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Description  |
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FIELD OF THE INVENTION
The invention relates to novel X-ray intensifying screens. More
specifically, the invention relates to fluorescent screens of the type
used to absorb an image pattern of X-radiation and emit a corresponding
pattern of longer wavelength electromagnetic radiation to imagewise expose
a radiographic element.
BACKGROUND OF THE INVENTION
In silver halide photography one or more radiation-sensitive emulsion
layers are coated on a support and imagewise exposed to electromagnetic
radiation to produce a latent image in the emulsion layer or layers. The
latent image is converted to a viewable image upon subsequent processing.
Roentgen discovered X-radiation by the inadvertent exposure of a silver
halide photographic element to X-rays. In 1913 the Eastman Kodak Company
introduced its first silver halide photographic element specifically
intended to be exposed by X-radiation--i.e., its first silver halide
radiographic element.
The medical diagnostic value of radiographic imaging is accepted.
Nevertheless, the desirability of limiting patient exposure to X-radiation
has been appreciated from the inception of medical radiography. Silver
halide radiographic elements are more responsive to longer wavelength
electromagnetic radiation than to X-radiation. As herein employed the term
"longer wavelength electromagnetic radiation" or "emitted radiation",
except as otherwise qualified, indicates electromagnetic radiation in the
300 to 1500 nm spectral range, including both the near ultraviolet and
blue regions of the spectrum to which silver halide possesses native
sensitivity and the visible and near infrared portions of the spectrum to
which silver halide is readily spectrally sensitized. Low X-ray absorption
by silver halide radiographic elements as compared to absorption of longer
wavelength electromagnetic radiation led quickly to the use of
intensifying screens. The Patterson Screen Company in 1918 introduced
matched intensifying screens for Kodak's first dual coated
(Duplitized.RTM.) radiographic element. An intensifying screen contains on
a support a fluorescent phosphor layer that absorbs the X-radiation more
efficiently than silver halide and emits to the adjacent radiographic
element longer wavelength electromagnetic radiation in an image pattern
corresponding to that of the X-radiation received.
The need to increase the diagnostic capabilities of radiographic imaging
while minimizing patient exposure to X-radiation has presented a
diligently addressed challenge of long standing in the construction of
both radiographic elements and intensifying screens. In constructing
intensifying screens the ideal aim is to achieve the maximum longer
wavelength electromagnetic radiation emission possible for a given level
of X-radiation exposure (which is realized as maximum imaging speed) while
obtaining the highest achievable level of image definition (i.e.,
sharpness or acuity). Since maximum speed and maximum sharpness in
intensifying screen construction are not compatible, actual screens
represent the best attainable compromise for their intended application.
The choice of a support for an intensifying screen illustrates the mutually
exclusive choices that are confronted in screen optimization. It is
generally recognized that supports having a high level of absorption of
emitted longer wavelength electromagnetic radiation produce the sharpest
radiographic images. Intensifying screens which produce the sharpest
images are commonly constructed with black supports or supports loaded
with carbon particles. Often transparent screen supports are employed with
the intensifying screen being mounted in a cassette for exposure along
with a black backing layer. In these screen constructions sharpness is
improved at the expense of speed by failing to direct to the adjacent
radiographic element a portion of the emitted longer wavelength
electromagnetic radiation that might otherwise be available for latent
image formation.
If a black or transparent intensifying screen support is replaced by a more
reflective support, a substantial increase in speed can be realized. The
most common conventional approach is to load or coat a screen support with
a white pigment, such as titania or barium sulfate. Juliano U.S. Pat. No.
3,787,238, Degenhardt U.S. Pat. No. b 4,318,001, and Ochiai U.S. Pat. No.
4,501,971, are offered as illustrative only, since the majority of well
drafted patents describing intensifying screen constructions mention at
least in passing similar options for support construction.
Even the best reflective supports identified by the art for intensifying
screen construction have degraded image sharpness in relation to imaging
speed so as to restrict their use to applications less demanding of image
definition. Further, many types of reflective supports that have been
found suitable for other purposes have been tried and rejected for use in
intensifying screens. For example, the loading of intensifying screen
supports with optical brighteners, widely employed as "whiteners", has
been found to be incompatible with achieving satisfactory image
definition.
