Magnetic Resonance Imaging (MRI) Practice Test

When is the phase encoding step performed during the MRI sequence?

1. After the frequency encoding

2. Prior to the frequency encoding

3. Simultaneously with frequency encoding

4. During the signal readout

The correct answer is: Prior to the frequency encoding

The phase encoding step is performed prior to the frequency encoding in the MRI sequence. Understanding the timing of these steps is crucial for the image formation process in MRI. In a typical MRI sequence, phase encoding involves the application of a gradient field that alters the phase of the spins in the tissue being imaged. This manipulation of the magnetic field is done after the initial excitation pulse and before the frequency encoding occurs. The sequence is designed so that different areas of the imaging slice are phase encoded separately, allowing for the spatial localization of the tissue being imaged. Once the phase encoding gradient is applied, the frequency encoding gradient is then turned on. This gradient differentiates the spins based on their frequency, contributing to how signals are received and processed during image reconstruction. By performing phase encoding prior to frequency encoding, MRI can accurately determine both the frequency and phase of the signals from different locations within the imaging slice, creating detailed images of the internal structures.