System and method of x-ray detection with a sensor

φ SYSTEM AND METHOD OR X-RAY DETECTION WITH A

SENSOR by pvn piin

Pearl Cohen Zedek Latzer

P-73573-1L 201765/2

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BACKGROUND

[0001] The present invention relates to x-ray imaging. More particularly, embodiments ofthe invention relate to automatic triggering of an x-ray sensor used in dentistry.

[0002] X-rays have been used in dentistry to image teeth and parts of the mouth for manyyears. In general, the process involves generating x-rays outside the patient’s oral cavity anddirecting the x-rays at an image receptor located in the patient’s mouth. The x-rays areattenuated differently by different parts of the patient’s dental structures (e.g., bone versus tissue)and this difference in attenuation is used to create an image, such as on film or by using anelectronic image sensor. In most cases, the x-ray source is triggered manually by the operator.

In other words, the capturing of an image is initiated by a technician or other person by, forexample, activating a switch. In the case of film-based systems, the image is captured as soon asthe film is exposed to x-ray radiation. So, there is no need to “activate” the film. Once the x-raysource is activated and the x-rays reach the film, an image is captured.

[0003] In electronic systems, the particular image captured depends on at least two factors:activation of the x-ray source and “activation” of the sensor. What constitutes “activation” of thesensor can vary based upon the type of sensor used, but in most cases “activation” occurs when acommand is provided to the sensor to either store or output its current image data (referred toherein as “image capture”). So, in some systems, there is an electrical link between the x-raysource and the sensor such that when the x-ray source is activated, a command is sent(simultaneously or nearly simultaneously) to the sensor to perform an image capture. Thus, it ispossible to generate a burst of x-ray radiation and be assured that an image will be captured bythe sensor during the relatively short period of x-ray exposure.

SUMMARY

[0004] Embodiments of the invention provide automatic triggering of an x-ray sensor. In anautomatic x-ray sensor, the sensor detects x-ray radiation from an x-ray source without requiringthat a particular trigger signal be sent to the sensor. Although no particular triggering signal is 1

Attorney Docket No. 026212-9022-US01 sent to an automatic x-ray sensor, some initializing signals may be sent to the sensor to activateor arm the sensor and indicate it should begin waiting to detect x-ray radiation.

[0005] The inventors have recognized many challenges with respect to automatic triggeringsystems. One challenge relates to false triggering based on dark current accumulation. As an x-ray sensor waits to detect x-ray radiation from an x-ray source, dark current and other noise canbuild charge on the sensor and, eventually, cause the sensor to incorrectly determine x-rayradiation has been received. This false triggering issue is amplified as the ambient temperaturenear the sensor increases because dark current increases with temperature.

[0006] Another challenge associated with automatic triggering systems relates to thealignment between the x-ray source and the sensor. In many instances, even with the use of apositioning system or mechanism, x-ray sensors (particularly those placed in the mouth (i.e., anintra-oral sensor)) are often misaligned. Thus, only a portion of the x-ray sensor is exposed toradiation. In many instances, this partial exposure is not sufficient to cause a simple threshold-based trigger to initiate image capture. Thus, a misalignment may not be recognized until the x-ray technician attempts to review images that he or she believes to have been created only todiscover that no such images have been created. The technician may then try to realign the x-raysource and sensor and reinitiate the imaging process. However, it may take several attempts tocapture a usable image and each attempt exposes the patient to additional doses of x-rayradiation. As is well-known, high doses of x-ray radiation can have severe adverse effects on anindividual’s health. So, unnecessary exposure to x-rays should be avoided.

[0007] Yet another challenge associated with automatic triggering systems is the relativelylarge variation in x-ray doses and dose rates that are provided to perform x-ray image formationin a receptor. The variation in dosages and dose rates is caused by a number of factors includingdifferences in x-ray sources. X-ray sources are manufactured by a number of differentmanufacturers and their designs and specifications have changed over time. Thus, the intensityof their outputs varies. For example, older x-ray machines usually generate relatively high x-raydoses with alternating dose rates while newer machines generate lower doses with more steadydose rates. The variation in x-ray doses and dose rates received at the sensor is also aconsequence of variations in anatomy (from patient to patient) and the distance of the source to 2

Attorney Docket No. 026212-9022-US01 the patient. As is known, the dose is dependent on the distance (d) between the source and thepatient by a factor of d2.

