The T1 relaxation time (also called spin lattice or longitudinal relaxation time), is a biological parameter that is used in MRIs to distinguish between tissue types. This tissue-specific time constant for protons, is a measure of the time taken to realign with the external magnetic field. The T1 constant will indicate how quickly the spinning nuclei will emit their absorbed RF into the surrounding tissue.
As the high-energy nuclei relax and realign, they emit energy which is recorded to provide information about their environment. The realignment with the magnetic field is termed longitudinal relaxation and the time in milliseconds required for a certain percentage of the tissue nuclei to realign is termed 'Time 1' or T1. Starting from zero magnetization in the z direction, the z magnetization will grow after excitation from zero to a value of about 63% of its final value in a time of T1. This is the basic of T1 weighted images.
The T1 time is a contrast determining tissue parameter. Due to the slow molecular motion of fat nuclei, longitudinal relaxation occurs rather rapidly and longitudinal magnetization is regained quickly. The net magnetic vector realigns with B0 leading to a short T1 time for fat.
Water is not as efficient as fat in T1 recovery due to the high mobility of the water molecules. Water nuclei do not give up their energy to the lattice (surrounding tissue) as quickly as fat, and therefore take longer to regain longitudinal magnetization, resulting in a long T1 time.

See also T1 Weighted Image, T1 Relaxation, T2 Weighted Image, and Magnetic Resonance Imaging MRI.

The basis of T1 weighted imaging is the longitudinal relaxation. A T1 weighted magnetic resonance image is created typically by using short TE and TR times.
The final image is a reflection of more than one of these pulse sequence parameters, weighted according to the type of sequence and its timing. T1 signals determine predominantly the contrast and brightness in this type of images but proton density will always contribute to the image intensity. The T1 dependence is mainly determined by the repetition time or any pre-pulses (such as in an inversion recovery pulse sequence).
Due to the larger longitudinal and transverse magnetization, fat has a higher signal and will appear bright on a T1 contrast MR image. Conversely, water has less longitudinal magnetization prior to a RF pulse, therefore less transverse magnetization after a RF pulse yielding low signal appearing dark on a T1 contrast image. Often, a paramagnetic contrast agent, a gadolinium compound, is administered, and both pre-contrast T1 weighted images and post-contrast T1 weighted images are obtained.

The T2 relaxation time (spin spin relaxation time or transverse relaxation time), is a biological parameter that is used in MRIs to distinguish between tissue types and is termed 'Time 2' or T2. It is a tissue-specific time constant for protons and is dependent on the exchanging of energy with near by nuclei. T2 weighted images rely upon local dephasing of spins following the application of the transverse energy pulse. T2 is the decay of magnetization perpendicular to the main magnetic field (in an ideal homogeneous field).
Due to interaction between the spins, they lose their phase coherence, which results in a loss of transverse magnetization and MRI signal. After time T2 transverse magnetization has lost 63% of its original value. This tissue parameter determines the contrast.
The T2 relaxation is temperature dependent. At a lower temperature molecular motion is reduced and the decay times are reduced.
Fat has a very efficient energy exchange and therefore it has a relatively short T2.
Water is less efficient than fat in the exchange of energy, and therefore it has a long T2 time.

See also T2 Weighted Image and Magnetic Resonance Imaging MRI.

T2* (also called T2 Star) is composed of molecular interactions (spin spin relaxation) and local magnetic field non-uniformities. Caused by this the protons precess at slightly different frequencies. The T2* effect cause a rapid loss in coherence and transverse magnetization. The T2* time is less than the T2 time.

See also T2* Time, T2 Star.

The portion of the MRI system that produces the RF current and delivers it to the transmitting coil (antenna). The RF signal produced by the transmitter is used to excite the protons in the imaging volume. Phase sensitive detectors (electronic device in which the output is dependent on the instantaneous difference in phase between two input signals) form part of the frequency synthesis pathway.