In vivo mapping of brain myo-inositol

Abstract

Myo-Inositol (MI) is one of the most abundant metabolites in the human brain located mainly in glial cells and functions as an osmolyte. The concentration of MI is altered in many brain disorders including Alzheimer's disease and brain tumors. Currently available magnetic resonance spectroscopy (MRS) methods for measuring MI are limited to low spatial resolution. Here, we demonstrate that the hydroxyl protons on MI exhibit chemical exchange with bulk water and saturation of these protons leads to reduction in bulk water signal through a mechanism known as chemical exchange saturation transfer (CEST). The hydroxyl proton exchange rate (k=600 s(-1)) is determined to be in the slow to intermediate exchange regime on the NMR time scale (chemical shift (∆ω)>k), suggesting that the CEST effect of MI (MICEST) can be imaged at high fields such as 7 T (∆ω=1.2×10(3)rad/s) and 9.4 T (∆ω=1.6×10(3) rad/s). Using optimized imaging parameters, concentration dependent broad CEST asymmetry between ~0.2 and 1.5 ppm with a peak at ~0.6 ppm from bulk water was observed. Further, it is demonstrated that MICEST detection is feasible in the human brain at ultra high fields (7 T) without exceeding the allowed limits on radiofrequency specific absorption rate. Results from healthy human volunteers (N=5) showed significantly higher (p=0.03) MICEST effect from white matter (5.2±0.5%) compared to gray matter (4.3±0.5%). The mean coefficient of variations for intra-subject MICEST contrast in WM and GM were 0.49 and 0.58 respectively. Potential overlap of CEST signals from other brain metabolites with the observed MICEST map is discussed. This noninvasive approach potentially opens the way to image MI in vivo and to monitor its alteration in many disease conditions.

Figure 1

The ¹H spectrum of 200mM MI. (A) showing the three –OH peaks downfield to bulk water proton resonance. Resonances from –CH groups are shown up field from water. The inset shows the chemical structure of MI. (B) The MI spectrum at pH 7.0 shows MI –CH resonances as well as broad –OH resonance. (C) Spectrum from PBS alone.

Figure 2

(A) Z spectra of MI at different concentrations (pH7.4) are showing asymmetry due to the CEST effect from exchangeable –OH protons of MI. (B) CEST asymmetry curves shows the CEST effect centered ~0.6ppm from the bulk water resonance.

Figure 3

A two-chamber phantom containing different concentrations of MI (2, 4, 6, 8, 10 and 12mM, pH 7.4) in the inner chamber and PBS in the outer chamber. Images were collected with a Hanning windowed saturation pulse of 6s duration and 75 Hz amplitude that was frequency selected at ±0.6ppm. Subtraction of the two images shows CEST contrast only from MI. (A) The MI signal overlay on the CEST image obtained at −0.6ppm. (B) Shows the linear dependence of MICEST effect on MI concentration with a slope of 0.74% per mM MI. (C) Depicts the changes in MICEST contrast with varying B₁and saturation pulse duration. The colorbar shows the MICEST contrast in percentage.

Figure 4

Phantom consisting of test tubes with solution of different metabolites at their physiological concentrations [glucose (3mM), glycogen (3mM), Glu (10mM), Creatine (10mM), GABA (2mM), and MI (10mM), pH=7.4] immersed in a beaker containing PBS. Only MI shows predominant CEST contrast and negligible contributions from other metabolites. The colorbar shows the MICEST contrast in percentage.

Figure 5

MICEST maps of phantom consisting of test tubes with the same concentration of MI (10mM) in different viscous media (1%, 2% and 3% agarose). The highest MICEST contrast was observed in 1% agarose medium followed by 2% and 3% at both 25°C and 37°C (A and B). (C) Shows the higher MICEST contrast in 1% agarose at 37 °C compared to 25°C. The colorbar shows the MICEST contrast in percentage.

Figure 6

Axial images of healthy human brain. (A) shows the T2 weighted image and figures B and C represent the corresponding B₀ and B₁ maps of the same slice. Figure D shows MICEST map from an axial slice of human brain. Figures E and F show the CEST maps from segmented white and gray matter regions, respectively.

Figure 7

The CEST asymmetric curves from the segmented white matter (WM) and gray matter (GM) regions of the brain. The dotted vertical line shows the peak of the MICEST at 0.6ppm.