Abstract:
An apparatus is provided for aligning a laser beam with a hidden target. In particular, a coupling is rotated which rotates a laser diode and simultaneously translates the diode to produce a spiral shaped laser beam path. The coupling is continuously rotated until it is determined that the beam has become aligned with the target.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to magnetic resonance imaging and, in particular, relates to a method for positioning a laser beam onto a predetermined location to identify a tissue to be imaged. 
     When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B 0 ), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B 1 ) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M z , may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment M t . A signal is emitted by the excited spins after the excitation signal B 1  is terminated. This signal may be received and processed to form an image. 
     When utilizing these signals to produce images, magnetic field gradients (G x  G y  and G z ) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well-known reconstruction techniques. 
     In modern MRI systems, a laser beam is used to define the center location, or sweet spot, of an image of human tissue to be imaged. However, due to mechanical interference with the MRI system, the laser emitting diode is typically mounted onto an outer surface of the MRI system. In order to direct the laser beam radially inwardly towards the patient, the beam is deflected off a mirror that is also mounted on the outer surface of the RF coils. An opening extends radially through the outer surface and provides a conduit for the laser beam to deflect off the mirror radially inwardly to identify the portion of the tissue that will correspond to the sweet spot of the image. Accordingly, once the laser has been calibrated, a patient may subsequently be placed in the MRI system and positioned so that the tissue to be imaged is identified by the laser beam. This will ensure that the sweet spot of the image will correspond to the desired tissue. 
     However, because the diode is positioned at an appreciable distance from the mirror, and because of the relatively small size of the mirror, and due to tolerances associated with manufacturing, it is highly unlikely that the beam will be in initial alignment with the mirror. For example, if the diode is mis-aligned by as little as ¼°, the beam will not hit the mirror. Furthermore, the laser will be periodically re-calibrated due to vibrations associated with operation of the MRI system. 
     Moreover, the laser must be adjusted to hit the precise point on the mirror that will yield the desired deflection. Accordingly, the position of the diode will need to be adjusted in the x and y directions so that the beam will deflect off the mirror. One method that could be used to properly align the laser with the mirror is to manually translate the diode in the x and y directions. The user will then rely on sight to determine when the beam becomes deflected off the mirror, which will become apparent when the beam extends through the MRI system in a predictable manner, and onto a predetermined calibration location. In particular, an adjustment lever extends from the laser assembly and out of the MRI system housing that may be manipulated to adjust the position of the diode. However, because the laser beam will be hidden when not properly aligned with the mirror, the user will be unaware what positional adjustments to the diode are necessary. Therefore, the user will essentially be blindly moving the diode at random until the beam hits the mirror. This is an unacceptably tedious, cumbersome, and time-consuming process. Furthermore, the sensitivity of the laser assembly hinders the fine adjustment of the laser beam using this method. 
     What is therefore needed is an improved method and apparatus for reliably and systematically manipulating a laser diode to align the output laser beam with a desired target location. 
     SUMMARY OF THE INVENTION 
     An apparatus for systematically aiming a laser beam to a target is presented having an outer housing extending generally along a central axis, a laser diode operable to emit the laser beam in the general direction of the target, and an adjustable coupling for mounting the laser diode to the outer housing and being operable to systematically move the laser beam in a search path that intersects with the target. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Reference is hereby made to the following figures in which like reference numerals correspond to like elements throughout, and in which: 
     FIG. 1 is a perspective view of a MRI system having a portion cut away to illustrate a laser system constructed in accordance with the preferred embodiment; 
     FIG. 2 is an exploded assembly view of the laser assembly illustrated in FIG. 1; 
     FIG. 3 is a sectional side elevation view of the laser assembly taken along lines  3 — 3  of FIG.  1  and shown in a contracted position; 
     FIG. 4 is a sectional side elevation view of the laser assembly illustrated in FIG. 3 shown in an extended position; and 
