Abstract:
A patient support table for breast MRI systems, the table including a base configured to slide into and out of the bore of an MRI system; a patient support member attached to the base and having a top surface shaped to support a patient in a prone position; two breast openings in the patient support member, the breast openings being sized and positioned to permit the patient&#39;s breasts to extend downward beneath the patient support member, at least a pair of breast immobilization paddles, one on each side of a breast opening, the immobilization paddles comprising a grid of breast contact elements that contact the breast while providing a plurality of openings through which a surgical tool may be inserted into the breast, and the paddles having a convex curvature along the Z direction (head to toe) so that the breast contacting surface of each paddle is further from the Y-axis center line of the breast opening midway along the Z extent of the paddle than at the Z extremities of the paddle.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application Ser. No. 60/332,563, filed on Nov. 21, 2001 (hereby incorporated by reference). This application incorporates by reference the disclosure of U.S. patent application Ser. No. 10/301,805, filed Nov. 21, 2002, entitled “Patient Support Table for Breast MRI System”. 
    
    
     BACKGROUND 
     The invention relates to magnetic resonance imaging (MRI) systems, particularly MRI systems for imaging breasts. 
     MRI systems typically employ a main magnet that produces a static magnetic field and gradient coils that superimpose spatially varying magnetic fields on top of the static field. The gradient coils are typically formed on a cylindrical insert that fits within the bore of the main magnet. Inside of the gradient coil insert there are RF transmitting and RF receiving coils (sometimes the same coil). The RF transmitting coil excites the molecules of the anatomy being imaged, and the RF receiving coil detects the response of the molecules. 
     MRI images can be used to guide biopsy and surgical tools. The anatomy of interest is imaged with one or more reference (“fiducial”) markers appearing in the image (or series of images). The surgical tool is installed in a known relationship to the reference markers (e.g., in a rotatable tool holder), and the tool is positioned so that upon insertion it will reach the lesion or other target found in the image (e.g., the tool holder is adjusted so that the target is at the center of rotation of the holder). Knowing the locations of the reference marker and the lesion makes IT possible to determine the position and orientation that the tool should be in for it to reach the target when inserted. 
     Often, the biopsy or surgical procedure is done after the patient is withdrawn from the MRI system, but the anatomy of interest is typically held stationary between the time that the images are taken and the surgery is completed. 
     This technique has been applied to some degree in biopsy of breast lesions. For example, in Schneider U.S. Pat. No. 5,855,554, MRI-transparent breast stabilization plates hold the breasts stationary, and reference markers built into the plates provide information for orienting the biopsy tool. 
     SUMMARY 
     In general, the invention features a patient support table for breast MRI systems, the table comprising a base configured to slide into and out of the bore of an MRI system; a patient support member attached to the base and having a top surface shaped to support a patient in a prone position; two breast openings in the patient support member, the breast openings being sized and positioned to permit the patient&#39;s breasts to extend downward beneath the patient support member, at least a pair of breast immobilization paddles, one on each side of a breast opening, the immobilization paddles comprising a grid of breast contact elements that contact the breast while providing a plurality of openings through which a surgical tool may be inserted into the breast, and the paddles having a convex curvature along the Z direction (head to toe) so that the breast contacting surface of each paddle is further from the Y-axis center line of the breast opening midway along the Z extent of the paddle than at the Z extremities of the paddle. 
     In preferred implementations, one or more of the following may be incorporated. The convex curvature may be greatest at the edge of the paddles corresponding to the base of the breasts at the chest wall. The convex curvature may be progressively less at locations on the paddle further from the edge corresponding to the base of the breasts at the chest wall. The paddles may be substantially flat without curvature at the edge opposite the edge with curvature. The convex curvature at the edge corresponding to the base of the breast may have a radius of curvature of at least 50 mm (preferably at least 100 mm). The paddles may be further shaped so that at the paddle edge closest to the breast opening, and to the chest when a breast is immobilized by the paddles, the paddles are flared outwardly with a radius of curvature greater than about 10 mm (preferably greater than about 15 mm). 
