Apparatus and method for reducing X-ray grid line artifacts

A X-ray imaging system has a moveable grid which rejects X-rays that were scattered by a body being imaged. During an X-ray exposure the grid is reciprocated to blur the shadow of the grid in the image. Upon commencement of an X-ray exposure, the grid is moved at decreasing rate in a first direction toward an end point of travel. When the grid is near the end point, the movement rate increases and continues at this faster rate before and after the grid reverses direction at the end point. A given distance after the direction reversal, the rate decreases to a slow constant rate.

BACKGROUND OF THE INVENTION 
The present application relates to X-ray imaging systems; and more 
particularly to such systems having grids to reject scattered radiation. 
An apparatus for creating X-ray radiographs is comprised generally of an 
X-ray source and an X-ray sensitive medium, such as a photographic film 
and screen combination, for recording an image produced by the varying 
transmission of X-rays directed through an imaged body. The intensity of a 
radiographic image at a given point on its surface is ideally a function 
of the absorptive characteristics of the image body along a straight line 
from the X-ray tube to that point on the image. For this relationship to 
hold, X-rays that have not traveled in a straight line from the X-ray tube 
to the medium, i.e. those that have been scattered within the body, must 
be blocked to prevent their contribution to the recorded image. 
Shielding the medium from scattered X-rays typically is done with a grid 
that is placed immediately above the medium, such as shown in U.S. Pat. 
No. 5,040,202. The grid contains channels that are oriented to pass only 
X-rays proceeding in straight lines from the X-ray tube. These channels 
are formed by rows of parallel vanes that are constructed of X-ray 
absorptive material. The vanes are separated either by low obsorptivity 
solid, such as plastic, or in certain instances by air gaps. 
The physical thickness of the grid vanes, as measured along the plane of 
the X-ray sensitive medium, cause some of the X-rays that would otherwise 
be passed by the grid to be blocked. The blocking of these X-rays produces 
shadow "grid lines" in the image. Even fine grid lines may be distracting 
and larger grid lines can obscure diagnostically significant details in 
the image. One method of reducing grid lines is to move the grid back and 
forth parallel to the plane of the X-ray sensitive medium using a DC motor 
with a cam shaft connected to the grid. The grid shadow thus is blurred by 
falling on different areas of the medium during the exposure. If the grid 
can be moved so that each area of the medium is eclipsed by the vane for 
an equal proportion of the exposure time, the grid lines effectively will 
be eliminated. 
In general, it is quite difficult to move the grid so that its vanes spend 
an equal amount of time over each area of the mediums. Reciprocating the 
grid at a constant speed with respect to the medium is one approach. The 
constant approach speed is upset when the grid changes direction and must 
be decelerated and then re-accelerated in the opposite direction. In 
previous reciprocating systems, the grid lines spent a disproportionate 
amount of dwell time near the ends of their travel, as compared with the 
center of the travel. As a result, faint grid lines appeared under each 
vane at the vane's point of direction reversal. 
Different techniques have been utilized to reduce the grid line shadows at 
the end of their reciprocating movement. For example, the aforementioned 
U.S. Patent describes modulating the X-ray beam synchronously with the 
grid motion to reduce the grid image at points of grid speed variation. 
Although this technique was successful, it required additional components 
of the circuitry for regulating the X-ray beam and a mechanism by which 
the modulation of the beam was synchronized with the movement of the grid. 
SUMMARY OF THE INVENTION 
An object of the present invention is to move the radiographic grid in a 
reciprocal manner to blur shadows of the grid lines on the image even for 
exposures during which end of travel points are reached and the direction 
of grid movement reverses. 
In order to achieve the desired objective the speed of the grid must be 
increased in a controlled manner at the end of the grid travel. For that 
purpose, a stepper motor is attached to move the grid in steps of a fixed 
incremental distance with the grid plane movement parallel to the plane of 
the X-ray medium on which an image is produced. A controller regulates the 
speed and direction of the motor and thus the grid to produce the 
following pattern of movement. 
