Solid state image sensors have replaced conventional film for capturing images in cameras. The image sensor typically includes a two-dimensional array of pixels. Each pixel includes a photodiode that records the light received at one point in the scene that is being recorded. To capture an image, each pixel is reset prior to the scene being imaged onto the sensor. After a predetermined exposure time, the image is blocked from measuring any further light and the charge stored by each photodiode is readout to provide an image of the scene.
In a conventional camera, the exposure is controlled by a shutter that is triggered in response to the user pushing a button. The image sensor is reset just before the shutter opens and is readout at a predetermined time after the shutter closes. These operations are synchronized to the user pushing the button.
In some applications, the imaging array must determine when the exposure begins without the aid of a synchronization signal such as the user pushing a button. For example, there has been considerable interest in replacing x-ray film images used in dentistry with digital images generated by CMOS image sensors. In these systems, the film that is placed in the patient's mouth is replaced by a CMOS image array that is covered with a layer of scintillation material that converts the x-rays to visible light that can be detected by the image sensor. Conventional x-ray systems using film do not require that the film exposure be synchronized with the x-ray source, since the x-ray pulse determines the exposure. Hence, conventional systems lack a synchronization system that can be used by the solid state image sensor.
To minimize the exposure of the patient to x-rays, the image sensor must be reset as close to the beginning of the x-ray pulse as possible so that a pulse of the minimum duration can be utilized. Any exposure that occurs prior to the image sensor entering the image accumulation mode is wasted, and hence, increases the exposure of the patient without generating a useful image.
In principle, the sensor could be reset sometime prior to the start of the pulse and just wait for the x-ray source to turn on. Unfortunately, the photodiodes in the imaging array generate a non-zero dark current which would be accumulating during the period between the reset and the start of the exposure. The accumulated dark current would result in an unacceptable background that could only be overcome by increasing the exposure. Hence, some other reset strategy is needed.
A number of systems have been proposed to deal with the synchronization of the imaging sensor with the x-ray pulse. The most straight forward approach would be to provide a synchronization signal similar to the pushbutton on a conventional camera. The imaging array could then be reset and the x-ray source triggered in the proper time sequence to minimize the exposure to the patient. Unfortunately, this strategy requires that the existing millions of x-ray machines already in place in dental facilities be modified at a considerable cost. Hence, some other form of triggering system has been sought.
In one class of triggering system, a separate set of detectors is used to detect the beginning of the x-ray exposure and trigger the reset, image acquisition, and readout when x-rays are detected. These additional detectors typically include additional photodiodes that are placed around the image sensor and are monitored to determine the start of the exposure. This type of system has three problems. First, the area of the separate sensors is relatively small, and hence, the sensitivity of the detection is less than ideal. In essence, the exposure sensors are equivalent to a few extra pixels in the image plane. The position of these sensors is behind the teeth or jaw bone, and hence, the time needed to provide a sufficient signal is of the order of the time needed to provide an image. Accordingly, the exposure of the patient to the x-rays is increased. Second, the sensors do not sample the entire image, and hence, the triggering decision is made on data that is not necessarily representative of the image. Finally, the sensors are often separate from the array, and hence, the cost of the sensor is increased.
In another class of prior art system, the imaging array is continually cycled. During each cycle, the imaging array is reset, allowed to accumulate charge for a predetermined period of time and then readout. If the image that is readout indicates the accumulation of a significant charge above that expected from the dark current, the system assumes that the exposure has begun, and the array is reset and allowed to accumulate the final image. This system has a better signal-to-noise ratio than systems based on a few small sensors, since the charge from a more representative set of photodiodes in the actual image is added together to make the triggering decision. Unfortunately, this system has a high power consumption due to the repeated readout cycles. The high power consumption is particularly problematic in applications that rely on battery power. In addition, the detection time is increased by the time needed to readout each image during the detection phase.