Patent Application
20060131669
The invention relates generally to imaging systems. In particular, the invention relates to thin film transistors for use in detectors of such imaging systems.
Non-invasive imaging broadly encompasses techniques for generating images of the internal structures or regions of a person or object that are otherwise inaccessible for visual inspection. For example, non-invasive imaging techniques are commonly used in the industrial field for inspecting the internal structures of parts and in the security field for inspecting the contents of packages, clothing, and so forth. One of the best known uses of non-invasive imaging, however, is in the medical arts where these techniques are used to generate images of organs and/or bones inside a patient which would otherwise not be visible.
One class of non-invasive imaging techniques that may be used in these various fields is based on the differential transmission of X-rays through a patient or object. In the medical context, a simple X-ray imaging technique may involve generating X-rays using an X-ray tube or other source and directing the X-rays through an imaging volume in which the part of the patient to be imaged is located. As the X-rays pass through the patient, the X-rays are attenuated based on the composition of the tissue they pass through. The attenuated X-rays then impact a detector that converts the X-rays into signals that can be processed to generate an image of the part of the patient through which the X-rays passed based on the attenuation of the X-rays. Typically the X-ray detection process utilizes a scintillator, which generates optical photons when impacted by X-rays, and an array of photosensor elements, which generate electrical signals based on the number of optical photons detected.
Some X-ray techniques utilize very low energy X-rays so that patient exposure can be extended. For example, fluoroscopic techniques are commonly used to monitor an ongoing procedure or condition, such as the insertion of a catheter or probe into the circulatory system of a patient. Such fluoroscopic techniques typically obtain large numbers of low energy images that can be consecutively displayed to show motion in the imaged area in real-time or near real-time.
However fluoroscopic techniques, as well as other low energy imaging techniques, may suffer from poor image quality due to the relatively weak X-ray signal relative to the electronic noise attributable to the detector. As a result it is typically desirable to improve the efficiency of the detection process, such as by reducing the electronic noise of the detector while in operation. Various aspects of the thin film transistors (TFTs) employed in the detector may contribute to the overall electronic noise. For example, the capacitance between the drain electrode and gate electrode of the TFT is a major component of the overall capacitance of the data line. This in turn, leads to two major noise sources associated with the data line, namely the Johnson noise associated with the resistance of the data line and the noise associated with the read out electronics. Further, the charge trapping currents in TFTs also contribute to the overall electronic noise.
Therefore, there is a need for reducing the electronic noise generated by electronic components in the detector.
In one aspect of the present technique, an X-ray imaging system is provided, where the X-ray imaging system includes an X-ray source configured to emit X-rays and, a detector. The detector includes an array of detector elements, where each detector element comprises a thin film transistor configured for use as a switch. The thin film transistor comprises a drain electrode and a source electrode that are not symmetric to one another. Also provided with the X-ray imaging system is a detection acquisition circuitry configured to acquire the electrical signals, a system controller configured to control at least one of the X-ray source or the detector acquisition circuitry, and an image processing circuitry configured to process the electrical signals to generate an image.
In another aspect of the present technique, an annular thin film transistor is provided, where the annular thin film transistor includes a layer of a semiconductor material, an annular source electrode disposed above the layer of the semiconductor material, a drain electrode disposed above the layer of the semiconductor material within the annular source electrode, and an active channel between the drain electrode and the annular source electrode, wherein a surface of the active channel comprises exposed semiconductor material.
In yet another aspect of the present technique, a serpentine thin film transistor includes a layer of a semiconductor material, a serpentine source electrode disposed above the layer of the semiconductor material, a drain electrode disposed above the layer of semiconductor material and substantially within a recess formed by the serpentine source electrode, wherein the drain electrode is configured to substantially conform to the recess, and an active channel between the drain electrode and the serpentine source electrode, wherein the active channel has a substantially consistent length, and wherein a surface of the active channel comprises exposed semiconductor material.
In still another aspect of the present technique, a method of manufacturing a detector for use in an imaging system is provided. The method includes forming an array of detector elements, where each detector element comprises a thin film transistor.
In another aspect of the present technique, a method of manufacturing an annular thin film transistor is provided. The method includes forming a layer of a semiconductor material, forming an annular source electrode disposed above the layer of the semiconductor material, forming a drain electrode disposed above the layer of the semiconductor material within the annular source electrode, and forming an active channel between the drain electrode and the annular source electrode.
In yet another aspect of the present technique, a method of manufacturing a serpentine thin film transistor includes forming a layer of a semiconductor material, forming a serpentine source electrode disposed above the layer of the semiconductor material, forming a drain electrode disposed above the layer of semiconductor material and substantially within a recess formed by the serpentine source electrode, and forming an active channel between the drain electrode and the serpentine source electrode.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
The X-ray source 12 is controlled by power supply/control circuitry 24 which furnishes both power and control signals for examination sequences. Moreover, detector 22 is coupled to detector acquisition circuitry 26, which commands acquisition of the signals generated in the detector 22. Detector acquisition circuitry 26 may also execute various signal processing and filtration functions, such as, for initial adjustment of dynamic ranges, interleaving of digital, and so forth.
