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Errors in the measurements of T2 using multiple-echo MRI techniques. I. Effects of radiofrequency pulse imperfections
Abstract:
In principle, multiple-echo magnetic resonance imaging (MRI) can be used to estimate the spin-spin relaxation time, T2, which can then be used for quantitative tissue characterization. Although multiple echoes can be used to enhance the signal-to-noise ratio in an image, by echo addition, rf pulse imperfections modify the echo amplitudes resulting in significant errors in the estimate of T2. Imperfect 180 degree pulses do not completely invert the transverse magnetization so that the magnitude of the transverse component is reduced and a longitudinal component is generated. Successive application of such imperfect pulses generates many components that interact in a complex manner. The amplitudes of successive echoes are affected whenever the transverse components refocus, whereas the longitudinal components may be rotated into the transverse plane by subsequent pulses and may often add to the image signal or give rise to an image artifact. These effects have been analyzed theoretically and have been demonstrated for a wide range of rf pulse imperfections using both simple and composite pulses, through computer simulations based on the numerical solution of the Bloch equations. The theoretical and simulation results have been substantiated through experiments performed on a mineral oil phantom using a resistive prototype MR scanner operating at 6.35 MHz. In this paper we report the effects of improper pulse amplitude or duration for nonselective rf pulses on resonance. We separately describe the other types of imperfections caused by off-resonance effects and the use of selective pulses.
Insights
Radiofrequency pulse imperfections in multiple-echo MRI cause errors in spin-spin relaxation time (T2) estimation, hindering quantitative tissue characterization. This study analyzes these effects to improve MRI accuracy.
Area of Science:
- Magnetic Resonance Imaging (MRI)
- Biophysics
- Medical Physics
Background:
- Multiple-echo MRI is a technique for estimating spin-spin relaxation time (T2), crucial for quantitative tissue characterization.
- RF pulse imperfections, such as imperfect 180-degree pulses, can significantly corrupt echo amplitudes.
- These imperfections lead to errors in T2 estimation, limiting the accuracy of MRI-based tissue analysis.
Purpose of the Study:
- To theoretically analyze and experimentally demonstrate the impact of radiofrequency (RF) pulse imperfections on T2 estimation in multiple-echo MRI.
- To investigate the effects of improper pulse amplitude or duration for nonselective RF pulses on resonance.
- To describe imperfections arising from off-resonance effects and the use of selective pulses.
Main Methods:
- Theoretical analysis of RF pulse imperfections and their effect on transverse and longitudinal magnetization.
- Computer simulations using numerical solutions of the Bloch equations to model various RF pulse imperfections.
- Experimental validation using a mineral oil phantom on a prototype MR scanner at 6.35 MHz.
Main Results:
- RF pulse imperfections significantly alter echo amplitudes, leading to substantial errors in T2 measurements.
- Imperfect pulses generate complex magnetization components that interact, affecting successive echo magnitudes.
- Simulations and experiments confirmed the theoretical predictions regarding pulse amplitude, duration, off-resonance, and selective pulse effects.
Conclusions:
- RF pulse imperfections are a critical source of error in quantitative T2 mapping using multiple-echo MRI.
- Understanding and mitigating these imperfections are essential for accurate MRI-based tissue characterization.
- This work provides a comprehensive analysis of pulse imperfections, paving the way for improved MRI techniques.