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.
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