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Least-squares reverse time migration (LSRTM) is powerful for imaging complex geological structures. Most researches are based on Born modeling operator with the assumption of small perturbation. However, studies have shown that LSRTM based on Kirchhoff approximation performs better; in particular, it generates a more explicit reflected subsurface and fits large offset data well. Moreover, minimizing the difference between predicted and observed data in a least-squares sense leads to an average solution with relatively low quality. This study applies L1-norm regularization to LSRTM (L1-LSRTM) based on Kirchhoff approximation to compensate for the shortcomings of conventional LSRTM, which obtains a better reflectivity image and gets the residual and resolution in balance. Several numerical examples demonstrate that our method can effectively mitigate the deficiencies of conventional LSRTM and provide a higher resolution image profile.

To assess the diagnostic value of T2 star-weighted angiography (SWAN) sequence for diffuse axonal injury (DAI) by virtue of correlation analysis between the number, volume and regional distribution of haemorrhagic lesions determined with SWAN sequence and clinical variables.Twenty-eight DAI patients were included in our study and were divided into subgroups in compliance with dichotomized clinical variables separately. Global and regional number, volume and distribution of haemorrhagic lesions were compared between groups by non-parametric Mann-Whitney U test (two tailed) and independent samples t test. Spearman's rank correlation analysis was performed to compare the dichotomized clinical variables with the global and regional number and volume of lesions.Patients with lower Glasgow Coma Scale (GCS) score (≤8, n = 16) or prolonged coma (4 days, n = 15) or abnormal pupillary light reflex (PLR, n = 15) had a greater global number (p ≤ 0.01) and apparent volume (p ≤ 0.01) of haemorrhagic lesions. In our study, haemorrhage extent in most brain regions, such as frontal white matter (FW), parietotemporaloccipital white matter (PTOW), corpus callosum (CC), thalamus (THAL), brain stem (BS), were greater in the lower GCS group, in the prolonged coma group and in the abnormal PLR group (p ≤ 0.05). Significant correlations were found between haemorrhage extent in global range and the dichotomized clinical variables (p ≤ 0.01). Correlations were also found between haemorrhage extent in most regions, such as FW, PTOW, CC, THAL and BS, and the dichotomized clinical variables (p ≤ 0.05). The number of involved regions was much more higher in the lower GCS group and the prolonged coma group (p0.0001).More accurate and objective assessment of injury can be obtained in DAI patients via SWAN sequence.