Abstract:
A variable angle slant hole (VASH) collimator for providing collimation of high energy photons such as gamma rays during radiological imaging of humans. The VASH collimator includes a stack of multiple collimator leaves and a means of quickly aligning each leaf to provide various projection angles. Rather than rotate the detector around the subject, the VASH collimator enables the detector to remain stationary while the projection angle of the collimator is varied for tomographic acquisition. High collimator efficiency is achieved by maintaining the leaves in accurate alignment through the various projection angles. Individual leaves include unique angled cuts to maintain a precise target collimation angle. Matching wedge blocks driven by two actuators with twin-lead screws accurately position each leaf in the stack resulting in the precise target collimation angle. A computer interface with the actuators enables precise control of the projection angle of the collimator.

Description:
GOVERNMENT LICENSE RIGHTS 
     This invention was made with government support under Management and Operating Contract No. DE-AC05-06OR23177 awarded by the Department of Energy. The United States Government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to radiation collimators and gamma and x-ray photon systems as applied to medical radiology and nuclear medicine. 
     BACKGROUND OF THE INVENTION 
     In radiation imaging, collimators are used to limit the passage of radiation beams to a specific angle in order to minimize detection of beams of scattered or secondary radiation. Single angle collimation provides 2-dimensional (2D) projection of radiation sources on an imager while the combining of multi angle projections provides 3-dimensional (3D) position of radiation sources by image reconstruction techniques. Hence, it is more desirable to achieve 3D images particularly in clinical nuclear imaging. Devices have been proposed for multi angle projection imagers using multiple detectors at different positions, or by moving the position of a single detector, or by fixing the location of a detector and using multiple collimators each of which has a different collimation angle. It is not always feasible to combine aspects of these methods due to physical interference with the imaging target. Previously proposed multi angle collimators typically include either variable angle slant hole (VASH) collimators or multi-view collimators. 
     Although the VASH provides an adjustable angle of collimation, the reported alignment mechanism of the multiple leaves clearly does not provide accurate alignment of each leaf, thereby resulting in a reduced size of the openings and an inaccurate collimation angle that reduces the output image quality. 
     Accordingly, it would be desirable to provide a multiple leaf collimator in which the openings are accurately aligned, that maintains a constant collimation angle, and thus achieves high output image quality. 
     OBJECT OF THE INVENTION 
     It is therefore an object of the present invention to provide a variable angle slant hole (VASH) collimator that enables accurate tomographic acquisition by multiple angle projection while the detector remains stationary. 
     A second object is to provide a multiple leaf collimator that provides multiple views through the breast, thyroid, heart, or similar body part. 
     A further object of the invention is to provide an improved VASH actuation device that eliminates mechanisms adjacent the patient. The compact nature of the multi-leaf collimator collimator and the fact that the mechanism can be positioned under covers on the side of the detector, away from the patient, makes it inherently safer to operate. This contributes greatly to the physical and emotional comfort of the patient. This is a significant improvement over prior mechanisms that use knife edge actuators, such as in breast imaging, in which the breast must be positioned between two knife blades. 
     A further object of the invention is to provide a multi-leaf collimator in which the openings can be maintained at a constant collimation angle and thus achieves high output image quality. 
     A further object of the invention is to provide a multi-leaf collimator that eliminates the need to rotate the detector. 
     A further object of the invention is to provide a multi-leaf collimator that can be adjusted to various viewing angles very quickly and with a high degree of precision. 
     A further object of the invention is to provide a multi-leaf collimator in which the aperture alignment is maintained and the motor doesn&#39;t strain when the collimator stack is turned vertically. 
     A further object of the invention is to provide a collimator in which the viewing angle can be changed very quickly to limit the radiation dose to the patient while acquiring the data for tomographic imaging. 
     A further object of the invention is to minimize the amount of time for acquiring multiple angle views of a patient that has been administered a radioisotope. 
     A further object of the invention is to provide a variable angle slant hole collimator that is compact in size to enable easy handling and turning to different orientations with respect to the patient. 
     BRIEF SUMMARY OF THE INVENTION 
     A variable angle slant hole (VASH) collimator provides collimation of high energy photon such as gamma ray used for radiological imaging of human. The VASH collimator is consist of multiple collimator leaves and alignment of each leave can provide various projection angles. Rather than rotate the detector around the subject, the VASH collimator allows tomographic acquisition while the detector remains stationary by multiple angle projection. An accurate alignment of leaves is required to provide maximum collimation efficiency. Individual leaf has own angled cut on each side and face matching wedge blocks driven by two actuators with twin-lead screws is providing an accurate position of each leaf resulting target collimation angle. Continuous collimation angle can be achieved by position control of wedge block. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top isometric view of the frame and driving mechanism for a variable angle slant hole (VASH) collimator according to the present invention. 
         FIG. 2  is a top view of the VASH collimator including the collimator stack, the frame, and the driving mechanism. 
         FIG. 3  is a top view of the VASH collimator with the motor and gearbox portion of the drive mechanism and the stack cover plates cut away. 
         FIG. 4  is an isometric view of a wedge block according to the present invention. 
         FIG. 5  is a side view of a wedge block according to the present invention. 
         FIG. 6  is a top view depicting the movement of the top leaf of a VASH collimator according to the present invention. 
         FIG. 7  is a top view depicting the movement of the middle leaf of a VASH collimator according to the present invention. 
         FIG. 8  is a side view depicting multi-leaf motion of a VASH collimator according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 1 , the present invention is a compact and efficient multi-leaf variable angle slant hole (VASH) collimator  10  in which the viewing angle of the apertures can be rapidly and accurately changed to provide various projection angles for 2-dimensional (2D) and 3-dimensional (3D) image reconstruction techniques. The VASH collimator  10  includes a frame  12  with opposing sides  14 , opposing ends  16 , and a front panel  18  with an opening  20  therein. A means  22  for positioning a plurality of collimator leaves (not shown) includes two motors  24  coupled by a gearbox  26  to a lead screw  28  at each end of the frame  12 . 
