a separate channel output  65  that may be attached to the MRI machine for the production of independent images or to be combined for improved signal to noise ratio. 
   Referring also to  FIG. 1  conductors  67 ,  68  and  70  are exposed on surfaces  18  in the form of connector tabs  78  that engage with corresponding connect slots  74  in seam surface  20  so that electrical continuity can be had across the seam interface  22  for conductors  67 ,  68  and  70  as they encircle the wrist. These points of connection are also shown in  FIG. 8 . The geometry of the hinge axis  16  allows a hinging action permitting clean engagement and disengagement of these electrical connectors. 
   Referring again to  FIGS. 1 and 2  and  3 , posts  79  with spring-loaded radially extending heads may be positioned on the right and on the left coil  10  along the seam interface  22  and positioned on the bottom of the shell  12  as shown in  FIG. 8 . These posts  79  may fit within a corresponding key slot  80  in a base unit  82  fitting against the upper surface of a patient table  84  and be held by the weight of the patient thereupon. In this way different orientations of the wrist coil  10  may be supported by the base, with the seam interface  22  vertically as shown in  FIG. 2  for the right hand, or inverted for the left hand, or with the seam plane horizontal. 
   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.