Patent Application
20220011450
The present specification generally relates to systems for scanning objects using X-rays, and more specifically to a system that uses a shutterless method for measuring an afterglow of scintillators used in X-ray imaging machines.
Scintillators are important components employed in imaging arrays used in medical and security scanners, and are commonly used as detectors in scanning systems. Scintillators are characterized by their conversion efficiency (photons/MeV), scintillation decay time, density, chemical stability, and afterglow. Afterglow is defined as the percentage luminescence intensity that remains for a prescribed time (e.g. 10 ms) after switching off the source of incident energetic radiation, such as neutrons or X-rays. Determining an amount of afterglow, and then minimizing or correcting that afterglow, is required for scans involving a rapidly changing field of view, otherwise a form of ‘ghosting’ is observed. Motion-related artifacts in radiological digital images can hinder security inspections as well as medical diagnoses.
An afterglow component is typically left after termination of sustained stimulating radiation incident on a scintillator. The prompt luminescent response falls rapidly to zero leaving an afterglow with a notably slower decay time. Given that afterglow builds up over tens or hundreds of milliseconds, the duration of the radiation pulse used and the method of afterglow measurement can influence the measured value of the afterglow. Therefore, for a given scintillator, comparisons with the afterglow measured for standard scintillators are important. In addition, afterglow measurements are essential for testing the performance and imaging capacity of scintillator detectors. The existing method of measuring afterglow employed in many scanning systems uses a mechanical lead shutter to block the stimulating radiation or X-ray beam that has been on for a finite amount of time, thus directing a specific dose of stimulating radiation toward and into the scintillators. The afterglow is measured immediately following the closure of the shutter. The time it takes to effectuate shutter closure can introduce error into the afterglow readings. Further, the shutter closure time can vary depending on normal wear and variations in environmental and operational parameters of the shutter mechanism over time that can effect performance and production schedules. Therefore, shutter error cannot be reliably quantified and factored into measurement.
Moreover, the shutter is a mechanical, moving part that requires routine maintenance which can be time consuming and expensive. Since the shutter is a pneumatically driven mechanical component, it is prone to failure as it has to rapidly close and open during measurements. Most shutters also have lead pieces for blocking the beam, which are difficult to maintain and keep in position within the shutter.
There is therefore a need for methods and systems for afterglow measurement that do not require mechanical parts, yet are capable of measuring and quantifying the afterglow with accuracy. There is also a need for methods and systems for afterglow measurement that can respond rapidly on the time scales of radiation emission and termination.
The following embodiments and aspects thereof are described and illustrated in conjunction with systems, tools and methods, which are meant to be exemplary and illustrative, and not limiting in scope. The present application discloses numerous embodiments.
The present specification discloses a method for measuring radiation afterglow for a first detector comprising a slow decay scintillator coupled to a first photodiode, said method comprising: providing a second detector comprising a fast decay scintillator coupled to a second photodiode; placing said first and said second detectors in the path of a radiation beam generated by a radiation source; using electronic means for switching on said radiation source; using an electrical circuit in contact with said first and said second detectors to produce a first and a second electrical signal corresponding to the detected radiation; using electronic means for switching off said radiation source; determining a first point in time when the second detector comprising said fast decay scintillator detects complete cessation of radiation; and measuring the detected radiation level for the first detector comprising said slow decay scintillator from said first point in time till the point in time when said first detector detects complete cessation of radiation.
Optionally, the radiation source is turned on for a time duration ranging from 0.1 to 30 seconds.
Optionally, a duration for which the radiation source is turned on is dependent upon the slow decay scintillator's fast and slow decay time constants.
Optionally, a duration for which the radiation source is turned on is dependent upon the slow decay scintillator's type and formulation.
Optionally, the slow decay scintillator in said first detector exhibits 10% afterglow in a response time ranging from 0.1 to 10 ms.
Optionally, the fast decay scintillator in said second detector exhibits 10% afterglow in a response time ranging from 0.1 to 10 ms.
Optionally, the complete cessation of radiation is determined from the electrical signals generated by the corresponding detectors. Optionally, an electrical circuit in contact with said first and said second detectors produces the first electrical signal and a second electrical signal corresponding to the detected radiation respectively.
The present specification also discloses a system for measuring radiation afterglow for a first detector comprising a slow decay scintillator coupled to a first photodiode, the system comprising: a second detector comprising a fast decay scintillator coupled to a second photodiode; a radiation source for generating a radiation beam which irradiates said first and said second detectors; electronic means for switching the said radiation source on or off; an electrical circuit in contact with said first and said second detectors to produce a first electrical signal and a second electrical signal corresponding to the detected radiation; and measurement means for measuring the first electrical signal for said first detector from the point in time when the second detector detects complete cessation of radiation till the point in time when said first detector detects complete cessation of radiation.
