Excitation and relaxation

One excitation, two simultaneous relaxations

The signal model begins with the static magnetic field B₀. It establishes a small net magnetisation of hydrogen nuclei along the longitudinal, or z, axis. An RF pulse near the nuclei’s resonant frequency rotates that net magnetisation by a chosen flip angle. A 90° pulse is a useful idealised shorthand for a conventional spin-echo example, not a property of every MRI sequence.

Once the magnetisation has a transverse component, two parts of its evolution matter at the same time. The longitudinal component grows back toward its equilibrium orientation along B₀. This recovery is described by T1, commonly called spin-lattice relaxation because it involves energy exchange with the surrounding molecular environment. The transverse component produces a detectable RF signal while the spins remain sufficiently phase-coherent. That coherence decays through microscopic interactions, a process described by T2, or spin-spin relaxation.

These are not sequential stages in which T1 finishes before T2 begins. They are two descriptions of the same post-excitation evolution: one follows recovery along z and the other follows loss of coherent magnetisation in the xy plane. The receive coil detects the changing transverse component; later gradients and reconstruction determine how that signal becomes spatially organised.

B0 establishes net longitudinal magnetisation
        |
        | RF pulse rotates it by a chosen flip angle
        v
Transverse magnetisation: an RF signal can be detected
        |\
        | \-- T1: longitudinal component recovers toward equilibrium
        \---- T2: transverse phase coherence decays

The time constants are tissue-dependent and the ordering is not a universal list of bright and dark tissues. At commonly used clinical settings, fat-like and fluid-like tissues often recover and lose transverse coherence at different rates, which supplies the raw material for contrast. The image does not display T1 or T2 directly; it displays a signal sampled at a chosen point in that evolving process.

T2, T2-star, and what the echo changes

The transverse signal can lose coherence for more than one reason. T2 describes the irreversible component associated with microscopic interactions. T2* also includes dephasing caused by static or slowly varying macroscopic field inhomogeneity, including effects related to magnetic susceptibility. It is therefore shorter than or equal to T2 in the usual idealised description and is especially relevant to gradient-echo acquisitions and susceptibility-sensitive applications.

A spin-echo sequence uses a refocusing pulse to reverse some phase dispersion caused by static field differences. That refocusing does not restore the irreversible T2 decay, so a spin-echo measurement and a gradient-echo measurement do not expose the same effective relaxation. The distinction matters when a phrase such as “T2-weighted” is interpreted as if it were a direct T2 measurement. It is a statement about which tissue differences the sequence makes most visible.

The next step is to make the sampling choice explicit. Timing, weighting, and sequence contrast explains how TR and TE select the balance between recovery and decay.

Sources

• Abi Berger, “Magnetic resonance imaging” — magnetic-field and RF signal formation, pulse sequences, and T1/T2-dependent contrast.
• Stanford Medicine Body MRI Research Group, MRI Physics Education Materials — public visual orientation on relaxation and MRI image formation.
• Ridgway JP. Cardiovascular magnetic resonance physics for clinicians: part I. J Cardiovasc Magn Reson. 2010;12:71. doi:10.1186/1532-429X-12-71. (Full text)