CT image features

1. CT image features

CT images consist of a certain number of pixels from black to white with different gray levels arranged in a matrix. These pixels reflect the X-ray absorption coefficient of the corresponding body element. The pixel size and number of images obtained from different CT devices are different. The size can be 1.0 × 1.0mm, 0.5 × 0. 5mm mm, or 256 × 256mm, or 65536 × 512mm. Obviously, the smaller the number of pixels, the more detailed the image is, 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 displayed in different gray scales, reflecting the degree of X-ray absorption of organs and tissues. Therefore, like black and white images shown in X-ray images, Black Shadows represent low-absorption areas, that is, low-density areas, such as the lungs; white shadows represent high-absorption areas, that is, high-density areas, such as bones. However, compared with X-ray images, CT has a high density resolution (density resolutiln ). Therefore, although the density difference between human soft tissue is small, the absorption coefficient is close to water, and can be compared and imaging. This is an outstanding advantage of CT. Therefore, CT can better display the organs consisting of soft tissue, such as the brain, spinal cord, mediastinum, lung, liver, gallbladder, pancreas, and pelvic organ, the image of the lesion is displayed on a good anatomical image background.

X-line images can reflect the density of normal and diseased tissue, such as high density and low density, but there is no concept of quantity. CT images not only show their density in different gray scales, but also show the degree of density by the absorption coefficient of the tissue on the X-ray, which has a concept of quantity. In practice, the density is converted to CT value instead of absorption coefficient, and the CT value is used to describe the density. The Unit is Hu (hounsfield unit ).

The water absorption coefficient is 10, the CT value is set to 0hu, the highest density of the body's bone cortex absorption coefficient is the highest, the CT value is set to + 1000hu, and the air density is the lowest, set to-1000hu. the CT values of different body density and various tissues are between-2000 Hu and + Hu.

From table 1-2-1, the CT values of the human soft tissue are mostly similar to those of water. However, due to the high density resolution of CT, the density difference is small and can be compared and developed.

The use of CT values not only describes the density of an image of a certain tissue, but also describes the density by their CT values.

CT images are layered images and commonly used cross-section images. Multiple consecutive layer images are required to display the entire organ. Through the use of CT equipment imaging reconstruction procedures, you can also reconstruct the coronary and vector level images.

2. CT device Overview

CT equipment consists of the following three parts: ① the scanning part is composed of an X-ray tube, a detector, and a scanning frame; ② the computer system stores the information data collected by scanning; ③ In the image display and storage system, images processed and reconstructed by computers are displayed on the TV screen or taken with multiple cameras or laser cameras.

 

Scan Part 1-2-4 and figure 1-2-5. The number of detectors has grown from the original one to up to 4800 today. Scanning methods also evolved from translation/rotation, rotation/rotation, rotation/fixation to the recently developed spiral CT scan ). Large computer capacity, fast operation, can achieve immediate reconstruction of images. Due to the short scanning time, motion can be avoided. For example, the interference of respiration motion can improve the image quality. The layer is continuous, so no lesions are missed, and 3D reconstruction is feasible, CT angiography (CT angiography and CTA) can be achieved by injecting contrast agent for angiography ). The scanning method used by ultra-high-speed CT scanning is completely different from that used by the former. The scanning time can be as short as 40 ms, and more images can be obtained per second. Due to the short scanning time and the availability of movie images, motion-induced artifacts can be avoided. Therefore, this method is suitable for patients with cardiovascular angiogram, pediatric and acute trauma that cannot be well performed.

3. Basic principles of CT Imaging

CT scans a layer of a certain thickness of the human body using an x-ray bundle. After the X-ray is received by the detector and converted to visible light, the X-ray is converted to an electrical signal, then, the analog/digital converter (analog/digital converter) is converted into a number, and the computer is input for processing. Image processing is like dividing a selected layer into cubes of the same size, called voxel. See figure 1-2-1. the X-ray attenuation coefficient or absorption coefficient of each element is obtained after calculation, and then arranged into a matrix, that is, a digital matrix.

Digital matrices can be stored on disks or discs. By digital/analog converter (digital/analog converter), each number in the digital matrix is converted into small blocks ranging from black to white, that is, pixels, which are arranged in a matrix, CT images. Therefore, CT images are reconstruction images. The X-ray absorption coefficient of each element can be calculated using different mathematical methods.