U.S. patent number 3,863,073 [Application Number 05/354,826] was granted by the patent office on 1975-01-28 for automatic system for precise collimation of radiation.
This patent grant is currently assigned to The Machlett Laboratories, Incorporated. Invention is credited to Howard G. Wagner.
United States Patent |
3,863,073 |
Wagner |
January 28, 1975 |
AUTOMATIC SYSTEM FOR PRECISE COLLIMATION OF RADIATION
Abstract
A radiographic system comprising a radiation source disposed to
direct a beam of radiation through an adjustable aperture in an
aligned beam-limiting device and onto an image receptor located at
a selected distance from the source, and automatic means for making
a unidirectional final adjustment of the aperture to provide the
beam with a cross-sectional area which conforms to the size of the
image receptor at the selected distance from the source.
Inventors: |
Wagner; Howard G. (New Canaan,
CT) |
Assignee: |
The Machlett Laboratories,
Incorporated (Stamford, CT)
|
Family
ID: |
23395056 |
Appl.
No.: |
05/354,826 |
Filed: |
April 26, 1973 |
Current U.S.
Class: |
378/91; 378/98;
378/150 |
Current CPC
Class: |
G05D
3/20 (20130101); A61B 6/06 (20130101); A61B
6/548 (20130101) |
Current International
Class: |
A61B
6/06 (20060101); G05D 3/20 (20060101); G03b
041/16 () |
Field of
Search: |
;250/511,512,513,514,402,416 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Borchelt; Archie R.
Assistant Examiner: Church; C. E.
Attorney, Agent or Firm: Meaney; John T. Murphy; Harold A.
Pannone; Joseph D.
Claims
1. A radiographic system comprising:
means for directing a beam of radiation from a source through an
adjustable shutter aperture and onto an image receptor installed at
a selected distance from the source; and
automatic means for adjusting the size of the aperture in
accordance with the size of the receptor and the source-to-image
receptor distance, said automatic means including mechanical drive
means for adjusting the shutter aperture and electronic means for
removing hysteresis from the drive
2. A radiographic system as set forth in claim 1 wherein the
automatic means includes receptor detecting means for producing an
electrical signal indicative of an installed image receptor and
source-to-image receptor detecting means for producing an
electrical signal indicative of a
3. A radiographic system as set forth in claim 2 wherein the
automatic means also includes sensing means for producing
electrical signals corresponding to the size of the image receptor,
the source-to-image
4. A radiographic system as set forth in claim 3 wherein the
automatic means includes electrical means for combining the
source-to-image receptor distance signal with the shutter aperture
signal, and producing an output signal corresponding to the
radiation field size at the plane of the image
5. A radiographic system as set forth in claim 4 wherein the
automatic means includes collimator monitoring means for
electrically controlling the mechanical drive means and comprising
a detector monitoring circuit means for receiving the electrical
indicating signals from the receptor detecting means and the
source-to-image receptor detecting means and for
6. A radiographic system as set forth in claim 5 wherein the
detector monitoring circuit means includes gating means for
producing an output signal indicative of an installed image
receptor and a selected
7. A radiographic system as set forth in claim 6 wherein the output
of the gating means is electrically connected to monostable
multivibrating means for producing an electrical pulsed output
signal indicative of changes in
8. A radiographic system as set forth in claim 7 wherein the
collimator monitoring means also comprises sensor monitoring
circuit means for receiving the field size signal from the
electrical combining means, and the receptor size signal from the
receptor sensing means, in addition to the output signal from the
gating means and the output signal from the monostable
multivibrating means in the detector monitoring circuit, nd
producing an electrical drive signal for operating the mechanical
drive
9. A radiographic system as set forth in claim 8 wherein the sensor
monitoring circuit means includes an electrical comparator having
respective input terminals connected to the electrical combining
means and to the receptor size means and means for producing an
electrical signal indicative of the difference between the
radiation field size and the
10. A radiographic system as set forth in claim 9 wherein the
sensor monitoring circuit means includes a flip-flop device having
respective terminals connected to the output of the comparator, the
output of said gating means, the output of said monostable
multivibrating means and having means for latching the flip-flop in
a particular mode of operation.
11. A radiographic system as set forth in claim 10 wherein the
sensor monitoring circuit means includes a transient protection
gating means for producing an electrical signal indicative of the
output of the flip-flop
12. A radiographic system as set forth in claim 11 wherein the
sensor monitoring circuit means includes the means for removing
hysteresis and comprises a one-shot multivibrating means connected
to the output of the transient protection gating means.
Description
BACKGROUND OF THE INVENTION
This invention relates generally to automatic collimating systems
and is concerned more particularly with a radiographic system
having adjustable beam collimating means for protecting a person
from overexposure to radiation.
It is well-known that internal organs of a human body, for example,
may be examined by exposing a preselected region of the body to
radiation, such as X-rays, for example, for a limited period of
time. However, the radiation should be confined to the specific
area of the body under examination in order to minimize exposure of
the patient to the radiation. This objective is best achieved by
precisely regulating the cross-sectional size of the beam
irradiating the area of the body being examined.
One type of X-ray apparatus particularly suitable for achieving
this objective is shown and described in U.S. Pat. No. 3,581,094
granted to L. F. Peyser, et al, and assigned to the assignee of
this invention. The apparatus disclosed therein includes a beam
limiting device or collimator having two orthogonally arranged
pairs of opposing pivotal plates which form a rectangular aperture.
The plates are made of X-ray absorbent material, such as lead, for
example, and serve to regulate the cross-sectional size and shape
of a beam passing through the aperture. In accordance with
electrical signals produced by sensing devices suitably located in
the apparatus, the rectangular aperture is adjusted to provide the
beam with a cross-sectional size conforming substantially to the
rectangular area of an X-ray film. Thus, when a patient is
positioned between the beam limiting device and the X-ray film,
only the portion of the patient's body selected for study should be
irradiated and imaged on the film.
Another type of beam limiting device, which is more suitable for
regulating the diametric size of an X-ray cone is shown and
described in U.S. Pat. No. 3,448,270 granted to L. F. Peyser and
assigned to the assignee of this invention. Briefly, this patent
discloses a beam limiting device having an exit aperture defined by
a thimble-like shutter comprising a plurality of X-ray absorbent
leaves arranged longitudinally in partial overlapping relationship
to form a frusto-conical structure. The leaves are pivotally
mounted and simultaneously adjustable to move into greater or
lesser overlapping relationship thereby defining the diametric size
of a variable aperture at the small diameter end of the
frsuto-conical structure. Thus, this beam-limiting device may be
adjusted to provide a cone of radiation passing through the
structure with the proper diameter for impinging on a circular
image receptor, such as the input screen assembly of an image
intensifer tube, for example.
The described beam-limiting devices generally include means for
adjusting the shutter aperture whereby an emerging X-ray beam is
provided with a cross-sectional size which conforms substantially
to the area of an image receptor. However, recent medical
investigations completed by the United States Public Health Service
indicate that the cross-sectional size of the X-ray beam, at the
plane of the image receptor, should conform even more closely to
the surface area of the image receptor than current practice
permits. Thus, it has been found desirable to provide automatic
means for setting the shutter aperture of the beam-limiting device
in a manner which will comply with the recommendations of the
United States Public Health Service. Automatic systems for
precisely setting the shutter aperture by means of accurate
mechanisms which overcome backlash and other forms of hysteresis
have proved unsatisfactory, because of the undesirable cost
involved and a tendency of the shutters to "hunt" when reaching a
desired setting for the aperture. Attempts have been made to
overcome this tendency to "hunt" by providing a "dead band" which
renders the system insensitive to small amounts of "overshoot" when
the shutters reach the desired setting. However, when the aperture
is being adjusted, and the shutters reach the desired setting, this
small amount of overshoot may result in the X-ray beam having a
cross-sectional area which exceeds the allowed tolerances for
conforming to the surface area of the image receptor.