Optionally, the measured first electrical signal is used for obtaining the radiation afterglow for the first detector.
Optionally, the electrical circuit in contact with said first and said second detectors is provided on a circuit board having a first top side in contact with the first detector and a second opposing bottom side in contact with the second detector. Optionally, the radiation beam passes through the circuit board to irradiate the second detector.
Optionally, a duration for which the radiation source is turned on is dependent upon the slow decay scintillator's fast and slow decay time constants.
Optionally, a duration for which the radiation source is turned on is dependent upon the slow decay scintillator's type and formulation.
Optionally, the radiation source is turned on for a time duration ranging from 0.1 to 30 seconds Optionally, the slow decay scintillator in said first detector exhibits 10% afterglow in a response time ranging from 0.1 to 10 ms.
Optionally, the fast decay scintillator in said second detector exhibits 1% afterglow in a response time ranging from 0.1 to 10 ms.
Optionally, the complete cessation of radiation is determined from the electrical signals generated by the corresponding detectors.
Optionally, the second detector is positioned below the first detector in a funnel configuration.
Optionally, the second detector is positioned at a side of the first detector in a parallel configuration.
The present specification also discloses a method of calibrating scan images of an object being scanned by using a scanning system comprising at least a radiation source and a detector, the detector comprising a fast scintillator coupled with a photodiode, the method comprising: turning on the radiation source to measure a full intensity of unobstructed light level detected by the detector; turning off the radiation source to measure a dark signal intensity comprising an afterglow signal detected by the detector; normalizing the measured afterglow signal by subtracting the measured dark signal intensity from the measured full intensity of unobstructed light signal intensity; and, obtaining calibrated scan images.
Optionally, the calibrated scan images have a resolution greater than a resolution of uncalibrated scan images.
Optionally, the radiation source has an energy ranging from 50 kV to 300 kV and power ranging from 0.5 mA to 10 mA.
Optionally, the dark signal intensity comprises a photodiode dark current.
Optionally, normalizing the measured afterglow signal is performed in-situ with the detectors remaining in place in the scanning system; Optionally, normalizing the measured afterglow signal is performed in-process during the operation of the scanning system in real time.
Optionally, the calibrated scanned images have a greater contrast characteristic than a contrast characteristic of uncalibrated scanned images.
The aforementioned and other embodiments of the present specification shall be described in greater depth in the drawings and detailed description provided below.
These and other features and advantages of the present specification will be appreciated, as they become better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:
In an embodiment, the present specification discloses a system that employs a shutterless method of measuring afterglow, in which the start and termination of the stimulating radiation or X-rays are controlled electronically at the source or the generator level. In an embodiment, a fast decay scintillator is used in the beam path to monitor and track the rise and fall of the stimulating radiation or X-rays to determine the dose and full cessation of the stimulating radiation or X-rays, which can be used to calculate the afterglow of a slow decay scintillator. The fast decay scintillator is used as a monitoring or tracking device to determine or compensate for radiation source decay. This method can also be used to calibrate and normalize scanned image data to produce an enhanced image.
The present specification discloses multiple embodiments. The following disclosure is provided in order to enable a person having ordinary skill in the art to practice the invention. Language used in this specification should not be interpreted as a general disavowal of any one specific embodiment or used to limit the claims beyond the meaning of the terms used therein. The general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the invention. Also, the terminology and phraseology used is for the purpose of describing exemplary embodiments and should not be considered limiting. Thus, the present specification is to be accorded the widest scope encompassing numerous alternatives, modifications and equivalents consistent with the principles and features disclosed. For purpose of clarity, details relating to technical material that is known in the technical fields related to the invention have not been described in detail so as not to unnecessarily obscure the present invention.
In the description and claims of the application, each of the words “comprise” “include” and “have”, and forms thereof, are not necessarily limited to members in a list with which the words may be associated. It should be noted herein that any feature or component described in association with a specific embodiment may be used and implemented with any other embodiment unless clearly indicated otherwise.
In addition, one of ordinary skill in the art would appreciate that the features described in the present application can operate on any computing platform including, but not limited to: a laptop or tablet computer; personal computer; personal data assistant; cell phone; server; embedded processor; digital signal processor (DSP) chip or specialized imaging device capable of executing programmatic instructions or code. It should further be appreciated that the platform provides the functions described in the present application by executing a plurality of programmatic instructions, which are stored in one or more non-volatile memories, using one or more processors and presents and/or receives data through transceivers in data communication with one or more wired or wireless networks. It should further be appreciated that each device has wireless and wired receivers and transmitters capable of sending and transmitting data, at least one processor capable of processing programmatic instructions, memory capable of storing programmatic instructions, and software comprised of a plurality of programmatic instructions for performing the processes described herein. Additionally, the programmatic code can be compiled (either pre-compiled or compiled “just-in-time”) into a single application executing on a single computer, or distributed among several different computers operating locally or remotely to each other.
