Figure 1.

Acquisition protocol for 5D relaxation–diffusion MRI. (a) Pulse sequence for acquiring images encoded for relaxation and diffusion in a 5D space defined by the echo time ${\mathit{\tau }}_{\mathrm{E}}$ , and $b$ -tensor trace $b$ , normalized anisotropy $b_{\mathrm{\Delta }}$ , and orientation ( $\mathrm{\Theta }$ , $\mathrm{\Phi }$ ). An EPI image readout block acquires the spin echo produced by slice-selective 90 and 180 ${}^{\circ }$ radio-frequency pulses. The 180 ${}^{\circ }$ pulse is encased by a pair of gradient waveforms allowing for diffusion encoding according to principles from multidimensional solid-state NMR (Topgaard, 2017) (red, green, and blue lines). The signal is encoded for the transverse relaxation rate $R_{\mathrm{2}}$ by varying the value of ${\mathit{\tau }}_{\mathrm{E}}$ . (b) Numerically optimized gradient waveforms (Sjölund et al., 2015) yielding four distinct $b$ -tensor shapes ( $b_{\mathrm{\Delta }}=-\mathrm{0.5}$ , 0.0, 0.5, and 1) (Eriksson et al., 2015).