Geometric distortion-free image intensifier system

A patient (12) is irradiated with x-rays (10) from an x-ray tube (A). X-rays that have passed through a region of interest of the patient are converted to a light image by a phosphor screen (14) on the dome shaped input face (16) of an image intensifier (B). The dome shaped input face causes the output optical image (20) of the image intensifier to be distorted with pin cushion distortion. The sweep pattern of the electron beam of a video pick up tube (24) of a video camera (C) is controlled by deflection plates (26a, 26b, 26c, 26d). A sweep control circuit (D) alters the sweep pattern such that electronic image representations generated by the video camera are distorted in a manner that is complementary to and cancels the image intensifier tube distortion. In this manner, the two distortions cancel and a man-readable image (28) displayed on a video monitor (22) is substantially distortion-free. The sweep control circuit alters conventional vertical sweep control voltage ramps (V) and horizontal control voltage ramps (H) by adding third order, parabolic terms: EQU V.sub.T =V+V+kV(V.sup.2 +H.sup.2) EQU H.sub.T =H+kH(V.sup.2 +H.sup.2) Amplifiers (50a, 50b50c, 50d) boost the voltage of the parabolically corrected sweep control signals and apply them to the sweep control plates in a push-pull manner.

BACKGROUND OF THE INVENTION 
The present invention relates to the image intensifier art. It finds 
particular application in conjunction with x-ray images that are monitored 
by a video camera and will be described with particular reference thereto. 
It is to be appreciated, however, that the invention may find utility in 
other applications without departing from the spirit and intent of the 
invention. 
Traditionally, x-rays were projected through a patient onto a flat sheet of 
photographic film to .produce film images. Now, the film is frequently 
replaced with an image intensifier and video camera to produce a video 
image. When the x-ray beam is projected on an image intensifier, rather 
than a flat sheet of film, there tends to be image distortion at the 
corners. More specifically, image intensifiers include a vacuum tube which 
has a spherical arc segment or dome on the receiving face. The dome moves 
further out of the traditional film plane and presents a more glancing 
angle to the x-rays with radial distance from the center of the dome. This 
produces "pin cushion" distortion in which the resultant image looks as if 
it were on a rubber sheet that was stretched outward at the corners. 
The distorted image on the domed input face of the image intensifier is 
converted to a smaller higher intensity image which, of course, retains 
the pin cushion distortion. The distorted image is viewed by a video 
camera. A video signal from the video camera is conveyed to a video 
monitor on which the distorted picture is displayed. 
Most commonly, video cameras include a magnetic sweep pattern control in 
order to maintain the accuracy of the high speed sweep pattern of a video 
tube. Although video cameras with electrostatic sweep controls are 
utilized for some applications, they have generally been considered 
undesirable for medical diagnostic imaging. First, controlling the ramp 
functions of the high voltages on the electrostatic plates required 
relatively slow, high voltage power transistors. The slow speed of the 
power transistors limits control of the video camera sweep pattern. 
Further, high capacitances involved in the sweep plates and the switching 
transistors limit the frequency response or switching times still further. 
Finally, the capacitance of the plates tends to be so large that the 
initial charging time at the beginning of each sweep produced a noticeable 
degradation of the sweep pattern. 
In early image intensifiers, which were only about 15-20 cm across, this 
pin cushion distortion was not very pronounced. Although accurate 
measurements could not be scaled off the image, radiologists found it 
acceptable for diagnostic purposes. As technology has progressed, the size 
of image intensifiers has also increased such that today image 
intensifiers up to about 40 cm in diameter are available. The larger the 
diameter of the image intensifier, the larger the dome and the more 
pronounced the pin cushion distortion becomes around the edges of the 
picture. 
Another drawback of the prior art image intensifiers is that the intensity 
of the distorted image decreases with radial distance from the center. 
Thus, edge portions of the image are not only distorted but also of lower 
intensity. 
