A method that uses the phase change of the macroscopic transverse magnetization vector (Mxy) due to the flow to suppress the background and highlight the vascular signal. The phase encoding uses a bipolar gradient field to encode the flow, i.e., two gradient fields of identical size and duration but opposite direction are applied between the level-selected gradient and the readout gradient after excitation of the RF pulse. For the proton population of the stationary tissue, the effects of the two gradient fields exactly cancel, so that the Mxy phase change of the stationary tissue at the moment of knife TE is equal to zero. As for the mobile proton population, the Mxy phase change of the mobile proton population is preserved at the TE moment due to the change in position when the gradient field is applied twice, so there is a phase difference with the stationary tissue, and this difference is used to form a phase contrast. During the application of the bipolar gradient field, the phase change of the mobile proton population accumulation is correlated with its flow velocity, and the faster the flow, the more obvious the phase change, and the phase difference is used to display the vascular images, i.e., to obtain PC-MRA images. The maximum phase change that PCMRA can reflect is 180 degrees, and if it exceeds 180 degrees, it will be mistaken for a reverse phase change, thus creating the illusion of reverse blood flow. If the blood flow rate is 50cm/s and the selected flow rate code is also 50cm/s, the phase change of its flow proton is exactly 180 degrees and the strongest signal is obtained; if the selected flow rate code is 40cm/s, the phase change of the flow proton is more than 180 degrees and the blood flow will be mistaken as reverse and present low signal. However, if the flow rate encoding is significantly smaller than the actual flow rate, the phase change of the fluid proton population is small, and the phase contrast between the flow and the resting tissue is poor. Therefore the key to PC MRA is the setting of the flow rate encoding. For fast blood flow we often choose larger flow velocity coding values, 80-200 cm/s; for medium velocity blood flow we often choose 40-80 cm/s, and for slow blood flow we often choose 10 cm/s. Note: Only the flow protons along the flow velocity coding direction will produce phase change, if the vessel is perpendicular to the coding direction it will not be visible on the PCMRA. The operator can select the encoding gradient along any direction, such as level selection direction, frequency encoding direction, phase encoding direction or all 3 directions. When flow is present in each direction, the flow encoding gradient needs to be applied along all 3 directions for acquisition, but in 3 times the time as when one direction is used. PC MRA generally requires 3 basic steps, i.e., acquisition of imaging information, subtraction, and display of the image. Features: 1. The images are divided into amplitude images and phase images; 2. The signal intensity of amplitude images is only related to flow velocity and does not have flow direction information, the faster the blood flow, the higher the signal, but cannot provide quantitative values of flow velocity; 3. Phase images, also known as flow images, the signal intensity of blood flow is not only related to flow velocity and can provide quantitative information of flow velocity, but also has flow direction information, positive blood flow shows high signal, the higher the flow velocity, the stronger the signal. The higher the flow velocity, the stronger the signal; the reverse blood flow shows low signal, the higher the flow velocity, the lower the signal; the resting tissue shows medium signal.4.After using the subtraction technique, the background tissue has no phase change, and the signal is almost completely rejected.5.As the phase change of blood flow can only be reflected in the flow velocity encoding gradient field direction, in order to reflect the real situation of blood flow in the blood vessel, it is necessary to apply flow velocity in the dimensional direction, phase encoding direction and frequency encoding direction. In order to reflect the real situation of intravascular blood flow, it is necessary to apply the flow-rate encoding gradient field in the level direction, phase encoding direction and frequency encoding direction. The conventional PC MRA is an amplitude image that shows the blood flow signal and thus the vessel structure. Phase images are mainly used for quantitative analysis of blood flow direction, flow velocity and flow. Compared with TOF MRA, PC MRA has advantages 1. good background tissue suppression, which helps to display small vessels; 2. good for displaying slow blood flow, which is suitable for examination of veins; 3. good for displaying vascular stenosis and aneurysm; 4. can be used for quantitative analysis of blood flow. Disadvantages: 1, time is longer than TOF MRA; 2, image processing is complicated; 3, need to determine the coding flow rate in advance, the coding flow rate is too small, easy to appear the illusion of reverse blood flow; coding flow rate is too large, the phase change of blood flow is too small, the signal is obviously weakened. The methods are 2D, 3D, and cine PC MRA (Cine). Flowmetry technique (P239) clinical application: relatively few, used for 1, venous 2, cardiac and macrovascular flow analysis 3, cerebrospinal fluid flow analysis. TOF MRA is mostly used for the examination of arterial lesions.