How Safe Is a CT Scan?

CT (Computed Tomography) is an electronic computer tomography. It uses precise collimated X-ray beams, -rays, ultrasound, etc., together with a highly sensitive detector to scan one after another around a part of the human body. It has the characteristics of fast scanning time and clear images, which can be used for the examination of various diseases; it can be divided into X-ray CT (X-CT) and -ray CT (-CT) according to the different rays used. ^[1]
CT CT scan is generally referred to as CT (Computed Tomography)

Chinese name: Computer tomography
Foreign name: Computed Tomography
Other name: CT machine

Imaging principle: Different tissues of the human body have different absorption and transmission of X-rays
Find the time: 1963
main application: Medical inspection, industrial inspection, security inspection

CT imaging principle

CT scans a certain thickness of a part of the human body with an X-ray beam.

Figure 1 CT principle After being converted into visible light, the X-rays are converted into electrical signals by photoelectric conversion, and then converted to digital by analog / digital converter, and then input to computer for processing. The process of image formation is like dividing a selected slice into several cuboids with the same volume, which is called voxel. ^[1]

The information obtained from the scan 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 an optical disk. A digital / analog converter converts each digit in the digital matrix into small squares ranging from black to white, that is, pixels, and arranges them in a matrix to form a CT image. Therefore, the CT image is a reconstructed image. The X-ray absorption coefficient of each voxel can be calculated by different mathematical methods. ^[1]

The working procedure of CT is as follows: it uses the extremely sensitive instrument to measure the human body according to the difference in the absorption and transmission of X-rays by different tissues of the human body, and then inputs the data obtained by the measurement into an electronic computer. After the data is processed, a cross-section or three-dimensional image of the body's examined part can be taken, and small lesions in any part of the body can be found. ^[1]

History of CT development

Since the discovery of X-rays, it has been used medically to detect human diseases. However, by

Figure 2CT machine Some organs in the human body have very little difference in the absorption of X-rays, so it is difficult to find the lesions of tissues that overlap by X-rays. As a result, scientists in the United States and Britain began to find a new thing to make up for the lack of X-ray technology to detect human lesions. ^[1]

In 1963, the American physicist Cormac found that the transmittance of different tissues of the human body to X-rays was different. In the study, some related calculation formulas were also obtained. These formulas laid the theoretical foundation for the subsequent application of CT. ^[1]

In 1967, British electronics engineer Hounsfield also started work on developing a new technology without knowing the results of Cormac's research. First, the pattern recognition was studied, and then a simple scanning device capable of enhancing the X-ray radiation source, which was later called CT, was used to perform experimental scanning measurements on the human head. Later, he used this device to measure the whole body and obtained the same effect. ^[1]

In September 1971, Huntsfield, in cooperation with a neuroradiologist, installed a device he designed and manufactured at a hospital outside London and began a head examination. On October 4, the hospital used it to examine the first patient. The patient is lying on his back in the fully awake state. The X-ray tube is installed above the patient and rotates around the examination site. At the same time, a counter is installed below the patient to reflect the absorption of X-rays on the counter by the electronic parts Computer processing makes the images of various parts of the human body displayed on the screen. This test was very successful. ^[1]

The first CT was born in 1972, which was only used for craniocerebral examination. In April, Huntsfield announced the results for the first time at the British Radiology Annual Conference, officially announcing the birth of CT. ^[1]

A whole body CT was made in 1974, and the scope of the examination was extended to the chest, abdomen, spine, and limbs. ^[1]

The first generation of CT machines used a rotate / translate mode to scan and collect information. Due to the use of a pen-shaped X-ray beam and only 1 or 2 detectors, less data is collected, long time is required, and the image quality is poor. ^[1]

The scanning method of the second generation CT machine has not changed from the previous generation, but the X-ray beam has been changed to a fan shape, and the number of detectors has been increased to 30. The scanning range has been expanded, the acquisition data has been increased, and the image quality has improved. Artifacts caused by physiological movements. ^[1]

The third-generation CT machine's controllers have surged to 300 ~ 800, and only rotate / rotate mode with the relative X-ray tube to collect more data. The scan time is within 5s, and the artifacts are greatly Decrease, the image quality is significantly improved. ^[1]

The fourth generation of CT machine controllers increased to 1000 ~ 2400, arranged in a ring and fixed. Only the X-ray tube rotates around the patient, that is, rotate / stationary mode, with fast scanning speed and image quality. high. ^[1]

The fifth generation CT machine shortened the scan time to 50 ms and solved the heart scan. An electron beam generated by an electron gun was directed at a ring-shaped tungsten target, and the detectors arranged in a ring collected information. The launched 64-slice CT can obtain a 64-slice image of the patient's body in only 0.33 s, with a spatial resolution of less than 0.4 mm, which improves the image quality, especially for the beating heart. ^[1]

