Apparatus and method for variable angle slant hole collimator
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.
1. A variable angle slant hole collimator, comprising:
a frame including opposing sides, opposing ends, and a front panel having an opening therein;
a plurality of substantially planar leaves stacked against said front panel in said frame, said stack including a top leaf with a face;
an array of apertures arranged in an identical pattern in each of said leaves, said leaves capable of alignment in an initial position wherein said apertures in said leaves are axially aligned with one another and at 90° with respect to the face of the top leaf;
a means for positioning each of said leaves in said stack of said leaves in a controlled manner such that each leaf slides at a predetermined rate with respect to the surrounding leaves in the stack; and
said means for positioning said stack includes a lead screw and two wedge blocks on each end of said frame and an actuator for driving said lead screws.
2. The variable angle slant hole collimator of claim 1 , further comprising
two ends and two sides on each leaf in said stack of leaves; and
said ends of each leaf include an angled cut that is unique to that leaf.
3. The variable angle slant hole collimator of claim 1 , including
a bottom leaf in contact with said front panel; and
said bottom leaf includes ends and sides that are at a 90° angle to each other.
4. The variable angle slant hole collimator of claim 1 , further comprising
a rail extending along each side of said frame;
an end bearing at opposing ends of said rails forming a pair of axially aligned end bearings at each end of said frame; and
one of said lead screws extending through each of said pairs of end bearings.
5. The variable angle slant hole collimator of claim 1 , wherein said actuator further comprises
a motor;
a gearbox; and
a computer interface for controlling the direction of rotation of said motor.
6. The variable angle slant hole collimator of claim 1 , further comprising
screw threads on said lead screws; and
a drive nut embedded in each of said wedges to accommodate the screw threads of the lead screws.
7. The variable angle slant hole collimator of claim 6 , further comprising a pillow block secured to said front panel for supporting said lead screw between each of said wedges.
8. The variable angle slant hole collimator of claim 7 , wherein the screw threads on one half of said lead screws are right-hand threaded and the screw threads on the opposing half of said lead screws are left-hand threaded.
9. The variable angle slant hole collimator of claim 1 , wherein each of said wedges includes a wedge body having a flat bottom surface, two sides, a top, and first end and a second end.
10. The variable angle slant hole collimator of claim 9 , further comprising a plate extending from said top of said wedge block.
11. The variable angle slant hole collimator of claim 10 , further comprising a stepped edge on said first end of said wedge block.
12. The variable angle slant hole collimator of claim 11 , wherein the length of each step in said stepped edge of said wedge matches the width of a corresponding leaf in said stack of leaves.
13. The variable angle slant hole collimator of claim 2 , wherein said angled cut on said ends of said leaves includes a substantially central apex and two angled surfaces extending to said sides of said leaves.
14. A method for aligning a multi-leaf collimator, comprising the steps of:
providing a frame including side rails, and two ends;
providing twin lead screws, two wedge blocks on each lead screw, coupling nuts, and a plurality of leaves with an array of apertures therein;
moving said wedge blocks and coupling nuts to the side rails of said frame;
fixing the wedge blocks to the coupling nuts;
stacking said leaves in said frame to form a multi-leaf stack;
adjusting the position of two wedge blocks on a first end of said frame toward each other until they are in contact with each other; and
moving the four wedge blocks in the desired direction with the desired distance, at the same speed, for setting a slant angle of the multi-leaf stack.
15. The method of claim 14 further wherein fixing the wedge blocks to the coupling nuts includes
providing a plurality of set screws;
setting the initial positions of said wedge blocks by sliding the wedge blocks on the coupling nuts; and
securing two set-screws through each wedge block to lock said wedge block to said coupling nut.
16. The method of claim 14 wherein said moving of said wedge blocks includes matching the speed and distance traversed of each wedge block.
17. The method of claim 14 which includes synchronizing the counter motion of each of said wedge blocks to avoid a gap between the edge of each leaf and each wedge block.
18. The method of claim 14 , further comprising
providing a stepped edge on a first end of said wedge block; and
matching the length of each step on said stepped edge of said wedge block equal to the width of a corresponding leaf in said stack of leaves.
19. The method of claim 14 , further comprising
providing an end on each leaf in said stack of leaves; and
providing an angled cut on the end of each leaf.
20. The method of claim 19 wherein said angled cut on said ends of said leaves includes a substantially central apex and two angled surfaces extending to said sides of said leaves.