Patent Description:
Imaging devices, such as an x-ray imager, have been used for diagnostic and treatment purposes. One type of x-ray imager is a diagnostic imager configured to operate with a diagnostic radiation source. Another type of x-ray imager is a high Detective Quantum Efficiency (DQE) detector that is configured for use with a treatment radiation source. An x-ray imager may also be configured for use with both diagnostic radiation beam and treatment radiation beam.

Creating a high DQE electronic portal imaging device (EPID) presents a significant technical challenge. One approach uses thick pixilated scintillator arrays that are coupled to a matrix of photodiodes. Incoming x-ray photons deposit energy into the scintillator which then produces optical photons via luminescence. These optical photons, which originate with random polarizations and direction vectors after the luminescence events, are transported throughout the scintillator where they can be reflected, refracted and scattered out of borders. Eventually, many photons will cross the boundary of the scintillator, and will reach the EPID's photodiodes. The photodiodes convert the photons into electrical current for readout and digitization. Despite the promise of the technology, performance of current EPIDs may be inadequate.

Current EPIDs employed in the field of radiotherapy utilize standard indirect flat panel design that includes a thin gadolinium oxysulfide (GOS) scintillator. The thickness of GOS scintillator is typically kept small to preserve the spatial resolution. The small thickness of the scintillator and high energies of megavoltage photon beams, used in radiotherapy field, yield low X-ray absorption within a scintillator. Low X-ray absorption limits the number of optical light produced in the scintillator and measured subsequently by a photo-diode matrix. Although a spatial resolution of the resulting digital image stays high, the signal to noise ratio (SNR) is degraded due to poor absorption characteristics. Current EPIDs suffer from low DQE (e.g., ~ <NUM>%) for MV imaging. Imaging tasks with such low quantum efficiency detectors are not very practical due to low contrast, especially when imaging soft tissue. Creation of an imaging device with a high DQE in the megavoltage range remains an important problemin the radiotherapy field.

One possible approach to increasing quantum efficiency (QE) is to try to make several identical detection layers for the EPID, and then stack them together. One example of such implementation is to have several identical detection layers folded under each other with the aim to catch unabsorbed X-rays in superior detection layers. While such configuration can increase DQE, manufacturing expense may make such a solution impractical. For example, if the EPID has four identical detection layers, the total DQE of the EPID can increase up to <NUM> times compared to a single layer EPID. The production cost may also increase up to four times, making higher efficiency benefits less attractive. <CIT> describes radiographic imaging apparatus and methods for operating the same including a first scintillator, a second scintillator, a plurality of first photosensitive elements, and a plurality of second photosensitive elements. The first scintillator can have first scintillator properties and the second scintillator can have second scintillator properties different from the first scintillator properties. <CIT> describes various two-panel radiographic imaging apparatus configurations, wherein a front panel and back panel have substrates, arrays of signal sensing elements and readout devices, and passivation layers. The front and back panels have scintillating phosphor layers responsive to X-rays. The X-ray apparatus has means for combining the signals of the X-ray images to produce a composite X-ray image. Furthermore, the composition and thickness of the scintillating phosphor layers are selected, relative to each other, for the diagnostic efficacy of the composite X-ray image.

An imaging apparatus having multiple radiation-detecting layers is described herein. The imaging apparatus has a first scintillator layer being a Gadolinium oxysulfide (GOS)-based scintillator that provides first image signals, and a second scintillator layer being a glass-based scintillator, such as LKH5 scintillator that provides second image signals. The first scintillator layer together with its associated photodiodes may provide a low- QE and high spatial resolution for the first image signals (forming a first image), and the second scintillator layer together with its associated photodiodes may provide a high-QE and low spatial resolution for the second image signals (forming a second image). Thus, a first image formed by the first image signals will be a noisy image with higher frequency details compared to a second image formed by the second image signals, while the second image will be a blurry image with a lower noise compared to the first image. The first scintillator layer may be thinner than the second scintillator layer.

The embodiments described herein are not limited to a two-layer detector configuration and two-image fusion. The detector can comprise more than two scintillator layers, which, as a result, would provide a plurality of digital images, which can be combined in the manners described herein. Scintillators can be made of clear or turbid material, as well as being structured in a pixelated or non-pixelated (slab) form.

Embodiments described herein teach how to combine these two inherently different images (i.e., (<NUM>) a high resolution (sharp) and low-efficiency (low signal-to-noise) image and (<NUM>) a low resolution (blurry) high-efficiency (high signal-to-noise) image) into one "fused" image. To combine the first and second image signals, several image fusion techniques may be employed in different embodiments. By means of non-limiting examples, the image fusion technique may take advantage of detector imaging characteristics, such as modulation transfer function (MTF), noise power spectrum (NPS), DQE, or any combination of the foregoing. In some embodiments, the first and second image signals may be combined in a way that maximizes an image's SNR and/or that minimizes spatial resolution losses (that may occur during image fusion).

In some embodiments, images from the multiple layers may be combined via weighted summation. The weighting may be dependent on spatial frequency, with the weights w(f) chosen to maximize SNR of the combined image for all spatial frequencies f. This effectively maximizes the combined image's DQE, which is a measure of image quality as a function of spatial frequency. The weights w(f) may be chosen based on the performance of each detector layer, where performance is dictated by spatial resolution (relating to MTF) and noise (relating to NPS). In some cases, the weights may be chosen to maximize DQE in the combined image. For example, if image n has performance described by <MAT>, then the optimal linear weighting for image n is wn(f) = [(MTFn/NFSn)/∑n(MFFn/NPSn)](f), where the summation is over all images. The combined image is then: Fm(f)-∑nwn(f) ·Fmn(f), where the linear weights give Im' the highest possible DQE for all frequencies. Each spatial frequency can be selected using 2D image filters. Using the two-layer GOS + glass-based scintillators the predicted MTF and NPS of each of the two images (from the respective two scintillators) may be used to determine optimal linear weights w(f) for each image. These weights result in a desirable (e.g., maximum) DQE across all frequencies.