     FIG. 5 is a schematic illustration of the path of the laser beam in accordance with the preferred embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring initially to FIG. 1, a laser system  20  includes a laser assembly  22  and mirror  30  that are mounted onto a surface  24  of a magnetic resonance device. In particular, the laser assembly  22  and mirror may be mounted onto an outer housing of a RF coil, and surrounded by an outer housing of the MRI system. The laser assembly  22  emits a laser beam  26  from a laser diode  28  (see FIG. 2) onto a mirror  30  having a deflection surface  32 , which deflects the beam radially inwardly, and through an aperture  23  extending through surface  24 , to identify what will be the sweet spot  36  of an acquired MR image. A calibration device having a calibration surface (not shown) may be inserted into the bore of the MRI system, and the laser adjusted such that the beam impinges on the surface to identify a predefined calibration point. 
     A patient  34  may then be placed in the MRI system and positioned such that the tissue to be imaged is identified by the laser beam  26 . It should be appreciated that the deflection surface  32  is relatively small, and the distance between the laser diode  28  and mirror  30  is great enough such that the laser assembly  22  must be precisely aligned to ensure that the beam will hit the mirror. Moreover, the laser beam  26  must hit the precise point on the deflection surface  32  that will yield the desired deflection. 
     In one embodiment, a magnetic resonance imaging system that is commercially available from General Electric Company under the trademark Open Speed™ (registration pending), the laser assembly  22  and mirror  30  are mounted onto the outer housing of the RF coil and spaced approximately 585.7 millimeters apart. The MRI system is configured to accept a mirror whose deflection surface  32  has a width of no more than approximately 5 mm. Additionally, because the deflection surface is angled approximately 45° with respect to the laser beam  26 , the deflection surface  32  presents an even smaller target for the laser beam  26 . For example, it has been found that if the laser diode  28  is misaligned by as little as 1°, the beam  26  will be translated by approximately 10 mm, or twice the width of the deflection surface  32 . Accordingly, the sensitivity is such that the laser diode  28  may be misaligned by no more than ¼° in order for the laser beam  26  to hit the deflection surface  32 . 
     Referring now to FIGS. 2 and 3, the components of the laser assembly  22  comprise a screw block  38  having an outer surface  39 , and a threaded annular opening  40  extending axially therethrough. The outer surface  39  is rectangular in cross-section and mounted onto the outer surface  24  of the MRI system. The opening  40  is threaded and configured to accept therein a corresponding plurality of threads  43  of a screw  42 . Accordingly, the screw  42  is translatable with respect to the screw block  38  by rotating the screw therein, about an axis of rotation  25 , via a thumb wheel  35  that is connected to the outer end of the screw  42  via a shaft  33 . The shaft  33  has a length sufficient such that the thumb wheel  35  extends beyond the outer housing of the MRI system so as to be accessible to a user. It should be appreciated, however, that the thumbwheel could be configured to receive the bit of a screwdriver and, in this configuration, the thumbwheel would not need to extend beyond the outer housing of the MRI system. The screw block  38  includes an opening  29  extending radially through the outer housing  39  that is configured to receive a set screw  31  to lock the position of the screw  42  with respect to the screw block  38  when the screw is in its desired position, as will be described in more detail below. 
     The screw  42  comprises an annulus, in accordance with the preferred embodiment, whose inner surface  44  includes a generally axially directed groove that forms a ramp  46 . In particular, the ramp  46  comprises an elongated groove that is formed in the inner surface  44 . The ramp  46  extends generally in the direction of the axis of rotation  25 , and is sloped such that the end of the ramp adjacent a laser beam emitting end  59  is closer to the axis of rotation than the opposite end of the ramp. In accordance with the preferred embodiment, the ramp  46  has a slope of 1° with respect to the axis of rotation  25 . 
     A laser module housing  48  is received in the opening of the screw  42 . It comprises an elongated hollow annular body  50  having an outer diameter smaller than the inner diameter of the screw  42  and it further defines a ball  52  having a spherical outer surface  54  disposed at one end. An axially directed opening  53  extends through the housing  48  and defines a laser beam emitting end  29  at the ball  52 . 
     A slide stud  58  is connected to the outer surface of the laser module housing  48  at the end opposite the ball  52  and extends radially outwardly therefrom. The slide stud  58  comprises a cylindrical body  60  having an outwardly disposed flange  62  whose outer diameter is greater than that of the cylindrical body  60 . In cross-section, the ramp  46  has a throat of reduced width with respect to the groove to accommodate the flange  62  of slide stud  58 . Accordingly, the flange  62  is locked in the ramp  46  with respect to radial movement, allowing the slide stud  58  to slide within the ramp when the screw  42  is rotated with respect to the screw block  38 . In operation, rotation of screw  42  additionally rotates the slide stud  58 , which in turn rotates the laser module housing  48  a laser module  64 , which will now be described. 
