JPH049533B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a current measuring device used in an X-ray CT apparatus.

In an X-ray CT system (Computed Tomography), a patient is irradiated with pulsed X-rays, and the transparent X-rays are detected by an X-ray detector equipped with an array of ionization chambers, which are many X-ray detection elements, and the resulting signal is detected. The system is configured to obtain tomographic images that look like slices of the patient. In this case, each part of the patient (e.g. head)
By irradiating X-rays and measuring the amount of transmitted
The reason why it is possible to obtain a tomographic image of a portion irradiated with radiation is that the amount of X-rays transmitted differs depending on each tissue of the human body.

On the other hand, in an X-ray CT system, pulsed
One ray is irradiated to the affected area, this transmitted X-ray is measured, then the irradiation angle of the X-ray is changed, and one pulsed X-ray is irradiated to the same affected area as before. The operation of measuring transmitted X-rays is repeated.

Therefore, it is fine if the X-ray pulse emitted from the X-ray generator is always at a constant level, but various factors (for example, the X-ray pulse immediately after the power is turned on and the The level of the X-ray pulses usually varies, and as the voltage applied to the X-ray generator changes due to power fluctuations, the level of the X-ray pulses also changes accordingly. It is necessary to normalize the amount of radiation transmitted by the amount of X-rays irradiated for each shot.

The above operation will be explained using figures. The diagram is an X-ray
This figure shows an example of the configuration of the main parts of a CT device and a current measuring device used therein.The external configurations of these are conventionally known, and the configuration shown in the figure is also used in the present invention. ing.

First, the operation of a conventional current measuring device and its drawbacks will be explained.

In the figure, reference numeral 1 denotes an X-ray generator that emits pulsed X-rays at once in a fan-shaped direction as shown in the figure.
L ₁ , L ₂ , and L ₃ are X-ray beams. 3 is a patient to be examined. Reference numeral 5 denotes an X-ray detector, which is provided to sandwich the patient 3 and face the X-ray generator. This X-ray detector 5 is constructed by arranging a large number of X-ray detecting elements A ₁ to A _o as shown in the figure. As the X-ray detection elements _A1 to _Ao , those with various detection principles can be used.
126085) can be used. The X-ray detector 5 outputs currents i _{1 -io} proportional to the energy level of the incident X-ray dose for each detection element A ₁ -A _o . B ₁ ～
B _o is a current/voltage converter, such as a capacitor. Each of these capacitors B ₁ ~
B _o and each of the X-ray detection elements A ₁ to _{A o} form an input channel circuit. C ₁ and C ₂ are multiplexers that selectively take out the voltages of the capacitors B ₁ to B _o and transmit them to the next stage. 6 and 7 are A/D
It is a converter that converts analog signals to digital signals. A memory 9 stores signals proportional to the X-ray dose output from the A/D converters 6 and 7. Reference numeral 10 denotes an arithmetic unit, which has an arithmetic function for standardization as described above.

In the figure, the input channel circuit is divided into two blocks, and A/D converters 6 and 7 are connected to each divided block.

Here, the input channel that measures the intensity of the original X-ray pulse emitted from the X-ray generator 1 (the intensity of the X-rays that directly reach the X-ray detector without passing through the patient) is called the reference channel. Make it. The X-ray detection elements of this reference channel are A ₁ and A _o in the figure.
Suppose that

In the apparatus shown in the figure having the above-described configuration, the conventional means performs standardization by the following calculation.

Original X-ray intensity=R ₆ +R ₇ /2 (1) The measured values of the A/D converter 6 are normalized using equation (2).

D ₆ /R ₆ +R ₇ /2=2/1+α ₇ /α ₆ ·D/R (2) The measured value of the A/D converter 7 is normalized using equation (3).

D ₇ /R ₆ +R ₇ /2 = 2/1 + α ₆ /α ₇・D/R (3) where R: Original X-ray intensity D: Transmitted X-ray amount of patient (subject) R ₆ : R is A・Value measured by D converter 6 R ₇ : Value measured by R by A/D converter 7 D ₆ : Value measured by D by A/D converter 6 D ₇ : Value measured by D by A/D converter 7 Measured value α ₆ : Gain of A/D converter 6 α ₇ : Gain of A/D converter 7 In the above case, as shown in equations (2) and (3),
If there is a difference or fluctuation in the gains of the two A/D converters 6 and 7, an error will occur in the measurement results.

The present invention has been made in view of the above points, and is intended to eliminate the influence of gain differences and fluctuations between A/D converters in a current measuring device using two or more A/D converters. We are trying to provide the means to do so.

The configuration of the current measuring device according to the present invention is the same as that shown in the figure as described above. However, the method of calculation in the calculator 10 is different from the conventional one.

In the present invention, the measured values of the A/D converters 6 and 7 are standardized separately. That is, the measured value of the A/D converter 6 is normalized using equation (4).

D ₆ /R ₆ = α ₆ ·D/α ₆ ·R=D/R (4) Further, the measured value of the A/D converter 7 is normalized using equation (5).

D ₇ /R ₇ =α ₇ ·D/α ₇ ·R=D/R (5) In this way, normalization calculations are performed between data obtained by the same A/D converter. As a result, as shown in equations (4) and (5), errors due to gain fluctuations and differences in gain of the A/D converters 6 and 7 do not occur.

In addition, although the above description has been made using an example in which two A/D converters are used, the present invention is applicable even if there are M number of A/D converters. In this case, the reference channel and the measurement channel can each be divided into M blocks, and standardization can be performed using data from the same A/D converter as described above.

As described above, according to the present invention, it is not necessary to make the gains of the plurality of A/D converters the same, and even if this gain changes, the normalized measurement value is equal to the gain of the above gain. unaffected.

[Brief explanation of drawings]

The figure shows the main parts of the conventional X-ray CT apparatus and the present invention, and an example of the configuration of the current measuring device used therein. 1...X-ray generator, 5...X-ray detector, _A1 ~
A _o ...X-ray detection element, _B1 ~B _o ...Current/voltage converter, _C1 , _C2 ...Multiplexer, 6,7...
A/D converter, 9... memory, 10... arithmetic unit.

Claims (1)

[Scope of Claims] 1. An X-ray generator, an X-ray detector configured by arranging a large number of X-ray detection elements, which is provided opposite to the X-ray generator with a subject in between, and each X-ray detector. A plurality of current/voltage converters that convert the ionizing current obtained from the detection element into voltage, and an A/D converter that divides the current/voltage converters into a plurality of blocks and connects one to each block. and an arithmetic unit that inputs signals from the A/D converter and performs calculations, in which each X-ray detection element group corresponding to each block is irradiated with X-rays that do not pass through the subject. A first measurement value R obtained by the AD converter 6 of the block based on the ionization current of the X-ray detection element A1 is provided.
6 and a second measured value D6 obtained by the AD converter 6 of the block based on the ionization current of the X-ray detection element irradiated with the X-rays that have passed through the subject, into the arithmetic unit, A current measuring device used in an X-ray CT device that calculates the second measured value/first measured value in the device.