According to the invention, there is provided an imaging apparatus according to claim <NUM>.

Optionally, the image combiner is configured to combine the first image signals and the second image signals in a way that increase signal-to-noise ratio while reducing spatial resolution loss.

Optionally, the image combiner is configured to combine the first image signals and the second image signals based on frequency-dependent weighting.

Optionally, the image combiner is configured to combine the first image signals and the second image signals based on frequency-dependent filtering.

Optionally, the image combiner is configured to combine the first image signals and the second image signals based on noise-dependent weighting.

Optionally, the image combiner is configured to combine the first image signals and the second image signals in image domain based on noise reduction.

Optionally, the image combiner is configured to apply a first weight factor for the first image signals, and a second weight factor for the second image signals.

Optionally, the first weight factor is between <NUM> and <NUM>.

Optionally, the first weight factor is <NUM>.

Optionally, the first weight factor has a first value below <NUM> for a first frequency or first frequency range, and a second value above <NUM> for a second frequency higher than the first frequency or for a second frequency range higher than the first frequency range.

Optionally, the image combiner is configured to combine the first image signals and the second image signals based on modulation transfer function (MTF).

Optionally, the image combiner is configured to combine the first image signals and the second image signals based on noise power spectrum (NPS).

Optionally, the image combiner is configured to combine the first image signals and the second image signals based on detective quantum efficiency (DQE).

According to the invention, the first scintillator layer is GOS-based.

According to the invention, the second scintillator layer is glass-based.

Optionally, the first scintillator layer is thinner than the second scintillator layer.

Optionally, the first scintillator layer and the second scintillator layer are stacked.

Optionally, the imaging apparatus further includes a third scintillator layer, wherein the first scintillator layer, the second scintillator layer, and the third scintillator layer are stacked.

Optionally, the imaging apparatus further includes a fourth scintillator layer, wherein the first scintillator layer, the second scintillator layer, the third scintillator layer are stacked, and the fourth scintillator layer are stacked.

Optionally, the image combiner is configured to combine a third image signals associated with the third scintillator layer with the first image signals and the second image signals.

Optionally, the imaging apparatus is configured to provide a detective quantum efficiency of <NUM>% or higher.

According to the invention, an imaging method according to claim <NUM> is provided.

According to the invention, there is also provided a product having a computer-readable medium according tot claim <NUM>.

Other and further aspects and features will be evident from reading the following detailed description.

The drawings illustrate the design and utility of embodiments, in which similar elements are referred to by common reference numerals. These drawings are not necessarily drawn to scale. In order to better appreciate how the above-recited and other advantages and objects are obtained, a more particular description of the embodiments will be rendered, which are illustrated in the accompanying drawings. These drawings depict only exemplary embodiments and are not therefore to be considered limiting in the scope of the claims.

It should be noted that the figures are not drawn to scale and that elements of similar structures or functions are represented by like reference numerals throughout the figures.

<FIG> illustrates a radiation system <NUM>. The system <NUM> is a treatment system that includes a gantry <NUM>, a patient support <NUM> for supporting a patient <NUM>, and a control system <NUM> for controlling an operation of the gantry <NUM>. The gantry <NUM> is in a form of an arm, but in other embodiments, the gantry <NUM> may have other forms (such as a ring form, etc.). The system <NUM> also includes a radiation source <NUM> that projects a beam <NUM> of radiation towards a patient <NUM> while the patient <NUM> is supported on support <NUM>, and a collimator system <NUM> for controlling a delivery of the radiation beam <NUM>. The collimator <NUM> may be configured to adjust a cross sectional shape of the beam <NUM>. The radiation source <NUM> can be configured to generate a cone beam, a fan beam, or other types of radiation beams in different embodiments.

As shown in the figure, the system <NUM> also includes an imager <NUM>, located at an operative position relative to the source <NUM> (e.g., under the support <NUM>). In the illustrated embodiments, the radiation source <NUM> is a treatment radiation source for providing treatment energy. In such cases, the treatment energy may be used by the imager <NUM> to obtain images. In order to obtain imaging using treatment energies, the imager <NUM> is configured to generate images in response to radiation having treatment energies (e.g., MV imager). In other embodiments, in addition to being a treatment radiation source, the radiation source <NUM> can also be a diagnostic radiation source for providing diagnostic (imaging) energy for imaging purpose. In further embodiments, the system <NUM> may include the radiation source <NUM> for providing treatment energy, and one or more other radiation sources for providing diagnostic energy. In some embodiments, the treatment energy is generally those energies of <NUM> kilo-electron-volts (keV) or greater, and more typically <NUM> mega-electron-volts (MeV) or greater, and diagnostic energy is generally those energies below the high energy range, and more typically below <NUM> keV. Also, in some embodiments, a treatment energy may be <NUM> MV or higher (e.g., <NUM> MV). In other embodiments, the treatment energy and the diagnostic energy can have other energy levels, and refer to energies that are used for treatment and diagnostic purposes, respectively. In some embodiments, the radiation source <NUM> is able to generate X-ray radiation at a plurality of photon energy levels within a range anywhere between approximately <NUM> keV and approximately <NUM> MeV. In other embodiments, the radiation source <NUM> may be configured to generate radiation at other energy ranges.

In the illustrated embodiments, the control system <NUM> includes a processing unit <NUM>, such as a computer processor, coupled to a control <NUM>. The control system <NUM> may also include a monitor <NUM> for displaying data and an input device <NUM>, such as a keyboard or a mouse, for inputting data. The operation of the radiation source <NUM> and the gantry <NUM> are controlled by the control <NUM>, which provides power and timing signals to the radiation source <NUM>, and controls a rotational speed and position of the gantry <NUM>, based on signals received from the processing unit <NUM>. In some cases, the control <NUM> may also control the collimator system <NUM> and the position of the patient support <NUM>. In addition, in some embodiments, the control <NUM> may be configured to control an operation of the imager <NUM>. Although the control <NUM> is shown as a separate component from the gantry <NUM> and the processor <NUM>, in alternative embodiments, the control <NUM> can be a part of the gantry <NUM> or the processing unit <NUM>.

