Introduction

High field ¹H MR spectroscopy in human brain has advantages in high sensitivity and spectral resolution. This feature can lead to accurate quantitation. In absolute quantitation of metabolites of human brains in ¹H MRS, tissue water is widely used as an internal concentration reference [1]. Water content in a volume of interest (VOI) is estimated from fractions of GM, WM and CSF with the literature values. However, pathological changes of water content will cause quantitation error. This problem can be resolved using an external concentration reference (Fig. 1). However, this method cannot be used at high B₀ field due to B₁ inhomogeneity.
One of important findings on B₁ that the transmission RF field and the reception sensitivity are represented as B₁⁺ and B₁^-, respectively and that these fields differ in spatial distribution at high B₀ field even when a transceiver RF coil is used [2]. While B₁⁺ can be measured, reception sensitivity of B₁^- cannot be measured in human brains because it is always associated with signal intensities. Then, reception sensitivities between the VOI in a human brain and the external reference cannot be compared at high B₀ field.
We have reported that the linear relationship exists between B₁⁺ and B₁^- around a uniform area even at high B₀ field [3]. Water content maps could be measured in human brains using this relationship at 4.7T. In this work, we will apply this relationship to absolute quantitation in ¹H MRS at high B₀ field and will demonstrate measurement of concentration in experiments using water phantoms.

Methods

Proposed absolute quantitation method
In our proposed method, a water phantom as a reference and a human brain are measured separately (Fig. 2). The following measurements are required to obtain a reception sensitivity in the VOI of a human brain; B₁⁺ maps both in the phantom and in the human brain, and localized water signal in a uniform area in the phantom. Form the finding that the linear relationship exists between B₁⁺ and B₁^- around a uniform area [3], the ratio of reception sensitivities of B₁^-s in uniform areas (red squares in Fig. 2), denoted as Coef_uniform can be expressed by the ratio of measured B₁⁺s.
$$\begin{align}Coef_{uniform}&=\frac{B_{1\ \ brain.uniform}^{-}}{B_{1\ \ ref.uniform}^{-}}\\&=\frac{B_{1\ \ brain.uniform}^{+}}{B_{1\ \ ref.uniform}^{+}}\tag{1}\end{align}$$
where subscript texts of brain.uniform and ref.uniform denote the uniform area inside the human brain and that inside the reference phantom, respectively. Next, B₁^-s can be compared between the VOI (yellow dotted square) and the uniform area (red dotted square) in the human brain using our previously reported ratio map method [4].
$$Coef_{VOI}=\frac{B_{1\ \ brain.VOI}^{-}}{B_{1\ \ brain.uniform}^{-}}.\tag{2}$$
From these equations, reception sensitivities can be compared between the VOI in the human brain and the uniform area (red square) in the reference phantom.
$$\begin{align}Coef&=Coef_{VOI}\times Coef_{uniform}\\&=\frac{B_{1\ \ brain.VOI}^{-}}{B_{1\ \ ref.uniform}^{-}}.\tag{3}\end{align}$$
Then, concentration of a metabolite in the human brain denoted as Conc_metab can be computed by
$$Conc_{metab}=Conc_{ref}\times \frac{PA_{brain.VOI.metab}}{Coef\times PA_{ref.uniform}}\tag{4}$$
where Conc_ref and PA_ref.uniform denote the concentration of water that is 55.5 M and a peak area of water in the uniform area in the phantom, respectively, and PA_{brain.VOI.metab} denotes PA of a metabolite in the localized ¹H spectrum of the human brain.
Experiments
All experiments were performed using a 4.7T whole-body MR system (INOVA, Agilent) by using a volume TEM coil both for transmission and reception. To demonstrate the proposed absolute quantitation method, we performed experiments with a cylinder water phantom with 150-mm diameter and 170-mm length as a reference (Fig. 3a) and a spherical saline phantom with 130-mm diameter for measuring (Fig. 3b). Absolute B₁⁺ maps were measured by the phase method where a set of two SE images were acquired using hyperbolic secant pulses form both excitation and refocusing [5]. The same conditions of RF power were used both in the phantom and in the human brain. Instead of the ratio map in a human brain, a reception sensitivity map in the saline phantom for measuring was measured by an adiabatic SE sequence [6] to avoid B₁⁺ inhomogeneity with condition of TR/TE = 4000/26 ms. Localized water signals both in the uniform area (red square in Fig. 3a) in the reference phantom and in the VOI (yellow dotted square) in the saline phantom were measured by the STEAM sequence with TR/TE = 15,000/4 ms. The VOI was set up around the edge of the phantom of non-uniform area (Fig. 3b). The voxel volumes were 8ml in the both phantoms.

Results & Discussion

Amplitudes of B₁⁺ in the uniform areas are 0.234 [KHz] in the reference phantom and 0.123 [KHz] in the saline phantom for measuring, respectively. The ratio of the VOI to the uniform area calculated from measured B₁^- map in the saline phantom was 0.776 (Fig. 3). The peak areas of the water spectra in arbitrary units were 141.8 in the reference phantom and 56.5 in the saline phantom, respectively. Then, a concentration of water in the saline phantom for measuring was calculated as 54.2 [M] using Eq. 4 whose error was -2.3% to 55.5 [M].

Conclusions

Our proposed method is useful for absolute quantitation in ¹H MRS at high B₀ field. This can also be applied in a human brain.

Acknowledgements

No acknowledgement found.