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 (V BIAS) to the flat panel; a current detection circuitry configured to monitor a first bias current (I BIAS) flowing through the flat panel under the bias voltage (V BIAS) and detect a change in the first bias current (I BIAS) caused by an onset of the X-ray exposure; and a gate driver circuitry configured to: supply a first voltage (V GL) 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 when the imager operates in the charge integration mode; characterised in that the gate driver circuitry is further configured to: supply a second voltage (V GH) to the control end of each pixel switch to fully turn on each pixel switch so as to couple the plurality of pixels to the virtual ground when the imager operates in an idle mode; and supply a third voltage (V STB) 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 (V STB) is smaller than the second voltage (V GH) and larger than a threshold voltage (V TH) of each pixel switch. 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 (V BIAS) to the flat panel; a current detection circuitry configured to monitor a first bias current (I BIAS) flowing through the flat panel under the bias voltage (V BIAS) and detect a change in the first bias current (I BIAS) caused by an onset of the X-ray exposure; and a gate driver circuitry configured to: supply a first voltage (V GL) 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 when the imager operates in the charge integration mode; characterised in that the gate driver circuitry is further configured to: supply a second voltage (V GH) to the control end of each pixel switch to fully turn on each pixel switch so as to couple the plurality of pixels to the virtual ground when the imager operates in an idle mode; and supply a third voltage (V STB) 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 (V STB) is smaller than the second voltage (V GH) and larger than a threshold voltage (V TH) of each pixel switch. supply a second voltage (V GH) to the control end of each pixel switch to fully turn on each pixel switch so as to couple the plurality of pixels to the virtual ground when the imager operates in an idle mode; and supply a third voltage (V STB) 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 (V STB) is smaller than the second voltage (V GH) and larger than a threshold voltage (V TH) of each pixel switch.