U.S. patent number 3,860,821 [Application Number 05/303,167] was granted by the patent office on 1975-01-14 for imaging system.
This patent grant is currently assigned to Raytheon Company. Invention is credited to Harrison H. Barrett.
United States Patent |
3,860,821 |
Barrett |
January 14, 1975 |
IMAGING SYSTEM
Abstract
An imaging system suitable for use with high energy nuclear
particles or photons such as gamma radiation and X-radiation. The
system provides means for illuminating an object with radiation as
well as spatially coding the illuminating radiation or emitted
radiation if the object is self-luminous, to provide composite
image representing the summation of the shadows from all points of
the source of illumination. Spatial modulation is accomplished by a
mask having a coded pattern of transparent and opaque regions
linearly scanned in time. The resulting signal has the
characteristics of a chirp waveform typical of pulse compression
radars. The composite image is readily decoded by a delay line
having a phase or delay characteristic complementary to that of the
spatial modulation pattern.
Inventors: |
Barrett; Harrison H.
(Lexington, MA) |
Assignee: |
Raytheon Company (Lexington,
MA)
|
Family
ID: |
26759286 |
Appl.
No.: |
05/303,167 |
Filed: |
November 2, 1972 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
77459 |
Oct 2, 1970 |
3748470 |
|
|
|
Current U.S.
Class: |
250/363.01;
250/363.06; 250/369; 378/2; 976/DIG.429; 250/366; 250/505.1;
378/145 |
Current CPC
Class: |
G01T
1/295 (20130101); G21K 1/025 (20130101); G01T
1/1641 (20130101); G01N 23/04 (20130101); G01T
1/1645 (20130101); A61B 6/4258 (20130101); G01T
1/2978 (20130101); G01T 1/1642 (20130101) |
Current International
Class: |
G21K
1/02 (20060101); G01N 23/04 (20060101); G01T
1/164 (20060101); G01T 1/00 (20060101); G01N
23/02 (20060101); G01T 1/29 (20060101); G01t
001/20 () |
Field of
Search: |
;250/363,366,369,494,505,509,511,514 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Borchelt; Archie R.
Attorney, Agent or Firm: Pannone; Joseph D. Bartlett; Milton
D. Warren; David M.
Parent Case Text
This is a division of application Ser. No. 77,459 filed Oct. 2,
1970 now U.S. Pat. No. 3,748,470.
Claims
What is claimed is:
1. A method of photographing a subject comprising the steps of:
illuminating said subject with a radiant energy emanating from one
of a plurality of spaced apart sites of said radiant energy, said
radiant energy being capable of propagating through said subject
with substantially no diffraction;
illuminating said subject from a second and from a third of said
sites with a radiant energy capable of propagating through said
subject with substantially no diffraction, said illuminating from
said second and from said third sites being directed such that a
region of space external to said subject is commonly illuminated
with said radiant energies which have propagated through said
subject and emanated from said first, said second and said third
sites;
inserting a mask in said commonly illuminated region of space, said
mask having regions substantially transparent to said radiant
energy spaced apart by regions relatively opaque to said radiant
energies;
imparting a relative motion between said mask and said subject;
positioning a radiation detector in spaced apart relation to said
mask and at a side of said mask away from said subject, the depth
of said mask being sufficiently small relative to the widths of
said transparent regions to permit illuminating radiant energy
emanating from separate ones of said sites to impinge upon a common
point of said detector; and
recording the intensities of said radiant energies propagating in
said commonly illuminated region of space.
2. A method according to claim 1 wherein said recording of the
intensities of said radiant energies is performed on a photographic
film.
3. A method according to claim 1 wherein said illuminating from
said first, said second and said third sites is performed
simultaneously, and said motion of said mask is in a transverse
direction to said subject.
4. A method according to claim 1 wherein the radiant energy
emanating from said first site is of a frequency equal to the
frequency of said radiant energy emanating from said second site,
and wherein said recording means includes means for scanning a
recorded shadow of said mask in a direction perpendicular to the
direction of said motion.
5. A method according to claim 1 wherein said recording medium
comprises a single detecting medium, said single detecting medium
being utilized for recording the intensities of each of the radiant
energies emanating from said first, said second and said third
sites, said recording being a composite recording having data
provided by each of said radiant energies emanating from said
first, said second and said third sites.
6. A method according to claim 5 further comprising the step of
decoding said composite recording for providing an image of said
subject.
7. A method according to claim 6 wherein said single detecting
medium comprises photographic film and said step of decoding
comprises the substeps of:
developing said film;
illuminating said film with radiant energy that propagates through
said film; and
filtering said radiation that has passed through said film.
8. A method according to claim 7 wherein said filtering is
accomplished by utilizing a delay line filter having a spatial
frequency response complementary to the spatial arrangement of said
plurality of sites.
9. A method according to claim 7 wherein said filtering is
accomplished by scanning said film to provide a scanned signal, and
processing said scanned signal by means of a filter having a
temporal response complementary to a signal formed by scanning a
shadow of said mask upon said recording medium.
10. A method of photographing a subject comprising the steps
of:
illuminating said subject with radiant energy capable of
propagating through said subject with substantially no
diffraction;
placing a detector of said radiant energy at a distance from said
subject in a path of propagation of said radiant energy;
masking a portion of said radiant energy to inhibit said masked
energy from reaching said detector, said masking being done at a
first region of space positioned between a source of said radiant
energy and said detector;
masking said radiant energy at a second and a third region of space
distant from each other and distant from said first region of
space, said detector being positioned such that shadows from said
first and said second and said third maskings are cast upon said
detector, each of said maskings being done with a mask sufficiently
thin to permit illuminating energy emanating from spaced apart
sites to impinge upon a common point of said detector; and
recording the intensities of said radiant energies incident upon
said detector.
11. A method according to claim 10 wherein said recording of the
intensity of said radiant energies is performed on a photographic
film.
12. A method according to claim 10 wherein said masking at said
first, said second and said third regions of space is performed
sequentially.
13. A method according to claim 10 wherein the first, the second
and the third maskings are of a chirp pattern, said recording being
a composite recording having data provided by said first, said
second and said third maskings.
14. A method according to claim 13 further comprising the step of
decoding said composite recording for providing an image of said
object.
15. A method according to claim 14 wherein said detector comprises
photographic film and said step of decoding comprises the substeps
of:
developing said film;
illuminating said film with radiant energy that propagates through
said film; and
filtering said radiation that has passed through said film.
16. A method according to claim 15 wherein said filtering is
accomplished by utilizing a filter having a spatial frequency
response complementary to the spatial arrangement of said plurality
of sites.
17. A method according to claim 15 wherein said filtering is
accomplished by scanning said film to provide a scanned signal, and
processing said scanned signal by means of a filter having a
temporal response complementary to a signal formed by a scanning of
an arrangement of said plurality of sites.
18. A method comprising the steps of:
introducing sources of radiation within a subject;
masking portions of said radiation emanating from said subject by
means of a mask placed adjacent said subject;
detecting such portions of said radiation as passes through said
mask, said detecting including a detecting of the positions of rays
of said radiation by a detector spaced apart from said mask, the
depth of said mask being sufficiently smaller than the spacing
between said detector and said mask to permit quanta of said
radiation propagating along separate paths to impinge upon a common
point of said detector; and
moving said mask in an oscillatory pattern.
