X-ray examination of imaging

X-ray images are composed of images with different gray levels from black to white. These images with different gray levels reflect the anatomical and pathological state of human tissue structure. This is the natural contrast on which X-ray inspection is based. For tissues or organs lacking natural contrast, a certain amount of substances with higher or lower density can be artificially introduced to produce artificial contrast. Therefore, natural contrast and manual contrast are the basis of X-ray examination. Include fluoroscopy and photography.

Perspective: perspective for short. This is a common X-ray examination method. Because of the low fluorescence brightness, fluoroscopy must generally be carried out in a dark room. Before perspective, we must adapt ourselves to the sight. Using image enhancement TV system, the image brightness is obviously enhanced and the effect is better. The main advantage of fluoroscopy is that it can rotate the patient's position and change the direction for observation; Understand the dynamic changes of heart, great vascular pulsation, diaphragm movement, gastrointestinal peristalsis and other organs; The perspective equipment is simple, easy to operate and low in cost, and can draw conclusions immediately. The main disadvantages are low screen brightness, poor image contrast and definition, and it is difficult to observe organs with small difference in density and thickness and parts with large density and thickness. For example, the skull, abdomen, spine, pelvis and other parts are not suitable for perspective. In addition, the lack of objective records is also an important shortcoming.

Radiography: The photos obtained are usually called plain films. This is the most widely used inspection method. The advantages are clear imaging, good contrast and high definition; Lesions are not easy to occur in parts with large density and thickness or small difference between density and thickness; It can be used as an objective record for comparison and reference during the second interview. The disadvantage is that each photo is just an instant X-ray image in one direction. In order to establish a three-dimensional concept, it is often necessary to take pictures in two directions perpendicular to each other, such as the front position and the side position. The observation of function is not as convenient and direct as perspective; The cost is slightly higher than perspective.

These two methods have their own advantages and disadvantages, which can improve the accuracy of diagnosis by cooperating with each other and learning from each other. Tomography: Ordinary X-ray film is the total projection of all images on the X-ray projection path. Some images cannot be displayed because they overlap with the images before and after them. Tomography can obtain the image of the tissue structure on the selected layer through special equipment and operation, while the structure that does not belong to the selected layer is blurred in the projection process. Its principle is shown in figure 1- 1-6. Tomography is usually used to identify lesions that are difficult to display on plain films, overlap more and are located in deeper parts. It is often used to know whether the internal structure of the lesion is damaged, sunken or calcified, whether the edge is sharp and the exact location and scope of the lesion; Show whether there is stenosis, blockage or dilatation of trachea and bronchial cavity; Observe the structure and pathological changes of the selected slices with contrast examination.

Flexible radiography: Molybdenum target tube balls that can emit soft X-rays are used to examine soft tissues, especially breasts.

Others: Special examination methods include ① magnifying photography, using microfocus to increase the distance between human body and photos to show subtle lesions; (2) Fluorescent photography, microfilm based on fluorescent imaging, is mainly used for collective physical examination; (3) Waveform recording photography, which records the pulsation of the heart and great vessels, diaphragm movement and gastrointestinal peristalsis in the form of waves with special devices.

During exposure, the X-ray tube moves in the opposite direction to the film, and the moving axis is on the plane of the selected layer. As a result, only one layer of the selected structure is always projected to a fixed position (A') on the film, so that the structure of this layer can be clearly developed, and during the exposure process, the position of this layer moves continuously on the film, forming a blurred image (B'). A considerable part of human tissue structure can not be displayed in ordinary examination only by its own density and thickness difference. At this time, substances higher or lower than the tissue structure can be introduced into the internal or surrounding space of organs to make them contrast and develop, which is called contrast examination. The introduced substance is called contrast medium. The application of contrast examination greatly expands the scope of X-ray examination.

(1) Contrast agents are classified into high-density contrast agents and low-density contrast agents according to their density.

1. High-density contrast agent is a substance with high atomic number and high specific gravity. Commonly used are barium and iodine.

Barium agent is medical barium sulfate powder, which is prepared by adding water and glue. According to the inspection site and purpose, according to the size and uniformity of powder particles and the amount of water and glue, different types of barium suspensions are prepared, and the concentration is usually expressed by weight/volume ratio. Barium sulfate suspension is mainly used for esophagography and gastrointestinal radiography, and double contrast examination of barium and gas can improve the diagnostic quality.

There are many kinds of iodine agents, including organic iodine and inorganic iodine preparations.

Injection of organic iodine water-based contrast agent into vascular display organs and large blood vessels has become a routine method for decades. It is mainly discharged from biliary tract or urinary tract through liver or kidney, so it is widely used for cholangiography and enhanced ct examination of bile duct and gallbladder, renal pelvis and urinary tract, arteries and veins. Ion contrast agents were used before 1970s. This kind of hypertonic ionic contrast agent can cause intravascular fluid increase and vasodilation, pulmonary vein pressure increase, vascular endothelial damage and great neurotoxicity, and may have toxic and side effects in use. In 1970s, nonionic contrast agent was developed, which has the advantages of relatively low permeability, low viscosity and low toxicity, greatly reducing the side effects, and is suitable for blood vessels, nervous system and enhanced CT scanning. Unfortunately, the cost is high and it is difficult to be widely used.

The above-mentioned water-soluble iodine contrast agents have the following types: ① ionic type, represented by meglumine diatrizoate; ② The nonionic type is represented by iohexol, iopamidol and Pierer; ③ nonionic dimer, represented by iotrolan.

Among non-mechanism iodinating agents, lipiodol contains 40% iodine, which is often used for examination of bronchus and fistula. Iodine oil absorbs slowly after filming, so try to suck it out after filming.

