When people go to the hospital, they often need to do CT or MRI, so what is the difference between these two tests? Why do doctors sometimes let me do CT, but sometimes let them do MRI? CT and MRI MRI are two very different examination methods MRI is the abbreviation of MagneticResnaneIamge, Chinese for Magnetic Resonance Imaging. MRI is to place the human body in a powerful magnetic field, through radiofrequency pulse excitation of hydrogen protons in the human body, nuclear magnetic resonance, and then accept the MRI signal from the protons, after the gradient field three directions of positioning, and then CT is the abbreviation of ComputerTomography, which is a computer-controlled X-ray tube and detector that rotates around a part of the body and uses the different densities of human tissues that appear under the X-ray for contrast imaging. MRI can form various images by different scanning sequences, such as T1-weighted image, T2-weighted image, proton density image, etc. There are also water imaging, water suppression imaging, fat suppression, diffusion imaging, spectral imaging, functional imaging, etc. CT can only discriminate tissues with density difference, and the soft tissue resolution is not high while MRI has better resolution of soft tissues, such as muscle, fat, cartilage, and fascia with different signals. Therefore, CT and MRI are very different examination methods. MRI is very effective in examining common cranio-cerebral diseases such as intracerebral hematoma, extracerebral hematoma, brain tumor, intracranial aneurysm, arteriovenous malformation, cerebral ischemia, intravertebral tumor, spinal cord cavitation and spinal hydrocele, etc. It is also effective in diagnosing posterior lumbar intervertebral disc protrusion and primary hepatocellular carcinoma, etc. It does not produce artifacts in CT detection, has no ionizing radiation, and has no adverse effects on the body. MRI also has shortcomings. The spatial resolution of MRI is not as good as that of CT, and patients with pacemakers or certain metallic foreign bodies cannot be examined with MRI. CT is more superior for the display of parts that are more difficult to show on plain film examination, such as concentric and large overlapping vascular lesions. Also CT is increasingly showing its superiority for the diagnosis of chest diseases with the application of high resolution. Contrast-enhanced scans are usually used to clarify the presence or absence of masses or enlarged lymph nodes in the mediastinum and hilum, and the presence or absence of bronchial stenosis or obstruction, and play a major diagnostic role in the diagnosis of primary and metastatic mediastinal tumors, lymph node tuberculosis, and central lung cancer. Interstitial and parenchymal lesions within the lungs can also be better demonstrated. Pleural, diaphragm and chest wall lesions can also be clearly displayed. With the development of technology CT is of great importance for the examination of the heart and blood vessels. In the heart, the diagnosis of pericardial lesions is the main concern. The heart cavity and the wall of the heart can be displayed. In the vascular area, calcification of coronary arteries, calcification of large vessel walls and aneurysm changes can be well shown by CT examination. Therefore the two examinations should complement each other, which is why sometimes CT is done after MRI, or MRI after CT.