FI98037C - Method for measuring exposure values of an X-ray imaging device for automatic control of the image values - Google Patents

Description

98037

A method for measuring the exposure values of an X-ray imaging device for the automatic adjustment of imaging values. - For the purpose of measuring the exposure to automatic exposure to the image recovery.

The invention relates to a method for measuring the exposure values of an X-ray imaging device for automatic adjustment of imaging values, in which radiation transmitted by an object to be imaged illuminates the pixels of a semiconductor sensor, in particular a CCD detector, in which charges proportional to exposure are formed.

CCD X-ray equipment, as well as film-based X-ray equipment, must be able to adjust the exposure in a controlled way so that the image is not overexposed and, on the other hand, to obtain the optimal amount of different grayscale for diagnostics.

In X-ray devices based on the use of a CCD (Charge Coupled Device) detector, the number of grayscale depends on the dynamics of the detector and amplifier electronics and the X-ray quantum-photoelectron conversion ratio provided by the scintillator layer and optics. If the exposure to the detector is not adjusted correctly, in the event of overexposure with the CCD, there will be an overflow of charge in the pixels, causing the image to "darken" and the spatial resolution to decrease. In the underexposure situation, the signal-to-noise ratio of the CCD cannot be optimally utilized, resulting in a reduction in the number of detectable grayscale and deterioration of the image quality.

For example, in the method known from U.S. Pat. No. 5,142,557, the exposure is adjusted by continuously reading *: · the photosensitive serial register of the CCD (without transferring data therefrom:

2 98037

In addition, it is known to adjust the exposure by using one or more separate semiconductor detectors behind the scintillator layer, the signal given by which is proportional to the exposure.

A weakness of the previously known methods is that the exposure has to be adjusted on the basis of some small detail which is poorly representative of the exposure of the image, whereby it is uncertain to obtain the correct exposure. For example, in the aforementioned U.S. patent, the exposure is adjusted based on the intensity at the edge of the CCD, in which area there may not be information relevant to the exposure control.

The accompanying figure shows a schematic diagram of a CCD, with reference to which the formation of an image with a CCD is normally described below. The photons are allowed to hit the field of view of the CCD for the exposure time t. The light causes a charge corresponding to the intensity on the surface of the CCD. The pixels in the image field are then controlled in the direction of the serial register so that the 1st row charge is transferred to the serial register, the 2nd row charge is transferred to the first row, etc. For 1.pixel, etc.

In the output amplifier, the charge is converted to a voltage corresponding to the intensity of the light hitting each pixel. The image is read in its entirety when all the rows have been alternately transferred to the serial register and the charge of each pixel has been transferred to the output in the above-mentioned manner. The reading of an image typically takes, for example, a 1024 * 1024 pixel CCD and a 1 MHz transmission path in Judo: Γ 1 second.

In principle, the exposure could be adjusted by taking successive. images with different current-voltage-shutter speed values and select the iteri- 3 98037 bat for the settings that give the best results. In practice, however, this is not possible due to the time taken to take successive images and the iteration. The total dose received by the subject also increases due to the taking of consecutive images.

The object of the invention is to provide a method for obtaining automatic exposure of a CCD X-ray device, by which, regardless of the thickness or density of the tissue layer, overexposure can be prevented and the exposure can be adjusted to be optimal for the dynamics of the CCD. With the method according to the invention, the current-voltage and exposure time values required for exposure can be optimized and at the same time the radiation dose received by the subject can be minimized.

This object and object of the invention is achieved on the basis of the features set forth in the appended claim 1. Preferred embodiments of the invention are set out in the dependent claims.

In the method according to the invention, the exposure is adjusted so that the CCD is exposed for a very short period of time compared to the normal exposure time of the shooting situation, for example with a 1024 * 1024 pixel CCD 6 = to / 1024 of the assumed required exposure time, or Thereafter, all the rows of the image field are transferred to the serial register without the charge received in the serial register being transferred to the output of the CCD in between. In doing so, the sum of each reservation in the vertical row is entered in the serial register. For example, the sum charge obtained at time t = to / 1024 represents ·: the average of the intensities for each vertical row at time t, • · ·· because the exposure time is shortened by a factor corresponding to the number of rows. As described above, the CCD can be read very quickly, with a read time of 1024 * 1024 pixels on a CCD at a frequency of 1 MHz for 2 ms. The contents of the serial register, ie the said sum charges 4 98037, are read as the measured value on the basis of which the exposure values are calculated. The parameters required for exposure can also be adjusted by comparing the measured and known optimal line-by-line values.

With the method according to the present invention, it is possible to measure the total intensity on the surface of the CCD also on the basis of light, for example, on two half-images or several sub-images. In this case, the exposure time required to estimate the parameters is the estimated exposure time (or current value) divided by the number of rows to be summed.

In addition, with the method according to the invention, it is also possible to sum together a different number of rows of the image produced by the same exposure. For example, if the charge summed to the shift register of the first half-image causes an overflow in the register (the series register typically holds a four-pixel charge of the image field), the charge of the remaining image can be summed to the sequence register (to find the correct exposure parameters) in several sizes, such as 1/4, As the sum of 8-, 1/16 sub-images (binary search).

The advantage of the method described above is that the signal hitting the pixels of the optional size area of the CCD image field can be quickly utilized when adjusting the X-ray device settings. In this case, local variations in the intensity of the image field do not adversely affect the adjustment of the exposure parameters. Said optional region may consist of one or more rectangular regions which are part of the field of view of the CCD detector used for imaging. In this case, the measured values ·; can be formed from several selective areas of the image field of the semiconductor sensor, and the approximate size of the object can be determined on the basis of these measured values. This size or pattern detection, in turn, can be used to adjust exposure values.

Claims (4)

1. A method for measuring the exposure values of an X-ray imaging device for automatically controlling the image values, in which method the radiation which pierced the object to be exposed exposes a semiconductor detector, in particular a CCD detector's pixel points, to which proportional charges are generated, characterized in that measurement results of exposure values are obtained from an area of any size for the semiconductor detector's image field, which before the actual exposure is exposed briefly or with a low radiation intensity compared to the exposure time or radiation intensity used in the exposure, and with the said exposure -ring proportional charges are transferred from the optional field of said pixel row to a series register pixel, where the charges from the multiple rows pixel elements are summed before the contents of the series register are read as said metric, on the basis of which the exposure used a exposure values;, · are counted. Method according to Claim 1, characterized in that said optional area consists of one or more rectangular areas which are part of the image detector field used in the exposure. «I • · · 3. A method according to claim 1 or 2, characterized in that said measuring values are formed from several sub-fields of the semiconductor detector image field and on the basis of these measuring values the approximate size of the object is defined. on the basis of which the exposure values are regulated. • · • · » 4. A method according to any one of claims 1-3, characterized in that said measuring values are formed from several optional sub-regions of the semiconductor detector's image field and the measured values obtained from different sub-regions are compared with each other and with a predetermined reference value and when detected.