19. A method according to claim 18 further comprising the steps
of:
scanning an image of said detected radiation; and
comparing said detected radiation with a reference to extract from
said detected radiation data about said subject, said comparing
being accomplished by passing said scanned image through a
dispersive delay line having a phase shift function complementary
to a pattern of said mask.
20. A method according to claim 18 wherein said detecting utilizes
a photographic film, said method further comprising the steps
of:
illuminating said film with radiation directed at said film from a
source of said illuminating radiation; and
filtering such portions of said illuminating radiations which
traverses said film.
21. A method according to claim 20 wherein said radiation within
said subject is of a sufficiently high frequency such that
substantially no diffraction occurs.
22. A method comprising the steps of:
introducing sources of radiant energy into a subject;
placing a detector of said radiant energy external to said subject;
and
simultaneously scanning said detector past said subject while
masking radiant energy emanating from said subject and incident
upon said detector, said masking being done with a mask having
relatively transparent and opaque regions and spaced apart from
said detector, the depth of said mask being sufficiently small
relative to the spacing between said mask and said detector and to
the widths of said transparent regions to permit rays of said
radiant energy propagating along different paths to impinge upon a
common point of said detector.
23. A method according to claim 22 further comprising a step of
detecting the locations of rays of said radiant energy which are
incident upon said scanned detector.
24. A method according to claim 23 further comprising the step of
filtering data obtained via said detector relative to the radiant
energy emanating from said subject.
Description
BACKGROUND OF THE INVENTION
This invention pertains to the focusing of radiant energy and, more
particularly, to radiation characterized by the presence of high
energy particles, particularly high energy photons such as in gamma
radiation.
In the past, focusing of radiation has been done by lenses where
the radiation is of a lower frequency, such as optical radiation,
and by means of pinhole cameras or collimator arrays where the
radiation if of a higher energy, such as gamma radiation. The
pinhole camera has been utilized because the index of refraction at
all materials is too small to permit lens construction.
One well-known type of camera for use with gamma radiation imaged
by a pinhole or parallel hole collimator array is the Anger camera
as disclosed in U.S. Pat. No. 3,011,057, which issued to H. O.
Anger on Nov. 28, 1961. While such a camera is in common usage
today, its performance is inferior to that of cameras customarily
used for optical radiation in that its resolution is substantially
lower and its effective aperture, no larger than a pinhole, is far
smaller than that of the wide aperture lenses commonly employed in
optical cameras today and can be increased at the expense of
resolution. Thus, there is a problem as to how to construct a
system responsive to high energy radiation which provides real-time
imaging of extended objects, such as might appear on a television
screen, and variable focusing, for the case where the
object-to-camera distance is variable.
SUMMARY OF THE INVENTION
This invention provides a system wherein an object emitting, or
illuminated by, high energy radiation, such as X-radiation, gamma
radiation, and nuclear radiation, is observed or imaged by
spatially coding the illumination so that there is received a
composite image having shaded regions, the shading being due to
both the shadows cast by the object itself as well as shadows due
to the spatial modulation of the radiation. The shading is the
variation over the image plane in the probability of arrival of
high energy photons. In one embodiment of the invention, the
spatial modulation of the radiation is accomplished by means of
mask or plate having regions which are transparent and regions
which are opaque to the radiation. The opaque regions may be
regarded as barrier elements which inhibit the passage of particles
such as gamma ray photons and nuclear particles. The invention is
particularly useful where the dimensions of the transparent and
opaque regions of such masks are, in practical devices, much larger
than a wavelength of the radiation which precludes the use of
interference or diffraction effects to alter the direction of a
ray. In a second embodiment of the invention, the spatial
modulation of the radiation is accomplished by illuminating the
object by means of a source of radiation comprising emitting areas
from which high energy particles emanate which are interspersed
among nonemitting areas from which no high energy particles
emanate. With both embodiments, the pattern of illuminated and
shaded areas has a code or predetermined format.
A detector assembly is positioned to intercept the radiant energy
which in the case of gamma radiation comprises a sequence of
photons, or quanta of radiant energy. An image of these rays is
formed on the face of the detector assembly, the image being
scrambled due to the spatial modulation. An image of the object
itself is provided by scanning the scrambled image on the face of
the detector assembly to provide a scan signal containing
information relating to the locations of the various portions of
the scrambled image. The scan signal is passed through a filter
having a transfer function which is conjugate to the scan signal
produced from a point source of radiation through the spatially
coded mask, that is, the temporal impulse response function of the
filter is the temporal inverse of the scan signal waveform, so that
there is a correlation between the filter and the spatial
modulation. Thus, for example, where the modulating elements have
the form of a series of opaque and transparent regions of
successively decreasing size, the scan signal has a form similar to
that of a chirped radar signal where the frequency is linearly
increasing; and accordingly, in this case the filter would have the
form of a pulse compression filter providing differential delays
between portions of the signal having differing frequencies. Thus,
the image of the radiant energy on the face of the detector
assembly would be decoded and compressed into a series of points
which are then displayed as the image of the object.
BRIEF DESCRIPTION OF THE DRAWINGS
The aforementioned objects and other features of the invention are
explained in the following description taken in connection with the
accompanying drawings wherein:
FIG. 1 is a pictorial representation of a system in accordance with
the invention for displaying a radiograph of a radioactive
object;
FIG. 2 is a block diagram of the imaging system of the
invention;
FIG. 3 is a pictorial view of a mask used in the invention;
FIG. 4 is a pictorial view of an alternative embodiment of the mask
utilized in the invention;
FIG. 5 is a plane view of a dispersive surface wave delay line
showing a variation in the spacing of fingers of a comb
structure;
FIG. 6 is a plane view of a portion of a surface wave delay line
showing a variation in the overlapping of the fingers in an
interdigital network;
FIG. 7 is an alternative embodiment of the imaging system of the
invention;
FIG. 8 is a further embodiment of the invention wherein a large
spatial frequency bandwidth is provided by a source of
radiation;
FIG. 9 is a detailed view of a source of radiation providing a
large spatial bandwidth in accordance with the invention; FIG. 10
is a diagrammatic view of a radiographic system employing spatial
filtering of an image formed with the aid of the source of FIG. 9;
and
FIGS. 11 and 12 are diagrammatic views of alternative embodiments
showing a mechanical scanner and an image intensifier.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to FIG. 1 there is shown a human patient 20 being
treated for a thyroid ailment with the aid the equipment responsive
to gamma radiation. As is well known, the thyroid gland tends to
absorb compounds of iodine which may be injected into the patient
or ingested by the patient. To provide a radiograph of a thyroid 21
a radio pharmaceutical containing iodine is administered to the
patient. The radioactive molecules of the radiopharmaceutical are
then distributed about the thyroid gland and emit gamma radiation,
the gamma radiation from each of the molecules forming a part of
the radiograph.
