Final answer:
In MRI, the initial RF-transmission pulse inverts the net magnetization of protons, which then return to equilibrium at various rates depending on the tissue type, allowing for detailed imaging contrast.
Step-by-step explanation:
The principle mechanism of inversion recovery imaging in magnetic resonance imaging (MRI) states that an initial RF-transmission pulse moves the net magnetization of protons to an inverted position. During this process, protons in the body are aligned with a strong magnetic field, and an RF pulse is applied to flip the protons into a higher energy state. When the RF pulse is turned off, the protons relax back to their equilibrium state, releasing energy in the form of radio waves that are detected to form images. Different tissues within the body relax at different rates, which allows the MRI to differentiate between various tissue types, providing highly detailed images. Techniques such as T1, T2, or proton density scans exploit these differences in relaxation times for improved image contrast.