By a process of trial and error over a development period of approximately
70 years the intensifying screen art has developed a bias for the
selection of reflective supports from a relatively limited class of
constructions and against regarding as suitable for intensifying screen
construction support elements that, though nominally reflective, were
developed for other, less demanding purposes.
During the last quarter century, a period in which the potentially
deleterious effects of even low levels of X-radiation exposure have been
publically called into question and a period in which every obvious
expedient and a virtual continuum of inventions have been pressed into
service to increase the capabilities of diagnostic radiographic imaging
while reducing patient X-ray exposure, there has existed in the art a
class of reflective supports that have never been suggested for use in
intensifying screens, hereinafter referred to as "stretch cavitation
microvoided" supports.
In 1964, Johnson U.S. Pat. No. 3,154,461, disclosed a polymeric film loaded
with microbeads of calcium carbonate of from 1 to 5 .mu.m in size. By
biaxially stretching the support, stretch cavitation microvoids were
introduced, rendering the support opaque.
Primary interest in stretch cavitation microvoided supports has centered on
imparting to polymer film supports paper-like qualities, as illustrated by
Takashi et al U.S. Pat. No. 4,318,950; Toyoda et al U.S. Pat. No.
4,340,639; Ashcraft et al U.S. Pat. Nos. 4,377,616 and 4,438,175; and H.
H. Morris and P. I. Prescott, "White Opaque Plastic Film and Fiber for
Papermaking Use," ACS Div. Org. Coatings Plastic Chemistry, Vol. 34, pp.
75-80, 1974.
More recently, stretch cavitation microvoided supports have been
investigated as possible replacements for photographic print supports, as
illustrated by Mathews et al U.S. Pat. Nos. 3,944,699 and 4,187,113 and
Remmington et al U.K. Patent Specifications 1,593,591 and 1,593,592.
Polypropylene microbeads are in one instance employed, but the preferred
microbeads are white pigment barium sulfate microbeads.
Pollock et al U.S. Ser. No. 47,821, filed May 5, 1987, titled SHAPED
ARTICLES FROM POLYESTERS AND CELLULOSE ESTER COMPOSITIONS, commonly
assigned, discloses stretch cavitation microvoided shaped articles, such
as films, sheets, bottles, tubes, fibers, and rods, wherein the polymer
forming the continuous phase is a polyester and the microbeads are a
cellulose ester.
From the 1960 filing of Johnson U.S. Pat. No. 3,154,461 until this
invention there has been no suggestion that stretch cavitation microvoided
supports would be suitable for the demanding requirements of radiographic
intensifying screens.
SUMMARY OF THE INVENTION
It is a recognition of this invention that superior intensifying screens
for use with silver halide radiographic elements can be constructed
exhibiting a balance of imaging speed and sharpness not heretofore
achieved in the art.
In one aspect, this invention is directed to an intensifying screen for
producing a latent image in a silver halide radiographic element when
imagewise exposed to X-radiation comprised of (i) a fluorescent layer
capable of absorbing X-radiation and emitting for latent image formation
longer wavelength electromagnetic radiation more readily absorbed by the
silver halide radiographic element when X-radiation and (ii) a support
capable of reflecting the longer wavelength radiation, characterized in
that at least one portion of the support is comprised of reflective
lenslets.
In a specific preferred implementation, the reflective portion of the
support is comprised of three distinct phases: (a) a polymeric continuous
phase transparent to the longer wavelength electromagnetic radiation, (b)
immiscible microbeads forming a dispersed second phase in the polymeric
phase, and (c) stretch cavitation microvoids forming reflective lenslets
concentrically positioned with respect the microbeads and having major
axes oriented parallel to the fluorescent layer. In a specific preferred
embodiment of this implementation the microbeads are themselves
transparent to the longer wavelength electromagnetic radiation.
