Image redirection and optical path folding

Details Image redirection and optical path folding Image redirection and optical path folding

1999-11-24

2002-03-05

Dunn, Drew (Department: 2882)

X-ray or gamma ray systems or devices

Electronic circuit

With display or signaling

C378S098800

Reexamination Certificate

active

06353657

ABSTRACT:

TECHNICAL FIELD
The present invention relates to image redirection in digital imaging and in particular to optical path folding in a digital x-ray imaging system.
BACKGROUND
In conventional x-ray imaging a photographic film is exposed to visible light in order to capture the image of the object being x-rayed. The x-rays are passed through the object and impinge on an imaging screen, such as a phosphor imaging screen. The phosphor imaging screen converts some of the radiation into a selected spectral component (typically visible light). The exposure of the photographic film to the spectral component from the phosphor imaging screen produces the image of the object on the photographic film.
Recent advances in x-ray imaging however have changed the focus from use of photographic films to electronic imaging sensors. Such a system is disclosed in U.S. Pat. No. 5,309,496, entitled “FILMLESS X-RAY APPARATUS AND METHOD OF USING THE SAME”, issued to Winsor, which is hereby incorporated herein by reference. In the preferred embodiment of Winsor, a video camera and a frame grabber are used to provide still x-ray images.
An alternative electronic imaging sensor structure, like a CCD (charge coupled device) camera, as shown in
FIG. 1
may also be used for x-ray imaging.
FIG. 1
shows a top view of a prior art imaging system
100
. Imaging system
100
comprises X-ray source
107
that emits X-rays to image object
108
. Imaging system
100
comprises a camera housing
106
. One side of camera housing
106
is preferably made up of a radio-transparent plastic and a phosphor imaging screen
101
. In the embodiment of the prior art system shown in
FIG. 1
, a CCD camera
103
is positioned at another side of camera housing
106
. CCD camera
103
includes a CCD chip
105
and a lens (or lens assembly)
104
.
As the phosphor imaging screen
101
does not convert all the x-rays, there is some residual portion of the x-rays left in the light coming from the phosphor screen. When imaging with a sensor which may be exposed to high energy x-ray radiation, there is the potential for the imaging sensor and associated electronics to be damaged by such x-ray radiation. Moreover, the radiation energy in the light may produce undesirable image artifacts in the captured image. The prior art system shown in
FIG. 1
solves this problem by redirecting the light from the phosphor imaging screen
101
in such a manner as to place the imaging sensor and its associated electronics out of the path of the x-ray radiation. The prior art imaging system shown in
FIG. 1
uses a single mirror element or prism
102
, including X-ray absorbing element
109
, to redirect the visible light towards the imaging sensor, such as CCD camera
103
. Typically, such mirrors are rectangular in shape. Therefore, in prior art systems, the light from the phosphor screen is redirected by a single mirror before being provided to the imaging sensor. This solves the problems associated with image artifacts and prevents damage to the imaging sensor that may be caused due to exposure to high energy radiation.
The size of the camera housing is dependent on the size of the lens as the housing has to be of sufficient size to accommodate the lens. Because the lens (or lens assembly) used in such a system is typically very long, the housing cannot be less than a predefined size. Moreover, the size of the lens significantly effects the overall cost of the x-ray imaging system. Furthermore, the optical distance of the CCD chip
105
from the phosphor screen
101
also depends on the lens. Therefore, the optical distance of the CCD chip from the phosphor screen also limits the size of the camera housing.
Therefore, there is a need in the art for a system and method for reducing the overall camera housing dimensions while also reducing the costs associated with digital x-ray imaging.
SUMMARY OF THE INVENTION
These and other objects, features and technical advantages are achieved by a system and method for digital imaging which utilizes optical path folding by redirecting the image more than once before providing it to an imaging sensor.
The optical distance of CCD chip
105
of
FIG. 1
from the imaging screen
101
is the sum of the lengths of the lens (or lens assembly) and the optical path. The optical path is the path taken by the light source from the imaging screen to the surface of the lens. Since the optical distance is predefined, therefore for a given optical distance, by increasing the length of the optical path, the length of the corresponding lens may be reduced. This is preferably accomplished by making the light from the imaging screen, such as a phosphor imaging screen used in x-ray imaging, travel a longer distance before coming in contact with the lens surface.
The length of the optical path may be increased without increasing the dimensions of the camera housing by redirecting the light from the phosphor screen more than once before it comes in contact with the lens surface. Thus, by redirecting the light several times within the camera housing, the length of the optical path can be increased thereby allowing reduction in the length of the lens for a given optical distance (or for a given imaging screen size). Multiple redirection of the light creates the effect of optical path folding.
Multiple redirections or folds of the light may be accomplished by using multiple redirecting elements within the camera housing. Such redirecting elements may be a mirror, a prism with a reflecting surface, or any other object capable of redirecting light. In a preferred embodiment of the present invention light from the phosphor screen is folded twice, once by using a first mirror which reflects the light to a second mirror which then redirects the light into a lens (or lens assembly) and then onto a CCD chip associated with an imaging sensor, such as a CCD camera.
The size of the housing may be further reduced by using multiple imaging sensors. Multiple imaging sensors may be used to capture different portions of a single image or imaging screen. In a preferred embodiment of the present invention, the phosphor imaging screen is divided into two portions and each portion of the screen or image is preferably captured by a different imaging sensor. Moreover, each imaging sensor preferably has at least two redirecting elements associated with it to redirect the light at least twice from the associated portion of the image. Preferably, the redirecting elements are shaped and/or positioned in such a manner so as to encompass the full field of view of the associated imaging sensor along an optical plane. In the case of multiple imaging sensors, the optical configuration used to direct the imaging rays of light onto the respective imaging sensors is an important consideration. The optical configuration should be such as to allow for overlapping fields of view for each imaging sensor so that after image capturing, different portions of the image may be stitched together, if desired, for example to create the entire image. The overlapping portions of the images provide a reference for stitching or joining together the different images.
Accordingly, it is a technical advantage of a preferred embodiment of the present invention to provide a smaller camera housing for a predefined imaging screen size.
It is another technical advantage of a preferred embodiment of the present invention that the size of the lens used can be reduced.
It is yet another technical advantage of a preferred embodiment of the present invention to reduce the cost of digital x-ray imaging due to reduction in the size of the lens.
It is still another technical advantage of a preferred embodiment of the present invention that the image does not have any artifacts.
It is still another technical advantage of a preferred embodiment of the present invention that the electronics components associated with an imaging sensor are not subjected to x-ray radiation.
It should be noted that although visible light is used as an example in the preferred embodiment, any spectral radiation may be d

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