JPS6327009B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a so-called computer tomography apparatus that uses a computer to construct an image of an X-ray absorption rate distribution in a predetermined tomographic plane of a subject.
In particular, it is possible to always accurately measure the transmitted X-ray intensity of the subject regardless of fluctuations in the X-ray tube output, and to obtain accurate X-ray absorption rates at each part within a predetermined tomographic plane.
The present invention relates to a line tomography device.

As is already known, this type of tomography equipment irradiates a desired cross-section of a subject with an X-ray beam, collects data regarding the intensity of transmitted X-rays, and uses a computer to process this data using a Fourier algorithm. Performing signal processing such as conversion to obtain the X-ray absorption rate at a predetermined position on the X-ray beam axis, constructing an image of the X-ray absorption rate distribution within the tomographic plane, and displaying it on a cathode ray tube. This is what I did.

In addition, in this type of device, the intensity of transmitted X-rays changes not only due to the amount of X-ray absorption by the body tissue of the subject, but also depending on the output of the X-ray source, making it difficult to obtain accurate transmitted X-ray intensity, and signal processing Since this will result in an error in the calculation of the X-ray absorption rate of the subject by the circuit, a reference detector is generally provided close to the X-ray source to monitor its output, and the connection between this detector and the detector that detects transmitted X-rays is By determining the ratio of outputs, variations in the output of the X-ray source are compensated for.

FIG. 1 is a block diagram showing an example of the above-mentioned conventional device of this type.

In the figure, X-rays emitted from an X-ray tube 1 transmit through a subject P, and the intensity of the transmitted X-rays is detected by an X-ray detector 2 installed opposite the X-ray tube 1, and is detected within the X-ray irradiation field. Data regarding transmitted X-ray intensity is obtained. This data is calculated using a logarithmic operational amplifier (Log.
Amp) 3 and then led to a signal processing circuit 4.

The X-ray tube 1 and the X-ray detector 2 are attached to a rotating frame (not shown) that rotates around the subject P, and by rotating this rotating frame, data regarding the transmitted X-ray intensity within a desired cross section can be obtained. can be collected.

From this data, the X-ray absorption rate of each part within the tomographic plane is determined by the signal processing circuit 4, and a two-dimensional distribution of the X-ray absorption rate is displayed on the CRT 5.

One input terminal of the Log.Amp3 has the X
The output of a reference detector 6 provided close to the wire tube 1 for monitoring its output is led. Therefore, the output signal E ₀₁ of Log.Amp3 becomes a transmitted X-ray intensity signal in which fluctuations in the output of the X-ray tube 1 are compensated.

In the above configuration, from the relationship between the output e _A1 of the X-ray detector 2 and the output e _R1 of the reference detector 6, Log.
The output E ₀₁ of Amp3 is calculated as follows.

The X-ray intensity I after passing through the object P is expressed by the following equation.

I=I ₀ e ^- 〓 ^x ……(1) However, I ₀ : X-ray intensity μ before entering the object: X-ray absorption coefficient of the object x: Thickness of the object X-ray detector 2 The output e _A1 and the output e _R1 of the reference detector 6 are as follows.

e _A1 = A ₁ I = A ₁ I ₀ e ^- 〓 ^x ………(2) However, A ₁ : Constant e _R1 = R ₁ I ₀ ………(3) However, R ₁ : Constant Also, both of the above outputs Log.Amp with e _A1 and e _R1 as inputs
The output E ₀₁ of 3 is as shown in the following equation.

E ₀₁ = −K ₁ loge _A1 /e _R1 ………(4) However, K ₁ : Constant Substituting equations (2) and (3) into equation (4) above, E ₀₁ becomes E ₀₁ = −K ₁ logA ₁ I ₀ e ^- 〓 ^x /R ₁ I ₀ = -K ₁ logA ₁ e ^- 〓 ^x /R ₁ ......(5).

According to the apparatus shown in FIG. 1 provided with the reference detector 6, from the above equation (5), X before entering the subject P
It can be seen that variations in the radiation intensity, ie the X-ray intensity I ₀ emitted from the X-ray tube 1, are compensated for.

