U.S. patent number 3,894,181 [Application Number 05/369,824] was granted by the patent office on 1975-07-08 for differential enhancement of periodically variable images.
This patent grant is currently assigned to Wisconsin Alumni Research Foundation. Invention is credited to Charles A. Mistretta, Michael G. Ort.
United States Patent |
3,894,181 |
Mistretta , et al. |
July 8, 1975 |
**Please see images for:
( Certificate of Correction ) ** |
Differential enhancement of periodically variable images
Abstract
Image signals are converted to video signals which are
differentially detected, integrated, subtracted and displayed to
display integrated differential features.
Inventors: |
Mistretta; Charles A. (Madison,
WI), Ort; Michael G. (Madison, WI) |
Assignee: |
Wisconsin Alumni Research
Foundation (Madison, WI)
|
Family
ID: |
23457085 |
Appl.
No.: |
05/369,824 |
Filed: |
June 14, 1973 |
Current U.S.
Class: |
378/98.11;
348/E5.089; 976/DIG.439 |
Current CPC
Class: |
H04N
5/3205 (20130101); G21K 4/00 (20130101) |
Current International
Class: |
G21K
4/00 (20060101); H04N 5/32 (20060101); H04n
007/18 () |
Field of
Search: |
;178/6.8,DIG.1,DIG.5,DIG.33,DIG.38 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Britton; Howard W.
Attorney, Agent or Firm: Burmeister, Palmatier &
Hamby
Claims
We claim:
1. Differential imaging apparatus,
comprising image producing means for alternately producing first
and second sequential image with identical portions and
differential portions,
means including a television system for converting said first and
second images into first and second video signals,
a video difference detector,
means for sequentially supplying said first and second video
signals corresponding to the first and second images to said video
difference detector,
said detector being operative to produce a first differential video
signal corresponding to the difference between said first and
second video signals,
an image storage device,
first writing means for writing an image in a positive sense on
said image storage device corresponding to said first differential
video signal,
means for deriving a second differential video signal from said
video difference detector corresponding to the difference between
said second and first video signals,
second writing means for writing a second image in a negative sense
on said image storage device corresponding to said second
differential video signal whereby said image storage device
additively integrates the differential portions of said
differential video signals while subtractively combining the
non-differential portions thereof,
reading means for reading the integrated image stored in said image
storage device,
and display means for displaying said integrated image.
2. Apparatus according to claim 1,
in which said image producing means includes X-ray apparatus for
alternately producing first and second X-ray images under
significantly different conditions whereby said images will have
differential portions.
3. Apparatus according to claim 1,
in which said image producing means comprises X-ray apparatus for
alternately producing first and second X-ray images with X-rays
having significantly different energies whereby said images will
have differential portions.
4. Apparatus according to claim 1,
in which said image-producing means comprises X-ray apparatus for
alternately producing first and second X-ray images using X-rays of
different energies,
said X-ray apparatus including an X-ray source and differential
X-ray filtration means for alternately filtering the X-rays from
the source for producing the different X-ray energies.
5. Apparatus according to claim 1,
in which said image-producing means comprises X-ray apparatus for
alternately producing X-ray images using X-rays having different
energies,
said X-ray apparatus including an X-ray source and means for
alternately energizing said X-ray source with two different
voltages to produce the different energies.
6. Apparatus according to claim 1,
in which said video difference detector includes a video
subtraction cathode ray tube.
7. Apparatus according to claim 6,
in which said cathode ray tube includes a target having a
dielectric layer over a conductive backplate.
8. Apparatus according to claim 7,
in which said cathode ray tube comprises means for causing an
electron beam to scan said dielectric layer to produce an
electrical charge image on said dielectric layer.
9. Apparatus according to claim 8,
including output means coupled to the backplate of said cathode ray
tube.
10. Apparatus according to claim 1,
in which said image storage device comprises a storage tube having
a target for storing electrical charge images.
11. Apparatus according to claim 10,
in which said storage tube comprises a target having a mosaic of
dielectric islands over a conductive backplate,
and means for causing an electron beam to scan said target.
12. Apparatus according to claim 11,
in which said first writing means includes means for energizing
said backplate with a first voltage which is sufficiently high to
produce writing with positive charges on said target by the
electron beam,
said second writing means including means for energizing said
backplate with a lower voltage such that the electron beam produces
writing with negative charges on said target.
13. Apparatus according to claim 12,
in which said reading means includes means for energizing said
backplate with a low voltage while deriving a video signal output
from said backplate.
14. Apparatus according to claim 12,
including means for alternately actuating said first and second
writing means through a plurality of cycles to produce enhanced
integrations of the differential signal portions.
