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BACKGROUND OF THE INVENTION
This application pertains to the art of video display devices and more
particularly to video display devices used to display readout images from
medical diagnostic equipment. The invention is particularly applicable for
displaying the pictures derived from data obtained by scintillation
cameras also commonly referred to as radioisotope, gamma, nuclear or
Anger-type cameras. Although the invention will be described with
particular reference to scintilltion cameras, it will be appreciated that
the invention has much broader application.
A persistence scope is a cathode ray tube on which excited regions of
phosphor persist or remain luminescent for an extended period of time
before fading into darkness. Heretofore, the persistence of luminescing
regions on the screen of cathode ray tubes has been achieved through the
use of a phosphor which continues to luminesce after the electron beam
excitation has been removed. Normally, the rate of fading is determined by
the phosphor and is adjusted by controlling the amount of excitation
energy in the electron beam.
One of the prior art problems has been the lack of ability of control
differences in displayed brightness or intensity. It is an inherent
limitation of persistence cathode ray tubes that only three or four
discernible shades of grey are displayable. The lack of distinctive levels
of shading make output data relatively difficult to read and interpret. In
scintillation cameras, for example, differences in intensity are important
indicia to the medical technician of variation in radioisotope
concentration. It is desirable to be able to determine these
concentrations accurately.
Another problem with the prior art persistence screens is size and cost.
Persistence screens are costly to manufacture even in small sizes and cost
increases rapidly with size. Commonly, only relatively small persistence
screen sizes are available rendering interpretations of the display
relatively inaccurate.
Further, persistence cathode ray tubes are electronically complex. This
complexity results in unreliability and difficulty of adjustment. It is
not uncommon for prior art persistence scopes to fail at a rate
approaching thirty percent per year.
SUMMARY OF THE INVENTION
The present invention contemplates a new and improved persistence type
display which overcomes all of the above referred problems and provides a
display which is large, easy to read, reliable and economical.
In accordance with the present invention, there is provided a persistence
scope in which the retention and fading of the displayed images are
controlled electronically.
In accordance with a more limited aspect of the invention, there is
provided an electronic storage matrix, a video monitor for displaying the
contents of the storage matrix, and supportive electronic circuitry for
adding and removing data from the contents of the storage matrix. Input
data such as from a scintillation camera is stored in the storage matrix
for display on a video monitor. The electronic circuitry at a variable
rate deletes information from the storage matrix. As information is
deleted and no longer displayed on the video monitor, the video image
fades with the appearance of a persistence phosphor.
In accordance with a more limited aspect of the invention, electronic
circuitry removes or fades the data at various rates consistent with the
rate of input data acquisition such as linearly, exponentially or in
various other selectable modes.
In accordance with yet another aspect of the invention, the input data to
the storage matrix may be weighted such as logarithmically to increase the
dynamic range of the persistence scope.
One major advantage of the invention is the improved output display. Not
only can a larger display be obtained but one with greater definition. The
video display monitor can display numerous shades of grey. Rather than the
three or four shades of grey available on a conventional CRT persistence
scope, the present invention can produce sixteen or more discernible
shades of grey.
Another advantage of the present invention is the use of digital
electronics. This simplifies and reduces the number of adjustments,
increases the reliability and simplifies the interfacing with other
digital electronic equipment such as video tape recorders, enhancement
processing apparatus, etc.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention may take physical form in certain parts and arrangements of
parts, a preferred embodiment of which will be described in detail in the
specification and illustrated in the accompanying drawings which form a
part thereof.
THE FIGURE is an overall system view of a persistence scope in accordance
with the present invention in combination with a scintillation camera.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the drawings wherein the showings are for the purposes
illustrating a preferred embodiment of the invention only and not for
purposes of limiting it. The figure shows a scintillation camera A in
conjunction with an electronic video persistence scope including
electronic persistence and control circuitry B and a video monitor C.
Scintillation cameras are conventional nuclear medicinal diagnostic
devices. A scintillation camera such as the one shown in U.S. Pat. No.
3,011,057 to Anger or in U.S. Pat. No. 3,911,278 to Stout are exemplary of
scintillation cameras which may be used.
In using a scintillation camera, a medical technician commonly injects a
fluid containing one or more radioisotopes into the patient's blood. After
a time, the technician monitors the distribution of each radioisotope in
the patient for such purposes as monitoring blood flow or the propensity
of organs or tissue to absorb isotope containing components of the fluid.
