The X-ray image is composed of images of different shades of gray from black to white. These different shades of gray reflect the anatomy and pathology of the body's tissues and structures. This is the natural contrast on which X-ray examinations are based. For tissues or organs that lack natural contrast, artificial contrast can be created by artificially introducing a certain amount of material that is higher or lower in density than it. Thus, natural and artificial contrasts are the basis of X-ray examination. This includes fluoroscopy and photography.
Fluoroscopy: Fluoroscopy for short. It is a commonly used X-ray examination method. Because of the low brightness of fluorescence, fluoroscopy is usually carried out in a dark room. Before fluoroscopy, it is necessary to darken the visual acuity. With the use of image-enhancing television systems, the brightness of the image is significantly enhanced for better results. The main advantage of fluoroscopy is that it can rotate the patient's position and change the direction of observation; understand the dynamic changes of organs, such as the heart, large blood vessel pulsation, diaphragm movement and gastrointestinal peristalsis, etc.; fluoroscopy equipment is simple, easy to operate, low cost, and can be immediately concluded. The main disadvantage is that the brightness of the screen is low, the contrast and clarity of the image is poor, and it is difficult to observe the organs with less difference in density and thickness, as well as the parts with greater density and thickness. For example, the skull, abdomen, spine, pelvis and other parts are not suitable for fluoroscopy. In addition, the lack of objective records is also an important disadvantage.
X-ray photography (radiography): the resulting picture is often called plainfilm. This is the most widely used method of examination. Advantages are clear imaging, contrast and clarity are better; not difficult to make the density, thickness or density, thickness of small differences in the site of the lesion is visualized; can be used as an objective record, easy to review the control and consultation. The disadvantage is that each photo is only an orientation and a momentary X-ray image, in order to establish a three-dimensional concept, often need to be perpendicular to each other, such as orthopantagonal and lateral photography; the observation of the functional aspects of the less convenient and direct than the fluoroscopy; the cost is slightly higher than the fluoroscopy.
These two methods have their own strengths and weaknesses, and they complement each other to improve the correctness of the diagnosis. Tomography: A plain X-ray film is a projection of the sum of all the images overlapping in the path of the X-ray projection. A portion of the image cannot be displayed because it overlaps with the image before or after it. Tomography, on the other hand, allows special devices and operations to obtain images of tissue structures at a selected level, while structures that do not belong to the selected level are blurred out in the projection process. The principle is shown in Figure 1-1-6. Volumetric photography is often used to visualize lesions that are difficult to visualize on plain films, that overlap a great deal, and that are in deeper areas. It is mostly used to find out whether the internal structure of the lesion is damaged, hollow or calcified, whether the edges are sharp and the exact location and extent of the lesion; to show whether the lumen of the trachea and bronchus is narrowed, blocked, or dilated; and to cooperate with the contrast examination in order to visualize the structure and lesions at selected levels.
Flexible radiography: the use of a molybdenum tube ball that emits soft X-rays for the examination of soft tissues, especially the breast.
Other: special inspection methods include ① magnification photography, the use of micro-focus and increase the distance between the human body and the photo to show the more subtle lesions; ② fluorescence photography, fluorescence imaging on the basis of microfilm, mainly used for group physical examination; ③ wave recording photography, the use of special devices to record the heart, large blood vessels in the form of waveforms of the heart, diaphragm movement and gastrointestinal peristalsis, and so on.
During exposure, the X-ray tube moves in the opposite direction to the film, and the axis of movement is in the plane of the selected level. As a result, in the part being examined, only a selected layer of structure is always projected on the film in a fixed position (A'), so that the structure of the level of clear imaging, while its front and back of the structure of the layers due to exposure, the position of the projection on the film is constantly moving and become a fuzzy image (B') The human body has a considerable portion of the structure of the tissues, relying only on their own differences in density and thickness can not be in the ordinary examination. B') In this case, a substance that is higher or lower than the tissue structure can be introduced into the organ or the surrounding space to create contrast for visualization, which is called a contrast test. The substance introduced is called contrastmedia. The use of contrast media has significantly expanded the scope of X-ray examinations.
