X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
2000-09-25
2001-11-27
Dunn, Drew (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S019000, C378S901000, C382S131000
Reexamination Certificate
active
06324244
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a computed tomography apparatus which includes a radiation source, a detector device and an analog-to-digital converter for converting the detector output signals into digital values.
SUMMARY OF THE INVENTION
Generally speaking, the photons received in the detector device, that is, light photons or X-ray photons, are processed in a data acquisition system. Therein, the analog signals from the photosensors are amplified and converted into digital signals for further processing.
OS 36 34 190 discloses an arrangement for digital radiography which includes an imaging device whose physical parameters can be varied by X-rays, a detector device which senses and converts such variations into an electric signal, and an analog-to-digital converter for converting the detector output signals into digital values. On the one hand such an analog-to-digital converter must have a comparatively high sampling frequency so as to enable the processing, within an acceptable period of time, of the large amounts of data that are liable to occur in digital radiography; on the other hand, the resolution of the analog-to-digital converter, that is, the number of binary positions with which the analog-to-digital converter outputs the digital sample value, must be large enough to enable the transfer of the complete information contents of an X-ray image. The detector output signals are digitized in a binary number whose absolute value is expressed as a number of positions. The time required for an analog-to-digital converter increases as a function of the number of binary positions required for each sample value. Therefore, for the further processing of such binary numbers it is desirable that the number of bits is small. To this end, OS 36 34 190 proposes that the digital values are always proportional to the square root of the detector output signal.
In order to establish such a non-linear relationship between the digital sampling value and the detector output signal, OS 36 34 190 proposes to provide the analog-to-digital converter with non-linear quantization stages. Alternatively, use can be made of an analog-to-digital converter having a linear characteristic by arrangement of a root extractor between the detector and the analog-to-digital converter. The known root extractor has a drawback in that it has a complex construction and hence is vulnerable.
On the basis of the described state of the art, it is an object of the invention to provide a computed tomography apparatus of the kind set forth in which the number of binary positions required for the encoding of the signal entering the analog-to-digital converter is reduced by means of components which overall have a simple construction and operate with a high degree of reliability.
In a computed tomography apparatus this object is achieved in that the analog-to-digital converter is preceded by a read-out amplifier having a plurality of gain factors, and in that there is provided a control system for the gain factor which, when a predetermined limit value of the integrated input signal is reached, automatically selects a suitable gain factor for the step-wise linear approximation of a proportional ratio between the digital values and the square root of the detector output signals.
The analog-to-digital converter is preceded by an amplifier which has a plurality of gain factors and in which linear quantization steps are performed. By appropriate selection of the gain factors a linear approximation of the root function is performed in that, after predetermined limit values of the integrated charge or values corresponding to the integrated charge have been reached, the root function is linearly approximated by switching over to a correspondingly lower gain factor. The amplification stages are adjusted by means of parallel-connected capacitors which are readily available components.
Overall this solution offers the advantage that input signals can be processed at a high speed, despite a wide dynamic range. Granted, analog-to-digital converters are known which are also capable of processing large numbers of bits, for example, 17 bits or more. However, as the number of bits is higher, an analog-to-digital converter becomes slower in proportion to 2
n
. When in conformity with the invention the individual local dynamic ranges of the read-out amplifier are reduced by way of the step-wise linear approximation of the root function, for example, to 12 without limiting the overall dynamic range of the amplifier, for example, amounting to 17 bits, the analog-to-digital converter need be designed overall for 12 bits only. The processing speed is then faster in comparison with a 17 bit analog-to-digital converter, but the ultimate information of the signal to be processed will not be restricted.
Moreover, it is an advantage that errors which are due to the inevitable dark currents of the photodiode and the offset of the amplifier can be corrected by way of mathematical processes according to the solution of the invention involving an amplifier; this is not possible in the prior art root extraction device with a corresponding root function.
The relevant gain factor is chosen so that the square root ratio between the detector output signals and the digital values is linearly approximated during the relevant interval. Overall, the limit value on the basis of which the control system switches over to a smaller gain factor is predetermined by the dynamic range that can be handled by the respective selected analog-to-digital converter. This means, for example, that in the case of a 12 bit analog-to-digital converter this limit value is reached for an integrated charge whose signal amounts to 12 bits. Moreover, the analog-to-digital converter is chosen in dependence on the noise of the smallest signal to be processed. The gain factor should preferably be determined by calculation in such a manner that the noise corresponding to the respective smallest signal in the relevant interval is still detected and that the number of bits of the respective largest signal does not exceed the number of bits that can be handled by the analog-to-digital converter.
In a first embodiment of the invention the amplifier input signals are summed as analog signals. When a predetermined maximum limit value of the analog output voltage is reached, the control system switches over to a respective lower gain factor. In a second embodiment of the invention digit-dependent switching takes place in that the integrated analog output voltage is expressed by a digit. In both embodiments the output voltage of the amplifier, or the digit value, is converted in the analog-to-digital converter so as to form a digital signal which is multiplied again by the selected gain factor during post-processing so as to enable further operation with the realistic signal.
In order to enhance the operating stability, the instant of switching over to a lower gain factor is limited, preferably by means of a restricted time window as disclosed in claim 6.
It is also proposed to reset the amplifier each time upon switching over to a different gain factor. Interference voltages which are unavoidable during switching over can thus be eliminated. The summed voltages are arithmetically processed.
When use is made of the CMOS technology, the read-out amplifier can be arranged in the direct vicinity of a photosensor (pixel) of the computed tomography apparatus delivering the input signal.
REFERENCES:
patent: 5220589 (1993-06-01), Gard
patent: 3634190 (1988-04-01), None
Lauter Josef
Schneider Stefan
Wieczorek Herfried Karl
Dunn Drew
U.S. Philips Corporation
Vodopia John F.
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