The scintillation camera is positioned adjacent the patient in such a
position that radioactive emissions from the isotope impinge upon it. The
scintillation camera includes a flat scintillation plate, any position on
which can be described by an x,y coordinate. Each time a radiation
emission impinges on the scintillation plate it fluoresces with a
characteristic intensity. Electronic circuitry in the camera senses the
fluorescence and as described in the Stout and Anger patents, supra,
produces analog signals inditing the x and y coordinates of the
scintillation. A third signal, commonly called a z signal indicates the
intensity of the scintillation. By comparing the z signal with one or more
known references, the camera can determine if each pair of x,y coordinate
signals represents a plurality of simultaneous flashes, or stray
radiation, or for some other reason is undesirable. This comparison
produces an unblanking signal from the camera to enable only desirable x,y
coordinate signals to be displayed. The unblanking signal can also be used
to indicate which of plural isotopes the scintillation represents.
In the past, each analog pair of x,y coordinate signals was conveyed to an
oscilloscope-type cathode ray tube, with a persistence phosphor. If an
appropriate unblanking signal was also produced, the electron beam of the
cathode ray tube would excite a small region or dot of the phosphor at a
location corresponding to the x,y coordinate represented. This small dot
of phosphor fluoresces for a period of time gradually decreasing in
intensity until it goes black.
In the present invention, a TV-type video monitor is used with a phosphor
which quickly stops fluorescing, sometimes called a short afterglow. To
retain the benefits of persistence phosphor effect, the present invention
includes circuitry which causes the luminescence of the video monitor to
appear to fade. This fading is achieved by exciting a region of the
phosphor corresponding to a x,y coordinate a plurality of times, each
successive time with less energy.
Looking now to THE FIGURE, the preferred embodiment will first be described
with a broad overview. As discussed above, a scintillation camera A
produces x coordinate, y coordinate and unblanking signals. A persistence
circuit B receives these signals and causes the signals to produce a
persistence scope display on a video monitor C.
Persistence circuit B includes a storage means D and new data input means
E. Each time a valid coordinate signal is received by persistence circuit
B, the new data input means E records in storage means D that an
additional coordinate signal was received for that location. Thus, the
storage means retains a record of how many scintillations are received
corresponding to each x,y coordinate.
The video monitor C cyclicly monitors the record retained in the storage
means. As the electron beam of the video monitor scans past the location
on its face corresponding to each x,y coordinate, it causes the phosphor
to fluoresce with an intensity corresponding to the number of coordinate
signals recorded in storage means D at an address corresponding to the
displayed x,y coordinate.
A persister F is by definition a means for removing information relating to
the receipt of coordinate signals from storage means D. In effect, the
persister causes the storage means to "forget" that some coordinate
signals were received.
Thus in operation, the new data input means E with the receipt of the each
coordinate signal builds up the stored record in storage means D of the
number of coordinate signals received corresponding to each x,y
coordinate. The persister to the contrary decreases the stored record of
the number of coordinate signals received corresponding to all coordinate
positions.
A control means G controls the rate at which the persister F removes
information from the storage means. The rate can be selected to be
proportional to the rate at which unblanking signals are generated by the
camera--, i.e. the rate of radiation impinging on the camera from the
injected isotope. Or the operator may select the rate as a function of
time alone.
Looking to the details of persistence circuit B, the output of the
scintillation camera is conveyed electronically such as by line 10 to its
input. Connected with the input is an analog-to-digital converter 12 for
converting the analog x and y coordinate signals into digital
representations. This conversion is enabled by the presence of an
appropriate unblanking signal. The number of bits in the digital
representations is, of course, a function of the degree of accuracy with
which the x and y coordinates are to be identified. It has been found that
a seven bit binary analog-to-digital converter is satisfactory with
scintillation cameras. The digital x and y coordinate representations may
be held temporarily in a latch 18.
The digital x and y coordinate representations from the analog-to-digital
converter forms the input of an address selection means 20. This address
selection means, such as address multiplexer, derives from the digital x
and y coordinate representation a corresponding address in the storage
means D.
The storage means in the preferred embodiment is a storage matrix 24 with a
plurality of addresses. There is an address corresponding to each digital
x,y coordinate representation generated by the analog-to-digital
converter. For a seven bit analog-to-digital converter, i.e. 2.sup.7
possible x values and 2.sup.7 possible y values, 2.sup.14 address should
be available. Further, the storage matrix must be able to count the number
of coordinate signals received corresponding to each address. Seven bits
of storage per address has been found satisfactory. Accordingly, a
128.times.128.times.7 storage matrix may be used to provide the above
storage.