CT equipment composition

CT equipment mainly has the following three parts:

The scanning part is composed of X-ray tube, detector and scanning frame; ^[2]
The computer system stores and collects the collected information and data; ^[2]
An image display and storage system, which displays and processes a computer-processed, reconstructed image on a television screen or captures the image with multiple cameras or laser cameras. ^[2]

From proposal to application, CT equipment is also constantly developing. Detectors have grown from one to many

Atlas 1 CT equipment and imaging (11 photos)

Up to 4,800 scanning modes have also been developed from translation / rotation, rotation / rotation, rotation / fixation to the newly developed spiral CT scan. The computer has a large capacity and a fast operation, and can immediately reconstruct the image. Because the scan time is short, artifacts caused by movement can be avoided, for example, the interference of breathing movement can improve the image quality; the layers are continuous, so that no lesions are missed, and three-dimensional reconstruction is feasible. Injection of contrast agent for angiography can be obtained CT angiography (Ct angiography, CTA). ^[2]

The scanning method used in ultra-high-speed CT scanning is completely different from the former. The scan time can be as short as 40 ms, and multiple frames of images can be obtained per second. Because the scan time is short, movie images can be taken, and artifacts caused by sports can be avoided. Therefore, it is suitable for cardiovascular angiography and patients who cannot cooperate well such as pediatric and acute trauma. ^[2]

CT related parameters

CT CT CT value

The CT value of a substance is equal to the difference between the attenuation coefficient of the substance and the absorption coefficient of water, then it is compared with the attenuation coefficient of water and multiplied by the indexing factor. The CT value of a substance reflects the density of the substance, that is, the higher the CT value of the substance, the higher the density of the substance.

That is CT value = × m-w / w

is the indexing factor. When the value is 1000, the unit of CT value is Heinz unit (Hu).

Different tissues in the human body have different attenuation coefficients, so their CT values are also different. The CT values of bone tissue, soft tissue, fat, water, gas, and water are about 0 Hu according to the CT value. ^[3]

CT resolution

The resolution of CT equipment is mainly divided into three types: spatial resolution, density resolution, and temporal resolution. The former refers to the smallest detail that can be resolved in the image, the medium refers to the smallest density difference that can be displayed, and the latter refers to the shortest time of body activity. spacing. ^[3]

CT layer thickness and distance

The former refers to the thickness of the scanning layer, and the latter refers to the distance between the centers of the two layers. ^[3]

CT partial volume effect

Because each layer has a certain thickness, tissues with different densities may be included in this thickness. Therefore, the CT value of each pixel actually represents the average number of CT values of various tissues in a unit volume, so it cannot reflect this. The true CT value of the organization. ^[3]

CT window width and window level

Normal or abnormal tissues have different CT values, ranging from -1000 to +1000 Hu, and the resolving power of the human eye is relatively limited. Therefore, when you want to display the details of a certain tissue structure, you should choose a tissue that is suitable for observation. Or the window width and window position of the lesion for the best display. ^[3]

CT field of view

The field of view (FOV) is divided into two types: scanning field (SFOV) and display field (DFOV). The scanning field is the range during X-ray scanning. The display field is the image range formed by data reconstruction. The scanning field is larger than the display field.

CT tube current, tube current

That is, tube current, tube current, KV and mAs determine the two parameters of the X-ray hardness and the number of photons. Increasing the KV value can increase the penetration of the X-ray. Increasing the mAs increases the amount of radiation. For patients of different ages and sizes, the corresponding examination options need to be selected.

CT Matrix

The CT matrix is used to reconstruct the image. There are several types of 256 x 256, 512 x 512, etc. The 512 x 512 matrix is commonly used.

CT noise

A uniform object is scanned. Within a certain ROI (region of interest), the CT value of each pixel is not the same but fluctuates around an average value. The change in CT value is noise. The CT value of the axial (tomographic) image shows a certain fluctuation. That is to say, the CT value is only viewed as an average value, and it may have an upper and lower deviation, and this deviation is noise. Noise is determined by the intensity of the radiation. That is, it is determined by the X-Ray quantum number reaching the detector. The greater the intensity, the lower the noise. Image noise depends on the amount of photon flux on the detector surface. It depends on the tube voltage, tube current, pre-filter and collimator aperture of the X-ray tube. The reconstruction algorithm also affects noise. ^[4]

CT signal to noise ratio

That is, the signal-to-noise ratio SNR , the ratio of the signal to the noise, and the appropriate reduction of noise can make the image better.

CT scan method

CT scanning methods are divided into three types: plain CT scan, contrast enhancement scan (CE) and contrast scan. ^[4]

CT plain scan

Plain scan refers to the common scan without contrast enhancement or contrast. Generally, CT scan is performed first.