Preferably before an x-ray exposure commences the grid is moved to one end 
of its travel so that the initial movement during the exposure will not be 
against the force of gravity. Upon commencement of an X-ray exposure, the 
grid is moved at a first rate in a direction toward the other end of the 
travel and thereafter the rate of movement periodically decreases. 
As the grid nears the other end point, the rate of movement increases, for 
example the rate doubles. When the grid reaches the other end point, the 
direction of movement reverses so that the grid then travels toward the 
opposite end point. In the preferred embodiment of the present invention, 
the rate of movement increases to an even higher rate for a period of time 
immediately after the direction reversal. Then the rate of grid movement 
reduces to about the second rate for a given distance before slowing again 
to a constant speed. If the X-ray exposure is long enough the grid 
reverses direction in a similar manner when it reaches either end point. 
By increasing the rate at which the grid moves just before and after the 
point where the direction of movement is reversed, the grid shadows at the 
ends of the reciprocal travel are significantly blurred. Thus this grid 
movement technique reduces the artifacts produced in the image by the 
radiographic grid lines.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, a radiographic system 10 includes an X-ray tube 11 
directed to project a beam of X-rays 14 through soft tissue 16 toward a 
conventional X-ray sensitive medium 18. After passing through the medium 
18, the X-rays are detected by an exposure detector 20, such as that 
described in U.S. Pat. No. 4,970,398 entitled "Focused Multi-Element 
Detector For X-Ray Exposure Control." 
A radiographic grid assembly 22 is located between the soft tissue 16 being 
imaged and the medium 18 to block scattered X-rays. The grid assembly 22 
is composed of a grid 26 formed by a series of spaced apart X-ray 
absorbing vanes 24 which are aligned or "focused" to the X-ray tube 11. 
The vanes 24 form channels of a given width and height which prevent 
scattered X-rays from reaching the medium 18. 
One side of grid 26 rides on base members 28 which permit the grid to move 
in the reciprocal directions indicated by arrows 30. A U-shaped drive 
bracket 32 is attached to the opposite side of the grid 26 from members 
28. A support 34 on fixed base 49 is positioned within the opening of the 
drive bracket 32 and has rods 36 and 38 extending outwardly therefrom 
through apertures in each leg of the bracket. The drive bracket 32 is able 
to slide on the rods 36 and 38 as the grid 26 reciprocates in the 
directions indicated by arrows 30. 
One leg 40 of drive bracket 32 has a threaded aperture therethrough which 
receives a threaded shaft 42 of a bidirectional stepper motor 44 that is 
attached to base 49. As the stepper motor 44 drives shaft 42, the 
radiographic grid 26 moves in one of the directions indicated by arrows 30 
depending upon the direction of rotation of that shaft. As is well-known, 
stepper motors provide a very accurate incremental movement of a shaft as 
will be described each time the motor is driven by a step signal. 
The grid assembly contains a mercury tilt switch 46 which closes when the 
grid assembly 22 is tilted into vertical orientation as occurs when the 
X-ray apparatus 10 is swiveled orthogonally to the orientation shown in 
FIG. 1. The tilt switch 46 is positioned at an angle of approximately 20 
degrees from horizontal in the orientation of the system 10 shown in FIG. 
1. When the edge 45 of the grid 26 is above the motor 44 by a given 
amount, the tilt switch 46 closes, providing a signal to a control circuit 
for the motor, as will be described. 
The grid assembly 22 also contains a electro-optic sensor 48 which produces 
a signal when the grid 26 is at either of the two extremes of its travel, 
known as "home positions." The electro-optic sensor 48 is mounted on base 
49 of the grid assembly 22 and is a standard device having a light 
emitting diode and a phototransistor with a gap therebetween, as shown in 
FIG. 2. A shutter plate 47 is mounted on the grid 26 so as to pass between 
the diode and phototransistor of the electro-optic sensor 48 as the grid 
moves. The shutter plate 47 is shown schematically in FIG. 2 and is 
slightly shorter than the maximum travel of the grid 26. Thus, when the 
grid is at either end of its travel, the shutter plate 47 will clear the 
electro-optic sensor 48 allowing the diode to illuminate the 
phototransistor, which produces a signal designated HOME. 
