Complications of nonlinear echo time spacing for measurement of T (2).

Some consequences of using nonlinear echo spacing in multi-echo sequences for measuring T(2) were investigated under the conditions of imperfect RF refocusing or diffusion losses. Although using nonlinear echo spacing has previously been shown to estimate T(2) more accurately, the effect of such spacing is shown to be detrimental when sequences use imperfect RF refocusing pulses. The progressive loss of transverse magnetization that results from imperfect refocusing will alter estimates of T(2) regardless of the echo spacing. However, when the echo spacing is nonlinear, this loss of magnetization also introduces non-mono-exponential T(2) components. Such an effect may distort relative amplitudes of a multi-component T(2) distribution or generate multiple T(2) components where they do not exist. Diffusion through inhomogeneous magnetic fields results in a similar loss of magnetization and T(2) distortion. For these reasons, the use of nonlinearly spaced echoes, while providing in theory a more appropriate sampling of transverse relaxation, is not appropriate for many imaging situations.