Fig. 3B–D shows the same 3 mm slice selective hp 83Kr images as Fig. 3A, but with a delay period

td between inhalation and start of the image acquisition ranging from 0.5 s to 1.5 s (td = 0 s in Fig. 3A). A new bolus of hp 83Kr was delivered for each of the images. As a clear trend observed directly in these PS 341 four images (Fig. 3A–D), the signal originating from the major airways was less affected by the delay time than the rest of the lung. The cause for the slower relaxation was presumably the smaller surface to volume (S/V) ratio in the airways as opposed to the alveolar space. Smaller airways were not resolved but contribute to the contrast observed in the MR images. Fig. 3E shows a T1 relaxation time map obtained from the td dependent signal decay of each volume element in Fig. 3A–D. The longitudinal relaxation time (averaged over 20 this website voxel) for the trachea is T1 = 5.3 ± 1.9 s and T1 = 3.0 ± 0.9 s for the main stem bronchus. The averaged relaxation times measured in lung parenchyma adjacent to the major airways and in the periphery of the lung are T1 = 1.1 ± 0.2 s and T1 = 0.9 ± 0.1 s respectively. The signal decays of selected voxel are shown in Fig. 4. The observed T1 data are in reasonable agreement with previous,

spatially unresolved bulk measurements of 83Kr T1 relaxation in excised rat lungs that also demonstrated that the addition of up to 40% of O2 did not significantly alter the T1 times [22]. SQUARE originates from surfaces but its effect is detected in the gas phase due to rapid exchange. It is however not known to what depth the alveolar surface, which is comprised Fossariinae of surfactant molecules and proteins, followed by a water layer, cell tissue, and the vascular system (filled with phosphate buffer solution in this work), is probed by the SQUARE effect. The relaxation of the krypton dissolved in extracellular water is too slow, i.e. T1 = 100 ms at 298 K [29], to be a major contributor to the observed T1 values in the alveolar region, given the small quantity of krypton dissolved in extracellular water. SQUARE may therefore originate from a deeper layer (i.e. cell tissue)

or may be caused by interactions of the krypton atoms with the outer surfactant layer. The answer to this question could have profound impact on potential usage of SQUARE for disease related contrast but its exploration is beyond the scope of this work. As Fig. 2 and Fig. 3 demonstrate, the extraction technique from low pressure (90–100 kPa) SEOP cells works well, generating reproducibly Papp = 2.0% with a line narrowed laser providing 23.3 W of power incident at the SEOP cell. This resulted in an approximately 10 fold increase in MR signal intensity as compared to the previously published results on hp 83Kr MRI in excised rat lungs [19]. An additional factor of 8.7 improvement in signal to noise ratio was achieved by using isotopically enriched to 99.925% 83Kr gas.