Thus, it is advantageous and desirable to provide a radiographic
system with a beam-limiting device and means for adjusting the
shutter aperture of the device in a manner which overcomes backlash
and other forms of mechanical and electronic hysteresis without
resorting to prohibitively expensive mechanisms and electronic
controls for achieving the desired accuracy.
SUMMARY OF THE INVENTION
Accordingly, this invention provides a radiation source disposed to
direct a beam of radiation through an adjustable beam-limiting
device and onto an image receptor located at a selected distance
from the source, and automatic means for adjusting the aperture to
limit the beam to a cross-sectional size which conforms accurately
to the size of the image receptor, at the plane of the
receptor.
The automatic means includes detecting means for producing
electrical signals indicative of the installation of an image
receptor in the holder and the selection of a desired
source-to-image receptor distance (SID). The automatic means also
includes sensing means for producing electrical signals indicative
of the image receptor size and the radiation field size at the
plane of the image receptor.
The automatic means also includes a collimator monitoring unit
including a detector monitoring circuit and a sensor monitoring
circuit. The detector monitoring circuit is provided with input
means for receiving electrical signals from the detecting means,
gating means for producing an electrical signal indicative of an
installed image receptor and a selected SID, and one-shot
multivibrating means for producing a predetermined time pulse
signal in response to a signal from the detector gating means.
The sensor monitoring circuit includes comparator means for
receiving electrical signals from the sensing means and producing
an output signal indicative of any adjustment required in the
aperture size, latched gating means for allowing further manual
closing of the aperture but preventing opening of the aperture to a
size that allows the radiation field to exceed the receptor except
in response to output signals from the detector monitoring circuit
and an opening signal from the comparator means, overshoot means
for opening the shutter aperture beyond any corrective adjustment
required by the comparator means and thereby causing closing of the
aperture for a unidirectional final adjustment thereof, and gating
means for determining whether the aperture is to close, open or
whether corrective adjustment is completed.
In one preferred embodiment, an X-ray source is disposed to direct
a generally conical X-ray beam through a substantially circular
aperture formed by a frusto-conical shutter in a beam-limiting
device and onto a circular input screen assembly of an image
intensifier tube located in a suitable holder at a selected
distance from the source. Detecting means are provided for
producing electrical signals indicative of the location of an image
intensifier tube in the holder and the selection of an SID. Sensing
means are provided for producing electrical signals indicative of
the diametric size of the useful portion of the input screen, the
distance of the input screen assembly from the source and the
diametric size of the aperture to limit the diametric size of the
conical X-ray beam such that it conforms to the circular area of
the useful portion of the input screen.
In another preferred embodiment, an X-ray source is disposed to
direct a generally conical X-ray beam through a substantially
rectangular aperture formed by two orthogonally disposed pairs of
opposing plates in a beam-limiting device and onto a rectangular
X-ray film clamped in a suitable holder at a selected distance from
the source. Detecting means are provided for producing electrical
signals indicative of the installation of a film bearing cassette
in the holder and the selection of an SID. Sensing means are
provided for producing electrical signals indicative of the length
and width of the film, the distance of the film from the source and
the length and width of the aperture. The collimator monitoring
unit is provided with a common detector monitoring circuit feeding
output signals into dual sensor monitoring circuits, one for
adjusting the length dimension of the shutter aperture and one for
adjusting the width dimension thereof. Thus, this collimator
monitoring unit provides means for limiting the cross-sectional
size of the X-ray beam such that it conforms to the area of an
X-ray film to within the desired accuracy.
BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of this invention, the following more
detailed description makes reference to the accompanying drawings,
wherein:
FIG. 1 is a perspective view of one type of radiographic system
embodying the invention;
FIG. 2 is a schematic view of the radiographic system shown in FIG.
1;
FIG. 3 is a block diagrammatic view of the basic components
required for practicing this invention in conjunction with the
system shown in FIG. 1;
FIG. 4 is a block diagrammatic view of the invention as adapted for
the system shown in FIG. 1;
FIG. 5 is a schematic diagrammatic view of a typical embodiment of
the invention as shown in FIG. 4;
FIG. 6 is a perspective view of another type of radiographic system
embodying the invention;
FIG. 7 is a schematic view of the radiographic system shown in FIG.
6;
FIG. 8 is a block diagrammatic view of the basic components
required for practicing this invention in conjunction with the
system shown in FIG. 6;
FIG. 9 is a block diagrammatic view of the invention as adapted for
the system shown in FIG. 6; and
FIG. 10 is a schematic diagrammatic view of a typical embodiment of
the invention shown in FIG. 9.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring more particularly to the drawings wherein like characters
of reference designate like parts throughout the several views,
there is shown in FIGS. 1 and 2 a radiographic system 10 comprising
an X-ray generator 12 which includes a hollow cylindrical housing
14 having an X-ray tube 16 longitudinally disposed therein. The
X-ray tube 16 may be of any conventional type embodying an electron
emitting cathode 18 and a spaced opposing anode 20 within the usual
evacuated envelope 22. The electrodes of X-ray tube 16 are
electrically connected to an X-ray control unit 24 by suitable
means, such as an interconnecting cable 25 having therein an
insulated conductor 26 which is connected to the anode 20 and two
insulated conductors 27 and 28 which are connected to respective
terminal ends of the cathode 18.
In operation, the X-ray control unit 24 supplies electrical current
through the conductors 27 and 28 to heat the cathode 18 to a
desired electron emitting temperature. By means of conductor 26 and
one of the conductors 27 and 28, the X-ray control unit 24 applies
suitable electrical potentials to the anode 20 and the cathode 18,
respectively, for the purpose of establishing a strong
electrostatic field therebetween. The electrostatic field
accelerates emitted electrons from the cathode 18 and, in a
well-known manner, focusses them onto the anode 20 in a relatively
small focal spot area 21 which functions as a point source for a
resulting cone 30 of X-radiation emanating therefrom. Thus, it may
be seen that the X-ray control unit 24 may prevent the generation
of X-ray cone 30 by withholding the electrical potentials applied
to the cathode 18 and the anode 20 of X-ray tube 16 by any suitable
means.
The cone 30 of X-radiation generated in tube 16 passes through an
aligned port 15 in the cylindrical wall of housing 14 and enters a
beam limiting device 32 or collimator which may be mounted over the
port 15 by any suitable means. The beam limiting device 32 may
comprise a housing 33 having an entrance aperture (not shown)
adjacent the port 15 of the X-ray generator 12 and an opposing exit
aperture 34 defined by a thimble-like shutter 36 which protrudes
outwardly of the housing. The shutter 36 is more fully described in
the aforementioned U.S. Pat. No. 3,448,270 granted to L. F. Peyser
and assigned to the assignee of this invention. Briefly, the
thimble-like shutter 36 comprises a plurality of longitudinally
extending leaves 37 made of X-ray absorbent material, such as lead,
for example, arranged in partial overlapping relationship to form a
hollow frusto-conical structure. The leaves 37 are pivotally
mounted at the ends adjacent the large diameter opening of the
frusto-conical structure and are operatively connected to a
rotatable ring 38. Rotation of the ring 38 moves the leaves into
greater or lesser overlapping relationship thereby varying the
diameter of the exit aperture 34 at the small diameter end of the
frusto-conical structure. Thus, the thimble-like shutter 36 limits
the diametric size of the X-ray cone 30 as it passes through the
beam-limiting device 32.