Description
This invention relates to the enhancement of differential features
which may be present in repetitive time-dependent images so that
the contrast of such differential features will be increased to a
point such that the differential features are clearly visible, even
though initially the differential features may have such low
contrast that they are scarcely visible.
The present invention will find many applications to situations
where certain features of images are periodically variable. For
example, the present invention is particularly well adapted for
enhancing the contrast and visibility of differential features in
X-ray images. Such differential features may be produced by
periodically changing the conditions under which the X-ray images
are produced. For example, two or more different X-ray filters may
be moved repeatedly into and out of the path of the X-rays so as to
vary the energy of the X-rays.
The X-ray conditions can also be varied by repetitively pulsing the
X-ray source to two or more different voltages so as to change the
energy of the X-rays. It is also possible to switch repetitively
between two or more X-ray sources having different energies. The
differential features in the X-ray images can also be produced by
time-dependent variations in the subject being X-rayed. For
example, such time-dependent variations may be caused by the heart
beat or the respiration of the subject.
The general object of the present invention is to enhance the
contrast and visibility of the differential features of
periodically variable images, particularly X-ray images.
In accordance with the present invention, a television system is
employed to convert the periodically variable image into video
signals. In this way, the differential features of the periodically
variable image are reproduced in the form of differences between
the sequential sets of video signals representing frames of the
television images. The sequential video signals are supplied to a
video difference detector which develops differential video
signals. Such signals are supplied to an integrating and
subtracting storage device which integrates the differential
features of the video signals, while subtractively combining the
non-differential features. The integrated video signals are then
supplied to a TV monitor which displays the integrated differential
features of the video images.
In the illustrative embodiment, first and second X-ray images
having differential features are produced by periodically varying
the conditions under which the X-ray images are produced. Such
conditions can be varied by moving different X-ray filters into and
out of the path of the X-rays. The sequential first and second
X-ray images are converted by a television camera into first and
second video image signals, which are supplied sequentially to a
video subtraction detector preferably utilizing a first video
storage tube in which the video images can be stored in the form of
electrical charges on a target. Such target preferably has a
dielectric layer over a conductive backplate.
After the first video signal has been written on the target, the
second video image is written with the result that the storage tube
develops a first differential video signal representing the
difference between the first and second video image signals. The
first differential video signal is then written positively on an
integrating and subtracting storage device, preferably utilizing a
second storage tube having a target which includes a mosaic of
dielectric islands on a conductive backplate. The first storage
tube then develops a second differential video signal representing
the difference between the second and first video image signals.
Such second differential video signal is written subtractively on
the target of the second storage tube.
This series of operations has the effect of integrating the
differential portions of the first and second video signals, while
subtractively combining the non-differential portions. The
integrated image can be read as desired from the second storage
tube and supplied to a television monitor which will produce a
visible display of the enhanced differential image features.
Further objects, features and advantages of the present invention
will appear from the following description, taken with the
accompanying drawings, in which:
FIG. 1 is a schematic block diagram of a system or apparatus for
the enhancement of differential images to be described as an
illustrative embodiment of the present invention.
FIG. 2 is a diagrammatic representation of the first video storage
tube employed in the video difference detector of FIG. 1.
FIG. 3 is a diagrammatic representation of the second video storage
tube employed in the integrating and subtraction storage devices of
FIG. 1.
FIG. 4 is a graph representing the writing characteristics of the
second storage tube, such graph being effective to illustrate how
the images can be written either positively or negatively on the
second storage tube.
FIG. 5 is a set of waveform and operational diagrams illustrating
the operation of the system of FIG. 1.
FIG. 6 is a graph in which the X-ray attenuation coefficients of
certain materials are plotted as a function of X-ray energy to
illustrate the manner in which differential X-ray images may be
produced.
FIG. 7 is a graph illustrating the manner in which X-rays of
different energy distributions can be produced by using different
X-ray filters.
As just indicated, FIG. 1 illustrates a system or apparatus 10 for
the enhancement of differential features in periodically variable
images. In this case, the images are produced by an X-ray system 12
utilizing an X-ray source 14, which may simply comprise an ordinary
X-ray tube. The X-ray source 14 is adapted to be energized by a
high voltage source 16, preferably in the form of an X-ray pulser
adapted to supply high voltage pulses to the X-ray source 14,
whenever a control pulse or signal is supplied to the X-ray pulser
16 over a control line 18.
The X-rays from the X-ray source 14 pass through the patient or
subject 20 to be X-rayed and then impinge upon an intensifier
screen 22 which produces a visible X-ray image.
