Free-sensing decay-like sequences : saturation recovery sequences and inversion recovery sequences for FID acquisition. Spin-echo sequences : Tissue T1 relaxation and T2 relaxation are intrinsically linked, but are two relatively independent and distinct processes, with significant differences in the mechanisms, manifestations and rates of their occurrence. (Occurring at the same time but relatively independent, they are two different things, do not think together) T1 relaxation needs to transfer energy from inside the proton group to other molecules outside the proton, which takes longer time. In contrast, the energy transfer of transverse relaxation occurs within the proton population, i.e. between protons and protons, and takes less time. Therefore, the T1 values of all tissues are much longer than their T2 values, generally hundreds to thousands of milliseconds for tissues, while the T2 values are only tens to more than a hundred milliseconds, and a few can reach hundreds of milliseconds. Generally, as the main magnetic field strength increases, the T1 value lengthens and the T2 value shortens. In clinical work, we can choose the TR and TE of the SE sequence within a certain range according to the need, TE is actually the waiting time between the excitation of the 90 degree RF pulse and the generation of the spin echo, TE is very short, transverse relaxation has not yet had time to occur, the acquired signal does not carry T2 information; if TE is very long, transverse relaxation has been completed, the acquired signal also does not carry T2 information; choose The selection of a suitably long TE allows the T2 relaxation of the tissue to have an impact on the contrast of the image. TR is actually the waiting time from one 90-degree pulse excitation to the next 90-degree pulse excitation, during this waiting process the echo signal has been acquired, and it is necessary to wait some more time before applying the next 90-degree pulse, (it can be understood that waiting is the T1 relaxation process, because T1 relaxation takes a long time) If the waiting time is long (choose a long TR), the next 90-degree pulse excitation When the macroscopic longitudinal magnetization vector has been fully recovered, the macroscopic transverse magnetization vector generated when the 90-degree pulse is given again does not carry information about the difference of T1 relaxation of different tissues, then the T1 relaxation of tissues will not have an effect on the image contrast, that is, a very long TR can eliminate the effect of T1 relaxation on the image contrast. If the TR is very short and all tissues have not had time to undergo T1 relaxation, then there is not enough macroscopic longitudinal magnetization vector for the next 90-degree pulse excitation, and the macroscopic transverse magnetization vector will not be generated after the 90-degree pulse excitation, and the coil will not detect the echo signal. If the TR is suitable, due to the different T1 relaxation rate of each tissue, the macroscopic longitudinal magnetization vectors recovered by different tissues at the next 90-degree pulse excitation will be different, and the macroscopic transverse magnetization vectors generated by different tissues after 90-degree pulse excitation will be different, and the acquired MR signal will carry the T1 relaxation information of the tissues. The T1-weighted image will be T1 all the time without considering T2 relaxation. T1 ….T1…. .T1…. T1…T1-weighted and T2-weighted are independent and do not interfere with each other, T1 is T1 and T2 is T2. By adjusting the TR and TE of the SE sequence, the T1 relaxation and T2 relaxation components contained in the MR image can be determined and different weighted images can be obtained. T1 weighting : In the SE sequence, a suitably short TR is selected so that the macroscopic longitudinal magnetization vector that has been recovered before the next 90 degree pulse excitation is different due to the different T1 relaxation rate of the tissue, then the macroscopic transverse magnetization vector produced after the next 90 degree pulse excitation is different, and then the 180 degree pulse focus pulse is immediately used to generate the echo, i.e., a very short TE is selected to record This difference in macroscopic transverse magnetization vector is actually the difference in longitudinal magnetization vector before the excitation of the 90-degree pulse, and this difference in longitudinal macroscopic magnetization vector between different tissues is caused by the difference in T1 relaxation between different tissues after the excitation of the previous 90-degree pulse and shutdown. Therefore, the spin-echo signal acquired after a 90-degree pulse using a 180-degree focus pulse actually records the difference in the longitudinal magnetization vector of tissues after the previous 90-degree pulse (different T1 values), so it is T1WI. Therefore, T1WI must have a very short TE and a suitable short TR. The shorter the T1 weight, the heavier the T1 weight. However, not the heavier the T1 weight is, the better it is to choose according to the need in the clinic. Generally, if the T1 relaxation difference between two tissues is to be distinguished to the greatest extent, the TR of the SE sequence is best chosen near the average of the T1 values of the two tissues, and the T1 contrast is best . the shorter the T1 value on T1WI, the higher its signal intensity. T1WI TE: 8~20ms TR: 200~600ms A suitably short TR to make T1 relaxation difference, followed by the shortest TE to record is T1WI, the shorter the TR, the heavier the T1 weight. Short TE eliminates T2 differences. T2 weighting: long TR, eliminating T1 differences. Suitable TE, T2WI, the longer the TE, the heavier the T2. TR generally 2000 ~ 2500ms, TE generally 50 ~ 150ms.(TE choose the average of the two tissue T2 values when T2 contrast is best ) The longer the T2 value, the stronger the signal. In reflecting the very long T2 values of liquid and soft tissue T2 differences for example water imaging, TE generally hundreds to 1000ms or more. Proton density weighted imaging (PDWI): first give 90 degrees, then choose a very long TR, T1 relaxation is completed, then remove the influence of T1, then macroscopic longitudinal magnetization vector back to normal, that is, the difference in proton density of different tissues, and then give 90 degrees, then immediately 180 degrees focus to generate echoes, that is, short TE to get is the proton density difference, that is, PDWI. long TR with T2 TR, short TE with T1 TE. The short TE is the same as the TE of T1. T1WI: suitable TR200~600ms shortest TE 8~20ms, the shorter the T1, the higher the signal. the shorter the TR, the heavier the T1. T2WI: longest TR2000~2500ms suitable TE 50~150ms, the longer the T2, the higher the signal. the longer the TE, the heavier the T2. PDWI: longest TR2000~2500ms shortest TE 8~20ms, the greater the proton density, the stronger the signal. SE sequence is the classical sequence of MRI. Advantages: ①Simple sequence structure, easy to interpret signal changes ②Good signal-to-noise ratio ③Good tissue contrast ④Low sensitivity to magnetic field inhomogeneity, thus slight magnetization rate artifacts ⑤T1WI using SE sequence is generally 2~5 minutes. Disadvantages: ①High energy of 90 degree pulses, long T1 relaxation time, and T2WI and PDWI require long TR, one echo is acquired with one excitation, thus the sequence acquisition time is long and T2WI requires more than 10 minutes ②Long time is prone to artifacts ③Long time cannot be dynamically enhanced scan ④In order to reduce artifacts, NEX needs to be increased to further extend the scan time. Therefore, SE sequences are seldom used for T2WI and PDWI, and generally SE sequences are used for T1WI with a scan time of 2~5 minutes, and are commonly used for cranial, bone and joint soft tissue, and spine (less moving parts). Currently, gradient echoes are mostly used as a common sequence for T1WI in high field machines, especially in the abdomen. (90 and 180 degree pulses are applied simultaneously to the level selection gradient, as this is the only way to know on which level the 90 and 180 degrees hit. The echo acquisition process simultaneously applies a frequency gradient – after reading out the gradient 90 degree pulse, the phase encoding gradient is applied before the echo acquisition and then turned off, leaving the phase different during the echo acquisition) The longer the TR, the higher the SNR; the longer the TE, the lower the SNR.