CT enhanced scan

Enhanced scanning refers to the method of injecting water-soluble organic iodine, such as 60% to 76% diatrizoate, into a high-pressure syringe through a vein and then scanning. After the blood iodine concentration is increased, the iodine concentration in the organ and the lesion may be different, forming a density difference, which may make the development of the lesion clearer. Methods are divided into the group injection method and the intravenous drip method. ^[4]

CT contrast scan

An angiographic scan is a method of angiography of an organ or structure before scanning. For example, injecting 8-10 ml of iodotrexam into the cerebral cistern or injecting 4-6 ml of air into the cerebellar angiography and then scanning, which is called a cerebrovascular angiography CT scan, can clearly show the cerebral cistern and its small tumors. ^[4]

CT image characteristics

Figure 3 CT image of the abdomen The CT image is composed of a certain number of pixels with different gray levels from black to white arranged in a matrix. These pixels reflect the X-ray absorption coefficients of the corresponding voxels. The pixel size and number of images obtained by different CT devices are different. The size can be 1.0 × 1.0 mm and 0.5 × 0.5 mm; the number can be 256 × 256, which is 65536, or 512 × 512, which is 262144. Obviously, the smaller the pixels, the larger the number, and the more detailed the composition image, that is, the higher the spatial resolution. The spatial resolution of CT images is not as high as that of X-ray images. ^[1]

CT images are expressed in different gray levels, reflecting the degree of absorption of X-rays by organs and tissues. Therefore, like the black-and-white image shown in the X-ray image, the black shadow represents a low-absorption region, that is, a low-density region, such as a gas-containing lung; the white shadow represents a high-absorption region, that is, a high-density region, such as bone. However, compared with X-ray images, CT has higher density resolution, that is, higher density resolution. Therefore, although the difference in the density of human soft tissues is small, although the absorption coefficient is close to that of water, it can be contrasted and imaged. This is the outstanding advantage of CT. Therefore, CT can better display organs composed of soft tissues, such as brain, spinal cord, mediastinum, lung, liver, gallbladder, pancreas, and pelvic organs, etc., and show lesion images on a good anatomical image background. ^[1]

X-ray images can reflect the density of normal and diseased tissues, 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 can use the absorption coefficient of tissue to X-rays to indicate the degree of density, which has a concept of quantity. In actual work, the absorption coefficient is not used, but it is converted into a CT value, and the density is expressed by the CT value. The unit is Hu (Hounsfield unit). ^[1]

The absorption coefficient of water is 10, and the CT value is set to 0 Hu. The highest density of the bone cortex in the human body is the highest, the CT value is +1000 Hu, and the air density is the lowest, -1000 Hu. The CT values of different density and various tissues in the human body are between 2000 divisions of -1000 Hu to +1000 Hu. ^[1]

CT images are tomographic images, often cross-sections. To display the entire organ, multiple consecutive tomographic images are required. Through the use of image reconstruction procedures on CT equipment, it is also possible to reconstruct slice images of the coronal and sagittal planes, and the relationship between organs and lesions can be viewed from multiple angles. ^[1]

CT advantages and disadvantages

CT advantage

CT diagnosis has been widely used in clinic because of its special diagnostic value. ^[1] And with the development of technological level and computer technology, CT has developed rapidly. Multi-row spiral CT has been put into practical use to 320 rows, and various manufacturers are also studying more advanced flat-panel CT. The combined product of CT and PET, PET / CT, is widely used in clinical practice, especially in the diagnosis of tumors. ^[3]

CT disadvantages

CT equipment is relatively expensive, the inspection cost is high, and the diagnostic value of some parts, especially qualitative diagnosis, is limited. Therefore, CT examination should not be regarded as a conventional diagnostic method. It should be reasonable on the basis of understanding its advantages. Selection application. In addition, the diagnostic radiation dose of CT is larger than that of ordinary X-ray machines, so CT examination is not suitable for pregnant women. ^[1]

CT nuclear magnetic resonance

Computerized tomography (CT) can accurately detect small differences in density between various tissues on a transverse anatomical plane, and is an ideal inspection method for observing bone joints and soft tissue lesions. In the diagnosis of arthritis, it is mainly used to check the spine, especially the sacroiliac joint. CT is superior to traditional X-ray examination in that it has high density resolution and can also be used for axial imaging. Due to the high density resolution of CT, soft tissues, bones, and joints can all be seen clearly. In addition, CT can be used for axial scanning, and some traditional X-ray images with difficult to distinguish joints can be "prototype exposed" on CT images. For example, the sacroiliac articular surface is tilted and bent, and there is overlap of other tissues. Although the x-ray film of the sacroiliac joint may meet the requirements in most cases, sometimes X-ray examination finds sacroiliitis Difficult, CT examination can be done for patients with problems. ^[3]