Located on the exterior of housing 33 is a knob 40 which may be
manually adjusted to rotate the ring 38 and thereby vary the
diameter of exit aperture 34 in a controllable manner. Thus, the
knob 40 provides means for regulating the cross-sectional size of
X-ray cone 30 when system 10 is operated in the manual mode. The
X-ray cone 30 emerging from the beam limiting device 32 may pass
through an aligned portion 42 of a patient 44 and be modified in
accordance with a density pattern formed by the internal organs of
portion 42, in a well-known manner. Consequently, the modified
X-ray cone 30 will convey an X-ray image of these internal organs
to an aligned image receptor 46 which may comprise an input screen
assembly 48 of an image intensifier tube 50, for example.
A suitable image intensifier tube 50 is shown and described in U.S.
Pat. No. 3,417,242 granted to R. W. Windebank and assigned to the
assignee of this invention. Briefly, the image intensifier tube
disclosed therein comprises a generally cylindrical envelope 52
closed at one end by an input faceplate 54 of preselected diametric
size, and at the other end by an output faceplate 56 which is
considerably smaller in diameter. Disposed adjacent input faceplate
54 is input screen assembly 48 which converts the incident X-ray
image to an equivalent electron image, in a well-known manner. The
electron image is electrostatically accelerated from the input
screen assembly 48 and focussed onto an output screen assembly (not
shown) which is disposed adjacent the output faceplate 56. The
output screen assembly converts the accelerated electron image, in
a well-known manner, into a bright visible image which is viewable
through the output faceplate 56. Thus, the internal organs of
portion 42 may be observed while being subjected to
X-radiation.
The image intensifier tube 50 may be provided with zooming means
comprising a coaxially aligned series of spaced electrodes (not
shown) disposed between the input screen assembly 48 and the output
screen assembly, as disclosed in the aforementioned Windebank
patent. These intermediate electrodes are maintained at respective
variable electrical potentials supplied by a remote control unit 71
having a plurality of potentiometers (not shown) therein which may
be adjusted by a single control knob 73. The potentials applied to
the intermediate electrodes shape the electrostatic field between
the input screen assembly 48 and the output screen assembly such
that only a portion of the input screen fills the entire output
screen. Consequently, the imaged portion of the input screen 48,
which constitutes the image receptor 46, is reduced in size.
From the foregoing description, it may be seen that only the
X-radiation impinging on the imaged portion of the input screen
assembly 48 is useful in forming the visible image on the output
screen of image intensifier tube 50. Consequently, if the
cross-sectional area of X-ray cone 30, at the plane of the image
receptor 46, is greater than the area of the imaged portion of the
input screen, the patient 44 is exposed to unnecessary radiation.
Therefore, in order to protect the patient 44 from such
overexposure to X-radiation, it is required that the
cross-sectional area of the X-ray cone 30 conform closely to the
useful area of the image receptor 46. From FIGS. 1 and 2, it may be
seen that the cross-sectional size of X-ray cone 30, at the image
receptor 46, is proportional to the diametric size of aperture 34
in beam-limiting device 32. Thus, the aperture 34 may be adjusted,
as by knob 40, for example, to provide the X-ray cone 30 with the
desired cross-sectional size. However, it has been found in a
survey conducted by the United States Public Health Service that
operators of this type of X-ray equipment tend to adjust knob 40 in
a direction which will provide an X-ray cone having a
cross-sectional area greater than the area of the image receptor.
Consequently, in the practice of this invention, the knob 40 is
reserved for use only during special diagnostic procedures by
highly skilled personnel, and automatic means are provided for
adjusting the cross-sectional size of X-ray cone 30 during routine
diagnostic procedures.
Therefore, in accordance with this invention, the image intensifier
tube 50 is supported in a suitable holder 60 having operably
mounted thereon a receptor detecting means, such as a pressure
actuated switch 62, for example. One side of switch 62 is connected
to the positive side 64 of a suitable polarized voltage source (not
shown), and the other side of switch 62 is connected, by means of a
wire lead 66, to a collimator monitoring unit 68. Consequently,
when an image intensifier tube, such as 50, for example, is
inserted into the holder 60, pressure actuated switch 62 will be
closed thereby sending an electrical signal through wire lead 66 to
the collimator monitoring unit 68. Thus, the electrical signal
produced by the receptor detecting means indicates that an image
receptor is installed in position for receiving a radiation image
of the subject 42.
The holder 60 is supported by a telescopic suspension post 70 on
which there is mounted the remote control unit 71 and an attached
receptor size sensing means, such as a potentiometer 72, for
example. Potentiometer 72 includes a conventional resistive element
74 having one end connected to a positive terminal 78 of a
polarized voltage source (not shown), and the other end connected
to electrical ground. Thus, there is established along the
resistive element 74 a graduated series of voltage value which may
be sensed sequentially by a rotatable wiper arm 76. The arm 76 may
be mechanically coupled to the control knob 73. As a result, the
remote control knob 73 positions the wiper arm 76 on the resistive
element 74 to sense a particular voltage value which is related to
the diametric size of the imaged portion of the input screen 48.
However, since there is a corresponding relationship between the
respective diametric sizes of the imaged portion of the input
screen 48 and the image receptor 46, the particular voltage value
sensed by wiper arm 76 also may be correlated to the diametric size
of image receptor 46. In this manner, the graduated series of
voltage values established along resistive element 74 may be
calibrated to indicate corresponding image receptor sizes, which
are selected by adjusting control knob 73. Accordingly, the voltage
value sensed by wiper arm 76 constitutes an electrical signal which
is indicative of the diametric size of image receptor 46 and which
is conducted through a connecting wire lead 80 to the collimator
monitoring unit 68.
The telescopic suspension post 70 suspends from one end of an
overhead carriage assembly 82 having a pair of spaced parallel
rails 84 along which a support block 86 is movable in a
conventional manner. The block 86 is attached to one end of a
second telescopic suspension post 88 which extends between the
rails 84, in spaced relation therewith, and is attached at the
opposing end to housing 33 of beam-limiting device 32. Thus, moving
the block 86 along the rails 84 carries the beam-limiting device 32
and attached X-ray generator 12 toward or away from the input
screen 48 of image intensifier tube 50. As a result, the
cross-sectional area of X-ray cone 30 at the plane of input screen
48 decreases or increases, respectively, in diametric size.
Consequently, when adjusting the cross-sectional area of X-ray cone
30 to conform to the diametric size of input screen 48, a
determination must be made of the distances between the focal spot
21 in X-ray generator 12 and the input screen 48 of image
intensifier tube 50, which may be referred to as the
"source-to-image receptor distance" or simply as "SID."
Therefore, in order to obtain an accurate measurement of the SID,
one end of a cable 90 is fixedly attached to support block 86 by
any convenient means, as by securing it to a pin 87 carried on
block 86, for example. Cable 90 extends along the carriage assembly
82 and is wound around a spring loaded pulley 92 in a well-known
manner. Thus, when the X-ray generator 12 is moved toward or away
from the image intensifier tube 50, the cable 90 is respectively
wound onto or drawn from the pulley 92. The resulting rotation of
pulley 92 turns an axial shaft 94 which carries adjacent one end
thereof a fixedly attached disc 96. The periphery of disc 96 is
provided with a plurality of regularly spaced teeth 98 which
sequentially engage a spring biased arm of an interrupter switch
100 thereby causing switch 100 to open and close at regular
intervals. One side of switch 100 is connected to a positive
terminal 102 of a polarized voltage source (not shown), and the
other side of switch 100 is connected through switches 122 and 124
and wire lead 104 to the collimator monitoring unit 68. Thus,
switch 100, 122 and 124 constitutes an SID detecting means which
produces an electrical signal indicative of a selected SID and
interrupts the signal when the SID is changing.
