Fast Spin Echo
Fast spin echo sequences reduce image (slice) acquisition time. This is done by using the time after the first echo to apply 180° pulses to obtain a different echo, called spin echo train. Basically, more echoes are received using the same time of repetition.
Using a somewhat similar process to fast spin echo sequences, in multi-echo spin echo sequences, after the spin echo is received, a 180° pulse is applied. The difference is that the echo train received thereafter has the same phase encoding, and is used to build a second image. Since the echo time differs between the 2 images, the second image will be more T2 weighted than the first. This will also reduce the number of repetitions (TR), thus reducing acquisition time. Hence, PD- and T2-weighted images can be obtained simultaneously.
Fast repetition of 180° pulses changes T2 relaxation times however, by perturbing a phenomenon called J coupling, which refers to a coupling of nuclei occurring with the spins, causing T2 lengthening. Thus, fat has a higher T2 signal in fast spin echo than in standard spin echo.
The risk of artifacts is increased.
Ultrafast spin echo sequences
Again using the spin echo concept, ultrafast spin echo sequence is an even faster technique. There is no repetition of pulses; single 90° pulses are applied, throughout the sample desired to image (as much needed to obtain enough information to obtain a slide). That pulse will generate image for a segment of the image and the 180° pulse is applied to obtain T2 and PD information. Time is greatly reduced, but increases noise and blurring. Time will be dependent on the number of image encodings required. They provide better results in non-moving structures, appearing as a T2-weighted hypersignal. They are commonly used for cholangio-MRI and uro-MRI.