X-ray or gamma ray systems or devices – Specific application – Holography or interferometry
Reexamination Certificate
2001-03-01
2004-10-12
Glick, Edward J. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Holography or interferometry
C378S062000, C378S087000
Reexamination Certificate
active
06804324
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to refractive imaging in general and x-ray refractive radiography in particular.
2. Discussion of Related Art
It is well known to use X rays for imaging the internal features of objects in those cases when the object is opaque in the visible optics domain, or when extremely high spatial resolution is necessary. Traditional x-ray imaging techniques are based on the absorption contrast, i.e., on the variation of the absorption factor of different parts of a sample. Therefore, the only way to increase a contrast of small objects in traditional x-ray images, is to increase the intensity of an x-ray beam. But this way is unacceptable in many cases. For example, strong x-ray beams cannot be used for visualization of the inner structure of integrated circuits because of their possible radiation damage, or for medical radiography for safety reasons. The refractive contrast, originating from the variation of the refractive indices of different parts of a sample, produces far more detailed images of the samples with small features. This type of x-ray imaging is commonly referred to as a phase contrast imaging (PCI). However, the direct beam, carrying practically no information about the object, if the latter is transparent to x rays, deteriorates the image, bringing additional noise into it. Therefore, the direct beam is undesirable.
One proposed way to suppress the direct beam is disclosed in U.S. Provisional Patent Application Serial No. 60/258,851, filed on Dec. 28, 2000, and U.S. Provisional Application Serial No. 60/272,354, filed Feb. 28, 2001, entitled “Dark-Field Phase Contrast Imaging” and by Vladimir V. Protopopov, the entire contents of each of which are incorporated herein by reference. In each of those applications, several embodiments of an imaging system are disclosed. One embodiment is shown in
FIGS. 1-2
where an x-ray tube
114
generates a beam
115
so that the long side of the focus
116
of the beam
115
is in the plane of incidence. The beam
115
is directed to a monochromator
118
that may be composed of two crystals
120
,
122
that are well known in the art. The two crystals
120
,
122
are selected so that they strongly disperse the beam
115
so as to generate highly parallel x-ray beams
100
. In the embodiment of
FIGS. 2 and 3
, the object
102
is preferably no larger than several millimeters so that the object
102
is fully covered by the x-ray beam
100
. Accordingly, there is no need to move the object
102
during imaging.
After the beam
100
interacts with the object
102
, the beam
104
is directed to an analyzer
110
that suppresses the intensity of the original wave or beam
106
by several orders of magnitude in a manner as schematically shown in FIG.
8
. The suppressed beam
106
and the refracted beam
108
are directed to the imaging plane
112
where a detector, such as an x-ray charge coupling device (CCD)
113
, receives the beams. The detector then sends a signal to a processor (not shown) that generates an image that is formed on a display (not shown).
One embodiment of an analyzer
110
that can suppress the intensity of the beam
106
is shown in FIG.
6
. In particular, the analyzer
110
of
FIG. 6
is a specially designed multilayer mirror
124
. The reflective coating of the x-ray multilayer mirror
124
is composed of many altering layers of materials with large and small atomic numbers. For instance, the layers
126
with large atomic numbers may be made of tungsten while the layers
128
with small atomic numbers may be made of boron-carbide, i.e., B
4
C. The thickness of the layers may differ, but they are typically of the order of 10 Å-50 Å. The interfacial roughness is equal to 5 Å.
As described in “X-Ray Multilayer Mirrors with an Extended Angular Range,” by Protopopov et al., Optics Communications Vol. 158 (1998), pp. 127-140, the entire contents of which are incorporated herein by reference, it is possible to control the shape of the angular and spectral reflection curves by altering the thickness of the layers
126
and
128
. Varying slightly the thickness of layers it is possible to make the partial reflected waves approximately counterphased at a specific grazing angle &thgr;, so as to obtain as small reflection at this angle as possible. Moreover, the total reflection can be made even less if not only the phases of the partial waves are opposite each to another, but the coming and reflected waves produce interference pattern whose maxima at this particular angle coincide with the layers of heavy material, introducing additional absorption. Thus, it is possible to design a mirror with deep (the reflectivity of the order of 10
−2
-10
−3
) and narrow (several arc seconds) resonant gap in the angular reflection curve as shown in
FIGS. 7
a-b
. The roles of reflection and absorption are clear from the solid and dashed curves, respectively, in
FIG. 7
a
. In addition, the sensitivity of the scheme with respect to the refracted beams
108
is determined by the sharpness of the reflection curve around the resonant angle &thgr;
r
. The sharpness of the gap in the reflection curve of the multilayer mirror
124
is sufficient to effectively detect small-contrast images.
If it is desired to image objects that are larger than 2 mm and have dimensions up to 150-200 mm, then a modified imaging system can be employed. This is advantageous for biological and medical applications. An embodiment of such an imaging system is shown in
FIGS. 3-5
. In this embodiment, the x-ray tube
114
works in the point projection mode. The width of the beam in the plane of incidence is limited by the x-ray tube focus, and is an order of magnitude less than in that for the imaging system of
FIGS. 1-2
. Consequently, the length of the mirror
110
in this direction may be much less than in the previous case.
As shown in
FIG. 3
, the x-ray tube
114
generates a beam
115
that is directed to the monochromator
118
that is composed of two crystals
120
,
122
that are similar to those described previously with respect to the imaging system of FIG.
2
. Again, the two crystals
120
,
122
are selected so that they strongly disperse the beam
115
so as to generate highly parallel x-ray beams
100
.
In the embodiment of
FIGS. 3 and 4
, the object
102
is preferably larger than the width of the x-ray beam
100
. Accordingly, there is a need to move the object
102
relative to the detector
113
during imaging as shown in FIG.
5
.
After the beam
100
interacts with the object
102
, the beam
104
is directed to an analyzer
110
that suppresses the intensity of the original wave or beam
106
by several orders of magnitude in a manner as schematically shown in FIG.
8
. The suppressed beam
106
and the refracted beam
108
are directed to the imaging plane
112
where a detector, such as an x-ray charge coupling device
113
, receives the beams. The detector then sends a signal to a processor (not shown) that generates an image that is formed on a display (not shown). The analyzer
110
preferably has a structure that is similar to that as the analyzer
110
used in the imaging system of
FIGS. 1-2
.
As shown in
FIG. 5
, the object
102
is scanned in the plane of incidence in the direction transversal to the x-ray beam
100
, so that each moment of time only a small fraction of the object is investigated. During each moment of time t the detector signal can be described by the matrix u
ij
(t), where i and j are the ordinal numbers of its sensitive elements. The signals corresponding to the same row j but different column i differ each from another only by the time delay equal to i&tgr;, where &tgr; is the time interval during which the object is shifted by a distance equal to a single detector element. Therefore, it is possible to average the signals from different columns if only take into account the delay. Such an averaging will raise the sensitivity and signal-to-noise ratio because the noise in the channels is uncorr
Martynov Vladimir V.
Platonov Yuriy
Brinks Hofer Gilson & Lione
Glick Edward J.
Keaney Elizabeth
Osmo, Inc.
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