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.
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