[0008] In one embodiment, the invention provides a method of automatically detecting x-rayradiation with an x-ray sensor. The method includes resetting a pixel array by removing storedcharge from the pixel array and measuring, by a processor, an elapsed time since resetting of thepixel array. The method also includes a processor executing a decision operation using theelapsed time and an average dark current trigger time, and determining that a threshold has beencrossed. The threshold being crossed indicates a predetermined amount of charge has beenstored on at least a portion of the pixel array. The method also includes determining, by theprocessor, that x-ray radiation has been received at a portion of the pixel array based on thedecision operation. Upon determining that x-ray radiation has been received, data is output fromthe pixel array to be used to generate an x-ray image.

[0009] In one embodiment, the invention provides an x-ray sensor that automatically detectsreceipt of x-rays. The x-ray sensor includes a processor, a pixel array, and a memory. Theprocessor is configured to reset a pixel array by removing stored charge from the pixel array andmeasure an elapsed time since resetting of the pixel array. The processor is also configured toexecute a decision operation using the elapsed time and an average dark current trigger time, andto determine that a threshold has been crossed. The threshold being crossed indicates apredetermined amount of charge has been stored on at least a portion of the pixel array. Theprocessor is configured to determine that x-ray radiation has been received at a portion of thepixel array based on the decision operation. Upon determining that x-ray radiation has beenreceived, the processor is configured to output data from the pixel array to be used to generate anx-ray image.

[0010] Other aspects of the invention will become apparent by consideration of the detaileddescription and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Fig. 1 illustrates a dental x-ray system including an x-ray source, an intraoral sensorlocated in a patient’s mouth, and a computer connected to the intraoral sensor. 3

Attorney Docket No. 026212-9022-US01 [0012] Fig. 2 illustrates an intraoral sensor.

[0013] Fig. 3 illustrates a circuit diagram of a pixel array according to some embodiments ofthe invention.

[0014] Fig. 4 depicts a timing diagram of x-ray exposure on a pixel array.

[0015] Fig. 5 depicts a flow chart of x-ray accumulation and output by a pixel.

[0016] Fig. 6 depicts a graph of dark current relative to temperature.

[0017] Fig. 7 illustrates a process of automatically detecting x-ray radiation.

[0018] Fig. 8 illustrates fixed reset timing diagrams.

[0019] Fig. 9 illustrates a process of automatically detecting x-ray radiation.

[0020] Fig. 10 illustrates a fully adaptive reset timing diagram.

[0021] Figs. 1 la-c illustrate a sensor receiving x-ray radiation.

DETAILED DESCRIPTION

[0022] Before any embodiments of the invention are explained in detail, it is to beunderstood that the invention is not limited in its application to the details of construction and thearrangement of components set forth in the following description or illustrated in the followingdrawings. The invention is capable of other embodiments and of being practiced or of beingcarried out in various ways. Additionally, the term processor as is used in this application tomean any of a microcontroller, programmable logic device (e.g., a field programmable gate array“FPGA”), a general purpose processor, specifically designed hardware (e.g., an applicationspecific integrated circuit “ASIC”), or a combination thereof.

[0023] Fig. 1 illustrates a dental x-ray system 10. The system includes an x-ray source 12.

In the embodiment shown, the source is located on an end 13 of a mechanical arm 15. Whenactivated, the x-ray source 12 generates an x-ray stream 16. (Of course, x-rays are generally 4

Attorney Docket No. 026212-9022-US01 invisible, but a representation of a stream is illustrated to facilitate understanding of theinvention.) In some applications, a removable collimator is used with a mechanical positioningdevice to help align the x-ray stream with an x-ray sensor.

[0024] As shown in Fig. 1, the arm 15 is positioned (e.g., by an operator) so that the x-raystream is directed to an intraoral sensor 20. The intraoral sensor 20 is shown located in themouth of a patient 21. In some embodiments, the intraoral sensor 20 includes a scintillator thatcoverts x-ray radiation to visible light and light detecting elements that convert the visible lightto electrons. In other embodiments, the sensor 20 is configured to convert x-rays to electronswithout a scintillator.

