FIGURE 3.

Acquisition timing and T ₂* weighting for zero echo time (ZTE)‐based sequences PETRA, WASPI and HYFI. (A) Time of acquisition in k‐space (i.e. the time after excitation when each k‐space point is acquired). For clarity, only part of the positive k‐space is displayed. It includes the inner k‐space (k < k_gap), colored in blue, red and green, respectively (c.f. Figure 1C), as well as part of the radially acquired outer k‐space in black (c.f. Figure 1B). (B) Modulation transfer functions (MTFs) in k‐space associated with T ₂* relaxation. In PETRA, the inner k‐space is acquired with single‐point imaging (SPI) (i.e. every single data point is acquired with a new radio‐frequency excitation and after the same dead time Δt). Different k‐space locations are reached by adapting the gradient strength, which is inversely proportional to the slope of the lines through the origin. In this way, all points in the inner k‐space have the same T ₂* weighting, forming a plateau in the MTF. The radially acquired signal in outer k‐space (black line) decays exponentially and is equivalent for all techniques (c.f. Figure 1B). In WASPI, the inner k‐space is acquired radially with strongly reduced gradient strength, creating heavy T ₂* weighting, a disruption at k_gap, and thus image artifacts. In HYFI, all points of the inner k‐space are acquired within a restricted time range t_acq to largely take advantage of the efficiency of radial encoding while limiting signal amplitudes to a range R sufficiently small to avoid image artifacts. Only a few central points are acquired with SPI