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.

A comparative study at 3 T of sequence dependence of T2 quantitation in the knee

“…Similarly, fast spin echo with 16 echo train lengths is significantly higher than fast spin echo with 8 echo train lengths in all tissues. Our FSE results correlate with the literature regarding slower T 2 decays, resulting in abnormally high signal intensities in later echoes due to stimulated echo pathway created by the imperfect refocusing pulses in a multi-echo sequence (5,7,14). Other factors that affect the quantification accuracy in a multiecho sequence include static field inhomogeneities and the error propagation introduced by off-resonance effects (15).…”

Biexponential parameterization of diffusion and T2 relaxation decay curves in a rat muscle edema model: Decay curve components and water compartments

“…Most imaging studies of T 2 relaxation with magnetic resonance imaging scanners, however, have been performed with more stringent limitations on minimal echo spacings due to the need for frequency encoded readouts. Also, unless specifically optimized for single slice imaging, measurements are often compromised somewhat by the deleterious effects of slice selective RF pulses and/or RF inhomogeneities over the imaged volume (11)(12)(13). Thus, for most quantitative MRI based T 2 studies, there has been a tendency to assume monoexponential T 2 relaxation and to employ the use of only a few echo times to measure single T 2 relaxation times from tissue (14 -19).…”

Simultaneous spin‐echo refocusing

“…1 and 2, the position of the primary echo of one slice oscillates with a frequency of two refocusing periods. A spoiling scheme with the same oscillation frequency (for a periodic spoiling scheme see (16)) might preserve the correctly positioned stimulated echoes to maintain maximum image SNR. In the following description, the moment of the gradient spoilers is given in unit measures, where one unit measure generates a phase distribution of 2 within an image voxel.…”