The invention relates to an X-ray detector and to an X-ray examination apparatus, which uses the detector. The detector is for providing image signals as well as exposure control signals. In particular, the invention relates to an X-ray examination apparatus in which exposure measurement circuitry is integrated with solid state X-ray detector circuitry, which enables real time control of the X-ray exposure during an image acquisition process.
It is well known that the X-ray exposure of a patient should be controlled as a function of the absorptivity of the tissue under examination. For example, overexposed areas of high brightness may occur in the image, for example caused by X-rays which are not (or only hardly) attenuated by the object to be examined, for example a patient. Tissue having a low X-ray absorptivity, for example lung tissue, will provide less attenuation and therefore requires less X-ray exposure to obtain an image of given contrast and to prevent saturation of the image detector.
Configurations of known X-ray examination apparatus are well known to those skilled in the art. Typically, the apparatus includes an X-ray source for irradiating a patient to be radiologically examined, by means of an X-ray beam. Due to local differences in the X-ray absorptivity within the patient, an X-ray image is formed. The X-ray detector derives an image signal from the X-ray image. In a detector using an optical sensor, the detector has a conversion layer or surface for converting the incident X-ray energy into optical signals. In the past, these optical signals have largely been detected by an image intensifier pick-up chain, which includes an X-ray image intensifier and a television camera.
A known X-ray examination apparatus of this type is disclosed in U.S. Pat. No. 5,461,658. This document additionally discloses an exposure control system in which an auxiliary light detection system utilizes local brightness values in the optical image in order to adjust the X-ray source. This auxiliary light detection system includes a CCD sensor for locally measuring the brightness in the optical image. The exposure control system derives a control signal from the measured brightness values, the control signal being used to adjust the X-ray apparatus in such a manner that an X-ray image of high diagnostic quality is formed and displayed, namely such that small details are included in the X-ray image and suitably visibly reproduced. The control signal controls the intensity and/or the energy of the X-ray beam and can also be used to control the amplification of the image signal. Both steps influence the signal level of the image signal directly or indirectly.
More recently, the use of a solid state X-ray detectors have been proposed. There are two basic configurations for such devices.
In a so-called xe2x80x9cindirectxe2x80x9d detector arrangement, the incident X-ray radiation is first converted into light. An array of photosensitive cells is provided, each comprising a light-sensitive element (photodiode), and a charge storage device (which may be a separate element or it may be the self-capacitance of the photodiode).
In a so-called xe2x80x9cdirectxe2x80x9d detector arrangement, an X-ray sensitive photoconductor is used to convert the X-rays directly into electrons. Since the photoconductor has no self-capacitance, a capacitor is fabricated by thin film techniques to act as a charge storage device.
During X-ray exposure, the light incident on each cell is stored as a level of charge on the charge storage device, to be read out at the end of the exposure period. The read out of charges stored effectively resets the image sensor, so this can only be carried out at the end of the X-ray exposure period. Thus, it is not possible to use the output signals from an image sensor of this type to control the exposure period in real time, because such outputs are only available at the end of exposure. The nature of the solid state image sensor device also prevents the type of feedback control described above using CCDs to be implemented.
One possible way to achieve dose control is to analyse the obtained image, and then to repeat the image acquisition process with a different exposure level. Of course, this increases the overall exposure of the patient to potentially harmful X-ray radiation, and is also not appropriate for rapidly changing images, or where images from different viewpoints are required in rapid succession.
External dose sensing arrangements have been proposed which are independent of the solid state image detector, but these can degrade the image quality. There is therefore a need for a dose sensing arrangement which enables real time dose control and which can be used with solid state image sensors.
According to the invention, there is provided an X-ray detector apparatus comprising an array of detector pixels, each pixel comprising a conversion element for converting incident radiation into a charge flow, a charge storage element and a switching device enabling the charge stored to be provided to an output of the pixel, and wherein a plurality of dose sensing pixels further comprise a dose sensing element, wherein charge flow from the conversion element during X-ray exposure results in a change in the charge stored on the charge storage element and also results in a dose sensing signal being generated which can be read out from the pixel.
This detector is preferably used in an X-ray examination apparatus comprising an X-ray source for exposing an object to be examined to X-ray energy. The detector receives an X-ray image after attenuation by the object to be examined.
The apparatus may further comprise a phosphor conversion layer for converting an incident X-ray signal into an optical signal, and the conversion element then comprises an optical sensor, such as a photodiode. The charge storage element may then be a separate element in parallel with the photodiode, or it may comprise the self-capacitance of the photodiode.
Alternatively, the conversion element may comprise a photoconductor, which converts the X-ray radiation directly into an electron charge flow.
The dose sensing pixels enable a dose signal to be obtained without reading the charges stored on the pixel charge storage elements, so that dose sensing can be carried out during exposure.
The pixels may be arranged in rows and columns, with rows of pixels sharing a row address line and columns of pixels sharing a column readout line, wherein the charge storage element is connected in series with the switching device between a common electrode for all pixels and the column readout line, the switching device being controlled by the row address line.
This is a known pixel configuration. In use, charge storage elements are all initially pre-charged. During exposure, the conversion element is isolated (because the switching device is turned off) and charge flow results in partial discharge of the charge storage element. The level of discharge is measured at the end of the cycle (by measuring the flow of charge required to recharge the capacitor) and represents the level of illumination. This known pixel configuration can be adapted in various ways to provide dose sensing pixels of the invention.
Defining a node between the charge storage element and the switching device, the dose sensing element of the dose sensing pixels may comprise a further charge storage element connected between the node and a dose signal readout line. As charges are supplied to this charge storage element, the charge flow can be measured by a charge sensitive amplifier at the end of the dose signal readout line. However, the charge stored can still be read out at the end of the exposure time, so that no image signal is lost.
Alternatively, the dose sensing element of the dose sensing pixels may comprise a transistor connected between a dose electrode common for all the dose sensing pixels and a dose signal readout line, the gate of the transistor being connected to the node. In this arrangement, the voltage on the charge storage capacitor is supplied as a gate voltage. The source-drain current can then be sampled to obtain this gate voltage, which is a measure of the state of charge of the charge storage element, and therefore represents the preceding level of incident X-ray radiation. Again, the measurement of the dose sensing signal does not destroy the image sensor signal on the charge storage element.
Preferably, the dose sensing signals for a plurality of dose sensing pixels are supplied to an individual dose signal readout line. For example, the dose signal readout lines may be parallel to the column readout lines and arranged alternately with the column readout lines.
The dose sensing pixels associated with an individual dose signal readout line may be arranged in a block, and wherein pixel dose outputs in the block are connected together in columns with column lines, and a single row connection line is provided for connecting together the pixel dose outputs of different columns in the block. This single row connection line enables the number of points at which dose signal lines cross to be minimised, which reduces cross talk.
The dose signal readout lines may alternatively be parallel to the row address lines and are then arranged alternately with the row address lines.
Preferably, all pixels are dose sensing pixels. This enables all pixels to have the same layout, which reduces image artifacts.
The invention also provides a method of using the X-ray examination apparatus of the invention, the method comprising:
exposing the object to be examined with X-ray radiation;
monitoring output signals from selected dose sensing pixels during the exposure;
halting the X-ray exposure in response to the dose sensing signal monitoring; and
reading out the charges stored on the charge storage elements to obtain an X-ray image.
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