I. Basic principles of magnetization transfer technique For general tissues, the object of mr imaging is actually the protons in water molecules. Water molecules have free water and bound water. The so-called free water is not attached to the protein molecules, and the free movement of water molecules sufficiently free; bound water is attached to the protein, its natural movement is restricted water molecules, that is, the water molecules of the protein hydration layer. Protein molecules and proton feed frequencies in bound water have a wide range and short t2 values, so there is almost no direct contribution to the signal of mr images. mr imaging generally uses the proton feed frequency in free water as the center frequency, if we apply a deviation from the center frequency of about 1000~1200hz to the tissue before the mr imaging sequence (which can be gre sequence or se sequence) saturation pulse, then the protons in the free water will not be excited, while the protein molecules and the protons in the bound water will be excited and gain energy. The energy received by the protein molecules and protons in the bound water from the RF pulse will be transferred to the free water around them, and we call this energy transfer as magnetization transfer. Due to the magnetization transfer, the free water that gained energy will be saturated, and when the mr imaging real RF pulse comes, this part of water molecules will no longer be able to receive energy, and the unsaturated free water can only be excited. Almost all kinds of tissues contain a certain amount of protein and bound water, due to the application of mt pre-pulse and the existence of mt phenomenon, the free water in these tissues will be saturated to different degrees, and therefore the signal intensity of the tissues will be reduced to different degrees. The amount of protein and bound water in each tissue is different, and the degree of signal intensity attenuation due to the mt effect will also differ, and this contrast due to the magnetization transfer phenomenon is called magnetization transfer contrast (mtc). After the application of mt pre-pulse, the signal intensity of normal skeletal muscle is attenuated by about 60%; brain white matter by about 40%; brain gray matter by about 30%; and blood by about 15%. In the early stage of some diseases, the free water content in some lesions does not change much, so there is often no obvious signal abnormality on conventional t1wi and t2wi, but if there is a difference in protein and bound water content between the lesion and normal tissue, it is possible to detect the lesion using mt technique. Second, the clinical application of mt technology at present mt technology is mostly used in the neurological system clinically, mainly in the following aspects. (a) for tof mratof mra technique uses blood inflow enhancement effect to create contrast between flowing blood and resting tissue, so the suppression of background tissue signal is very important, using conventional tof mra technique, the background tissue signal is often insufficiently suppressed, and small diameter vessels cannot be displayed due to poor contrast between them and resting tissue. With the mt technique, the signal of the resting tissue is better suppressed, and the blood signal is attenuated to a small extent, thus increasing the contrast between the resting tissue and the blood and allowing small vessels to be clearly displayed. However, the mt pre-pulse needs to occupy a period of time in the tr interval, so after applying the mt technique, the tr needs to be extended by 10~20ms, and thus the scan time is extended accordingly. (ii) For enhancement scanning mt technique can suppress the signal of tissue, but mri contrast agent can shorten the t1 value of tissue, and its short t1 effect acts on free water, which is not related to the suppression of tissue signal by mt technique. After the application of the mt technique, the signal of the enhanced tissue was not significantly attenuated, while the signal of the unenhanced tissue was suppressed, thus increasing the contrast between the two and allowing some slightly enhanced tissues to be better displayed. It has been found that the enhancement of a single-dose brain enhancement scan with the mt technique applied is close to that of a triple-dose enhancement scan without the mt technique applied. It should be noted that some lesions with the mt technique may show increased relative signal and high signal without contrast injection, and this should be noted when evaluating the enhanced images after the mt technique is applied. It is best to perform a flat scan with the mt technique applied before performing an enhanced scan with the mt technique applied for control. (iii) Application of magnetization transfer ratio The magnetization transfer ratio (mtr) can be calculated by performing mr scans without and with the mt technique applied and measuring the signal intensity values at the same site using the region of interest, while keeping the other imaging parameters identical. mtr = (si -simt)/si, where si indicates the signal intensity of the tissue on the image without the mt technique applied, and simt indicates the signal intensity of the tissue with the mt technique applied. The mtr images can also be obtained by calculating all images using a computer. mtr is currently used mostly in the study of multiple sclerosis (ms) and azygosheim’s disease (ad). It has been found that the mtr of ms lesions is significantly reduced compared to normal brain white matter, averaging about 25% (normal brain white matter is about 40%). Studies on white matter of the brain that showed normal signal on t2wi in ms patients also found that the mtr of these white matter was also significantly reduced, which shows that the examination of mtr is more sensitive than conventional mri for ms. A study of patients with early ad found that mtr in both hippocampus and parahippocampal gyrus was significantly decreased in ad patients compared to controls.