Simultaneous T₂ -weighted real-time MRI of two orthogonal slices

MR guidance is used during therapy to detect and compensate for lesion motion. T₂ -weighted MRI often has a superior lesion contrast in comparison to T₁ -weighted real-time imaging. The purpose of this work was to design a fast T₂ -weighted sequence capable of simultaneously acquiring two orthogonal slices, enabling real-time tracking of lesions.

To generate a T₂ contrast in two orthogonal slices simultaneously, a sequence (Ortho-SFFP-Echo) was designed that samples the T₂ -weighted spin echo (S^- ) signal in a TR-interleaved acquisition of two slices. Slice selection and phase-encoding directions are swapped between the slices, leading to a unique set of spin-echo signal conditions. To minimize motion-related signal dephasing, additional flow-compensation strategies are implemented. In both the abdominal breathing phantom and in vivo experiments, a time series was acquired using Ortho-SSFP-Echo. The centroid of the target was tracked in postprocessing steps.

In the phantom, the lesion could be identified and delineated in the dynamic images. In the volunteer experiments, the kidney was visualized with a T₂ contrast at a temporal resolution of 0.45 s under free-breathing conditions. A respiratory belt demonstrated a strong correlation with the time course of the kidney centroid in the head-foot direction. A hypointense saturation band at the slice overlap did not inhibit lesion tracking in the semi-automatic postprocessing steps.

The Ortho-SFFP-Echo sequence delivers real-time images with a T₂ -weighted contrast in two orthogonal slices. The sequence allows for simultaneous acquisition, which could be beneficial for real-time motion tracking in radiotherapy or interventional MRI.