Various means may be employed to cause the X-ray image to be
periodically variable so that the X-ray image will have
differential features which may be enhanced by the system 10. For
example, the X-ray pulser 16 may be constructed and arranged to
supply sequential pulses of different voltages to the X-ray source
14 so that the energy of the X-rays will be varied in a
time-dependent manner. It is also possible to switch between two or
more different X-ray sources which produce X-rays of different
energies. Such X-ray sources may be of the monochromatic of
monoenergetic type.
Perhaps the easiest method of changing the energy of the X-rays is
by variable filtration. This method is employed in the X-ray system
12 of FIG. 1, in which a variable filter device 24 is adapted to be
interposed between the X-ray source 14 and the patient 20. The
filter device 24 comprises at least one filter which can be moved
into and out of the beam of X-rays.
As shown, the variable filter device 24 comprises a rotatable
filter disc 26 having at least two filter segments 26a and 26b,
which contain different materials having different characteristics
as to X-ray absorption so that the energy of the X-rays passing
through the filter disc will be changed when the filter segments
are changed. For example, the filter segments 26a and 26b may
contain cerium and iodine. Various other materials may be employed
as desired. The cerium may be in the form of cerium foil or a layer
of coating of cerium dioxide. The iodine may be in the form of
sodium iodide applied as a coating or layer with a suitable binder
on a supporting member having a low X-ray absorption.
FIG. 7 illustrates the different X-ray energy distributions which
are produced by using filters containing cerium and iodine. It will
be seen that FIG. 7 comprises a curve 28 which represents the
energy distribution of the unfiltered X-rays. A second curve 28a
represents the energy distribution when the cerium filter is used,
while a third curve 28b represents the energy distribution when the
iodine filter is used. It will be noted that the curves 28a and 28b
constitute relatively sharp peaks so that they represent X-ray
spectra which may be regarded as quasi-monoenergetic. The peak of
the cerium curve 28a is at higher energy then the peak of the
iodine curve 28b.
Provision is made for moving the filter segments 26a and 26b into
and out of the X-ray beam. In this case, the filter disc 26 is
adapted to be rotated by a motor 30 connected to the disc 26 by a
shaft 32. For synchronizing purposes, a cam 34 may be mounted on
the shaft 32 to operate a control switch 36. The cam 34 has two
different lobes 34a and 34b corresponding to the filter segments
26a and 26b. Various other means may be provided for synchronizing
purposes such as a commutator, or optical encoder.
The absorption of the X-rays by the patient or subject 20 varies
slightly when the two filter segments 26a and 26b are being used
due to the difference in the energy levels of the X-rays. While
this is true to a slight extent as to ordinary soft tissue, it is
true to a greater extent as to portions of the patient's body which
contain significant quantities of a contrast medium, such as
iodine.
These differences in X-ray absorption are illustrated in FIG. 6, in
which the X-ray absorption coefficient (K) is plotted against the
X-ray energy for water and iodine. The absorption curve for water
is approximately applicable to soft tissue. It will be noted that
the absorption curve for iodine has a discontinuity 38 at a
particular X-ray energy. This discontinuity 38 is often referred to
as the K edge.
The use of the cerium and iodine filters 26a and 26b has the effect
of shifting the X-ray energy above and below the discontinuity or K
edge 38 so that there will be a significantly greater differential
between the two X-ray images as to iodine containing tissue than as
to ordinary soft tissue. The iodine containing tissue may be the
thyroid gland, for example, which naturally contains iodine in
quantities which are sufficient to render the thyroid gland clearly
visible by the system of the present invention. Alternatively,
iodine may be injected in small quantities into the circulatory
system so that the blood vessels will be rendered visible.
Various other filters and contrast media may be employed. For
example, xenon gas in small quantities may be inhaled by the
patient to render the respiratory system visible.
The X-ray images on the intensification screen 22 are converted
into video image signals by a television system 40, including a
television camera 42 which may be fo conventional construction. The
first and second X-ray images, produced by the use of the first and
second filters 26a and 26b, produce first and second video signals
which contain small differences corresponding to the differences
between the X-ray images.
The video signals from the TV camera 42 are amplified by a video
amplifier 44 and are supplied through a video switch or gate 46 to
a video difference detector 48. The video switch 46 is controlled
by pulse signals received over a control line 46a so that the video
signals will be supplied to the video difference detector 48 on a
selective basis. The video difference detector is also under the
selective control of voltage pulses or signals supplied by a
control line 48a.