The shaft 94 is operatively connected to a rotatable drum 106 which
carries on its outer periphery a series of irregularly spaced
landings 108, each of which engages a spring biased arm of a
respective switch 110. Closing one of the switches 110 sends an
electrical current through a respective series connected lamp 112
thereby illuminating the lamp to indicate that a particular SID
value is selected. For example, standard radiographic procedures
generally require the selection of one of four conventional SID
values, namely 36 inches, 40 inches, 48 inches, and 72 inches,
respectively. Thus, each of the landings 108 is precisely
positioned on the periphery of drum 106 to light a respective lamp
112 and indicate the associated SID to within the required degree
of accuracy. However, the landings 108 may be positioned on the
periphery of drum 106 to indicate other SIDs, if desired. Also,
more than the four landings 108 may be carried by the drum 106 to
indicate respective SID values in addition to the conventional SID
values generally selected.
The axial shaft 94 of pulley 92 also is mechanically coupled to a
wiper arm 118 of a potentiometer 116 whereby turning of the shaft
94 results in rotating the wiper arm 118 correspondingly. The wiper
arm 118 slidingly engages a conventional resistive element 120
having one end connected to wire lead 104 and through normally
closed switches 122 and 124 to the positive terminal 102 of a
polarized voltage source (not shown). The other end of resistive
element 120 is connected to electrical ground. As a result, there
is established along the resistive element 120 as graduated series
of voltage values which are calibrated to correspond accurately to
associated SID values. Consequently, when the shaft 94 positions
the wiper arm 118 on the resistive element 120, the wiper arm 118
senses a particular voltage value which corresponds accurately to
the SID selected. Thus, the potentiometer 116 constitutes an SID
sensing means for producing an electrical signal indicative of a
selected source-to-image receptor distance.
However, if the block 86 is moved along the rails 84 beyond a
maximum SID limit, such as 80 inches, for example, the wiper arm
118 carries a dielectric cam member 126 which opens the switch 122
thereby preventing current flow through the resistive element 120
and the wire lead 104. Similarly, if the block 86 is moved along
rails 84 to within a minimum SID limit, such as thirty inches, for
example, the cam member 126 opens switch 124 thereby preventing
current flow through the resistive element 120 and wire lead 104.
These switches 122 and 124 constitute at least in part limiting
means for preventing the emission of X-radiation, in the automatic
mode, when the generator 12 is too far from or too close to the
patient 44.
Mounted in the housing 33 of beam-limiting device 32 is a
potentiometer 130 having a conventional resistive element 132 which
is connected at one end to electrical ground. The other end of
resistive element 132 is connected by means of wire lead 128 to the
wiper arm 118 of potentiometer 116. Thus, the voltage value sensed
by the wiper arm 118 is applied across the resistive element 132 of
potentiometer 130. The resistive element 132 is slidably engaged by
a wiper arm 134 which is operatively connected to ring 38 for
rotation therewith when shutter aperture 34 is being adjusted, as
previously described. In this manner, the wiper arm 134 senses the
size of shutter aperture 34 and is positioned accordingly on the
resistive element 132. Since the voltage applied across the
resistive element 132 corresponds to a selected SID within the
allowed range set by switches 122 and 124, respectively, the wiper
arm 134 senses a particular voltage value which corresponds to the
field size associated with the selected SID, as provided by the
size of the shutter aperture 34. Thus, the potentiometer 130
constitutes a shutter aperture sensing means which in conjunction
with the SID sensing means produces an electrical signal indicative
of the radiation field size at the plane of the image receptor when
the SID is within the allowed range. This electrical signal is fed
through a connecting wire lead 136 to the collimator monitoring
unit 68.
Also mounted in the housing 33 of beam-limiting device 32 is a
reversible motor 138 having a drive shaft 139 suitably geared to
rotate the ring 38 and thereby adjust the shutter aperture 34. The
motor 138 is energized by an electrical signal conducted through a
wire lead 140 from the collimator monitoring unit 68. Thus, as
shown in FIG. 3, the collimator monitoring unit 68 receives
constant value voltage signals from the receptor detecting means 62
and the SID detecting means 100, 122 and 124 respectively. The
collimator monitoring unit 68 also receives variable value voltage
signals from the receptor size sensing means 72 and the radiation
field size sensing means, 130 in conjunction with 116,
respectively. From these input signals, the collimator monitoring
unit 68 generates an electrical signal which drives the motor 138
in the proper direction for adjusting the shutter aperture 34 as
required. While the shutter aperture 34 is being adjusted, the
field size sensing means is adjusted accordingly, thus completing a
servo loop. However, adjustment of the shutter aperture 34 must be
completed before X-ray cone 30 is generated in order to protect the
patient 44 from exposure to excess radiation. Therefore, the
collimator monitoring unit 68 sends an electrical signal through a
connecting wire lead 142 to the X-ray control unit 24 for the
purpose of preventing generation of X-ray cone 30 until the shutter
aperture 34 is completely adjusted.
As shown in FIG. 4, the collimator monitoring unit 68 includes a
detector monitoring circuit 144, a sensor monitoring circuit 146,
and an indicator/interlock circuit 148.
The detector monitoring circuit 144 may include two input
transistors 150 and 152, respectively, the base of transistor 150
being connected to the SID detecting means 100 122 and 124 and the
base of transistor 152 being connected to the receptor detecting
means 62. The transistors 150 and 152 have their respective
emitters connected to electrical ground and their respective
collectors connected to a source 154 of positive potential. The
collector of transistor 150 is connected to an SID inverter 156
which has its output connected to an input lead 157 of a detector
gate 160. Similarly, the collector of transistor 152 is connected
to a receptor inverter 158 which has its output connected to an
input lead 159 of detector gate 160.
The output of receptor inverter 158 also is applied to a conductor
164 which extends into the indicator/interlock circuit 148 to
connect to the input of a manual indicator inverter 170 and also to
an input lead 173 of a safety interlock gate 172. Thus, the
conductor 164 constitutes one output lead of the detector
monitoring circuit 144. The output of detector gate 160 is
connected to the input of a one-shot multivibrator 162, and through
an inverter 169 to a conductor 166 which constitutes a second
output lead of the detector monitoring circuit 144. The output of
the one-shot multivibrator 162 is applied to a conductor 168 which
constitutes a third output lead of the detector monitoring circuit
144.
The sensor monitoring circuit 146 is provided with an input
comparator 180 having input leads 182 and 184, respectively. The
lead 182 is connected to the wiper arm 134 of the shutter aperture
sensing potentiometer 130 through wire lead 136 and the lead 184 is
connected to the wiper arm 76 of receptor size sensing
potentiometer 72 through wire lead 80. Thus, an electrical signal
indicative of the radiation field size at the selected SID is
applied to the lead 182, when the selected SID is within the
allowed range. Similarly, an electrical signal indicative of the
diametric size of image receptor 46 is applied to the lead 184.
These two signals are compared electrically in the comparator 180;
and if one signal is larger than the other, an electrical signal
corresponding to the difference between the two signals is applied
to an output lead 186 of comparator 180.
The output lead 186 of comparator 180 and the output leads 166 and
168, respectively, of the detector monitoring circuit 144 are
connected to the input of a flip-flop gate 188 having a latching
feedback loop 190. The output of the flip-flop gate 188 is
connected to the input of a sequencing inverter 192 which has its
output connected to an input terminal of a sequencing gate 194. A
second input terminal of the sequencing gate 194 is connected to
the output lead 166 of detector monitoring circuit 144. The output
of sequencing gate 194 is connected to the input of an overshoot,
one-shot multivibrator 196 and through a lead 199 to the input of
an open drive OR gate 198. The output of OR gate 198 is connected
to the input of a motor drive circuit (open) 200, and also to the
input of a ready indicator inverter 202.
The output lead 186 of comparator 180 and the output lead 166 of
detector monitoring circuit 144 also are connected to respective
input terminals of a close drive gate 204. A third input terminal
of close drive gate 204 is connected to the output of ready
indicator inverter 202 and to an input terminal of a ready
indicator gate 210. The output of close drive gate 204 is connected
to the input of a close drive inverter 206 and to a second input
terminal of ready indicator gate 210. The output of close drive
inverter 206 is connected to the input of a motor drive circuit
(close) 208.