     The laser module  64  is disposed within the laser module housing  48  and attached thereto. It includes the laser emitting diode  28  mounted at one end, and the laser beam  26  is emitted generally along the central axis  25  of the laser module housing  48  from its emitting end  59 . An electrical wire  55  extends from the other end of the module  64  opposite the diode  28  and is connected to a power supply (not shown) that activates the diode. The wire may  55  be supported within the shaft  33 , or may alternatively hang out the rear end of the laser assembly  22 . 
     Referring particularly to FIGS. 3 and 4, inner and outer ball socket retainer plates  68  and  70  are connected to each other and mounted to the front face  37  of the screw block  38  via screws  45  or the like. Each plate  68  and  70  has an axially aligned hollow interior which together form concave surfaces. The concave surfaces  72  and  74  for a spherical connection so as to entrap the ball  52  on the housing  48  to form a socket  76  that retains the housing  48  and permits the ball  52  to swivel therein. This ball and socket connection permits the housing  48  and enclosed laser module  64  to be moved to adjust the direction of the emitted laser beam  26 . 
     Referring now also to FIG. 4, when the screw  42  is rotated counterclockwise, such as by rotating thumb wheel  35 , the screw will move axially outwardly in the “z” direction with respect to the screw block  38 . The assembly will move from its contracted position illustrated in FIG. 3 to its extended position illustrated in FIG.  4 . As the screw  42  is rotated, the slide stud  58  will rotate therewith which, in turn will additionally rotate the laser module housing  48  and laser module  64 . Alternatively, the laser module  64  could be rotatably fixed such that the housing  48  rotates with respect to the laser, causing the laser diode  28  to tilt, as will become apparent to one having ordinary skill in the art from the description below. 
     The slide stud  58 , laser module housing  48 , and laser module  64  will not translate in the axial direction along with the screw  42 , however, as the ball  52  is axially fixed in the socket  76 . Accordingly, the slide stud will slide along the ramp  46  as the screw  42  is translated with respect to the laser module housing  48 . It should be appreciated that a lubricant may be applied to the ramp  46  to allow the slide stud  58  to more easily slide therealong. 
     Because the ramp  46  is sloped, the rear end of the laser module housing  48  will be moved radially outward from the central axis  25  as the screw is rotated to swivel, or wobble, the housing  48  in the ball and socket connection. The directed laser  26  will therefore be aimed increasingly away from the central axis  25 . As the thumb wheel  35  is rotated to translate the screw  42  within screw block  38 , the housing  48  is wobbled such that the laser beam  26  maps out a spiral path on any target in its path, as shown in FIG.  5 . The amount of this wobble, and hence the radius of the resulting beam path  80 , is at least partially dependent on the pivot radius of the laser module housing  48 , which is the distance D 1  between the center  73  of ball  72  and the center of slide stud  58 , as will be described in more detail below. 
     In accordance with the preferred embodiment, the spiral path of the laser beam  26  is the result of the laser module  64  rotating along with the laser module housing  48 . Alternatively, if the laser module  64  is fixed with respect to rotational movement, as described above, the laser diode  28  would be progressively tilted with the rotation of the housing  48 . 
     In operation, when the laser assembly  22  is initially installed in an MRI system, or when the laser is to be periodically calibrated, the laser beam  26  will initially be pointed towards an origination location  78  that offset from the mirror  30  in both the “x” and “y” directions. However, because the laser beam  26  and mirror  30  are hidden from the user&#39;s vision, the user will not know how the orientation of the laser diode  28  is to be adjusted so as to translate the beam towards a destination location  82  disposed on the deflection surface  32  of mirror  30 . Rather, the user will only be able to observe the calibration surface and infer that the beam  26  is properly aligned with the destination location  82  when the beam identifies a target on the calibration surface (not shown) that is positioned within the MRI system. 
