Abstract:
The invention is directed to a method and apparatus for ion source positioning and adjustment. According to one embodiment, the invention relates to an apparatus for ion source positioning and adjustment. The apparatus comprises a bottom plate, a middle plate and a top plate, wherein the top plate is coupled to the middle plate by at least one adjustment member for causing the top plate to move in a first direction, wherein the at least one adjustment member positions the top plate in a predetermined position with respect to the middle plate; and the middle plate is coupled to the bottom plate by a worm gear assembly for causing the middle plate to move in a second direction with respect to the bottom plate.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates generally to the field of cyclotron design for radiopharmacy and more particularly to a method and apparatus for ion source positioning and adjustment. 
   Hospitals and other health care providers rely extensively on positron emission tomography (PET) for diagnostic purposes. PET scanners can produce images which illustrate various biological process and functions. In a PET scan, the patient is initially injected with a radioactive substance known as a PET isotope (or radiopharmaceutical). The PET isotope may be  18 F-fluoro-2-deoxyglucose (FDG), for example, a type of sugar which includes radioactive fluorine. The PET isotope becomes involved in certain bodily processes and functions, and its radioactive nature enables the PET scanner to produce an image which illuminates those functions and processes. For example, when FDG is injected, it may be metabolized by cancer cells, allowing the PET scanner to create an image illuminating the cancerous region. 
   PET isotopes are mainly produced with cyclotrons, a type of circular-shaped particle accelerators.  FIG. 1  illustrates the operation of a known cyclotron for isotope production. The cyclotron comprises two hollow D-shaped metal electrodes  102  and  104  that are placed in a magnetic field B. The two electrodes  102  and  104  are separated by a small gap  103 , across which an alternating electric field E is applied. The cyclotron usually operates at high vacuum (e.g., 10 −7  Torr). In operation, a negative ion  108  is initially extracted from an ion source  106  near the center of the cyclotron. Confined by the magnetic field, the ion  108  starts moving in a circular path. A radio frequency (RF) high voltage source rapidly alternates the polarity of the electric field E, so that the ion  108  is accelerated each time it crosses the gap  103 . As it acquires more kinetic energy, the ion  108  follows a spiral course  110  until it is eventually directed to a target material to produce desired PET isotopes. 
     FIG. 2  illustrates the operation of a known plasma-based ion source  200  used in cyclotrons for isotope production. As shown, the ion source  200  comprises an ion source tube  204  positioned between two cathodes  202 . The ion source tube  204  may be grounded while the two cathodes  202  may be biased at a high negative potential with a power source  212 . The ion source tube  204  may have a cavity  208  into which one or more gas ingredients may be flowed. For example, a hydrogen (H 2 ) gas of certain pressure may be flowed into the cavity  208 . The voltage difference between the cathodes  202  and the ion source tube  104  may cause a plasma discharge  210  in the hydrogen gas, creating positive hydrogen ions (protons) and negative hydrogen ions (H − ). These hydrogen ions may be confined by a magnetic field  220  imposed along the length of the ion source tube  204 . A puller  216 , biased with a power source  214  at an alternating potential, may then extract the negative hydrogen ions through a slit opening  206  on the ion source tube  204 . The extracted negative hydrogen ions  218  may be further accelerated in the cyclotron (not shown) before being used in isotope production. 
   Traditionally, after positioning and adjustment of the slit opening, the only way to determine whether the position is acceptable is by measuring the ion source output. In order to measure the ion source output, the cyclotron chamber has to be pumped down to an acceptable vacuum level. In one cyclotron, for example, it takes about an hour to reach such a vacuum level. If measurement of the ion source output reveals that the slit opening has not been accurately positioned, the cyclotron chamber has to be re-opened to allow re-adjustment. Unfortunately, a simple reading of the ion source output does not offer a clear indication as to which direction or by how much the ion source tube should be adjusted. A service engineer usually has to adjust the position in small increments and repeat the pump-and-measure process for several times until a desired ion source output is measured. One iteration can take 2-3 hours. For an inexperience service engineer, it may take several iterations to achieve an acceptable level of ion source output. Therefore, the traditional approach for ion source positioning and adjustment can be very time-consuming. Even when an acceptable level of ion source output has been achieved, it is seldom clear whether an optimal position of the ion source tube has been reached. 