In some embodiments, the system <NUM> may be a treatment system configured to deliver treatment radiation beam towards the patient <NUM> at different gantry angles. During a treatment procedure, the source <NUM> rotates around the patient <NUM> and delivers treatment radiation beam from different gantry angles towards the patient <NUM>. While the source <NUM> is at different gantry angles, the collimator <NUM> is operated to change the shape of the beam to correspond with a shape of the target tissue structure. For example, the collimator <NUM> may be operated so that the shape of the beam is similar to a cross sectional shape of the target tissue structure. In another example, the collimator <NUM> may be operated so that different portions of the target tissue structure receive different amount of radiation (as in an IMRT procedure).

In the illustrated embodiments, the system <NUM> also includes an imaging device <NUM> having an imaging source <NUM> and an imager <NUM>. The imaging device <NUM> is configured to obtain one or more images of an internal part of the patient <NUM>. The image(s) obtained by the imaging device <NUM> may be used to setup the patient <NUM>, monitor a position of the patient <NUM>, track a target within the patient <NUM>, or any combination of the foregoing. In some cases, the imaging device <NUM> may be configured to obtain images of an internal fiducial <NUM> of the patient <NUM>. The internal fiducial <NUM> may be an internal structure inside the patient <NUM>. In some embodiments, the internal structure may move in correspondence (e.g., in sync) with a target of the patient <NUM> that is desired to be treated. In such cases, the internal structure may be used as a surrogate for determining a position and/or movement of the target during treatment of the patient <NUM>, and motion management based on the surrogate may be employed in some cases. Thus, the internal fiducial <NUM> may be imaged by the imaging device <NUM> (and/or by the radiation source <NUM> and imager <NUM>) that functions as a position monitoring system during a treatment of the patient <NUM>. By means of non-limiting examples, the internal fiducial <NUM> may be an anatomical surrogate, such as bony structure, a vessel, a natural calcification, or any other items in a body. As discussed, the imaging device <NUM> and/or the imager <NUM> may also be used for target tracking and/or patient positioning. In some embodiments, the control <NUM> may be configured to control an operation of the imaging device <NUM>. For example, the control <NUM> may provide one or more control signals to activate the imaging source <NUM>, and/or to operate a readout and control circuit in the imager <NUM>.

In some embodiments, the imaging device <NUM> may be an x-ray device. In such cases, the imaging source <NUM> comprises a radiation source. In other embodiments, the imaging device <NUM> may have other configurations, and may be configured to generate images using other imaging techniques. For example, in other embodiments, the imaging device <NUM> may be an ultrasound imaging device, a MRI device, a tomosynthesis imaging device, or any of other types of imaging devices. Also, in the above embodiments, the imaging device <NUM> is illustrated as being integrated with the treatment machine. In other embodiments, the imaging device <NUM> may be a separate device that is separate from the treatment machine. In addition, in some embodiments, the imaging device <NUM> may be a room-based imaging system or a couch-based imaging system. In either case, the imaging device <NUM> may provide any form of imaging, such as x-ray imaging, ultrasound imaging, MRI, etc. Furthermore, in other embodiments, the imaging device <NUM> may provide in-line imaging in the sense that it may be configured to acquire images along the same direction as the treatment beam. For example, a dual-energy source (integrating the treatment source <NUM> and the imaging source <NUM>) may be provided to provide imaging energy for generating an image, and to provide treatment energy to treat a patient along the same direction. In such cases, the imager <NUM> may replace the imager <NUM>, or may be integrated with the imager <NUM> to form a hybrid-imager, which is configured to provide kV and MV imaging. In still further embodiments, the imaging device <NUM> and/or the imaging device <NUM> may be configured to provide dual energy imaging and any form of energy-resolved imaging to increase contrast in x-ray images. For example, a first part of an image may be generated using a first energy, and a second part (e.g., a more relevant part that includes a target) of the same image may be generated using a second energy that is higher than the first energy. As a result, the second part of the image may have higher contrast compared to the first part. However, the overall dose involved in generating the whole image may be reduced compared to the situation in which the entire image is generated using the second energy.

<FIG> illustrates an imaging apparatus <NUM> in accordance with some embodiments. The imaging apparatus <NUM> may implement as the imager <NUM> of <FIG> in some embodiments. The imaging apparatus <NUM> includes a first scintillator layer <NUM>, a second scintillator layer <NUM>, a first photodiode layer <NUM>, a second photodiode layer <NUM>, and an image combiner <NUM>.

The first scintillator layer <NUM> is configured to receive radiation <NUM> and generate first photons in response to the radiation <NUM>. The radiation <NUM> may be treatment radiation having an energy level that is sufficient for treatment of a patient. The first photodiode layer <NUM> includes first photodiode elements <NUM> that are configured to convert first photons into first electrical signals for readout by a readout circuit. The first electrical signals may be considered as first image signals forming a first image. The first scintillator layer <NUM> may be pixelated or non-pixelated.

The second scintillator layer <NUM> is configured to receive radiation after it has passed through the first scintillator layer <NUM>, and to generate second photons in response to the radiation. The second photodiode layer <NUM> includes second photodiode elements <NUM> that are configured to convert second photons into second electrical signals for readout by the readout circuit. The second electrical signals may be considered as second image signals forming a first image. The second scintillator layer <NUM> may be pixelated or non-pixelated.

