A hybrid x-ray detector includes: a first detector that is adapted to receive x-rays, the first detector absorbs a first portion of the x-rays and allows a second portion of the x-rays to pass through the first detector; and a second detector that is adapted to receive the second portion of the x-rays. A radiation imaging system includes: an x-ray source that produces x-rays; and an image detector assembly that is adapted to receive the x-rays, the image detector assembly having a hybrid x-ray detector that includes: a first detector that is adapted to receive the x-rays, the first detector absorbs a first portion of the x-rays and allows a second portion of the x-rays to pass through the first detector; a second detector that is adapted to receive the second portion of the x-rays; and an x-ray source. A method of operating a hybrid x-ray detector, the method includes: receiving x-rays at a first detector; absorbing a first portion of the x-rays; passing a second portion of the x-rays through the first detector; and receiving the second portion of the x-rays at a second detector.

BACKGROUND OF INVENTION

This invention relates generally to a system and method for differentiating material characteristics using an imaging system and more particularly to a system and method for differentiating material characteristics using a hybrid x-ray detector.

Energy discrimination adds significantly to the capabilities of an x-ray imaging detector. Energy discrimination improves signal to noise and potentially allows different materials to be distinguished. Current energy discrimination applications are limited to radiography (RAD) (digital radiography and computed radiography), mammography, and some dual energy computed tomography products. One strategy is to separate two detectors with a thin plate of metal, which preferentially absorbs low energy x-rays. Low energy x-rays then provide a relatively larger signal to the first detector and higher energy x-rays provide a relatively larger signal in the second detector. Another approach is to have two separate x-ray exposures and using different x-ray spectra for each. These approaches have in common that multiple images are acquired sequentially, and combined to produce one or more processed images representing the additional energy information. However, this approach only works well for single shot exposures, such as in RAD and mammographs.

For cardiac imaging, it is desirable to image small objects, such as guide wires and catheters, which are moving in real time, and to do so in thick patients, for which image quality is a major challenge. For this type of imaging, both high spatial resolution and sensitive material discrimination are desirable. Catheters are becoming smaller and harder to visualize in the rapidly moving heart. Thus, sequential imaging approaches to energy discrimination are not suitable for this application.

It is also noted that there also exist direct conversion detectors, such as thin film direct conversion detectors and single crystal direct conversion detectors. Thin film direct conversion detectors typically consist of polycrystalline materials, which are not capable of energy discrimination. Single crystal direct conversion detectors are more expensive to produce and are only available in small sizes. They typically have poor performance at the high x-ray flux rates required, and the small size requires tiling to produce large areas, which can result in gaps between tiles. The gaps cause data to be absent from the image, and can result in a misdiagnosis if critical anatomy is not correctly imaged.

SUMMARY OF INVENTION

The above discussed and other drawbacks and deficiencies are overcome or alleviated by a hybrid x-ray detector that includes: a first detector that is adapted to receive x-rays, the first detector absorbs a first portion of the x-rays and allows a second portion of the x-rays to pass through the first detector; and a second detector that is adapted to receive the second portion of the x-rays. A radiation imaging system includes: an x-ray source that produces x-rays; and an image detector assembly that is adapted to receive the x-rays, the image detector assembly having a hybrid x-ray detector that includes: a first detector that is adapted to receive the x-rays, the first detector absorbs a first portion of the x-rays and allows a second portion of the x-rays to pass through the first detector; a second detector that is adapted to receive the second portion of the x-rays. A method of operating a hybrid x-ray detector, the method includes: receiving x-rays at a first detector; absorbing a first portion of the x-rays; passing a second portion of the x-rays through the first detector; and receiving the second portion of the x-rays at a second detector.

DETAILED DESCRIPTION

Referring toFIG. 1, an x-ray system10, which incorporates a hybrid x-ray detector12, has an x-ray source14that provides an x-ray beam16passing through an area18of a patient20. The x-ray beam16is attenuated along its many rays by the internal structure of the patient20to then be received by the detector12, which extends generally over an area in a plane perpendicular to the central ray of the x-ray beam16.

Hybrid x-ray detector12includes an energy integrating detector22and an energy discriminating detector24(shown inFIG. 2). Energy integrating detector22adds all of the signals that land on each pixel and accumulates the signal in a storage device26. Energy integrating detector22then reads out the sum of the energies that have landed on the pixel during the exposure to x-rays. A characteristic of the energy integrating detector22is that it provides a high resolution image giving the principal view of the anatomical data. Energy integrating detector22may be any type of detector that detects x-rays in the manner described above.

In an exemplary embodiment, energy integrating detector22is a scintillator detector coupled to light sensitive element, such as an array of photodiodes28. The array28is divided into a plurality of individual cells30arranged rectilinearly in columns and rows. As will be understood to those of ordinary skill in the art, the orientation of the columns and rows is arbitrary, however, for clarity of description it will be assumed that the rows extend horizontally and columns extend vertically.

