The blood has a long T1 value of about 1200 ms at 1.5T field strength, and therefore presents a relatively low signal. Due to the long T2 value of blood, it may show high signal in T2WI.
MRA provides not only morphological information about the blood vessels, but also quantitative information about the direction, velocity and flow of blood. All MRA techniques utilize or target some aspect of the characteristics of blood flow for imaging.
I. Common forms of blood flow.
Most MRA techniques utilize the flow effects of blood for imaging. Some MRA techniques, even if they do not utilize the flow effects of blood for imaging, still have flow characteristics that affect the blood signal.
1.Flat flow (ideal state, does not actually exist).
2.Laminar flow
3.Turbulent flow (vortex formation)
Blood flow in blood vessels is usually laminar and turbulent at the same time or alternately. NR=ρDV/η (NR is Reynolds number, representing the ratio of inertial force and viscosity, ρ is blood density, D is vessel diameter, V is the average velocity of blood flow, η is blood viscosity) NR<2000 tends to be laminar; NR>3000 tends to be turbulent. The larger the vessel diameter, the faster the flow velocity and the lower the viscosity, the more likely it is to lead to turbulent flow. In addition, vessel stenosis, vessel wall roughness, vessel bifurcations, and vessel turns or tortuosity can lead to turbulence.
The blood flow signal depends on the blood flow form, blood flow direction, blood flow velocity, pulse sequence and imaging parameters.
Second, the blood flow that exhibits low signal
1, flow-space effect: the direction of blood flow is perpendicular to the scan level (the longer the TE/2, the more obvious the flow-space effect).
2, the signal attenuation caused by the movement of the proton group position within the scan level. (The scanning level is parallel to the blood flow direction.) The 180 degree pulse can reject the proton out-of-phase caused by the constant inhomogeneity of the main magnetic field. Although the blood flow within the scanning level remains within the scanning level for TE/2 time period, the position of the proton population at the level changes compared to that at the 90 degree pulse, and the main magnetic field environment in which it is located changes. 180 degree pulse cannot correct the out-of-phase proton population caused by the main magnetic field inhomogeneity, so the signal of the flowing proton population is attenuated compared to that of the stationary tissue.
3, Out-of-phase caused by the difference in laminar flow velocity.
4, out-of-phase caused by molecular rotation due to laminar flow.
5.Turbulent flow. It causes irregular motion of blood flow in direction and speed, and thus the proton population within the voxel will be out of phase and the MR signal will be significantly attenuated. Turbulence is likely to occur at the distal side of stenoses, vascular bifurcations, vascular turns, aneurysms, and other sites.
6. Long T1 characteristics of blood flow. In some TR and TE very short ultra-fast T1WI, flow has little effect on the signal of blood, and it is mainly its T1 value that determines the signal of blood. The T1 value of blood is very long, about 1200ms at 1.5T field strength, so it presents relatively low signal.
Third, the blood flow that exhibits high signal
1.Inflow enhancement effect. If the blood flow is perpendicular or basically perpendicular to the scanning level, while the selected TR is relatively short, so that the proton population in the resting tissue within the level does not have enough time to occur sufficient longitudinal relaxation, saturation phenomenon occurs, and thus the signal attenuation occurs. For the blood flow, there is always an unexcited proton population flowing into the scanning plane, which produces a stronger signal after excitation by the RF pulse and shows a high signal compared to the resting tissue. The inflow enhancement effect is often seen in gradient-echo sequences and can also occur in spin-echo sequences. In two-dimensional multilevel scans, the inflow effect of blood flow is strongest within the first layer in the upstream direction of blood flow with high signal, while the signal gradually decreases within the other layers in the direction of blood flow due to the gradual increase of the saturated proton population in the blood flow.
2. Diastolic pseudo-gating phenomenon. The blood flow velocity at the end of diastole is very slow and little affected by the flow, mainly affected by the blood T1 value and T2 value, which can show an increased signal or even present a high signal. tr coincides with the cardiac cycle.
3.Even-echo effect. when SE multi-echo imaging, blood flow shows low signal on the image of odd echoes and high signal on the image of even echoes. It is also called even-echo phase recombination. It is well known that the incoming frequency and phase of protons are related to the magnetic field strength, and a change in the position of protons in a gradient magnetic field will cause a change in the incoming frequency and phase. If the proton population moves along the phase encoding direction, the even linearly changing gradient magnetic field can cause the proton population whose phase has been discrete to reunite in phase again, thus appearing as a higher intensity blood flow signal. The even-echo effect is often seen on hepatic SE multi-echo sequences, e.g., the hepatic veins and intrahepatic portal branches exhibit low signal in the first echo (PD) and high signal in the second echo (T2WI).FSE, because it uses continuous 180-degree pulses to produce echo chains of varying length, actually half of the echoes in the echo chain belong to odd echoes and the other half are even echoes, so that using FSE for T2WI, there will also be even-echo effect, such as in liver FSE T2WI, the hepatic vein or intrahepatic portal vein branches can often show high signal.
4, Very slow blood flow. The blood flow in vessels such as paravertebral plexus or pelvic plexus is very slow, and the out-of-phase or flow-void effect caused by flow is not obvious, then the signal of blood flow in these vessels has little relationship with the flow itself, but mainly depends on the T1 and T2 values of blood, which can show high signal in T2WI due to the long T2 value of blood.
5. Blood flow exhibits high signal on gradient echo sequences. Unlike the SE sequence, the echo of the GRE sequence is generated by the switching of the gradient field, and the switching of the gradient field does not require the level selection, so the blood flow subjected to small-angle excitation to generate macroscopic transverse magnetization vector can still feel the switching of the gradient field and generate the echo, even though it leaves the scanning level, as long as it does not exceed the effective range of the effective gradient field and the acquisition coil, and thus does not show a flow null but presents a relatively high signal intensity.
6.Balance-SSFP sequence using ultra-short TR and TE shows high signal, TR<5ms, TE<2ms, even for fast arterial blood flow, the flow (including laminar and turbulent flow) has little effect on the image. On this sequence, the signal intensity of the tissue depends on T2/T1, so the longer T2 value of blood is shown, and both arterial and venous blood present high signal.
7. Use of contrast agents. The gradient echo T1WI sequence of ultrashort TR and ultrashort TE, the signal of blood is little affected by the flow, but mainly depends on the T1 value, because the TR is very short, the general tissue presents lower signal due to saturation, then the T1 value of blood is shortened by the intravenous mass injection contrast agent (significantly shorter than the T1 value of fat), and this time the blood is high signal.