Another drawback of the prior art image intensifier and video camera 
systems is that a mechanical adjustment procedure is necessary in order to 
square the resultant image within the video monitor. That is, the 
orientation of the video monitor image is determined by the angular 
orientation of the video camera relative to the image intensifier. The 
video camera had to be physically moved relative to the image intensifier 
in order to eliminate an undesirable tilt or canting of the resultant 
video image relative to the video monitor. 
The present invention provides a new and improved image intensifier system 
which substantially eliminates pin cushion distortion. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, an image intensifier is connected 
with a video camera whose scanning or sweep pattern is controlled by a 
sweep control circuit. The image intensifier inherently produces an output 
image display with a known distortion. The video camera sweep circuit 
control alters the sweep pattern of the video camera to create a 
substantially equal and opposite distortion effect. In this manner, the 
image intensifier and video camera distortions cancel producing a 
substantially distortion-free resultant image representation. 
In accordance with another aspect of the present invention, the video 
camera sweep control circuit includes an electronic image rotation means 
for electronically adjusting the video camera sweep pattern such that the 
resultant image is rotated, preferably to a preselected orientation. 
In accordance with another aspect of the present invention, the video 
camera sweep control circuit controls the dwell time of the video camera 
sweep such that it reads edge portions of the distorted image for a longer 
duration than center portions. In this manner, the inherent intensity 
fall-off adjacent the edges of the distorted image intensifier image is 
corrected by the change in sweep pattern. 
In accordance with another aspect of the present invention, the video 
camera sweep control circuit performs a third order correction. 
In accordance with yet another aspect of the present invention, the video 
camera utilizes a three-way improved electrostatic sweep control, rather 
than the more common magnetic sweep control. First, the electrostatic 
sweep voltage is controlled by a high speed, relatively low voltage 
transistor. Zener diodes or the like are utilized to boost the controlled 
voltages which are applied to the electrostatic plates. Second, the 
controlled voltages to the sweep plates are controlled by transistors 
arranged in a common base configuration to reduce the capacitance, hence, 
increase the frequency response. Third, the controlled voltage switching 
circuitry of the sweep control circuit includes a constant current source 
to increase the speed with which the highly capacitive sweep deflection 
plates of the video camera can be brought to the controlled charge. 
One advantage of the present invention is that it corrects for image 
intensifier distortion. 
Another advantage of the present invention is that it accurately and 
precisely controls the sweep pattern of electrostatic video cameras. 
Another advantage of the present invention is that the distortion corrected 
image can be rotated electronically without reintroducing distortion. 
Yet another advantage of the present invention is that it corrects for 
intensity inconsistency in the output of image intensifiers. 
Still further advantages of the present invention will become apparent to 
those of ordinary skill in the art upon reading and understanding the 
following detailed description.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
A source of penetrating radiation A, such as an x-ray tube directs a swatch 
of radiation 10 through a region of interest of a subject 12. The 
intensity of radiation that traverses the region of interest varies as a 
shadowgraphic projection of the x-ray absorptive properties of the 
materials in the region of interest. An image intensifier B converts this 
x-ray shadowgraph into a pin cushion distorted optical image. That is, 
x-rays impinge upon a phosphor screen 14 mounted to a dome shaped input 
face 16 of the image intensifier. The phosphor screen converts the x-rays 
into light of the visible spectrum that is projected onto a target 18 as 
an optically distorted image 20. 
A video camera c converts the output optical image into a video signal or 
other electronic image representation. The electronic image representation 
can be stored on tape or disk, subject to various types of processing, 
displayed on a video monitor 22, or the like. The video camera includes a 
video pick-up tube 24 whose sweep pattern is controlled by electrostatic 
deflection plates 26a, 26b, 26c, 26d. 
A sweep control circuit D modifies a conventional, linear beam sweep 
pattern to introduce a second distortion in the video camera image 
representation, which second distortion is equal and opposite to the image 
intensifier optical distortion. In particular, the modified sweep pattern 
generally follows the arcuate lines of a pin cushion distorted grid image 
20 rather than conventional straight, parallel lines. Moreover, this 
arcuate sweep pattern becomes slower at points more radially outward from 
the center of the video pick-up tube. The lighter intensity peripheral 
portions of the intensifier image 20 become balanced with the intensity of 
central portions of the image in a resultant video image 28. 
