What do we know about magnetic resonance signals?

It is a new high-tech imaging examination method in recent years, and it is a new medical imaging diagnosis technology that has been applied to clinic since the early 1980 s, without ionizing radiation (radioactive) damage; No bone artifacts; Multi-directional (horizontal axis, coronal plane, sagittal plane, etc. ) and multi-parameter imaging; High soft tissue resolution; The unique advantages such as vascular structure can be displayed without using contrast agent. Therefore, it is called another major development in the field of medical imaging after X-ray and CT. The principle of magnetic resonance imaging is abstract and complex, and the professional theory is difficult to understand. Generally speaking, imaging technology is to collect the signals generated by human tissues (mainly water accounting for 65%) under the action of magnetic field and reconstruct images. T 1 weighted image (T 1WI) and T2-weighted image (T2WI) are often mentioned in MRI reports and are the most basic elements in image diagnosis. T 1 and T2 are the physical change characteristics of tissues after receiving a series of pulses in a certain time interval. Different tissues have different T 1 and T2, which depends on the reaction of hydrogen protons in tissues to RF pulses applied by magnetic fields. By setting the imaging parameters (TR and TE ), TR MRI, TR is the repetition time and interval time of RF pulses, and TE is the echo time, that is, the time from the application of RF pulses to the reception of signals, all in milliseconds (ms), and images (T 1 weighted image or T2-weighted image) representing tissue characteristics can be made. T 1 weighted image (T 1WI) highlights the difference of tissue T 1 relaxation (longitudinal relaxation). T 1WI mainly reflects the difference of longitudinal relaxation of tissue. T2-weighted images (T2WI) highlight the differences in T2 relaxation (lateral relaxation) of tissues. T2WI mainly reflects the difference of transverse relaxation of tissues. How to distinguish T 1 weighted image from T2 weighted image? The values of TE and TR in the observed image can be distinguished (Figure 1): TE can be 20ms shorter and 80ms longer, and TR can be 600ms shorter and 3000+ms longer. T 1WI weighTEd images have shorTEr te and TR values, while T2WI weighted images have longer te and TR values. Short T 1 and long TR are proton-weighted images. Knowing the signal characteristics of water and fat in graph 1 is helpful to distinguish T 1 weighted images from T2-weighted images, which is especially valuable when the images do not display the characteristic TE and TR values. Observe the structure of fluid such as ventricle, cerebrospinal fluid and bladder. If the liquid is bright (white), this is a T2-weighted image, and if the liquid is dark (black), this is a T 1 weighted image. Specifically, the signals (colors) of ventricles and cisterns were observed by craniofacial MRI (Figures 2 and 3); Observe the signal of cerebrospinal fluid around spinal cord in cervical, thoracic and abdominal spinal canal (Figure 4); Observe bladder signal in pelvis (Figure 5); Observe the signal of a small amount of joint fluid in the joint space of limbs (Figure 6); Signals of gallbladder and stomach can be observed in the abdomen (Figure 7). If the liquid is bright, other structures are not like T2-weighted images, and TE and TR are short, which may be gradient echo images. Figure 2, Figure 3, Figure 4, Figure 5, Figure 6 and Figure 7 generally think that the high signal on T 1 weighted images is mostly caused by bleeding or adipose tissue. However, recent studies show that T 1 weighted image signals can still be seen in a variety of intracranial lesions, including tumors, cerebrovascular diseases, metabolic diseases and some normal physiological conditions. T 1WI observation and anatomy were good. T2WI is helpful for observing lesions and sensitive to bleeding. There are relatively few artifacts (but patients are prone to exercise because of the long imaging time). The imaging speed is slow. MRI can show lesions that CT can't, and it is superior to CT in showing diseases of brain, spine and spinal cord. It can display the structure of blood vessels without using contrast agent, so it is unique in distinguishing blood vessels, masses, lymph nodes and blood vessel structures. It also has several times higher soft tissue resolution than ct, and can sensitively detect the change of water content of tissue components, so it is often more effective and earlier to find lesions than CT.