Also called homo/inverse phase imaging, it is now being used more and more widely. We already know that the signals from MRI of the human body originate from two main components: water and fat. Hydrogen protons in water molecules are chemically bonded with O-H bonds, while hydrogen protons in fat molecules are chemically bonded with C-H bonds. The difference in the distribution of the electron cloud around the hydrogen protons in these two structures results in the hydrogen protons in the water molecule feeling a slightly higher intensity of the magnetic field, which ultimately results in the hydrogen protons in the water molecule advancing at a slightly faster frequency than those in the fat molecule, with a difference of 3.5 ppm, which corresponds to 150 Hz/T. This difference in the advancing frequency increases as the field strength is increased.1.5 T, the hydrogen protons in water molecules advance at a slightly faster rate than those in fat molecules.2 The difference is 3.5 ppm, equivalent to 150 Hz/T. This difference in the advancing frequency is greater as the field strength increases. Hydrogen protons have an incoming frequency 225 Hz faster. Current clinical chemical shift imaging techniques mostly use 2D scrambled phase GRE T1WI sequences, where different TEs are selected to obtain positive and negative phase images. In-phase TE = 1000ms ÷ [150Hz/T × field strength] Inverse-phase TE = In-phase TE ÷ 21.5T, in-phase TE = 1000 ÷ (150 × 1.5) = 4.4ms Inverse-phase TE = 2.2ms (understood as: 2.2ms of time the hydrogen protons in the water molecule go around, while the hydrogen protons in the fat molecule go around half a time —– Inverse-phase. After another 2.2ms, i.e. 4.4ms, the water goes two turns and the fat goes one turn —– in the same phase). In practice, the closer the selected TEs are, the better the results. It is desirable for chemical shift imaging to acquire in-phase images in the same sequence for comparison. The in-phase image is actually a normal scrambled-phase GRE T1WI, and the in-phase image can be compared with the in-phase image to make a preliminary judgment of whether the tissue or lesion contains lipid and its approximate proportion. At present, in the new MRI instrument above 1.5T, using the scrambled GRE T1WI sequence, the dual echo technique can be used to obtain the same anti-phase image in the same scan at the same time, and the images obtained are more comparable. Chemical shift imaging can also utilize other sequences such as Balance-SSFP sequence. 1, inverse-phase image characteristics: ① water and fat mixed tissue signal significantly attenuated. ② Pure fat tissue signal is not significantly attenuated. Such as subcutaneous fat, mesentery, omentum and so on. ③Hook edge effect. The edge of the surrounding organs rich in fat tissue will appear as a black line, outlining the outline of the organs. Because the signal of the general organs mainly from water molecules, and the signal of the surrounding adipose tissue mainly from fat, so in the inverse image, the organs and the surrounding adipose tissue, the signal drop is not obvious, but in the interface between the two pixels at the same time sandwiched between the organs (water molecules) and the fat, and therefore the signal on the inverse image is significantly reduced, thus appearing to outline the edges of the effect. 2, Clinical application of chemical shift imaging technology. ① Differential diagnosis of adrenal lesions. Because adrenal adenomas often contain lipids, the inverse phase is significantly reduced, with a sensitivity of 70~80% and specificity of 90~95%. ② Diagnosis and differential diagnosis of fatty liver, the sensitivity exceeds that of conventional MRI and CT. ③ Determination of the presence of steatosis in focal liver lesions. Because those who develop steatosis in focal liver lesions are mostly hepatocellular adenoma or highly differentiated hepatocellular carcinoma. ④ It helps in the diagnosis and differential diagnosis of other lipid-containing lesions such as renal or hepatic vascular smooth muscle lipoma. It is important to note that the chemical shift imaging technique itself does not distinguish whether lipids are intracellular or extracellular. Therefore, inverse phase signal attenuation does not indicate that the cell contains lipids. IV.Dixon technique. The use of in-phase and anti-phase image, but also can produce a separate “water” or “fat” signal image. W: water signal intensity F: fat signal intensity I with: in-phase signal intensity I anti-: anti-phase signal intensity, then: I with = W + FI anti- = W – F so that the two equations, respectively. W-F In this way, the two equations are added and subtracted, resulting in: W = (I with + I against)/2 F = (I with – I against)/2 can be carried out separately for water or fat imaging, also known as water-fat separation imaging, called Dixon technique. Not only can it be used for scrambled phase GRE T1WI sequences, but also SE or FSE sequences can be employed. With SE or FSE sequences, the Dixon technique makes it easier to achieve fat suppression at low field strengths. The SE sequence uses a 180-degree focusing pulse to acquire the echo signal. The 180-degree focusing pulse reverses the phase difference between the water proton and the fat proton before the pulse is applied, and this phase difference disappears completely at the moment of echo generation after the pulse is applied (TE), so that with the conventional echo acquisition technique of SE or FSE sequences, no matter how the TE is selected, the in-phase image is obtained and an inverse-phase image cannot be obtained. Inverse-phase images are obtained. The prerequisite for the 180-degree focusing pulse to eliminate the phase difference between water and fat protons is that the readout gradient fields before and after the 180-degree focusing pulse must cancel each other out when the echo reaches its peak. If the position of the readout gradient field is kept unchanged and the 180-degree focusing pulse of the SE sequence is shifted forward by f/2ms, a spin echo will be generated at a point fms earlier than the original TE, and this spin echo will only experience the readout gradient field before the 180-degree pulse but not after the 180-degree pulse, and thus the 180-degree focusing pulse will not be able to remove the phase difference between the water proton and the fat proton. difference, so this echo will be an in-phase echo; and reaching the original TE moment, the readout gradient field after the 180-degree pulse comes into play and counteracts the readout gradient field area before the 180-degree pulse, and thus another in-phase echo will be generated, and the in-phase and in-phase images can be reconstructed using these two echoes. According to the Dixon formula described earlier, by adding these two images and dividing by 2, an image of water will be obtained , and by subtracting the two images and dividing by 2, an image of fat will be obtained. In low-field machines, SE or FSE can be easily performed using the Dixon method to obtain water-fat separation images, with more applications in bone and joint systems.