FIG. 2 depicts a control circuit 50 for operating the stepper motor 44. 
Control circuit 50 includes microcomputer 52 that contains a 
microprocessor, random access memory, read only memory and associated 
components. The program for controlling the operation of the grid assembly 
22 is stored within the read only memory of the microcomputer 52. The 
microcomputer 52 receives an EXPOSURE signal via line 53 from a 
conventional main control system (not shown) of the radiographic system 
10. This EXPOSURE signal goes to an active logic level when the main 
control system initiates an X-ray exposure and remains at the active logic 
level until the main control system determines that the X-ray exposure 
should be terminated. 
The main control system for the radiographic apparatus 10 receives a signal 
designated AT SPEED from the microcomputer 52 indicating that the grid 26 
has reached a normal operating speed. This signal may be produced a given 
interval of time after the microcomputer 52 begins activating the stepper 
motor 44. In another implementation of the present invention, the 
microcomputer 52 ramps up the speed of stepper motor 44, in which case the 
AT SPEED signal is produced when microcomputer 52 has ramped the stepper 
motor up to the full operating speed. The microcomputer 52 also receives 
the signal from tilt switch 46 and a HOME signal from the opto-sensor 48 
which indicates when the grid 26 is in one of the home positions. 
The microcomputer 52 responds to these input signals by producing a set of 
output signals which controls the direction and speed of the stepper motor 
44. The application of power to the stepper motor 44 is governed by a 
conventional stepper motor driver 54. The microcomputer 52 produces an 
ON/OFF signal which activates the stepper motor driver 54. The direction 
in which the stepper motor 44 is to rotate shaft 42 is determined by a 
DIRECTION signal from the microcomputer 52 and each time that the stepper 
motor is to incrementally advance in that designated direction, the 
microcomputer sends a STEP signal pulse to the stepper motor driver 54. 
The stepper motor driver 54 responds to these signals from the 
microcomputer 52 by applying power to the appropriate coils of the stepper 
motor 44. 
The common terminals for the coils of the stepper motor 44 are connected by 
resistors 55 and 56 to node 58. Another resistor 60 is connected to node 
58 to form a voltage divider with resistors 55 and 56 between the stepper 
motor 44 and a source of positive voltage V.sup.+. A switch 62 of relay 64 
is connected across resistor 60. When the relay switch 62 is in an open 
position, the voltage divider formed by resistors 55, 56 and 60 applies a 
relatively low voltage to the stepper motor 44. Whereas when the relay 
switch 62 is closed and resistor 60 is shorted, a higher voltage is 
applied to the stepper motor 44. The level of voltage applied to the 
stepper motor determines the energy and thus the force that is exerted by 
the stepper motor on the grid 26. As will be seen, the force will be 
varied in order to compensate for the gravitational effects on the grid 
26. 
Relay 64 is controlled by a digital ENERGY signal from the microcomputer 
52. The logic level of the ENERGY signal is stored within a latch 68 which 
has an output that drives the coil 66 of relay 64. 
The stepper motor 44 must move the grid 26 fast enough so that the grid 
vane pattern is sufficiently blurred to be indiscernible on the exposed 
radiograph. Since the grid vane pattern is uniformly repetitive, the 
minimum required velocity is inversely proportional to the spacing between 
the adjacent vanes of the grid. The greater the vane spacing, the faster 
the grid must move. This relationship is given by the mathematical 
expression V.sub.min =C/(T.sub.ex S), where V.sub.min is the minimum 
threshold grid velocity, T.sub.ex is the exposure time, and S is the grid 
vane spacing. C is a proportionality constant that is dependent upon, 
among other things, characteristics of medium 18, film development 
processing and the specific X-ray apparatus employed. A value for C is 
derived from empirical test data for the specific configurations of the 
X-ray apparatus 10. The relationship of grid velocity to exposure time is 
plotted by the dashed line in FIG. 3. As can be seen, the shorter the 
exposure time, the greater the required velocity of the grid apparatus. 