   Unfortunately, ion source adjustment is hardly avoidable since an ion source typically has a limited lifetime and requires periodical replacement. During a scheduled service, the cyclotron needs to be opened up to allow access to the ion source. However, since the cyclotron usually becomes radioactive during isotope production, it is necessary to wait for the radiation to decay to a safe level before starting the service. The wait for the radiation decay can sometimes last ten hours, for example. The safe level of radiation usually depends on how long a service engineer will be exposed. That is, a job that takes a short time can be started at a higher radiation level (i.e., after a shorter decay time) than one that takes a long time. Therefore, the shorter it takes to position and adjust a new ion source, the faster a scheduled service may be completed. 
   In view of the foregoing, it would be desirable to provide a more efficient solution for accurate positioning and adjustment of an ion source tube. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention is directed to a method and apparatus for ion source positioning and adjustment that overcomes drawbacks of known systems and methods. 
   According to one embodiment, the invention relates to an apparatus for ion source positioning and adjustment. The apparatus comprises a bottom plate, a middle plate and a top plate, wherein the top plate is coupled to the middle plate by at least one adjustment member for causing the top plate to move in a first direction, wherein the at least one adjustment member positions the top plate in a predetermined position with respect to the middle plate; and the middle plate is coupled to the bottom plate by a worm gear assembly for causing the middle plate to move in a second direction with respect to the bottom plate. 
   According to another embodiment, the invention relates to a method for ion source positioning and adjustment. The method comprises: coupling an ion source tube to a top plate of an adjustment tool, wherein the top plate is coupled to a middle plate by at least one adjustment member for causing the top plate to move in a first direction; installing the adjustment tool by attaching a bottom plate of the adjustment tool to a chamber of a cyclotron; adjusting the at least one adjustment member until the top plate is at a predetermined position with respect to the middle plate; and driving a worm gear that causes the middle plate to move in a second direction with respect to the bottom plate, until a desired output of the ion source tube is measured. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order to facilitate a fuller understanding of the present invention, reference is now made to the appended drawings. These drawings should not be construed as limiting the present invention, but are intended to be exemplary only. 
       FIG. 1  illustrates the operation of a known cyclotron for isotope production. 
       FIG. 2  illustrates the operation of a known plasma-based ion source used in cyclotrons for isotope production. 
       FIGS. 3 and 4  illustrate an exemplary ion source adjustment tool according to an embodiment of the invention. 
       FIG. 5  illustrates the exemplary ion source adjustment tool as installed in a cyclotron according to an embodiment of the invention. 
       FIG. 6  is a mechanical diagram illustrating various parts of the exemplary ion source adjustment tool. 
       FIG. 7  illustrates an exemplary driving unit for use with the exemplary ion source adjustment tool according to an embodiment of the invention. 
       FIG. 8  illustrates various parts of the exemplary driving unit. 
       FIG. 9  illustrates an exemplary hand control unit for use with the exemplary driving unit according to an embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Reference will now be made in detail to exemplary embodiments of the invention, examples of which are illustrated in the accompanying drawings. 
   In an ion source similar to the one shown in  FIG. 2 , positioning of the slit opening relative to the puller is a significant factor affecting ion extraction. The position of the ion source tube usually has to be accurate within a fraction of a millimeter. Accurate positioning of the ion source tube usually depends on three parameters: its distance to the puller (or “longitudinal position”), the lateral position of the slit opening relative to the puller, and angle of the slit opening with respect to the ion source body. Of these three parameters, the lateral position of the slit opening is usually most significant. The distance to the puller and the lateral position may be accurately and efficiently adjusted based on the method and apparatus described hereinafter. The angle of the slit opening may be fixed easily by a special angle tool during installation of the ion source tube. 
     FIGS. 3 and 4  illustrate an exemplary ion source adjustment tool according to an embodiment of the invention.  FIG. 3  shows the front side of the exemplary ion source adjustment tool, and  FIG. 4  shows the back side. 
