Magnetic Resonance Imaging of Diffusion in the Presence of Background Gradients and Imaging of Background Gradients

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Abstract The measurement of the self-diffusion coefficient D by an NMR technique that uses an applied gradient G A can be corrupted by systems that have a background magnetic field gradient G 0 and also by imaging gradients G 1 , when used in an imaging mode. In a nonimaging mode, the corrupting cross term G A · G 0 can be eliminated in the diffusion measurement by use of an alternating-pulse-field-gradient (APFG) sequence that allows an accurate and uncorrupted measurement of D . A Carr-Purcell echo train enables the measurement of the expectation value, 〈 D G 2 0 〉; assuming D and G 0 to be uncorrelated will allow 〈 G 2 0 〉 to be determined. An image of D or of 〈 D G 2 0 〉 may be obtained without the corrupting G A · G 0 and G A · G I terms by appending a standard imaging sequence to an APFG sequence or a Carr-Purcell sequence, respectively; assuming D and G 0 to be uncorrelated will allow (〈 G 2 0 〉 to be determined within each pixel. Measurements of D and 〈 G 2 0 〉 and their images are made in apple flesh in which minute air bubbles are shown to produce the large 〈 G 2 0 〉. Their values in an 81 g Golden Delicious apple at a measuring frequency of 100 MHz were D = 1.42 × 10 −5 cm 2 /s and [formula] = 8.9 G/cm.