Thus, to adequately blur the vane shadows, the stepper motor control 
circuit 50 must be capable of a velocity range which is sufficiently great 
to accommodate the entire range of possible exposure times. 
In conventional radiographic systems, the duration of the exposure is 
controlled by a feedback loop in which detector 20 senses radiation 
flowing through the medium 18 and produces a signal indicative of the 
level of radiation. The detector signal is used by the main control system 
to determine when a proper exposure has occurred and when to shut off the 
X-ray tube 11. Therefore, at the commencement of a given X-ray exposure, 
the duration of that exposure is unknown. In order to accommodate this 
unknown exposure duration, the grid 26 is initially moved at a relatively 
high velocity which decreases over the exposure time as shown by the solid 
line in FIG. 3. 
With reference to FIG. 4A, the speed of the grid 26 is determined by the 
control program which is executed by the microcomputer 52. The initial 
section of the program ensures that the grid is placed into the proper 
home position in expectation of an X-ray exposure. The orientation of the 
grid 26 at the start of an exposure is important as it is undesirable to 
initially move the grid upward against the force of gravity. Therefore, 
between X-ray exposures, the microcomputer 52 monitors the TILT signal to 
detect the orientation of the grid assembly 22. 
The state of the tilt switch 46, as indicated by the logic level of the 
TILT signal, is checked at step 100 in order to sense the orientation of 
the grid assembly 22. If the tilt switch is not closed, the grid assembly 
is in either the horizontal position or an angular position with the motor 
above edge 45. In this case the home position to be used is toward the 
motor and the program execution advances to step 102. At that point the 
microcomputer activates the stepper motor driver 54 to retract the grid 26 
into a home position. During retraction, the grid 26 moves toward the 
stepper motor 44 until the vane 47 on the grid clears the electro-optic 
sensor 48 so that the sensor produces an active HOME signal. Then at step 
103, the microcomputer 52 reverses the DIRECTION signal for the stepper 
motor driver 54 to prepare for movement upon the start of an exposure. At 
this time the grid no longer moves, as STEP signal pulses are not being 
applied to the stepper motor driver 54. 
If the tilt switch 46 is found closed at step 100, as occurs when the grid 
assembly 22 begins to be tilted vertically with edge 45 significantly 
above the stepper motor 44, the program branches to step 104. In this 
event, the grid 26 is advanced away from the stepper motor and into the 
home position at the opposite end of grid travel, where the electro-optic 
sensor 48 produces an active HOME signal. Thereafter, the DIRECTION signal 
is set at step 106 to produce movement of the grid 26 toward the stepper 
motor 44. 
Once the grid is in the appropriate home position, the microcomputer 52 
checks for an active EXPOSURE signal at program step 108. When this signal 
is inactive, the program execution returns to step 100 to monitor the tilt 
switch 46. 
At the beginning of an X-ray exposure, the microcomputer 52 receives an 
active EXPOSURE signal on line 53 from the main X-ray system controller. 
This causes the program to advance to step 109 where variables and 
counters used in controlling the stepper motor 44 are initialized. 