A third input terminal of ready indicator gate 210 is connected to
the output conductor 166 of detector monitoring circuit 144. The
output of ready indicator gate 210 is connected to the input of a
ready indicator circuit 212 and through a conductor 219 to th input
of a ready interlock inverter 214. The output of inverter 214 is
connected through lead 175 to a second input terminal of safety
interlock gate 172. The output of safety interlock gate 172 is
connected to the input of a safety interlock circuit 176 which, in
turn, activates an exposure hold indicator circuit 178 and an
exposure hold circuit 225.
A typical embodiment of the circuits shown in FIG. 4 is provided in
FIG. 5 for purposes of illustrating this invention and utilizes
conventional devices which are readily available on the commercial
market.
Thus, in operation, when an SID is selected, the SID detecting
switch means 100, 122, 124 send a constant value voltage signal to
the base of transistor 150 thereby rendering it conductive and
connecting the collector of transistor 150 to ground. The resulting
drop to zero potential at the input of SID inverter device 156
causes it to send a logic One signal through the lead 157 to an
input terminal of a detector Nand gate 160. However, the Nand Gate
160 does not produce a readiness output signal until a similar
logic One signal is received at its other input terminal from the
receptor inverter device 158.
If an image receptor is not in position to receive a radiation
image, the receptor detecting switch 62 will not be closed to send
a constant value voltage signal to the base of transistor 152.
Consequently, transistor 152 will remain nonconductive, and the
potential of source 154 will be applied to the input of receptor
inverter device 158. As a result, the inverter device 158 will send
a logic Zero signal through the lead 159 to the connected input
terminal of detector Nand gate 160 which, accordingly, will not
produce an output readiness signal. However, the logic Zero signal
produced by the inverter device 158 also will flow through the
output lead 164 to the manual indicator inverter 170 in the
indicator/interlock circuit 148. Therefore, the inverter 170 will
produce a logic One signal which will flow through a load resistor
216 to the base of a transistor 218 in the "Manual Operation"
indicator 174. Consequently, the transistor 218 will conduct and
thereby illuminate a Manual Operation indicator lamp 220.
The logic Zero signal applied to the input of inverter 170 also
will pass through lead 173 to an input terminal of a safety
interlock Nand gate 172. As a result, the Nand gate 172 will
produce at its output a logic One signal which will be applied
through a load resistor 221 to the base of a transistor 222 in the
safety interlock circuit 176. Accordingly, the transistor 222 will
be rendered conductive and permit current to flow through a series
connected relay coil 224. Consequently, an "exposure hold" circuit
225 will be completed by the closing of relay contacts 223 thereby
allowing X-ray exposures to be taken in the manual mode of
operation. Relay contacts 226 also open, thereby extinguishing the
"Exposure Hold" indicator lamp 228. Thus, this invention provides
means for automatically returning system 10 to a manual mode of
operation and allowing X-ray exposures to occur when an image
receptor is not in position for automatic operation.
If, however, an image receptor is supported in the holder 60, the
receptor detector switch 62 will be closed and will apply a
constant value voltage signal to the base of transistor 152.
Accordingly, the transistor 152 will be rendered conductive thereby
connecting the collector of transistor 152 to ground. The resulting
drop to zero potential at the input of receptor inverter 158 will
cause it to send a logic One signal to the input of the manual
indicator inverter 170 and through lead 173 to the safety interlock
Nand gate 172. Consequently, the inverter 170 will produce a logic
Zero signal which will render transistor 218 non-conductive and
extinguish the "Manual Operation" indicator lamp 220. The logic One
signal applied to the input of inverter 170 also will be applied
through lead 173 to the connected input terminal of safety
interlock Nand gate 172. Consequently, when the other input lead
175 of Nand gate 172 is at a logic One, the Nand gate 172 will
produce a logic Zero signal which will render transistor 222
non-conductive and deenergize relay coil 224. Accordingly, the
"exposure hold" circuit will be actuated by the opening of relay
contacts 223 and X-ray emission will be prevented by the collimator
monitoring unit 68. Relay contacts 226 also close thereby
illuminating the "Exposure Hold" indicator lamp 228.
The output of detector Nand gate 160 is connected to an inverter
169 which is connected in series with the output lead 166 of the
detector monitoring circuit 144. The output of detector Nand gate
160 also is connected in series with an inverter 163 which is
connected to an input terminal of a Nand gate 161. A second input
terminal of Nand gate 161 is connected to the positive side of a
capacitor 167 having a negative side connected to ground, and also
is connected through a resistor 165 to the output of detector Nand
gate 160. Thus, with no image receptor in position, the resulting
logic Zero signal applied to the input lead 159 causes Nand gate
160 to produce at its output a logic One signal. As a result, the
inverter 169 applies a logic Zero signal to the output lead 166,
and the inverter 163 applies a logic Zero signal to the connected
input terminal of Nand gate 161. Also, the logic One signal at the
output of Nand gate 160 will be applied through resistor 165
directly to the other input terminal of Nand gate 161 and will
charge the capacitor 167 to a logic One voltage value.
Then, when an image receptor is installed in the holder 60, the
resulting logic One signal applied to lead 159 in conjunction with
the logic One signal to lead 157 causes the Nand gate 160 to change
its output signal from logic One to logic Zero. As a result, the
inverter 169 applies a logic One signal to the output lead 166, and
the inverter 163 applies a logic One signal to the connected input
terminal of Nand gate 161. However, the capacitor 167 maintains the
other input terminal of Nand gate 161 at the stored logic One
signal also for the time required to discharge the stored signal
through the resistor 165. Thus, for the RC time of the resistor 165
and capacitor 167 network, both input terminals of Nand gate 161
will have a logic One signal. Consequently, during that short
interval of time the output of Nand gate 161 will drop to a logic
Zero and then will return to a logic One when capacitor 167 has
discharged to a logic Zero voltage value. Accordingly, whenever an
image receptor is changed and an SID is selected, the Nand gate 161
will produce a negative going pulse at its output. The timed pulse
produced by Nand gate 161 will be applied to output lead 168 of the
detector monitoring circuit 144.
The flip-flop gate 188 comprises two Nand gates 185 and 187,
respectively, which have their outputs connected to respective
input terminals of a third Nand gate 189. Flip-flop 188 is latched
in one of two operating conditions by latching loop 190 which
connects the output of Nand gate 189 through a series resistor 181
to an input terminal of Nand gate 185 and also to the positive side
of a capacitor 183 having its negative side connected to ground.
The flip-flop 188 is pulsed momentarily by the negative going pulse
applied to output lead 168 which is connected to the other input
terminal of Nand gate 185. When thus pulsed, the operating
condition of flip-flop 188 is determined by the electrical signal
applied to comparator output lead 186 which is connected to one of
the input terminals of Nand gate 187. The other input terminal of
Nand gate 187 is connected to the output lead 166 which has a
constant logic One signal applied to it when an image receptor is
in position and and SID is selected.
If the receptor size signal at the input lead 184 of comparator 180
is larger than the field size signal at the input lead 182, the
output lead 186 will have applied to it an electrical signal which
is equivalent to a logic Zero and indicates that the shutter
aperture 34 should be opened. As a result, the Nand gate 187 will
produce at its output a logic One signal. Also, while the negative
going pulse applied to output lead 168 is at a logic Zero level,
the Nand gate 185 will produce at its output a logic One signal.
