Synopsis

The method for quantitative water content mapping of a human brain at high magnetic field was proposed. We demonstrated that B₁^- is proportional to B₁⁺ in a uniform area even on a nonuniform image measured at 4.7T. B₁^-s of the reference phantom and a human brain can be compared by measurable B₁⁺s in uniform areas and water content of human brain can be computed from that comparison. Our method was validated in the experiments of the mixture phantom of H₂O and D₂O. Quantitative water content maps of human brains were obtained by our method.

Introduction

MR images measured at high static magnetic field have advantages in high sensitivity and contrast. However, they suffer from non-uniform signal intensity (SI) due to B₁ inhomogeneity. Both of transmission and reception fields that are represented as B₁⁺ and B₁^-, respectively differ in spatial distribution at high field. While B₁⁺ can be measured, B₁^- cannot be measured in human brains because it is always associated with SI. SI in an image of a human brain cannot be calibrated by an external ¹H reference because the reception sensitivities of B₁^-s cannot be compared between a human brain and the reference. Then, quantitative water mapping in human brain is difficult at high field.

Figure 1 shows a T₁ weighted image and a B₁⁺ image of a human brain measured with a volume RF coil at 4.7T. Both images show uniform patterns only in the central area on that non-uniform image. While B₁^± can be expressed as [(B_1xyMag² ± B_1xyPhase²)/2]^1/2 where B_1xyMag = [(B_1x²+B_1y²)/2]^1/2 and B_1xyPhase = B_1xB_1ysin(θ_x-θ_y),¹ we thought that the phase term of B_1xyPhase will be neglected and the equivalence between B₁⁺ and B₁^- will hold in that uniform area. Then, unmeasurable B₁^- will be calculated by this relationship.

In this work, we will demonstrate that relationship between B₁⁺ and B₁^- in the phantom experiments. We will also propose a method of quantitation of water content using this relationship and will map water content in human brains.

Methods

All experiments were performed using a 4.7T whole-body MR system (INOVA, Agilent) by using a volume QD TEM coil both for transmission and reception. To demonstrate the equivalence in B₁ in the uniform area, we performed experiments with three saline spherical phantoms with 104-mm, 130-mm and 164-mm diameters adding 0.1% CuSO₄. All B₁⁺ maps were measured by the phase method² under the same conditions of RF transmission power. For B₁^- measurement we used the multiecho adiabatic spin echo (MASE) sequence³ to avoid B₁⁺ inhomogeneity with condition of TR/TE = 4000/26, 52, 78, 104, 156, 182 ms. By fitting a single exponential curve by the equation of SI = M₀^MASEexp(-TE/T₂) in each pixel on the six MASE images, M₀^MASE and T₂ maps were generated. Then, the magnitude values of B₁⁺ and the SIs of M₀^MASE in the central uniform region were compared between three phantoms (Fig. 2).

The water reference phantom was measured before human studies. SI is proportional to water concentration and B₁^-. When B₁^- may be calculated from measurable B₁⁺ in the uniform area, both B₁^-s of the reference and the human brain can be compared. Then, the water concentration in the human brain can be computed by that comparison. To validate this proposed method, we did experiments of the phantom with the water content adjusted to 0.846 by mixture of H₂O (500.3 g) and D₂O (100.5 g).

Human brain measurements were performed on five healthy female (38 ± 2.6 years) subjects. B₁⁺ mapping and MASE imaging were also performed with T₁ mapping by IR Turbo Flash sequence in all subjects. After curve fitting, T₁ correction and B₁^- correction by the ration map method,⁴ M₀^MASE images were generated. The water content in the central area, shown as the red square in Fig. 4 was computed by the proposed method and the quantitative water content map of the human brain was generated.

Results & Discussion

Figure 3 shows the relationship between magnitude values of B₁⁺ and SI of M₀^MASE images in the central area at 4.7T in all three phantoms. Significant linear relationship was shown with correlation coefficient of 0.9998 between phantoms having varied diameters. It means that B₁^- is proportional to B₁⁺ in a uniform area in a sample at 4.7T. In the validation experiments of the mixture phantom, calculated water content was 0.832, close to the water content of 0.846. Figure 4a shows the M₀^MASE image of the human brain. The B₁^- map (Fig. 4c) was calculated from the B₁⁺ map (Fig. 4b) by the ratio map method.⁴ After water content was calibrated in the central area shown as the red square in Fig. 4, quantitative water content map was generated (Fig.4 d, e). The fractions of water averaged on images measured from five subjects were 0.76 ± 0.01for gray matter and 0.60 ± 0.01 for white matter. The differences between reported values of 0.78 and 0.65⁵ are less than 10 %.

Conclusions

Acknowledgements

References

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