Then a "step" routine is called at program step 112 to produce incremental 
movement of the stepper motor 44. The step routine is shown in FIG. 5 and 
commences at program step 121 to check whether the main X-ray system 
computer is signalling that the exposure should continue, as indicated by 
an active EXPOSURE signal. Microcomputer 52 turns off the stepper motor 44 
at step 122 if an exposure is not occurring, and the program returns to 
step 100. Otherwise during an exposure, the subroutine branches to step 
124 where the microcomputer 52 produces a pulse of the STEP signal which 
causes the stepper motor driver 54 to move the stepper motor 44 one fixed 
increment in the direction indicated by the DIRECTION signal. A count of 
the steps during each movement cycle of the grid is maintained in the 
memory of microcomputer in order to know the position of the grid 26. At 
the commencement of the exposure, this count was zero and thereafter is 
incremented by one each time program step 125 is executed. 
The rate of grid movement is determined by the interval of time between 
STEP signal pulses, the shorter the interval the faster the rate. The 
pulse interval is determined by a delay timer that is implemented as a 
conventional software routine executed by the microcomputer 52. This timer 
is loaded at step 126 with a delay period. At the beginning of the 
exposure (step 109), this delay period is set to a very short interval so 
as to produce maximum velocity of the grid as determined by the shortest 
allowable exposure time. As will be described, the delay period is 
incremented by a given amount in the early portion of the exposure so as 
to decrease the speed of the motor 44 and thus the grid 26 during the 
exposure time. Then at step 128, the delay timer is repeatedly inspected 
until it reaches zero, at which time the step routine terminates and 
returns to the point in the main program on FIG. 4A at which it was 
called. 
Upon returning at this time, the program execution enters step 114 where 
the microcomputer 52 determines whether it is time to reverse the 
direction of the grid 26. Since each pulse of the STEP signal produces a 
fixed incremental movement of the stepper motor 44, the count of the step 
pulses indicates how far the grid 26 has moved. Thus, the number of pulses 
between the home position and the point of movement at which direction 
reversal should begin is known. Therefore, at step 114, that number of 
pulses is compared to the value in the step counter, called STEP COUNT. If 
the two values are not equal, the program execution advances to step 116. 
At this time, the microcomputer 52 checks a flag which during the initial 
stage of the grid movement (prior to point 61 on FIG. 3) has a zero value. 
This causes step 118 to be executed where the step period is incremented 
by a given amount to slow the grid speed, as shown by the solid line in 
FIG. 3. Once the step period has been incremented, the program execution 
returns to step 112 to once again call the step routine to incrementally 
advance the stepper motor 44 and thus the grid 26. This loop through 
program steps 112-118 continues until the STEP COUNT reaches a value which 
indicates that reversal of the grid 26 should begin. 
Reversal of the grid 26 starts at point 61 in FIG. 3 when the grid is 
approaching the extreme end of its travel from the home position. Then, 
the program execution advances to step 120 where the step period is set to 
a much shorter fixed value designated as "FAST." This significantly 
shorter step period approximately doubles the grid velocity between points 
61 and 62 as shown in FIG. 3. At program step 130, the step routine is 
called once again to produce movement of the stepper motor and grid at 
this faster speed. This action results in movement of the grid 26 at an 
increased speed near the ends of travel in order to prevent grid lines in 
the X-ray image due to dwell of the grid at the extreme points. 
During the reversal procedure at the ends of grid travel, a separate count 
of movement steps is maintained in a memory location designated "reverse 
count." At step 132, the reverse count is incremented by one. Operation at 
the FAST speed continues for a number of motor steps, for example eight. 
During this time, the program repeatedly loops through steps 130-134. When 
eight steps at the FAST speed have occurred, program execution advances 
from step 134 to step 136. 
It is now time to reverse the direction of the grid 25 and the 
microcomputer 52 changes the logic level of the DIRECTION signal at step 
136. As noted previously, if the grid assembly 22 has been tilted 
significantly from the horizontal, higher energy must be applied to the 
stepper motor 44 when the grid 26 is travelling upward against the force 
of gravity. Thus, if the TILT signal is active, the ENERGY signal must be 
set at step 138 to indicate high energy is required due to reversal of the 
movement direction. The reversal of the grid direction occurs at time 62 
in FIG. 3 and as illustrated, the initial operation in the reverse 
direction occurs at even a faster speed than was occurring prior to time 
62. Thus, at program step 140, microcomputer 52 sets the step period to an 
even shorter value designated "VERY FAST." The fixed values for FAST and 
VERY FAST are stored in a data table within the internal ROM of the 
microcomputer. 
