What are the characteristics of X-ray computed tomography?

CT is the abbreviation of ComputedTomongraphy, which is called Computed X-ray Tomography in Chinese. CT has become an indispensable means of non-invasive X-ray examination.

The basic principle of CT imaging is that X-ray beams are irradiated from multiple directions along the selected fault plane of a certain part of the human body, and some X-rays are absorbed by tissues and accepted by detectors, and the transmitted X-rays are measured. After digital comparison, the absorption coefficient of tissue per unit volume in this plane is calculated, and then the image is reconstructed.

Compared with conventional X-ray examination technology, CT examination technology has a high sensitivity of 100 times, especially for soft tissues with similar density. That is to say, the resolution of CT on tissue density is higher than that of X-ray, but the spatial resolution is generally not necessarily higher than that of X-ray imaging, so it can not completely replace X-ray examination at present. In addition, CT technology is developing towards high speed and simplification.

At present, CT still has the following limitations: ①CT equipment is huge and expensive, and the examination cost is expensive.

② Contrast-enhanced imaging may cause allergy and even death. According to the domestic statistics, the fatal reaction is 0.5‰ for adults and 0.1‰ for those under 20 years old. The nonfatal reaction was 4.04‰.

③ The examination results of some parts are not good, such as posterior cranial fossa lesions, digestive tract lesions, cell lesions, etc.

④ Receive a certain amount of X-rays. Normal CT value of human tissue (with water as reference standard).

Normal CT value of human tissue

Air-1000Hu fat -50~-90Hu water -20~+25Hu soft tissue 20~60Hu blood (fresh) 20~60Hu Chen Gui blood 60~80Hu blood clot 60~80Hu bone and calcification 80~ 1 000Hu white matter C-25 ~ 34Hu gray matter C-30 ~ 40hu+32 ~ 43Hu lung -500~-0~70Hu kidney C-20 ~ 60hu+60 ~ 120 Hu uterus 40~80Hu bladder (full of urine) 5 ~/kloc-

MRI is the abbreviation of MagneticResonanceImaging, which is called "magnetic * * vibration or magnetic * * * vibration imaging" in Chinese. It used to be called "nuclear magnetic * * * vibration", and it can also be called * * * yoke photography. MRI is a novel imaging method, which has the characteristics of strong tissue contrast, high spatial resolution, multi-plane anatomical structure display, no radiation damage and so on, and is particularly sensitive to physiological changes.

In recent years, medical imaging technology has developed rapidly, and there are four main imaging diagnosis methods, including nuclear medical imaging methods based on functional examination (γ photography and single photon CT) and three methods based on morphology: X-ray (including CT), ultrasound and MRI. Especially because of the appearance of MRI, the level of image diagnosis has reached a new stage, and the clinical application of MRI is more and more extensive, which can complement the above examination methods, and some parts even exceed CT examination.

However, MRI examination is prohibited for those with pacemakers and nerve stimulators, those who have undergone aneurysm surgery and intracranial aneurysm clipping, those who have undergone heart surgery with artificial heart valves, those who have metal foreign bodies in their eyeballs or those who have implanted metal prostheses in their inner ears.

Patients with various metal implants, pregnant women, critically ill patients who need to use life support systems, epileptic patients and claustrophobic patients should be treated with caution during the examination.