FIGURE 1.

Zero echo time (ZTE) data acquisition. (A) The gradient G applied for radial center‐out encoding is ramped up before radio‐frequency (RF) excitation. The initial part of the signal cannot be acquired due to the dead time Δt (white dots), which leads to a k‐space gap of size k_gap. (B) After each excitation, a different radial spoke is acquired to fill a 3D k‐space volume. The dead time leads to a spherical gap of radius k_gap in the center of k‐space. (C) PETRA, WASPI and HYFI all provide the missing data using additional acquisitions at lower gradient strengths. However, these techniques differ in acquisition timing and geometry. In PETRA, the inner k‐space (inside the gap) is acquired single‐pointwise on a Cartesian grid. In WASPI, a second set of radial acquisitions is performed with strongly reduced gradient strength. HYFI consists of a Cartesian single‐point core surrounded by several radially acquired shells. Note that the figure partially reuses schemes of previous publications by the same author ²² , ²³