However, the X-ray intensity detected by the reference detector 6 is the amount of X-rays emitted from the X-ray tube 1, and depends on the power supplied to the X-ray tube 1, that is, the tube voltage V and tube current mA. It does not represent directly related X-ray intensity.

Therefore, if the tube voltage and tube current change due to fluctuations in power supply voltage, etc., and the X-ray quality changes with the dose, it is impossible to compensate for the change in X-ray quality due to fluctuations in the tube voltage, and the signal processing circuit An error occurs when calculating the absorption coefficient rate.

That is, the quality of X-rays changes due to changes in tube voltage, and the transmittance of X-rays changes. However, not only the quality of X-rays but also the amount of X-rays changes due to changes in the X-ray tube voltage.

Further, since the reference detector is expensive, the device becomes complicated and expensive.

In FIG. 1, the X-ray beam irradiated to the subject from the X-ray tube 1 is in the shape of a thin fan, and the X-ray detector 2 is composed of, for example, a collimator, a scintillator, and a photomultiplier tube. A plurality of collimators are arranged so as to direct them to the focal point of the X-ray tube 1. The high voltage generator 8 of the X-ray tube 1 is a high voltage power source for energizing the X-ray detector, and the reference numeral 9 is a high voltage power source of the reference detector 9.

In view of the above, this invention automatically compensates for variations in the X-ray intensity emitted from an X-ray tube, that is, not only the X-ray amount, but also the X-ray quality, and accurately adjusts the transmitted X-ray intensity regardless of these variations. An object of the present invention is to provide an X-ray tomography apparatus in which errors caused by changes in X-ray intensity, especially X-ray quality, do not intervene in calculation processing of X-ray absorption rates of various parts of desired tomographic planes of a subject. The tube voltage and tube current of the X-ray tube are detected, and the transmitted X-ray intensity signal is compensated using the detected signals.

The present invention will be explained below with reference to the embodiment shown in FIG. Note that the same components as in FIG. 1 are given the same numbers.

In the figure, reference numeral 10 denotes an operational amplifier (hereinafter referred to as OP, Amp), and its input terminals each have the tube voltage V detected by the detection circuit 11 installed in the high voltage generator 7 for energizing the X-ray tube 1, and the tube voltage V detected by the A current mA is applied, and its output is connected to the other input terminal of Log.Amp3, to which the output of the X-ray detector 2 is applied.

As the tube voltage V and tube current mA detection circuit 11 provided in the high voltage generator 7 can be a well-known one, the configuration thereof will be omitted.

In the above configuration, the output E ₀₂ of Log.Amp3 is as follows. In other words, the output e _R2 of OP.Amp10
is e _R2 = R ₂ V ⁿ mA (6) where, R ₂ : constant V : X-ray tube voltage mA : X-ray tube current n : constant. Note that if, for example, an operational amplifier model 433 manufactured by ANALOG DEVICES is used as the OP.Amp 10, the output of the above formula can be obtained. The X-ray intensity I ₀ emitted from the X-ray tube 1 is expressed by the following equation.

I ₀ = B ₁ V ⁿ mA ......(7) However, ₁ : Constant Therefore, by substituting equation (7) into equation (2), e _A1 becomes e _A1 = A ₁ B ₁ V ⁿ mAe ^- 〓 ^x ......(8) becomes.

Output of X-ray detector 2 e _A1 and output of OP.Amp10
The output E ₀₂ of Log.Amp with e _R2 as input is E ₀₂ = −K ₂ loge _A1 / e _R2 ......(9), so by substituting the above equations (6) and (8), the following equation is obtained. become that way.

E ₀₂ = -K ₂ logA ₁ B ₁ V ⁿ mAe ^- 〓 ^x /R ₂ V ⁿ mA = -K ₂ log A ₁ B ₁ e ^- 〓 ^x /R ₂ ...... (10) In the above, B ₂ = R ₂ /B ₁ , the above equation becomes E ₀₂ = −K ₂ logA ₁ e ^- 〓 ^x /B ₂ ...(11).

Comparing equation (11) with equation (5) above, it is clear that the output of Log.Amp 3 in the configuration shown in FIG. 2 and the reference detector 6 shown in FIG. Since the outputs of Log.Amp3 have the same characteristics, if the configuration is as shown in FIG. 2, it is possible to eliminate the expensive reference detector that is indispensable in conventional devices of this type.

