Time of flight, based on the inflow enhancement effect of blood.TR shorter fast scrambled-phase GRE T1WI sequences are acquired, and the resting tissue within the imaging volume or level is saturated by repeated excitation with small magnetization vectors, which suppresses the resting background tissue, whereas the blood outside the imaging is not saturated by RF pulses, and when the blood inflows into the imaging volume or level it has a higher signal and a better contrast between it and the resting tissue. I. 2D TOF MRA Zhang Chengjun, Department of Radiology, Chaoyang Downtown Hospital, Liaoning Province, China Successive thin sweeps with layer thickness of 2-3 mm, and then post-processing of the original image, TR 20-30ms in 1.5T, shortest TE to reduce flow out-of-phase, and selection of RF pulses with a larger angle of 40-60 degrees to suppress signals from the background tissue. Advantages: 1, short TR and large inversion angle, good background suppression; 2, single-layer acquisition, less saturation of blood flow within the layer, which is conducive to the display of venous slow blood flow. 3, fast speed, single layer 1-5s. Disadvantages: 1, lower spatial resolution in the direction of the layer, voxels are larger, the flow out-of-phase is more pronounced, especially by the turbulence, and is prone to the corresponding artifacts. 2, post-processing is not effective. 3, Easy to voxel shadow distortion due to interlayer alignment error caused by original image distortion. Methods to improve the quality of 2D TOF MRA: 1. Sweep as thin as time and signal-to-noise ratio allow; 2. Keep the scanning level perpendicular to the direction of blood flow; 3. Try to use the technique for vessels that are heading in a relatively straight direction; 4. Cardiac gating to reduce beating artifacts. Second, three-dimensional TOF MRA TR 25-45ms, TE 6.9ms (equivalent to inverse phase images to minimize the signal of fat), excitation angle 25-35 degrees. Advantages: 1, high spatial resolution, especially in the direction of the level, the original image layer thickness can be <1mm; 2, voxels are small, flow out of phase is relatively light, and less affected by turbulence. 3, high signal-to-noise ratio. 4, good post-processing. Disadvantages: 1, the saturation of blood flow is more obvious, which is not conducive to the display of slow blood flow; 2, in order to reduce the saturation effect of blood flow needs to narrow the excitation angle, the background suppression effect is not as good as 2D TOF MRA; 3, the scanning time is relatively long. Magnetization transfer technology can be added to suppress the background image, but the time will be extended. Three-dimensional TOF MRA of blood flow saturation phenomenon should not be ignored, saturation phenomenon has two main effects 1, slow blood flow signal is obviously weakened, 2, the signal of the distal side of the blood flow in the volume is obviously weakened, but the following strategies can be used: ① narrowing the excitation angle, which will inevitably result in a poor suppression of the background tissue; ② the use of TONE or RAMP ③ overlap more than one thin-layer block acquisition ④ Sliding Ky interlacement acquisition technique ⑤ inverse Blood flow acquisition. Clinical application 1, blood vessel travel. The direction of travel is relatively straight, such as the neck and lower extremity blood vessels ---- two-dimensional, while the travel of tortuous blood vessels such as the cerebral artery is three-dimensional effect. 2, blood flow velocity. Speed such as most of the arteries, especially the head and neck arteries more three-dimensional, while the slow blood flow speed of the veins more two-dimensional. 3, the length of the target blood vessels. Small with three-dimensional, length of large vessels such as lower extremity vessels with two-dimensional. Clinical: Cerebral arteries ---- three-dimensional; carotid arteries - two-dimensional or three-dimensional; lower extremities ---- two-dimensional; veins - two-dimensional. Two-dimensional speed, abdominal blood vessels, especially veins, can be acquired in segments with multiple breath-holds. MRA acquired with TOF technique shows both arteries and veins, but sometimes overlap. Saturation bands are added upstream of the blood flow to selectively display arteries or veins. Considerations for analyzing TOF MRA: 1. It is normal if it is smooth and neat, with no stenosis.2. Artifacts of stenosis can occur ---- out of phase due to turbulence, vessel turns ---- siphon segments of the internal carotid artery, and vessel bifurcations ---- bifurcation of the internal and external carotid arteries.3. The degree of stenosis is often exaggerated because stenosis is susceptible to turbulence, which can result in loss of signal.4. Aneurysms are likely to be missed. Because there is much turbulence in the aneurysm, resulting in signal loss.5. Care should be taken to observe the thin-layer raw image.6. Enhancement when an artifact is considered.