Intraspinal injection of fatty acid iodide meglumine diatrizoate can be used for myelography, but now nonionic dimer iodine solution has been used.

2. Low-density contrast agents are substances with low atomic number and small specific gravity. Clinical applications include carbon dioxide, oxygen and air. In human body, carbon dioxide is absorbed fastest and air is absorbed slowest. Neither air nor oxygen can be injected into the bleeding organ to avoid air embolism. It can be used for angiography of subarachnoid space, joint capsule, abdominal cavity, chest cavity and soft tissue space.

(2) There are two ways to compare.

1. Direct introduction includes the following ways; ① Oral method: barium meal examination of esophagus and gastrointestinal tract; ② Perfusion methods: barium enema, bronchography, retrograde cholangiography, retrograde urography, fistula, pyogenic cavity radiography and hysterosalpingography; ③ Puncture injection method: it can be injected into organs or tissues directly or through catheters, such as angiocardiography, arthrography and myelography.

2. The indirectly introduced contrast agent is first introduced into a specific tissue or organ, and then absorbed and concentrated in the organ to be imaged, thereby developing it. Including absorption and excretion. Absorption, such as lymphangiography. Excretive, such as intravenous cholangiography or intravenous pyelography and oral cholangiography. In the first two cases, after intravenous injection of contrast agent, the contrast agent accumulates in liver and kidney, and then excretes into bile duct or urinary tract. The latter is that after oral administration of contrast agent, the contrast agent enters the blood circulation through the intestine, then reaches the liver and gallbladder and is discharged into the gallbladder, that is, photography is taken during the accumulation process, which is rarely used now.

(3) Preparation before inspection and handling of contrast reaction. Various contrast inspections have corresponding preparation and precautions before inspection. It must be strictly implemented and carefully prepared to ensure the inspection effect and patient safety. Rescue drugs and instruments should be prepared for emergency use.

Among contrast agents, barium is safer, and gas embolism should be prevented during gas radiography. After venous gas embolism, the patient should be placed in the left lateral position immediately to prevent gas from entering the pulmonary artery. In contrast reaction, iodine contrast agent allergy is more common and serious. When choosing iodine contrast agent for angiography, the following points are worth noting: ① To know whether the patient has contraindications for angiography, such as severe heart and kidney diseases and allergic constitution; (2) Do a good job of explanation and strive for patients' cooperation; ③ Contrast agent allergy test: General intravenous injection of 1ml30% contrast agent and observation of 15min. If chest tightness, cough, shortness of breath, nausea, vomiting and urticaria occur, it is positive and not suitable for comparative examination. However, it should be pointed out that although there are no such symptoms, reactions can also occur during angiography. Therefore, the key lies in the ability to prepare and rescue allergic reactions; ④ Prepare for rescue. Serious reactions include peripheral circulation failure and cardiac arrest, convulsion, laryngeal edema, pulmonary edema and asthma attack. In this case, angiography should be stopped immediately, and anti-shock, anti-allergic and symptomatic treatment should be carried out. Oxygen should be given for dyspnea, norepinephrine should be given for peripheral circulation failure, and heart massage should be given immediately for cardiac arrest. CT images are made up of a certain number of pixels with different gray levels from black to white arranged in a matrix. These pixels reflect the X-ray absorption coefficient of the corresponding voxel. Different CT devices have different pixel sizes and numbers. The size can be 1.0× 1.0mm, ranging from 0.5×0.5mm；; The number can be 256×256, that is, 65536, or 5 12×5 12, that is, 262 144. Obviously, the smaller the pixels, the more the number, the more detailed the image, that is, the higher the spatial resolution. The spatial resolution of CT images is not as high as that of X-ray images.

CT images are expressed in different gray levels, which reflects the degree of X-ray absorption by organs and tissues. Therefore, like the black-and-white images displayed in X-ray images, black shadows represent low absorption areas, that is, low density areas, such as lungs; White shadow indicates high absorption area, that is, high density area, such as bone. However, compared with X-ray images, CT has high density resolution, that is, high density resolution. Therefore, although the density difference of human soft tissue is small, and the absorption coefficient is mostly close to water, it can also form contrast and imaging. This is the outstanding advantage of CT. Therefore, CT can better display the organs composed of soft tissues, such as brain, spinal cord, mediastinum, lung, liver, gallbladder, pancreas and pelvic organs, and display the images of lesions on a good anatomical image background.

X-ray images can reflect the density of normal and diseased tissues, such as high density and low density, but there is no concept of quantity. CT images not only show their density at different gray levels, but also show their density through the absorption coefficient of X-rays by tissues, which has a quantitative concept. The absorption coefficient is not used in practical work, but converted into CT value, and the density is explained by CT value. The unit is the Hounsfield unit.

The absorption coefficient of water is 10, and the CT value is 0Hu. The bone cortex with the highest human density has the highest absorption coefficient, and the CT value is+1000Hu, while the air density is the lowest, which is-1000Hu. The CT values of different densities and tissues in the human body range from-1000Hu to+1000Hu, with 2000 divisions.

As can be seen from the upper right picture, the CT value of human soft tissue is mostly similar to that of water, but because of the high density resolution of CT, although the density difference is small, it can also be compared and developed.

The use of CT values can not only describe the density of tissue images with high density or low density, but also explain the degree of density with their CT values.

CT image is a slice image, commonly used in cross section. In order to display the whole organ, multiple continuous slice images are needed. Coronal and sagittal images can also be reconstructed using the image reconstruction program on CT equipment.