In some embodiments, each photodiode element <NUM>/<NUM> may include one or more amorphous silicon (a:Si) detector. Also, in some embodiments, the photodiode element <NUM>/<NUM> may be implemented using a photodiode. In this specification, the term "photodiode" refers to one or more electrical circuit element(s) on a detector pixel that are associated with converting photon energy into electrical signals. This can include, but is not limited to, photodiode(s), switching transistor(s), amplification transistor(s), direct conversion element, indirect conversion elements, photon counting elements, or a combination thereof. In some embodiments, signal from each photodiode element <NUM>/<NUM> forms a pixel in an image. In other embodiments, a binning circuit is optionally provided to combine the signals from two or more photodiode elements to form each pixel in the image. For example, the binning circuit of the imager <NUM> may be configured to provide 2X2 binning, 3X3 binning, 4X4 binning, 1X2 binning, 1X4 binning or binning of other number of pixels. The binning circuit may be implemented as a part of the readout circuit in some embodiments. In some embodiments, the readout circuit may be communicatively connected to the control <NUM>, or another separate control, for controlling an operation of the readout circuit. Also, in some embodiments, the readout circuit may be included as a part of the image combiner <NUM>, or may be communicatively coupled to the image combiner <NUM>.

As shown in <FIG>, the image elements <NUM> is secured to a first substrate <NUM>, and the image elements <NUM> are secured to the second substrate <NUM>. In the illustrated embodiments, the image elements <NUM> of the first photodiode layer <NUM> are closer to a first side (e.g., a top side) of the substrate <NUM> than to a second side (e.g., a bottom side) of the substrate <NUM>. Also, the image elements <NUM> of the first photodiode layer <NUM> are closer to a first side (e.g., a top side) of the substrate <NUM> than to a second side (e.g., a bottom side) of the substrate <NUM>. In other embodiments, the image elements <NUM> of the first photodiode layer <NUM> are closer to the second side (e.g., the bottom side) of the substrate <NUM> than to the first side (e.g., the top side) of the substrate <NUM>. Also, in other embodiments, the image elements <NUM> of the first photodiode layer <NUM> are closer to the second side (e.g., the bottom side) of the substrate <NUM> than to the first side (e.g., the top side) of the substrate <NUM>. The substrates <NUM>, <NUM> may be made from glass, plastic, or other materials.

According to the invention, the first scintillator layer <NUM> is GOS-based. For example, the first scintillator layer <NUM> may be a Gadolinium oxysulfide scintillator layer. Also, the second scintillator layer <NUM> is glass-based. For example, the scintillator layer <NUM> may be a LKH5 scintillator. Although LKH5 scintillator is mentioned here as an example, it should be noted that the scintillator layer <NUM> may be any glass high density scintillator.

As shown in <FIG>, the first scintillator layer <NUM> is thinner than the second scintillator layer <NUM>, and the first scintillator layer <NUM> and the second scintillator layer <NUM> are arranged relative with each other in a stacked configuration. For example, in some embodiments, the first scintillator layer <NUM> may have a thickness that is between <NUM> and <NUM> (e.g., <NUM>), and the second scintillator layer <NUM> may have a thickness that is between <NUM> and <NUM> (e.g., <NUM>). In other embodiments, the first and second scintillator layers <NUM>, <NUM> may have other thicknesses. In the illustrated embodiments, the first scintillator layer <NUM> is above the second scintillator layer <NUM> so that the first scintillator layer <NUM> receives the radiation <NUM> before the second scintillator layer <NUM>. In other embodiments, the second scintillator layer <NUM> may be arranged above the first scintillator layer <NUM> so that the second scintillator layer <NUM> receives the radiation <NUM> before the first scintillator layer <NUM>.

The image combiner <NUM> is configured to obtain the first image signals and the second image signals, and combine them to form a combined image. Because the first and second photodiode layers <NUM>, <NUM> separately receive photos from the respective scintillator layers <NUM>, <NUM>, and create separate first and second image signals that correspond with the respective scintillator layers <NUM>, <NUM>, the image combiner <NUM> can separate process (e.g., by apply weight factors, filtering, etc.) the first and second image signals before combining them. In some embodiments, the image combiner <NUM> may be configured to combine the first image signals and the second image signals in a way that increase signal-to-noise ratio while reducing spatial resolution loss. Also, in some embodiments, the image combiner <NUM> may be configured to combine the first image signals and the second image signals based on frequency-dependent weighting. Alternatively, or additionally, the image combiner <NUM> may be configured to combine the first image signals and the second image signals based on frequency-dependent filtering. Alternatively, or additionally, the image combiner <NUM> may be configured to combine the first image signals and the second image signals based on noise-dependent weighting.

In some embodiments, the image combiner <NUM> is configured to apply a first weight factor for the first image signals, and a second weight factor for the second image signals. For example, the first weight factor may be between <NUM> and <NUM>, and more preferably between <NUM> and <NUM>, (e.g., <NUM>). The second weight factor may be a value that is equal to <NUM> minus the first weight factor. Also, in some embodiments, the first weight factor may have a first value below a threshold for a first frequency or first frequency range, and a second value above the threshold for a second frequency higher than the first frequency, or for a second frequency range higher than the first frequency range. The threshold may be between <NUM> and <NUM>, and more preferably between <NUM> and <NUM>, such as <NUM>).

In some embodiments, the image combiner <NUM> may be configured to combine the first image signals and the second image signals based on modulation transfer function (MTF), noise power spectrum (NPS), detective quantum efficiency (DQE), or any combination of the foregoing. As discussed, the image combiner <NUM> may be configured to combine the first image signals and the second image signals based on frequency-dependent weighting and/or filtering. By means of non-limiting examples, the frequency-dependent weighting and/or filtering may be based on MTF, NPS, DQE, or any combination of the foregoing.

Also, in some embodiments, the image combiner <NUM> may be configured to combine the first image signals and the second image signals based on noise-dependent weighting factors, which can be estimated either by calculating the first-order statistics of noises in both images or by iteratively enforcing a regularization constraint such as the total variation of output image. Furthermore, in some embodiments, the image combiner <NUM> may be configured to combine the first image signals and the second image signals based on weighting factors, wherein the weighting factors may be frequency-dependent, noise-dependent, or a combination of the foregoing. Also, in some embodiments, the image combiner <NUM> is configured to combine the first image signals and the second image signals to maximize DQE (or SNR) for the combined image. In addition, in some embodiments, the image combiner <NUM> may be configured to combine the first image signals and the second image signals based on weighting that account for imaging task (e.g., task transfer function), and/or observer model (e.g., eye filter, human observer, hoteling observer, etc.).