In another preferred implementation, the reflective portion of the support
is comprised of spherical or spheroidal beads transparent to the lower
wavelength electromagnetic radiation dispersed in a polymeric continuous
phase, wherein the refractive index of the beads exceeds the refractive
index of the continuous phase. In a specific embodiment of this
implementation, the beads are spheres and have a higher refractive index
than that of the surrounding continuous polymeric medium, with of ratio of
the higher refractive index of the sphere and the lesser refractive index
of the surrounding continuous polymeric phase being in the range of from
1.7 to 2.1.
In an additional preferred implementation, the lenslets are gas filled
cells having minor axes normal to the fluorescent layer, with the ratio of
the major to minor axes being in the range of 1.5:1 to 10:1.
The invention is based on the discovery that a novel and improved
relationship of speed and sharpness can be realized when an intensifying
screen is constructed employing a support having at least one reflective
portion containing reflective lenslets which are either spherical or
oriented with their major axes parallel to the fluorescent layer of the
intensifying screen.
The invention is based on the further recognition that (a) stretch
cavitation microvoided supports, (b) supports containing transparent
spherical or oriented spheroidal beads of properly chosen refractive
indices, or (c) properly oriented and proportioned gas filled cells are
capable of providing the reflective lenslets required.
The invention is further based on the identification of specific stretch
cavitation microvoided supports having superior properties as reflective
intensifying screen supports.
The invention is still further based on the discovery that intensifying
screens of increased speed and sharpness can be constructed by employing
supports containing lenslets in the form of retroreflective spheres.
Finally, the invention is directed to certain radiographic intensifying
screens produced by advantageous combinations of fluorescent layers and
reflective lenslet supports.
The invention, including advantages of specific selections and
combinations, can be more fully appreciated by reference to the
description of preferred embodiments and the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of an imaging arrangement;
FIG. 2 is a schematic diagram of a dual coated radiographic element and
intensifying screen pair assembly;
FIG. 3 is a perspective view in section illustrating a preferred embodiment
of a reflective lenslet support;
FIG. 4 is a perspective view in section illustrating an alternate
construction of a reflective lenslet support;
FIG. 5 is a sectional view of a single lenslet of the support of FIG. 3;
FIG. 6 is a sectional view taken along section line 6--6 in FIG. 5;
FIG. 7 is a sectional view of a single lenslet of the support of FIG. 4;
FIG. 8 is a graphical view illustrating the change in size of microvoids
surrounding microbeads as a function of the stretch ratio;
FIGS. 9, 10, and 11 are photomicrographs of reflective lenslet supports
formed of a polyester continuous phase, cellulose ester microbeads forming
a second phase, and reflective microvoid lenslets bordering the
microbeads;
FIGS. 12 and 13 are reflection diagrams;
FIGS. 14 and 15 illustrate reflections from selected spheres; and
FIG. 16 is a plot of modulation transfer factors (MTF) versus cycles per
millimeter, showing preferred standards of performance high definition
imaging applications.
DESCRIPTION OF PREFERRED EMBODIMENTS
A typical arrangement for examining human tissue with X-radiation is
illustrated in FIG. 1. Tissue 1 to be examined radiographically, in this
instance a mamma (breast), is located between an exposure and compression
arrangement 3 and an exposure grid 5. Beneath the grid is located an
exposure recording assembly 7.
The exposure and compression arrangement is comprised of a radiation input
window 9 (the output window of an X-radiation generating tube) and an
output window 11 (the input window for supplying X-radiation to the
subject), which are each substantially transparent to X-radiation. The
output window acts as a compression element so that the mamma is held well
compressed during examination. A wall 13 formed of a material having low
penetrability to X-radiation joins the input window and defines with it an
X-radiation field emanating from a tube or other conventional source,
shown schematically as emanating from focal spot 15.
Unscattered X-radiation passing through the input and output windows and
tissue to the grid is indicated by the solid arrows 17. Collisions of
X-radiation with matter within the tissue results in part in absorption of
the X-radiation and in part in redirecting the X-radiation.
Redirected--i.e., scattered X-radiation--is illustrated schematically by
dashed arrows 19.