In accordance with the invention a radiograph is formed by means of
an imaging system 22 comprising a detector assembly 24 responsive
to gamma radiation, a processor 26 for deriving information from
each of the high energy photons of the gamma radiation as they
impinge on the detector assembly 24, and an output display 28 for
showing a radiograph of the thyroid 21. In addition, a mask 30
having an array of apertures of predetermined sizes, indicated
generally by the numeral 32, which are arranged in a predetermined
manner as will be shown in greater detail hereinafter with
reference to FIGS. 3 and 4, is utilized instead of the pinhole
aperture or collimator array commonly used for photography of high
energy particles. The mask 30 creates a scrambled or coded image at
the face of the detector assembly 24 as will be described
hereinafter with reference to FIG. 2, and accordingly, the
processor 26 incorporates means for decoding the scrambled image.
The mask 30 serves the dual functions of providing increased
aperture and permitting a higher resolution than has hithertofore
been possible with such large apertures. The larger aperture admits
high energy gamma ray photons at a rate far greater than possible
with a single pinhole aperture or collimator and thereby reduces
the time required to make a radiograph.
Referring now to FIG. 2 there is shown a block diagram of the
imaging system 22 of the invention in which an object 34 such as
the radioactive thyroid 21 emits radiation in the direction of the
mask 30 and detector assembly 24. The detector assembly 24 has a
form similar to that of the Anger camera as disclosed in the
aforementioned patent to Anger and utilizes a scintillator 36, such
as a cesium iodide crystal in the form of a plate, which is
illuminated by radiation shown as rays 38A-D from the object 34. As
is well known, a scintillator emits light from the point of impact
of a high energy particle which may be a high energy photon from
gamma radiation or X-radiation as well as an impact from a nuclear
particle such as a proton or a neutron. An array of detector
elements 40 are arranged to intercept the light, for example the
light rays 42 emitted from the sites of such impacts on the
scintillator 36. As disclosed in the aforementioned patent to Anger
the detector elements 40 are connected to a matrix 44 of resistors
which provide the X and Y coordinates. Currents are induced in the
resistors (not shown) proportional to the intensity of the light
received at the respective detector elements 40. Since the
intensity of light impinging upon any one of the detector elements
40 is related to the angle of arrival of the light rays 42 from a
point of impact on the scintillator 36 to a detector element 40,
the currents in the resistor matrix 44 are related to the location
of the point of impact on the scintillator 36. Thus, the resistor
matrix 44 is able to provide the X and Y coordinates of the impact
location as shown on lines 46 and 48. Since the detector assembly
24 is responsive to nuclear particles as well as high energy
photons, the imaging system 22 can form a radiograph of an object
radiating high energy in the form of nuclear particles as well as
high energy photons.
The operation of the imaging system 22 is readily distinguished
from well known optical imaging systems which commonly employ
reflecting surfaces or lenses to produce images. In the case of a
lens used in such optical systems the index of refraction of the
material used in the lens varies with frequency and approaches
unity for the higher radiation frequencies as are found in
X-radiation and gamma radiation as well as for nuclear particles.
Thus, in high energy radiation imaging systems the use of optical
lenses is precluded in that there is insufficient index of
refraction to bend the rays of radiation to form an image. And
similarly with the reflecting surfaces of mirrors used in optical
systems, at the higher energy radiation, such as X-radiation and
particularly gamma radiation and nuclear radiation, the reflecting
surfaces tend to be no longer reflecting and such radiation
proceeds to travel straight through the material used in the
reflecting surface.
The mask 30 and the detector assembly 24 do not use the focusing
effect associated with the bending of rays of radiation as is done
in the typical optical system. The image formed on the scintillator
36 is produced by techniques of geometric optics analogous to that
of the pinhole camera in which all rays of radiation are straight.
Accordingly, the operation of the mask 30 is to be distinguished
from that of defraction gratings in optical systems of the prior
art.
The operation of the mask 30 can be explained as follows. Consider
a point source such as source 50 of high energy radiation located
on the object 34. The source 50 emits quanta of radiation, either a
photon or nuclear particle, which travels from the object 34 to the
mask 30. The quanta of energy passes through the mask 30 in the
event that it is emitted in a direction towards an aperture 32, or
is stopped by the mask 30 in the event that it is emitted in a
direction towards an opaque section of the mask. The source 50
emits the quanta of energy sequentially and at irregular intervals.
If the scintillator 36 were to have a very long persistence time
compared to the mean interval between the emitted quanta of radiant
energy then it would be observed that an image or shadow of the
mask 30 would gradually appear on the scintillator 36 as successive
quanta of radiation passed through the apertures 32 to impact upon
the scintillator 36.
As a practical matter, cesium iodide scintillators do not have
sufficient persistence time for forming an image when illuminated
by an object such as the radioactive thyroid 21. Accordingly, the
X-Y coordinate position data of each of the impact points must be
processed in a manner which preserves the data of the individual
impact points until such time as there are a sufficient number of
these points to provide a usable image. Thus, for example, the X-Y
coordinate data on lines 46 and 48 could be processed by a computer
(not shown) which provides a memory address code for each of the
impact points, or as is shown in the preferred embodiment of FIG. 2
the processor 26 utilizes a first storage tube display 52
responsive to the X-Y coordinate data on lines 46 and 48. The first
storage tube display 52 has a cathode ray tube (not shown) with a
long persistence storage screen, hereinafter referred to as the
first storage screen 54, which as is well known, emits light in
response to the impact of electrons from the electron gun of the
cathode ray tube. The details of the first storage tube display 52
are not shown since they are well known. The first storage tube
display 52 additionally contains a circuit responsive to the
presence of the electrical signals on lines 46 and 48 for
energizing the beam of the cathode ray tube to illuminate the first
storage screen 54 at the point corresponding to the X and Y
coordinates. In this way the sequential impact of quanta of radiant
energy from the source 50 upon the scintillator 36 are transformed
into an image on the first storage screen 54, the image having the
form of the shadow of the mask 30 corresponding to illumination of
the mask 30 from the point source 50. Since a radioactive object
such as the radioactive thyroid 21 has many points which serve as
sources of radiation, each such point being a small volume of
radioactive material, a multiplicity of images are formed and
superposed on the first storage screen 54 in response to
illumination of the mask by each of these sources such as the
sources 50, 56 and 58. It is thus apparent that the image appearing
on the first storage screen 54 is in fact a scrambled or coded
image of the object 34 since it bears little, if any, resemblance
to the object 34 and yet contains all the information as to the
form of the object 34. The next step in forming a radiograph of the
object 34 is therefore the unscrambling or decoding of the image on
the first storage screen 54.
The decoding of the scrambled image on the first storage screen 54
can be done, for example, by a computer (not shown) which processes
the X-Y coordinate data in accordance with programs utilizing the
mathematics of convolution and Fourier transform operations, or as
is shown in the preferred embodiment of FIG. 2, by means of a
scanning technique which utilizes the matched filter or pulse
compression technique of radar systems. The processor 26
unscrambles the image in a two-step procedure in which it first
unscrambles the image in the horizontal dimension and then
unscrambles the image in the vertical dimension.
