Low field slice-selective ZTE imaging ofultra-short T2 tissues based on spin-locking

Magnetic Resonance Imaging (MRI) of hard biological tissues is very challenging due to small proton abundance and to ultra-short T 2 decay times, especially at low magnetic fields where sample magnetization is weak. While several pulse sequences, such as Ultra-short Echo Time(UTE), Zero Echo Time (ZTE) and SWeep Imaging with Fourier Transformation (SWIFT), have been developed to cope with ultra-short lived MR signals, only the latter two hold promise of imaging tissues with sub-millisecond T 2 times at low fields. All these sequences are intrinsically volumetric, thus 3D, because standard slice selection using a long soft radio-frequency pulse is incompatible with ultra-short lived signals. The exception is UTE, where double half pulses can perform slice selection,although at the cost of doubling the acquisition time. Here we demonstrate that spin-locking is a very versatile and robust method for slice selection for ultra-short lived signals, and present three ways of joining this pulse sequence to ZTE imaging of the selected slice. With these tools,we demonstrate slice-selected 2D ex vivo imaging of the hardest tissues in the body at low field (260mT) within clinically acceptable times.