     By rotating the screw  43 , the user is able to utilize the resulting spiral path  80  of the laser beam  26  to locate the destination location  82 . Assuming the radius of spiral  80  produces sufficient coverage for the laser beam  26 , as indicated by the distance D 2  in FIG. 5, the laser beam will eventually hit the destination location  82 . It should be apparent that the number of revolutions the beam  26  makes as it spirals outward must be sufficient that the distance between adjacent radii in the spiral path  80  is less than the target size. This radial resolution is determined primarily by the slope of the ramp  46 . When the target is hit, the beam  26  will deflect approximately 90° and extend radially inwardly through the opening  23  in the housing  24  and onto the calibration surface. The user will then cease rotating the screw  42 , and will tighten the set screw  31  to prevent the screw  42 , and laser beam  26 , from slipping during use of the MRI system. 
     As can be seen in FIG. 5, the illustrated spiral  80  is sufficiently tight so as to prevent the beam  26  from passing over the destination location  82 , and additionally has sufficient coverage to ensure that the target is within the range of the laser beam. Spiral tightness is defined as the distance D 3  between individual passes of the laser beam  26 . However, if the width of the destination location  82  with respect to the beam is less than the spiral tightness, the user will run the risk of the beam  26  passing over the target. Additionally, the destination location  82  may be missed if the spiral has insufficient coverage. For example, if the destination location  82  is disposed far from the origination point, and the spiral  80  that is produced after the screw  42  is fully extended is not sufficiently large, the beam  26  will not reach the target. 
     It should be appreciated in this regard that several characteristics of the laser assembly may be varied that will affect the properties of the spiral  80  in a predictable manner, thereby affecting the probability of the laser beam becoming successfully aligned with the destination location  82 . For instance, referring to Table 1 below, the tightness of the and maximum coverage of the spiral  80  may be adjusted by varying the configuration of the laser assembly  22  which will, in turn vary the sensitivity of the laser assembly by varying the spiral coverage per unit length of screw movement as well as the maximum number of screw turns until the screw  32  is fully extended. It should be appreciated in Table 1 that the parameters of the laser assembly are assumed to be varied without changing the length of the screw  32 . 
     Examples of characteristics of the laser assembly  22  that could be varied include the pitch of the threads (threads/inch) of screw  42 , the slope of the ramp  46 , and the pivot radius of the laser module housing  48 . 
     In accordance with the preferred embodiment, the threads of screw  42  have a pitch diameter of ¾″ and a corresponding pitch of 16 (threads/inch), the slope of ramp  46  is 1°, and the pivot radius is 29.4 mm, which yields a spiral coverage of 9.2 mm for every half-inch that the screw  42  is translated. If the thread pitch is increased, the spiral tightness will also increase while maintaining the coverage constant. If the slope is increased, the spiral tightness will decrease while increasing the spiral coverage. If the pivot radius is increased, the spiral tightness will increase while the available coverage is maintained constant. 
     Therefore, if increased spiral coverage is desired, the user could increase the slope of the ramp  46 . However, doing so would also decrease the spiral tightness. Accordingly, depending on the size of the destination location  82 , the thread pitch or pivot radius may be increased to yield a tighter spiral  80 . It should be appreciated, however, that increasing the pivot radius would also decrease the coverage. Accordingly, it may be desirable to increase the thread pitch. 
     
       
         
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Spiral Property 
                 Thread Pitch ↑ 
                 Slope ↑ 
                 Pivot Radius ↑ 
               
               
                   
               
             
             
               
                 Tightness 
                 ↑ 
                 ↓ 
                 ↑ 
               
               
                 Coverage 
                   
                 ↑ 
                 ↓ 
               
               
                 Coverage per 
                   
                 ↑ 
                 ↓ 
               
               
                 Unit Screw Shift 
               
               
                 Max No. Turns 
                 ↑ 
                   
                   
               
               
                   
               
               
                 ↑ indicates an increase: ↓ indicates a decrease:  indicates no change  
               
             
          
         
       
     
     The invention has been described in connection with what are presently considered to be the most practical and preferred embodiments. However, the present invention has been presented by way of illustration and is not intended to be limited to the disclosed embodiments. Accordingly, those skilled in the art will realize that the invention is intended to encompass all modifications and alternative arrangements included within the spirit and scope of the invention, as set forth by the appended claims.