   The exemplary ion source adjustment tool may comprise three plates: a top plate  13 , a middle plate  12 , and a bottom plate  11 . The top plate  13  may be coupled to the middle plate  12  by a knurled screw  19 . The knurled screw  19  may go through the top plate  13  and into the middle plate  12 , such that, when the knurled screw  19  is turned, the top plate  13  may slide back or forth with respect to the middle plate  12 . Movement of the top plate  13  may be a linear movement along the ±X directions. A stop screw  18  placed next to the knurled screw  19  may control a relative position of the top plate  13  with respect to the middle plate  12 . This relative position may vary for different cyclotrons. The stop screw  18  may go through the top plate  13  and may act as a stop when it touches a back part of the middle plate  12 . The stop screw  18  may be adjusted to control how far it extends to touch the middle plate  12 . Apart from the combination of a knurled screw and a stop screw, other mechanisms known in the art may also be used to control the relative position of the top plate  13  with respect to the middle plate  12 . For example, a single knurled screw may be used, together with markings along the edges of top plate  13  and/or the middle plate  12 , to adjust the relative position. 
   The middle plate  12  may be coupled to the bottom plate  11  by a worm gear assembly  304 . The worm gear assembly  304  may cause the middle plate  12  to rotate slightly around a pivot  302 . The rotation is typically so small that the tip of the middle plate  12  can be viewed as moving along the ±Y directions. Details of the worm gear assembly  304  and its operation will be described in connection with  FIGS. 6 and 7 . 
     FIG. 5  illustrates the exemplary ion source adjustment tool as installed in a cyclotron according to an embodiment of the invention.  FIG. 5  shows a portion of the cyclotron chamber. The exemplary ion source adjustment tool may be installed in a magnet pole valley  402 , for example. The installation may be done by attaching the bottom plate  11  to the magnet pole surface. The top plate  13  may be coupled to an ion source assembly  408 , particularly an ion source tube (not shown). The pipes  404  may include water-cooling pipes and gas lines for providing plasma-producing gases such as hydrogen. A flexible shaft, hidden in a copper tube  406 , may be coupled, via a coupling  23 , to the worm gear assembly on one end, and be coupled to a driving unit on the other end outside the cyclotron chamber. The driving unit may comprise a motor for turning the flexible shaft in either direction, thereby causing the worm gear assembly to move the middle plate  12  back and forth in the lateral directions (i.e., ±Y directions). Since the range of movement caused by the worm gear assembly is only a couple of millimeters while the ion source tube is about 50 mm away from the pivot  302 , the movement of the ion source tube is effectively a linear motion. 
   To replace the ion source, the top plate  13 , with the old ion source tube attached, may be removed from the chamber. Then, the old ion source tube may be replaced by a new one. An angle tool may be used to facet the slit opening on the new ion source tube in an appropriate angle. Next, the top plate  13 , with the new ion source tube attached, may be re-installed in the magnet pole valley  402 . Since the stop screw  18  “remembers” the relative position between the top plate  13  and the middle plate  12 , such position may be easily restored by tightening the knurled screw  19  until the stop screw  18  touches the middle plate  12 . A feeler gauge (not shown) may used to quickly ascertain that the original distance (approximately 1.5 mm, for example) between the puller and the ion source tube has been restored. Once the cyclotron chamber has been closed and pumped down to an acceptable vacuum level, an output of the new ion source may be measured, for example, with an ion probe. Based on the measured output (i.e., the ion probe current), the worm gear assembly may be continuously adjusted from outside the cyclotron chamber to move the middle plate  12  (and thus the top plate  13  and the ion source tube attached thereto) in the ±Y directions, until a desired ion source output is measured. For example, the ion source tube may be initially moved in one direction (e.g., +Y direction). If the ion probe current increases, the ion source tube may be kept moving in the same direction. If the ion probe current starts to drop, that is, it passes a maximum value, the ion source tube may have passed an optimal position. The ion source adjustment tool may control the ion source tube to move in an opposite direction until a maximum value is measured for the ion probe current. Apart from the adjustment upon installation of a new ion source, the optimization may also be performed during operation of the cyclotron. 
   Since the ion source tube&#39;s longitudinal position has been restored upon installation, and the lateral position is remotely and continuously adjustable while the cyclotron chamber is under high vacuum, service time required for the ion source may be significantly shorter than with the traditional approach. As a result, the service engineer(s) may have much less radiation exposure. Due to the faster and easier installation, highly skilled service engineers are no longer necessary for consistent results. 
