SE sequence, when NEX is 1, a 256*256 image needs to be excited 256 times using 90 degree pulses, which means 256 TRs need to be repeated, so it takes a long time.
FSE Features.
① ELT is the acceleration factor or time factor, and the time takes 1/ELT.
② The time interval from the midpoint of the 90-degree pulse to the midpoint of the echo that fills the center of K space is defined as the effective TE. If the first echo is filled in the center of K space, i.e., a very short TE is chosen, the effect of T2 relaxation on the image contrast will be basically eliminated, and the obtained will be T1WI or PDWI; if the last echo in a long echo chain is filled in the center of K space, i.e., a very long effective TE is chosen , the result will be a very heavily weighted T2WI. Generally the longer the ELT, the lower the tissue contrast of the image.
③In addition, the first echo is strong and decreases sequentially, so the image is blurred.
④Higher signal of fatty tissue
⑤Insensitive to magnetic field inhomogeneity, but unfavorable to the detection of some lesions that can increase magnetic field inhomogeneity, such as hemorrhage
⑥Increased energy deposition.
The T2 value of the tissue will be prolonged, generally by about 30%. Therefore, when FSE is performed, the TE of T2WI should be selected by adding 30% to the T2 value of the target tissue.
Adjustment of FSE parameters.
① The TR of FSE is longer than SE, and the adjusted TE in FSE is the effective TE.
② ELT is not the longer the better, generally the longer the ELT the faster the imaging speed and the more weighted T2WI such as water imaging can be performed, but the soft tissue contrast is reduced, the blur effect is increased, the signal-to-noise ratio is reduced, the fat signal is increased, the SAR value is increased, and the magnetization transfer effect is more pronounced.
③Echo gap ES if shortened, then the TE difference of each echo is reduced, soft tissue contrast may be improved, blur effect is reduced, increased ETL may increase the acquisition speed, but its disadvantages fat tissue signal is higher, SAR value is higher, magnetization transfer effect is increased, which may increase saturation and may reduce image contrast.
④Focusing pulse angle: 180-degree pulse has the best polyphasic effect. Theoretically, the focusing pulses of both SE and FSE sequences should use 180-degree focusing pulses. However, FSE is special and needs to be improved: one is because 180 focus pulse has high energy, if the echo chain of FSE sequence uses 180 degree focus pulse, then SAR will definitely increase, and the longer the ELT the higher the SAR; the shorter the ES the higher the SAR; the shorter the TR the higher the SAR; the more layers are scanned the higher the SAR; the higher the field strength the higher the SAR (SAR value is proportional to the square of the field strength). is proportional to the square of the field strength). One way to solve the problem of SAR increase is to reduce the angle of the focusing pulse. The second is to reduce the focus angle at the front of the echo chain, and then gradually increase it, so that the amplitude of each echo tends to be close to each other and reduce the blurring effect (GE selects Tailored RF in imaging options).
options, Tailored RF is selected).
Clinical application of FSE sequences
① FSE T1WI (ELT 2~4)
② FSE T2WI with short ELT (ELT 5~10)
③ FSE T2WI with medium ELT (ELT 10~20)
④ FSE T2WI with long ELT (ELT >20)
1, FSE T1WI A shorter ELT is chosen because the longer the ELT, the more TE-length echo signals in the echoes filling the K-space, and thus will increase the contamination of the image by T2 relaxation and reduce the T1 contrast. For FSE T1WI, the first echo signal in the echo chain should be filled in the center of the K-space (the shortest effective TE is selected) to minimize the effect of T2 relaxation on the image contrast. Usual FSE T1WI: TR 200~600ms Effective TE 8~15ms ELT 2~5
Advantages: fast, even breath-hold scanning, such as ELT = 4 TR = 300ms phase encoding step = 160 NEX = 2, the scan time TA = 300 * 160 * 2 / 4 = 24s
Disadvantages: contaminated by T2 relaxation, the T1 contrast of the image is not as good as SE T1WI; blurring effect; speed is still slower than gradient echo, and when breath-hold scanning is required, the number of layers that can be scanned in one breath-hold is limited.
Uses: ① lower requirements for T1 contrast, to show structure-oriented parts, such as the spine, large joints, bone and soft tissue, pelvis, heart, etc.; ② poor patient tolerance, requiring faster scanning speed; ③ dynamic enhancement scanning of the pituitary gland; ④ breath-hold scanning of the body. When the T1 contrast requirements are high, such as brain tissue and abdominal organs T1WI, generally do not use FSE T1WI, but use SE T1WI.
2, short ELT of FSE T2WI sequence ELT is 2 ~ 10, in practice ELT is usually 5 ~ 10. usually used as PDWI and T2WI. advantages: ① compared with SE, fast, TA generally 2 ~ 7min ② ELT is short, T2 contrast is better, close to SE T2WI ③ insensitive to magnetic field inhomogeneity, no obvious magnetic sensitivity artifacts. The disadvantage is that the scanning speed is not fast enough, and there are motion artifacts when imaging the body part.
3, medium ELT of FSE T2WI sequence ELT is 10~20 Features: faster, 1~5min; slightly poorer T2 contrast. The FSE sequence of medium ELT of the new MRI instrument has basically replaced the FSE of short ELT as the most commonly used T2WI sequence, which is widely used. This is generally used.
4, long ELT FSE T2WI sequence ELT>20 Features: faster, TA for 20s to 3min, can hold the breath to scan; due to the long ELT, the image is more blurred, poor T2 contrast; bad breath hold has motion artifacts.
Uses: Body breath-hold T2WI for cases where respiratory rhythm is not well controlled leading to short respiratory trigger or moderate ELT for FSE T2WI failure; abdominal water imaging paired with respiratory trigger technique. In the liver for the identification of cystic, solid lesions. (A true water-rich lesion or a solid lesion with a high water content.) Identify small hepatocellular carcinoma or hemangioma. prolonged TE, prolonged ELT, can be identified by breath-hold T2WI.