JPH04164439A - Radiation quality hardening correction method in x-ray ct device - Google Patents

Abstract

PURPOSE:To obtain a good picture quality with no artifact by selecting a non- linearity correction data set corresponding to the X-ray tube temp. at the time, when actual scan measurement is made, among a plurality of different non- linearity correction data sets, and performing a radiation quality hardening compensation. CONSTITUTION:First the tube temp. T at the starting time of scan measurement is entered, and if judgment is T<=t1, a non-linearity correction data set P1 is selected. If judgment is otherwise and if T<=t2, a non-linearity correction data set P2 is selected. If T>t2, a non-linearity correction data set P3 is selected. When the selection is finished, the scan measurement data is subjected to a radiation quality hardening compensation using the selected data set P. Thereby radiation quality hardening correction is made using optimum correction data always even though there is focal shifting associate with change in the tube temp., and a good picture quality with no artifact is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a radiation hardening correction method in an X-ray CT apparatus that performs nonlinearity correction for incident X-rays of an X-ray detector.

[Prior Art] X-rays used in an X-ray CT device are usually polychromatic X-rays, so as the X-rays pass through the object to be inspected and attenuate, the high-energy portions of the X-ray spectrum become relative. It grows larger and hardens.

When X-rays having such an energy distribution are incident on an X-ray detector, the detector output becomes an integral value regarding energy including the energy response characteristics of the detector. From this, the logarithm of the detector output is not proportional to the integral value of the linear absorption coefficient, but is affected by the hardening of the X-ray quality, and exhibits nonlinear rather than linear characteristics with respect to the transmission thickness. Become.

By the way, the image reconstruction algorithm assumes that attenuation due to X-ray transmission is proportional to the integral value of the linear absorption coefficient, so if image reconstruction is performed using detector output with such nonlinear characteristics, , specific artifacts occur. For example, in the case of a circular object to be inspected, the CT numbers of the image reconstruction cross section will differ between the center and the periphery.

Therefore, it is necessary to perform radiation hardening correction as a preprocessing of the measurement data.The method is to obtain a correction function that turns nonlinear characteristics into linear characteristics in advance, and then apply correction to the detector output. It is. This reduces the artifacts mentioned above. (edited by Yoshinori Iwai, published by Coronasha)
See "CT Scanner", pages 131-132. ) [Problems to be Solved by the Invention] The conventional radiation hardening correction method described above has the following problems. That is, in order to perform the radiation hardening correction described above, each measurement channel of the X-ray detector has different nonlinearity correction data (the nonlinearity correction data of all measurement channels are collectively referred to as a nonlinearity correction data set). Nonlinear characteristics change due to focus movement as the temperature of the X-ray tube increases.

Therefore, if the nonlinearity correction data (nonlinearity correction data set) obtained by measuring when the X-ray tube is not hot is used to correct the measurement data when the X-ray tube is hot, the Due to the difference in nonlinear characteristics, there was a risk that ring artifacts would occur. The same goes for using nonlinearity correction data (nonlinearity correction data set) obtained by measuring when the X-ray tube is hot to correct measurement data when the X-ray tube is not hot. , there has been a demand for improvement in this regard.

The present invention can always perform radiation hardening correction using optimal nonlinearity correction data (nonlinearity correction data set) even when the focal point shifts due to changes in tube temperature, resulting in good image quality without artifacts. An object of the present invention is to provide a method for correcting radiation hardening in an X-ray CT apparatus that provides the following.

[Means for solving the problem] The above purpose is to generate multiple types of nonlinearity correction data sets for different X-ray tube temperatures, which are a collection of nonlinearity correction data set for each measurement channel of an X-ray detector. During actual scan measurement, select the nonlinearity correction data set corresponding to the X-ray tube temperature at that time from among the multiple types of nonlinearity correction data sets mentioned above, and perform radiation hardening correction using this. This is achieved by

Since there is no way to directly confirm focal point movement, the present invention utilizes the fact that there is a correspondence between X-ray tube (hereinafter simply referred to as tube) temperature and focal point movement (see Fig. 5). The tube temperature is used as a guideline for focal point movement.

In this case, the temperature of the tube is constantly measured, for example, by attaching a thermocouple or the like to the surface or inside of the tube.