The first and second video image signals are supplied sequentially
to the video difference detector 48, which is constructed and
arranged to produce an output corresponding very closely to the
difference between the first and second video image signals. Thus,
the differential features of portions of the video image signals
are enhanced, while the identical or non-differential features are
cancelled or nearly so.
The first differential video signal, as thus produced by the video
difference detector 48, is supplied through another electronic
video switch or gate 50 to an integrating subtraction and storage
device 52. The video switch 50 is under the selective control of
pulses received over a control line 50a. Similarly, the storage
device 52 is selectively controlled by pulses or voltages received
over a control ine 52a.
When the first differential video signal is supplied to the storage
device 52, an electronic image corresponding to such signal is
written and stored in a positive sense in the storage device
52.
It will be recalled that the first differential video signal
represents the difference between the first and second video image
signals which are supplied sequentially to the video difference
detector 48. A second differential video signal of opposite phase
is then produced by reversing the sequence so that the second video
image signal is followed by the first video image signal, as
applied to the video difference detector 48.
Putting this another way, the first and second video differential
signals are sequentially produced by supplying the first video
image signal, followed by the second video image signal, followed
by the reapplication of the first video image signal.
After the first video difference signal has been written positively
in the storage device 52, the second video differential signal is
written negatively therein. Due to the opposite phase of the first
and second signals, the differential portions of the signals are
integrated by the storage device 52, while the small remaining
non-differential portions are subtractively combined. Thus, a
further cancellation of the non-differential portions is produced
by the storage device 52. In this way, the storage device 52 still
further enhances the contrast between the differential and
non-differential portions of the video signals.
The storage device 52 is read to produce an output video signal
representing the integrated and subtracted image stored in the
storage device. This output video signal is supplied to a
television monitor 54 which produces a visible display of the image
stored in the storage device 52. In such image, the differential
features of the periodically variable X-ray images are greatly
enhanced so that they become clearly visible, even though they may
have been scarcely visible or even invisible in the original X-ray
images.
The control pulses or signals for controlling the operating
sequence of the system 10 may be supplied by a control pulse
generator 56. The previously mentioned control lines 46a and 50a
are illustrated as extending from the control pulse generator 56 to
the electronic video switches or gates 46 and 50.
The vertical synchronizing pulses for the television system 40 are
supplied to the control pulse generator 56 for synchronizing
purposes over a control line 58 extending from the vertical sync
pulse generator 60. The sync pulses are also supplied to the sweep
generator 62 of the TV system. Such sweep generator 62 supplies the
sweep or scanning signals to the TV camera 42, the video difference
detector 48 and the integrating subtraction and storage device
52.
The control pulse generator 56 is also synchronized with the
rotation of the X-ray filter disc 26. For this purpose, a control
line 64 extends between the cam-operated switch 36 and the control
pulse generator 56.
In the illustrated system 10, the video difference detector 48 and
the integrating subtraction and storage device 52 are controlled by
supplying different operating voltages thereto. For this purpose,
the system 10 may include a power supply 66. In this case, the
power supply 66 has outputs to supply 6, 20, 30, 50 and 340 volts.
It will be understood that these voltages may be varied and that
the above-mentioned voltages are cited merely by way of
example.
Means are preferably provided to switch or key the operating
voltages so that they may be selectively supplied to the control
lines 48a and 52a leading to the video difference detector 48 and
the integrating subtraction and storage device 52. In the
illustrated system 10, such means take the form of electronic
switches or gates 68, 70, 72, 74, 76 and 78. As shown, the
electronic switches 68, 70, 72, 74 and 76 are connected to the 6,
20, 30, 50 and 340 volt outputs of the power supply 66,
respectively. The electronic switch 78 is also connected to the 340
volt output. The control line 48a for the video difference detector
48 is connected to the electronic switches 72 and 76, which thus
control the supply of 30 volts and 340 volts to the detector 48.
The control line 52a for the integrating subtraction and storage
device 52 is connected to the electronic switches 68, 70, 74 and
78, which thus control the supply of 6, 20, 50 and 340 volts to the
storage device 52.
The electronic switches 68, 70, 72, 74, 76 and 78 are preferably
controlled by pulses or signals supplied by the control pulse
generator 56. Such control pulses may be supplied over control
lines 68a, 70a, 72a, 74a, 76a and 78a. Further details of the
control pulse generator 66 will be given presently.
FIG. 2 illustrates an electronic storage tube 80 which may be
employed in the video difference detector 48 of FIG. 1. It will be
understood that other difference detecting devices may be
employed.
The storage tube 80 is of a type which has been used as a moving
target indocator for radar systems or other surveillance systems.
Tubes of this type are manufactured by Princeton Electronic
Products, Inc. and Hughes Aircraft Company.