     Two stack alignment rails  30 , which will serve as guides for a stack of collimator leaves, extend along each side  14  of the frame. Two end bearings  32  positioned in each end of the rails  30  form paired end bearings that are axially aligned and enable rotation of the lead screws  28  with respect to the rails and frame. Two wedge blocks  34  are positioned on each lead screw  28  at the two ends  16  of the frame  12 . The wedge blocks  34  include an inner end  36  and an outer end  38 . A drive nut  40  is embedded in each wedge block to accommodate the screw threads of the lead screws  28 . The outer end  38  of each wedge block  34  includes set screws  42  for locking each wedge block to its respective drive nut  40 . A pillow block  44  supports each lead screw approximately mid-way between the two end bearings  32 . Each wedge block  34  includes a clearance notch  46  for accommodating the pillow blocks  44  at full inward travel of the wedge blocks. A side mount  48  extends from one side  14  of the frame  12  and includes two bores  50  therein to accommodate attachments for imaging specific body parts, such as compression paddles (not shown) for breast imaging. One of the rails  30  includes two brackets  52  for mounting of the gearboxes  26  and motors  24  thereto. 
     Referring to  FIG. 2 , a coupler  54  including one or more screws  56  secures each motor  24  and gearbox  26  to their respective lead screw  28 . A plurality of substantially planar leaves, of which the top leaf  58  having a face  60  is visible, are stacked against the front panel  18  to form a stack  61  of leaves. An array  62  of apertures  64  are arranged in an identical pattern in each of the leaves  58 , with the leaves capable of alignment in an initial position wherein the apertures  64  in the leaves are axially aligned with one another and at 90° with respect to the face  60  of the top leaf. Each leaf  58  includes sides  66  and ends  68 . The rails  30  maintain the contact with the sides  66  of the leaves  58  thus keeping the stack of leaves in alignment. The ends  68  of each leaf  58  include an angled cut  70  that is unique to that leaf. Most preferably the angled cut  70  on the ends of the leaves includes a substantially central apex  72  and two angled surfaces  74  extending to the sides  66  of each leaf. The bottom leaf (not shown) is preferably not angled, but includes ends that are at 90° with respect to its sides. A computer interface  76  may be used for controlling and synchronizing the direction of rotation of each of the motors  24  to change the position of the leaves  58  and thus the alignment of the apertures  64  to a desired slant angle. A stack cover plate  77  extends from each wedge block  34  and maintains the stack  61  of leaves  58  in place on the frame  12 . 
     Preferably, with reference to  FIG. 3 , a reverse thread is included on the shaft of the lead screws  28  on opposing sides of the pillow block  44 , such as the screw threads on one half of the lead screws are right-hand threaded  78   a  and the screw threads on the opposing half of the lead screws are left-hand threaded  78   b.    
     Referring to  FIGS. 4 and 5 , each wedge block  34  includes a wedge body  80  having a flat bottom surface  82 , two sides  84 , a top  86 , and a first end  88  second end  90 . The bottom surface  92  is at 90 degrees with respect to the sides  84  and the first end  88  is cut with a desired angle. A stepped edge  92  is provided on the first end  88  of the wedge block. Preferably, the length L of each step  94  in the stepped edge matches the thickness of a corresponding leaf in the stack of leaves held by the frame  12 . The wedge block  34  further includes a side bore  96  for accommodating the drive nut  40  and two apertures  98  for screw attachment of the cover plate  77  (see  FIG. 2 ). 
       FIGS. 6-8  depict the actuation mechanism of the VASH collimator  10  including movement of the top leaf ( FIG. 6 ), movement of the middle leaf ( FIG. 7 ), and a side view of multi-leaf motion ( FIG. 8 ). The top leaf  58  and middle leaf  101  have different side profiles, with the top leaf  58  including a length d and the middle leaf  101  including a length d/2, resulting in a displacement of x and x/2 for the two leaves, respectively. A continuous slant angle can be achieved by positioning of the four wedge blocks  34 . The bottom leaf with a straight side does not move. The other leaves have an angled cut and the tangent value of each angle is a linear step to make the stepped displacement linear. The sliding wedge blocks  34  each include a stepped profile with each step being the same thickness as a corresponding leaf so that there is no gap between the sliding wedge  34  and the side of each leaf as shown in  FIG. 8 . This will provide a maximum efficiency for wedging motion. The desired angle of the slant hole collimation is selected by moving the four sliding wedges  34 , such that the motion moves each leaf in one direction with a different displacement. 
     As shown in  FIG. 1 , the two wedge blocks  34  on each end  16  are moving in opposite directions of each other by the rotation of the twin-lead screw  28 . At the same time, the two wedge blocks  34  on the opposing end  16  are moving in the opposite direction of those on the first end in order to compensate leaf motion. There is a pillow block  44  at the middle of each twin-lead screw to prevent bending of the lead screws  28 . Each wedge block  34  has a cover plate  77  to ensure all leaves are stacked at the same height as the wedge blocks  34 . Accurate motion and counter motion of the wedge blocks  34  at each end  16  of the VASH collimator provides an exact amount of space for each leaf resulting in an accurate positioning and setting of the collimation angle for all the apertures in the leaves. 
     The range of slant angles is determined by the cut profile of opposing sides of each leaf, the matching profile for the sliding wedge block  34 , and by the sliding/travel range of the wedge blocks. Since the displacement by wedge motion of each leaf is continuous, the collimation angle can be set continuously within a pre-defined range. On one end  16  of the leaves, the two wedges  34  are moving in opposite direction of those on the opposing end of the leaves to balance the force. Actuators for the twin-lead screws  28  are driven by a motor  24  with a high ratio gear box  26 . 
     The present invention further includes a method for aligning a collimator, the method including: 
     a. providing a frame including side rails, and two ends; 
     b. providing twin lead screws, two wedge blocks on each lead screw, coupling nuts, and a plurality of leaves with an array of apertures therein; 
     c. moving the wedge blocks and coupling nuts to the side rails of the frame; 
     d. fixing the wedge blocks to the coupling nuts; 
     e. stacking the leaves in the frame to form a multi-leaf stack; 
     f. adjusting the position of two wedge blocks on a first end of the frame toward each other until they are in contact with each other; and 
     g. moving the four wedge blocks in the desired direction with the desired distance, at the same speed, for setting the slant angle of the multi-leaf stack. 
     Preferably the leaves  58  and the frame  12  are constructed of tungsten primarily for its ability to stop radiation penetration, and also for non-dulling properties, abrasion resistance, and mechanical strength. Preferably the apertures  64  are photo-etched square holes, but may be hexagonal or circular shaped. The size of the apertures can be selected for the desired levels of sensitivity and resolution, with larger apertures leading to higher sensitivity but lowering the resolution. The thickness of the leaves may also be selected for the application, with thicker leaves leading to lower resolution. The energy of the radioactive tracer determines the number of leaves required, with a taller stack leading to higher resolution and a shorter stack leading to higher sensitivity. Although the disclosure herein depicts the apertures parallel to one another, it is within the scope of the invention to provide apertures at various angles, such as sloping the apertures higher toward the edge of each leaf. 
     The theoretical analysis of VASH is as follows:
         (a) The maximum travel range of wedge motion is limited by following equation
 