On the other hand, nonlinearity correction data is measured in advance at multiple tube temperatures with different focal positions, and is obtained by calculation as multiple types of nonlinearity correction data sets. The nonlinearity correction data set is selected based on the measured value), and radiation hardening correction is performed on the measurement data.

Note that it is not realistic to prepare a nonlinearity correction data set that corresponds to all X-ray tube temperatures, so in reality, for each nonlinearity correction data set, the range of tube temperatures from which it is selected is determined. Set.

[Effect X at multiple tube temperatures with different focal point movement amounts!
The nonlinearity correction data for each measurement channel of the I detector is
It is determined in advance by measurement or calculation, and is prepared as a plurality of types of nonlinearity correction data sets.

During actual scan measurement, a nonlinearity correction data set corresponding to the X-ray tube temperature (temperature range) at that time is selected from among the multiple types of nonlinearity correction data sets mentioned above, and radiation hardening correction is performed using this. .

According to the above, even when there is a focal shift due to changes in tube temperature, radiation hardening is always corrected using the optimal nonlinearity correction data (nonlinearity correction data set), and good image quality without artifacts is obtained. It will be done.

C Example] Hereinafter, the present invention will be described in detail with reference to the drawings.

First, before a detailed description of the present invention, a general radiation hardening correction method will be described. FIG. 6 is a front view of the X-ray source-detector system of the X-ray CT apparatus, and FIGS. 7 and 8 are side views thereof. In these figures, 1 is a tube, 1a is an X-ray focus, and 1b is an anode. 2 is a collimator, 3 is a phantom, and 4 is an X-ray detector.

As mentioned earlier, in order to correct the nonlinearity due to radiation hardening, the amount of X-ray attenuation due to the object's transmission with monochromatic X-rays is calculated, and the
) The ratio with the amount of X-ray attenuation due to transmission is set as nonlinearity correction data (nonlinearity correction data set for nonlinearity correction data of all measurement channels). During actual scan measurement, radiation hardening correction is performed by calculating the obtained nonlinearity correction data and scan measurement data.

However, in each measurement channel of the X-ray detector 4,
The sensitivity characteristics for the line in the slice thickness direction are different.

As shown in FIG. 7 (when the tube 1 is cold) and FIG. 8 (when the tube 1 is hot), the slice thickness of the X-ray detector 4 is The incident direction of X-rays shifts, causing a change in the nonlinearity correction data. Therefore, if there is a large difference between the tube temperature at the time of nonlinearity correction data measurement and the tube temperature at the time of actual scan measurement,
A sufficient correction effect may not be obtained, and on the contrary, there is a possibility that ring artifacts may occur.

The present invention always performs optimal correction by selecting a nonlinearity correction data set according to the tube temperature.
An example of this will be described below.

FIG. 2 shows a tube temperature measurement system. Here, a thermocouple 5 is used as a sensor for measuring the temperature of the tube, and is attached to the tube 1 to measure the temperature of the tube. In this case, the preferred location for attaching the thermocouple 5 is the anode portion of the container of the bulb 1. In practice, if it is difficult to attach it to this location, it may be installed near the base of the anode rotating shaft on the surface of the bulb.

The tube temperature detection signal from the thermocouple 5 is amplified by an amplifier 6, sampled by a sample hold circuit 7, A/D converted by an A/D converter 8, and then sent to an image processing device W9 to determine the tube temperature. Entered as data. In the image processing device 9, the tube temperature data is taken as the tube temperature T.

FIG. 3 shows the tube temperature and focal point movement during measurement of nonlinearity correction data. Since a plurality of types of the above-mentioned correction data are required, three types of correction data will be described here as an example. The magnitude relationship of the tube temperatures at that time is Tl<T2<T3, and the tube temperatures Tl, T2. The nonlinearity correction data set based on the nonlinearity correction data measured at T3 is PL, P2. P
Set it to 3.

In addition, the temperature threshold for selecting the nonlinearity correction data set is set to tl
, t2, and the tube temperature at the time of actual scan measurement is T, the temperature conditions for selecting and using each correction data set P1 to P3 are as follows. That is, (1) T≦t1 for the correction data set P1, (2) tl<T≦t2 for the correction data set P2, and (2) t2<T for the correction data set P3.

In addition, when measuring correction data, the relationship between the tube temperature Tl, T2°T3 and the temperature threshold value tl, t2 for selecting the correction data set is T
1≦t1, tl<T2≦t2, and t2<T3.

