Patent Number: 061817732
Section: description

DETAILED DESCRIPTION OF INVENTION The invention will next be illustrated with reference to the figures wherein similar numbers indicate the same elements in all figures. Such figures are intended to be illustrative rather than limiting and are included herewith to facilitate the explanation of the apparatus of the present invention. FIG. 1 shows a schematic arrangement in which a source of X-ray radiation 10 provides a beam 18 of X-rays. A target 12 (i.e. a patient in the case of medical diagnostic imaging) is placed in the X-ray beam path. The radiation emerging through patient 12 is intensity modulated because of the different degrees of X-ray absorption in various parts of the patient's body. Cassette enclosure 14, containing radiation sensor 16, intercepts the modulated X-ray radiation beam 18'. Radiation detector 16 absorbs X-rays that penetrate the cassette enclosure 14, and produces a digital image in accordance with the above-referenced patent. A radiation anti-scatter device 20, known in the art as a bucky, comprising an anti-scatter grid attached to a holder, is typically placed between target 12 and cassette 14 to focus the modulated X-ray beam to prevent scattered X-rays from impinging the sensor at undesirable angles. Standard bucky grid architecture comprises a set of parallel vanes. The bucky is typically placed so that it moves in a vertical or horizontal plane orthogonal to the length of the vanes. According to this invention the bucky is moved over the detector in a single stroke during a time period that exceeds the radiation exposure duration. This is obtained by imparting to the moving bucky a decelerating velocity profile preferably one that asymptotically approaches zero. The velocity profile, by necessity, includes an accelerating first period. The accelerating first period must be such as to accelerate the bucky to its maximum velocity quickly enough so as not to unreasonably delay the onset of the actual patient exposure, and not to use up an excessive fraction of the available grid displacement. Typical acceleration times are of the order of a few milliseconds, preferably between 0.001 and 0.5 seconds. The exact time is determined by practical limitations related to the physical environment of a specific installation and equipment available. In general, it is desirable that the grid move between 0.1 and 1.5 cm during the accelerating period, and that the decelerating portion of the grid movement lasts for about 2 seconds and translates the grid another 1 to 5 cm. The acceleration velocity profile may be linear or non-linear, as desired. A linear profile has the advantage of requiring only a constant force to accelerate the grid. In FIGS. 2A-C, there are shown graphs of time versus velocity graph 30, and time versus displacement graph 32, of an exemplary moving bucky. Each graph depicts the same motion, wherein the time period shown in 2B is 10.times. that shown in 2A, and 2C is 10.times. the period in 2B. As illustrated the grid is first accelerated to a first, high velocity, preferably prior to initiating the radiation exposure, and then decelerated again preferably during the exposure. For the first time period, velocity profile 30 conforms to the general equation: EQU V=K.sub.1 t for t equal to or less than 0.005 sec. (1) where: V=velocity in cm/second PA1 K=2236 and PA1 t=time in seconds. PA1 V=velocity in cm/second PA1 K.sub.2 =25, and PA1 m=0.5 PA1 t=time in seconds. For a second time period, for t greater than 0.005 sec. and less than 2 seconds the velocity profile 30 conforms to the general equation: EQU V=K.sub.2 (1000 t).sup.-m (2) where: Referring now to FIG. 3, there is shown an exemplary radiation anti-scatter device 40 of the present invention, showing a grid 42 and grid driver mechanism 44 for imparting motion onto the grid. As shown in FIG. 3, grid driver 44 comprises a motor 46, which may be a variable speed DC motor typical of motors well-known in the art, and a variable-pitch screw 48 that is threaded through a "nut" 50 adapted to mesh with the variable pitch of the screw. Thus, as motor 46 turns screw 48 in the direction of arrow A, nut 50, connected by bracket 51 to grid 42, travels in the direction of arrow B and moves the grid along track 45. Although described as having both a variable speed motor 46 and variable pitch screw 48 with respect to FIG. 3, an alternate grid movement system may comprise a fixed speed motor with a variable pitch screw or any mechanical variable drive coupling known in the art, such as for example, lever/cam or wheel/crank systems. Furthermore, the grid movement system may comprise a variable speed motor with a fixed mechanical coupling. A variable drive coupling and variable speed motor are preferred, however, to promote a operator-changeable accelerating or decelerating velocity profile. Usually, the radiation blocking elements 52 in the grid are parallel to each other and the grid is oriented so that the blocking elements are also parallel to the alignment of sensors 56 of the detector 54, in one direction (i.e. row or column). The motion of the grid is, usually, perpendicular to the grid radiation blocking elements (also known as vanes). Because the grid is moving relative to the detector, any Moire patterns created are transient in nature lasting only a few milliseconds, not long enough to be captured by the detector. An alternate arrangement is shown in FIG. 4. Grid 58 again comprises a plurality of vanes 60 and the motion of the bucky is along arrow B, perpendicular to the orientation of the vanes. The underlying direct radiography panel 