In some embodiments, the image combiner <NUM> may be configured to combine the first image signals contributed by the first scintillator layer <NUM>, and the second image signals contributed by the second scintillator layer <NUM>, in frequency domain. In other embodiments, the image combiner <NUM> may be configured to combine the first image signals and the second image signals in image domain. For example, the image combiner <NUM> may be configured to combine the first image signals and the second image signals in image domain based on noise reduction. In further embodiments, the image combiner <NUM> may be configured to combine the first image signals and the second image signals in both frequency and image domains. For example, the image combiner <NUM> may be configured to perform image fusion in both the frequency domain and image domain one after another (e.g., in frequency domain first and then followed by image domain, or vice versa), or simultaneously. Also, in some embodiments, the image combiner <NUM> may perform image fusion in the frequency domain by application of various frequency filters that depend on MTF, NPS, DQE, or any combination of the foregoing. Alternatively, or additionally, the image combiner <NUM> may perform image fusion in the image domain based on a direct or an iterative technique designed to reduce noise in the fused image and preserve spatial resolution.

In some embodiments, before combining the first image signals and the second image signals, the image combiner <NUM> may be configured to (<NUM>) seal off bad pixels and correct images with Dark Field (DF) and Flat Field (FF) datasets, (<NUM>) remove scatter from images (e.g., with polynomial detrending), (<NUM>) normalize images to a common background, (<NUM>) register the images with respect to each other, or any combination of the foregoing. In some cases, to seal off bad pixels (e.g., due to photodiode being dead or underperformed), the image combiner <NUM> may correct bad pixels such that new values of the bad pixels will be similar to neighboring pixels. In one implementation, interpolation between the neighboring pixels may be performed to determine new values for the bad pixels. In other embodiments, one or more of the above features may be performed by a module that is coupled upstream with respect to the image combiner <NUM>.

In some embodiments, the imaging apparatus <NUM> is configured to provide a detective quantum efficiency (DQE) of <NUM>% or higher, or more preferably <NUM>% or higher.

Although the imaging apparatus <NUM> has been described as having two scintillator layers <NUM>, <NUM>, and two corresponding photodiode layers <NUM>, <NUM>, in other embodiments, the imaging apparatus <NUM> may have more than two scintillator layers, and more than two photodiode layers. For example, in some embodiments, the imaging apparatus <NUM> may further include a third scintillator layer and a corresponding third photodiode layer (for generating third image signals in response to photons from the third scintillator layer). In such cases, the first scintillator layer <NUM>, the second scintillator layer <NUM>, and the third scintillator layer may be stacked. For example, the third scintillator layer and the third photodiode layer may be placed below the second substrate <NUM>. Alternatively, the third scintillator layer and the third photodiode layer may be placed between the first substrate <NUM> and the second scintillator layer <NUM>. In other embodiments, the third scintillator layer and the third photodiode layer may be placed above the first scintillator layer <NUM>.

In further embodiments, the imaging apparatus <NUM> may further include a fourth scintillator layer and a corresponding fourth photodiode layer (for generating fourth image signals in response to photons from the fourth scintillator layer), wherein the first scintillator layer <NUM>, the second scintillator layer <NUM>, the third scintillator layer are stacked, and the fourth scintillator layer are stacked.

<FIG> illustrates a method <NUM> of combining image signals. In some embodiments, the method <NUM> may be performed by the imaging apparatus <NUM> of <FIG>. The method <NUM> includes obtaining first image signals generated by a first scintillator layer, the first image signals having a first quantum efficiency and a first spatial resolution (item <NUM>). The method <NUM> also includes obtaining second image signals generated by a second scintillator layer (item <NUM>). In the illustrated example, the second image signals have a second quantum efficiency and a second spatial resolution, wherein the first quantum efficiency is lower than the second quantum efficiency, but the first spatial resolution is higher than the second spatial resolution. The method <NUM> further includes electronically processing the first image signals and the second images by an image combiner to combine the first image signals and the second image signals forming a combined image (item <NUM>).

In some embodiments, in item <NUM>, the first image signals and the second image signals are combined in a way that increase signal-to-noise ratio while reducing spatial resolution loss.

In some embodiments, in item <NUM>, the first image signals and the second image signals are combined based on frequency-dependent weighting.

In some embodiments, in item <NUM>, the first image signals and the second image signals are combined based on frequency-dependent filtering.

In some embodiments, in item <NUM>, the first image signals and the second image signals are combined based on noise-dependent weighting.

In some embodiments, in item <NUM>, the first image signals and the second image signals are combined in image domain based on noise reduction.

In some embodiments, in item <NUM>, an image combiner applies a first weight factor for the first image signals, and a second weight factor for the second image signals.

In some embodiments, in the method <NUM>, the first weight factor is between <NUM> and <NUM>. In some embodiments, the first weight factor is <NUM>.

In some embodiments, in the method <NUM>, the first weight factor has a first value below <NUM> for a first frequency or first frequency range, and a second value above <NUM> for a second frequency higher than the first frequency or for a second frequency range higher than the first frequency range.

In some embodiments, in item <NUM>, the first image signals and the second image signals are combined based on MTF.

In some embodiments, in item <NUM>, the first image signals and the second image signals are combined based on NPS.

In some embodiments, in item <NUM>, the first image signals and the second image signals are combined based on DQE.

In the method <NUM>, the first scintillator layer is GOS-based.

In the method <NUM>, the second scintillator layer is glass-based.

In some embodiments, in the method <NUM>, the first scintillator layer is thinner than the second scintillator layer.