     Other features and advantages will be apparent from the following detailed description and drawings. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIGS. 1–3  are perspective views (photographs) of the breast immobilization paddles and associated mechanisms of a preferred embodiment of the invention. 
         FIG. 4  is a perspective view of the same paddles and mechanisms. 
         FIG. 4A  is a cross section view taken at  4 A— 4 A in  FIG. 4  showing the flare of the upper edge of the paddles. 
         FIGS. 5–6  are views of a single breast paddle. 
         FIG. 7  is an enlarged cross section through a grid element of a breast paddle. 
         FIG. 8  is a view of a breast paddle with marker deformations shown. 
         FIG. 9  is an elevation view, partially broken away, of the paddle adjustment mechanism. 
         FIG. 10  is an enlargement of a portion of  FIG. 9 . 
         FIG. 11  is a perspective view of the surgical tool holder and adjustment elements. 
         FIG. 12  is another view of the surgical tool holder adjustment elements. 
         FIG. 13  is an elevation view of the surgical tool holder with tool installed. 
         FIG. 14  is a perspective view (photograph) of the MRI reference marker. 
         FIG. 15  is a cross-sectional view of the MRI reference marker. 
     
    
    
     DETAILED DESCRIPTION 
     A description of the patient support table and the MRI system can be found in the disclosure of U.S. patent application Ser. No. 10/301,805, filed Nov. 21, 2002,. The surgical tool holder of the invention is installed in the vicinity of the breasts between the chest support member  44  and the base member  20 . 
       FIGS. 1–4  show the five column shaped RF coil housings  30  that extend between the chest support member  44  and base plate  19 , and one pair of breast immobilization paddles  56 . Another pair is installed on the other side of the base plate. The columns  30  also serve to support the breast paddles and their adjustment mechanisms. 
     The breast paddles have unique shapes designed to improve patient comfort, breast immobilization, and biopsy access. At their upper edges  102  ( FIG. 4 ), which contact the chest wall on the left and right sides of each breast (extending nearly all of the way through the breast openings), the paddles are flared outwardly in the X direction, with a radius of curvature R 1  of about 20 mm ( FIG. 4A ). The radius of curvature may be different than 20 mm, but it should be greater than 10 mm, and preferably greater than 15 mm. This shape has the advantages of greater comfort, and greater conformity with the breast so that this portion of the breast tissue is captured and immobilized. Openings  104  in the breast paddles extend close to the upper edge  102 , so that a surgical tool may be inserted close to the chest wall. 
     Another respect in which the paddles are shaped is the curvature of the upper edge along the Z direction (head to toe). This is best seen in  FIG. 5 . This curvature has a radius R 2  of curvature of about 150 mm. Preferably the radius of curvature is greater than 50 mm, and more preferably greater than 100 mm. The curvature is designed to match the breast anatomy, while also permitting access by a surgical tool. The curvature is substantially the same as the curvature of the side of the breast opening through which it extends. Both paddles have a similar curvature at this upper edge. The curvature becomes progressively less until at the lower edge  106  of the paddle there is substantially no curvature, and the paddle is flat. 
     The grid elements extending across the extent of the paddles have several important features. Preferably they are oriented on the diagonal, as at this orientation they line up better with the periphery of the breast, as illustrated in  FIG. 6 . This means that along the periphery, nearly entire grid openings are spanned by the breast. Were the grid elements to extend vertically and horizontally, the breast periphery would be broken up between a greater number of grid elements, making it harder and less convenient for the surgeon to align the surgical tool for insertion to a lesion along the periphery. Although not necessary in the preferred embodiment disclosed herein, the diagonal orientation of the grid elements has the further advantage that by moving the tool holder only in either X or Y directions, without rotating it (as is possible in the preferred embodiment shown) it is possible to find an insertion path that avoids hitting the grid. That would not usually be true if the grids were oriented vertically and horizontally. 
     The cross sectional shape of the grid elements is preferably selected so that the edges that contact the breast tissue have a curvature with a radius of curvature R 3  in the range of 0.5 to 5.0 mm, and preferably 1.0 to 3.0 mm. In  FIG. 7 , a cross section through one grid element is shown enlarged. The edge of the grid that contacts the breast tissue is to the right of the grid element, and it has corners each with a radius of curvature R 3 . This curvature tends to adhere better to the breast tissue, so that the skin is held taut between the grid elements. Preferably, the openings formed by the grid elements are in the range of 10 mm to 30 mm. 
