Magnetization Transfer (MT – Magnetization Transfer) is a selective tissue signal inhibition technique. Principle: Water molecules in the human body exist in two different states, freely moving water molecules (free water) and water molecules bound to macromolecular proteins (bound water), which we also call the free pool and bound pool, respectively. Since bound water is bound to macromolecules with T2 values less than 1 ms (usually only tens of microseconds), conventional MRI techniques can only capture the signal of free water, but not that of bound water. The pre-saturation pulse of MT is a pulse that deviates from the resonant center frequency of the tissue, and its center frequency deviates from the resonant frequency of free water by 1000-2000 Hz (generally 1200 Hz, in some equipment can be set in a certain range by itself). Set the deviation value of MT saturation pulse center frequency). When such an MT pre-saturation pulse is applied to the tissue, only the bound water is excited and saturated in the tissue, while the free water is almost unaffected. Since bound water does not produce a signal in conventional MRI, the saturation of bound water by itself does not affect the MR signal of the tissue. However, the protons in the binding water and the protons in the free water are always in a dynamic equilibrium with rapid chemical exchange, so that the saturated binding water will transfer the energy obtained from the MT pulse to the protons in the free water, resulting in the saturation of the free water, and when the real imaging pulse is applied, this part of the saturated free water will not be able to produce the signal, which eventually leads to the attenuation of the tissue signal. This process is actually the transfer of the saturated magnetized state from the bound water to the free water, and is therefore referred to as magnetization transfer or magnetization transfer. Clinical application: When MT saturation pulses are applied to tissues before the real imaging pulse, tissues rich in macromolecular structures such as brain and muscle, the MT effect is obvious and the tissue signal is significantly attenuated; while in tissues containing few macromolecules, such as cerebrospinal fluid, blood, bone marrow and adipose tissue, the MT effect is weak and the tissue signal is only slightly attenuated. In other words, we can use MT technique both to increase the tissue contrast and to indirectly reflect the protein content and its changes in tissues. 1, for increasing the contrast of TOF MRA TOF MRA technique uses blood inflow enhancement effect to create contrast between flowing blood and resting tissues, so the suppression of background tissue signal is very important. Using 3D TOF MRA technique, the background tissue signal is often not sufficiently suppressed, and small diameter vessels cannot be displayed due to poor contrast between them and resting tissues. With the MT technique, the signal from the resting tissue is better suppressed, while the blood signal is minimally attenuated, 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, and after applying the MT technique, the TR needs to be extended by 10~20ms, thus the TA is extended accordingly. 2, for enhanced scanning. MT technique can suppress the signal of the tissue, but MRI contrast agent can shorten the T1 value of the 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 MT technique was applied, 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. One study found that the enhancement of a single-dose brain enhancement scan image with the MT technique applied was close to that of a triple-dose enhancement scan image without the MT technique applied. It should be noted that some lesions with MT technique may show high signal due to their relative signal increase before contrast injection, which needs to be noted when evaluating the enhanced images after MT technique is applied. It is best to perform a flat scan with the MT technique applied before the enhancement scan for control. 3. Application of magnetization transfer rate. The magnetization transfer technique can also indirectly or even semi-quantitatively reflect the change of macromolecular protein content in the tissue. Commonly used index of magnetization transfer rate (MTR): MTR=(M0-Ms)/M0*100% (M0 is the signal intensity value on the image without magnetization transfer, Ms is the signal intensity value on the image after adding the magnetization transfer pulse) By this index, it can objectively reflect the changes of structural integrity within the brain tissue. MTR is currently used mostly in the study of multiple sclerosis (MS) and Alzheimer’s disease (AD). MTR is significantly reduced in MS lesions compared to normal brain white matter, with an average of about 25% compared to 40% in normal brain white matter. Studies of white matter in MS patients that exhibited normal signal on T2WI also found a significant reduction in MTR in these white matter brains as well. A study of patients with early AD found that the MTR of both the hippocampus and the parahippocampal gyrus was significantly reduced in AD patients compared to controls.