In some embodiments, in the method <NUM>, the first scintillator layer and the second scintillator layer are stacked.

In some embodiments, the method <NUM> provides a detective quantum efficiency of <NUM>% or higher, or more preferably <NUM>% or higher.

As discussed, in some embodiments, the combining of the first image signals and the second image signals may be performed based on MTF, NPS, DQE, or any combination of the foregoing. <FIG> illustrate imaging characteristics of a GOS-based scintillator and imaging characteristics of a glass-based scintillator particularly showing MTF, NPS, and DQE as functions of, or in relation with, frequencies. In particular, <FIG> illustrates relationship between MTF values and frequency values. <FIG> illustrates relationship between MPS values and frequency values. <FIG> illustrates relationship between DQE values and frequency values. As shown in <FIG>, the MTF value associated with the first scintillator layer <NUM> ( GOS-based scintillator is generally higher than the MTF value associated with the second scintillator layer <NUM> ( glass-based scintillator). This is because due to the grainy structure of GOS image, the MTF characteristics are very good, i.e., it provides "high" spatial resolution compared to the glass-based image (e.g., LKH5 image). The glass-based image, on the other hand, suffers from light blurring and therefore yields a "low" spatial resolution. Also, as shown in <FIG>, the DQE value associated with the first scintillator layer <NUM> ( GOS-based scintillator) is lower than the DQE value associated with the second scintillator layer <NUM> ( glass-based scintillator). The above information may be utilized by the image combiner <NUM> in combining the first image signals and the second image signal, such that the resulting combined image will have high resolution features (e.g., higher than that of the second image signals) carried over from the first images signals, and high DQE (or SNR) (e.g., higher than that of the first image signals) carried over from the second image signals. In one embodiment such image combination can be performed using frequency-dependent filtration or weighting where the weighting factor (w) is determined by MTF and NPS characteristic curves. In this case MTF determines frequency-dependent "resolution" of the image while NPS curve suggests amount of noise generated in the system and, furthermore, identifies detective quantum efficiency. For example, low efficiency high resolution of GOS-based scintillator and high efficiency and low resolution of glass-based scintillator suggest that during image fusion it would be preferable to use high frequency content of GOS-based scintillator image and low frequency content of glass-based scintillator image. It means that GOS-based scintillator image should have a lower weight at low frequency region and higher weight at high frequency region, while the remaining "<NUM>-w" weight would be applied to glass-based scintillator image, respectively. In some embodiments, the image combiner <NUM> is configured to combine the first image signals and the second image signals to maximize DQE (or SNR) for the combined image, while preserving spatial resolution. In one implementation, first image (first image signals) and second image (second image signals) may be normalized (by the image combiner <NUM> or by a normalization unit coupled upstream to the image combiner <NUM>) prior to being combined based on the following: <MAT>.

Then the image combiner <NUM> may perform frequency-dependent linear combination of the first and second images based on the following: <MAT> <MAT> <MAT>.

The DQE of the combined image (DQE<NUM>) may then be maximized by the image combiner <NUM> according to the following: <MAT>.

In other embodiments, the image combiner <NUM> may be configured to combine the first and second images by maximizing SNR of the resulting image based on Rose noise model. According to the Rose noise model, the SNR may be defined as a product of a contrast-to-noise ratio (CNR) and the square root of the number of pixels N in the area of interest, as follows: <MAT>.

The CNR may be expressed in terms of signal (Is) and background (Ib) intensities and their standard deviations σs and σb: <MAT>.

In some cases, a figure-of-merit (FOM) may be utilized to optimize the SNR with respect to the integral dose D delivered to the patient, wherein FOM may be defined as: <MAT>.

<FIG> illustrates weight factor as function of frequency for the imaging apparatus <NUM>. As shown in the figure, the optimal weighting (weight factor) for the first image signals has different values depending on the frequency values. If a weight factor of <NUM> is applied for the first image signals contributed by the first scintillator layer <NUM>, it provides a good approximation to the optimal values for a majority of the applicable frequency range (e.g., from <NUM> to <NUM>^-<NUM>). Accordingly, in some embodiments, the image combiner <NUM> may be configured to apply a first weight factor of <NUM> for the first image signals contributed by the first scintillator layer <NUM>, and a second weight factor of <NUM> (= <NUM> - <NUM>) for the second image signals contributed by the second scintillator layer <NUM>, regardless of the frequency values. In other embodiments, the image combiner <NUM> may be configured to apply a first weight factor that is between <NUM> and <NUM> for the first image signals, and a second weight factor that is <NUM> minus the first weight factor for the second image signals. Furthermore, in other embodiments, the image combiner <NUM> may be configured to apply a first weight factor having a first value for the first image signals for a first frequency range, and a first weight factor having a second value for the image signals for a second frequency range. In such cases, the image combiner <NUM> may also be configured to apply a second weight factor having a third value (that is equal to <NUM> minus the first value of the first weight factor) for the second image signals for the first frequency range, and a second weight factor having a fourth value (that is equal to <NUM> minus the second value of the first weight factor) for the second image signals for the second frequency range. In some embodiments, the first frequency range may be below a frequency threshold, and the second range may be above the frequency threshold.

<FIG> illustrates exemplary results of image fusion. The image <NUM> is based on image signals generated from a GOS-based scintillator. The image <NUM> is based on image signals generated from glass-based scintillator. As shown in the figure, the image <NUM> has a relatively higher resolution compared to the image <NUM>, but the image <NUM> is generated with a relatively lower quantum efficiency compared to the image <NUM>. Thus, the image <NUM> has a strong grainy noise structure and low SNR compared to the image <NUM>. This is consistent with the MTF curves presented in <FIG>. Namely, the GOS-based scintillator produces the MTF curve with slow-dropping tail in high frequency region. The MTF of the glass-based scintillator drops down much quicker than the GOS-based scintillator.

