Phase‐Contrast MRI With Hybrid One‐ and Two‐Sided Simultaneous Three‐Directional Flow‐Encoding and Velocity Spectrum Separation (HOTSPA+)

A phase-contrast MRI (PC-MRI) acceleration technique was developed using hybrid one- and two-sided simultaneous three-directional flow-encoding and velocity spectrum separation (HOTSPA+). HOTSPA+ simultaneously encodes blood flow in three directions, using one-sided encoding for the z direction and two-sided encoding for both the x- and y-directions. By this encoding method, spectra of blood flow velocities in three directions can be separated from each other by temporal frequency filtering, accelerating the acquisition, and improving the flow temporal resolution. To demonstrate the benefits of HOTSPA+, common carotid data were acquired from 10 healthy volunteers and compared with the standard PC-MRI and HOTSPA, which is a previously proposed technique that can simultaneously encode flow up to two directions. Compared to the standard PC-MRI, acquisition time using HOTSPA+ was reduced by threefold, while the flow measurements showed strong correlations with R² = 0.969 for the total volumetric flow and R² = 0.936 for the peak velocity. The total volumetric flow showed a bias of -0.09 mL (relative bias error of 3.18%), and the peak velocity demonstrated a bias of -0.16 cm/s (relative bias error of 0.18%). When using the same acquisition time, HOTSPA+ acquired higher peak velocity and more accurate mean velocity than the standard PC-MRI, while measuring similar total volumetric flow (R² = 0.971, bias = -0.06 mL, relative bias error = 1.19%), indicating that HOTSPA+ can also have higher flow temporal resolution than conventional methods. By simultaneously encoding the flow in three directions and using frequency filtering to separate and recover the velocities, our proposed HOTSPA+ technique can significantly reduce the scan time while accurately measuring the peak flow velocity and total volumetric flow.