Each time the address selecting means 20 addresses one of the addresses of
the storage matrix 24, the value stored at the selected address is fed out
of the storage means on line 26. This output line is connected to the new
data input means E which includes modifying means 30 for modifying the
value. Modifying means 30, in the preferred embodiment, is a means for
increasing the stored value through addition, multiplications, etc. As
will be explained below, several methods of modifying this value may be
used. In a first embodiment, modifying means 30 is a digital adder. Each
time the receipt of new data causes the storage matrix to be addressed,
the value stored at that address is conveyed by line 26 to modifying means
30 where a constant is added to it. A convenient constant has been found
to be the integer 4. The new increased value with the constant added is
transferred to a data input multiplexer 34. Upon receiving a control
signal, such as from address selection means 20, that the storage matrix
was addressed for purposes of adding new incoming data, multiplexer 34
returns the new increased value to the storage matrix and substitutes it
for the value at that address.
To generate the video display, a video address and synchronization
generator 40 cyclicly causes the values stored at each address in storage
matrix 24 to be marshalled onto line 26. A display processor 42 translates
each value on line 26 into a corresponding shade of grey for the video
monitor. For example, by working with the four most significant bits of
each value, sixteen shades of grey may be designated, however, more or
fewer shades may be generated. The video processor may also perform such
functions as averaging values corresponding to adjacent x or y coordinates
to generate interpolated data for the video monitor.
The digital representation from the display processor of the shade of grey
which corresponds to the addressed value is converted to an analog signal
by digital-to-analog converter 44.
Video address and synchronization generator 40 also coordinates the
scanning of the electron beam of the video monitor with the addressing of
the storage matrix. As the grey tone for each value emerges from converter
44, the generator 40 causes the electron beam of the video monitor to be
aimed at a position corresponding to the x,y coordinate corresponding to
the address from which the value producing the grey tone was obtained. The
synchronization of the scanning is in the form of a composite sync signal
which is added with the grey scale analog value by an analog summer 46.
This sum forms a composite video signal for controlling video monitor C.
One way of coordinating the acquisition of new data with the display of old
data, is for the generator 40 to allow a short time interval between video
line scans. During this interval, new data which was temporarily held
during the line scanning in latch 18 or a sample and hold circuit is added
to the values in the storage matrix.
As data values are marshalled onto line 26 for video display, each value is
also received by persister F. Persister F may take various forms for
altering the magnitude of each value according to various functions. The
persister includes means 50 for altering the values. In the preferred
embodiment, altering means 50 reduces the values, such as by subtraction,
multiplying by a fraction, dividing, etc.
The reduced value from the output of the altering means 50 is conveyed to
data input multiplexer 34. Multiplexer 34 returns the diminished value
from the persister to the corresponding address, if signaled by address
selection means 20 that the storage matrix was addressed for circulating
values to the video monitor. If the storage matrix were addressed for new
data input multiplexer 34 would, of course, return the values from
modifying means 30 to the storage matrix.
In the first embodiment of the new data input E in which it adds the
constant 4 to the value for each new input signal, persister F subtracts
the constant 1 from each value as it is circulated to the video monitor.
This enables even a stored value indicative of only a single event, to be
attenuated over several passes of the data through the video monitor.
As alluded above, there are several alternate implementations for handling
the data. For example, the persister F may be implemented with a
multiplication means, such as a digital multiplier which multiplies each
value by a fraction less than 1, e.g. 0.9. In this implementation, the
values are each lowered by a preselected percentage in each pass of the
data. This achieves negative exponential attenuation of the stored values.
An advantage of an exponential attenuation is that addresses at which the
stored values are increasing rapidly have their values attenuated rapidly.
Addresses at which the stored values are increasing slowly have their
values attenuated more slowly. Thus, regions with a high rate of data
acquisition attenuate more rapidly than regions with a low rate of data
acquisition.
As another alternative, the persister may be implemented with a read-only
memory (ROM). Each value on line 26, acts to address the ROM, in response
to which the ROM conveys to multiplexer 34 a value corresponding to that
ROM address. The ROM implementation can be used to achieve a linear
attenuation, exponential attenuation, or other nonlinear or partially
linear attenuations determined by the programming of the ROM.
The new data input means E may alternately be implemented with a
multiplication means, such as a digital multiplier, which multiplies each
value by a preselected constant greater than 1. As yet another
alternative, new data input means may be implemented with a read-only
memory (ROM). Each stored value acts as an address to the ROM causing a
larger value to be generated by the ROM for return to that address of
memory matrix 24. The ROM can be programmed to cause the memory matrix to
store the logarithm of the number of scintillations at each x,y
coordinate. To achieve such a logarithmic conversion, the ROM is
preprogrammed to implement a transfer function of the type generally
designated by the formula
1-e.sup.-x.
