The return to equilibrium (high energy protons returns to the low energy state) within the lattice is named the T1, spin lattice or longitudinal relaxation. During the time T1, the spinning protons realign with the external magnetic field with an exchange of their energy, resulting in heat. The value of the T1 time depends of the tissues ability for energy exchange.

See also Longitudinal Relaxation Time.

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.

Every tissue in the human body has its own T1 and T2 value. This term is used to indicate an image where most of the contrast between tissues is due to differences in the T1 value.
This term may be misleading in that the potentially important effects of tissue density differences and the range of tissue T1 values are ignored.
If the machine parameters are chosen, so that TR less than T1 (typically under 500 ms) and TE less than T2 (typically under 30 ms), a power series expansion of the exponential functions and then neglecting second and higher order terms yields
Mxy = Mxy0 TR/T1
thus the expression becomes independent of T2 and yields the condition for T1 weighting. Therefore a T1 contrast is approached by imaging with a short TR, compared to the longest tissue T1 of interest and short TE, compared to tissue T2 (to reduce T2 contributions to image contrast). Due to the wide range of T1 and T2 and tissue density values that can be found in the body, an image that is T1 weighted for some tissues may not be so for others.
Lesions with short T1 are (bright in T1 weighted sequences):
fat (lipoma, dermoid)
sub-acute haemorrhage (metHb)
paramagnetic agent (Gd, pituitary)
protein-containing fluid (colloid cyst)
metastatic melanoma (melanotic).

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.

(THRIVE) THRIVE is an optimized fast T1 weighted 3 dimensional imaging technique combining sensitivity encoding, large volume coverage and uniform fat suppression. THRIVE improves for example dynamic liver, small bowel, breast, prostate and pancreas MRI, providing isotropic images with high resolution in short breath-hold times.