Magnetic Resonance Imaging (MRI) Board Practice Exam

RF pulse timing sets how tissues recover and dephase between excitations, which is the primary way image contrast is formed. After each pulse, tissues with different T1 and T2 relaxation times recover longitudinal magnetization and accumulate transverse magnetization at different rates. The repetition time (TR) and, if used, inversion time (TI) determine how much recovery has occurred before the next excitation, shaping T1-weighted contrast. The echo time (TE) controls how much transverse magnetization has decayed before signal readout, shaping T2-weighted contrast. Inversion recovery sequences, where a 180-degree pulse is followed by a defined TI, exploit this timing to null or emphasize signals from specific tissues, producing pronounced contrast differences. Other aspects like signal-to-noise ratio depend on factors such as coil design, field strength, voxel size, and averaging; acquisition speed is set by sequence timing and hardware constraints beyond just RF timing; and gradient performance relates to hardware capabilities and timing of gradient waveforms rather than RF timing alone.