Low-power imager with autosensing function
An imager includes a flat panel configured to collect charges when the imager operates in a full-power charge integration mode. The imager switches to a low-power standby mode immediately after each image acquisition in the full-power charge integration mode. Bias current flowing through the flat panel is monitored in the standby mode. The imager switches to the full-power charge integration mode when detecting a change in the bias current indicating onset of an X-ray exposure.
1. An imager with an auto-sensing function, comprising:
a flat panel configured to collect charges during an X-ray exposure when the imager operates in a charge integration mode, the flat panel comprising a plurality of pixels each having a photodiode and a pixel switch;
a bias supply circuitry configured to supply a bias voltage to the flat panel;
a current detection circuitry configured to monitor a first bias current flowing through the flat panel under the bias voltage and detect a change in the first bias current caused by an onset of the X-ray exposure;
a gate driver circuitry configured to:
supply a first voltage to a control end of each pixel switch to fully turn on each pixel switch so as to couple the plurality of pixels to a virtual ground when the imager operates in an idle mode;
supply a second voltage to the control end of each pixel switch to turn off each pixel switch so as to isolate the plurality of pixels from the virtual ground when the imager operates in the charge integration mode;
supply a third voltage to the control end of each pixel switch to weakly turn on each pixel switch when the imager operates in a standby mode, wherein the third voltage is smaller than the first voltage and larger than a threshold voltage of each pixel switch.
2. The imager of claim 1 , further comprising:
a charge amplifier circuitry driven by a second bias current and configured to output pixel photodiode signals acquired during the X-ray exposure, wherein:
the second bias current is set to a first value of when the imager operates in the standby mode;
the second bias current is set to a second value of when the imager operates in the charge integration mode; and
the first value is smaller than the second value.
3. The imager of claim 1 , further comprising:
a low-impedance boost circuitry configured to rapidly connect the control end of each pixel switch to the second voltage immediately after the imager switches from the standby mode to the charge integration mode.
4. The imager of claim 3 , further comprising:
a control circuitry configured to provide timing signals to the gate driver circuitry in order to selectively turn on or turn off each pixel switch; and
a latch circuitry coupled to the current detection circuitry and configured to activate the low-impedance boost circuitry when the current detection circuitry detects the change in the first bias current.
5. The imager of claim 3 , further comprising:
a control circuitry coupled to the current detection circuitry and configured to:
provide timing signals to the gate driver circuitry in order to selectively turn on and turn off each pixel switch; and
activate the low-impedance boost circuitry when the current detection circuitry detects the change in the first bias current.
6. A method of operating an imager having a flat panel, the flat panel comprising a plurality of pixels each having a photodiode and a pixel switch, the method comprising:
configuring the imager to operate in a charge integration mode;
switching the imager to a standby mode immediately after acquiring a first image in the charge integration mode;
monitoring a first bias current flowing through the flat panel in the standby mode;
switching the imager to the charge integration mode in response to detecting a change in the first bias current indicating an onset of an X-ray exposure;
supplying a first voltage to a control end of each pixel switch to turn off each pixel switch so as to isolate the plurality of pixels from a virtual ground during the charge integration mode; and
supplying a second voltage to the control end of each pixel switch to weakly turn on each pixel switch during the standby mode, wherein the second voltage is smaller than the first voltage and larger than a threshold voltage of each pixel switch.
7. The method of claim 6 , further comprising:
driving a charge amplifier circuitry of the imager with a second bias current to output pixel photodiode signals acquired during the X-ray exposure; and
setting the second bias current to a first value when the imager operates in the standby mode or setting the second bias current to a second value when the imager operates in the charge integration mode, wherein the first value is smaller than the second value.
8. The method of claim 6 , further comprising:
activating a low-impedance boost circuitry of the imager when detecting the change in the first bias current so as to rapidly connect the control end of each pixel switch to the first voltage.
9. The method of claim 6 , further comprising:
turning off a wireless communication channel between the flat panel and an X-ray generator during the standby mode.
10. The method of claim 6 , further comprising:
acquiring a second image in the charge integration mode immediately after acquiring the first image; and
compensating offset voltages of the plurality of pixels based on the first image and the second image.
11. The method of claim 6 , further comprising:
acquiring a second image in the standby mode after acquiring the first image in the charge integration mode; and
compensating offset voltages of the plurality of pixels based on the first image and the second image.
12. The method of claim 6 , further comprising:
keeping the flat panel under a same bias condition during the charge integration mode and the standby mode.