The first step in the decoding is accomplished by means of a first
vidicon 60 and a first delay line 62. The first vidicon 60
horizontally scans the first storage screen 54 and provides an
output signal consisting of successive horizontal line scans. The
first vidicon 60 utilizes a linear sweep rate when scanning storage
tube screens that are flat; a nonlinear sweep rate is utilized for
curved storage tube screens in order to cancel the effect of the
curvature so that the output signal from the first vidicon 60 has
the characteristics of a linear scan. The waveform of the signal
provided by each line scan of the first vidicon 60 corresponds to
the shadow cast upon the scintillator 36 and is readily visualized
in the case of illumination of the mask 30 by a single source of
high energy radiation such as the source 50.
The shadow cast by the mask upon the scintillator 36 comprises a
succession of light and dark areas as can be visualized by
examining the axonometric view of the mask 30 as shown in FIG. 3 as
well as the diagrammatic sectional view taken through a line of
apertures 32 as shown by the mask 30 of FIG. 2. The mask 30
comprises an array of apertures 32 or relatively transparent areas
formed within a base material such as lead which is relatively
opaque to high energy radiation, the opaque portion being
designated by numeral 64. Since the opaque portion 64 is a
relatively thin film in the case of X-radiation or gamma radiation,
the opaque portion 64 is supported by a rigid substrate 65 of
relatively transparent material such as a material of low atomic
number, for example, aluminum.
In an alternative embodiment of the mask indicated by numeral 66
and shown partially cut away in FIG. 4, apertures 68 do not pass
completely through the base material so that there is a small
amount of opacity even in the relatively transparent region of the
mask 66. The embodiment of FIG. 4 represents one method of reducing
the effect of radiation resulting from Compton scattering within
the object 34 of FIG. 2 since, as is well known, the radiation
resulting from the Compton scattering is of a lower energy than the
direct radiation from the source 50. Accordingly, the mask 66 shown
in FIG. 4 can provide an image on the first storage screen 54
having greater definition than does the mask 30 of FIG. 3. With
either the embodiment of the mask as shown in FIG. 3 or that shown
in FIG. 4 substantially the same shadow pattern is developed by the
mask on the scintillator 36 in response to radiation emanating from
the source 50.
Referring again to FIGS. 2 and 3 the configuration of the apertures
32 and their arrangement may be readily explained by considering an
aperture array of one dimension such as the row of apertures 70A-D.
For ease of reference in this description of the aperture
arrangement, each of the apertures 32 are further designated by
individual numerals followed by letters, the numeral indicating the
row position and the letter indicating the column position. The row
of apertures 70A-D are configured and arranged so that when the
image of the row of apertures 70A-D on the first storage screen 54
is scanned, a chirp waveform similar to that utilized in a pulse
compression radar system appears on the output signal of the first
vidicon 60.
Recalling that a linear scan is utilized by the first vidicon 60,
the output signal of the first vidicon 60 has the form of a square
wave in which the period of the square wave is linearly increasing
with time or linearly decreasing with time. The first delay line 62
to which the signal is applied is responsive to the repetition
frequency of the square wave, so that for purpose of this analysis
the higher order harmonics of the square wave can be disregarded.
Therefore, the chirped square wave may be regarded as a chirped
sine wave having a frequency which is linearly increasing with time
as the image of the apertures 70A-D is scanned in the direction
from aperture 70A to aperture 70D, the instantaneous frequency of
the chirped sign wave being linearly decreasing in time when the
image of the apertures 70A-D is scanned in the direction from
aperture 70D toward aperture 70A. As shown in FIG. 3 the dimensions
of contiguous opaque and transparent regions of the mask differ
only slightly in the direction of scanning, with the spacings
between the respective apertures 70A-D decreasing linearly as do
the widths of the apertures 70A-D.
In the dimension perpendicular to the direction of scanning, which
for ease of reference may be referred to as the height of the
apertures, is uniform form along the row of apertures 70A-D. The
height of the apertures in the next row of apertures, namely the
height of the apertures 72A-D is also of uniform magnitude along
the row of apertures 72A-D, but is smaller than the height of the
apertures in the row of apertures 70A-D. The heights of the
successive rows and of the spacings between the successive rows
decreases linearly so that the height of the row of apertures 74A-D
is smaller than that of the row of apertures 72A-D, and similarly
the height of the row of apertures 76A-D is smaller than that of
the row of apertures 74A-D. In this way a vertical scanning of the
image or shadow of the mask 30 results similarly in a chirped
waveform.
Returning now to FIG. 2 the horizontal scanning of the first
storage screen 54 by the first vidicon 60 results in a chirped
waveform for the reasons described hereinabove with reference to
FIG. 3. The chirped square wave of the first vidicon 60 is applied
to the first delay line 62 which is frequency dispersive and
furthermore has a phase or time delay characteristic which is the
inverse (or the mirror image) of the chirped square wave. Signals
of different frequencies experience different time delays in
progressing through the first delay line 62. The first vidicon 60
together with the first delay line 62 may be regarded as a
transmission medium through which portions of the image on the
first storage screen 54 are sequentially transmitted, the medium
being characterized by a differential delay imparted to the various
portions of the image. As is well known from the theory of matched
filters and the pulse compression filters utilized in radar systems
(for example, see the articla entitled "The Theory and Design of
Chirp Radars" by J. R. Klauder, A. C. Price, S. Darlington and W.
J. Albersheim in the Bell System Technical Journal of July, 1960,
Volume 39, pages 745-808), a filter having an impulse response
which is the inverse of the time waveform of the input signal
applied to the filter provides an output signal in the form of a
narrow pulse. In the case of a wide bandwidth input signal, such as
the chirped waveform of the imaging system 22, the output signal of
such a filter approximates an impulse. Accordingly, the output
signal of the first delay line 62, assuming the mask 30 to be
illuminated by the single source 50, may be regarded as an impulse
which corresponds to the position of the source 50. The impulse is
displayed on the screen 78 of a second storage tube display 80. The
position of the displayed impulse on the second storage screen 78
is in response to the location of the source 50 relative to the
mask 30 and the detector assembly 24. Thus, the imaging system 22
can show the direction to a source of high energy particles.
If the mask 30 were illuminated by high energy radiation from the
source 56 which is spaced apart from the source 50, the resulting
image on the first storage screen 54 would differ from the image
obtained by illumination of the mask by source 50. The chirped wave
form signal being produced by the first vidicon 60 in response to
the scanning of the image produced by the source 56 differs from
that associated with source 50 in that the occurrence of a
particular instantaneous value of pulse repetition frequency is
attained at a different instant of time relative to the interval of
scanning by the first vidicon 60. Accordingly, the output pulse
from the first delay line 62 corresponding to illumination by
source 56 occurs at a different time relative to the scanning
interval of the first vidicon 60 than would the output pulse
corresponding to the illumination by the source 50. Thus the
display on the second storage screen 78 shows a point representing
the location of source 56 at a location which differs from that of
the image point which represented the location of the source
50.
