Currently, both Inversion Recovery (IR) and Fast Inversion Recovery (FIR) are generally used for spin echo. (Inversion Recovery) I. Principle of Inversion Recovery: First, the 180 degree excitation, i.e., the inversion pulse. (i) The longitudinal relaxation process is prolonged and the T1WI contrast is increased. ②After the 180-degree inversion pulse, during the relaxation process, its longitudinal magnetization vector is reversed to 0 and then to the maximum. When a 90-degree pulse is given at the moment when the longitudinal magnetization vector of a tissue reaches 0, there is no macroscopic longitudinal magnetization vector and therefore no transverse magnetization vector is generated, and the tissue does not produce a signal. Using this, the signal of a tissue with a certain T1 value can be selectively suppressed. Zhang Chengjun, Department of Radiology, Chaoyang Central Hospital, Liaoning Province, China II. Inversion recovery sequence: It is a T1WI sequence with a 180-degree inversion pre-pulse added before SE. The 180-degree inversion pre-pulse is given first, and at the appropriate moment, a 90-degree pulse is added to excite it, after which a 180-degree focus pulse is applied immediately and an echo is acquired. the time from the 180-degree inversion pulse to the midpoint of the 90-degree pulse is called the inversion time TI. the 90-degree to 180-degree time is called TE, and the interval between two adjacent 180-degree inversion pre-pulses is called TR. in order to ensure that before each 180-degree inversion pre-pulse In order to ensure that the longitudinal magnetization vector of each tissue can basically return to equilibrium before each 180-degree inversion prepulse, it is required that the TR is long enough (TR-TI>5T1), at least equivalent to the TR length of SE T2WI or FSE T2WI. Therefore, the T1 contrast and weights in the IR sequence are not determined by TR, but by TI. The selected TI is close to the T1 value of the tissue to obtain a better T1 contrast. Features: ①Best T1 contrast, significantly higher than SE T1WI. ②Only one echo is acquired in one inversion, and TR is long, so TA is long, and TA is equivalent to T2WI. clinically general T1WI, not widely used, is mainly used to increase T1 contrast between gray and white matter of the brain, and has high value for myelin development studies in children. ir sequences can also be used for fat suppression (STIR, not suitable for The IR sequence can also be used for fat suppression (STIR, not suitable for enhancement scans) or water suppression (Flair), but due to the long time, it is now mostly done with fast inversion recovery sequences. Fast inversion recovery sequence (FIR): It is a 180-degree pre-pulse + FSE (T1WI, T2WI, PDWI) Features: ① There is ELT, fast speed. ② with ETL presence, contaminated by T2, T1 contrast decrease. ③with ELT, with blurring effect. ④Compared with FSE T1WI, the T1 contrast of FIR T1WI is improved due to the application of 180-degree inversion pre-pulse. ⑤Selecting different TI can selectively suppress the signals of tissues with different T1 values. Under the condition that the TR is guaranteed to be long enough (TR>5T1), the TI value for suppressing the signal of a certain tissue is equal to 69% of the T1 value of that tissue, which is generally calculated as 70%. Use: 1, STIR (short TI inversion recovery) FIR sequence short inversion time FIR sequence is mainly used for T2WI fat suppression (low field with more, because the frequency difference between water and fat in low field MRI is relatively small, with the frequency saturation method of fat compression effect is not good) (high field STIR fat compression is used for off-center parts and parts with irregular morphology (e.g., soft tissues of the neck). The fat T1 value is very short, 1.5T, and the fat T1 value is 200~250ms, and the macroscopic longitudinal magnetization vector of the fat tissue takes 70% of its T1 value, i.e. 140~175ms, after the 180-degree reversal pulse if the TR is long enough, i.e. 140~175ms, if a 90-degree pulse is applied at this time (i.e. TI=140~175ms), as there is no macroscopic longitudinal magnetization vector, there is no macroscopic transverse magnetization vector generation, there is no signal, and the fat signal is suppressed, and the use of a very short TI is the origin of the name of the sequence. 1.5T, STIR FIR Generally TI is chosen to be about 150ms, and TR is greater than 2000ms. STIR technique for compression lipid is suitable for low field machines. In fact, STIR technology can also be used for gradient echo, EPI, etc. for compression of fat. 2, FLAIR sequence (Fliud attenuated inversion recoverry) Liquid inhibition inversion recovery (Blackwater sequence) On brain or spinal cord T2WI, when the lesion is relatively small and close to the cerebrospinal fluid (such as cortical lesions, paraventricular lesions), lesions presenting slightly high signal or high signal are often masked by the higher signal cerebrospinal fluid and cannot be If the signal of cerebrospinal fluid can be suppressed on T2WI, the lesion can be fully exposed. Initially, we can determine whether the T1 value of the lesion is close to that of pure water. FLAIR sequence is actually a FIR sequence with long TI, because the T1 value of cerebrospinal fluid is very long, at 1.5T, the T1 value of cerebrospinal fluid is 3000~4000ms, choose TI=(3000~4000)*70%=2100~2800ms, at this time the macroscopic longitudinal magnetization vector of cerebrospinal fluid is 0, at this time give 90 degree pulse, the cerebrospinal fluid is not excited, no signal, the signal is suppressed, followed by actually an FSE. clinically, 1.5T general TI is chosen 2100~2500ms, TR needs to be more than 3~4 times of TI, ELT and effective TE are similar to FSE T2WI. 3, FIR T1WI is actually a short ELT of FSE T1WI preceded by a 180 degree inversion pre-pulse to enhance the T1 contrast of the image. ge is called T1 FLAIR. is a 180 inversion pre-pulse before FSE T1WI —–FIR T1WI) clinically mainly used for T1WI of brain parenchyma, gray-white matter contrast T1 FLAIR is stronger than SE T1WI or FSE T1WI but weaker than IR T1WI. 1.5T General TR=2000~2500ms TI=750ms ELT=4~8. IV. Single excitation fast inversion recovery sequence Using a combination of 180 degree inversion prepulse + single excitation FSE. (IR-SS-FSE) Application: ① Use STIR pressed fat ② FLAIR pressed water ③ Choose the appropriate TI and use the shortest TE (fill the earliest echo in the echo chain to the center of K space) can obtain IR-SS-FSE ultra-fast T1WI, these sequences are ultra-fast imaging sequences, mainly used for patients who cannot cooperate with the examination. V. Multi-inversion pre-pulse sequences The previously mentioned are a 180 degree inversion pre-pulse, we can use 2~3 180 inversion pre-pulses in each execution of the sequence, which is called double inversion or triple inversion pulse technique, using this technique can selectively suppress the signal of 2~3 tissues according to the different T1 values. In fact the multi-inversion pre-pulse technique can be used both in combination with pre-FSE and with fast gradient echo. 1, Using the adjustment of the inversion time of the two inversion pre-pulses of double inversion FSE, the signal of cerebrospinal fluid and cerebral white matter can be selectively suppressed while highlighting cerebral gray matter, and the signal of cerebrospinal fluid and cerebral gray matter can be suppressed while highlighting cerebral white matter. 2. Double inversion – black blood in the heart (not suppressing both tissues, secondly, using blood flow, mainly suppressing the signal of blood), on the basis of which, triple inversion is used for lipid compression. 1.5T T1 value Fat 225ms Pure water 3500ms