Radiographic imaging apparatus and method

1. A radiographic imaging apparatus comprising an X-ray source for projecting X-ray radiation into an examination region and a photon counting X-ray detector for receiving X-ray radiation after passing through said examination region and converting the received X-ray radiation into detector signals,

wherein said X-ray source comprises

a cathode for emitting an electron beam,

a rotary X-ray anode having a number of radial slits and a target layer provided on a surface of said rotary X-ray anode in between said radial slits for emitting X-ray radiation when hit by said electron beam, and

a drive unit for rotating said X-ray anode,

and

wherein said photon counting X-ray detector comprises

a direct conversion X-ray detection unit for receiving the X-ray radiation and outputting an electrical signal,

a photon counting unit for generating, from said electrical signal, said detector signal representing the number of photons of the received X-ray radiation, and

a persistent current sensing and correction unit for sensing a persistent output current in a blanking interval during which no X-ray radiation is emitted by said X-ray source and for using the sensed persistent output current to correct a detector signal generated by said photon counting unit in a subsequent measurement interval during which X-ray radiation is emitted by said X-ray source.

2. A radiographic imaging apparatus as claimed in claim 1 , wherein the width of the radial slits, of the rotary anode is configured such that during a blanking interval the persistent output current can be completely sensed by said persistent current sensing and correction unit.

3. A radiographic imaging apparatus as claimed in claim 1 , wherein the radial slits of the rotary anode have a minimum slit width of FS+(R×Ω×T), wherein FS is the focal spot size on the anode, R is the radius of the focal track on the rotary anode, Ω is the angular speed of the rotary anode and T is the minimum time required for completely sensing the persistent output current by said persistent current sensing and correction unit.

4. A radiographic imaging apparatus as claimed in claim 3 , wherein R is in the range from 5 cm to 35 cm, Ω is in the range from 2π×50 Hz to 2π×400 Hz, and T is in the range from 0.1 μsec to 100 μsec.

5. A radiographic imaging apparatus as claimed in claim 1 , wherein said the angular width of a slit is in the range from 0.5 mm to 3 mm.

6. A radiographic imaging apparatus as claimed in claim 1 , wherein said persistent current sensing and correction unit comprises

an amplifier coupled between the output of said direct conversion X-ray detection unit and the input of said photon counting unit for amplifying said electrical signal, and

a sample and hold unit for receiving said amplified electrical signal and generating, during the blanking interval, a compensation signal coupled to the output of said direct conversion X-ray detection unit for dynamically adjusting the electrical signal to compensate the persistent output current.

7. A radiographic imaging apparatus as claimed in claim 6 , wherein said sample and hold unit comprises a switch for enabling a dynamic adjustment of the electrical signal during a blanking interval by use of the dynamically generated compensation signal and for keeping the compensation signal constant during a subsequent measurement interval.

8. A radiographic imaging apparatus as claimed in claim 7 , wherein said switch is controlled by a blanking signal to be synchronously with the blanking interval switched on and off.

9. A radiographic imaging apparatus as claimed in claim 7 , wherein said switch is controlled by a blanking signal to be asynchronously with the measurement intervals of the photon counting X-ray detector and wherein a reference measurement is used to correct for X-ray flux variation caused by flux blanking.

10. A radiographic imaging apparatus as claimed in claim 7 , wherein said switch is controlled by a blanking signal to be synchronously with the measurement intervals of the photon counting X-ray detector and between two successive measurement intervals.

11. A radiographic imaging method comprising

projecting X-ray radiation into an examination region by use of an X-ray source comprising a cathode for emitting an electron beam, a rotary X-ray anode having a number of radial slits and a target layer provided on a surface of said rotary X-ray anode in between said radial slits for emitting X-ray radiation when hit by said electron beam, and a drive unit for rotating said X-ray anode,

receiving X-ray radiation after passing through said examination region by use of a direct conversion X-ray detection unit,

outputting an electrical signal from said direct conversion X-ray detection unit,

converting the electrical signal into a detector signal representing the number of photons of the received X-ray radiation.

sensing the persistent output current of the photon counting unit in a blanking interval during which no X-ray radiation is emitted by said X-ray source, and

using the sensed persistent output current to correct a detector signal generated by said photon counting unit in a subsequent measurement interval during which X-ray radiation is emitted by said X-ray source.