The grid is equipped with vanes 21, which are relatively impenetrable by
the X-radiation and arranged parallel to the unscattered X-radiation. The
vanes permit almost all of the unscattered X-radiation to pass through the
grid uninterruted. X-radiation that has been slightly redirected is
capable of passing through the grid also, but the most highly scattered
X-radiation, which if left alone, would produce the greatest degradation
in image sharpness, is intercepted and deflected by the vanes. The
thickness and spacing of the vanes is exaggerated in FIG. 1 for ease of
illustration. By vane construction and spacing the desired balance between
the attenuation of X-radiation supplied to the exposure recording assembly
and the sharpness of the image can be realized. To minimize X-ray
attenuation the grid can be entirely eliminated, but a grid is usually
preferred to improve sharpness. Suitable exposure grids are known and
commercially available.
In FIG. 2 the exposure recording assembly is shown in greater detail. A
conventional case or cassette used to compress the elements of the
assembly into close contact is not shown. The assembly consists of three
separate elements, a dual coated silver halide radiographic element 23, a
front intensifying screen 25 intended to be positioned between the
radiographic element and an exposing X-radiation source, and a back
intensifying screen 27.
As shown, the dual coated radiographic element consists of a support 29
including subbing layers 31 and 33 coated on its opposite major faces.
Silver halide emulsion layers 35 and 37 overlie the subbing layers 31 and
33, respectively. Overcoat layers 36 and 39 overlie the emulsion layers 35
and 37, respectively.
As shown, the front intensifying screen is comprised of a support
consisting of a substrate portion 41 and an interposed layer portion 43, a
fluorescent layer 45, and an overcoat layer 47. Similarly, the back
intensifying screen as shown is comprised of a support consisting of a
substrate portion 49 and an interposed layer portion 51, a fluorescent
layer 53, and an overcoat layer 55. Anticurl layers 57 and 59 are on the
major faces of the front and back screen substrate portions 41 and 49,
respectively, opposite the fluorescent layers.
In use, X-radiation enters the image recording assembly through the front
screen anticurl layer 57 and substrate portion 41 passing uninterrupted to
fluorescent layer 45. A portion of the X-radiation is absorbed in the
front screen fluorescent layer. The remaining X-radiation passes through
the overcoat layers 47 and 36. A small portion of the X-radiation is
adsorbed in the silver halide emulsion layer 35, thereby contributing
directly to the formation of a latent image in the emulsion layer.
However, the major portion of the X-radiation received by the emulsion
layer 35 passes through the support 29 and associated subbing layers 31
and 33 to the remaining silver halide emulsion layer 37. Again, a small
portion of the X-radiation is absorbed in the remaining silver halide
emulsion, thereby contributing directly to the formation of a latent image
in this emulsion layer, and, again, the major portion of the X-radiation
received by the emulsion layer 37 passes through the overcoat layers 39
and 55 to the fluorescent layer 53 of the back screen. The major portion
of the X-radiation striking the back screen fluorescent layer is absorbed
in this layer.
Exposing X-radiation is principally absorbed in the fluorescent layers 45
and 53 and reemitted by the fluorescent layers as longer wavelength
electromagnetic radiation more readily absorbed by the silver halide
radiographic element 23. Longer wavelength electromagnetic radiation
emitted by the front intensifying screen fluorescent layer 45 exposes the
adjacent silver halide emulsion layer 35. Longer wavelength
electromagnetic radiation emitted by the back intensifying screen
fluorescent layer 53 exposes the adjacent silver halide emulsion layer 37.
These longer wavelength electromagnetic radiation exposures primarily
account for the latent image formed in the silver halide emulsion layers.
From the foregoing, it is apparent that all of the layers above the
fluorescent layer 53 must be penetrable by X-radiation to at least some
extent. While the silver halide emulsion layers usefully absorb some
X-radiation, the only other usefully absorbed X-radiation occurs in the
front intensifying screen fluorescent layer. Thus, the supports and
overcoat and subbing layers overlying the back intensifying screen are
chosen to be as nearly transparent to exposing X-radiation as possible.