In the preferred embodiment, the sweep control circuit D adds a parabolic 
correction to the normal vertical and horizontal saw tooth sweep voltages. 
That is: 
EQU Vt=V+kV (V.sup.2 +H.sup.2) (1a) 
EQU Ht=H+kH (V.sup.2 +H.sup.2) (1b), 
where V is the normal or uncorrected vertical sweep voltage and H is the 
normal or uncorrected horizontal sweep voltage, and k controls or 
determines the amount of correction. When k is equal to zero, conventional 
saw tooth vertical and horizontal sweep voltages are applied and an 
uncorrected image is produced. The value of k is increased, such as by 
adjusting a variable potentiometer or resistor until the resultant image 
28 meets preselected criteria of accuracy and linearity. Of course, a 
third order parabolic correction of this form might also be provided in a 
magnetically controlled sweep tube type video camera or by shifting and 
averaging pixel values of a CCD type video camera. 
In the preferred embodiment, the sweep control circuit D includes a 
vertical deflection ramp generator 30 and a horizontal deflection ramp 
generator 32. A mixed parabola generator includes circuit means 34 which 
calculates the sum of the squares of the vertical and horizontal ramp 
voltages. A multiplying or other magnitude adjusting means 36 multiplies 
the function by a selectable constant k. A first multiplying means 38 
multiplies the vertical ramp voltage by the sum of the squares and a 
second multiplying means 40 multiplies the horizontal ramp by the sum of 
the squares function. A first adding means 42 adds the vertical voltage 
ramp to the parabolic product of multiplying means 38 and a second adding 
means 44 adds the horizontal ramp voltage to the parabolic product of 
multiplying means 40. In this manner, a vertical deflection voltage as 
described by Equation (a) and a horizontal deflection voltage as described 
by Equation (1b) are defined. 
The parabolically adjusted sweep control signals are split and inverted by 
inverters 46 and 48. The inverted and uninverted parabolically adjusted 
sweep control signals are amplified by four substantially identical 
amplifiers 50a, 50b, 50c, 50d. The amplified inverted and uninverted 
vertical sweep signals are applied to the vertical sweep plates 26a, 26b 
and the amplified horizontal sweep signals are applied to the horizontal 
sweep plates 26a, 26b. Because all four amplifier circuits are 
substantially the same, only one is described herein in detail, which 
detailed explanation applies analogously to all four circuits. 
The amplifiers increase the sweep voltages to provide a sweep voltage of 
.+-.200 volts on a 600 volt pedestal. A transistor 52, which can handle 
400 volts, is interconnected with a pair of zener diodes 54, 56. These 
zener diodes each shift the voltage by 200 volts such that the transistor 
52 only sees a 400 volt voltage swing, even although the plate voltages 
are swinging between 400 and 800 volts. The transistor 52 is connected 
with a common base configuration to eliminate a substantial portion of the 
capacitance. Because the frequency response of an amplifier is 
proportional to 1/2.pi.RC, minimizing the capacitance maximizes the 
frequency response of the amplifier. 
In order to reduce the effects of the capacitance of the deflector plates, 
denoted electrically as C.sub.26, an isolation transistor 58 disconnects 
the plate capacitance c.sub.26 from the system capacitance c.sub.sys. When 
transistor 52 is open, there is approximately 800 volts on the plates, 
hence on capacitor C.sub.26. When the transistor 52 closes to ground, the 
voltage on C.sub.26 drops to about 400 volts. When the transistor 52 
closes, the current flows through transistor 52 and when it is open, the 
current charges capacitor C.sub.sys. The output potential E.sub.out is 
related to a charging current I.sub.in flowing into C.sub.sys. I.sub.in is 
established by the constant current source 60. Then E.sub.out may be 
expressed by the equation: 
##EQU1## 
Some of this current also flows to the deflection plates or capacitor 
C.sub.26. In order to bring the plates quickly to the applied potential, 
the isolation transistor 58 has a high current gain, e.g. 100:1, so that 
the output current is very high and capacitor C.sub.26 charges quickly. 
