Linear Combination of Multiecho Data: Short T₂ Component Selection

The myelin sheath, which is wrapped around the axons in the brain, can be affected by many diseases, resulting in cognitive and physical disability. Other work showed water in the myelin sheath has a T₂ ≈ 15 ms. The current standard technique to estimate the fraction of myelin water in vivo is to collect multiecho data and fit the decay curves using a nonnegative least-squares (NNLS) algorithm. A new algorithm was developed to calculate optimized coefficients which were used to linearly combine multiecho data to estimate the myelin water signal. A set of simulations showed the new technique was accurate over a broad range of myelin water signal. The myelin water fraction from brain regions in scans from five volunteers, estimated by the linear combination method, agreed with the myelin water fraction estimated by the standard technique. The strength of the new technique is that the linear combination does not assume an underlying T₂ model and is 20,000 times faster than NNLS.