During operation the rows of cells30are scanned one at a time by scanning circuit32so that exposure data from each cell30may be read by read-out circuit34. Each cell30independently measures the intensity of radiation received at its surface and thus the exposure data read-out provides one pixel of information in an image40to be displayed on a display42, such as a monitor, normally viewed by the user.

Energy integrating detector22also includes a bias circuit44that controls a bias voltage to the cells30. Each of the bias circuit44, scanning circuit32, and read-out circuit34, communicates with an acquisition control and image processing circuit46, which controls operation of circuits44,32, and34by means of an electronic processor. The acquisition control and image processing circuit46also controls the x-ray source14, turning it on and off and controlling the current and thus the fluence of x-rays in beam16and/or the voltage and hence the energy of the x-rays in beam16.

Referring toFIG. 2, energy discriminating detector24distinguishes the energy spectrum of the incident radiation. The discrimination of energy may occur in any manner known in the art. For instance, each photon may be analyzed separately so that the energy of each photon is distinguished. Also, when the absorption characteristics of the material that the x-rays initially pass through are known, then the overall spectrum of the photons may be analyzed. Energy discriminating detector24adds characterization information to the high resolution image. For instance, energy discriminating detector24provides the supplemental data, such as a highlighting the position or otherwise enhancing the visibility of the catheter, etc. Energy discriminating detector24may be any type of detector that discriminates energy.

In an exemplary embodiment, the energy discriminating detector24is a direct conversion detector, which is a tiled, single crystal direct conversion detector, like the one described in U.S. Pat. No. 6,408,050. The count rates on the direct conversion detector may be quite low, allowing counting of individual x-rays and/or energy discrimination. In another exemplary embodiment, energy discriminating detector24is a thin film detector that would have limited material decomposition functionality, since the energy discrimination would simply be the difference in spectrum between the initial and final photon absorption (beam hardening within the detector). The direct conversion detector adds characterization information to the high resolution image. Another example of energy discriminating detector24is a scintillator made of a crystalline scintillator material and one or more light-sensitive elements, such as a photodiode or the like. In each of the exemplary embodiments, energy discriminating detector24would be used to detect metal, calcium, or other material in the image.

Energy integrating detector22and energy discriminating detector24are coupled to an image processor70, which receives the image data from both detectors22and24. Once image processor70receives the image data, image processor70produces the image data into one or more processed images. In at least one of those images, clinically relevant information, such as a catheter or arterial calcification, is enhanced. An analysis72of those images then may occur. The analysis72takes the processed image or images and analyzes the characteristics of the images. The analysis72may include combining two or more processed images into processed and analyzed images. The initial processed images may be displayed at display42and/or may be stored at a storage device76. In addition, the processed and analyzed images may be displayed at display42and/or may be stored at storage device76. The processed and analyzed images may be used in any manner.

For instance, there are a number of uses for the processed and analyzed images. The first use for the processed and analyzed images is to produce a visual display for the user with improved visibility and/or conspicuity of the clinically relevant portion of the image and sends this image to display42. One way of providing improved visibility is to provide different color for the clinically relevant portion of the image, i.e., the catheter being red, so that the clinically relevant portion of the image stands out and is enhanced.

The second use for the processed and analyzed images is to produce a score representing some clinically relevant risk factor. For example, a calcium score representing the total amount and distribution of calcium in the arteries, which bears some relation to the clinical risk of future heat attack.

The third use for the processed and analyzed images is to perform some Computer Aided Diagnosis or Computer Aided detection on the images. For example, automatically determining the location of arterial blockages, or highlighting to the user suspicious locations in the image.

Referring toFIG. 3, hybrid x-ray detector12is shown in more detail. In an exemplary embodiment, hybrid x-ray detector12includes energy integrating detector22fabricated on a front side48of a substrate50. Front side48receives x-ray beam16. Energy discriminating detector24is located at a back side52of substrate50and may either be attached to back side52of substrate50or there may be a space between back side and energy discriminating detector24; however, there is no attenuating plate or similar type of object between energy discriminating detector24and substrate50. Energy discriminating detector24includes electronics56, which may be energy sensitive electronics for measuring and counting photons. Electronics56counts photons or distinguishes the energy with some degree of energy resolution. Examples of such electronics include an Application Specific Integrated Circuit.

Substrate50is preferably a low x-ray attenuating substrate, such as a low barium glass substrate or polymer substrate. The low attenuation allows a sufficient number of x-rays to pass through energy integrating detector22and be received by the energy discriminating detector24. For instance, low attenuation is approximately 80% to 90% transmission of the incident x-rays.

Energy discriminating detector24does not need to be very large, as it is used to supplement the primary x-ray image formed by energy integrating detector22and to highlight certain specific features, such as tracking the catheter tip. Thus, energy discriminating detector24is not required to image the entire field of view. For instance if a cardiac detector is 20 cm by 20 cm, energy discriminating detector24may be approximately 5 cm by 5 cm or 10 cm by 10 cm. However, the smaller energy discriminating detector24is merely to save cost and thus, energy discriminating detector24may also be the same size as energy integrating detector22.

