What process causes the MRI signal to fade over time?

Prepare for the ASMIRT MRI Accreditation Test with comprehensive quizzes, flashcards, and in-depth explanations. Get familiar with exam format and content, ensuring your success!

The correct answer is associated with relaxation, which refers to the processes that occur after the radiofrequency (RF) pulse is turned off. In MRI, the signals emitted by the hydrogen nuclei in the body diminish over time due to two main relaxation processes: T1 (longitudinal) relaxation and T2 (transverse) relaxation.

During T1 relaxation, the spins of the hydrogen nuclei return to the equilibrium state, which involves the absorption of energy from the surrounding lattice. This process takes time and is characterized by T1 relaxation time specific to the tissue's properties. On the other hand, T2 relaxation pertains to the loss of phase coherence among the spins, which occurs due to interactions between the spins and their environment, leading to a decay of the transverse magnetization.

Both T1 and T2 processes contribute to the overall signal decay observed in an MRI scan. As the time elapses after the RF pulse, the signal intensity decreases, prompting the need to understand and optimize these relaxation times for better image quality in clinical practice. Thus, the phenomenon described in the question directly relates to these relaxation processes, making it the correct choice.

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