Thus, with both input terminals of Nand gate 189 at logic One, it
produces at its output a logic Zero signal which will be applied to
the connected input terminal of Nand gate 185. Consequently, the
output of Nand gate 185 will be maintained at logic One even when
the negative going pulse in lead 168 returns to a steady logic One
value. Therefore, the flip-flop device 188 is latched in an
operating condition which permits opening of the shutter aperture
34.
The latched logic Zero signal at the output of the flip-flop 188
causes the sequencing inverter 192 to send a logic One signal to
the connected input terminal of a sequencing And gate 194. The
function of And gate 194 is to determine whether the flip-flop 188
has been unlatched and allowed to flip to a shutter opening
operative condition due to a transient pulse from some other
source, such as a momentary power failure, for example.
Consequently, the other input terminal of And gate 194 is connected
to the output lead 166 of detector monitoring circuit 144 to insure
that an image receptor is in position and an SID has been selected.
Therefore, with a logic One signal on each of its input terminals,
the And gate 194 will produce at its output a logic One signal.
Thus, it may be seen that the values of resistor 181 nad capacitor
183 are selected to provide an RC time constant longer in duration
than any expected transient pulse in order to avoid unintended
unlatching of the flip-flop 188 and a consequent undesirable
opening of the shutter aperture 34. Typical values selected for the
resistor 181 and capacitor 183 are 330 ohms and 150 microfarads,
respectively, to provide an RC time constant of approximately 50
milliseconds. Therefore, the values of resistor 165 and capacitor
167 in the detector monitoring circuit are selected to provide an
unlatching pulse of longer time duration than the RC time constant
of resistor 181 and capacitor 183. Accordingly, typical values
selected for resistor 165 and capacitor 167 may be 330 ohms and 200
microfarads, respectively, to provide a pulse duration of
approximately 65 milliseconds.
The overshoot one-shot multivibrator 196 comprises an And gate 193
having one input terminal connected through an inverter 191 to the
output of sequencing And gate 194, and another input terminal
connected to the positive side of a capacitor 197 and through a
resistor 195 to the output of sequencing And gate 194, the negative
side of capacitor 197 being connected to ground. Thus, the logic
One output signal produced by sequencing And gate 194 charges the
capacitor 197 accordingly and is applied to the connected input
terminal of And gate 193. The same logic One signal at the input of
inverter 191 causes it to apply a logic Zero signal to the other
input terminal of And gate 193. Thus, with a logic One and a logic
Zero signals at its respective input terminals, the And gate 193
applies a logic Zero signal to a connected input terminal of
open-drive Or gate 198.
However, the logic One signal produced by sequencing And gate 194
also is applied through a by-pass lead 199 to the other input
terminal of Or gate 198. As a result, the Or gate 198 sends a logic
One signal through a load resistor 201 in motor drive circuit 200
to the base of a transistor 203 thereby rendering it conductive.
Consequently, current will flow through a relay coil 205 which
energizes the motor 138 in the housing 33 of beam-limiting device
32. Accordingly, the motor 138 will rotate ring 38 to pivot the
shutter leaves 37 into lesser overlapping relationship thereby
opening the shutter aperture 34. Since the rotation of ring 38 also
moves the wiper arm 134 slidingly along the resistive element 132
of aperture size sensing potentiometer 130 correspondingly until
the field size signal applied to the input lead 182 of comparator
180 equals the receptor size signal applied to input lead 184. As a
result, the electrical signal applied to output lead 186 of
comparator 180 will change from logic Zero to logic One.
When the electrical signal applied to output lead 186 of comparator
180 has changed to logic One, the flip-flop device 188 will change
it to its other operating condition and will produce an output
logic One signal at the input of sequencing inverter 192. The
flip-flop device 188 will be latched in this operating condition.
The logic One signal at the output of flip-flop device 188 will
cause the inverter 192 to send a logic Zero signal to the connected
input terminal of sequencing And gate 194. Consequently, the And
gate 194 will produce at its output a logic Zero signal which will
cause the inverter 191 to apply a logic One signal to the connected
input terminal of overshoot And gate 193. Since the capacitor 197
is charged to a logic One signal, both input terminals of the And
gate 193 will be at a logic One during the discharge time of
capacitor 197. Consequently, the And gate 193 will apply a logic
One signal to the connected input terminal of Or gate and cause it
to continue to apply a logic One signal to the base of transistor
203, even though the other input terminal of Or gate 198 is now at
logic Zero.
As a result, the motor 138 will continue to pivot the shutter
leaves 37 and the aperture 34 will open beyond the corrective
adjustment required initially by the comparator 180. Accordingly,
the ring 38 will slide the wiper arm 134 along the resistive
element 132 to produce at the input lead 182 of comparator 180 a
field size signal which is larger than the receptor size signal.
Therefore, the electrical signal applied to output lead 186 will
remain at a logic One which indicates that the shutter aperture
should be closed down. The logic One signal applied to output lead
186 will not affect the output of flip-flop device 188 which will
remain latched with a logic One output. Consequently, when the
capacitor 197 is discharged, the overshoot And gate 193 will
produce at its output a logic Zero signal. Thus, with both inputs
of the Or gate 198 at logic Zero, it will apply a logic Zero to the
base of transistor 203 thereby rendering it non-conductive and
stopping motor 138.
The logic Zero signal produced by Or gate 198 will cause the
connecting close drive inverter 202 to apply a logic One signal to
the connected input terminal of close drive Nand gate 204. A second
input terminal of Nand gate 204 is connected to output lead 186
which is at a logic One. A third input terminal of Nand gate 204 is
connected to output lead 166 which also is at a logic One thereby
insuring that an image receptor is in position and an SID is
selected. Consequently, with logic One signals on all three input
terminals of the Nand gate 204, it will apply a logic Zero signal
to the input of inverter 206, which, accordingly, will apply a
logic One signal through a load resistor 209 to the base of a
transistor 207 in the motor drive circuit (close) 208. As a result,
current will flow through a relay coil 211 which will energize
motor 138 to rotate the ring 38 and pivot the shutter leaves into
greater overlapping relationship thereby closing the aperture
34.
The corresponding movement of wiper arm 134 along resistive element
132 will produce at the input lead 182 of comparator 180 a field
size signal which will be equal in magnitude to the receptor size
signal applied to input lead 184. Consequently, the electrical
signal applied to output lead 186 will change to a logic Zero which
will be applied to the connected input terminal of Nand gate 204.
As a result, Nand gate 204 will send a logic One signal to inverter
206 which will apply a logic Zero signal to the base of transistor
209 thereby rendering it non-conductive. Thus, relay coil 211 will
be deenergized and the motor 138 will stop.
The logic Zero signal now applied to output lead 186 is prevented
from changing the flip-flop 188 to a shutter opening operative
condition by the absence of an unlatching pulse from the one-shot
multivibrator 162 in the detector monitoring circuit 144.
Consequently, the flip-flop 188 will remain latched in a shutter
closing operative condition which permits further closing of the
shutter aperture 34 by manually adjusting knob 40. However, any
attempt to open the shutter aperture 34 beyond the diametric size
of image receptor 46, as by knob 40, for example, will move the
wiper arm 134 along resistive element 132 of aperture size sensing
potentiometer 130. As a result, the field size signal applied to
input lead 182 of comparator 180 will become larger in magnitude
than the receptor size signal applied to input lead 184.
Accordingly, the electrical signal applied to output lead 186 of
comparator 180 will change from logic Zero to logic One which will
be applied to the connected input terminal of Nand gate 204 and
result in the motor 138 closing the shutter aperture 34 down to its
automatically set dimension, as previously described. Consequently,
the field size signal applied to input lead 182 of comparator 180
again will equal the receptor size signal on input lead 184, and
the electrical signal applied to output lead 186 will change from
logic One to logic Zero.
The logic Zero signal applied to output lead 186 of comparator 180
will be delivered to the connected input terminal of Nand gate 204.