The imaging system 22 is linear and accordingly superposition
applies so that illumination of the mask 30 by both sources 50 and
56 produces an image on the first storage screen 54 which has the
form of the superposition of the two individual images resulting
from illumination by source 50 and source 56. Similarly, the output
signal of the first vidicon 60 attained upon a scanning of the
first storage screen 54 is the superposition of two chirp
waveforms. The first delay line 62 responds to the superposition of
the two chirped waveforms in the same manner that it responds to
each of the waveforms individually and accordingly provides two
output pulses corresponding in time to the locations of the sources
50 and 56. Thus, there appears on the second storage screen 78 two
image points corresponding to the locations of the source 50 and
56. By an extension of the superposition principle it becomes
apparent that with a multiplicity of sources of high energy
radiation in the object 34, such as for example, the individual
radioactive molecules of the radio pharmaceutical within the
thyroid gland 21, a multiplicity of image points appear on the
second storage screen 78, each of these points corresponding to the
locations of the individual sources of the high energy radiation in
the object 34. Thus, there appears on the second storage screen 78
a partially unscrambled image of the object 34, the image being
unscrambled in the horizontal direction due to the decoding by the
first vidicon 60 and first delay line 62 but still being scrambled
in the vertical dimension.
The second step in the decoding of the image of the mask 30 is
performed by the second vidicon 82 and the second delay line 84.
The second vidicon 82 scans the image on the second storage screen
79 in the vertical direction and provides a corresponding chirp
waveform which is applied to the second delay line 84. The second
delay line 84 operates in the same manner as does the first delay
line 62 and, accordingly, it responds to the chirp waveform signal
from the second vidicon 82 by providing a set of output pulses
corresponding to the locations in the vertical plane of the sources
of high energy radiation of the object 34. The output pulses of the
second delay line 84 are transmitted to the output display 28 which
shows a fully unscrambled or decoded image of the object 34. Thus,
it is seen that the first vidicon 60 and the first delay line 62
resolve the locations of the sources such as the sources 50, 56 and
58 in the horizontal direction while the second vidicon 82 and
second delay line 84 resolve the locations of the sources 50, 56
and 58 in the vertical direction.
Referring now to FIGS. 5 and 6 there are shown two plan views of
delay lines. In the preferred embodiment of FIG. 2, the first delay
line 62 and the second delay line 84 are identical since the mask
30 of FIG. 3 has the same configuration and arrangement of
apertures 32 in both the rows and the columns. Accordingly, FIGS. 5
and 6 are equally applicable to both the first delay line 62 and
the second delay line 84. The delay line 86 of FIG. 5 comprises an
elongated piezoelectric crystal 88 upon which is mounted a pair of
interdigital electrical networks, one interdigital network 89A
serving as the input for generating surface acoustic waves on the
crystal 88 and a second interdigital network 89B located at the
output end of the crystal 88 for extracting an electrical signal
from the crystal 88. The input interdigital network 89A comprises a
pair of opposed interlaced combs 90 and 92 having fingers 94, four
of which are designated 94A-D, which are spaced in accordance with
a predetermined format. In addition, the length of the fingers 94
may be varied in a prescribed format as shown with the fingers 96
of delay line 97 in FIG. 6. The varying degrees of overlapping
between the fingers 96 of the opposed combs 98 and 100 provide
varying degrees of coupling of energy between the interdigital
network 102 and the crystal 104. The design of a delay line such as
the delay line 86 is described in the Pat. to J. H. Rowen U.S. Pat.
No. 3,289,114 which issued on Nov. 29, 1966, and in an article
entitled "Tapping Microwave Acoustics for Better Signal
Processing," by L. Altman, J. H. Collins and P. J. Hagon, which
appeared in Electronics p. 94 et. seq. Nov. 10, 1969.
The input terminals to the delay line 86 comprise extensions of the
pair of combs 90 and 92. An input electrical signal having a
voltage V.sub.IN is applied across the two terminals. The fingers
94A and 94B are spaced apart a distance X while the fingers 94C and
94D are spaced apart by a distance Y. The electrical energy in the
input signal is coupled into the crystal 88 and transformed into
mechanical energy of the crystal 88 at a first frequency dependent
on the spacing X and at a second frequency dependent on the spacing
Y. The mechanical energy is indicated by a wave 106 shown by a
series of wavey arrows. Thus, energy is coupled at the first
frequency between fingers 94A, 94B and the crystal 88, and at the
second frequency between the fingers 94C, 94D and the crystal
88.
The reverse mechanism mainly the conversion of the mechanical
energy of the crystal 88 into electrical energy occurs at the
output end of the delay line 86. In the output interdigital network
89B there are also a pair of fingers spaced at a distance X,
namely, fingers 108A and 108B, and similarly there are also a pair
of fingers spaced at a distance Y, namely, fingers 108C and 108D.
Mechanical energy at the first frequency is coupled out from the
crystal 88 by means of fingers 108A and 108B, and that at the
second frequency by means of the fingers 108C and 108D The delay
line 86 is made dispersive to provide different delays at different
frequencies by arranging the input and output interdigital networks
89A and 89B such that they are the mirror images of each other with
respect to the centerline of the crystal. Thus, for example, the
two spacings of distance X are symmetrically located relative to
the centerline of the crystal 88 and similarly the two spacings of
distance Y are symmetrically located relative to the centerline of
the crystal 88, however the spacings of distance X are further away
from the centerline than the spacings of distance Y. As a result,
energy at the first frequency traverses a greater portion of the
crystal 88 than does energy of the second frequency, and
consequently experiences a greater delay. Thus where the signal
voltage V.sub.IN is a chirped signal in which the instantaneous
frequency is increasing, the energy at each of these frequencies is
selectively delayed with the result that substantially all of the
energy appears at the output terminals at the same instant of time.
Thus, the output voltage V.sub.OUT is a pulse of energy
approximating an impulse.
The delay line 86 is commonly referred to as a surface wave delay
line since accoustic energy in the form of mechanical vibrations
travel along the surface of the crystal as indicated by the wave
106. The delay line 86 can be designed to approximate various
filter characteristics by adjusting the amount of overlap between
adjacent fingers of opposed combs as shown by fingers 96 of in FIG.
6. For example, if it is desired to pass energy at one frequency
but attentuate energy at a second frequency than a relatively large
amount of overlap is provided for the first frequency and a minimal
amount of overlap is provided at the second frequency. In this way
a relatively large amount of energy is coupled at the first
frequency and passes through the delay line 86 while a minimal
amount of energy is coupled at the second frequency resulting in
attenuation at that frequency.
Returning again to FIG. 2 a scan controller 110 coordinates the
scannings of the first and second vidicons 60 and 82, the second
storage tube display 80 and the output display 28 so that each of
these scannings occur with the correct temporal relationship.