This logarithmic conversion allows the video persistence scope to display
images varying over a large dynamic range. For example, the same display
has been found capable of representing regions receiving 2000
scintillations per minute and other regions receiving 2,000,000
scintillations per minute, i.e. a 1000:1 dynamic range. For a logarithmic
display in which each grey shade represents a change in the rate of data
acquisition by a factor of 2, ten shades of grey produce a display with a
dynamic range of 1024:1.
The display processor 42 may also transfer stored nonlogarithmic values to
logarithmic representation to improve the dynamic range of the display.
Further, the display processor may cause the video monitor to display
either positive or negative image by displaying the data or (1-data).
A major advantage of the present video persistence scope over a CRT with a
persistence phosphor is the operator's flexibility in controlling the rate
of fading. To control the fade rate, a persistence or fading control means
G is provided. The control means can control the persistence in numerous
manual and automatic ways. These ways include altering the amount by which
the magnitude of the values is reduced in each pass. Alternately, the
percent of passes in which the magnitude is reduced may be altered, that
is, persister could be caused to reduce the magnitude of only some of the
cyclic passes through the storage matrix for producing the video display.
Fade rates in which the intensity is attenuated by 50% in one quarter to
ten seconds are preferred.
In the preferred embodiment, the frequency with which persistence operation
occurs controls the rate of fading. The operator may select either a
manual or automatic mode. In the manual mode, a variable voltage supply 62
is varied by the operator to select the fading rate. The selected voltage
is converted to a frequency proportional to the selected voltage by a
voltage to frequency converter 64. The output from the voltage to
frequency converter 64 triggers a monostable circuit 66 which provides a
series of output pulses in tune with the selected frequency. The pulses
from the monostable circuit 66 are channelled in the manual mode to a
counter 68. Each time counter 68 reaches preselected number it resets
itself and signals a persistence control circuit 60. Thus, circuit
components 62, 64, 66 and 68 form a timing means for periodically
generating a control signal whose rate is controlled by component 62.
Upon receipt of the control signal, persistence control circuit 60 causes
the next full cyclic pass of values from storage matrix 24 to the video
monitor to be decreased by persister F. By adjusting the voltage, hence,
the frequency, the speed or rate with which the counter 68 reaches a
preselected level may be varied. This, in turn, varies the percent of
passes in which the persister is enabled.
In the automatic mode, the rate of persistence or fading is automatically
adjusted as a function of the rate of data accumulation. To achieve this,
the unblanking signals from the scintillation camera and the series of
pulses from the monostable circuit 66 are fed to an AND gate 70. The
output of AND gate 70 is indicative of the coincidence of monostable
pulses and unblanking signals. The higher the data acquisition rate, the
more coincident events occur.
The output of AND gate 70 is connected to a counter means 72 for counting
radiation impingements on the camera. Each time counter 72 reaches
predetermined count, it generates a control signal which causes control
means 60 to enable the persister to decrease the next full cyclic pass of
values from the storage matrix to the video monitor. In this manner, the
rate at which stored values are decreased by the persister is consistent
with and proportional to the rate at which radiation is received by the
scintillation camera A.
The control means G include a variety of operator actuated controls, such
as means for selecting manual or automatic fading, the subtractive,
multiplicative or ROM persistence implementation, a persistence constant,
the mode of new data input, a new data input constant and other operator
controls.
One of the other controls is an erase control 74. The erase control causes
the control means to zero the values at all addresses of the storage
matrix. This may be done by setting the magnitude of all the elements of
the storage matrix to 0 directly or by causing the persister to decrease
each value by 100% such as by using a multiplicative persistence constant
of 0.
Another control function is a freeze mode initiated by a freeze means 78.
The freeze means causes the present stored values to be rendered
inviolate, so that the present display will remain unchanged. One
implementation of the freeze mode is to block the analog-to-digital
converter 12 from digitizing new data and blocking persister F from
reducing present values. Alternately, data input multiplexer 34 may be
blocked from transferring new values from the persister F or new data
input E to the storage matrix. Or the persister and new data input means
can have their modes and constants adjusted so that the incremental
increase for each new coordinate signal is zero and the persistence
decrease is zero.
Examples of circuit chips which may be used for new data input means E and
the persister F include adders designated 74LS83, multipliers designated
74LS284/285, PROM's designated Intel 3624, or numerous other solid state
circuit chips.
It will be appreciated that with alternate logic implementations the
persister may increase the magnitude of the stored values. Likewise, the
new data input may decrease the magnitudes of the stored values.
The invention has been described with reference to the preferred
embodiment. Clearly numerous modifications and alterations will occur to
others upon reading and understanding of this specification. It is our
intent to include all such modifications and alterations insofar as they
come within the scope of the pending claims or the equivalence thereof.
* * * * *
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