Speaker
Description
Introduction:
Magnetic resonance imaging of short signal-lifetime samples comes with several challenges1, namely lower signal and the need for short acquisition windows. The apparent transverse relaxation time ($T_2^*$) of water-content in cement paste has been measured2 to be <0.3ms at 3T: this suggests imaging at low field (<0.5T) where $T_2^*$ is expected to be longer, permitting 2D imaging of water penetration in a clay sample. However, proton imaging at low field prohibits the use of low flip-angles if high signal is desired, so a short echo-time (TE) pulse sequence using 90° flip angles was implemented with a water and clay sample at 74mT.
Method:
The x-centric pulse sequence3 consists of acquiring each half of every k-space line separately, from the centre outwards in the readout/kx direction: this halves the acquisition duration and ensures the centre of k-space is acquired first, minimizing signal decay caused by $T_2^*$ relaxation. This pulse sequence was used to image water distribution in a 12mL bentonite clay sample on a 74mT MRI system and compared with the traditional gradient echo (GRE) sequence. Eight $T_2^*$-weighted images were obtained using 8 different TEs=0.5ms…10ms. Bulk relaxation measurements of the longitudinal ($T_1$) and apparent transverse relaxation times were also performed for increasing water content (1mL increments).
Results:
The $T_1$ relaxation was around 10ms and was largely independent of water content; the $T_2^*$ relaxation was proportional to the amount of water in the clay (3 to 5ms). The x-centric pulse sequence was 2.5 times more efficient than GRE. A 2D $T_2^*$ map was generated from eight $T_2^*$-weighted x-centric images: the global mean $T_2^*$ value was 6.4$\pm$3.2ms.
Conclusion:
We have shown that x-centric was able to image the water content in the bentonite clay with minimal $T_2^*$-weighting. To our knowledge, this is the first attempt to image water-content in bentonite clay4. The $T_2^*$ dependence on water content suggests that a $T_2^*$ map also represents a regional water absorption/content map. The short $T_1$ measured here should allow for rapid real-time 2D and 3D imaging of water penetration in porous materials, and the significantly longer $T_2^*$ at this field strength alleviates the imaging issues caused by this fast signal decay.
References:
1 Muir et al. MRC (2013); 2 Sakai et al. OJCE (2017); 3 Ouriadov et al. MRM (2015); 4 Fagan et al. MRI (2005)
| Keyword-1 | MRI |
|---|---|
| Keyword-2 | Clay |
| Keyword-3 | Water Absorption |