   Referring now to  FIG. 6 , there is shown a mechanical diagram illustrating various parts of the exemplary ion source adjustment tool. In addition to the top plate  13 , the middle plate  12  and the bottom plate  11 , the exemplary ion source adjustment tool may comprise screws  14  for fastening the bottom plate  11  to a magnet pole surface inside the cyclotron chamber, for example. Screws  17  may pass through the collars  16  and may be threaded into the nuts  15 , so as to fasten the middle plate  12  to the bottom plate  11 . Note that the holes  28  and  29 , which host the collars  16 , are slightly different in size. The hole  28  is slightly larger than the hole  29 , thereby allowing a limited rotation of the middle plate  12  around the hole  29 . The hole  29  corresponds to the pivot  302  shown in  FIGS. 3-5 . The worm gear assembly  304  may comprise a base  20  that is attached to the bottom plate  11 . The base  20  may comprise a shaft around which a gear  21  may rotate. A worm  22  (driving gear) may be coupled to the gear  21  (driven gear) for causing its rotation. There may be a large gear ratio between the worm  22  and the gear  21 . That is, several turns of the worm  22  may cause one turn of the gear  21 . Thus, fine adjustment of the gear  21  may be achieved through the worm  22 . A shaft component may be attached to and rotate with the gear  21 . The shaft component may comprise a shaft  24  that is not aligned with the gear  21 &#39;s center of rotation. That is, the shaft  24  is intentionally made to be off-centered. The shaft  24  may pass through a track  30  in a plate  25  which is attached to the middle plate  12  with two screws  26 . Thus, when the worm  22  is turned (e.g., in the δ-direction), it drives the gear  21 , causing the shaft  24  to rotate (e.g., in the θ-direction). As the shaft  24  rotates, it slides in the track  30 , causing the middle plate  12  to rotate around the hole  29  (or pivot  302 ). Since the top plate  13  is coupled to the middle plate  12  by the knurled screw  19  and by two bolts  27 , the slight rotation of the middle plate  12  may cause the top plate  13 , as well as an ion source tube attached thereto, to move laterally, in the ±Y directions. In operation, the worm  22  is typically coupled to a flexible shaft (not shown) through the coupling  23 . 
   The flexible shaft may be coupled to a driving unit located outside the cyclotron chamber.  FIG. 7  illustrates an exemplary driving unit  700  for use with the exemplary ion source adjustment tool according to an embodiment of the invention.  FIG. 8  illustrates various parts of the exemplary driving unit  700 . The exemplary driving unit  700  may comprise a motor assembly  810 . A flexible shaft  804 , shielded and guided by a copper tube  802 , may be coupled to the motor assembly  810  through a coupling  806  and a collar component  808 . The motor assembly  810  may further comprise an interface connector  812  to accommodate a connection to a hand control unit. 
     FIG. 9  illustrates an exemplary hand control unit  900  for use with the exemplary driving unit  700  according to an embodiment of the invention. The exemplary hand control unit  900  may comprise an interface connector  904 . A matching cable (e.g., a D-sub cable) may be used to connect the interface connector  904  with the interface connector  812 , thereby putting the driving unit  700  within control of the hand control unit  900 . The hand control unit  900  may comprise a first switch  902  for causing the driving motor to change its direction of rotation, and a second switch  906  for causing the driving motor to rotate. In operation, after a new ion source tube is positioned with the adjustment tool, the cyclotron chamber may be closed and pumped down. Then, the ion source may be activated and its output measured. The hand control unit  900  may now be used to control the driving unit  700  which in turn drives the worm gear assembly. With the hand control unit  900 , the lateral position of the ion source tube may be continuously changed in either direction. This may allow an optimal lateral position to be found that corresponds to a desired output from the ion source. 
   While the foregoing description includes many details and specificities, it is to be understood that these have been included for purposes of explanation only, and are not to be interpreted as limitations of the present invention. It will be apparent to those skilled in the art that other modifications to the embodiments described above can be made without departing from the spirit and scope of the invention. Accordingly, such modifications are considered within the scope of the invention as intended to be encompassed by the following claims and their legal equivalents.