When would the Ernst angle be less than 90°?

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The Ernst angle is the flip angle that maximizes the signal from a spin-echo MRI sequence, and it depends on the repetition time (TR) and the longitudinal relaxation time (T1) of the tissue being imaged. When TR is less than T1, the Ernst angle will be less than 90 degrees. This relationship arises because a shorter TR compared to T1 allows for less time for the longitudinal magnetization to recover before the next excitation pulse.

In this scenario, if TR is less than T1, not enough time has elapsed for the magnetization to fully return to equilibrium before the next RF pulse is applied. The result is that the tissue is less magnetized than it would be with a longer TR, thus requiring a smaller flip angle to achieve optimal signal. This is why the Ernst angle would be less than 90 degrees; it reflects the balancing act between maximizing signal and the timing of magnetization recovery.

On the contrary, if TR were to exceed T1, the Ernst angle would typically approach or exceed 90 degrees as more longitudinal magnetization would be available, allowing for more effective excitation and signal return. Other options suggest scenarios that do not accurately reflect the conditions under which the Ernst angle can be determined effectively,

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