In the depicted exemplary embodiment, one or both of the power supply/control circuitry 24 and detector acquisition circuitry 26 are responsive to signals from a system controller 28. In some exemplary systems it may be desirable to move one or both of the detector 22 or the X-ray source 12. In such systems, a motor subsystem may also be present as a component of the system controller 28 to accomplish this motion. In the present example, the system controller 28 also includes signal processing circuitry, typically based upon a general purpose or application specific digital computer. The system controller 28 may also include memory circuitry for storing programs and routines executed by the computer, as well as configuration parameters and image data, interface circuits, and so forth.
Image processing circuitry 30 is also present in the depicted embodiment of the X-ray imaging system 10. The image processing circuitry 30 receives acquired projection data from the detector acquisition circuitry 26 and processes the acquired data to generate one or more images based on X-ray attenuation.
One or more operator workstation 32 is also present in the depicted embodiment of the X-ray imaging system 10. The operator workstation 32 allows an operator to initiate and configure an X-ray imaging examination and to view the images generated as part of the examination. For example, the system controller 28 is generally linked to operator workstation 32 so that an operator, via one or more input devices associated with the operator workstation 32, may provide instructions or commands to the system controller 28.
Similarly, the image processing circuitry 30 is linked to the operator workstation 32 such that the operator workstation 32 may receive and display the output of the image processing circuitry 30 on an output device 34, such as a display or printer. The output device 34 may include standard or special purpose computer monitors and associated processing circuitry. In genetal, displays, printers, operator workstations, and similar devices supplied within the system may be local to the data acquisition components or may be remote from these components, such as elsewhere within an institution or hospital or in an entirely different location. Output devices and operator workstations that are remote from the data acquisition components may be linked to the image acquisition system via one or more configurable networks, such as the internet, virtual private networks, and so forth. As will be appreciated by one of ordinary skill in the art, though the system controller 28, image processing circuitry 30, and operator workstation 32 are shown distinct from one another in
Referring now to
Turning now to
Further, the scintillation-based detector 35 includes a scintillator 44, which, when exposed to X-rays, generates the optical photons detected by the photosensitive regions 40. As illustrated in this embodiment, a conductive layer 54 disposed on a dielectric layer 56 is disposed between the scintillator 44 and the array of photosensor elements 38. Vias 58 electrically couple the conductive layer 54 to the top surface of each element of the array of photosensor elements 38 to allow a common bias to be applied to each photosensor element.
In embodiments employing a direct conversion detector, as opposed to a scintillation-based detector 35 discussed above, a photoconductor (such as of selenium, lead oxide, lead iodide, mercuric iodide, and so forth) is utilized in place of the scintillator. Similarly, simple storage capacitors are utilized in place of the photosensitive diodes in such a direct conversion detector. Other aspects of such a direct-conversion detector, including the use of data and scan lines, vias and bridges, and the use of TFTs 42, are similar or analogous to scintillation-based detector 35 described above and, therefore, may also benefit from the present technique as described herein.
In accordance with the present invention, and as discussed in greater detail below, the TFTs include a source electrode and a drain electrode that are not symmetric to one other. In certain embodiments, the drain electrode is smaller that the source electrode. This asymmetry allows a reduction in drain-to-gate capacitance, particularly relative to the source-to-gate capacitance, to the extent that these capacitances are a function of the overlap of the gate electrode with each of the drain and source electrodes, respectively. As will be appreciated by those skilled in the art, reducing the drain-to-gate capacitance generally reduces the noise associated with the TFT, thereby increasing the signal-to-noise ratio (SNR).
For example, in one embodiment, the TFT 42 is a structure in which the source electrode partially or completely encloses the drain electrode. For simplicity, such a structure will be referred to as an annular TFT 60 herein, though, as will be appreciated by those skilled in the art, the annular source electrode 62 may be any enclosing shape such as, oval, rectangular, square, etc., as opposed to circular. Similarly, the enclosed drain electrode 64 may be other shapes than circular. For simplicity, however, the annular TFT 60 described herein, and depicted in
Referring now to
The annular TFT 60 is coupled to vertically offset data lines (not shown) by electrically conductive vias 58, such as shown with respect to the disc-shaped drain electrode 64 in
In the illustrated embodiment of
Further, in the depicted embodiment of
In another embodiment, the TFT 42 is a structure in which the source electrode and drain electrode are differently sized. In such an embodiment, the source and drain electrodes may also be interleaved. For simplicity, such a structure will be referred to as a serpentine TFT 78 herein. For example, referring now to
In addition, as will be appreciated by those of ordinary skill in the art, the source electrode 80 and the drain electrode 84 are separated by an active channel 98, typically formed by etching a portion of the semiconductive layer 82. As will be appreciated by those of ordinary skill in the art, the active channel 98 has a width, where the width is a distance traversed by the active channel 98 in a direction parallel to the source and drain electrodes 80 and 84. In one embodiment, the width of the active channel 98 is in a range from about 15 microns to about 150 microns. In the illustrated embodiment of
While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