At steps 142 and 144, the step routine is called to produce an incremental 
movement of the grid 26 at this higher speed and the reverse count is 
incremented by one. The VERY FAST speed occurs for a relatively short 
period of time, for example four movement steps, which is sufficiently 
long to ensure blurring of the vanes 24 at the turn-around location. Thus, 
when the step count reaches a value of 12 at step 146, as occurs at time 
63, the program execution advances to step 150 on FIG. 4B. 
The step period is once again set to the FAST value to decrease the speed 
of the grid. Then at steps 152, 154 and 156, eight more steps at this 
intermediate speed occur as indicated between points 63 and 64 in FIG. 3. 
Time 64 occurs at the end of the reversal process, where the reverse count 
is zeroed at step 158 to prepare for the next reversal process. Then, grid 
speed is reduced dramatically by setting the step period to a relatively 
large value, designated SLOW, at step 160 to move the grid even slower 
than occurred immediately prior to the direction reversal at time 61. The 
speed maintained constant at this SLOW level by setting the flag to one at 
step 162 so that the step period will no longer be incremented at program 
step 118 as happens prior to time 61. 
Movement at this fixed SLOW speed is accomplished by continuously calling 
the step routine at program step 164 until the step count indicates at 
step 166 that the direction of the grid should be reversed again as it is 
approaching the home position. 
When the step count indicates that the grid is a fixed distance, for 
example eight steps, from the home position, the program execution 
advances to step 168 where the step period is once again set to the FAST 
value. The step routine is called at step 170 to advance the grid one 
increment of the stepper motor. Following the step routine, the reverse 
count is incremented and a determination is made at step 174 whether the 
grid has reached the home position as indicated by the HOME signal from 
the electro-optic sensor 48. The program execution continues looping 
through step 170-174 until the home position is reached. 
Upon reaching the home position, microcomputer 52 acts as a comparator by 
comparing the STEP COUNT to a value designated CYCLE COUNT, which is the 
nominal number of movement steps which occur during a cycle of grid 26. If 
the actual STEP COUNT is not within a given tolerance, e.g. .+-.10, of the 
CYCLE COUNT, the microcomputer sets an error indicator at step 178. 
In either event, the program execution then advances to step 180 to once 
again reverse the direction of the grid 26 by changing the logic level of 
the DIRECTION signal to the stepper motor driver 54. Then at step 82, if 
the TILT signal is active, the ENERGY signal is set to a low logic level 
to decrease the energy applied to the stepper motor 44 since the new 
direction of travel will be downward and not against the force of gravity. 
Then the step period is set to the VERY FAST value at step 184 and steps 
186, 188 and 190 produce four increments of movement at that very fast 
speed. Then the step period is set to the FAST value at step 192 and at 
steps 194, 196 and 198, eight steps, for example, occur at the fast speed. 
When those steps have been completed, the grid may once again travel at 
the slow speed. Therefore, the reverse count is zeroed at step 200 and the 
step period is set to the SLOW value at step 202. The program execution 
then returns to step 112 to begin another reciprocal cycle of the grid 
movement. 
As can be seen from the graph of FIG. 3, the motor speed increases just 
before reversal of the grid direction. Immediately following the reversal, 
the grid is moved at an even higher speed for a short amount of time and 
then at an intermediate speed, before returning to a normal slow speed. 
This rapid movement of the grid before and after direction reversal, 
eliminates the shadows which previously occurred due to dwell of the grid 
at the points of reversal. Furthermore, the comparison of the actual 
number of movement steps to the nominal amount for a grid cycle provides 
an indication of when the grid is not moving satisfactorily.