X-ray filter system for medical imaging contrast enhancement

Details X-ray filter system for medical imaging contrast enhancement X-ray filter system for medical imaging contrast enhancement

2001-01-23

2003-09-02

Dunn, Drew A. (Department: 2882)

X-ray or gamma ray systems or devices

Beam control

Filter

C378S062000, C378S156000, C378S157000

Reexamination Certificate

active

06614878

ABSTRACT:

FIELD OF THE INVENTION
The present invention pertains generally to devices and methods for imaging the internal features of an object. More particularly, the present invention pertains to improved devices and methods for imaging the internal features of an object while using a conventional broadband X-ray source. The present invention is particularly, but not exclusively, useful for producing an enhanced-contrast image of the internal features of an object by using filtered X-ray radiation.
BACKGROUND OF THE INVENTION
The ability to image the internal features of an object is important in many applications. Two examples include medical diagnosis and the non-destructive testing of structural components to detect configuration or discover internal flaws. In all applications, it is desirable to produce an image having high contrast and spatial resolution. Radiation within the X-ray spectrum is often used to image internal features because of the ability of X-ray radiation to penetrate matter and because different matter absorbs X-ray radiation at different rates. Typically, a conventional X-ray source produces an emission of X-ray radiation having a broad range of energies. In conventional imaging applications, these X-rays are directed through the object for subsequent capture by a film or detector. The imaging films and detectors used are responsive to the intensity of the radiation received, and thus are able to produce an image of the internal features of the object when those internal features have differing absorption characteristics.
To enhance the contrast and spatial resolution of an image, contrast agents are often used. Specifically, these agents include chemical elements that have absorption rates that are significantly different than the constituents of the object to be imaged. For example, iodine can be administered within the body as a contrast agent. Once administered, the iodine is selectively absorbed by certain tissues or is present within the blood vessels. Subsequently, when an X-ray image is formed, areas of the body with large amounts of iodine will absorb relatively greater amounts of X-ray radiation than areas of the body without iodine. Thus, contrast agents can be used with good efficacy to increase both the contrast and the spatial resolution of the image.
FIG. 1
shows the variation of absorption coefficient with radiation energy for a typical chemical element. In simple terms, the absorption coefficient decreases as the energy increases until an energy is reached that is sufficient to knock a K-shell electron from it's orbit. At this energy, E
K-EDGE
, the value of the absorption coefficient jumps. For purposes of the present disclosure, the term K
EDGE
is used to denote the energy at which the jump in absorption coefficient occurs. Continuing with
FIG. 1
, it can be seen that further increases in energy again result in a gradual decrease in absorption coefficient.
The present invention recognizes that the variation in absorption coefficient near K
EDGE
can be utilized to increase image contrast. Specifically, the present invention recognizes that image contrast can be increased by first introducing a contrast agent having a known K
EDGE
into the object. Next, a first image can be formed using monochromatic radiation having an energy just less than K
EDGE
for the contrast agent (such as radiation having an energy, E
1
, in
FIG. 1
) and a second image formed using monochromatic radiation having an energy just greater than K
EDGE
for the contrast agent (such as radiation having an energy, E
2
, in FIG.
1
). When this is done, the resulting two images can be compared (subtracted) to produce a high contrast image of the internal features of the object.
One way to produce the monochromatic X-ray radiation needed to conduct the above described subtraction method is to use a crystal monochromator. Unfortunately, when a crystal monochromator is used in conjunction with a conventional X-ray source, the intensity of the resulting monochromatic radiation is so reduced that the radiation is insufficient for almost all practical imaging uses. One way to produce monochromatic radiation at a suitable intensity for imaging is to pass the high intensity radiation from a synchrotron source through a double crystal monochromator. As can be expected, producing radiation with a synchrotron source is very expensive. Further, the beam produced by the synchrotron source/double crystal monochromator is fixed in direction and, consequently, cannot be rapidly moved as required in a typical tomographic scan.
In light of the above, it is an object of the present invention to provide devices and methods suitable for the purposes of producing a digital image signal that is substantially equivalent to an image signal obtained by passing quasi-monochromatic radiation (i.e. radiation having a narrow energy band) through an object. It is another object of the present invention to provide devices and methods for producing images of the internal features of an object having enhanced contrast and high spatial resolution. It is yet another object of the present invention to provide devices and methods for use in conjunction with standard computer tomography or angiography equipment to enhance the contrast and increase the spatial resolution of the images produced. It is yet another object of the present invention to provide devices and methods for enhancing image contrast and increasing spatial resolution that can be used with a variety of different contrast agents. Yet another object of the present invention is to provide an X-ray Filter System For Medical Imaging Contrast Enhancement which is easy to use, relatively simple to manufacture, and comparatively cost effective.
SUMMARY OF THE PREFERRED EMBODIMENTS
The present invention is directed to a system for creating an image of the internal features of an object. Specifically, the present invention is directed at imaging an object that contains a contrast agent. For the present invention, the system includes an X-ray source configured to produce a spectrum of X-ray radiation. An optional collimator may be provided to collimate the radiation emitted from the X-ray source into one or more beams. As such, each beam emanates from the X-ray source in a slightly different direction, and consequently, along a separate path. For the present invention, the X-ray source is oriented relative to the object to direct all such paths towards the object. Further, a mechanism is provided to move the X-ray source relative to the object to cause each radiation beam emanating from the X-ray source to successively travel on different paths through the object. For example, the X-ray source can be slideably mounted on a circular track that extends around the object.
A detector array is positioned on the opposite side of the object to interpose the object between the X-ray source and the detector array. Preferably, the detector array includes a plurality of detectors, one detector for each beam that emanates from the X-ray source/collimator assembly.
Further, a mechanism can be provided to move the detector array as the X-ray source is moved. Specifically, the detector array can be moved to allow each detector to track a single X-ray beam, as that X-ray beam travels on successive paths through the object. In response to the receipt of an X-ray beam, the detector produces an electrical signal that is proportional to the intensity of the radiation received.
An important aspect of the present invention is that the X-ray radiation is filtered between the X-ray source and the detectors. Specifically, for each X-ray beam on each path, the beam is successively filtered four times, each time with a different filter. Each time the beam is moved to a new path, the beam is once again filtered four times. Each time the beam is successively filtered four times, four different electrical signals are produced by a detector.
For the present invention, these four electrical signals can be manipulated by a processor to produce an image signal for the path. Once an

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