Simultaneous T -weighted real-time MRI of two orthogonal slices.

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

Methods: To generate a T contrast in two orthogonal slices simultaneously, a sequence (Ortho-SFFP-Echo) was designed that samples the T -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.

Results: In the phantom, the lesion could be identified and delineated in the dynamic images. In the volunteer experiments, the kidney was visualized with a T 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.

Conclusion: The Ortho-SFFP-Echo sequence delivers real-time images with a T -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.