In this case, the output E ₀₂ of Log.Amp3 is the amount and quality of X-rays due to changes in the tube voltage V and tube current mA of X-ray tube 1, as is clear from equations (6), (8) and (11) above. This is a compensated transmitted X-ray detection signal, and as a result, the subsequent signal processing circuit 4 calculates the X-ray absorption rate of each part within a given tomographic plane of the subject without causing errors due to changes in X-ray dose and quality. is calculated.

On the other hand, the relationship between the linear absorption coefficient μ of the subject and the X-ray tube voltage V is expressed by the following equation in a narrow range of fluctuations in the X-ray tube voltage.

μ=K ₃ /V〓 ………(12) K ₃ : Constant α : Constant For example, in the conventional apparatus shown in Fig. 1, the X-ray tube voltage V
and decrease the x-ray tube current mA to increase the x-ray intensity
Even if I ₀ is kept constant, the X-ray absorption coefficient μ changes as shown in equation (12) above, so the output E ₀₁ of Log.Amp changes as shown in equation (5) above. On the other hand, in the apparatus of the present invention shown in FIG. 2, if n in equations (6) and (8) above is set to n ₁ = n + α, even if the )(11), the output E ₀₂ of Log.Amp can be kept almost constant.

In the above embodiment, in order to speed up the collection of data regarding the intensity of transmitted X-rays within a predetermined tomographic plane of the subject P, a fan-shaped X-ray beam is irradiated from the X-ray tube, and a plurality of X-ray beams are emitted within this irradiation field. Arrange the detector,
Although both the X-ray tube and the detector are configured to rotate around the subject, the present invention is not limited to the above-mentioned embodiment. An X-ray tube and a single X-ray detector are arranged facing each other, and data regarding transmitted X-ray intensity is collected by combining scanning by parallel movement of both and rotation of both around the subject. It's okay.

As described above, according to the present invention, variations in the dose and quality of X-rays emitted from the X-ray tube due to changes in tube current and tube voltage are automatically compensated for. Since it is possible to collect data on the transmitted X-ray intensity within a given tomographic plane of the subject regardless of fluctuations, it is possible to obtain accurate X-ray absorption rates of each part within the tomographic plane, and to obtain tomographic images with excellent image quality. can.

[Brief explanation of drawings]

FIG. 1 is a block diagram schematically showing a conventional device, and FIG. 2 is a block diagram schematically showing an embodiment of the present invention. 1: X-ray tube, 2: X-ray detector, 3: Logarithmic operational amplifier Log.Amp, 4: Signal processing circuit, 5: CRT,
7: High voltage generator for energizing the X-ray tube, 8: High voltage power supply for the X-ray detector, 10: Operational amplifier OP.Amp, 1
1: Tube voltage V and tube current mA detection circuit.

Claims (1)

[Scope of Claims] 1. An X-ray beam from an X-ray tube is irradiated onto a desired tomographic plane of a subject, and the transmitted X-ray intensity of the subject at that time is measured by opposing the X-ray tube and the subject with the subject in between. The tube voltage of the X-ray tube in an X-ray tomography apparatus is configured to construct an image of the X-ray absorption rate distribution within the tomographic plane by detecting the transmitted X-ray intensity signal with an X-ray detector and processing the transmitted X-ray intensity signal. a detection means for detecting the tube current; a means for obtaining an output signal related to the intensity of X-rays emitted from the X-ray tube based on a detection signal of the detection means; and an output signal of the means and the X-ray detector. An X-ray tomography apparatus comprising: means for determining a ratio between the output signal of the X-ray tube and the output signal of the X-ray tube, the X-ray tomography apparatus comprising means for determining a ratio of the X-ray output signal to the output signal of the X-ray tube, the X-ray tomography apparatus comprising: means for determining the ratio of the X-ray output signal to the output signal of the X-ray tube; 2. The X-ray tomography according to claim 1, wherein the means for determining the ratio is a logarithmic operational amplifier that performs a logarithmic operation on the detection signal of the detection means and the output signal of the X-ray detector. Device.