Background R₂′ is an MRI measure of microscopic magnetic field inhomogeneity, and is increased by the paramagnetic effect of iron and the diamagnetic effect of myelin. R₂′ may detect features of multiple sclerosis (MS) not evident with conventional MRI.

Methods Multiecho T₂ and T₂* weighted sequences were obtained from 21 healthy controls (nine men, 12 women; mean age 36 years) and 28 MS patients (seven men, 21 women; 18 relapsing remitting, 10 secondary progressive; mean age 42 years). T₂ and T₂* relaxation time maps were created from the multiecho sequences, and R₂′ maps were created using the formula R₂′ = R₂*−R₂ = 1/T₂*−1/T₂. R₂′ was measured in MS white matter lesions and in regions of interest in normal appearing white matter (NAWM) and grey matter in all subjects.

Results R₂′ was reduced in NAWM in MS compared with controls (9.5/s vs 10.1/s, p=0.05). R₂′ was additionally reduced in lesions, both T₁ isointense (8.5/s vs 9.5/s, p=0.02) and T₁ hypointense (7.7/s vs 9.5/s, p=0.003) compared with NAWM. R₂′ tended to be higher in the basal ganglia of MS patients compared with controls, and was significantly higher in the caudate nucleus in secondary progressive MS (12.9/s vs 10.9/s, p=0.03). Increased T₂ lesion volume predicted an increase in R₂′ in the caudate (β=0.412, p=0.02).

Conclusions Reduction in R₂′ in NAWM and lesions is consistent with a decreases in myelin, tissue iron and/or deoxyhaemoglobin. Increased caudate R₂′ in patients with secondary progressive MS is consistent with increased iron deposition, as corroborated by other techniques.