Beat phenomena of oscillating readouts

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Biomedical Engineering
• Physics

Background:

• Gradient systems in MRI are crucial for spatial encoding.
• Mechanical resonances within these systems can introduce unwanted oscillations.
• Echo-planar imaging (EPI) relies on fast oscillating gradients, making it susceptible to these mechanical effects.

Purpose of the Study:

• To demonstrate how mechanical resonances in MRI gradient systems create slowly varying, erroneous magnetic field gradients.
• To explain the observed phase variations in oscillating readouts, specifically in EPI sequences.
• To investigate the impact of these temporal variations on image quality.

Main Methods:

• Acquired 1D EPI projections of a static phantom across a range of EPI frequencies (1121–1580 Hz) on 3T clinical MRI systems.
• Eliminated static B0 inhomogeneity phase errors using complex division of oppositely-polarized EPI readouts.
• Analyzed temporal phase evolution, correlating it with gradient system mechanical resonances via the gradient modulation transfer function.
• Evaluated the impact of time-varying mechanical resonance effects on EPI using echo-planar spectroscopic imaging.

Main Results:

• Observed a 'beat phenomenon' causing slowly varying phase errors, with temporal frequency equaling the difference between EPI and mechanical resonance frequencies.
• Measured maximum erroneous oscillating phase of ±0.5 radians at 1281 Hz EPI frequency.
• Echo-planar spectroscopic imaging revealed time-dependent field-of-view (FOV) stretching and compression due to these effects.

Conclusions:

• Oscillating EPI readouts can produce low-frequency, erroneous phase contributions explained by the beat phenomenon.
• Accurate image reconstruction in EPI may necessitate incorporating these beat effects into phase-correction algorithms.
• Understanding and mitigating mechanical resonance artifacts is vital for high-fidelity MRI.