A diode 62 and resistor 64 hold the voltage on the base of a transistor 66 
constant such that a constant, high current flows through resistor 68 and 
transistor 58. 
Amplifier means 50b produces a complementary signal to that produced by 50a 
for application to the other vertical deflection plate. Analogously, 
amplifiers 50c and 50d produce complementary voltage signals for 
application to the horizontal deflection plates. An x,y interchanging 
means 70 interchanges the horizontal and vertical deflection signals and 
deflection plates 26a, 26b, 26c, and 26d to produce a 90.degree. image 
rotation. A vernier rotation control 72a and 72b are provided for 
continuous rotation from 0 to .+-.45.degree.. Alternately, the rotation 
control 72a and 72b may be interconnected with the image intensifier B or 
a support for the patient such that in diagnostic procedures in which the 
subject and image intensifier are moved relative to each other, the sweep 
pattern is not rotated but the image on the video monitor 22 remains 
constant. 
Although a parabolic correction is utilized in the preferred embodiment, 
the exact shape of the correction is determined by the shape of the dome 
of the image intensifier. In the preferred embodiment, the dome is a 
spherical segment as illustrated by curve 80 of FIG. 2. The x and y 
distance from an arbitrary point on the curve to the point where a radial 
line through the curve point intersects the flat picture plan can be 
described by the coordinates .DELTA.r.sub.y,.DELTA.r.sub.x where: 
##EQU2## 
Stated in terms of the tangent of the angle of the radius: 
##EQU3## 
By similar triangles, .DELTA.r.sub.y can be stated as 
##EQU4## 
To simplify the mathematics, one can define the radius of the dome as 
unity: 
##EQU5## 
Substituting the value of .DELTA.r.sub.x in Equation (6b) into Equation 
(6a) expresses the value of .DELTA.r.sub.y in terms only or r. 
##EQU6## 
Using a Taylor series expansion of the form (1.+-.x).sup.31n =.-+.nx+. . 
., .notident.r.sub.y becomes: 
##EQU7## 
Thus, the displacement .DELTA.r.sub.y along the flat picture plane is 
proportional by a constant k times the cube of r. 
With reference to FIG. 3, a corresponding correction .DELTA.V deflection 
voltage V and correction .DELTA.H to the horizontal deflection voltage H 
can be readily determined by conventional trigonometry: 
EQU r.sup.2 =V.sup.2 +H.sup.2 (9) 
##EQU8## 
Substituting Equation (8d) into Equations (11a) and (11b) 
##EQU9## 
Substituting Equation (10) into Equations (12a, 12b), the corrections 
.DELTA.V and .DELTA.H can be expressed solely in terms of V and H: 
EQU .DELTA.V=k(V.sup.2 +H.sup.2)V (13a) 
EQU .DELTA.H=k(V.sup.2 +H.sup.2)H (13b). 
The corrected sweep values V.sub.T, H.sub.T are the sum of the original 
sweep value and these corrections: 
EQU V.sub.T =V+.DELTA.V (14a) 
EQU H.sub.T =H+.DELTA.H (14b) 
EQU V.sub.T =V+kV(V.sup.2 +H.sup.2) (15a) 
EQU H.sub.T =H+kH(V.sup.2 +H.sup.2) (15b). 
If an input lens or dome of a different contour is utilized, the 
appropriate corrections are calculated analogously. 
It is to be further appreciated that additional refinement is obtainable 
through the use of the vernier rotational control 72a and 72b as shown in 
FIGURE Specifically through the use of the vernier rotation control 
Equations (15a) and (15b) are further refined to: 
EQU V.sub.r =V+kV(V.sup.2 +H.sup.2).+-.KH (16a) 
EQU H.sub.T =H+kH(V.sub.2 +H.sub.2).+-.KV (16b). 
The invention has been described with reference to the preferred 
embodiment. Obviously, modifications and alterations will occur to others 
upon reading and understanding the preceding detailed description. It is 
intended that the invention be construed as including all such alterations 
and modifications insofar as they come within the scope of the appended 
claims or the equivalents thereof.