An electronic mechanism 408 is configured to turn on the radiation source 404 to expose the scintillator 401 to radiation 403, or to turn off the radiation source 404 to stop the path of stimulating radiation 403 and thus measure the afterglow. In an embodiment, electronic mechanism 408, when activated, provides an instantaneous command to turn on or off the source 404, however, the actual output of the radiation source 404 takes approximately 50-200 milliseconds to rise and fall. In an embodiment, the electronic mechanism 408 comprises an electronic signal such as, but not limited to, a transistor-transistor logic (TTL) signal generated by a software algorithm and controlled by a signal controller for turning the source 404 on or off. As an example, X-ray generators having an energy ranging from 50 kV to 300 kV and power ranging from 0.5 mA to 10 mA take approximately 50-200 milliseconds to rise and then fall. In an embodiment, the output of the radiation source 404 takes one or more microseconds to rise and fall. In various embodiments, the source 404 is kept on or off depending upon on the slow decay scintillator's 401 fast and slow decay time constants. In an embodiment, the source 404 is turned on for 10-20 seconds and then turned off, and afterglow is measured immediately after. For example, the afterglow may be measured at 20, 50, 100, 200 and 500 ms. Once the radiation source is turned off, the fast decay sensor 406 tracks the radiation source 404 as it falls, while the slow decay scintillator 401 lags in detection. In various embodiments, the radiation beam 403 passes through the PCB 405 and impinges on the fast decay sensor 406. Since fast decay sensors such as fast decay scintillators 406 respond to stimulating radiation in nano or micro seconds as known in the art, they can track the rise and fall of the slow radiation sources that take milliseconds to rise and milliseconds to fall. In an embodiment, after the full cessation of stimulating radiation is determined by fast decay sensor 406, the afterglow of the slow decay scintillator 401 is measured. Thus, the present specification uses a fast decay sensor/scintillator 406 as a monitoring or tracking device to be able to determine the decay time of the radiation source 404.
In various embodiments the planar configuration shown in
As is known, visible light photons emitted by a scintillator in response to stimulating radiation or by recombination of hole-electron pairs, are absorbed by a photodiode coupled with the scintillator and converted into electrons resulting in a current flow. When the stimulating radiation, which in an embodiment is a beam of X-rays, is turned off, the light output of the scintillator decays in two phases, wherein phase one corresponds to a fast decay and phase two corresponds to a slow decay. An intensity (or number) of light photons emitted by the scintillator in the two exponentially decaying phases may be obtained by the following equation:
N=Lf*e{circumflex over ( )}(−t/τf)+Ls*e{circumflex over ( )}(−t/τs) (1)
where N represents the number of light photons or intensity of light; τf and τs are the fast and slow decay time constants respectively; Lf represents an initial full intensity of the scintillator before the termination of X-rays and Ls represents an initial intensity of afterglow at a start of the slow decay phase.
The first decay phase (fast decay) comprises a rapid drop of signal intensity from a full light output intensity to a low light output intensity, whereby the low light output intensity is on the order of less than 3% of the full light output intensity. A decay time of the fast decay phase is measured from the instance of time the stimulating radiation is turned off to the instance of time when the light output intensity reaches e{circumflex over ( )}(−1) of the full light output intensity Lf. The fast/primary decay time may vary from multiples of 10 nanoseconds to multiples of 10 microseconds. The second decay phase (slow decay) begins after the first, fast decay phase and is known as the afterglow. The second, slow decay phase is the result of trapped holes and electrons in crystalline defects or impurities of the scintillator that are released and recombined, thus emitting photons. The rate of release of holes and electrons from the scintillator is dependent on a thermalization rate in the scintillator.
In an embodiment of the present specification, the afterglow is measured as percentage of incident stimulating radiation by using the following equation:
A(t)=100*(I(t)−Idark)/(Ilight−Idark) (2)
where A(t) represents afterglow intensity at a time t; I(t) represents a light current measured at the time t; Idark represents a current measured at the scintillator before the stimulating radiation (X rays) is turned on; and Ilight represents the current measured at the scintillator corresponding to a full intensity of light while stimulating radiation, in other words, when the X-rays are turned on or being emitted.