Magnetic resonance imaging (MRI) is based on the interaction of radio waves and hydrogen nuclei in a strong magnetic field. When magnetic resonance was introduced, it quickly became an imaging tool useful in the diagnosis of many diseases, including the skeletal muscle system. The musculoskeletal system is best suited for magnetic resonance imaging because of its wide range of tissue density contrasts. In the diagnosis of bone, joint, and soft tissue lesions, magnetic resonance imaging has significantly higher soft tissue contrast than CT due to its imaging parameters several times greater than CT and a high degree of soft tissue resolution. Through its multi-directional planar imaging function, the use of high-resolution surface coils can significantly improve the imaging quality of various joint parts, and display subtle results that cannot be distinguished by other imaging examinations such as nerves, tendons, ligaments, blood vessels, and cartilage. . The disadvantage of magnetic resonance imaging in the bone and joint system is that it is not specific for the qualitative diagnosis of bone and soft tissue lesions, and the imaging speed is slow during the examination process. Patients' voluntary or involuntary activities can cause motion artifacts and affect diagnosis. ^[3]

X-ray photography, CT, and magnetic resonance imaging can be called troikas. The three organically combine to expand the scope of the current imaging examination and improve the diagnostic level. ^[3]

The main purpose of CT

Medical check

CT examination has higher diagnostic value for central nervous system diseases, and its application is widespread. Intracranial tumors, abscesses and granulomas, parasitic diseases, traumatic hematomas and brain injuries, cerebral infarction and cerebral hemorrhage, and spinal canal tumors and intervertebral disc prolapse are well diagnosed and the diagnosis is more reliable. Therefore, in addition to cerebral angiography, brain angiography is still used to diagnose intracranial aneurysms, vascular dysplasia and cerebrovascular occlusion, and to understand the blood supply arteries of brain tumors. Spiral CT scans can obtain relatively fine and clear vascular reconstruction images, that is, CTA, and can achieve three-dimensional real-time display, which is expected to replace conventional cerebral angiography. ^[3]

CT is also valuable in the diagnosis of head and neck disease. For example, intraorbital mass lesions, early sinus cancer, small cholesteatoma of the middle ear, destruction and dislocation of the ossicular bone, slight damage to the labyrinth of the inner ear, congenital abnormalities in the ear, and early detection of nasopharyngeal cancer. However, patients with obvious lesions and who can be diagnosed with plain radiographs do not need CT. ^[3]

For the diagnosis of chest disease, CT examination with the application of high-resolution CT has increasingly shown its superiority. Contrast-enhanced scans are usually used to determine whether the mediastinum and hilar have masses or enlarged lymph nodes, and whether the bronchi are narrow or obstructed. It is of great help in the diagnosis of primary and metastatic mediastinal tumors, lymph node tuberculosis, and central lung cancer . Interstitial and parenchymal lesions in the lung can also be better displayed. CT is more advantageous for plain radiographs, such as concentric, large vessels overlapping lesions. Lesions of the pleura, diaphragm, and chest wall can also be clearly displayed. ^[3]

CT examination of the heart and large blood vessels, especially the latter, is of great significance. The heart is mainly the diagnosis of pericardial lesions. Display of heart cavity and heart wall. The scan time is generally longer than the cardiac cycle, which affects the sharpness of the image and has limited diagnostic value. However, CT examination of coronary artery and heart valve calcification, large vessel wall calcification, and aneurysm changes can be well displayed. ^[3]

CT examination of abdominal and pelvic diseases is increasingly widely used, mainly for the diagnosis of liver, gallbladder, pancreas, spleen, peritoneal cavity and retroperitoneal space, and urinary and reproductive system diseases. Especially space occupying lesions, inflammatory and traumatic lesions. Gastrointestinal lesions are extravaginal invasion and metastasis to nearby and distant places. CT examination is also of great value. Of course, the condition of gastrointestinal lumen still depends on barium angiography and endoscopy and pathological biopsy. ^[3]

Industrial inspection

The development of modern industry makes CT play a significant role in non-destructive testing and reverse engineering.

The results of non-destructive testing of products using industrial CT show that industrial CT technology has a high detection sensitivity for various common defects such as stomata, inclusions, pin holes, shrinkage holes, and delamination, and can accurately determine the size of these defects Give its location in the part. Compared with other conventional non-destructive testing technologies, the spatial and density resolution of industrial CT technology is less than 0.5%, and the imaging accuracy is high. It is not limited by the type and geometry of the workpiece material. It can generate three-dimensional images of material defects. , Material uniformity, accurate measurement of micro-porosity, and overall micro-cracks, inclusions, pores, abnormal large grains and other defects detection has great research and application value. ^[5]

Security inspection

In addition to medical and industrial applications, CT equipment can also be used in the detection of security, air transportation, harbor transportation, large cargo container case devices, etc. ^[6]