It is also apparent that the overcoat layers 36 and 47 separating the front
intensifying screen fluorescent layer and the emulsion layer adjacent
thereto as well as the overcoat layers 39 and 55 separating the back
intensifying screen fluorescent layer and the emulsion layer adjacent
thereto are preferably transparent to the emitted longer wavelength
electromagnetic radiation. Being transparent to both X-radiation and
longer wavelength electromagnetic radiation, the overcoat layers 36, 47,
39, and 55, though preferred for other reasons, are not needed for imaging
and can be omitted.
To realize the advantages of the present invention only one of the two
intensifying screens in the exposure recording assembly 7 need contain a
reflective lenslet support. If only one of the two intensifying screens
employs a reflective support, it is preferred that the back screen be a
reflective lenslet support. Because of the superior imaging properties
attainable with intensifying screens containing reflective supports
satisfying the requirements of the invention, it is specifically
recognized that both the front and back intensifying screens of the
exposure recording assembly can contain reflective lenslet supports
satisfying the requirements of the invention.
It is, of course, recognized that in the simplest possible combination one
intensifying screen satisfying the requirements of the invention and a
radiographic element containing only one silver halide emulsion layer are
capable of producing a radiographic image. In other words, the exposure
recording assembly 7 can be simplified by removing all of the layers and
elements above or below the support 29. With the elimination of one
intensifying screen, imaging speed is, of course, lowered. However,
crossover, which is a well recognized source of unsharpness in
radiographic elements containing dual coated emulsion layers is also
eliminated, and the improved properties of the reflective lenslet support
satisfying the requirements of the invention is capable of boosting
imaging speed with the least possible reduction in sharpness.
Nevertheless, in their preferred use, to realize the sharpest possible
images at the highest attainable imaging speeds, the intensifying screens
of this invention are employed as one or both members of a front and back
intensifying screen pair intended to be employed in combination with a
dual coated silver halide radiographic element, as described above.
Specifically preferred radiographic elements are those which exhibit the
highest attainable speeds in relationship to sharpness--e.g., tabular
grain radiographic elements which exhibit a crossover of less than 10
percent and, optimally, less than 1 percent crossover, more specifically
identified below. Additionally, for the reasons set forth below,
fluorescent layers that satisfy the higher performance requirements of the
art produce in combination with the reflective lenslet supports required
by this invention intensifying screens that exceed the performance
capabilities of conventional intensifying screens.
In FIG. 2 the intensifying screens 25 and 27 are shown as including
substrate portions 41 and 49 and interposed layer portions 43 and 51,
respectively. Further, anticurl layers 57 and 59 are shown associated with
the substrate portions. Anticurl layers are, of course, a practical
convenience rather than a requirement for screen construction and can be
eliminated when the substrate portions are sufficiently rigid to resist
curl.
In one form of the invention, when the intensifying screen support includes
both substrate and interposed layer portions, the substrate portion is the
reflective lenslet portion of the support and the interposed layer portion
is a conventional transparent subbing layer or combination of subbing
layers. In the preferred reflective lenslet substrate constructions the
presence of the lenslets not only increases the reflectivity of the
substrate, but also improves its texture for adhesion of the fluorescent
layer. Thus in a specifically preferred form of the invention no subbing
layer is required, and the interposed layer can be eliminated, resulting
in a unitary reflective lenslet support.
In an alternate form, the substrate portion can be a conventional
transparent support, preferably a transparent polymeric film support, and
the interposed layer portion can constitute the reflective lenslet portion
of the support. A further possible variant is to supplement the
reflectivity of the interposed reflective lenslet layer portion with a
reflective substrate portion, which can also be a reflective lenslet
portion or can take another reflective form known to be useful in the
construction of intensifying screens.
For simplicity the discussion which follows is directed to the unitary
relective lenslet support construction noted above. The applicability of
the description to the alternate support constructions set forth above is
readily apparent.
Stretch Cavitation Microvoided Supports
FIG. 3 illustrates a unitary reflective lenslet support 60 which has been
biaxially oriented [biaxially stretched, i.e., stretched in both the
longitudinal (X) and transverse (Y) directions], as indicated by the
arrows. The support 60 is illustrated in section, showing microbeads 62
contained within circular microvoids 64 in the polymeric continuous matrix
66. The microvoids 64 surrounding the microbeads 62 are theoretically
regular in shape, but on microscopic examination often show
irregularities, particularly when the random spacing of the microbeads
results in two or more microbeads being located in close proximity.
