Factors affecting T1 and T2 relaxation times of different tissues are generally based on molecular motion, size and interactions.
The protons giving rise to an NMR signal are mainly those in cell water and lipids (i.e. protons that are free to move), while those in protein and solids usually do not contribute to signal. This is due to the motional characteristics of molecules that dictate spin-lattice and spin-spin relaxation times. The MR imaging is an image of the magnetized tissue, with the brightness indicating the level of magnetization. A tissue has a high signal (white/bright) if it has a large transverse component of magnetization. A large transverse magnetization induces a large amplitude in the receiver coil (which is located in the transverse plane). A tissue gives a low signal (black/dark) if it has a small transverse component of magnetization.
The value of the T1 time constant represents the time for the longitudinal magnetization to regrow to 63% of its maximum value. A 90 degree RF pulse makes the spin-up and spin-down populations equal, while T1 relaxation attempts to restore the population back to equilibrium values. The source of the relaxation is fluctuating magnetic fields in the local environment (the surrounding lattice) that the protons find themselves in.
The most common source of lattice fields is the dipole-dipole interaction between neighboring spins. That is to say, a spinning proton can exert a magnetic field on its neighbor. As the molecule tumbles through space, the distance between protons varies giving rise to a fluctuating field. If the field has some components matched with the Larmor frequency, protons in the higher-energy spin-down state can revert to the spin-up state by giving up their excess energy to the lattice.
The presence of paramagnetic ions can provide a powerful relaxation mechanism. Paramagnetic ions contain an unpaired electron and therefore have a magnetic moment due to electron spin providing a large fluctuating field. This reduced the T1 time. Examples include transition group elements Fe, Mn, Cu, Cr and Gd.
T1 time constant
The T1 time constant is dependent on a number of factors:
- specific tissue (type of nuclei, the mobility of nuclear species and present of macromolecules)
- magnetic field strength (T1 time increases with field strength)
- presence of paramagnetic ions/molecules - e.g. Gd (shortens T1 time)
During relaxation, the magnetization is changing, and the image brightness mirrors this change. Saturation renders the image dark and relaxation recovers the brightness. For example, fat has a short T1 value, CSF has a long T1 value, and so fat will appear bright in T1-weight images, and CSF will be dark.
The creation of an image at a time where the T1 curves of the tissues are widely separated (i.e. at short repetition time/TR [Short TRs do not permit full longitudinal recovery of tissues] and long echo time/TE), it will produce an image that has high contrast between these tissues. This is termed T1 contrast, and the corresponding images are called T1-weighted.
- 1. Jerrold T. Bushberg, John M. Boone. The Essential Physics of Medical Imaging. (2011) ISBN: 9780781780575
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