JPH04292145A - X-ray ct device - Google Patents

Abstract

PURPOSE:To improve measuring accuracy by providing an integrator between an X-ray detector and an A/D converter so that the whole capacity of an integral capacitor can be selected in plural stages by a switch operation, and constituting an amplification factor switching circuit consisting of a series circuit of the integral capacitor and the switch. CONSTITUTION:When a body to be examined does not exist and only air exists between each of an X-ray source and an X-ray detector, and an output from the X-ray detector is maximum switches S1, S2 are closed in advance and a value of the whole capacity C of an integral capacitor is determined so as not to exceed the maximum output voltage of an operational amplifier 6 of an integrator. At the time of output of the detector in a regular measurement, amplification of the output of the integrator becomes necessary, but by turning off the switch S2 in this case, the whole capacity C of the integral capacitor becomes small, and the output of the integrator becomes large. For instance, when a value of C is set to 1/2, the output becomes two folds, noise integration caused by the integrator is constant irrespective of the value of C, and when the output of the integrator is amplified to two folds, it becomes 1/2 fold. Also, at the time of small output of the X-ray detector, S3 is turned off in addition to the switch S2 and the whole capacity of a prescribed integral capacitor is denoted as C1.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an X-ray CT apparatus with particularly improved measurement accuracy.

0002

2. Description of the Related Art FIG. 2 is a circuit diagram showing a schematic configuration of an X-ray CT apparatus. In FIG. 2, 1 is an X-ray source, 2 is a subject,
Reference numeral 3 denotes an X-ray detector having a multi-channel configuration, which is disposed opposite to the X-ray source 1 with the subject 2 in between, and rotates together with the X-ray source 1.

In the illustrated configuration, X-rays emitted from an X-ray tube 1 are transmitted through a subject 2, and the transmitted X-rays are transmitted to a detector 3.
incident on . The detector 3 generates an electric charge according to the amount of incident X-rays, and the electric charge is stored in the capacitor 4. The charge stored in the capacitor 4 is given to the next stage by turning on the switch 5.

The number of such capacitors 4 and switches 5 is equal to the number of channels of the detector 3.
N, the signal is applied to an integrator 8 composed of an operational amplifier 6 and an integrating capacitor 7.

The integrator 8 amplifies the input signal with an amplification factor determined by the capacitance of the integrating capacitor 7 and outputs the amplified signal. The output of this integrator 8 is a PGA (Programmable
After the output level is adjusted by the Gain Amplifier) 11, the ADC (Analog Digit)
an image processing device 1 which converts the data into digital data by an al Converter 12 and reconstructs a tomographic image;
Sent to 3.

Note that 9 is a reset switch for the integrator 8;
10 is a current limiting resistor. The reset switch 9 turns on the output every time the switch 5 is turned on and the resulting output is given to the ADC 12 via the integrator 8 and A/D converted.
N and resets the integrator 8.

[0007]

[Problems to be Solved by the Invention] The above-mentioned prior art has the following problems. That is, in the above conventional technology,
The output V of the integrator 8 is expressed as V=Q, where Q is the charge sent from the capacitor 4 and C is the capacitance of the integrating capacitor 7.
Takes a value expressed as /C. Since the output range of this integrator 8 is limited by the output voltage of the operational amplifier 6, the object 2 is not present and there is only air between the X-ray source 1 and the detector 3.
The value of the capacitance C must be determined so that the output from the detector 3 is within the maximum voltage of the operational amplifier 6 when it is at its maximum.

However, with the capacitance C of the capacitor 7 determined in this way, the output from the detector 3 due to the X-rays when actually passing through the abdomen of the subject 2 becomes much smaller. Therefore, before sending the output of the integrator 8 to the ADC 12,
9 to minimize the quantization error. According to this, along with the output of the detector 3, the integrator 8
There was a problem in that the noise components generated in the process were also amplified, resulting in lower measurement accuracy.

An object of the present invention is to provide an X-ray detector that can amplify only the X-ray detector output signal component without increasing the noise component of the integrator when the X-ray detector output signal is small, and improve measurement accuracy. The purpose of the present invention is to provide a line CT device.

0010

[Means for Solving the Problems] The above object is to provide an X-ray CT apparatus equipped with an integrator between an X-ray detector for detecting transmitted X-rays from a subject and an A/D converter. This is achieved by providing an amplification factor switching circuit consisting of a series circuit of an integrating capacitor and a switch so that the total capacity of the integrating capacitor provided therein can be externally selected in a plurality of stages by operating a switch.

[0011]

[Operation] The amplification factor switching circuit provided in the integrator consists of a series circuit of an integrating capacitor and a switch, and the total capacity of the integrating capacitor provided in the integrator can be externally selected in a plurality of stages by operating the switch.

Now, depending on the value of the accumulated charge Q in the capacitor in front of the integrator at the maximum output of the X-ray detector, the maximum The value of the total capacitance C of the integrating capacitor (Cmax)
decide. The setting is made so that this can be obtained without any operation of the switch of the amplification factor switching circuit (switch ON state).

