The three-dimensional MRI can be obtained by multi-slice imaging where different frequencies of radio frequency pulses are used within the time interval TR-TE. Then the images are stacked to obtain the three-dimensional MRI. The sequences used are spin-echo sequence and gradient echo sequence. In a classic “spin-echo-sequence” as shown in Fig 4.11, the magnetization is first rotated 90 degrees away from equilibrium through the use of radio waves. A short waiting period follows with dephasing caused by inhomogeneity. Subsequently the magnetization is turned an additional 180 degrees with radio waves. After a little more waiting time and refocusing, the signal is measured. This course is usually repeated. In multi-slice imaging there will be different frequencies applied and it depends on the ratio TR/TE.
The spin-echo sequence is time-consuming about 8 min and cannot be used for the abdominal imaging where the patient has to hold the breath. The gradient-echo sequence is used instead as there is no repetition of 1800 and a frequency gradient in the negative direction is used to de-phase as shown in Fig 4.12 and then the phase roll will gradually rewind until the spins are back in phase.