Accordingly, in the operation of the second storage tube display 80
the successive horizon deflections of the cathode ray tube beam are
delayed from the corresponding horizontal line scans of the first
vidicon 60 by an amount of time equal to the minimum time delay of
the first delay line 62, that is, the amount of time required for
energy to first appear at the output of the first delay line 62 in
response to a signal from the first vidicon 60. The scanning of the
second vidicon 82 is delayed until the full image has been composed
upon the second storage screen 78. The operations of the output
display 28 and of the second vidicon 82 are delayed by an amount
equal to the minimum delay time of the second delay line 84. The
aforesaid temporal relationships among the various scannings assure
that the images provided on the various displays are appropriately
centered relative to the display. It is also apparent that,
alternatively, the horizontal and vertical scanning procedures may
be interchanged such that the image on the first storage screen 54
is vertically scanned while the image on the second storage screen
78 is horizontally scanned.
By way of alternative embodiments it is noted that the first
storage tube display 52 and the first vidicon 60 could be replaced
by a single well-known scan converter tube (not shown) comprising
read and write electron beams and a storage screen if the
persistence of the storage screen is sufficient to permit the
development of the image of the mask 30 during the successive
impacts of quanta of radiant energy upon the scintillator 36. The
use of scan converter tubes is well known and is not shown in the
figures. Similarly, the second storage tube display 78 and the
second vidicon 82 may be replaced with a single scan converter
tube.
Focusing of the imaging system 22 of FIG. 2 to provide the desired
spacings between the object 34, the mask 30 and the detector
assembly 24 is accomplished as follows. The object 34, mask 30 and
detector assembly 24 are spaced apart such that the shadow or image
of the mask 30 due to illumination by a single source of the high
energy radiation is smaller than the scintillator 36. Thereby, each
of the shadows of mask 30 corresponding to illumination by
successive sources, such as sources 50, 56 and 58, fall wholly
within the area of the scintillator 36. If the size of the mask 30
approximates that of the scintillator 36 than it is apparent that
the shadow due to illumination by source 58 may lie wholly within
the area of the scintillator 36 while the shadow due to
illumination by source 50 has its upper edge outside the area of
the scintillator while the shadow due to illumination by source 56
has its lower edge outside the area of the scintillator. As a
result with an overly large mask not all of the sources such as the
sources 50 and 56 receive the full benefit of the mask 30 during
the formation of the images of these sources by the imaging system
22. On the other hand, a large mask permits increased resolution in
that a greater range of aperture sizes may be formed within the
mask.
The spatial bandwidth, and hence, the resolution attainable with
the imaging system 22 is determined by the difference between the
smallest aperture size and the largest aperture size of the mask
30. A large number of apertures 32 are utilized to insure small
gradations in size between adjacent apertures, thereby providing a
smooth transistion in the spatial frequency domain from the lowest
spacial frequency to the highest spatial frequency and, thereby
further providing output signals from the first and the second
vidicons 60 and 82 which have a smooth spectral distribution. As a
result, the delay lines 62 and 84, having temporal impulse response
functions which are the inverse of the vidicon output signals,
function as pulse compression filters providing minimal side lobes.
If the spatial bandwidth is retained but the number of apertures 32
in the mask 30 is decreased, that is there are larger jumps in size
between adjacent apertures 32, then the extent of the side lobes in
the output signals from the delay lines 62 and 84 increases.
Therefore, it is seen that it is desirable to use a large mask yet
retain a sufficiently small size such that all the sources of the
high energy radiation provide shadows which fall within the area of
the scintillator 36. It is convenient to regard the field of view
of the imaging system 22 as being the maximum spacings between
sources of the high energy radiation such that there is no
reduction in the attainable resolution due to an extension of a
shadow of the mask 30 beyond the area of the scintillator 36.
Another advantage of a large mask is the increased aperture of the
imaging system 22 due to the fact that more rays of radiant energy
are intercepted by the mask 30 and processed. A larger aperture
means decreased viewing time so that, in the case of the patient 20
of FIG. 1 being treated for a thyroid ailment, less exposure time
is required to obtain the radiograph of the thyroid gland 21. In
particular, it is noted that the mask 30 provides a greater
aperture or efficiency than does either the pin hole version of the
Anger camera or the collimator version of the Anger camera, both
disclosed in the aforementioned patent to Anger. As compared to the
pinhole camera, the imaging system 22 attains a greater efficiency
because there is a larger total aperture due to the summation of
all the apertures 32 in the mask 30; and with reference to the
collimator version of th Anger camera, the imaging system 22
attains a greater efficiency due to the fact that a relatively
large number of high energy photons strike the septal separations
within the collimator so that only those photons directed in a
direction parallel to the collimator axis reach the
scintillator.
For precise focusing of the imaging system 22 on the object 34, a
predetermined scan rate provided by rate selector 112 is applied by
means of the scan controller 110 to the first vidicon 60, the scan
rate being selected so that the image of a point source on the
first storage screen 54 is scanned in a time interval of a preset
value independently of the size of this image. It is readily
apparent tha the dimensions of this image are proportional to the
dimensions of the mask 30 and, furthermore, related to the distance
from the object 34 to the mask 30 and from the mask 30 to the
scintillator 36. For example, with reference to the patient 20 of
FIG. 1, if the patient were to move away from the imaging system
22, then the image formed upon the first storage screen 54 would
become smaller. If the scan rate applied by the scan selector 112
were to remain at a preset value, then it is apparent that the
image, due to its reduced size, would be scanned in a
correspondingly reduced interval of time with the result that the
frequency components occurring in the output signal of the first
vidicon 60 would be scaled to a correspondingly higher value which
may greater than that for which the first delay line 62 has been
designed. To compensate for this motion of the patient 20 of FIG.
1, either the mask 30 is to be positioned further away from the
detector assembly 24 by suitable means (not shown) thereby
restoring the image on the first storage screen 54 to its original
size, or alternatively, by reducing the scan rate applied by the
scan selector 112 so that the reduced size image is scanned in a
time interval having the preset value. Since the field of view of
the imaging system 22 is dependent on the relative distances from
the object 34 to the mask 30 and from the mask 30 to the
scintillator 36, it is preferable to adjust the focusing by means
of the scan rate applied by the scan controller 110 to the first
vidicon 60.
The following relationships, indicated mathematically, are useful
in the design of the imaging system 22. The compression ratio
resulting from the use of the chirped signal from the first vidicon
60 and the conjugate time delay characteristic of the first delay
line 62 may be expressed as the ratio of the width of an image on
the first storage screen 54, corresponding to a point source of
radiation, to the width of the mask 30. The compression ratio C is
given by
C = L.sub.a BW.sub.s
where l.sub.a is the width of the mask 30 and BW.sub.s is the
spatial frequency bandwidth which is given as the difference
between the minimum and the maximum spatial frequencies of the mask
pattern. For example, a uniform arrangement of 10 equal apertures
within a distance of one inch would give a spatial frequency
spectrum described by a single line of value 10 line-pairs per
inch. As a further example consider a mask having a chirped pattern
in which the apertures are spaced at a rate of 200 apertures per
inch near one edge of the mask and at a rate of 100 apertures per
inch near the opposite edge of the mask. In this example, the
spatial frequency bandwidth is 100 line-pairs per inch.
