Self-Triggering CMOS Image Sensor for Dental X-Ray
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Solution Overview
Problem
Conventional solid-state image sensors in dental x-ray systems face challenges in synchronizing exposure with x-ray pulses without increasing patient exposure, as they generate dark current during reset periods, and existing solutions either require costly modifications to x-ray machines or increase power consumption and exposure time.
Innovation Solution
An image sensor system with a controller that switches between monitoring and image capture modes, utilizing a large 'super pixel' configuration to detect changes in light intensity and dark current, allowing for precise timing of exposure initiation without additional detectors or high power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of time
If the image sensor is reset close to the beginning of the x-ray pulse to minimize patient exposure, then the exposure time is reduced, but dark current accumulates during the reset period causing unacceptable background noise
Solution Approach 1:
The system performs preliminary actions by continuously monitoring photodiode charge levels before the actual image capture. The controller detects when sufficient charge has accumulated during a preliminary period and triggers reset at the optimal moment, allowing the sensor to be ready immediately when the x-ray pulse begins without waiting time that would generate dark current.
Solution Approach 2:
The system uses feedback from the photodiode charge levels to control the reset timing. The controller continuously monitors the charge accumulation and uses this feedback to determine the precise moment to trigger reset, ensuring minimal delay while preventing dark current accumulation. This closed-loop control optimizes the balance between exposure time and noise reduction.
2Loss of time
If a synchronization signal is provided to trigger reset and x-ray source in proper time sequence, then the exposure can be minimized, but existing x-ray machines require costly modifications
Solution Approach 1:
The image sensor system performs self-service by autonomously detecting the optimal reset timing based on its own internal charge accumulation monitoring. The controller uses the photodiode's own charge levels as the trigger signal, eliminating the need for external synchronization signals or modifications to the x-ray machine's control system. This self-triggering mechanism integrates seamlessly with existing equipment.
Solution Approach 2:
The system makes the image sensor multi-functional by using it both for image capture and for triggering its own reset operation. The photodiode serves dual purposes: capturing the x-ray image and providing the timing signal for optimal reset. This eliminates the need for separate synchronization hardware and reduces system complexity.
3Reliability
If separate detectors are used to detect the beginning of x-ray exposure and trigger reset, then synchronization is achieved, but the area of sensors is small reducing sensitivity and increasing exposure time
Solution Approach 1:
The system merges the functions of the image sensor and the detection sensor into a single integrated component. The photodiode that captures the x-ray image also serves as the detection element that triggers the reset operation. This consolidation eliminates the need for separate detectors, increases the effective sensing area, and reduces the time delay associated with signal transmission between separate components.
4Measurement precision
If the imaging array is continually cycled with repeated readout to detect exposure, then signal-to-noise ratio is improved, but power consumption increases significantly
Solution Approach 1:
The system performs preliminary charge accumulation and monitoring before the actual image capture cycle. By detecting charge accumulation during a preliminary phase and triggering reset at the optimal moment, the system achieves high signal-to-noise ratio without requiring repeated readout cycles. This preliminary action eliminates the need for continuous cycling and reduces power consumption significantly.
Solution Approach 2:
The system extracts the triggering function from the main image capture cycle. Instead of continuously cycling through readout to detect exposure, the system separates the detection function into a preliminary monitoring phase. This extraction allows the main capture cycle to be triggered precisely when needed, eliminating redundant readout operations and reducing power consumption while maintaining measurement precision.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach minimizes patient exposure by accurately detecting the start of the x-ray pulse, reducing unnecessary exposure and power consumption, while maintaining high signal-to-noise ratio and sensitivity, and is compatible with existing x-ray machines.
Implementation Method 1
Each pixel includes a photodiode that records the light received at one point in the scene that is being recorded
Implementation Method 2
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
Data Source
AI summary
An image sensor and method for using the image sensor to capture an image are disclosed. The image sensor includes an imaging array, a first block amplifier and a controller. A first plurality of pixels in the imaging array includes pixels having a photodiode connected to a first node by a gate transistor, a reset transistor connected between the first node and a reset node, a pixel amplifier having an input connected to the first node and an output, and an output gate for connecting the pixel amplifier output to an output bus. The sensor has a monitoring mode and an image capture mode. In the monitoring mode the reset node is connected to the first block amplifier whose output is monitored and used to trigger the image capture mode when the output exceeds a predetermined threshold.