Full name: computed tomography
CT is a full-featured instrument for detecting medical conditions, which is short for electronic computerized X-ray tomography.
The working procedure of CT is like this: according to the different tissues of the human body to the X-ray absorption and transmission rate of different, the application of highly sensitive instruments to measure the human body, and then the measurement of the data obtained by inputting electronic computers, electronic computers to process the data, can be taken down the human body to be examined part of the cross-section or three-dimensional image, found in any part of the body of the small lesions.
The invention of CT
Since the discovery of X-rays, medical science has used them to detect diseases in the human body. However, because some organs in the human body absorb X-rays differently, X-rays are difficult to detect lesions in tissues that overlap back and forth. So, the United States and the British scientists began to look for a new thing to make up for the lack of X-ray technology to check the human body lesions. 1963, the United States physicist Cormac found that different tissues of the human body on the X-ray transmission rate is different, in the study also came up with some relevant formula, these formulas for the later application of CT laid the theoretical foundation. 1967, the British electronic engineer Hen Sfeld, unaware of the results of Cormac's research, also began the development of a new technology. He first studied pattern recognition, and then built a simple scanning device, later known as CT, to enhance the X-ray radiation source for experimental scanning and measurement of the human head. Later, he used this device to measure the whole body, obtaining the same results.In September 1971, again in collaboration with a neuroradiologist, Hengsfeld began head examinations by installing the device he had designed and built in a hospital outside London.On October 4, the hospital examined its first patient with it. The patient is fully awake in the case of supine, X-ray tube installed in the patient's upper, around the inspection site rotation, while the patient below the installation of a counter, so that all parts of the human body on the amount of X-ray absorption is reflected in the counter, and then through the processing of electronic computers, so that all parts of the human body from the screen image shows up. The test was very successful, in April 1972, Hengsfeld in the United Kingdom Radiology Annual Meeting for the first time announced the results, officially announced the birth of CT. This news caused a great shock to the scientific and technological community, the successful development of CT is known as since the discovery of X-rays by R?ntgen, the most important achievements in diagnostic radiology. As a result, Hengsfeld and Cormac*** were awarded the 1979 Nobel Prize in Physiology or Medicine. Today, CT is widely used in medical diagnostics.
The basic principle of CT imaging
CT is to scan a certain thickness of the human body with an X-ray beam, the detector receives X-rays through the level, transformed into visible light, converted into electrical signals by the photoelectric conversion, and then through the analog/digital converter (analog/digital converter) into a digital input to the computer processing. The processing of image formation, such as the selected level is divided into a number of rectangles of the same volume, called the voxel (voxel), see Figure 1-2-1. Scanning of the information obtained by the calculation of the X-ray attenuation coefficient or absorption coefficient of each voxel, and then arranged into a matrix, that is, the digital matrix (digital matrix), digital matrices can be stored on a disk or CD-ROM. The digital matrix can be stored on a disk or CD-ROM. The digital/analog converter converts each digit in the digital matrix into small squares of varying shades of gray from black to white, i.e., pixels, and arranges them according to the matrix, which constitutes a CT image. Therefore, CT images are reconstructed images. The X-ray absorption coefficient of each voxel can be calculated by different mathematical methods.
CT equipment
CT equipment has the following three main parts: ① scanning part of the X-ray tube, detector and scanning frame; ② computer system, the scanning of the information collected by the data storage operations; ③ image display and storage system, computer processing, reconstruction of the image will be displayed on the TV screen or with a multi-frame camera or laser camera will be ingested images. The detector has grown from the original 1 to as many as 4800 now. Scanning methods have also evolved from pan/rotate, rotate/rotate, rotate/fixed to the newly developed spiral CT scan. The large capacity of the computers and the fast computation allow for immediate reconstruction of the images. Due to the short scanning time, the artifacts produced by motion can be avoided, for example, the interference of respiratory motion, which can improve the image quality; the level is continuous, so it does not miss the lesion, and it is feasible to reconstruct in three dimensions, and the injection of contrast agent for angiography can be obtained from the CT angiography (Ct angiography, CTA). The scanning method used in ultra-high-speed CT scanning is completely different from the former. The scanning time can be as short as 40ms or less, and multiple images can be obtained per second. Because the scanning time is so short, cinematographic images can be taken, and artifacts caused by motion can be avoided, it is suitable for cardiovascular angiography as well as for pediatric and acute trauma patients who are unable to cooperate well.
CT image characteristics
CT images are composed of a certain 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 (spatial resolution) 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, which contain a lot of gas, and white shadows indicate areas of high absorption, i.e., high density, such as 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 tissues, such as the brain, spinal cord, mediastinum, lungs, liver, bile, pancreas, and pelvic organs, etc., and show the image of the lesion on the background of a good anatomical image.
The x-ray image can reflect the density of normal and diseased tissues, such as high density and low density, but there is no quantitative concept. CT image not only shows the density of different shades of gray, but also can be used to illustrate the degree of the absorption coefficient of the tissues on the x-ray, with 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 in the body of the highest absorption coefficient of the bone cortex, CT value of +1000Hu, and the lowest density of the air, set at -1000Hu. The density of the human body and a variety of tissues in the CT value of the body resides in the -1000Hu to +1000Hu of the 2,000 divisions between.
CT images are level images and are commonly used in cross section. In order to show the entire organ, multiple consecutive level images are required. Through the use of reconstruction programs for images on the CT device, coronal and sagittal level images can also be reconstructed, allowing multiple views of the organ and lesion relationships.
CT examination techniques
Subdivided into plain CT scan, contrast enhancement (CE), and contrast scan.
(1) plain CT scan is a normal scan without contrast enhancement or imaging. The plain scan is usually done first.
(2) Contrast-enhanced scan It is a method of injecting water-soluble organic iodine agents, such as 60%-76% pantethine glucosamine 60 ml, intravenously and then scanning. After the iodine concentration in the blood increases, the concentration of iodine in the organ and the lesion can produce a difference, forming a density difference, which may make the lesion appear more clearly. The method is divided into several kinds of mass injection method, static drip method and static injection and static drip method.
(3) Contrast scanning is the method of first making the contrast of organs or structures, and then scanning. For example, injecting 8-10 ml of iodotrast or 4-6 ml of air into the cerebral pool for cerebral pool imaging and then scanning is called cerebral pool imaging CT scan, which can clearly show the cerebral pool and the small tumors in it.