[0025] As best seen by reference to Fig. 2, the sensor 20 also includes an array of pixels 22.Each pixel produces an electric signal in response to light (from the scintillator) or x-rayradiation impinged upon it. In one embodiment, the sensor 20 includes one or more analog-to-digital (“A/D”) converters to covert analog signals generated by the pixels to digital signals.These signals are provided to a processor 23 (such as a programmable, electronicmicroprocessor, FPGA, ASIC, or similar device). In some embodiments, the A/D converters areimplemented as part of the pixel array 22, as part of the processor 23, or as separate componentsbetween the pixel array 22 and the processor 23. In the embodiment shown, the processor 23 isconnected to memory 24 (ROM and RAM) and an input-output interface 25. The sensor 20 alsoincludes one or more electronic circuits for power supply, driving the pixel array, and driving theoutput (e.g., circuits located in the VO interface 25). To facilitate the illustration of theconnections between pixel array 22 and components 23, 24, and 25, the array 22 is shown ascovering only a portion of the sensor 20. However, in most applications, the array 22 isgenerally coextensive with the sensor 20 and the components 23, 24, and 25 are located behindthe array 22, rather than to the side of the array 22 (as shown in Fig. 2).

[0026] Referring back to Fig. 1, a wire, cable, or similar connecter 27 of the sensor 20connects the sensor 20 to a computer 30. The computer 30 includes various components,including a processor or similar electronic device 32, an input/output interface 34, and memory36 (e.g., RAM and ROM). In one particular embodiment, the input/output interface 34 is aUniversal Serial Bus (“USB”) connection and the connector 27 is a USB cable. In other 5

Attorney Docket No. 026212-9022-US01 embodiments, a wireless connection is made between the sensor 20 and computer 30. Fig. 1illustrates that image data captured by the sensor 20 and processed by the computer 30 is sent toa display 38 and viewed as image 40. (Image 40 is drawn more distinctly than an x-ray imagewould typically appear.) [0027] Fig. 3 depicts an exemplary diagram of the pixel arcay 22. The pixel array 22 isdepicted as a 3 x 3 pixel array for explanation purposes, but, the pixel array 22 generally hasmore than nine pixels. The pixel array 22 includes pixels 42, a reset switch 44, and a sensingswitch 45. Each pixel includes a reset/sense switch 46, an integrating element 48, a read-outamplifier 50, and a read-out switch 52. The integrating element 48 integrates charge in responseto receiving energy based on x-rays, dark current, and noise, as will be described in more detailbelow. While we use the term charge in this description, broadly speaking, the charge representsthe amount of energy received at the integrating element 48. In other embodiments, electrons,holes, or other electrical signals, whether analog or digital, that represent the amount of energyreceived at the integrating element 48. Each pixel also receives one of the row select signals 54and column select signals 56. The row select line 54 controls the read-out switch 52. The chargestored on each integrating element 48 can be read using the row select signals 54 and columnselect signals 56 and interpreted to generate an x-ray image 40 as described above. In someembodiments, the integrating element 48 is erased upon a read out (a “destructive” read). Inother embodiments, the integrating element 48 is not erased upon a read out (a “non-destructive”read).

[0028] The pixel array 22 has four general function states: 1) a reset state, 2) a detectingstate, 3) an integrating state, and 4) read-out state. In the reset state, the charge stored on theintegrating element 48 of each pixel 42 is removed by setting the integrating elements 48 to thereference voltage (e.g., 2 volts). The integrating elements 48 are set to the reference voltage byclosing the reset/sense switch 46 and the reset switch 44, while leaving the sense switch 45 andread-out switch 52 open.

[0029] In the detecting state, the reset/sense switch 46 and the sensing switch 45 are closedto connect integrating elements 48 to sensing line 47, while the reset switch 44 and the read-outswitch 52 are left open. In the detecting state, the pixel array’s collective charge is measured to 6

Attorney Docket No. 026212-9022-US01 determine whether a threshold has been crossed, which may indicate receipt of x-ray radiation.Each integrating element 48 begins with a voltage approximately equal to the reference voltagefrom the reset state. Thereafter, as charge is integrating at the integrating element 48 from x-rayenergy, dark current, and noise, the voltage at the integrating element 48 decreases. Therefore,the collective voltage across the entire pixel array (referred to as the “diode voltage,” since theentire pixel array 22 can be viewed as a meta diode) measured across the sense line 47 andground 59 decreases as the voltage at any integrating element 48 decreases. In someembodiments, only a portion of the pixels 42 are connected to the sense switch 45 during thedetecting state. In other embodiments, additional sense switches 45 are provided in the pixelarray 22, and each sense switch 45 is connected to a particular portion of pixels 42. Thus, aparticular portion of the pixels 42 may be sensed to have crossed a voltage threshold, as opposedto sensing across the entire pixel array 22.

