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Principles of Mammography Imaging

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Update time : 2023-09-10 17:22:00
1. Simulated mammography imaging principle
(1) Basic concepts
1. The mammary gland is soft tissue photography, which requires the use of soft X-rays with low energy, long wavelengths, and weak ability to penetrate substances, that is, X-rays produced by tube voltages below 40kV.
2. As the kV value of the tube voltage decreases, the absorption of X-rays by the substance becomes Compton absorption, which gradually decreases, and photoelectric absorption increases. In photoelectric absorption, the photoelectric absorption coefficient (μ) is proportional to the fourth power of the atomic number (Z).
3. Two substances with the same thickness but different absorption coefficients have different X-ray contrasts due to the different absorption coefficients (μ) of tissues to X-rays.
From the above table it can be concluded that:
1) In the structure of the human body, there are certain differences in the physical properties (density, thickness) or chemical properties (atomic number) between two adjacent tissues, which is the basis for the formation of photo contrast.
2) The size of the ray contrast depends on the difference of the line absorption coefficient (μ). The line absorption coefficient is not only related to the effective atomic number of the material to be irradiated, but also related to the density (ρ) of the material to be irradiated and the wavelength of X-rays. (λ) related, the relationship is: μ=Kλ3Z4ρ
3) The lower the tube voltage, the greater the X-ray contrast after passing through muscle and fat, and the stronger the photo contrast. As the tube voltage increases, the X-ray contrast decreases significantly.
4) When the wavelength of X-rays is in the range of 0.062~0.093nm, the maximum contrast (Kx) value is obtained between muscle and fat.
(2) Principles and characteristics of mammography
1. Structural characteristics of mammary glands
1) The tissue structure of the human body can be represented by four main substances, namely gas, fat, muscle and bone. Gas density is the lowest, bone density is the highest, and fat and muscle are in between. We call fat, muscle and skin soft tissue.
2) The general anatomy of the breast includes the nipple, areola, skin, fat, breast lobe, lactiferous duct, and suspensory ligament of the breast, all of which are soft tissue structures with similar densities and lack of natural contrast. In order to increase its absorption difference to the line and obtain breast structure images with good contrast, soft X-ray photography technology must be selected.
2. Principles of mammography
1) Mammography uses a molybdenum target X-ray machine, and the anode target surface of the X-ray tube is made of molybdenum (Mo), which can generate soft X-rays. Its frame structure is designed according to the physiological characteristics of the mammary gland.
2) The tube voltage adjustment range of the mammography camera is 20~40kV. When the tube voltage is around 35kV, molybdenum can produce K series characteristic radiation (marker radiation). The average energy of the characteristic radiation of the K system is 20keV, and the wavelength of the X-ray released during the electron energy transition of 20keV is about 0.063nm. Just within the ideal wavelength range for maximum X-ray contrast in soft tissue photography.
3) The K-series characteristic radiation is the strongest part of all the radiation produced by the molybdenum target, that is, the two peaks in the X-ray energy spectrum produced by the molybdenum target X-ray tube, with a narrow range, constant wavelength, and strong monochromaticity, suitable for breast cancer photography.
4) In order to ensure the imaging effect and quality of mammography, the imaging system structure should also have the following characteristics:
① The focus of the X-ray tube should be controlled below 0.5mm.
② The cassette is made of materials with a small absorption coefficient.
③ The fluorescent body of the intensifying screen can absorb soft rays, the crystal particles are fine, and only one-sided rear screen is used.
④ The X-ray film is a single-emulsion film that matches the screen-gel system and a special mammography film with a large r value.
⑤ 0.03mm molybdenum/0.025mm rhodium is commonly used for window filtration to adapt to different density mammography options.
⑥ The grid is usually 80LP/cm ultra-dense grain grid or high penetration unit grid (HTC).
⑦ Implement pressurization technology.
2. Digital Mammography
Two types of flat-panel detectors, amorphous silicon and amorphous selenium, are currently most used in breast DR imaging equipment, and their imaging principles are the same as those described in Section 2 DR. However, due to the relatively high requirements of mammography, the following points should be noted:
1. For digital mammography, higher X-ray energy can produce better image quality with a smaller dose, especially for dense breasts. The dual-target X-ray tube is based on this purpose, molybdenum/tungsten , Molybdenum / rhodium double-target X-ray machines are more common.
2. Mammography needs to detect and classify very small objects, especially microcalcifications can be as small as 100~200μm, any flat panel detector must be able to image these very small microcalcifications of interest. Therefore, the pixel size range of the flat panel detector should be between 50 and 100 μm.
3 In order to meet the needs of clear display of breast tissue from thick to thin at the same time, on a typical breast image, 3100 gray levels can be distinguished. In order to prevent the system from losing image information, the system should provide a dynamic range of more than 14bit. The tissue structure and thickness of small breasts are different. In order to reduce the retake rate, reduce the radiation dose, and provide high-quality image quality, automatic exposure control (AEC) should be used.
4. The DQE of direct conversion detectors is higher than that of indirect conversion detectors.
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