The spin echo sequence or spin echo (SE) is the most widely used in MRI. An initial 90° RF pulse is applied to put the hydrogen protons in phase. This is followed by a 180° RF pulse to refocus the dephasing protons and to get a signal (echo). The time between the 90° RF pulse and the echo is called echo time (TE). This time can be divided into two parts: TE/2 (between the 90° and the 180° RF pulses) and 2TE/2 (from the 180° RF pulse to the echo). The time between two 90° RF consecutive pulses is called repetition time (TR).

The relaxation of the protons is strongly influenced by the neighboring atoms, so by varying the repetition time (TR) and the echo time (TE), it is possible to enhance the image (white indicating a strong signal and  black the lack of one). It is thus possible to obtain images based on the longitudinal relaxation (T1 weighted), on the transverse relaxation (T2 weighted) and on the proton density (density weighted).

T1 weighted sequence

T1 is defined as the time taken for 63.2% of the protons to realign with the magnetic field Bo (the remaining 36.8% take 25 times longer); in other words, to recover the longitudinal magnetization. T1 is also called spin-lattice relaxation because it releases the energy captured by protons (spins) to the surrounding tissues. It depends on the magnetic field (Bo) and the precession frequency (wo).

As the TR time corresponds to the growth of the longitudinal magnetization (LM), long TR times allow the recovery of the LM of all the protons. Therefore, if we want to enhance contrast between protons belonging to different tissues with T1 weighted sequences, the TR must be short. For this reason, it is said that the TR maximizes contrast in T1. The fatty tissues have shorter relaxation times than other tissues because the hydrogen protons of the fat are in resonance. This means that the fat molecules have a slow frequency movement (similar to the precession frequency) so the hydrogen protons can release the energy easily. On the other hand, free water has a longer relaxation time because its hydrogen protons are not in resonance. This means that the water molecules have high frequency movements (higher than the precession frequency) so it takes more time for the water hydrogen protons to release their energy. To obtain a T1 weighted image in a field of 1.5 T, we should use a TR of 450 to 600 ms and a TE of 12 to 25 ms.

The hydrogen protons of fat are the ones that release energy more rapidly (hyperintense signal). They are followed by bone marrow, white matter, gray matter, muscle, free water, ligaments, cortical bone and air (hypointense signal).

T2 weighted sequence

T2 is defined as the time taken for 63.2% of the protons to dephase; in other words, the time required for the transverse magnetization (TM) to reduce to 36.8% of its original value. The imperfections in the homogeneity of the magnetic field created by the static field of magnets induce an intense dephasing of the protons. This decrease of the signal is called T2*.

As T2 is the result of interactions between the protons (spins), it is also called spin-spin relaxation. Because each proton has its own magnetic field, they influence each other, decreasing the signal. In the case of free water, the rapid movements of small molecules cancel the local magnetic fields, allowing the hydrogen protons to dephase homogenously. T2 relaxation is slow, giving a bright signal.

If we want to enhance contrast in T2, it is necessary to have a long TE. For a T2 weighted image in a field of 1.5 T, we should use a TR of 3500 to 4000 ms and a TE of 100 to 120ms.

Due to the fact that T2 indicates the synchronism in the relaxation (loss of transverse magnetization), the highest signal (hyperintense) corresponds to free water (especially CSF). It is followed by fat, bone marrow, gray matter, white matter, muscle, ligaments, compact bone and air (hypointense signal). Myelin lipids have a very short T2 because they remain immobile within highly structured membranes.

Proton density weighted sequence (D).

The intensity of the signal depends on how many hydrogen protons there are in any particular case. By using a long TR and a short TE, we minimize both T1 and T2. In these images, the signal is related to the number of hydrogen protons rather than to a relaxation process. The hyperintense signal corresponds to free water, followed by fat, gray matter, white matter, muscle, cortical bone and air (hypointense signal).