[0030] In the integration state, all switches (44,45, 46, and 52) are open. The pixel array 22integrates the charges created by the x-ray radiation as well as by the undesirable noisecomponents (e.g., dark current).

[0031] In the read-out state, a signal is provided to a column select line 56 (either j, j + 1, or j+ 2). In addition, a signal is provided along a row select line 54 (either i, i + 1, or i + 2) to aparticular row of pixels. In response, the read-out switches of the selected row of pixels isclosed. The charge stored on the integrating elements 48 of the row of pixels is output along theoutput paths 57. The indication provided to the particular column select line 56 serves to choseone of the output paths 57 and allows the charge output along the chosen output path 57 to beinput to the A/D converter 58. The A/D converter 58 converts the analog signal received from apixel and outputs a digital signal to the processor 23. By repeating this process for each pixel 42through providing signals to the appropriate row select line 54 and column select line 56, theentire pixel array 22 is read out.

[0032] In some embodiments, multiple pixels are read out in parallel. For instance, in someembodiments, the A/D converter 58 converts multiple analog signals from pixels 42 to digitalsignals simultaneously and forwards the digital signals along a multi-bit bus to the processor 23.In other embodiments, individual pixel A/D converters are provided within each pixel, as 7

Attorney Docket No. 026212-9022-US01 opposed to a single A/D converter 58. In some embodiments, the charge integrating onintegrating elements 48 increases (rather than decreases) the voltage stored across eachintegrating element. In this embodiment, the reset signal removes the stored charge on eachintegrating element 48 by causing the voltage across each integrating element 48 to be set toground. Additionally, the diode voltage increases, rather than decreases, as the pixel array isexposed to x-ray radiation, dark current, and other noise. Thus, the threshold voltage is set to avalue above the reset value and is crossed upon the diode voltage increasing to a level above thethreshold.

[0033] Fig. 4 depicts a time line 80 for the source 12 emitting an x-ray stream 16, the sensor20 detecting the x-ray stream, and capturing of the image by the processor 23. Before time 82,the pixel array 22 is prepared by repeatedly switching between the reset state and detection state.At time 82, the source 12 begins to emit an x-ray stream 16. At time 84, the sensor 20 detectsthe x-ray stream 16 and begins integrating charge generated in response to the x-ray stream 16.At time 86, the x-ray radiation emitted from source 12 has concluded. Between times 86 and 88,the pixel array 22 remains in an integration state. In some embodiments, the integration timebetween times 86 and 88 is included to avoid a premature readout that would result in lost x-rayinformation. At time 88, the charge integrated at the pixel array 22 is read by the processor 23and, in some embodiments, stored in memory 24. In one embodiment, the period between times82 and 84 is in the range of 1 to 6 milliseconds; the period between times 84 and 86 is in therange of 20 to 600 milliseconds; the period between times 86 and 88 is in the range of about 200hundred milliseconds; and the period between times 88 and the end of the pixel read out isapproximately one second. Generally, the integration period between times 84 and 88 is set to belonger than the length of time the source 12 emits x-ray radiation. In some embodiments,however, the sensor 20 provides circuitry or software to detect the end of x-ray radiation and theread-out operation beginning at time 88 occurs before time 86 and closer to the end of the x-rayradiation.

[0034] The signal received at a pixel of pixel array 22 includes two main portions: abackground signal and a signal generated as a result of incident x-ray radiation. The backgroundsignal is mostly a consequence of 1) dark current, 2) other parameters, and 3) noise. When thesum of the signals on the pixel array 22 cross a trigger threshold level, the sensor 20 detects an x- 8

Attorney Docket No. 026212-9022-US01 ray or performs additional steps to determine whether an x-ray has been received, as will bedescribed below.

