Characteristics of high kV photography and matters needing attention in its use

High kV photography is a photography method that uses high-energy X-rays generated by tube voltage above 120kV to obtain rich X-ray photographic images in a small density range.

When high-energy X-rays pass through limbs, their absorption attenuation mode and absorption coefficient are different from those of ordinary energy X-rays, which leads to different contrast changes and thus obtains different effects from ordinary X-rays. The X-ray produced by the diagnostic X-ray machine is a continuous spectrum. Under a certain nominal tube voltage, photoelectric absorption and Compton-Wu Xun You absorption each account for a certain proportion.

In the photon energy range of 10 ~ 100 kev, with the increase of photon energy, the X-ray absorption of limbs is decreasing, while Compton-Wu Xun You absorption is increasing. When the photon energy is 10keV, the photoelectric absorption is above 95%, and Compton-Wu Xun You absorption is 5%. When the photon energy increases to 100keV, Compton-Wu Xun You absorption exceeds 95%. At this time, the absorption of X-rays by limbs is not affected by the atomic number of the constituent substances, but is related to the number of electrons per gram and photon energy of the substances. The total absorption coefficients of continuous rays and marked rays produced by human tissues at a tube voltage of 60 ~ 100 kV include photoelectric absorption coefficients and scattering absorption coefficients of different percentages.

Precautions:

1, after the voltage of the X-ray tube is switched on, the radiation dose rate of the useful beam should quickly reach the specified value to ensure that the deviation between the radiation received by the patient's treatment site and the calculated value (the product of the predetermined radiation time and the normal radiation dose rate) is not more than 5%. Otherwise, an electric shutter must be installed in the window of the X-ray machine and used in conjunction with the automatic timer. The protective thickness of shutters should not be less than 10 half-value layer.

2. At any treatment position, the focus of the X-ray tube and the center of the fixed diaphragm should be on the same straight line with the central axis of the light collecting tube. The asymmetry of useful beam irradiance in irradiation field is less than 5%.

3. The X-ray tube must be installed firmly, and the tube head assembly should be able to lock in any required position.

4, the setting of the treatment room must fully consider the safety of the surrounding environment. The intensive treatment room must be separated from the control room. The treatment room should have sufficient use area, generally not less than 30 square meters. Do not pile up sundries indoors.

Extended data

In photoelectric absorption, because the effective atomic number of bones is higher than that of soft tissues, the photoelectric absorption coefficient of bones is directly proportional to the fourth power of the effective atomic number of substances, and the photoelectric absorption coefficient of bones is higher than that of soft tissues, so the contrast index between them is large and the photos show high contrast.

In scattering absorption, the difference of absorption coefficient between bone and muscle and the difference of absorption coefficient between muscle and fat is due to the increase of tube voltage and the decrease of contrast index. When the tube voltage is above 120kV, the contrast index of human tissues is low. This is because the percentage of scattering absorption is higher than that of photoelectric absorption. Calculated in grams, the scattering and absorption energy of bones and soft tissues are equal. At this time, only the difference of absorption coefficient caused by the density difference of each tissue forms the image contrast.