What are the factors that cause blurred X-ray photos?

(1) focus area The focus of an X-ray tube is not a point in geometry, but an area that changes with the tube capacity. The generated X-ray photographic image must contain penumbra. The appearance of penumbra blurs the outline of the image and affects the clarity of the photo.

The larger the focal zone, the larger the penumbra and the worse the definition.

The size of the penumbra depends on the distance between the focus, the object and the film, that is, h = b.a? . f .

According to the formula, in order to obtain an image with small penumbra and high sharpness, it is required that the focal area (f) is small, the distance between the subject and the focal sheet (b) is small, and the distance between the focal point and the film (a+b) is large.

Due to the inclination of the anode surface of the X-ray tube, the focus area used in photography is different from the area where electrons hit the anode. The former is the effective focal zone and the latter is the actual focal zone, and the effective focal zone is smaller than the actual focal zone.

(2) Anode effect Due to the inclination of the anode surface, the X-ray intensity and effective focal area emitted from the window of the bulb tube are unevenly distributed, and the X-ray intensity and effective focal area near the anode end are smaller than those at the cathode end. This effect is called anode effect.

(3) The focal distance is large, the penumbra is small, and the definition is high.

X-rays are not parallel beams because they radiate in a cone. Therefore, the image irradiated on the film must have a blurred penumbra. If the focal plane is farther away, the light tends to be parallel and the penumbra naturally becomes smaller.

However, in practical application, it is impossible to increase the focal distance indefinitely. Because the X-ray intensity was weakened by inverse square law. To obtain the same density will greatly increase the photographic conditions, which are limited by the capacity of the machine. Therefore, in the actual photography technology, according to the photography requirements of different parts and the capacity of the machine used, several focal distance are specified. For example, the distance of chest photography is 150~200cm, and the focal distance of general parts is 75~ 100cm.

(4) Object-film distance When the focal area and film distance are fixed, the penumbra increases with the increase of object-film distance. On the contrary, the closer the subject is to the film, the smaller the penumbra, the smaller the magnification distortion and the higher the definition.

However, we can also use the image magnification produced by the subject being far away from the film to carry out the magnification photography technology.

To sum up, our conclusion is that there are requirements in photography technology: ① the subject should be as close to the film as possible; (2) Use as large a focal length as possible; ③ Use the smallest focus.

2. Motion blur

During the X-ray irradiation, the X-ray tube, the irradiated object and the film should be kept still. If one of the factors moves, the image will be blurred, which is caused by two factors: ① the movement of X-ray tube, table top and cassette; ② The illuminated object moves.

The movement of the subject is divided into two categories, one is physiological, such as breathing, heartbeat, gastrointestinal peristalsis, spasm and so on. Generally uncontrolled, only breathing can be solved by holding your breath; One is unexpected, such as posture and movement.

3. Object absorption blur

The fuzzy absorption of objects is caused by the lack of good sharp edges in human tissues. That is to say, when X-rays pass through the edge of an object, their absorption changes gradually. In order to explain this phenomenon, we can assume that X-ray is a point light source and there are three object shapes: cone, square and sphere. They are the same substance, the same thickness, and the same photographic distance irradiated by X-rays. The edge of vertebral body is parallel to X-ray. Its absorption blur is very small, and the density of the image edge changes suddenly, so the definition is the highest. Obviously, the absorption of X-rays by a cube changes along the outside, with little absorption of X-rays at the edges and more absorption at the lower corners. The image boundary of a circular object is blurred because the reflection density changes gradually along the image boundary.

As mentioned above, sharpness refers to whether the density change is clear or gradual. Clear means high sharpness, and gradient means low sharpness. When it is necessary to measure small round or oval tissues, such as measuring the width of blood vessels in angiography. The influence of absorption blur is more prominent.

Take blurred photos

Including the influence of film particles and the influence of intensifying screen performance.

Generally speaking, the larger the film particles, the thicker the latex film and the worse the image definition, which is not significant without using the intensifying screen. In practical work, the intensifying screen is mostly used in photography, which seriously affects the clarity of the image.