[0035] Fig. 5 shows a model 90 of the x-ray accumulation and the signal output by a singlepixel(u,v) of the pixel array 22. The charge built up on the pixel includes three components: 1)x-ray signals 92 from source 12, which may have been converted to light by a scintillator orsimilar device; 2) random noise 94 from various sources, and 3) dark cument 96. The threecomponents are integrated at the pixel, the function of which is represented at integrator 98. Theintegrator 98 is depicted as integrating element 48 in Fig. 3. The pixel, which is a CMOS devicein one embodiment, converts the integrated charge into voltage at block 100. The block 100 isdepicted as read-out amplifier 50 in Fig. 3. Thereafter, a read out of the pixel occurs in steps 104and 106 by reading out the columns followed by the rows of the pixel array 22. Alternatively,the rows are read out followed a read out of the columns. Regardless of the column-row order,the voltage read out of the pixel array 22 is converted by an A/D converter 108, which may bewithin the pixel array or outside of the pixel array 22. The A/D converter 108 is depicted asA/D converter 58 in Fig. 3. Blocks 102a-c illustrate that the pixel array may also be subject torandom noise, but the effect is normally negligible. Gain may be added to the signal eitherbefore the A/D 108, within the A/D 108, or digitally after the A/D 108.

[0036] Fig. 6 shows the influence of temperature on dark current as is known in thesemiconductor imaging field. As the temperature of the pixel array 22 of sensor 20 increases(either on a particular portion or globally), the dark current increases. For instance, dark currentdoubles approximately every 7-11 degrees Celsius. The temperature of pixel array 22 can, forinstance, increase when placed inside a patient’s mouth for an intraoral x-ray, increase whenexposed to sunlight, or decrease when exposed to a disinfectant (such as alcohol). The darkcurrent, if allowed to accumulate, will eventually cause the trigger threshold of the pixel array 22to be crossed and trigger an image capture when no x-rays have been received.

[0037] Figs. 7 and 8 depict a process 150 of preventing dark current accumulation fromfalsely triggering pixel array 22 and fixed reset timing diagrams 151a and 151b. The processbegins at step 152 and proceeds to a reset step 154. In the reset step 154, the variable “t” is resetto 0 and the pixel array 22 is reset. To reset the pixel array 22, the charge is drained off each 9

Attorney Docket No. 026212-9022-US01 pixel as described above with reference to Fig. 3. After the reset, charge begins to accumulatefrom sources such as dark current or an x-ray stream 16. In step 156, the process determines ifvariable t is equal to a predetermined “reset time.” If the variable t is not equal to the reset time,the process proceeds to step 158. In step 158, the process determines whether chargeaccumulated in the pixel array 22 has exceeded the trigger threshold. If not, the process proceedsto step 160, where variable t is increased by one (i.e., t = t + 1).

[0038] After step 160, the process returns to step 156 and again determines whether variablet is equal to the reset time. If no x-ray stream 16 is received by pixel array 22 over apredetermined amount of time (reset time), the process steps 156-160 will have repeated enoughtimes such that t will equal reset time in step 156. Processing then proceeds to the reset step 154.and the pixel array 22 is reset such that dark current charge is eliminated from the pixel array 22.The process returns to steps 156-160 to await receipt of an x-ray stream 16. Exemplary resettimes may be approximately 1 millisecond. The reset time may be stored in the processor 23during manufacture of the sensor 20 or at another time before installation of the sensor 20(installation occurs when the sensor is connected to a user’s computer 30). In someembodiments, the reset time is updated in the field to accommodate for different x-ray doses andto account for aging and/or use of the sensor 20.

[0039] In step 158, if the charge accumulated in the pixel array 22 exceeds the triggerthreshold, the process 150 determines that an x-ray stream 16 has been received by the pixelarray 22. Thereafter, in step 162, the pixel array 22 is read by the processor 23 and, in step 164,output to the computer 30.

[0040] Fig. 8 depicts timing diagrams 151a and 151b for the reset signal 181 (sent to pixelarray 22 during reset step 154) and the charge accumulation in pixel array 22. As chargeaccumulates in the pixel array 22, the diode voltage 182 declines from the initial value. Asshown in timing diagram 151a, after the reset time passes (t = reset time in step 156), assumingno x-ray stream 16 has been received by the pixel array, the reset signal pulses and the pixelarray 22 is reset. If, however, an x-ray stream 16 is received, the diode voltage will drop fasterand cross below the threshold voltage 183 and the x-ray stream 16 will be detected (step 158), asshown in timing diagram 151b. In the embodiment shown in Fig. 8, a reset time of 300 10

Attorney Docket No. 026212-9022-US01 microseconds, threshold voltage 183 of 0.5 V, and initial voltage of 2 V are used. In otherembodiments, different values may be used.