Consequently, Nand gate 204 will produce at its output a logic One
signal which will be sent to a connected input terminal of ready
indicator And gate 210. As stated previously, the Or gate 198 will
have at its output a logic Zero signal which will result in an
inverter 202 applying a logic One signal to a second connected
input terminal of ready indicator And gate 210. Also, the output
lead 166 of detector monitoring circuit 144 will apply a third
logic One signal to a connected input terminal of ready indicator
And gate 210, thereby insuring that an image receptor is in
position to receive an X-ray image and that an SID has been
selected. Thus, with logic One signals on all three of its
terminals, the And gate 210 will apply a logic One signal through a
load resistor 213 to the base of a transistor 217 in ready
indicator circuit 212. Consequently, the transistor 217 will be
rendered conductive thereby permitting current flow through the
filament of a series connected lamp 212. As a result, the "Ready"
indicator lamp 212 will be illuminated to signify that the shutter
aperture 34 has been completely adjusted to limit an X-ray cone 30
to a cross-sectional size which conforms to the diametric size of
the image receptor 46.
The logic One signal produced at the output of ready indicator And
gate 210 also will be applied through a conductor 219 to the input
of a safety interlock inverter 214. As a result, inverter 214 will
send a logic Zero signal through lead 175 to a connected input
terminal of a safety interlock Nand gate 172. As stated previously
a logic One signal on output lead 164 of detector monitoring
circuit 144 will be applied through lead 173 to the other input
terminal of Nand gate 172. Consequently, the Nand gate 172 will
produce at its output a logic One signal which will be applied
through a load resistor 221 to the base of a transistor 222 in
safety interlock circuit 176. As a result, transistor 222 will be
rendered conductive thereby permitting current flow through a relay
coil 224, which will open contacts 226 in the "Exposure Hold"
indicator 178 and will close contacts 223 in the "Exposure Hold"
circuit 225. Thus, the "Exposure Hold" lamp 228 will be
extinguished and the "Exposure Hold" circuit will be completed
thereby permitting the emission of X-rays from the source 21 in
X-ray generator 12.
The total elapsed time for completing the entire described cycle
from the detection of an image receptor 46 and a selected SID to
the release of an "Exposure Hold" on X-ray emission has been found
to be in the range of approximately one-tenth of a second to
approximately 1 second. Changing the image receptor or the SID
causes the described beam-limiting system to recycle and open the
shutter aperture 34 to the maximum allowed dimension. During a
shutter opening cycle, the automatic system of this invention will
open the shutter aperture to a size greater than the required
dimension, because of an overshoot drive pulse supplied by the
one-shot multivibrator 196 in the sensor monitoring circuit 148.
Then, after stopping the drive motor 138, the beam-limiting system
of this invention will close the shutter aperture 34 down to the
correct dimension. During a shutter closing cycle, the automatic
beam-limiting system of this invention will merely close the
shutter aperture 34 down to the correct dimension. Thus, when
adjusting the shutter aperture, the automatic system of this
invention, causes the shutters to go through a unidirectional final
adjustment whereby the shutters always approach the correct
dimension from the same direction. In this manner, mechanical
backlash, electronic lag and other forms of hysteresis are removed
from the drive system when adjusting the shutter aperture 34.
Consequently, the erroneous effects due to hysteresis are avoided.
It has been found that a beam-limiting system embodying this
invention can regulate the diametric size of the X-ray cone with
respect to the area of the image receptor such that the sum total
of all deviations regardless of sign is well within two percent of
the SID.
It is to be understood however, that the automatic system of this
invention is adaptable for use with other types of radiographic
systems. There is shown in FIGS. 6 and 7, an X-ray system 230
comprising an X-ray table 231 whereon a patient (not shown) may be
disposed for X-ray examination. Slidably inserted beneath the
surface of table 231 is an image receptor holder comprising a tray
233 having on its upper surface opposing clamps 235 and 237 which
are slidably mounted for simultaneous slidable movement toward and
away from one another in a wellknown manner.
An image receptor comprising a rectangular, film bearing cassette
239 is inserted between the clamps 235 and 237, respectively, and
then the clamps are slidingly moved into abutting relationship with
opposing edges of the cassette 239. The clamp 235 carries a
pressure actuated switch 232 which is closed when the clamp 235
abuts the adjacent edge of cassette 239. Thus, the switch 232
constitutes a receptor detecting means which produces a constant
value electrical signal indicative of the installation of an image
receptor in the holder. This constant value electrical signal is
fed to a collimator monitoring unit 234 by means of a connecting
wire conductor 236.
A cable 238 is suitably attached at one end to the clamp 235 and is
wound around a spring loaded pulley 240 which is rotatably mounted
at the back of tray 233. Mechanically coupled to the pulley 240 for
rotation therewith is a wiper arm 242 having an end portion which
slidably contacts a resistive element 244 of a potentiometer 246.
The resistive element 244 is connected at one end to a positive
voltage source 249 and at the other end to electrical ground.
Consequently, there is established along the resistive element a
graduated series of voltage values which may be calibrated to
correspond with the dimension of a cassette 239 held between the
clamps 235 and 237. This dimension generally, is referred to as the
"cross-table" or simply "cross" dimension of the cassette 239, the
other orthogonal dimension of the cassette being referred to as the
"longitudinal" or simply "long" dimension.
Although the clamps 235 and 237 do not abut respective edges the
X-ray film in the cassette there is a corresponding relationship
between the dimensions of the film and the dimensions of the
cassette whereby the graduated voltage values established along the
resistive element 244 can be correlated to the film sizes
associated with resistive cassette sizes. Thus, the potentiometer
246 constitutes a "cross" receptor sensing means which produces a
variable electrical signal indicative of the "cross" dimension of
the X-ray film in cassette 239. This electrical signal is fed to
the collimator monitoring unit 234 by means of a wire conductor 248
having one end electrically connected to the wiper arm 242 and the
other end connected into the collimator monitoring unit.
One end of a pivotal arm 250 lightly contacts a side edge of the
cassette 239 and rotates a wiper arm 252 having an end portion
slidably contacting a resistive element 254 of a potentiometer 256.
The resistive element 254 is connected at one end to a positive
voltage source 249 and at the other end to electrical ground. Thus,
in a manner similar to that described for potentiometer 246, there
is established along the resistive element 254 a graduated series
of voltage values which may be calibrated to correspond to
respective "long" dimensions of an X-ray film and which may be
sensed by the slidable arm 252. Accordingly, the potentiometer 256
constitutes a "long" receptor sensing means which produces a
variable electrical signal indicative of the "long" dimension of
the X-ray film in cassette 239. This electrical signal is fed to
collimator monitoring unit 234 through a wire conductor 258 having
one end connected to the wiper arm 252 and the other end connected
into collimator monitoring unit 234.
An upright support post 260 has extending outwardly therefrom a
carriage assembly 262 which is mounted for slidable movement along
the post 260, such that it can be moved toward and away from the
table 231. Attached to the carriage assembly 262 by suitable means,
as by pin 264, for example, is an end portion of a cable 266 which
extends downwardly into a housing 268 mounted on the table 231. The
cable 266 is wound around a spring-loaded pulley (not shown) in the
well-known manner and rotates the pulley as the carriage assembly
262 is moved relative to the X-ray table 231. Mechanically coupled
to the pulley for rotation therewith is an interrupter disc 94
having a plurality of teeth 96 on the periphery thereof whereby an
interrupter switch 100 is intermittently closed and opened in a
regular manner as the carriage assembly 262 is moved relative to
the table 231. Thus the switch 100 constitutes an SID detecting
means which produces an electrical signal indicative of a change in
SID, as set forth in detail in the previously described embodiment.
This electrical signal is fed to the collimator monitoring unit 234
through an interconnecting conductor 270.