The field of view mentioned hereinabove is given in the following
equation:
F.sub.V = (s.sub.1 /s.sub.2) (l.sub.i - l.sub.a) - l.sub.a
where F.sub.V is the field of view, s.sub.1 is the distance between
the object and the mask, s.sub.2 is the distance between the mask
and the image plane at the face of the scintillator 36, and l.sub.i
is the length of the image plane at the scintillator 36. The
resolution in line-pairs per inch is given by
R.sub.. = BW.sub.F R.sub.i (s.sub.2)/(2s.sub.1)
where R.sub.o is the resolution in the horizontal dimension of the
image on the first storage screen 54 in line-pairs per inch,
BW.sub.F is the fractional bandwidth of the first delay line 62
which is the bandwidth of the delay line divided by the maximum
frequency of the delay line, and R.sub.i is the minimum resolution
of the scintillator 36 which depends on such factors as the
thickness of the scintillator 36.
By way of example in constructing the preferred embodiment of the
imaging system 22, the first and the second delay lines 62 and 84
are each operated over a frequency of from 2.8 megahertz to 4
megahertz, and are fabricated from a quartz crystal 81/2 inches in
length; each of the interdigital networks 89A and 89B comprise a
pair of opposed combs, each comb having approximately 100 fingers.
The mask 30 in the case of X-radiation is a thin lead film of
approximately 3 microns (3.times.10.sup.-.sup.6 meters) in depth.
The transparent substrate 65 (seen in FIG. 3) for supporting the
thin film is fabricated from a 1/8 inch thick plate of aluminum.
For gamma radiation at a energy of 100 Kev the thickness of the
lead film is approximately one-half millimeter. The mask 30 has a
square shape, each side being 2 inches long. There are 100
apertures along a side giving a total number of apertures of
10,000.
The number of apertures that can be placed upon a 2 inch square
mask is limited by the thickness of the mask, since it is desired
to provide an aperture size which is much greater than the depth of
the mask to avoid producing a structure similar to the collimator
shown in the aforementioned patent to Anger. As shown in FIG. 2 the
rays of radiation indicated by the lines 38A-D fan out from the
source 50 through the apertures in the mask 30 to illuminate the
scintillator 36; such illumination of the scintillator 36, namely,
by diverging rays, would be precluded by the collimator structure
shown in the aforementioned patent to Anger.
The size of the mask 30 is smaller than the size of the
scintillator 36 for reasons which can be readily appreciated by the
following example. If the scintillator 36 were to have a width of 4
inches and the mask were to have a width of 2 inches, then for an
equal spacing of the mask between the object 34 and the
scintillator 36 a point source on the object 34 could completely
illuminate the scintillator 36 with an image or shadow of the mask
30. Then, if a second source were positioned alongside the first
source to illuminate the scintillator 36, the shadow cast by the
mask 30 due to the second source would not fall wholly upon the
scintillator 36. In view of the equations given above for the field
of view and the resolution, it is apparent that the imaging is more
readily accomplished if the mask size is less than half the size of
the scintillator 36. For example, a 2 inch maks and an 8 inch or 10
scintillator may be utilized.
Referring now to FIG. 7 there is shown an alternative embodiment of
the imaging system which may be utilized for providing a radiograph
of the object 34. The radiation from object 34 passes through the
apertures in the mask 30 to impinge upon a photographic film 122
carried by a reel assembly 124. An aperture stop 126 delineates the
boundaries of the image formed upon the film 122. In this
embodiment the radiograph is a negative rather than the positive
provided in FIG. 2.
The film 122 is developed by any suitable means (not shown) so that
the image on the film can be illuminated by a light beam. A beam of
light 128 is provided by lantern 130 and collimated by lens 132 to
illuminate the image on the film 122 which has been formed in
response to radiation passing through the mask 30. In this
embodiment the film serves as both the detector assembly 24 and the
first storage tube display 52 of the embodiment of FIG. 2. The
remaining portions of the embodiment of FIG. 7, such as the vidicon
134, which corresponds to the first vidicon 60 of FIG. 2 are the
same as those shown in FIG. 2.
Referring now to FIGS. 8 and 9 there is shown an alternative
embodiment of the invention wherein the spatial modulation of the
high energy radiation which was provided by the mask 30 of FIG. 2
is now provided by a source of radiation 136 which comprises a
novel arrangement of emissive material such as radioactive material
138 deposited on a substrate 140 and selectively etched to provide
areas of radiation having the same shape and configuration as the
apertures 32 of the mask 30 of FIGS. 2 and 3. The object 142 is
partially opaque so that points of the object 142 such as the
points 144, 146 and 148 are illuminated by the source of radiation
136 to form an image upon the detector assembly 24. The detector
assembly 24, processor 26 and output display 28 shown in FIG. 8 are
the same as those utilized in FIG. 2. In FIG. 2 each point source,
such as the source 50, is transformed to a shadow of the mask 30 at
the detector assembly 24. Analogously, in FIG. 8 each point of the
object 142 is transformed into an image at the detector assembly
24, the image depending on the form of the pattern of radioactive
material 138 deposited upon the substrate 140 of the source
136.
Referring now to FIG. 10 there is an X-ray radiographic system 150
utilizing a source 152 of X-rays to be described hereinafter, a
photographic film 154 carried by a reel assembly 156, and an object
158 to be illuminated by the source 152 for forming an image on the
film 154. After forming the image on the film 154 the film is
developed by any suitable means (not shown) and positioned such
that the image falls within a beam of light 160 formed by a lantern
162 and collimating lens 164 for processing by the optical system
166. The optical system 166 is of a well known form and is often
used for extracting information from a radiograph. The optical
system 166 comprises a pair of lenses 168 and 170 with a spatial
optical filter 172 placed between the lenses, and a screen 174 upon
which a filtered manifestation of the image appears. As is well
known spatial filtering is used in a manner analogously to the
filtering of time domain signals to extract those portions of the
signal having a desired frequency characteristic and suppressing
other portions of the signal having other frequency
characteristics. The spatial filter 172 has portions of varying
opacity to inhibit the passage of selected spacial frequencies. In
this way certain features of a radiograph are made more readily
visible.
Of particular interest is the fact that a broad band optical signal
can produce greater definition in a radiograph when proper
filtering is employed. A point source of high energy radiation
provides a relatively large spatial bandwidth. On the other hand a
relatively large source provides a relatively narrow spatial
bandwidth. As is well known, the sources of radiation which most
closely approximate the point source and therefore have the largest
spatial bandwidth provide a radiograph with the best definition or,
equivalently, the clearest picture. The use of the novel source 136
(of FIG. 9) of this invention as the source 152 in the radiographic
system 150 of FIG. 10 provides radiation having a large spatial
bandwidth, the extent of the bandwidth being directly related to
the number of radiant regions (such as the regions of radioactive
material 138) per unit area and their configuration. In particular,
the source as shown in FIG. 9 provides a chirp wave form bandwidth
characteristic analagous to that obtained by use of the mask 30 in
the imaging system 22 of FIG. 2. This broad bandwidth may be
utilized in conventional spatial filtering techniques such as that
shown in FIG. 10 for enhancing the definition of an object, or
alternatively may be utilized in the system of FIG. 8 as described
hereinbefore.