[0041] Although the fixed-timing process 150 of Fig. 7 works in some instances, moreadaptive techniques may be employed. If the chosen reset time is too long, dark current willaccumulate and trigger an image capture when no x-ray stream has been received. If the chosenreset time is too short, not enough charge will be able to be integrated on the pixel array 22 tosignify receipt of x-ray radiation. Thus, the trigger threshold will not be crossed and the x-raystream 16 will not be detected. Furthermore, the reset time cannot automatically adjust fordifferent environmental settings during operation. For instance, as shown in Fig. 6, dark currentis highly dependent on temperature. The potential for temperature fluctuations increases thedifficulty of selecting an appropriate reset time. Additionally, a reset time that is appropriate in afirst setting may not be appropriate in a second setting. One additional issue is that differentamounts of radiation may be emitted depending on the source 12, the distance between thesource 12 and sensor 20, the alignment of the source 12 and sensor 20. As such, the reset timemust be short enough to avoid false triggers by accumulated dark current and long enough toaccommodate low doses of x-ray radiation.

[0042] Figs. 9-10 depict an adaptive timing process 250 that address these concerns (Fig. 10illustrates a timing diagram 251, which is discussed below). The process 250 of Fig. 9 begins atstep 252 and proceeds to a reset step 254. In reset step 254, the variable i is reset to 0 and thepixel array 22 is reset by pulsing the reset signal 256. After the reset step, the sensor is allowedto accumulate dark current until the diode voltage 258 crosses the trigger threshold 260 in step262. A counter counts the number of clock pulses 264 between the reset step 254 and the darkcurrent causing the trigger threshold 260 to be crossed and sets to = number of clock pulses 264.This value to is the initial dark current trigger time. The values of tj are also referred to as a“time series.” [0043] The process 250 proceeds to step 266, where the pixel array is again reset and i = i + 1 by pulsing reset signal 256. In step 268, the process 250 determines whether the triggerthreshold has been crossed by determining if the trigger value 270 has changed to a logic high.Step 268 is repeated until the trigger value 270 is changed to a logic high. Upon the trigger value 11

Attorney Docket No. 026212-9022-US01 270 becoming a logic high, step 272 sets tj = the number of clock pulses 264 that have elapsedsince the reset step 266. In step 276, TAvg (the running average of tj from i = 0 to i) is calculated.Additionally, the standard deviation of Tavg from time i = 0 to i is calculated in step 276. Tavgrepresents the average dark current trigger time. The average dark current trigger time is theaverage elapsed time between a reset of the pixel array 22 and the diode voltage crossing thethreshold 260 due to dark current. In some embodiments, TAvg is simply set equal to tj.i or is therunning average of tj for only maximum number of previous tj values (e.g., tj from i = i-20 to i).

[0044] Thereafter, in step 278, TAvg- h is compared with a multiple of the standard deviationof Tavg· If Tavg - h is greater than n times the standard deviation of Tavg, an x-ray is detected.The value of fine tuning variable “n” is selected to adjust the detection process. In someembodiments, 0 < n < 1, meaning that small variations from the Tavg will result in an x-raydetection. In other embodiments, n > 1, and only large variations from Tavg will result in an x-ray detection. In still other embodiments, n = 1, and any variation from TAvg greater than thestandard deviation will result in an x-ray detection. Upon detection of an x-ray, the process 250proceeds to step 280, where the pixel array 22 is read by the processor 23 and, in step 282, outputto the computer 30. If in step 278, however, t; - TAvg is less than the product of n and thestandard deviation of TAvg, the process returns to step 266 to reset the pixel array and sets i = i +1.

[0045] In some embodiments, the comparison of step 278 simply compares the difference of

Tavg and tj with a predetermined value (e.g., 0, 1,2, etc.). If the difference between TAvg and tjis greater than the predetermined value, the method 250 will determine an x-ray has beenreceived at the sensor 20.

