Experimental study of bias in apparent exchange rate measurements

Purpose/Introduction: Double diffusion encoding (DDE)1 MRI offers a new possibility to study microstructural characteristics noninvasively. In DDE, two independent pairs of diffusion gradients are applied between excitation and acquisition.2 The technique has been proposed to study molecular exchange,3 mapping the apparent exchange rate (AXR).4-6 However, the same sequence can be used to retrieve microstructural information arising from restricted diffusion.2,7 The coupling of these two effects may compromise AXR results, as suggested by a numerical simulation.8 These calculations suggest that the restriction effect biases exchange measurements in samples comprising impermeable spheres with diameter[10 lm. The present work provides experimental confirmation of evidence for this phenomenon using a water-in-oil emulsion phantom.