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Physics Facts - What is T1?

A "T1 weighted image" is familar to everyone using MRI. But what exactly is T1 ? Hallmarq's Nick Bolas explains

A T1 weighted image is one of the common basic types of image. T1 weighted images highlight fat and are sensitive to the effect of paramagnetic contrast agents. Because T1 weighted images are quick to collect, have a high signal:noise ratio, and show high signal intensity from fat in brain, nervous tissue and bone they are often good for anatomical information.

But what actually is T1?

Remember back in chemistry (or physics) class you learned about spectroscopy. Energy from heat or a chemical reaction promotes electrons into a higher orbit, and as they fall back down they emit electromagnetic radiation. The exact energy emitted determines the frequency (the colour of the emitted light).

Something similar happens in MRI. The RF pulse promotes the nucleus to a more energetic spin state, and as it falls back down it emits electromagnetic radiation. The exact frequency (a radiofrequency of about 63MHz for a 1.5T magnet) is determined by the exact energy change, which itself is determined by the exact magnetic field strength.

But there is a huge difference: while visible light radiation is the most important way for electrons to lose energy, the loss to radiation in MRI is only an infinitesimal fraction of the energy in the spin states of all the nuclei. The vast majority of energy the nuclei gain from the RF pulse is lost by magnetic interactions with their surroundings. Just as an RF pulse of exactly the right frequency can transfer energy to the nucleus, so any magnetic field fluctuation of exactly the right frequency (and for a long enough duration) can transfer energy away. In a solid or liquid the random motions of nearby atoms and molecules creates a continually fluctuating magnetic field, and at random a component of this will be just right to interact with the nucleus. Hence another name for T1, the "spin-lattice" relaxation time, as energy is transferred from the nuclear spin to the surrounding lattice (in a crystal) or less structured surroundings in biological material.

The intensity and frequency of the random field fluctuations depend on the nature and mobility of the atoms and molecules in the imaged object. In a solid, the structure may be so rigid that vibrations fall mostly at frequencies that do not interact, so T1 can be long. In a gas the surroundings are so far away, on an atomic scale, that they barely interact and again T1 can be long. But in the liquid or semi-liquid conditions of most biological tissue the fluctuations are at approximately the correct frequency, and so T1 relaxation is fairly fast and sensitive to slight variations in the molecular environment. Rapidly diffusing, small molecules such as water move so fast that many of the fluctuations average out, making T1 relaxation of water somewhat slower than that of fat. The longer, more slowly moving fat molecules see less averaging out and relax slightly faster. Paramagnetic agents such as Gadolinium create a strong local magnetic field, increasing the interaction and speeding up the T1 relaxation.

In a T1 weighted image, RF pulses are applied in quick succession (short TR). Material with a long T1 absorbs the energy but is not able to lose it, becoming saturated and giving little signal back. At progressively shorter T1 times the nuclei are increasingly able to absorb and re-emit the RF energy, giving a stronger signal and a brighter image. At very short T1s the energy is lost before it can be detected, appearing dark - though in fact, T2 effects are more often responsible for dark regions, even in T1 weighted images (no weighting is perfectly 100%).