An MR imaging method for simultaneous measurement of gaseous diffusion constant and longitudinal relaxation time

A magnetic resonance imaging method for simultaneous and accurate determina tion of gaseous diffusion constant and longitudinal relaxation time is pres ented. The method is based on direct observation of diffusive motion. Initi ally, a slice-selective saturation of helium-3 (He-3) spins was performed o n a He-3/O-2 phantom (9 atm/2 atm), A time-delay interval was introduced af ter saturation, allowing spins to diffuse in and out of the labeled slice. Following the delay interval a one-dimensional (1-D) projection image of th e phantom was acquired. A series of 21 images was collected, each subsequen t image having been acquired with an increased delay interval. Gradual spre ading of the slice boundaries due to diffusion was thus observed. The proje ction profiles were fit to a solution of the Bloch equation corrected for d iffusive motion, The fitting procedure yielded a value of D-3He = 0.1562 +/ - 0.0013 cm(2)/s, in good agreement with a measurement obtained with a modi fied version of the standard pulsed-field gradient technique. The method al so enabled us to accurately measure the longitudinal relaxation of 3He spin s by fitting the change of the total area under the projection profiles to an exponential. A value of T-1 = 1.67 s (2 T field) was recorded, in excell ent agreement with an inversion recovery measurement. (C) 1999 Elsevier Sci ence Inc.