[0046] In other embodiments, step 276 is replaced by a plurality of sub-steps (not shown),and each sub-step includes a comparison of the difference of Tavg and tj with a uniquepredetermined value (e.g., 0,1, 2, etc.) or dynamic value (standard deviation). Using theplurality of comparisons enables the process 250 to detect both 1) high-dose rate, short durationx-ray exposures and 2) low-dose rate, long duration x-ray exposures. To detect high-dose rate,short duration exposures, one sub-step may include a detection algorithm that focuses only onthe most recent tj values. To detect low-dose rate, long duration exposures, another sub-step may 12

Attorney Docket No. 026212-9022-US01 include a detection algorithm that analyzes tj values over a longer period of time. The sub-stepsare executed in parallel and, if any sub-step indicates that an x-ray is detected, the process 250proceeds to step 280. For instance, where variable X is greater than variable Y, a first sub-stepfor detecting a high-dose rate, short duration exposure, may detect an x-ray if the difference ofTavg and tj is greater than X. A second sub-step for detecting a low-dose rate, long durationexposure, may detect an x-ray if the differences of TAvg and tj, TAvg and tj_i, TAvg and tj.2, TAvgand ti_3, and TAvg and are all greater than Y. X and Y may be predetermined static values ormay be based in part on dynamic values such as the standard deviations of TAvg, but usingdifferent fine tuning variables n. A third sub-step may indicate an x-ray simply by determiningthat the difference between tj and tj.i is greater than a variable Z. In this third sub-step, thevariable Z should be relatively large such that it is greater than any likely variation causedmerely by noise.

[0047] In some embodiments, steps 262 and 268 have timeout limits whereby the sensor 20will produce a timeout signal after a predetermined amount of time if the threshold 260 is notcrossed. Thus, the timeout limits prevent the sensor 20 from waiting an infinite amount of timewhen an error prevents the threshold 260 from being crossed.

[0048] In some embodiments, the TAvg or tj value are used by the processor 23 as anindication that the sensor 20 is over-heated (i.e., from being exposed to direct sunlight). Forinstance, if TAvg is too low, either in a single instance or over a predetermined number ofiterations of dark current causing the diode voltage to cross the threshold 260, the processor 23concludes that the sensor 20 is over-heated. Appropriate warning signals, alerts, or otherinformation is provided to a user upon detecting that the sensor 20 is over-heated.

[0049] Referring now to Fig. 10, the timing diagram 251 includes trigger value 270, whichindicates that the diode voltage 258 has crossed the trigger threshold 260. A time is measuredby, for instance, counting clock pulses 264. The variable tj is then set to the number of clockpulses counted, as described for process 250. The reset signal 256 pulses to reset the pixel array22 and to reset the diode voltage 258. In the embodiment shown in Fig. 10, a trigger threshold260 of 0.5 V and initial diode voltage 258 of 2 V are used. In other embodiments, differentvalues are used. 13

Attorney Docket No. 026212-9022-US01 [0050] In some embodiments of process 250, the sensor 20 is configured to be in an armedstate or disarmed state. When the sensor 20 is in a disarmed state, the process 250 proceedsnormally except that the decision in step 278 is always determined to be false and the processreturns to step 266 regardless of the values of TAVg, tj, n, and the standard deviation of TAVg-When sensor 20 is armed, the decision in step 278 is executed normally (if TAvg- t, is greaterthan the product of the tuning variable n and the standard deviation of TAVg the process proceedsto step 280). However, the values calculated while the sensor 20 was disarmed continue to beused in the armed state [0051] In some embodiments, a constant gain level is applied to the data output from thepixel array 22. The gain level alters the rate of change 259 of the diode voltage 258 (see Fig. 10). For instance, a higher gain level will increase the rate of change 259 such that less chargeintegration at the integrating elements 48 is necessary to cause crossing of the threshold voltage260. A lower gain level, in contrast, will decrease the rate of change 259 such that more chargeintegration at the integrating elements 48 is necessary to cause crossing of the threshold voltage260. In some embodiments, the gain level is altered based on tj levels. For instance, if t; levelsare too short, the gain level may be decreased causing the average dark current time to increase.Generally, the result will include a greater difference between TAvg (average dark current time)and the tj value when an x-ray is received at the sensor. If tj levels are too long, the gain levelmay be increased such that enough x-ray associated charge is integrated to generate a low noisex-ray image.

