What is the main characteristic of T2 decay?

The main characteristic of T2 decay is that it is indeed an exponential process resulting from spin interactions among neighboring hydrogen nuclei in a magnetic field. T2 decay, or transverse relaxation, occurs as the spins of the protons lose phase coherence in the transverse plane due to interactions with each other and their surrounding environment. This loss of coherence leads to a decrease in the signal measured in MRI over time, and the decay curve follows an exponential pattern, which is a fundamental feature of T2 relaxation.

The other choices touch on different concepts within MRI physics. While energy transfer to the environment is a consideration in relaxation processes, it does not specifically describe T2 decay. The time taken to regain T1 equilibrium pertains to T1 relaxation, which is about the recovery of longitudinal magnetization, not T2. Finally, the alignment of spins along the longitudinal axis refers to the initial alignment seen during the application of the magnetic field before any relaxation processes occur, and it relates to T1 rather than T2. Thus, the identification of T2 decay as an exponential process driven by spin interactions accurately captures its essence in MRI physics.