GB/T 35839-2018Non-destructive testing—Test method for measuring industrial computed tomography (CT) density (English PDF)

4.1 Test personnel

4.1.1 Personnel engaged in industrial CT density measurement shall meet the […]

5.2 Method 2

Select a density standard sample in the density comparison test specimens as the density reference sample of density measurement. According to the average CT density of the specified area of the density reference sample AND the proportionality coefficient K which is set by the CT system (generally 1000 ~ 5000 according to the overall parameters of the CT system), perform normalization to the average CT density of the designated area of each density comparison test specimen according to Formula (3), so as to obtain the average CT value in the area:

Where:

-- average CT number of the specified area of the density comparison test specimen;

-- average CT density of the specified area of the density comparison test specimen;

-- average CT density of the specified area of the density standard sample;

K -- proportionality coefficient that is set by the CT system.

Use the average CT number of the specified area of each density comparison test specimen AND the known density rho of the standard sample; […]

7.3 Density measurement

7.3.1 Select the same density reference sample that is used in the density calibration (the density comparison test specimen that is closest to the density of the to-be-test substance); place the density reference sample near the test area of the to-be-test object to perform CT scan, at the same time, as the measured object; or place the density reference sample individually at the same scanning position (with the test area of the to-be-test object), and use the same processing parameters as the to-be-test object, to scan the density comparison test specimen separately.

7.3.2 CT scanning process conditions of density measurement, such as specimen placement, scanning mode, scanning parameters (tube voltage, tube current, focus size, filtering method, slice thickness, diameter of the field of view, scan matrix, sampling time, etc.) and reconstruction parameters, shall be exactly the same as those of the density comparison test specimens.

7.3.3 In consideration of the stability of the equipment itself, in the actual detection process, according to the situation, periodically re-scan the density comparison test specimen; determine the change of the CT value of the density comparison test specimen; if necessary, modify the functional relation between the CT number and the density, so as to improve the detection accuracy.

7.3.4 Obtain the average CT density of the specified area (the number of pixels in the specified area should not be less than 100 and not more than 2/3 of the entire standard sample image area) from the CT image of the density reference sample.

7.3.5 Obtain the average CT density of the to-be-test area (the number of pixels in the area should be not less than 100) from the CT image of the to-be-test object. If Method 2 is used to perform the density measurement, perform normalization according to Formula (3) to obtain the average CT number of the to-be-test area.

7.3.6 Obtain the average material density of the area according to the average CT number of to-be-test area and the calibrated functional relation.

7.3.7 Repeat the measurement of different areas of the same uniform material for at least 5 times; use the average value of the densities which were measured from 5 times as the density value of the material; and calculate the standard deviation of the density measurement.

C.1 Calculation process

C.1.1 Set the detection parameter to the highest density resolution, and use industrial CT to detect the density comparison test specimen.

C.1.2 For the measurement of the average CT number of each density standard sample of the density comparison test specimens in a certain range (the measurement range should be a circular area; the number of pixels in each circular area should be no less than 100, but no more than 2/3 of the entire standard sample image area; the number of measurements should not be less than 5), it is recommended to use the least squares to establish the functional relation [Formula (1)] between the average CT number and the linear attenuation coefficient;

C.1.3 Multiply the density of each sample by the mass attenuation coefficient to obtain the linear attenuation coefficient µ1 of the sample, and respectively select the mass attenuation coefficient of a set of energy ranges (the range shall include the equivalent energy of the system) to calculate the linear coefficient of each sample, so as to obtain the linear attenuation coefficient of each sample at different energies.

C.1.4 Use the functional relation [Equation (1)] to calculate the linear attenuation coefficient µ2 of each sample; respectively calculate the correlation coefficient BETWEEN the linear attenuation coefficient µ1 AND the linear attenuation coefficient µ2 under different energies.

C.1.5 Select the energy corresponding to the maximum value of the correlation coefficient as the equivalent energy of the CT system.

C.2 Calculation example

Use the industrial CT system of 9 MeV to measure the density of three substances, namely methyl methacrylate (H8C5O2), tetrafluoroethylene (C2F4) and aluminum (aluminum, Al); and respectively measure the average CT number of the three substances; use the least squares to calculate the coefficients k and c of Formula (1); THEN use Formula (1) to calculate the linear attenuation coefficient µ1 of each substance; check the table to gain the mass attenuation coefficient of each substance at 3 700 keV ~ 4 000 keV; THEN multiply the corresponding density to gain the linear attenuation coefficient µ2 […]