FIG. 4 shows an example of measurement of nonlinearity correction data (nonlinearity correction data set for all measurement channels of the X-ray detector 4) of the tube temperature T1. In FIG. 4, first, in step 401, the tube temperature T is input, and in step 402, it is determined whether the temperature is lower than the target temperature T1. If it is low, go to step 403
The tube temperature is increased by radiation exposure. Tube temperature T is T1
Repeat steps 401-403 until higher.

If the tube temperature T becomes higher than T1, the process proceeds to step 404, where the tube is left to cool down, the tube temperature T is inputted at step 405, and it is determined whether the tube temperature T is equal to the target temperature T1 at step 406. do. Step 404~ until they are equal.
Step 406 is repeated, and when they become equal, the process proceeds to step 407, where nonlinearity correction data (nonlinearity correction data set) is measured, and the process ends.

In the same way, set the target temperature to 72. T3, and measure their nonlinearity correction data (nonlinearity correction data set).

In reality, the nonlinearity correction data measured as described above is not used as is for radiation hardening correction, but the amount of X-ray attenuation (theoretical value) due to phantom transmission of monochromatic X-rays, which can be theoretically calculated, is , the value divided by the measured nonlinearity correction data is used for radiation hardening correction. Therefore, calculations for this purpose are performed after step 407.

FIG. 1 shows an example of selection of a nonlinearity correction data set used when performing radiation hardening correction on measurement data during actual scan measurement, which is the main means of the method of the present invention. In FIG. 1, first, in step 501, the tube temperature T at the start of scan measurement is input, and if it is determined in step 502 that T≦t1, the process proceeds to step 503, and the nonlinearity correction data set P1 is input. select. If T≦t1 is not satisfied, the process advances to step 504.

If it is determined in step 504 that T≦t2, the process proceeds to step 505, and nonlinearity correction data set P2 is selected. When T≦t2, in other words, if t2<T,
Proceeding to step 506, the nonlinearity correction data set P3 is selected.

This completes the selection of the nonlinearity correction data set P.

The image processing device 9 performs the above processing and also performs radiation hardening correction on the scan measurement data using the selected nonlinearity correction data set P.

[Effects of the Invention] According to the present invention, during actual scan measurement, a nonlinearity correction data set corresponding to the X-ray tube temperature at that time is selected from among a plurality of types of nonlinearity correction data sets; Since the radiation hardening correction is performed using the method, even if the focal point shifts due to changes in the tube temperature, the radiation hardening correction is always performed using the optimal nonlinearity correction data set, and good image quality without artifacts can be obtained. effective.

Furthermore, the processing required by the method of the present invention can be easily performed using, for example, the circuit and calculation means shown in FIG. It also has the effect of becoming.

[Brief explanation of drawings]

Fig. 1 is a flowchart showing an example of nonlinearity correction data set selection in the method of the present invention, Fig. 2 is a block diagram showing an example of a tube temperature measurement system applied to the method of the present invention, and Fig. 3 is a flowchart showing an example of nonlinearity correction data set selection in the method of the present invention. A diagram for explaining the tube temperature and focus movement when measuring correction data, Figure 4 shows the tube temperature T
Flowchart showing an example of measurement of nonlinearity correction data (nonlinearity correction data set) in No. 1, Fig. 5 is a graph showing the relationship between tube temperature and focus movement, and Fig. 6 is X-ray CT.
A front view of the X-ray source-detector system of the apparatus, and FIGS. 7 and 8 are respectively side views thereof. 1... X-ray tube, 1a... X-ray focal point, 1b... anode, 2... collimator, 3... phantom, 4...
- X-ray detector, 6... Amplifier, 7... Sample/hold circuit, 8... A/D converter, 9... Image processing device, P. P1 to P3...Nonlinearity correction data set.

Claims (1)

[Claims] 1. Prepare multiple types of nonlinearity correction data sets for different X-ray tube temperatures, which are a collection of nonlinearity correction data set for each measurement channel of the X-ray detector, and use the above multiple sets during actual scan measurement. A radiation quality in an X-ray CT apparatus characterized in that a nonlinearity correction data set corresponding to the X-ray tube temperature at that time is selected from among various nonlinearity correction data sets, and radiation quality hardening correction is performed thereby. Hardening correction method.