Research article
T2* measurements in human brain at 1.5, 3 and 7 T

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Abstract

Measurements have been carried out in six subjects at magnetic fields of 1.5, 3 and 7 T, with the aim of characterizing the variation of T2* with field strength in human brain. Accurate measurement of T2* in the presence of macroscopic magnetic field inhomogeneity is problematic due to signal decay resulting from through-slice dephasing. The approach employed here allowed the signal decay due to through-slice dephasing to be characterized and removed from data, thus facilitating an accurate measurement of T2* even at ultrahigh field. Using double inversion recovery turbo spin-echo images for tissue classification, an analysis of T2* relaxation times in cortical grey matter and white matter was carried out, along with an evaluation of the variation of T2* with field strength in the caudate nucleus and putamen. The results show an approximately linear increase in relaxation rate R2* with field strength for all tissues, leading to a greater range of relaxation times across tissue types at 7 T that can be exploited in high-resolution T2*-weighted imaging.

Introduction

The recent availability of ultra-high field magnets operating at fields of 7 T and above for human imaging [1] requires reoptimization of pulse sequences and imaging protocols for operation at elevated field [2]. This optimization can only be carried out efficiently with an accurate knowledge of the relaxation times of the tissues involved. Accurate knowledge of T2* relaxation times in brain tissue is important, in general, for setting the optimal echo time in gradient echo sequences and, in particular, for choosing the echo time that maximizes blood-oxygenation-level-dependent contrast in functional magnetic resonance imaging (fMRI) experiments [3], [4]. In addition to providing useful information for pulse sequence and fMRI paradigm optimization, there is considerable interest in using relaxation time measurements to evaluate local iron content in human brain [5], [6], [7], [8]. Such measurements are valuable because the state and the concentration of iron, particularly in deep grey matter structures, have both been shown to vary with age and to be changed in neurodegenerative disorders, such as Parkinson's disease and Alzheimer's disease [5], [6], [7], [8]. T2* shows particular sensitivity to iron content, which is enhanced at increased magnetic field strength.

Here we describe the measurement of the T2* value of different tissues in the normal human brain at 7 T and the comparison of these measurements with results obtained at 3 and 1.5 T on the same six volunteers. Accurate measurement of T2* in the presence of macroscopic magnetic field inhomogeneity is difficult because of enhanced signal decay due to intravoxel dephasing [9], [10], which leads to underestimation of T2*. The approach employed here [10] allowed the signal decay due to through-slice dephasing to be measured and removed from data, thus facilitating precise measurement of T2* even at ultra-high field.

Section snippets

Methods

Scanning was carried out on Philips Achieva systems operating at 7, 3 and 1.5 T. T2* maps were produced using the modified echo planar imaging (EPI) sequence shown in Fig. 1B. Here, phase-encoding gradient blips shown in the conventional EPI sequence of Fig. 1A are replaced with a phase-encoding gradient pulse applied immediately following slice selection, and the sequence is repeated n times with different phase-encoding gradient strengths so as to span k-space. The resulting multishot

Results and discussion

Fig. 3 shows corrected and uncorrected T2* maps measured from approximately the same slice position in the same subject at three different field strengths. Maps showing the variation of the S0 value in the same slices are also shown for comparison. These have some T1 weighting but mainly show the variation of proton density and the effect of B1 inhomogeneity, with the latter being most evident as would be expected in 7-T data. The fitting procedure ensures that the effect of B1 inhomogeneity is

Conclusions

Measurements of T2* relaxation times in the human brain have been made on the same six subjects at fields of 1.5, 3 and 7 T. The effect of through-slice dephasing due to magnetic field inhomogeneity occurring on a large length scale was taken into account in data analysis. The resulting corrected T2* values provide a better measure of the effect of microscopic field variations that are more intrinsic to the tissue under investigation and are therefore more useful in modeling general signal

Acknowledgments

Grant support from the UK Medical Research Council (G9900259) is acknowledged. We also acknowledge the help of Dr. Matthew Clemence (Philips Medical Systems) in implementing the T2* mapping sequence.

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