As shown, the storage tube 80 is employed in an operating circuit
including a video input line 82 and a video output line 84. The
video signals from the TV camera 42, arriving through the amplifier
44 and the video switch 46, are applied to the input line 82.
During the first TV frame, similar video signals appear on the
ouput line. However, the magnitude of the output video signals
decreases with each passing frame, as the tube 80 comes to
equilibrium. Any changes in the input video signals are transmitted
with full magnitude, but the unchanged or non-differential portions
of the video signals are largely cancelled out.
The first storage tube 80, as illustrated in FIG. 2, has a spectial
target 86 but otherwise may be similar in construction to a
conventional vidicon cathode ray camera tube used in TV cameras.
The target 86 is scanned by an electron beam or cathode ray,
produced by a conventional electron gun 88, which may include a
cathode 88K and three grids 88G1, 88G2 and 88G3. The input line 82
is preferably connected to the cathode 88K, and also preferably to
the first and second grids 88G1 and 88G2. Thus, the electron beam
is modulated by the video input signals.
Means are provided to deflect the electron beam in the storage tube
80. Either magnetic or electrostatic deflection may be employed.
For illustrative purposes, the storage tube 80 is shown as having
horizontal and vertical deflection plates 90 and 92, which may be
supplied with sweep signals from the TV sweep generator 62 of FIG.
1. However, magnetic deflection coils can be employed instead of
the deflection plates. One or more magnetic focusing coils may also
be employed.
The target 86 of the first storage tube 80 may take the form of an
electrically conductive backplate or signal plate 94 having a thin
dielectric layer or facing 96 thereon. A collector electrode 98 is
provided adjacent the target 86.
The conductive backplate 94 may be made of doped silicon, while the
dielectric facing 96 may comprise a thin layer of silicon dioxide
(SiO.sub.2) grown thereon. The dielectric layer 96 is adapted to be
charged electrostatically by the electron beam so that
electrostatic television images can be written electrostatically on
the layer 96 by the electron beam.
In the illustrative arrangement of FIG. 2, a load in the form of a
resistor 100 is connected between the backplate 94 and the control
line 48a, to which different power supply voltages may be applied
by the electronic switches 72 and 76. If desired, a coupling
capacitor 102 may be connected between the backplate 94 and the
video output line 84.
There is capacitive coupling only between the charged front surface
of the dielectric layer 96 and the backplate 94.
The first X-ray image, produced, for example, with the cerium
filter 26a, is converted into video signals by the TV camera 42 and
may be written in the form of electrostatic charges on the
dielectric layer 96 of the target 86 in the storage tube 80. During
the first television frame, the electron beam distributes charges
on the dielectric layer 96, corresponding to the video signals. Due
to the capacitive coupling through the thin dielectric layer 96,
the charging of the layer 96 produces displacement currents to the
backplate 94 through the load resistor 100 so that video signals
are supplied to the output line 84. During subsequent frames, a
state of equilibrium tends to be established between the video
voltages on the cathode 88K and the voltages due to the charges on
the front surface of the dielectric layer 96. As the state of
equilibrium develops, the charging currents along the electron beam
tend to drop to zero so that the video output currents also tend to
drop to zero. Thus, the stable or non-differential portions of the
video signals tend to be cancelled out.
Before full equilibrium is established, the first X-ray image is
replaced with the second X-ray image by switching from the cerium
filter 26a to the iodine filter 26b. Due to the resulting change in
the energy of the X-rays, there is generally some change in the
X-ray image so that the second image differs slightly from the
first image, particularly in areas where a contrast agent, such as
iodine, may be present.
The changes in the video signals on the cathode 88K of the first
storage tube 80 produce changes in the charges on the front surface
of the dielectric layer 96. The changes in the charges produce
corresponding video output signals on the blackplate 94 and the
output line 84.
Thus, the first storage tube 80 produces differential video output
signals corresponding to the differential features, as between the
first and second X-ray images. The non-differential features are
largely cancelled out. Because full equilibrium was not achieved in
the writing of the first video image, the cancellation is not
complete. However, it is desirable to avoid reaching full
equilibrium, because with only partial equilibrium it is possible
to detect video signal changes which are both positive and negative
in sign.
The first storage tube 80 produces a great enhancement of the
differential portions of the video signals, corresponding to the
differential features of the X-ray images. The differential video
signals from the first storage tube 80 are integrated by the
integrating subtraction and storage device 52.