The image <NUM> is obtained from taking an average of the image <NUM> and the image <NUM>, which is the same as applying a <NUM> weight factor to the image <NUM>, applying a <NUM> weight factor to the image <NUM>, and adding them. The image <NUM> is obtained by applying a <NUM> weight factor to the image <NUM>, applying a <NUM> weight factor to the image <NUM>, and combining them. The image <NUM> is obtained by determining optimal weighting through optimal balance of preserving spatial frequency (relating to MTF) and noise power reduction (NPS), applying the optimal weighting to the image <NUM> and the image <NUM>, and combining them. As shown in the figure, the application of the <NUM> weight factor provides an image fusion result that is similar to that achieved through optimization. Thus, both <NUM> weight factor and optimal weighting provide images that retain high-DQE features of the second image (e.g., the glass-based image associated with the second scintillator layer <NUM>) in the expense of spatial resolution, in that the MTF is lower than the <NUM> weight factor, but is higher than the second image alone.

<FIG> illustrates image fusion performed using simulation performed on phantom and MV beams. The GOS image is based on image signals generated from a GOS-based scintillator. The LKH5 image is based on image signals generated from LKH5 scintillator (which is an example of the second scintillator layer <NUM>). As shown in the figure, the GOS image has a relatively higher resolution compared to the LKH5 image, but the GOS image is generated with a relatively lower quantum efficiency compared to the LKH5 image. Thus, the GOS image has a strong grainy noise structure and low SNR compared to the LKH5 image. The GOS+LKH5 image is obtained from taking an average of the GOS image and the LKH5 image, which is the same as applying a <NUM> weight factor to the GOS image, applying a <NUM> weight factor to the LKH5 image, and adding them. The "<NUM>*GOS + <NUM>* LKH5" image is obtained by applying a <NUM> weight factor to the GOS image, applying a <NUM> weight factor to the LKH5 image, and combining them. The optimal image is obtained by determining optimal weighting through optimal balance of preserving spatial frequency (relating to MTF) and noise power reduction (NPS), applying the optimal weighting to the GOS image and the LKH5 image, and combining them. As shown in the figure, the application of the <NUM> weight factor provides an image fusion result that is similar to that achieved through optimization. Thus, both <NUM> weight factor and optimal weighting provide images that retain high-DQE features of the second image (e.g., the LKH5 image associated with the second scintillator layer <NUM>) in the expense of spatial resolution, in that the MTF is lower than the <NUM> weight factor, but is higher than the second image alone.

<FIG> illustrates another image fusion performed using simulation performed on phantom and MV beams. The GOS image is based on image signals generated from a GOS-based scintillator. The LKH5 image is based on image signals generated from LKH5 scintillator (which is an example of the second scintillator layer <NUM>). As shown in the figure, the GOS image has a relatively higher resolution compared to the LKH5 image, but the GOS image is generated with a relatively lower quantum efficiency compared to the LKH5 image. Thus, the GOS image has a strong grainy noise structure and low SNR compared to the LKH5 image. The GOS+LKH5 image is obtained from taking an average of the GOS image and the LKH5 image, which is the same as applying a <NUM> weight factor to the GOS image, applying a <NUM> weight factor to the LKH5 image, and adding them. The "<NUM>*GOS + <NUM>* LKH5" image is obtained by applying a <NUM> weight factor to the GOS image, applying a <NUM> weight factor to the LKH5 image, and combining them. The optimal image is obtained by determining optimal weighting through optimal balance of preserving spatial frequency (relating to MTF) and noise power reduction (NPS), applying the optimal weighting to the GOS image and the LKH5 image, and combining them. As shown in the figure, the application of the <NUM> weight factor provides an image fusion result that is similar to that achieved through optimization. Thus, both <NUM> weight factor and optimal weighting provide images that retain high-DQE features of the second image (e.g., the LKH5 image associated with the second scintillator layer <NUM>) in the expense of spatial resolution, in that the MTF is lower than the <NUM> weight factor, but is higher than the second image alone.

In some embodiments, the combined image achieved using image fusion technique described herein provides better soft tissue visualization compared to if only thick glass-based scintillator layer is used. Also, in some embodiments, the combined image achieved using image fusion technique described herein allows for better detection of small (high frequency) features compared to if only thick glass-based scintillator layer is used (as in current EPID imagers). In further embodiments, the combined image achieved using image fusion technique described herein achieves noise reduction and edge enhancement that is better compared to current EPID imagers. In addition, in some embodiments, the image fusion technique described herein provides lower-dose MV imaging compared to current EPID imagers. In some embodiments, the image fusion technique described herein may be performed during a treatment procedure, e.g., for soft tissue visualization, patient positioning, etc..

In some embodiments, the first scintillator layer <NUM> and the second scintillator layer <NUM> may be configured to provide detective quantum efficiency (DQE) of at least <NUM>%. One exemplary configuration providing such <NUM>% DQE may utilize a <NUM> thick GOS-based scintillator and a <NUM> thick glass-based scintillator, wherein the GOS-based scintillator would yield about <NUM>% DQE, and the glass-based scintillator would result in about <NUM>% DQE. The thickness and efficiency of glass-based scintillator are not limited to numbers discussed, and can have higher or lower values in other embodiments. DQE is a measure of a combined effects of the signal (related to image contrast) and noise performance of an imaging system. In some cases, DQE may be expressed as a function of spatial frequency. In other embodiments, the DQE may be improved to achieve higher values, such as <NUM>% or greater. For example, in other embodiments, the thickness of the first scintillator layer <NUM> and/or the thickness of the second scintillator layer may be increased. Also, in some embodiments, the imaging apparatus <NUM> may include additional scintillator layer(s), as discussed, for providing higher DQE.

In some embodiments, the first scintillator layer <NUM> and the second scintillator layer <NUM> provides a DQE that is higher than a single layer EPID (which may have only <NUM>% DQE). Also, in some embodiments, the first scintillator layer <NUM> and the second scintillator layer <NUM> may be configured to provide a DQE that is higher than a four-layer design that has four layers of GOS-based detector. Accordingly, the two-layer design reduces complexity of the system and associated cost.

