X-ray tomography

1. Tomography: In conventional X-ray photography, various tissues overlap with each other, which may cover up tiny lesions and even fail to show the lesions in soft tissue organs such as brain and liver. In complex tissues and organs, such as the spine, it is more difficult to find early lesions. The traditional fault uses physical methods to make the tissue image of the selected thickness clear and the upper and lower structures blurred, but it still overlaps with the tissue image of the imaging layer. Therefore, it is not an ideal tomography method and has been eliminated. CT can only image the fault tissue with a certain thickness, and the adjacent upper and lower layers have no overlap and interference with the imaging tissue, which can overcome many shortcomings that X-ray plain films can't overcome, and is an ideal fault imaging method. Cross-sectional direction commonly used in CT. In order to display the whole organ, multiple continuous cross-sectional images are often needed. Through image post-processing and reconstruction, coronal, sagittal, oblique and three-dimensional images can be obtained.

2. Spiral scanning: Spiral CT is one of the major advances in modern CT technology. The traditional CT scanning method is that the X-ray tube and detector rotate around the body, and two-dimensional cross-sectional data are collected every rotation period to reconstruct the image. The scanning is actually discontinuous. When traditional CT scans the chest, abdomen and pelvis, patients need to hold their breath every time. After each scan, the CT tube is reset, and the table moves to the next position to scan the next slice. When acquiring the next image, the patient needs to hold his breath again. If the patient holds his breath differently during the two scans, some organs (such as lung and liver) may not be scanned at all, so small lesions cannot be found. Spiral CT has solved this problem, using the principle of electric slip ring, the tube does not need to be reset. The X-ray tube rotates continuously around the scanned object, which is exposed by slip ring technology, and the scanned object passes through the scanning hole with scanning bed at a uniform speed. At the same time, the detector continuously collects the attenuation data of X-rays in three-dimensional space after they pass through the human body, and then reconstructs a two-dimensional tomographic image by computer. This way of collecting data is also called volume sampling or volume sampling. According to the number of detector rows, spiral CT can be divided into single row, double row and multi-row. At present, 16 row spiral CT has been applied in clinic. The tube of spiral CT scans around the body once, which can not only complete the data acquisition and reconstruction of 16 slice image at the same time, but also complete the scanning of an organ with a thickness below 0.5mm in a few seconds, and complete the post-processing of the image in near real time, such as multi-directional reconstruction and three-dimensional reconstruction. This provides a new method for the diagnosis and research of cardiovascular, liver, tissue perfusion, bone structure and other organ systems that need accurate large-scale imaging.

3.CT value: the concept of quantitative density is used to describe the unit of CT image density. It is the relative value obtained by a certain mathematical transformation of the X linear attenuation coefficient measured by the detector. The unit is the Hounsfield unit. At present, the general concept of CT value is that the CT value of water is 0Hu, that of bone cortex with the highest human density is+1000Hu, that of air with the lowest density is-1000 Hu, and that of other tissues is between-1000 Hu ~+ 1000 Hu. For example, the CT values of soft tissues are mostly +20 ~+50 Hu, and those of adipose tissues are mostly -40 ~-90 Hu.

4. Density and gray scale: CT images also show the density difference of different gray-white tissues (gray scale is 16), which is usually represented by white as the high-density part of the image, and black as the low-density part of the image. Because CT image is a computer-processed image, its so-called density is only relative, and it can also be displayed in reverse color.

5. Window width and window level: window width, WW) refers to the range of CT values displayed in the image. Window level (WC) refers to the midpoint of the range of CT values displayed by the window width, and the average CT value of the tissue to be displayed is usually set as the window level.