Referring toFIG. 4, in another exemplary embodiment, energy integrating detector22includes a partially thinned area60in the region in front of energy discriminating detector24. Both energy integrating detector22and energy discriminating detector24absorb x-rays16. Substrate50also absorbs a small amount of x-rays16, thereby possibly leaving an insufficient number of photons to form an image with the desired image quality. By having partially thinned area60, energy integrating detector22will absorb about 75% of x-rays16and allows about 25% of x-rays16to pass through to energy discriminating detector24. Then energy discriminating detector24absorbs about 90–95% of those x-rays that passed through energy integrating detector22. Thus, the combined absorption for both energy integrating detector22and energy discriminating detector24is preferably approximately 97% to 99% of x-rays16. In order to get good image quality, it is desirable to have at least 90% of x-rays16absorbed by both energy integrating detector22and energy discriminating detector24.

The absorption of x-rays16at energy integrating detector22is reduced in the partially thinned region60, providing an increased number of x-rays16for energy discriminating detector24to improve the image quality. Accordingly, improved photon statistics to energy discriminating detector24is obtained. In addition, the signal from energy discriminating detector24may be combined with the signal from energy integrating detector22to produce the conventional anatomical image, while the signal from the energy discriminating detector alone may be used to generate the target-specific image highlighting.

The amount of thinning in partially thinned area60depends on the image processing selected for the configuration. Partially thinned area60allows enough x-rays16to pass through to energy discriminating detector24; however, too much thinning will also reduce the signal to noise ratio in the energy integrating detector22and potentially allow excessive flux to the energy discriminating detector24.

Referring toFIG. 5, in another exemplary embodiment, energy discriminating detector24and electronics56are disposed on energy integrating detector22, which is disposed on front side48of substrate50. Leads62for electronics56may cross energy integrating detector22. Energy discriminating detector24may or may not be coupled to energy integrating detector22and a space may also exist between energy discriminating detector24and energy integrating detector22.

In this embodiment, energy discriminating detector24receives x-rays16first and would not absorb a significant number of x-rays16, thereby leaving a sufficient number of x-rays for energy integrating detector22. As with the embodiment ofFIG. 4, the image displayed to the user could be formed by the sum of the data from energy discriminating detector24and energy integrating detector22. In addition, because of the attenuation of the primary x-rays16by the metalization and energy discrimination electronics56, and the increased radiation damage on energy discrimination electronics56, a radiation hard silicon process should be used in the fabrication of the energy discrimination electronics. Moreover, it is preferable in this embodiment not to have a partially thinned area for energy integrating detector22so that energy integrating detector22will absorb as much as possible of the x-rays16. Because energy discriminating detector24receives x-rays16first, it is desirable to absorb as much of the remaining x-rays16at energy integrating detector22as possible.

In addition, as seen by the exemplary embodiments, the order of the energy integrating detector and the energy discriminating detector does not matter. Thus, energy integrating detector can receive the x-rays first or the energy discriminating detector may receive the x-rays first. The important feature is that the detector that receives the x-rays first absorbs only a portion of the x-rays so that the second portion of the x-rays pass through to second detector. In addition, the energy integrating detector22absorbs the bulk of the x-rays16and forms a high resolution primary anatomical (density) image. Energy discriminating detector24adds characterization information to the high resolution image.

Referring toFIG. 6, an exemplary method200of operating the x-ray hybrid x-ray detector is illustrated. At step202, the energy integrating detector receives x-rays. At step204, the energy integrating detector converts a first portion of the x-rays to light. The amount of x-rays that are converted to light is controlled by choosing the thickness of the energy integrating detector. For instance, the thinner the energy integrating detector, the more x-rays that will pass through the energy integrating detector and the thicker the energy integrating detector, the less x-rays that will pass through the energy integrating detector. At step206, a second portion of the x-rays passes through the energy integrating detector. At step208, the energy discriminating detector receives a second portion of the x-rays. In an exemplary embodiment, the energy discriminating detector receives a small number of x-rays. At step210, the x-rays that strike the energy discriminating detector are individually counted and characterized as to their energy. This allows different materials to be characterized. In an exemplary method, the energy integrating detector provides the anatomical data and the energy discriminating detector provides the supplemental data, such as a highlighting the position or otherwise enhancing the visibility of the catheter, etc. In addition, the energy integrating detector provides a high resolution image and the energy discriminating detector adds characterization information to the high resolution image.

The advantages of the hybrid x-ray detector are the use of an energy integrating detector and an energy discriminating detector together. The energy integrating detector forms the primary image, and the energy discriminating detector highlights certain features. Energy discriminating detectors are typically unable to image at high count rates. Thus, the energy integrating detector detects about 90% to 95% of the photons and provides a high resolution image, while the energy discriminating detector receives a low number of photons and can then discriminate the photons and count them, thereby detecting supplementary features, such as a catheter. In addition, the energy discriminating detector may have gaps between the imaging regions of adjacent tiles, which may cause image data to be missing. By combining the energy integrating detector with the energy discriminating detector, there are no gaps because the energy integrating detector forms the primary image and the energy discriminating detector is used to highlight certain features. Thus, all of the desired information is captured in the image formed by the two detectors.