The storage device 52 may comprise a second electronic storage tube
104, as shown in FIG. 3. While various cathode ray storage tubes
may be employed, the illustrated tube is of the silicon storage
type having a special target 106 comprising an electrically
conductive backplate or signal plate 108 with a moasic 110 thereon
of dielectric islands. Preferably, the backplate is made of doped
silicon, while the mosaic 110 comprises islands of silicon dioxide
(SiO.sub.2) selectively grown thereon.
Aside from the target 106, the second storage tube 104 may be
similar to a conventional vidicon cathode ray camera tube as used
in television cameras. A collector electrode 112 is provided
adjacent the target 106.
The mosaic on the target 106 is scanned by an electron beam or
cathode ray produced by a conventional electron gun 114 having a
cathode 114K and three grids 114G1, 114G2 and 114G3. Either
magnetic or electrostatic deflection may be employed. In this case,
deflection coils 116 are provided to produce magnetic deflection.
Alignment coils 118 may also be provided.
The differential video signals from the first storage tube 80 may
be supplied to the second storage tube 104 by way of the output
line 84, the video switch 50, and an input line 120, which in this
case is connected to the first grid 114G1 of the second storage
tube 104. Thus, the electron beam current is modulated by the
differential video signals.
The output of the storage tube is preferably derived from the
backplate 108 of the target 106. Thus, the backplate 108 is coupled
to an output line 122, preferably through a coupling capacitor 124.
In the illustrated arrangement, a load in the form of a resistor
126 is connected between the backplate 108 and the control lead 52a
of FIG. 1. It will be recalled that the various power supply
voltages are supplied to the lead 52a by the electronic switches
68, 70, 74 and 78.
The second storage tube 104 is employed to write electrostatic
images on the mosaic 110 of the target 106 corresponding to the
differential video signals from the first storage tube 80 by
applying such differential video signals to the input line 120,
which transmits the signals to the first grid 114G1 of the storage
tube 104. The images can be written in either a positive or a
negative sense, depending upon the voltage which is supplied to the
backplate 108.
The ability to write either positively or negatively is shown by
the characteristic curve of FIG. 4, in which the secondary emission
coefficient of the target island mosaic 110 is plotted as a
function of the target island mosaic voltage. When the electron
beam impinges upon the target islands of the mosaic 110, secondary
electrons are emitted by the target islands in increasing numbers
with increasing target island voltage.
As plotted in FIG. 4, the secondary emission coefficient in the net
number of secondary electrons emitted for each primary electron
supplied by the electron beam. When the coefficient is greater than
zero, the electron beam writes images with positive charges on the
target mosaic 110 because each primary electron from the electron
beam causes the emission of more than one secondary electron from
the target island mosaic 110. When the coefficient is negative, the
electron beam writes images with negative charges because each
primary electron causes the emission of less than one secondary
electron on the average. The backplate voltage at which the
coefficient is zero may be called the crossover voltage. Above
crossover, which is about 30 volts for the characteristic curve
shown in FIG. 4, the electron beam causes a net deposit of positive
charges on the islands of the mosaic 110. Below crossover, the
electron beam causes a net deposit of negative charges.
While the differential video signals produced by the first storage
tube 80 can be written on the target 106 of the second storage tube
104 in either positive or negative charges, it is preferred to
write the differential video signals in positive charges. This is
done by applying the differential video signals to the input line
120, while maintaining the target voltage above the crossover
voltage. For example, a target voltage of 50 volts is employed in
this case and is supplied to the backplate 108 by the electronic
switch 74. The control pulse generator 56 supplies a pulse over the
control line 74a to the electronic switch 74 to actuate it so that
the 50 volt output of the power supply 66 will be connected to the
line 52a leading to the backplate 108 of the second storage tube
104.
The electrostatic image stored on the target mosaic 110 of the
second storage tube 104 can be read by applying a low voltage to
the backplate 108, while scanning the target mosaic 110 with the
electron beam. This reading procedure produces video signals on the
backplate 108 because the electrical charges on the target mosaic
110 modulate the electron beam as it is caused to scan the target
mosaic. In this case, for example, a backplate voltage of 6 volts
is employed as the reading voltage. It will be understood that this
voltage may be varied over a considerable range. When it is desired
to read the image stored on the target mosaic 110 on the second
storage tube 104, the 6 volt output of the power supply 66 is
connected to the control line 52a by the electronic switch 68 in
response to a control pulse supplied over the line 68a by the
control pulse generator.
During the reading operation, the video signals from the backplate
108 of the second storage tube 104 are supplied to the television
monitor 54 over the output line 122. The visible image produced by
the monitor 54 corresponds to the difference between the first and
second X-ray images produced by the use of the different filter
elements 26a and 26b.
