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What Can Be Detected By CT Scan?

May. 16, 2024

Basic imaging principles of CT


CT is to use X-ray beam to scan a certain thickness of the human body, the detector receives through the layer of X-ray, into visible light, from photoelectric conversion to electrical signal, and then analog/digital converter into digital, input computer processing.The image formation process is like dividing the selected layer into thousands of cuboids of the same volume, called voxel. The scanned information is calculated to obtain the X-ray attenuation coefficient or absorption coefficient of each voxel, and then arranged into a matrix, that is, a digital matrix. The digital matrix can be stored in a magnetic disk or optical disk. The digital/analog converter converts each number in the digital matrix into small squares with grayscale ranging from black to white, that is, pixels, and arranges them in a matrix to form a CT image. Therefore, CT images are reconstructed images. The X-ray absorption coefficient of each voxel can be calculated by different mathematical methods.



Basic components of CT


CT equipment mainly consists of the following three parts: the scanning part consists of X-ray tubes, detectors and scanning frames; the computer system stores and operates the information data collected by scanning; the image display and storage system processes and reconstructs the The image is displayed on a television screen or captured using a multi-frame or laser camera.


The number of detectors has grown from one to 4,800 today. Scanning methods have also evolved from translation/rotation, rotation/rotation, rotation/fixation, to the newly developed spiral CT scan. The computer has large capacity and fast operation, and can reconstruct the image immediately.


Due to the short scanning time, artifacts caused by motion, such as interference from respiratory movements, can be avoided, which can improve image quality; the slices are continuous, so lesions will not be missed, and three-dimensional reconstruction is possible. Injection of contrast agent for angiography can obtain 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 less than 40ms, and multiple frames of images can be obtained per second. Because the scan time is very short, movie images can be captured, and artifacts caused by motion can be avoided. Therefore, it is suitable for cardiovascular imaging examinations and examinations of patients who cannot cooperate well, such as children and acute trauma.



Characteristics of CT tomographic images


(1)Different from the direct physical imaging of two-dimensional X-ray fluoroscopy and photography technology, CT images are images calculated and reconstructed by computers according to a certain mathematical model. The quality of the image depends entirely on the algorithm used.


(2)As far as image sections are concerned, the image obtained by two-dimensional X-ray fluoroscopy and photography technology is a frontal image of the human body obtained by X-rays penetrating the human body from front to back, while the CT tomographic image is a horizontal section seen from top to bottom. .


(3)The images obtained by two-dimensional X-ray perspective and photography technology do not have the shape, size and spatial relationship between objects, which is difficult to understand. CT technology allows us to see the inside of the human body, making traditional flat medical images three-dimensional.


(4)CT images are composed of a certain number of pixels with different grayscales 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 images obtained by different CT devices are different. The size can range from 1.0 Obviously, the smaller the pixels and the greater the number, the more detailed the image is, that is, the spatial resolution is high. The spatial resolution of CT images is not as high as that of X-ray images.


(5)CT images are represented by different grayscales, reflecting the extent of X-ray absorption by organs and tissues. Therefore, like the black and white image shown in the X-ray image, the black shadow represents the low-absorption area, that is, the low-density area, such as the lungs containing a lot of air; the white shadow represents the high-absorption area, that is, the high-density area, such as bones. However, compared with X-ray images, CT has higher density resolution, that is, it has high density resolution. Therefore, even though the density difference of human soft tissue is small and the absorption coefficient is close to water, it can still form contrast and image. 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, gallbladder, pancreas and pelvic organs, etc., and display images of lesions on a good anatomical image background.



CT examination scope


(1)Head:cerebral hemorrhage, cerebral infarction, aneurysm, vascular malformation, various tumors, trauma, bleeding, fracture, congenital malformation, etc.


(2)Chest:various tumors of the lung, pleura and mediastinum, tuberculosis, pneumonia, bronchiectasis, lung abscess, cyst, atelectasis, pneumothorax, fracture, etc.


(3) Abdominal and pelvic cavity:tumors of various solid organs, trauma, bleeding, liver cirrhosis, gallstones, urinary tract stones, hydrops, bladder and prostate lesions, certain inflammations, deformities, etc.


(4) Spine and limbs:fractures, trauma, bone hyperplasia, intervertebral disc disease, spinal stenosis, tumors, tuberculosis, etc.


(5) Three-dimensional reconstruction imaging of bones and blood vessels.


(6) CTA (CT vascular imaging):Takayasu arteritis, arteriosclerosis obliterans, aortic aneurysm and dissection, etc.


(7) Thyroid diseases:thyroid adenoma, thyroid adenocarcinoma, etc.


(8) Others:ophthalmic and orbital tumors, trauma; paranasal sinusitis, nasal polyps, tumors, cysts, trauma, etc.


Due to the high resolution of CT, organs and structures can be clearly visualized, and lesions can be clearly displayed. Clinically, CT diagnosis of the nervous system and head and neck has been used early, and has great diagnostic value for brain tumors, brain trauma, cerebrovascular accidents, brain inflammation and parasitic diseases, congenital brain malformations, and brain parenchymal lesions.


In the diagnosis of facial features, it has diagnostic value for intraframe tumors, sinus and throat tumors, especially inner ear developmental abnormalities.


In the diagnosis of the respiratory system, the diagnosis made by CT examination is relatively reliable for the diagnosis of lung cancer, examination of mediastinal tumors and the internal structure of the tumor, as well as whether there is lymph node metastasis in the hilus and mediastinum.


It also has diagnostic value in the examination of the heart, great vessels and musculoskeletal system.


In terms of bone and joint diseases, most cases can be diagnosed through simple and economical routine X-ray examinations, so CT examinations are relatively rarely used.

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