FIG. 4 also illustrates a unitary reflective lenslet support 70 which has
been unidirectionally oriented (stretched in one direction only, as
indicated by the arrow). Microbeads 72 are contained between microvoid
lobes 74 and 74'. The microvoid lobes in this instance form at opposite
sides of the microbeads as the sheet is stretched. Thus, if the stretching
is done in only the longitudinal direction (X) as indicated by the arrow,
the microvoids will form on the leading and trailing sides of the
microbeads. This is because of the unidirectional orientation as opposed
to the bidirectional orientation of the sheet shown in FIG. 4. This is the
only difference between the supports of FIGS. 3 and 4.
Attention is particularly directed to the texture of the upper surfaces of
the reflective lenslet supports in each of FIGS. 3 and 4.
FIGS. 5 and 6 are sectional views which illustrate on an enlarged scale a
single reflective lenslet, microbead 80 being entrapped within the
polymeric continuous matrix 82 and encircled by microvoid 84. This lenslet
shape results from the support being stretched in both the X and Y
directions.
FIG. 7 is a view similar to FIG. 5, except illustrating in enlarged form
microbead 90 entrapped in the polymeric continuous matrix 92, having
formed on opposite sides thereof microvoid lobes 94 and 94', which are
formed when the support is stretched only in the direction of the arrow X.
The foregoing description is generally applicable to stretch cavitation
microvoided articles capable of being employed as reflective lenslet
supports in the intensifying screens of this invention. The description
that follows provides a further illustration of this form of the invention
by referring to specific, preferred embodiments--specifically, to the
choice of superior reflective lenslet supports from among the polyester
continuous phase or matrix and cellulose acetate microbead shaped articles
disclosed by Pollock et al U.S. Ser. No. 047,821, filed May 5, 1987, cited
above.
FIG. 8 is an enlargement illustrating a specific manner in which microvoids
can be formed in a polyester continuous matrix as the support is stretched
or oriented. The formation of the microvoids 100 and 100' around
microbeads 102 is illustrated on a stretch ratio scale as the support is
stretched up to 4 times its original dimension. For example, as the
support is stretched 4 times its original dimension in the X direction
(4X), the microvoids extend to the points 104 and 104', respectively.
FIGS. 9 and 10 are actual photomicrographs of sections of a reflective
lenslet support according to this invention which has been frozen and
fractured. The continuous polymeric matrix, microbeads, and microvoids are
obvious. FIG. 11 is an actual photomicrograph of a section of support
oriented in one direction. The scale of these photomicrographs is
indicated at the top of each in micrometers (.mu.m).
In this preferred form of the invention the reflective lenslet supports are
comprised of a continuous thermoplastic polyester phase having dispersed
therein microbeads of cellulose ester which are at least partially
bordered by voids. The supports are conveniently in the form of sheets or
film. The polyester is relatively strong and tough, while the cellulose
acetate is relatively hard and brittle.
More specifically, the present invention provides supports comprising a
continuous thermoplastic polyester phase having dispersed therein
microbeads of cellulose ester which are at least partially bordered by
voids, the microbeads of cellulose acetate being present in an amount of
10-30% by weight based on the weight of polyester, the voids occupying
2-50% by volume of the shaped article, the composition of the shaped
article when consisting only of the polyester continuous phase and
microbeads of cellulose ester bordered by voids characterized by having a
Kubelka-Munk R value (infinite thickness) of 0.90 to 1.0 and the following
Kubelka-Munk values when formed into a 3 mil (76.2 microns) thick film:
Opacity: about 0.78 to about 1.0
SX: 25 or less
KX: about 0.001 to 0.2
Ti: about 0.02 to 1.0
wherein the opacity values indicate that the article is opaque, the SX
values indicate a large amount of light scattering through the thickness
of the article, the KX values indicate a low amount of light | | |