It will be recalled that the difference between the first and
second video signals, corresponding to the first and second X-ray
images, is produced by supplying the first and second video signals
sequentially to the first storage tube 80. After this difference
has been obtained, it is preferred to reapply the first video
signals to the input line 82 of the first storage tube 80 so as to
obtain the inverse difference. Thus, the sequential application of
the second and first video signals to the first storage tube 80
results in the production of second differential video output
signals on the output line 84 of the first storage tube 80. The
phase of the second differential video signals is opposite from the
phase of the first differential video signals. Thus, a positive
going differential feature in the first differential video signals
will be replaced by a negative going differential feature in the
second differential video signals, and vice versa.
Inasmuch as the first differential video signals were written in a
positive sense on the target mosaic 110 of the second storage tube
104, it is preferred to write the second differential video signals
in a negative sense, by reducing the backplate voltage to a value
below the crossover voltage, while modulating the electron beam
with the second differential video signals. In this case, for
example, a power supply voltage of 20 volts is supplied to the
backplate 108 by the electronic switch 70 over the supply line 52a
in response to a control pulse supplied by the control pulse
generator 56 over a control line 70a. Due to the low backplate
voltage, the electron beam of the second storage tube 104 writes
the second differential video signals as negative charges, which
tend to neutralize or cancel the previously written positive
charges. Thus, the non-differential features of the second video
signals tend to cancel out the corresponding non-differential
features of the first video signals because these non-differential
features are approximately equal and opposite in sign.
However, the differential features in the second differential video
signals, being opposite in phase from the corresponding
differential features of the first differential video signals, do
not have the same tendency to neutralize or cancel the previously
written signals. Instead, the differential features, as stored on
the target mosaic 110, are enhanced in contrast and visibility
relative to the background of non-differential features.
This cycle may be repeated as many times as desired to produce
progressive integration of the differential features as stored on
the mosaic 110 of the second storage tube 104. With each cycle, the
contrast between the differential features and the non-differential
features is enhanced. Generally, the full contrast provided by the
television system is achieved after 2 to 50 cycles, depending on
the initial contrast of the differential feature between the first
and second X-ray images. With the system of the present invention,
an X-ray image differential amounting to loss than 1% can be
integrated and enhanced to full contrast.
It will be understood that in each cycle the first and second video
signals, corresponding to the first and second X-ray images are
supplied to the input line 82 of the first storage tube 80, which
then produces first differential video signals at the output line
84. These differential video signals are then written as positive
charges distributed on the target mosaic 110 of the second storage
tube 104 with the backplate voltage above crossover. The first
video signals are then reapplied to the input line 82 of the first
storage tube 80, which produces the second differential video
signals on the output line 84. The second differential video
signals are written as negative charges on the target mosaic 110 of
the storage tube 104 with the backplate voltage below crossover. By
this cycle, the differential features of the X-ray images are
enhanced, while the stable or non-differential features are largely
cancelled out.
When it is desired to detect very small contrast changes in the
X-ray images, the signal-to-noise ratio of the input video signals
becomes a limiting factor. With a repetitive periodic contrast
change, the difference detection process can be repeated several
times and the resulting difference signals can be integrated on the
second storage tube 104 so as to improve the signal-to-noise
ratio.
In detecting very small contrast changes, it has been found that an
account must be taken of the fact that the thermionically emitted
cathode electrons in the first storage tube 80 have an inherent
energy distribution following the Boltzman statistical principles.
Due to this factor, it is undesirable to allow the first storage
tube 80 to reach a state of equilibrium between the dielectric
target layer and the cathode because any small new voltage change
would be buried under the Boltzman distribution and would thus be
invisible. Therefore, it is advantageous to introduce the second
video signals corresponding to the second image before equilibrium
is reached. Due to the fact that equilibrium is not reached, there
is a residual signal in addition to the difference signal. In the
second storage tube 104, the difference signal is integrated, but
the residual or direct current signal is subtracted or
cancelled.
In this regard, the detailed procedure or mode of operation of the
system is illustrated in FIG. 5. As illustrated, the mode of
operation involves a four-step cycle. FIG. 5 illustrates the first
four-step cycle and part of the second cycle. It is generally
desirable to use from 2 to about 50 cycles, depending upon the
specific application.
As illustrated in the diagrams of FIG. 5, Images number 1 and
number 2 are identical with the exception of an additional white
bar in Image number 2. During Step I, which may comprise one
television frame, the first storage tube 80 of the video difference
detector is cleared by applying a high voltage, such as 340 volts,
to the backplate 94. This voltage is applied by the electronic
switch 76 in response to a pulse from the control pulse generator
56. The application of the high voltage causes the electron beam to
cover the dielectric layer with a uniform charge.