Rotatable with the disc 100 is a drum 106 having disposed on the
periphery thereof a series of irregularly spaced landings 108 which
actuate respective switches 110 to illuminate respective lamps 112
which indicate when a respective SID value has been reached, as
previously described. Also rotatable with the disc 100 is a wiper
arm 118 having an end portion slidably contacting a resistive
element 120 of a potentiometer 116. The resistive element 120 is
connected at one end to a positive voltage source 102 and at the
other end to electrical ground. Thus, as previously described,
there is established along the resistive element 120 a graduated
series of voltage values which may be calibrated to correspond to
respective source-to-image receptor distances. The wiper arm 118
senses a particular value associated with a selected SID and
delivers it through a conductor 272 to a beam-limiting device 274
which is mounted over the port 15 of X-ray generator 12.
The X-ray generator 12 is movably supported on rails 276 and 278 of
the carriage assembly 262 such that the X-ray source 21 in
generator 12 is aligned with the center of the film bearing
cassette 239 in tray 233. The X-ray source 21 emits a conical beam
31 of X-radiation which egresses through port 15 and passes through
the beam-limiting device 274. The device 274 comprises two
orthogonically disposed pairs, 280 and 282, respectively, of
opposing shutter plates. The plates 280 and 282 are made of X-ray
absorbent material, such as lead, for example, and are pivotally
mounted to move simultaneously toward and away from one another, as
adjusted by respective control knobs 284 and 286, respectively,
mounted on the exterior of the beam limiting device. Thus, the
plates 280 limit the "long" dimension of a rectangular aperture 288
formed by the shutter plates and the plates 282 limit the "cross"
dimension. In this manner, the conical beam 31 is provided with a
rectangular cross-sectional area which, at the plane of the film
bearing cassette 239, conforms to the area of the X-ray film in the
cassette.
To align the source 21 with the center of the cassette 239 there is
provided a visible light centering means comprising a mirror 290
centrally located on the axial centerline of the beam-limiting
device 274 and disposed at an angle thereto so as to reflect
visible light from a suitable source 292 through the rectangular
aperture 288. The source 292 is located off-axis a sufficient
distance to project by means of mirror 290 a virtual image which is
optically located at the point source 21 of X-radiation. In this
manner, it may be determined by the field of visible light at the
plane of the cassette 239 whether the source 21 and beam-limiting
device 274 are aligned with the cassette 239. Thus, it may be seen
that the beam-limiting device 274 is suitable for collimating a
beam of visible light as well as an X-ray beam.
The mirror 290 is left in place during an X-ray exposure in order
to provide the required filtration of soft X-rays. However, it is
pivotally mounted so as to be rotated off axis when necessary, such
as to examine the X-ray emitting surface of anode 20, for example.
Therefore, in order to insure that the mirror 290 is in position
during an X-ray exposure, there is provided a pressure actuated
switch 289 which is closed by the mirror 290 when it is rotated off
axis. The switch 289 is connected by means of conductors 294 to the
X-ray control unit 24 whereby X-ray emission will be delayed until
the mirror 290 is rotated back into position for filtering out soft
X-radiation.
In the practice of this invention, there is provided within the
housing of beam-limiting device 274, two motors, 296 and 298,
respectively, each of which functions in a manner similar to that
of motor 138 in the previously described embodiment. Thus, the
motor 296 pivots plates 280 toward and away from one another to
automatically limit the "long" dimension of aperture 284; and the
motor 298 rotates the plates 281 toward and away from one another
to limit the "cross" dimension of aperture 288. The motors are
electrically connected through a cable 300 to the collimator
monitoring unit 234. Also, the motor 296 is mechanically connected
to the wiper arm 302 of a "long" aperture sensing potentiometer 306
whereby rotation of the shaft of motor 296 to vary the "long"
dimension of shutter aperture 288 also moves the wiper arm 302
correspondingly along a resistive element 304 of potentiometer 306.
Similarly, the motor 298 is mechanically connected to the wiper arm
308 of a "cross" aperture sensing potentiometer 312 whereby
rotation of the shaft of motor 298 to vary the "cross" dimension of
shutter aperture 288 also moves the wiper arm 308 correspondingly
along a resistive element 310 of potentiometer 312.
Since the particular voltage value indicative of the selected SID
is impressed across the resistive elements, 304 and 310,
respectively, there will be established along each of the resistive
elements a graduated series of voltage values which may be
calibrated to correspond to respective field size values at the
plane of the selected SID. Thus, the wiper arm 302 will sense a
particular voltage value indicative of the field size provided by
the opening between "long" limiting shutter plates 280; and the
wiper arm 308 will sense a particular voltage value indicative of
the field size provided by the opening between "cross" limiting
shutter plates 282. The wiper arms are connected electrically
through respective conductors 316 and 318 to the collimator
monitoring unit 234.
Thus, as shown in FIG. 8, the collimator monitoring unit 234 will
receive constant value voltage signals from the receptor switch
detecting means 232 and from the SID switch detecting means 100
122, and 124. The collimator monitoring unit 234 also will receive
variable voltage signals from the "long" receptor sensing means
246, the "cross" receptor sensing means 256, the "long" aperture
sensing means 306, and the "cross" aperture sensing means 312. With
these signals, the collimator monitoring unit 234 generates an
electrical signal to drive the "long" adjustment motor 296, and
another electrical signal to drive the "cross" adjustment motor
298. In this manner, the shutter aperture 288 is automatically
adjusted, prior to X-ray emission, to provide the X-ray beam with a
rectangular cross-sectional size which, at the plane of the image
receptor, conforms to the rectangular area of the X-ray film in
cassette. However, in order to insure that a patient is not exposed
to X-radiation before the aperture 288 is completely adjusted, the
collimator monitoring unit 234 is connected through a conductor 315
to the X-ray control unit 24 whereby X-ray emission may be delayed
until adjustment of the aperture 288 is completed.
As shown in FIG. 9, the "long" sensor monitoring circuit 146a is
identical to the senosr monitoring circuit 146 described in the
previous embodiment, and the "cross" sensor monitoring circuit,
designated by block 146b, is identical to the circuit 146a. The
detector monitoring circuit 144a is identical to the detector
monitoring circuit 144 described in the previous embodiment, except
the output leads 166a and 168a are connected to associated
components in the two sensor monitoring circuits, 146a and 146b,
respectively. The indicator/interlock circuit 148a is similar to
indicator/interlock circuit 148 in the previous embodiment, except
the safety interlock inverter 214 in the previous embodiment is
changed to a ready interlock Nand gate 314 tp provide means for
determining when both the "long" and the "cross" shutter
adjustments are completed.
An illustrative embodiment of the circuits shown in FIG. 9 is
provided in FIG. 10 and utilizes conventional devices which may be
of the integrated circuit type and may be mounted on a control
panel of the printed circuit type, for example. A comparison of the
illustrative embodiment shown in FIG. 5 with the illustrative
embodiment shown in FIG. 10 indicates that the devices utilized to
form circuit 144 are identical to the devices utilized to form
circuit 144a. Similarly, the devices utilized to form circuit 146
are identical to the devices of circuit 146a and would be identical
to the devices of circuit 146b if shown. Also, the devices utilized
to form circuit 148 are identical to the devices utilized to form
circuit 148a except the inverter 214 in circuit 148 is changed to a
Nand gate 314 in circuit 148a, as previously described. Thus, the
circuits 144a, 146a, 146b and 148a will function in a similar
manner to that described for the illustrative embodiment shown in
FIG. 5.
From the foregoing, it will be apparent that all of the objectives
of this invention have been achieved by the structures shown and
described. It will be also apparent, however, that various changes
may be made by those skilled in the art without departing from the
spirit of the invention as expressed in the appended claims. It is
to be understood, therefore, that all matter shown and described is
to be interpreted as illustrative and not in a limiting sense.
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