(I) Contrast media are divided into two categories, high-density contrast media and low-density contrast media, according to their density.
1. High-density contrast agents are substances with high atomic number and high specific gravity. Barium and iodine are commonly used.
Barium is a medical barium sulfate powder, which is prepared by adding water and glue. According to the examination site and purpose, according to the powder particle size, uniformity, and the amount of water and glue formulated into different types of barium suspension, usually by weight/volume ratio to indicate the concentration. Barium sulfate suspension is mainly used for esophageal and gastrointestinal imaging, and can be used for barium gas double contrast examination to improve the quality of diagnosis.
There are many types of iodine agents, widely used, divided into organic iodine and inorganic iodine preparations.
Organic iodine aqueous contrast agent injected into the blood vessels to show organs and large vessels, has a history of several decades, and has become a routine method. It is mainly through the liver or kidney from the biliary tract or urinary tract, and thus widely used in the bile duct and gallbladder, renal pelvis and urinary tract, arteries and veins of the contrast and CT enhancement, etc. Before the 1970s are used in ionic contrast media. Before the 1970s, ionic contrast agents were used, which are hypertonic and can cause increased intravascular fluid and vasodilatation, elevated pulmonary venous pressure, endothelial damage and neurotoxicity, and toxic side effects can occur in the use of non-ionic contrast agents, which have the advantages of relative hypo-osmolality, low-viscosity, and low-toxicity, which greatly reduce toxic side effects, and are applicable to the vascular system, the nervous system, and the contrast-enhanced CT scans. Unfortunately, the cost is high and it is still difficult to be used universally.
The above water-soluble iodine contrast agents are of the following types: (1) ionic, represented by urografin (urografin); (2) non-ionic, represented by iodophenylhexanol (iohexol), iopromide (iopromide), iopamidol (iopamidol); (3) non-ionic dimers, represented by iotrolan (iotrolan).
Non-mechanism iodine agent, Bush chemical oil (lipoidol) containing iodine 40%, commonly used in bronchial, fistula subglottis input ovarian angiography and so on. Iodized oil is absorbed very slowly after contrast, so it should be sucked out as much as possible after the completion of the contrast.
Fatty acid iodide iodophenyl ester (pantopaque), can be injected into the vertebral canal for myelography, but now with non-ionic dimer iodine water.
2. Low-density contrast agents are substances with low atomic number and low specific gravity. Used in the clinic are carbon dioxide, oxygen, air and so on. Carbon dioxide is the fastest absorbing agent in the human body, and air is the slowest absorbing agent. Air and oxygen cannot be injected into the bleeding organ to avoid air embolism. It can be used for imaging of the subarachnoid space, joint capsule, abdominal cavity, thoracic cavity and soft tissue spaces.
(ii) Imaging modalities There are two modalities.
1. direct introduction including the following ways; ① oral method: esophagus and gastrointestinal barium meal examination; ② perfusion method: barium enema, bronchography, retrograde cholangiography, retrograde urography, fistulas, pus cavities, and uterine tubal imaging, etc.; ③ perforation and injection: can be injected into the organs or tissues directly or through the catheter, such as cardiovascular angiography, arthrogram, and myelogram, etc..
2. Indirect introduction of contrast medium is first introduced into a specific tissue or organ, and then absorbed and gathered in an organ to be contrasted, so as to make it visible. Including absorption and excretion of two types. Absorptive such as lymphangiography. Excretory such as intravenous cholangiography or intravenous pyelography and oral cholangiography. The former two are injected intravenously and then the contrast agent collects in the liver and kidneys and is excreted into the bile ducts or urinary tract. The latter is after oral administration of contrast medium, the contrast medium is absorbed into the blood circulation through the intestinal tract, and then to the liver and gallbladder and discharged into the gallbladder, that is, in the process of accumulation of photography, which is now used sparingly.