 Tw= 0.5* Wc−Wb  
   where Tw is the travel range of wedge block, Wc is the width of collimator leaf, and Wb is the width of wedge block.   (b) The profile of the top leaf is defined by P which is the half of the difference between the longest length of leaf and the shortest length of leaf. (d in  FIG. 6 ).   (c) Movement of each leaf from the zero degree (perpendicular to the detector) collimation to the maximum angle, M, is defined by following
 
 M=P*Tw/Wc  
   (d) The maximum collimation angle A is defined by
 
 A =tan−1( M /(( N− 1)* T ))
   where N is the number of the leaf and T is the thickness of a single leaf.       

     Example 
     As an example, a VASH collimator constructed for breast imaging included 50 leaves that were each 0.25 mm in thickness for a total stack height of 12.5 mm. A tungsten frame held the leaves. Two lead screws, each being ½ right-hand threaded and ½ being left-hand threaded, were driven by a motor and gearbox to change the viewing angle +/−28 degrees. There are two end bearings per end, one supporting each end of each screw. A pillow block supported each screw in the center. A drive nut was embedded in each wedge block to accommodate the screw threads of the lead screws. Each wedge included a clearance notch for pillow block clearance. A gamma transparent cover was used to cover the collimator stack, both for keeping the apertures clear and shielding the patient from moving parts of the collimator. A brushless motor available from Micromo of Clearwater, Fla., was used to power the lead screws and wedge blocks. The gearing ratio of the motor and gearbox was 134:1. The motion of the wedges was limited at each end of the lead screws by high current. Driving the motors required 100 mA for the first motor and 150 mA the second motor. The end pushing the wedge blocks to the center was the drive motor. The drive switches end-to-end to change the direction of leaves and thus the viewing angle. In the preferred embodiment, an elapsed time of one minute was required to move the viewing angle from +28 to −28. If preferred, by changing the gearbox, the elapsed time can be reduced to 20 seconds. Four (2×2) apertures were provided to each scintillator element. 
     The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiments herein were chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.