Implementation of the X-centric pulse sequence at low field for MRI of water penetration in clay

Abstract

Although the relaxation time constants of free water are relatively long, the relaxation of water in concrete and other sedimentary materials is significantly shorter. Dissolved ions and porous environments can cause increased magnetic susceptibility effects, leading to the apparent transverse relaxation time T₂^⁎ of this water to decrease drastically, from seconds to less than a millisecond. The longer T₂^⁎ of the low field regime (less than 0.5 T) should allow for 2D and even 3D imaging of water content in these types of materials; developing a suitable technique for imaging of short-T₂^⁎ samples would permit imaging of porous rocks and concrete.

A 12 mL wet bentonite clay sample was placed within a syringe and allowed to absorb increasing volumes of standing water. This progressing absorption was imaged on a 73.5 mT magnetic resonance imaging (MRI) system using the X-Centric pulse sequence. This pulse sequence is a modified version of the common gradient echo (GE) pulse sequence, in which each half of k-space is acquired separately, from the centre outwards in the readout direction, ensuring minimal T₂^⁎-weighting of the resulting image and allowing for 2D imaging within the short time frame of the shorter T₂^⁎ of water in the clay. Bulk relaxation measurements of T₂^⁎ and the longitudinal relaxation time T₁ were performed for increasing water content, with a mean T₁ of 12.0 ± 1.1 ms and mean T₂^⁎ of 4.5 ± 0.7 ms; 2D imaging of the clay sample was performed with both GE and X-Centric. In addition, a 2D T₂^⁎ map was generated from eight X-Centric images taken at different echo times.

The X-Centric pulse sequence was demonstrated to be an effective imaging method for short signal-lifetime samples, such as water trapped in bentonite clay. The ease of implementation, minimal diffusion-weighting and T₂^⁎ weighting of the k-space centre, and considerable gains in signal-to-noise ratio and imaging efficiency position this pulse sequence as a viable alternative or complement to conventional GE acquisitions. Additionally, the short echo-time of the X-Centric pulse sequence allows it to be used effectively with non-proton MRI, including ²³Na and fluorinated gases (e.g., ¹⁹F) where the T₂^⁎-decay is a potentially significant source of signal decay.