(C) the treatment of pre-examination preparations for contrast reactions All kinds of contrast examinations have corresponding pre-examination preparations and precautions. Must be strictly enforced, carefully prepared to ensure the effectiveness of the examination and patient safety. Should be prepared to rescue drugs and equipment, in case of emergency.
Among contrast agents, barium is safer, and air embolism should be prevented during gas contrast. After the occurrence of intraventricular gas embolism, the patient should be placed in the left lateral position immediately to avoid gas entering the pulmonary artery. Among the contrast reactions, iodine contrast allergy is more common and serious. In the selection of iodine contrast agent for imaging, the following points are worth noting: ① to understand the patient has no contraindications to imaging, such as serious heart, kidney disease and allergies, etc.; ② make a good job of explaining, and strive for the cooperation of the patient; ③ contrast allergy test, generally 1ml of 30% of the intravenous injection of the contrast medium, observation for 15 minutes, such as chest tightness, cough, shortness of breath, nausea, vomiting and urticaria, etc., then it is positive, should not be Contrast examination. It should be noted, however, that reactions can occur during imaging despite the absence of these symptoms. Therefore, the key lies in the preparation and ability to rescue anaphylactic reactions; ④ Be prepared to rescue serious reactions, including peripheral circulatory failure and cardiac arrest, convulsions, laryngeal edema, pulmonary edema and asthma attacks. In such cases, the imaging should be terminated immediately and anti-shock, anti-allergy and symptomatic treatment should be administered. Oxygen should be administered for respiratory distress, norepinephrine for peripheral circulatory failure, and immediate cardiac massage for cardiac arrest. A CT image consists of a number of pixels of different shades of gray, from black to white, arranged in a matrix. These pixels reflect the X-ray absorption coefficient of the corresponding voxel. The size and number of pixels in the image varies from one CT device to another. The size can be 1.0 × 1.0 mm, 0.5 × 0.5 mm, etc.; the number can be 256 × 256, that is, 65,536, or 512 × 512, that is, 262,144, etc.. Obviously, the smaller the pixel, the greater the number, the more detailed the composition of the image, that is, the spatial resolution (spatialresolution) is high.The spatial resolution of CT images is not as high as that of X-ray images.
CT images are represented in different shades of gray, reflecting the degree of absorption of X-rays by organs and tissues. Thus, as with the black-and-white images shown on X-ray images, black shadows indicate areas of low absorption, i.e., low density, such as the lungs, and white shadows indicate areas of high absorption, i.e., high density, such as the bones. However, CT has a high density resolution (density resolutiln) compared to X-ray images. Therefore, although the difference in density of human soft tissues is small, although the absorption coefficient is close to water, it can also form a contrast and imaging. This is the outstanding advantage of CT. Therefore, CT can better display organs composed of soft tissue, such as the brain, spinal cord, mediastinum, lungs, liver, gallbladder, pancreas, and pelvic organs, etc., and show the image of the lesion on the background of a good anatomical image.
The x-ray image reflects the density of normal and diseased tissues, such as high density and low density, but there is no quantitative concept. CT images not only show the density of different shades of gray, but also can be used to illustrate the degree of the absorption coefficient of the tissue on the x-ray, which has a quantitative concept. In practice, the absorption coefficient is not used, but is converted to a CT value, which is used to illustrate the density. The unit is Hu (Hounsfield unit).
Water absorption coefficient of 10, CT value of 0Hu, the highest density of the human body in the highest absorption coefficient of the bone cortex, CT value of +1000Hu, and the lowest density of air, set at -1000Hu. Different densities in the human body and a variety of tissues in the CT value of the residence of -1000Hu to +1000Hu of the 2000 divisions of the
By the right above figure It can be seen that the CT value of human soft tissue is similar to water, but because CT has a high density resolution, so the density difference is small, but also can form a contrast and visualization.
The use of CT values, so that in describing the density of a tissue image, not only can be used to describe the density of high-density or low-density, and can be used to illustrate the extent of their CT values flat density.
CT images are level images, often in cross section. In order to show the entire organ, multiple consecutive level images are required. Coronal and sagittal level images can also be reconstructed through the use of reconstruction programs for images on the CT device.