In some embodiments, a product including a medium storing a set of instructions is provided. An execution of the instructions causes an imaging method to be performed. The imaging method includes: obtaining first image signals generated by a first scintillator layer, the first image signals having a first quantum efficiency and a first spatial resolution; obtaining second image signals generated by a second scintillator layer, the second image signals having a second quantum efficiency and a second spatial resolution, wherein the first quantum efficiency is lower than the second quantum efficiency, but the first spatial resolution is higher than the second spatial resolution; and electronically processing the first image signals and the second images by an image combiner to combine the first image signals and the second image signals forming a combined image.

<FIG> is a block diagram illustrating an embodiment of a particular machine <NUM> that can be used to implement various features described herein. In some embodiments, the particular machine <NUM> may be considered as an example of a processing system. In some embodiments, the processing system <NUM> may be used to implement the processing unit <NUM> of <FIG>. The processing system <NUM> may also be used to implement a control that controls an operation of the imaging apparatus <NUM>, and/or a control that controls an operation of the treatment machine. In further embodiments, the processing system <NUM> may be used to implement a component of the imaging apparatus <NUM>, such as the image combiner of the imaging apparatus <NUM>.

Processing system <NUM> includes a bus <NUM> or other communication mechanism for communicating information, and a processor <NUM> coupled with the bus <NUM> for processing information. The processor system <NUM> also includes a main memory <NUM>, such as a random access memory (RAM) or other dynamic storage device, coupled to the bus <NUM> for storing information and instructions to be executed by the processor <NUM>. The main memory <NUM> also may be used for storing temporary variables or other intermediate information during execution of instructions to be executed by the processor <NUM>. The processor system <NUM> further includes a read only memory (ROM) <NUM> or other static storage device coupled to the bus <NUM> for storing static information and instructions for the processor <NUM>. A data storage device <NUM>, such as a magnetic disk, solid state disk, or optical disk, is provided and coupled to the bus <NUM> for storing information and instructions.

The processor system <NUM> may be coupled via the bus <NUM> to a display <NUM>, such as a flat screen monitor, for displaying information to a user. An input device <NUM>, including alphanumeric and other keys, is coupled to the bus <NUM> for communicating information and command selections to processor <NUM>.

In some embodiments, the processor system <NUM> can be used to perform various functions described herein. According to some embodiments, such use is provided by processor system <NUM> in response to processor <NUM> executing one or more sequences of one or more instructions contained in the main memory <NUM>. Those skilled in the art will know how to prepare such instructions based on the functions and methods described herein. Such instructions may be read into the main memory <NUM> from another processor-readable medium, such as storage device <NUM>. Execution of the sequences of instructions contained in the main memory <NUM> causes the processor <NUM> to perform the process steps described herein. One or more processors in a multi-processing arrangement may also be employed to execute the sequences of instructions contained in the main memory <NUM>. In alternative embodiments, hard-wired circuitry may be used in place of or in combination with software instructions to implement the various embodiments described herein. Thus, embodiments are not limited to any specific combination of hardware circuitry and software.

The term "processor-readable medium" as used herein refers to any medium that participates in providing instructions to the processor <NUM> for execution. Such a medium may take many forms, including but not limited to, non-volatile media, volatile media, and transmission media. Non-volatile media includes, for example, optical, solid state or magnetic disks, such as the storage device <NUM>. A non-volatile medium may be considered an example of non-transitory medium. Volatile media includes dynamic memory, such as the main memory <NUM>. A volatile medium may be considered an example of non-transitory medium. Transmission media includes coaxial cables, copper wire and fiber optics, including the wires that comprise the bus <NUM>. Transmission media can also take the form of acoustic or light waves, such as those generated during radio wave and infrared data communications.

Common forms of processor-readable media include, for example, a flexible disk, hard disk, magnetic tape, or any other magnetic medium, a CD-ROM, any other optical medium, any other physical medium with patterns of holes, a RAM, a PROM, and EPROM, a FLASH-EPROM, solid state disks any other memory chip or cartridge, a carrier wave as described hereinafter, or any other medium from which a processor can read.

Various forms of processor-readable media may be involved in carrying one or more sequences of one or more instructions to the processor <NUM> for execution. For example, the instructions may initially be carried on a magnetic disk or solid state disk of a remote computer. The remote computer can load the instructions into its dynamic memory and send the instructions over a network, such as the Internet. The processing system <NUM> can receive the data on a network line. The bus <NUM> carries the data to the main memory <NUM>, from which the processor <NUM> retrieves and executes the instructions. The instructions received by the main memory <NUM> may optionally be stored on the storage device <NUM> either before or after execution by the processor <NUM>.

The processing system <NUM> also includes a communication interface <NUM> coupled to the bus <NUM>. The communication interface <NUM> provides a two-way data communication coupling to a network link <NUM> that is connected to a local network <NUM>. For example, the communication interface <NUM> may be a local area network (LAN) card to provide a data communication connection to a compatible LAN. In any such implementation, the communication interface <NUM> sends and receives electrical, electromagnetic or optical signals that carry data streams representing various types of information.

Claim 1:
An imaging apparatus (<NUM>) comprising:
a first scintillator layer (<NUM>) together with associated photodiodes configured to provide first image signals with a first quantum efficiency and a first spatial resolution (<NUM>);
a second scintillator layer (<NUM>) together with associated photodiodes configured to provide second image signals with a second quantum efficiency and a second spatial resolution, wherein the first quantum efficiency is lower than the second quantum efficiency, but the first spatial resolution is higher than the second spatial resolution (<NUM>); and
an image combiner (<NUM>) configured to combine the first image signals and the second image signals;
characterized in that:
the first scintillator layer (<NUM>) is GOS-based, and wherein the second scintillator layer (<NUM>) is glass-based.