Longitudinal relaxation rate ( R 1 ) in lung: a systematic review and meta-analysis

The lung magnetic resonance longitudinal relaxation rate $R_1$ varies with disease but reported values in healthy subjects (HS) also differ markedly between studies.

To evaluate the reported values and variances of observed $R_1$ accounting for field strength $B_0$ , acquisition method, and disease.

A systematic literature search was performed to identify studies quantifying $R_1$ or its reciprocal $T_1$ . The relationship between field strength and observed R₁ was analysed in all healthy subject data. Data were fit to the heuristic equation $R_1=B\ast {(B}_0^A)$ . The variances between-study $\mathrm{bs}$ , within-study-between-subject $\mathrm{wsbs}$ , and within-study-within-subject $\mathrm{wsws}$ , in HS were assessed. Linear correlation between observed R₁ and TE was also investigated.

Fifty-nine papers were selected, 50 quantifying R₁ in HS and 20 in disease, representing 1559 human and non-human subjects, with $B_0$ ranging from 0.2 T to 9.4 T. In HS, the result of the fit on all healthy subjects was A = -0.227 ± 0.020 s^-1 T^-1, B = 0.923 ± 0.014 s^-1 with variance components $bs>wsbs>wsws$ . The reference values for R₁ at $B_0$ between 0.55 T and 7 T were derived. For inversion recovery with gradient echo readout at 1.5 T, a previously observed negative relationship between $R_1$ and TE was confirmed.

We provide reference values for lung $R_1$ across all commonly used field strengths. The variation in HS lung observed $R_1$ reported in different studies in different centres likely reflects methodological differences. Investigators wishing to compare lung $R_1$ values with previous reports should take account of this irreproducibility, and multicentre projects should standardise to minimise between-centre variance.