Referring now to FIG. 11, there is shown an alternative embodiment
of the imaging system of the invention wherein the scanning of the
scrambled image on the face of the detector assembly 24 is
accomplished in a two-step procedure in which the horizontal
scanning is done mechanically and the vertical scanning is done
electronically. In this embodiment the mask 30 of FIG. 2 is
replaced with a mask 176 having a single column of apertures 178
which provides a scrambled image having a height similar to the
height obtained with the embodiment of FIG. 2, but having a width
which is sufficiently narrow so that the scrambled image
approximates a line image. A collimator 180 comprising a single
slot 182 cut within a lead block 184 is used to collimate the rays
of radiation emanating from the object 34 so that only those rays
within the column of apertures 178 can pass to the detector
assembly 24.
The mask 176 is mounted upon a substrate 186 similar to the rigid
substrate 65 of FIG. 3. The collimator 180, the mask 176, and a
mask substrate 186 are supported upon a movable rack 188 which is
slidably mounted upon a track 190 affixed to a block 191 and
tightened in position against the track 190 by means of set screw
192. The movable rack 188 is utilized to position the mask 176 and
the collimator 180 for focusing the image of the object 34 in a
manner similar to that described with reference to the embodiment
of FIG. 2.
In the embodiment of FIG. 11 the detector assembly 24 and a storage
tube display 194 function in the same manner as the detector
assembly 24 and the first storage tube display 52 of FIG. 2. A
vidicon 196 is programmed by a scan controller 198 to scan a single
vertical line scan repetitively, rather than the sequence of
horizontally displaced vertical line scans that is associated with
a television raster type of scan. The output signal of the vidicon
196 is processed by a delay line 200 and subsequently displayed on
an output display 202 in a manner similar to that described in FIG.
2 with reference to the second delay line 84 and the output display
28. Each point of the scrambled image of the line scan displayed on
the storage screen 204 corresponding to a point, such as point 206,
on the object 34 is compressed by the delay line 200 into a single
point of the line displayed on the output display 202.
The mechanical scanning in the horizontal direction is accomplished
by means of a mechanical scanner 208 comprising a rod 210, slidably
mounted through the block 191 and a threaded rod 212, passing
through a tapped hole in the block 191, which support the movable
rack 188. The rod 210 and the threaded rod 212 are supported at
their first ends by a mount 214 and at their opposite ends by a
second mount, similar to mount 214 but not shown in the Figure. The
theaded rod 212 is journalled in the mount 214 and passes through
the mount 214 to make contact with a gear 216 by which the threaded
rod 212 is rotated in the manner of a worm drive to impart a
horizontal displacement in the position of the block 191 in
accordance with the amount of rotation of the theaded rod 212 and
the gear 216. The gear 216 is driven by a motor 218 through a
pinion 220 mounted on the shaft (not shown) of the motor 218 and
meshing with the gear 216. The motor 218 is a well known form of
electric motor, such as a shunt wound motor, wherein the direction
of rotation of the motor shaft can be varied electrically, as for
example, by reversing the direction of current which energizes the
rotor winding while retaining the direction of current which
energizes the stator winding. In this way the movable rack 188 can
be moved back and forth in the horizontal direction.
An electrical signal representing the position of the block 191 is
provided by a potentiometer 222 mechanically connected to gear 216
via a gear train 224, indicated diagrammatically in FIG. 11, having
a pinion 226 which meshes with the gear 216. In this way rotation
of the potentiometer shaft (not shown) is proportional to the
rotation of the threaded rod 212 and, therefore, to the
displacement of the block 191.
The horizontal and vertical scanning are coordinated by means of
the scan controller 198 which provides a signal along line 230 to
the storage tube display 194 for erasing the line image on the
storage screen 204 after each vertical scan of the vidicon 196 so
that a new scrambled image in the form of a vertical line on the
storage screen 204 can be composed for each position of the block
191. The output display 202 has a storage screen to permit direct
viewing of the information provided by the successive line scans.
The mechanical scanner 208 is driven in response to signals along
line 232 provided by the scan controller 198. Signals from the
potentiometer 222 representing the position of the block 191 are
transmitted to the scan controller 198 along line 234. Each line
scan by the vidicon 196 is provided in response to a signal on line
236 from the scan controller 198. The scanning rate is selected by
means of a rate selector 238 which connects with the scan
controller 198 and functions in a manner similar to that shown in
FIG. 2 with reference to the rate selector 112 for focusing the
imaging system.
The output display 202 employs a well known cathode ray tube (not
shown) for displaying an image of the object 34. Deflection signals
for the cathode ray tube of the output display 202 are provided in
accordance with signals from the scan controller 198 along line
240. The vertical deflection signals for the output display 202
correspond to the vertical deflection signals of the vidicon 196,
and the horizontal deflection signals for the output display 202
correspond to the signals along line 234 from the potentiometer
222.
It should be noted that the image displayed on the output display
202 of FIG. 11 differs from that displayed by the output display 28
of FIG. 2 in that the output display 28 provides compression in two
dimensions while the output display 202 of FIG. 11 provides an
image which is compressed in only the vertical direction.
Compression in the vertical direction, only, has occurred by virtue
of the fact that the mask 176 of FIG. 11 provides only a single
column of apertures 178, while in the imaging system 22 of FIG. 2
the mask 30 has a two-dimensional array of both columns and ros of
apertures 32.
Referring now to FIG. 12 there is shown an alternative embodiment
of the imaging system of FIG. 2 wherein the detector assembly 24
and the first storage tube display 52 are replaced by an image
intensifier 242 comprising a scintillator 244, a glass plate 246
contiguous to the scintillator 244 and supporting a photocathode
248 in the form of a thin film, and an anode 250 which are enclosed
by an envelope 252 for maintaining a vacuum between the
photocathode 248 and the anode 250. A differenence of potential is
maintained between the photocathode 248 and the anode 250 by a
suitable voltage source (not shown). Electrons emitted by the
photocathode 248 are focused by suitable means, such as a well
known magnetic deflection system (not shown) concentric to the
envelope 252 for providing an image on a screen 254.
The object 34 and mask 30 are positioned in front of the image
intensifier 242. In response to radiation emitted by object 34 and
passing through apertures 32 in mask 30 to the scintillator 244,
the scintillator 244 emits optical photons which interact with the
photocathode 248 causing it to emit electrons. The sites on the
photocathode 248 from which the electrons emanate correspond to the
sites on the scintillator 244 at which high energy photons from
object 34 impact. Accordingly, the image on the screen 254 has the
same form as the image appearing on the first storage screen 54 of
FIG. 2. The screen 254 is then scanned by vidicon 256 in the same
manner as the storage screen 54 of FIG. 2 is scanned by the first
vidicon 60. The remainder of this alternative embodiment of the
imaging system is the same as that of the imaging system 22 of FIG.
2 and is therefore not shown in FIG. 12.
It is understood that the above described embodiments of the
invention are illustrative only and that modifications thereof will
occur to those skilled in the art. Accordingly, it is desired that
this invention is not to be limited to the embodiments disclosed
herein, but is to be limited only as defined by the appended
claims.
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