     Some of the grid elements have differently shaped skin-contacting surfaces, with the intention that the different indentation shapes they produce will be visible in the MR image. Thus, the unique indentation pattern becomes a reference marker. This is particularly useful when a surgical tool is to be inserted manually without the benefit of the tool holder. The surgeon can locate the paddle opening closest to the lesion or target using the pattern of unique indentations. 
     Two types of indentations are shown in the embodiment. Ridges that extend along a grid element above the level of the other elements, and nubs that extend above the other elements at intersections of grid elements.  FIG. 8  shows the same paddle as  FIG. 6 , with one possible layout of ridges  112  shown. Nubs  110  are placed at all grid intersections other than intersections with ridges  112 . Different layouts of ridges and nubs can be used. The ridge layout shown in the figure was chosen so that when vertical slices oriented as shown at  114  are taken there would always tend to be two ridges that could possibly show up in the image. 
     The breast paddles can be adjusted in at least three ways to allow the paddles to conform to the patient&#39;s breast, and thereby hold it immobile with the skin taut. A paddle can be rotated about the Z axis, and translated left to right (X direction) at each end. By applying different amounts of translation at each end, it is possible, to some degree, to rotate the paddles about the Y axis. Thus, to some extent the paddles can be translated along the X axis, and rotated about the Z and Y axes. 
     The mechanisms for accomplishing the adjustment are shown in  FIGS. 9–10 , but various other types of mechanisms could be substituted, as the more important feature is that movement in a plurality of directions is made possible. In  FIGS. 9–10  the mechanisms are partially disassembled.  FIG. 10  is an enlargement of a portion of  FIG. 9 . In  FIGS. 1–2  they are assembled. 
     To translate a paddle at one end, the handle  120  at that end is squeezed, which has the effect of lifting gear train  122 , which flexes by bending at narrowed region  124 . Lifting the gear train  122  disengages it from mating gear segment  126 , to allow translation. By translating the two ends of a paddle differently, one can achieve both X translation and Y-axis rotation. 
     To rotate a paddle about the Z axis, knobs  128  at each end of the paddle must be pulled out in the Z direction. This has the effect of disengaging two sets of gear teeth  130  and permitting rotation of the paddle. 
     The surgical tool holder  50  and supporting structure is shown in  FIGS. 4 ,  11 – 12 .  FIGS. 11–12  show the tool holder installed in two different locations, one for medial access and the other for lateral access. The tool holder supports a surgical tool such as a biopsy needle (shown in  FIGS. 11–12 ). The tool holder is supported on one of two posts  54 ,  56 . Post  54  is used for lateral access, and post  56  for medial access. The posts are supported on adjustable support elements  150 ,  152 ,  154 , which are secured, in turn, to base plate  19 . Post  54  may be removed, and post  56  installed using an extension support element  152  when medial access is desired. 
     The tool holder may be translated in all three directions (X, Y, Z). Adjustment in the X and Z directions is accomplished by loosening the appropriate knob  160 , and translating the appropriate support element  150 ,  154 . The movement of the support elements is accurately measured by optical encoding sensors  170  (Z movement) and  172  (X movement). Adjustment in the Y direction is accomplished by loosing knob  161 , and moving the holder up and down on post  54  (or  56 ). A third optical encoder on the post records the Y translation. At any chosen X, Y, Z location, the tool holder  50  can also be rotated about an X axis. 
     We have found that the optical encoders may incorporate light emitting diode (LED) elements, or other elements that contain small amounts of material that can distort MRI images. 
     We have been able to place the optical encoders far enough away (e.g., grater than 40 mm) from the anatomy being imaged (the breasts) that the distortion is reduced to an insignificant level. Other types of encoders could be used instead of optical encoders. All that is necessary is that the encoder observe some sort of indicia that moves by it, and put out a digital output based on observation of the moving indicia. 