During the first and second steps of only the first cycle, the
second storage tube 104 is primed and erased as will be evident
from Parts IIA and IIB of FIG. 5. The priming step involves
applying a high voltage, such as 340 volts, to the backplate 108.
The electron beam then distributes positive charges over the entire
mosaic 110. The high voltage is applied to the backplate 108 by the
electronic switch 78 in response to a pulse from the control pulse
generator 56. The erasing step involves the application of a
voltage substantially below crossover to the backplate 108. The
electron beam then progressively removes the positive charges and
distributes negative charges. In this case, the low voltage is
derived from the 20 volt output of the power supply and is supplied
to the backplate 108 by the electronic switch 70 in response to a
control pulse from the control pulse generator 56.
In the second step, Image number 1, serving as a reference image,
is written on the video difference tube 80, as indicated in Parts
IB, IC and ID of FIG. 5. For this operation, the high voltage of
340 volts is reduced to a low voltage, such as 30 volts, on the
backplate 94. This low voltage is applied by the electronic switch
72, in response to a pulse from the control pulse generator 56. The
video switch 46 is actuated by a pulse from the control pulse
generator so that the video signals are supplied to the cathode and
first grid of the difference detector tube 80.
Part IE of FIG. 5 illustrates the output video waveform at the
backplate of the difference tube 80 as Image number 1 is written.
It will be seen that the magnitude of the video signals decreases
exponentially as a state of equilibrium is approached. The video
output signal does not go to zero because full equilibrium is not
achieved.
During the third step, involving television Frame 7, the second
video signals, corresponding to X-ray Image number 2, are supplied
to the cathode and the first and second grides of the video
difference detector tube 80, as will be evident from Parts IC, ID
and IE of FIG. 5. As a result, a large video differential output
corresponding to the white bar and some residual signal from Image
number 1 is produced at the output of the difference tube 80. This
video differential signal is shown in Part IE, Frame 7.
As will be evident from Part IIC of FIG. 5, this differential video
signal is supplied to the grid of the second storage tube 104 and
is written on the mosaic 110 of the tube in positive charges. The
video switch 50 is actuated by a pulse from the control pulse
generator. The positive writing is achieved by applying a voltage
above crossover to the backplate 108. As illustrated, such voltage
is derived from the 50 volt output of the power supply 66 and is
supplied to the backplate 108 by the electronic switch 74 in
response to a control pulse from the generator 56.
In Step 4, the video input signals corresponding to Image number 1
are reapplied to the video difference detector tube 80, as
indicated in Parts IC and ID of FIG. 5. As a result, the output
video waveform shown in Part IE contains a second difference signal
which is phased oppositely from the first difference signal. This
difference signal is applied to the grid of the second storage tube
104, as illustrated in Part IIC, Frame 9, and is written negatively
by applying a voltage below crossover to the backplate 108. Such
voltage may be derived from the 20 volt output of the power supply
66 and may be supplied to the backplate 108 by the electronic
switch 70 in response to a control pulse from the generator 56. The
video switch 50 is actuated by the control pulse generator 56.
It will be understood that by this procedure the differential
portions of the video signal are integrated and enhanced on the
screen of the second storage tube 104, while the non-differential
or residual portions are largely cancelled out. This will be
evident from Part IID of FIG. 5, Frames 10-12.
When the second storage tube 104 is not being primed, erased or
written, the second storage tube is switched to a reading mode by
applying a low voltage, such as 6 volts, to the backplate 108, as
will be evident from Parts IIA and IIB. This reading voltage is
applied by the electronic switch 68 in response to control pulses
from the generator 56. The stored video signals corresponding to
Image number 1 are read in Frame 8 of Part IID. The enhanced and
subtracted video signals are read in Frames 10, 11 and 12 of Part
IID.
Reverting to Frame 9, Part IE of FIG. 5, it will be seen that the
residual image is still in the output of the video difference
detector tube 80, but, in addition, there is a small dip at the
location of the white bar. This is due to the fact that the white
bar introduced in Frame 7 tended to drive the dielectric of the
difference detector more negative than the surrounding elements.
This is beneficial because a "black" bar written negatively on the
silicon storage tube is equivalent to subtracting less charge from
the target on the negative write step and thus makes the final
white bar more prominent with respect to the surrounding
regions.
In the second and subsequent cycles, all four steps (with the
exception of the priming and erasing of the silicon storage tube
104) are then repeated. Due to the integrating effect of the tube
104, an increasing difference signal is stored on the mosaic 110 of
the tube.
* * * * *