One of ordinary skill in the art would appreciate that the present method for afterglow measurement may be applied to any kind of inorganic scintillators including, but not limited to, BaF or barium fluoride; BGO or bismuth germinate; CdWO4 or cadmium tungstate; CaF2(Eu) or calcium fluoride doped with europium; CaWO4 or calcium tungstate; CsI: undoped cesium iodide; CsI(Na) or cesium iodide doped with sodium; CsI(Tl) or cesium iodide doped with thallium; Gd2O2S or gadolinium; LaBr3(Ce) (or lanthanum bromide doped with cerium); LaCl3(Ce) (or lanthanum chloride doped with cerium3(Ce); PbWO4 or lead tungstate; LuI3 or lutetium iodide; LSO or lutetium oxyorthosilicate (Lu2SiO5); LYSO (Lu1.8Y0.2SiO5(Ce)); YAG(Ce) or yttrium aluminum garnet; ZnS(Ag) or zinc sulfide and ZnWO4 or zinc tungstate, which is similar to CdWO.
In an embodiment, the present system and method of afterglow measurements may be used to calibrate and normalize raw scanned image data to produce an enhanced image in scanning systems. For this purpose, acceptable afterglow limits at specific decay time constants are specified for a given scanning system. In various embodiments, if no object is placed in the path of the radiation beam produced by an X-ray source, during a scan, the radiation source may be turned on/off for measuring the unobstructed light level, and dark level including the afterglow. By using the measured afterglow, the scanning system may be calibrated and the raw scanned image data may be normalized for obtaining scanned images of enhanced quality. At the production level, detectors may be tested and screened for the specified limits, using the present afterglow measurement system. In embodiments, the radiation source is turned on to measure a full intensity of an unobstructed light level and then is turned off to measure a dark intensity which includes the afterglow and a photodiode dark current. Subsequently, the measured signal is normalized by subtracting the dark signal intensity from the unobstructed light signal intensity. The normalized signal represents the full (100%) light signal intensity.
In another application, an afterglow percentage is determined for a given scanning system using the methods of present specification. These measured levels may be compensated in-situ with the detectors remaining in place in the scanning system; or in-process during the operation of the scanning system in real time. In embodiments, normalization may be applied at the time of image generation by the scanning system software.
In embodiments, by compensating for the measured levels of afterglow, an enhancement is observed in the resolution and contrast characteristics of the image, otherwise, the afterglow may hide some features of the object being scanned. In an embodiment, compensating for the measured levels of afterglow comprises normalizing the measured signal by subtracting the dark signal intensity from the unobstructed light signal intensity, as explained above. Smears or streaks in the edges of a scanned image are caused by afterglow and by removing the afterglow these artifacts are tuned out, thus rendering a sharper and clearer image. In most scanning systems, calibration or normalization is performed at startup and before scanning images, in order to correct for too less or too much light and for different materials with various attenuations and to compensate for non-linearity. This calibration may also incorporate adjustments on account of afterglow. This normalization may be periodically determined and executed by the system software. As an example,
Referring to
This effect of afterglow can be accurately measured using the present method and can be calibrated into the system software to remove errors in image generation. In embodiments, the radiation source is turned on to measure a full intensity of unobstructed light level and then is turned off to measure a dark intensity which includes the afterglow and a photodiode dark current. In embodiments, by compensating for the measured levels of afterglow, image resolution and contrast are enhanced. In an embodiment, compensating for the measured levels of afterglow comprises normalizing the measured signal by subtracting the dark signal intensity from the unobstructed light signal intensity, as explained above. Once the level of afterglow at different time constants is determined, the afterglow may be removed from the image data using a predefined algorithm in order to produce sharper images. Each pixel of the scintillation detector of the scanning system outputs a light signal that may be calibrated to remove the afterglow signal from the light signal that is detected while the object is scanned.
In one embodiment, the method of present specification may be incorporated in a software implementation on the same afterglow measurement system that uses a mechanical shutter for measurement. In an embodiment, where the method of present specification is implemented in a measurement system that uses a mechanical shutter for measurement, the X-ray source is turned on for a predetermined amount of time before opening the mechanical shutter. The X-ray source is turned off only after the mechanical shutter is closed. In an embodiment, the X-ray source is turned on at least two seconds before opening the mechanical shutter.
In this manner, the same system may be used with or without shutter, with the present method providing a backup mechanism in case the shutter mechanism fails.
The above examples are merely illustrative of the many applications of the system of present specification. It should be also understood by those of ordinary skill in the art that while the present system is described with respect to X-ray scanning systems, the afterglow measurement method and system of the present specification may be implemented for low, medium or high-energy X-rays, gamma rays, neutrons or other type of radiation.
The above examples are merely illustrative of the many applications of the system of present specification. Although only a few embodiments of the present invention have been described herein, it should be understood that the present invention might be embodied in many other specific forms without departing from the spirit or scope of the invention. Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive, and the invention may be modified within the scope of the appended claims.