     Referring to  FIG. 13 , the tool  52  is oriented at an angle with respect to the X axis of rotation  172 , and is positioned in the holder  50  so that the tool is inserted along an insertion direction  170  that intersects with the X axis of rotation  172  at target location  174 . The goal of the MRI imaging, and adjustment of the tool holder, is to move the holder to a location at which the target location  174  is coincident with the lesion or anatomical target  176  found in the MRI image. The holder  50  has a base that is fixed to the post  54 , and a rotatable member  55  to which the tool  52  is attached. The surgeon is able to rotate the holder about axis  172  without changing the ultimate target of the tool. Thus, rotation can be used to find an insertion path  170  that avoids grid elements  180  of the paddles. 
     In operation, a series of MRI images are taken with a reference marker installed in the tool holder. The reference marker  300  preferably has a 3-dimensional cross hair configuration, as shown in  FIGS. 14 and 15 . The interior of the cube is filled with a liquid that shows up in the MRI image, and the polymer rods  306  extending through the cube in three orthogonal directions remain transparent in the image. It is not necessary that exactly the crosshair configuration shown be used, so long as the reference marker has a shape that permits its location to be determined from the images (preferably its X, Y, and Z location from one reference marker). Other shapes than cross hairs can accomplish that objective. Also, the arrangement in which the surrounding material is visible and the marking elements are transparent could be reversed, so that the marking elements (the cross hairs in the embodiment shown) could be made visible in the MRI image (e.g., by filling hollow rods with an MRI-visible material). 
     A preferred MRI visible material for use in constructing the reference marker is a hydrogel (superabsorbent) material, as it tends to expand and fill all cavities. But any of a wide variety of MRI-visible materials can also be used. 
     Preferably, the intersecting rods (or other elements) forming the reference marker are sized and positioned so that their intersection is visible on a single image slice and so that the elements appear in a plurality of adjacent slices. Having the intersection appear in a single slice improves accuracy. Having the elements appear in a plurality of adjacent slices makes it easier for the person observing the MRI slices to find the slice in which the intersection appears, as if it only appeared in a single slice it could be missed during review. 
     It is also preferable that the reference marker be shaped so that the MRI visible area grows in size from image slice to image slice as one approaches the image slice containing the intersection. This increases the likelihood that the reference marker will be seen in the image. If a cube shaped member  300  ( FIG. 15 ) is used, this can be achieved by orienting the cube  300  so that a diagonal is parallel to the plane of the image slices  302 ,  304 . In this orientation, the cross-section of the cube (which is what is visible, as it is filled with MRI-visible material) appearing in each slice  302  grows in size as one approaches the slice  304  containing the intersection. 
     The thickness of the intersecting rods (or other elements) should be fairly narrow for accuracy reasons. Preferably, the rods have a diameter on the order of the voxel size of the image (a voxel representing the three-dimensional volumetric granularity of the MRI system). If the rod thickness is much less than a voxel, it will not be seen, whereas if it is much greater, accuracy will be lost. For example, with a voxel size in the range of 1.0 to 2.0 mm, the rod thickness would preferably also be about 1.0 to 2.0 mm. 
     Software is used to permit an operator to locate the reference marker and the target (e.g., lesion). Straightforward mathematical operations are then performed to determine how much X, Y, and Z translation, and in which direction of translation, the tool holder needs to be moved so that the surgical tool will access the target upon insertion. A read out of the amount and direction of translation can be provided, and the operator can then adjust the tool holder&#39;s position until the outputs of the optical encoding sensors produce the specified translation. This can be facilitated by providing for zeroing of the displays prior to movement of the holder, so that all that is necessary is for the operator to move the holder until the displays read out the X, Y, and Z translations specified by the imaging software. 
     Once the holder has been moved to the correct location, the surgeon rotates the tool holder to find an insertion path that avoids the grid elements of the paddle or other obstruction, and initiates the surgical procedure. One option is to repeat the MRI imaging with the surgical tool inserted, and in that way confirm that the tool hit the anatomical target. 
     Other embodiments of the invention are within the following claims, and additional features described above and in the drawings form part of the invention and may be made the subject of claims.