62 comprises a plurality of sensors 66 aligned along a first direction (here in rows 64 of sensors 66). The angle a between vanes 60 and rows 64 of sensors 66 is approximately 45 degrees, as shown in FIG. 3. Thus, the angle (90-.alpha.) between the motion along arrow B and the orientation of the rows of pixels is also approximately 45 degrees. Although an approximate 45-degree orientation is shown herein, angle a may be any non-parallel or non-orthogonal angle that minimizes Moire pattern artifacts in a radiograph produced by the imaging system of which the bucky is a component. Referring now to FIG. 5, the invention comprises a radiographic diagnostic imaging system 100 which includes a source 110 of penetrative radiation for emitting a radiation beam 118 along a path through a target 112. The radiation source is captured by a detector 162 positioned in the beam path for receiving the radiation; Detector 162 is a direct radiographic detector comprising a plurality of radiation sensors 164 arrayed in rows and columns of the type described in U.S. Pat. No. 5,319,206 issued to Lee et al. on Jun. 7, 1997. According to the present invention, there is placed in front of the detector 162, between the detector and the target 112, an anti-scatter grid 140 having a plurality of radiation absorbing elements, vanes 160. In the illustration the vanes 160 are oriented parallel to the detector's columns of sensors. However this is not critical, and the vanes can be oriented at an angle to the detector rows and columns, as illustrated in FIG. 3. The anti-scatter grid is mounted so as to be moveable relative to the detector and radiation beam through a supporting and moving mechanism represented by block 146. The drive shown is given by way of illustration rather than limiting the way in which the variable speed profile is achieved. A any other mechanical or electromechanical arrangement that will provide the necessary motion to the antiscatter grid, that is will accelerate and decelerate the grid at the required rates, preferably in accordance with the equations given earlier in this description, may be used. The motion imparted by the mechanism is in the direction of the arrow "A" and is preferably in a direction perpendicular to the vanes 160. The system further comprises a controller 170 adapted to synchronize the radiation exposure to the motion of the grid. Controller 170, which may be a computer, is used to begin the radiation emission from source 110 when the grid velocity is at a desired point, preferably right after it has reached its maximum and the deceleration cycle has just begun. The invention also comprises a method whereby grid generated artifacts are reduced by moving the anti-scatter grid unidirectionally during the full radiation exposure using a continuously decreasing rate of movement of the grid. This is done by imparting a single stroke motion to the grid whereby the grid is first accelerated to a first maximum velocity and then decelerated with a decelerating velocity profile, preferably one which approaches zero asymptotically. For example, the decelerating velocity profile may comprise V=K.sub.2 t.sup.-m. The accelerating speed profile is not important so long as it can produce the desired velocity within a short time, of the order of a few milliseconds. The accelerating profile may be a linear function such as V=K.sub.1 t The variables are as described above, and more preferably V(cm/sec)=2,236 t(sec) for t less than or equal to 0.005 seconds and V=25*(t*1,000).sup.-0.5 for t greater than 0.005 seconds and less than or equal to 2 seconds where V is in cm/sec and t is in seconds. The method steps include moving the grid in a direction perpendicular to its vanes with the grid oriented so that it traverses the detector in a direction perpendicular to the detector rows or columns of sensors when the grid vanes are aligned with either the rows or columns of the detector. Alternatively, the grid may be moved in a direction that is at an acute angle to its vanes. In still an alternate embodiment the motion of the grid may be perpendicular to its vanes but with the grid vanes forming an acute angle with the rows or columns of the detector. This angle is preferably selected to be 45.degree.. The advantage of the last two alternatives is that the dead spaces between detector columns (or rows) never align with the grid vanes therefore further reducing the Moire pattern formation as the grid travels over the detector. The disadvantage is that it is more complicated to implement this type of oblique translation of the grid in existing equipment, and may require a larger grid. In practicing the present method, the beginning of the x-ray exposure is timed to assure that the grid is moving at a sufficient velocity during the exposure. Such timing may comprise an initial delay to allow the grid to reach a predetermined speed, it may comprise a chosen start time to produce a desired average velocity, or it may preferably comprise a chosen start time so that the x-ray generator radiation emission pulses begin at maximum velocity (point 34 on FIG. 2) just as the grid begins decelerating. The method of controlling the grid may comprise starting the radiation exposure at any position in the grid motion optimized for a particular grid, radiation source, or examination procedure. Those skilled in the art having the benefit of the teachings of the present invention as hereinabove set forth, can effect numerous modifications thereto. These modifications are to be construed as being encompassed within the scope of the present invention as set forth in the appended claims wherein