Next, when the detector outputs in normal measurement, the ADC
If the integrator output V needs to be amplified in order to suppress the quantization error, the amplification factor switching circuit is turned off to reduce the total capacitance C of the integrating capacitor. This increases the integrator output V. For example, change the value of C to 1/
2, the output V is doubled and the value of C is reduced to 1/4.
By doing so, the output V is quadrupled.

At this time, the noise component caused by the integrator is constant regardless of the value of C, so if we focus on the noise component, the output V of the integrator can be transferred to the subsequent circuit (such as the above-mentioned PGA).
Compared to when amplified by
When it is amplified twice, it becomes 1/2, and when it is amplified four times, it becomes 1/4, and the measurement accuracy is improved.

Furthermore, by further decreasing the value of C, it is possible to reduce the minute output (accumulated charge Q of the capacitor before the integrator).
Even when the signal is small), the measurement signal can be extracted from the integrator without being buried in noise, and the dynamic range is also expanded.

[0016]

Embodiments Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a circuit diagram showing a main part of an embodiment of an X-ray CT apparatus according to the present invention. In this Figure 1, 4 to 7,
9, 10, 12, and 13 are the same as in FIG. 2, respectively. 8 also represents an integrator as in FIG. 2, but in the present invention it is provided with an amplification factor switching circuit 21.

This amplification factor switching circuit 21 constitutes an integrating capacitor capacity switching circuit consisting of a series circuit 24 of an integrating capacitor 22 and a switch 23, so that the total capacitance C of the integrating capacitor can be selected in a plurality of stages by operating a switch from the outside. That's what happens. In this case, one or more series circuits 24 are provided depending on the desired number of switching stages of the total capacitance C of the integrating capacitor. In the illustrated example, two circuits are provided, and are connected in parallel to the basic integrating capacitor 7 so that the total capacitance C of the integrating capacitor can be switched in three stages. Next, the operation will be explained. In FIG. 1, the lower switch 23 is set to S1.
, the upper switch 23 is S2, the capacitance of the basic integrating capacitor 7 is C1, the capacitance of the capacitor 22 connected to the switch S1 is C2, and the capacitance of the capacitor 22 connected to the switch S2 is C3. Also, C1=C2,C3
=2C1.

[0018] Now, the subject 2 is not present, and the X-ray source and
When there is only air between the X-ray detectors and the output from the X-ray detector is at its maximum, switches S1 and S2 are closed.
The integrator output is prevented from saturating.

[0019] Next, when there is a subject with high X-ray absorption between the X-ray source and the X-ray detector, it is necessary to amplify the integrator output. , the total capacitance C of the integrating capacitor is C1
Since +C2+C3=4C1, C1+C2=2C1. Therefore, from V=Q/C, the output V of the integrator 8 is 2
Therefore, the quantization error during A/D conversion by the ADC 12 can be reduced without increasing the noise component of the integrator 8.

Furthermore, when the output of the X-ray detector is small, by turning off switch S3 in addition to switch S2, the total capacitance of the integrating capacitor becomes C1, and the output of the integrator is four times that of the initial state. Obtained from 8.

The output from the integrator 8 is converted into digital data by the ADC 12 and sent to the image processing device 13 that reconstructs the tomographic image. Since the amplification factor of the signal generator 8 is changed, the magnitude of the signal is modified in accordance with the change before being sent to the image processing device 13.

[0022] Note that the ON/OFF of switches S1 and S2
The selection operation may be configured to be performed manually by the operator while looking at the subject, or may be performed automatically by a control circuit (not shown) according to the imaging conditions set by the operator, such as the tube current setting value. It may also be configured so that it is carried out separately.

[0023]

[Effects of the Invention] As explained above, according to the present invention,
The optimal integrator output can be obtained depending on the magnitude of the X-ray detector output.Moreover, when the X-ray detector output is small and it is amplified, the X-ray detector output can be obtained without amplifying the noise caused by the integrator.
Since only the radiation detector output signal can be amplified, it is possible to provide an X-ray CT apparatus with a high dynamic range and high measurement accuracy.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing a main part of an embodiment of the device of the present invention.

FIG. 2 is a circuit diagram showing a schematic configuration of a conventional device.

[Explanation of symbols]

3. X-ray detector 4 Capacitor 5 Switch 6 Operational amplifier 7 Integrating capacitor 8 Integrator 12 ADC 13 Image processing device 21 Amplification factor switching circuit 22 Integrating capacitor 23 Switch 24 Series circuit

Claims (1)

[Claims] Claim 1: An X-ray CT equipped with an integrator between an X-ray detector that detects transmitted X-rays from a subject and an A/D converter.
In the device, the integrator is equipped with an amplification factor switching circuit consisting of a series circuit of an integrating capacitor and a switch so that the total capacity of the integrating capacitor provided therein can be selected in multiple stages by operating a switch from the outside. line CT
Device.