[0052] In some embodiments, the average dark current trigger times calculated in processes250 are used by the processor 23 to estimate the temperature at which the sensor 20 is operating.The calculated temperature can be used, among other reasons, to create temperature records ofthe sensor 20 and to warn the user that the sensor 20 is operating at a temperature outside ofacceptable temperature ranges. The temperature records are used to identify thermal stressesplaced on the sensor 20 (e.g., stresses caused by spraying the sensor with a disinfectant) or forother maintenance analysis. Furthermore, the calculated temperature can be used to scale anoffset image of the sensor 20, predict an offset image of the sensor 20, or both. 14

Attorney Docket No. 026212-9022-US01 [0053] In other embodiments, a desired integration time for the pixel array 22 is estimated byanalyzing the time t; between a reset of the pixel array 22 to the receipt of x-rays at the pixelarray 22 (as determined by method 250). The time tj is analyzed to estimate the dose rate. Theshorter tj, the higher the estimated dose rate because of the reduced amount of time it took for thethreshold to be crossed. Once an estimated dose rate is determined, the integration time (i.e., thetime between times 84 and 88 of Fig. 4) can be properly adjusted. For example, if the processor23 estimates a high dose rate, a shorter integration time is used. If the processor 23 estimates alow dose rate, a longer integration time is used. Adjusting the integration time based on anaccurately estimated dose rate results in less dark current in the pixel array 22 from longintegration times, yet prevents short integration times that cut-off integration of x-ray signals tooearly.

[0054] The processes 150 and 250 use detection processes based on the cumulative chargeacross the entire pixel array 22. While measuring the charge on the entire pixel array providesadequate detection in some situations, the level of the cumulative charge integration is altered ifthe x-ray field does not cover the entire pixel array 22. When the x-ray field does not cover theentire pixel array 22 (also referred to as a “cone cut”), the amount of integration due to x-rays isreduced proportionally to the portion of the pixel array 22 that was not covered, but the effects ofdark current are still integrated across the entire pixel array 22. Thus, x-rays may not be detectedif the x-ray source is not properly aligned to the pixel array 22. For instance, in Fig. 1 la, the x-ray source 12 is properly aligned and the pixel array 22 is completely within a circle 400 of x-raystream 16. In Figs. 1 lb and 1 lc, however, the circle 400 of x-ray stream 16 does not reach theentire pixel array 22. Rather, only a portion of the pixel array 22 receives x-ray radiation. If lessthan the entire pixel array 22 receives x-ray radiation, a lower amount of charge will be built upon the pixel array 22 than if the entire pixel array was within the circle 400 (as shown, forexample, in Fig. 11a). Thus, in Figs. 1 lb and 11c, the trigger threshold may not be crosseddespite receipt of x-ray radiation.

[0055] In some embodiments, to account for misaligned x-ray sources, the detectionprocesses 150 and 250 monitor multiple sections of the pixel array 22 independently. Forinstance, the pixel array 22 of Figs. 1 la-c includes 14 sections, four of which are labeled sections402, 404, 406, and 408 for exemplary purposes. Thus, if an x-ray is detected on any of the 15

Attorney Docket No. 026212-9022-US01 sections, the processes will detect an x-ray for the entire pixel array 22 and an image capture willtake place. Thus, the automatic detection processes account for a misaligned x-ray source 12 andsensor 20.

[0056] In some embodiments, one or more of the plurality of sections of the pixel array 22being independently monitored are kept in the detecting mode after detection of x-rays, while theremainder of the pixel array 22 sections are switched to the integration mode. The informationprovided by the few sections that remain in the detecting mode can be used to 1) confirm no falsetrigger has occurred, 2) detect A/C x-ray pulse patterns, and 3) detect the end of x-ray radiationbeing received at the pixel array 22. As discussed above, detecting the end of x-ray radiation canbe used to more closely tailor the duration of integration of the pixel array 22 to the duration ofx-ray exposure. More closely tailoring the duration of integration reduces the time periodbetween times 84 and 88 to more closely match the duration of the x-ray exposure.

[0057] Although the detection processes described above are directed to human dentistry, insome embodiments the processes are used with x-ray sensors intended for: veterinaryapplications; non-dental applications; and imaging of inanimate objects. Furthermore, in someembodiments, the processor 23 and memory 24 of sensor 20, or their associated functions, resideor are executed within the computer 30.

[0058] Although the timing diagrams and processes were described with particular logicstates, e.g., logic high and logic low, embodiments of the invention contemplate using alternativesignal orientations to signal similar events. For instance, the trigger value 270 becomes a logichigh upon the diode voltage 258 crossing trigger threshold 260 in Fig. 10. However, in someembodiments, the trigger value 270 becomes a logic low to indicate the trigger threshold 260 hasbeen crossed.

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